Manual compression bone screws

Abstract

The bone screw may be insertable into a bone and may have a proximal member, a distal member having bone engaging threads, the distal member configured to slidably engage the proximal member such that a variable length cavity is defined in the proximal and distal members, and a tension member residing at least partially within the variable length cavity. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates and urges the distal member to move towards the proximal member. The proximal and distal members may be interconnected to share bending loads with a bending transfer mechanism.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to bone fixation devices, systems, and methods. More particularly, the present disclosure relates to bone screws capable of applying compressive forces to surrounding bone. [Background technology]

[0002] Surgical procedures involving fixation of bone portions with bone screws and fasteners can fail or loosen over time due to bending loads, multi-axial forces, and / or off-axis loading scenarios that may be applied to the bone screws during the healing process. Existing bone screws and fasteners may not provide sufficient fixation and strength to overcome these bending loads, multi-axial forces, and / or off-axis loading scenarios.

[0003] Additionally, it has been observed that fracture healing, fusion of bone segments, and other forms of bone formation are promoted by pressure applied across a bone interface. Existing bone fixation systems often provide pressure when initially applied, but this pressure then diminishes over time due to subsidence, resorption, movement of the involved bone segments, loosening of fasteners, and / or other factors. Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, bone fixation devices, systems, and methods having improved fixation, strength, and bone loading characteristics are desirable. [Means for solving the problem]

[0005] The various bone fixation devices, systems, and methods of the present disclosure have been developed in response to the current state of the art, and in particular in response to problems and needs in the art that have not yet been fully addressed by currently available bone fixation devices, systems, and methods. In some embodiments, the bone fixation devices, systems, and methods of the present disclosure may provide improved bone fixation and stabilization between two or more bone portions and / or implants.

[0006] According to some embodiments, a bone screw may be insertable into a bone and may have a proximal member, a distal member having bone-engaging threads, the distal member configured to slidably engage the proximal member such that a variable length cavity is defined in the proximal and distal members, and a tension member at least partially residing within the variable length cavity. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member. At least one of the proximal and distal members may have a bending transfer mechanism moving proximally or distally from the torque transfer mechanism that transfers torque from the proximal member to the distal member while allowing slidable engagement of the distal member relative to the proximal member. The proximal and distal members may be interconnected to share bending loads with the bending transfer mechanism.

[0007] The torque transmission mechanism may have a torque transmission surface that is oriented closer to radial than circumferential with respect to a cross section perpendicular to the longitudinal axis of the bone screw.

[0008] A proximal portion of the distal member proximal to the bone engaging thread may have a larger major diameter than a minor diameter of the bone engaging thread.

[0009] The proximal member may have a proximal member stop mechanism. The distal member may have a distal member stop mechanism configured to abut the proximal member stop mechanism to prevent further extension of the tension member when the tension member is at its maximum length. The tension member may be at least partially formed from a superelastic material. The maximum length may be selected so as not to exceed a superelastic strain level of the superelastic material.

[0010] The distal end may have a distal member coupling interface that may be inserted through the proximal end of the distal member and then coupled to the distal member from within the variable length cavity.

[0011] The proximal and distal members may cooperate to define a weakest cross section with respect to bending stress distal to the center of the bone screw.

[0012] The weakest section may be located immediately adjacent the bone engaging thread.

[0013] The proximal member may have a proximal inner surface defining a proximal portion of the variable length cavity. The distal member may have an extension within the proximal portion extending proximally of the torque transfer mechanism. The extension may include a bending transfer mechanism that may have an engagement surface that presses against the proximal inner surface in response to a bending load applied between the proximal and distal members.

[0014] The proximal member may have a proximal shank and a head that is wider than the proximal shank. The engagement surface may be closer to the head than the torque transfer mechanism.

[0015] The extension may have a distal end having an engagement surface and a relief having an outer diameter smaller than the engagement surface and distal to the engagement surface. The proximal inner surface may have a projection that projects into the relief toward a longitudinal axis of the bone screw. The projection may function as a proximal member motion stop feature. The relief may define a shoulder that functions as a distal member motion stop feature configured to abut the proximal member stop feature to prevent further extension of the tension member when the tension member is at its maximum length.

[0016] According to some embodiments, a bone screw may be insertable into a bone and may have a proximal member, a distal member having bone-engaging threads, the distal member configured to slidably engage the proximal member such that a variable-length cavity is defined in the proximal and distal members, and a tension member at least partially residing within the variable-length cavity. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates, urging the distal member to move toward the proximal member. At least one of the proximal and distal members may have a torque transfer mechanism that transfers torque from the proximal member to the distal member while allowing slidable engagement of the distal member relative to the proximal member. The torque transfer mechanism may have a torque transfer surface oriented in a direction closer to a radial direction than a circumferential direction with respect to a cross section perpendicular to a longitudinal axis of the bone screw.

[0017] The distal member may have a proximal end that extends proximally to a proximal portion of the variable length cavity formed in the proximal member.

[0018] The proximal portion of the distal member may have an extension within the proximal portion of the variable length cavity that extends proximally of the torque transfer mechanism.

[0019] The torque transfer mechanism may have a spline defined by a proximal member and a distal member. The torque transfer mechanism may have a plurality of proximal member teeth extending toward the distal member, the proximal member teeth having a torque transfer surface, and a plurality of distal member teeth extending toward the proximal member such that the distal member teeth intersect the proximal member teeth, the distal member teeth having a torque receiving surface that receives torque from the torque transfer surface.

[0020] The torque receiving surfaces may also be oriented closer to radial than circumferential.

[0021] According to some embodiments, a bone screw may be insertable into a bone and may have a proximal member, a distal member having a bone-engaging thread, the distal member configured to slidably engage the proximal member such that a variable-length cavity is defined in the proximal and distal members, and a tension member residing at least partially within the variable-length cavity. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates, urging the distal member to move toward the proximal member. A proximal portion of the distal member proximal to the bone-engaging thread may have an outer diameter greater than a minor diameter of the bone-engaging thread.

[0022] The distal member may have a torque transfer mechanism that receives torque from the proximal member while allowing slidable engagement of the distal member relative to the proximal member. The torque transfer mechanism may be disposed proximal and adjacent to the bone engaging thread.

[0023] The proximal member may have a proximal shank and a head that is wider than the proximal shank. The distal member may have a distal shank proximal to the bone-engaging threads. At least one of the proximal and distal shanks may have an outer diameter no smaller than a major diameter of the bone-engaging threads.

[0024] According to some embodiments, a bone screw may be insertable into a bone and may have a proximal member, a distal member having bone engaging threads, the distal member configured to slidably engage the proximal member such that a variable length cavity is defined in the proximal and distal members, and a tension member at least partially residing within the variable length cavity. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates, urging the distal member to move toward the proximal member. The tension member may be at least partially formed from a superelastic material. The proximal member may have a proximal member stop mechanism. The distal member may have a distal member stop mechanism configured to abut the proximal member stop mechanism to prevent further extension of the tension member when the tension member is at a maximum length. The maximum length may be selected to not exceed a superelastic strain level of the superelastic material.

[0025] The maximum length may further be selected so as not to exceed the fatigue limit of the superelastic material.

[0026] A proximal member stop mechanism may be disposed proximal of the torque transfer mechanism that transfers torque from the proximal member to the distal member while allowing slidable engagement of the distal member relative to the proximal member.

[0027] The proximal member may have a proximal shank and a head that is wider than the proximal shank. The proximal member stop may be closer to the head than the torque transmitting mechanism.

[0028] The proximal member may have a proximal inner surface defining a proximal portion of the variable length cavity. The distal member may have an extension extending proximally within the proximal portion. The extension may have a distal end and a relief distal to the distal end and having a smaller outer diameter than the distal end. The proximal inner surface may have a proximal member stop feature that may have a projection that projects into the relief toward a longitudinal axis of the bone screw. The relief may define a shoulder that functions as the distal member stop feature and is configured to abut the projection when the tension member is at its maximum length.

[0029] According to some embodiments, a bone screw may be insertable into a bone and may have a proximal member, a distal member having bone engaging threads configured to slidably engage the proximal member such that a variable length cavity is defined in the proximal and distal members, and a tension member residing at least partially within the variable length cavity. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates, urging the distal member to move towards the proximal member. The distal end may have a distal member coupling interface that may be inserted through the proximal end of the distal member and then coupled to the distal member from within the variable length cavity.

[0030] The distal member may have a distal inner surface defining a distal portion of the variable length cavity. The distal inner surface may define an internal thread. The distal member coupling interface may have external threads engagable with the internal threads.

[0031] The proximal end of the tension member may have an enlarged portion. The proximal member may have a proximal shank, a head that is wider than the proximal shank, a proximal inner surface that defines a proximal portion of the variable length cavity, and a proximal opening that provides access through the head to the proximal portion of the variable length cavity. The proximal opening may be sized to allow passage of the distal end of the tension member therethrough. The proximal opening may define a shoulder upon which the enlarged portion rests upon coupling the distal member coupling interface to the distal member.

[0032] The proximal member may have a proximal shank, a head wider than the proximal shank, a proximal inner surface defining a proximal portion of the variable length cavity, and a proximal opening through the head providing access to the proximal portion of the variable length cavity. The distal member may have a distal inner surface defining a distal portion of the variable length cavity and a distal opening providing access to the distal portion of the variable length cavity. The proximal and distal openings may be sized to allow a K-wire to pass therethrough.

[0033] According to one embodiment, a method for fixing a first bone portion to a second bone portion may include forming holes in the first bone portion and the second bone portion and inserting a bone screw into the holes. The bone screw may have a proximal member, a distal member, and a tension member having a proximal end coupled to the proximal member and a distal end coupled to the distal member. The method may include applying a torque to the bone screw to move the distal member away from the proximal member such that the tension member elongates and urges the distal member to move toward the proximal member. The torque may be transferred from the proximal member to the distal member via a torque transfer mechanism. A bending transfer mechanism moving proximally or distally from the torque transfer mechanism may be positioned to share bending loads between the proximal and distal members.

[0034] The torque transmission mechanism may have a torque transmission surface that is oriented closer to radial than circumferential with respect to a cross section perpendicular to the longitudinal axis of the bone screw.

[0035] The distal member may have a bone engaging thread. A portion of the distal member proximal to the bone engaging thread may have a distal outer diameter at least as large as a proximal outer diameter of a distal-most end of the proximal member.

[0036] The proximal member may have a proximal member stop feature. The distal member may have a distal member distal stop feature. The tension member may be at least partially formed from a superelastic material. Applying torque may include abutting the distal member stop feature against the proximal member stop feature to prevent further extension of the tension member at a maximum length of the tension member. The maximum length may be selected so as not to exceed a superelastic strain level of the superelastic material.

[0037] Inserting the bone screw into the hole may include inserting a proximal member and a distal member into the hole, and after inserting the proximal member and the distal member into the hole, inserting a tension member into the hole such that a proximal end of the tension member is coupled to the proximal member and a distal end of the tension member is coupled to the distal member.

[0038] The distal member may have a bone-engaging thread. Inserting the bone screw into the hole may include engaging a second bone portion with the bone-engaging thread. The proximal and distal members may cooperate to define a weakest cross section, with respect to bending stresses, located directly proximal to the bone-engaging thread.

[0039] The distal member may be configured to slidably engage the proximal member such that a variable length cavity is defined in the proximal and distal members. The proximal member may have a proximal inner surface defining a proximal portion of the variable length cavity. The distal member may have an extension in the proximal portion extending proximally of the torque transfer mechanism. The extension may have a bend transfer mechanism that may include an engagement surface. Arranging the bend transfer mechanism to share the bend load may include disposing the engagement surface to press against the proximal inner surface in response to a bend load applied between the proximal and distal members.

[0040] The extension may have a distal end having an engagement surface and a relief distal to the engagement surface having an outer diameter smaller than the engagement surface. The proximal inner surface may have a projection that projects into the relief toward a longitudinal axis of the bone screw. The projection may function as a proximal member motion stop feature. The relief may define a shoulder that functions as the distal member motion stop feature. Application of torque may cause the distal member stop feature to abut against the proximal member stop feature preventing further extension of the tension member at its maximum length.

[0041] According to one embodiment, a method for fixing a first bone portion to a second bone portion may include forming holes in the first bone portion and the second bone portion and inserting a bone screw into the holes. The bone screw may have a proximal member, a distal member, and a tension member having a proximal end coupled to the proximal member and a distal end coupled to the distal member. The method may further include applying a torque to the bone screw to move the distal member away from the proximal member such that the tension member elongates and urges the distal member to move toward the proximal member. Applying a torque to the bone screw may include transmitting the torque from the proximal member to the distal member via a torque transmission mechanism having a torque transmission surface oriented in a direction closer to the radial direction than the circumferential direction with respect to a cross section perpendicular to a longitudinal axis of the bone screw.

[0042] The distal member may have an extension proximal to the torque transmitting mechanism that extends proximally into the proximal member.

[0043] The torque transfer mechanism may include a plurality of proximal member teeth on the proximal member extending toward the distal member. The proximal member teeth may have a torque transfer surface. The torque transfer mechanism may also include a plurality of distal member teeth on the distal member extending toward the proximal member such that the distal member teeth intersect the proximal member teeth. The distal member teeth may have a torque receiving surface. Transferring torque from the proximal member to the distal member may include receiving torque from the torque transfer surface at the torque receiving surface.

[0044] The torque receiving surfaces may also be oriented closer to radial than circumferential.

[0045] According to one embodiment, a method for fixing a first bone portion to a second bone portion may include forming holes in the first bone portion and the second bone portion and inserting a bone screw to insert the bone screw into the hole. The bone screw may have a proximal member with a proximal member stop mechanism, a distal member with a distal member stop mechanism, and a tension member formed at least in part of a superelastic material. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member. The method may further include applying a torque to the distal member away from the proximal member to urge the distal member to elongate and move toward the proximal member until the distal member stop mechanism abuts the proximal member stop mechanism, preventing further elongation of the tension member at a maximum length of the tension member. The maximum length may be selected so as not to exceed a superelastic strain level of the superelastic material.

[0046] The maximum length may further be selected so as not to exceed the strength limits of the superelastic material.

[0047] The step of applying a torque may include transmitting the torque from the proximal member to the distal member via a torque transmitting mechanism distal to the proximal member stop mechanism.

[0048] The proximal member may have a proximal shank and a head that is wider than the proximal shank. The proximal member stop may be closer to the head than the torque transmitting mechanism.

[0049] The proximal and distal members may cooperate to define a variable length cavity. The proximal member may have a proximal inner surface within a proximal portion of the variable length cavity. The distal member may have an extension extending proximally into the proximal portion of the variable length cavity. The extension may have a relief. The proximal inner surface may have a protrusion that projects into the relief toward a longitudinal axis of the bone screw. The protrusion may function as a proximal member motion stop feature. The relief may define a shoulder that functions as a distal member motion stop feature. Applying a torque may include abutting the distal member stop feature against the proximal member stop feature to prevent further extension of the tension member at the maximum length of the tension member.

[0050] According to one embodiment, a method for fixing a first bone portion to a second bone portion may include forming holes in the first bone portion and the second bone portion and inserting proximal and distal members of a bone screw into the holes. The method may further include, after inserting the proximal and distal members into the holes, inserting a tension member of the bone screw into the holes such that a proximal end of the tension member is coupled to the proximal member and a distal end of the tension member is coupled to the distal member. The method may further include, after the tension member is inserted into the hole, applying a torque to the bone screw to move the distal member away from the proximal member such that the tension member elongates and urges the distal member to move toward the proximal member.

[0051] The proximal and distal members may cooperate to define a variable length cavity. Inserting the tension member into the hole may include inserting the tension member into the variable length cavity.

[0052] The distal member may have a distal inner surface defining an internal thread. The distal member coupling interface may include an external thread. Inserting the tension member into the variable length cavity may include engaging the internal thread with the external thread.

[0053] The proximal end of the tension member may have an enlarged portion. The proximal member may have a proximal shank, a head wider than the proximal shank, a proximal inner surface defining a proximal portion of the variable length cavity, and a proximal opening through the head providing access to the proximal portion of the variable length cavity. The proximal opening may define a shoulder. Inserting the tension member into the variable length cavity may further include inserting a distal end of the tension member through the proximal opening and positioning the enlarged portion to rest on the shoulder of the proximal opening.

[0054] The proximal member may have a proximal opening providing proximal access to the variable length cavity. The distal member may have a distal opening providing distal access to the variable length cavity. The method may further include inserting a K-wire into the hole such that the K-wire is secured to the second bone portion prior to inserting the bone screw into the hole. Inserting the proximal and distal members into the hole may include passing a proximal end of the K-wire through the distal and proximal openings.

[0055] According to one embodiment, a method for fixing a first bone portion to a second bone portion may include forming holes in the first bone portion and the second bone portion and inserting a bone screw into the holes. The bone screw may include a proximal member with a proximal member stop mechanism, a distal member with a distal member stop mechanism, and a tension member formed at least in part of a superelastic material. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member. The method may further include inserting the bone screw to a first insertion depth generating a first compressive load and inserting the bone screw to a second insertion depth generating a second compressive load similar to the first compressive load. The second insertion depth may be greater than the first insertion depth. The method may further include inserting the bone screw to a third insertion depth generating a third compressive load greater than the second compressive load. The third insertion depth may be greater than the second insertion depth.

[0056] The difference between the first and second insertion depths may be between 1 mm and 4 mm.

[0057] According to one embodiment, a method of accommodating shortening between a first bone portion and a second bone portion secured together by a bone screw may include forming a hole in the first bone portion and the second bone portion and inserting a bone screw into the hole such that a first compressive load is created. The bone screw may include a proximal member with a proximal member stop mechanism, a distal member with a distal member stop mechanism, and a tension member formed at least in part of a superelastic material. The tension member may have a proximal end coupled to the proximal member and a distal end coupled to the distal member. The bone screw may generate a second compressive load less than the first compressive load upon a first total shortening between the first bone portion and the second bone portion. The bone screw may generate a third compressive load similar to the second compressive load upon a second total shortening between the first bone portion and the second bone portion that is greater than the first total shortening. The bone screw may generate a fourth compressive load less than the third compressive load upon a third total shortening between the first and second bone portions that is greater than the second total shortening.

[0058] The difference between the second total shortening and the third total shortening may be between 1 mm and 4 mm.

[0059] These and other features and advantages of the present disclosure will become more fully apparent from the following description and appended claims, or may be learned by the practice of the devices, systems and methods described herein. [Brief description of the drawings]

[0060] Exemplary embodiments of the present disclosure will become more fully apparent from the following description taken in conjunction with the accompanying drawings, in which: With the understanding that these drawings depict only exemplary embodiments and therefore should not be considered as limiting the scope of the present disclosure, exemplary embodiments of the present disclosure will be described with additional specificity and detail through the use of the accompanying drawings, in which: [Figure 1A] FIG. 1A is a perspective view of a bone screw, according to one embodiment. [Figure 1B] FIG. 1B is a side view of a bone screw, according to one embodiment. [Figure 1C]FIG. 1C is a front view of a bone screw, according to one embodiment. [Figure 1D] FIG. 1D is a rear view of a bone screw, according to one embodiment. [Diagram 2] FIG. 2 is an exploded perspective view of the bone screw of FIG. [Figure 3A] 3A is a side view of the bone screw of FIG. 1; [Figure 3B] 3B is a side view of the proximal member of FIG. 1. FIG. [Figure 3C] 3C is a side view of the distal member of FIG. 1. FIG. [Figure 3D] 3D is a side view of the tension member of FIG. 1. FIG. [Figure 4] 4 is a front elevational cross-sectional view of the bone screw of FIG. 1; [Diagram 5] 5 is another cross-sectional front elevation view of the bone screw of FIG. 1; [Figure 6A] 6A is a side cross-sectional view of the bone screw of FIG. 1 upon initial insertion into bone. [Figure 6B] FIG. 6B is a side cross-sectional view of the bone screw of FIG. 1 upon further insertion to tension the bone screw. [Figure 7] FIG. 7 is a flow chart illustrating a method of inserting a bone screw into a bone, according to one embodiment. [Figure 8] FIG. 8 is a flow chart illustrating a method of inserting a bone screw into a bone along a guidewire, according to one embodiment. [Figure 9A] FIG. 9A is a diagram illustrating stress versus strain for an exemplary superelastic material. [Figure 9B] FIG. 9B is a diagram showing the change in insertion torque and screw length during screw insertion. [Figure 10A] FIG. 10A illustrates the change in screw compression during screw insertion. [Figure 10B] FIG. 10B illustrates the change in screw decompression during relative shortening between the bone portions. [Figure 11A] FIG. 11A is a perspective view of a bone screw according to another embodiment. [Figure 11B]FIG. 11B is a side view of a bone screw according to another embodiment. [Figure 11C] FIG. 11C is a front view of a bone screw according to another embodiment. [Figure 11D] FIG. 11D is a rear view of a bone screw according to another embodiment. [Figure 12] FIG. 12 is an exploded perspective view of the bone screw of FIG. [Figure 13A] 13A is a side view of the bone screw of FIG. [Figure 13B] 13B is a side view of the proximal member of FIG. [Figure 13C] 13C is a side view of the distal member of FIG. [Figure 13D] 13D is a side view of the tension member of FIG. [Figure 14] 14 is a front elevational cross-sectional view of the bone screw of FIG. 11. [Figure 15] 15 is another cross-sectional front elevation view of the bone screw of FIG. [Figure 16A] 16A is a side cross-sectional view of the bone screw of FIG. 11 upon initial insertion into bone. [Figure 16B] FIG. 16B is a side cross-sectional view of the bone screw of FIG. 11 upon further insertion to tension the bone screw. [Figure 17] FIG. 17 is a flow chart illustrating a method of inserting a bone screw into a bone, according to one embodiment. [Figure 18] FIG. 18 is a side view of a bone screw according to yet another embodiment.

[0061] It should be understood that the drawings are for purposes of illustrating the concepts of the disclosure and may not be drawn to scale. Moreover, the drawings depict illustrative embodiments and are not intended to represent limitations on the scope of the disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] Exemplary embodiments of the present application are best understood by reference to the drawings, in which like parts are designated with like numerals throughout. It will be readily understood that the components of the present disclosure, as generally described and illustrated, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of embodiments of implants, systems, and methods, as illustrated in the drawings, are not intended to limit the scope of the disclosure, but are merely representative of exemplary embodiments of the disclosure.

[0063] The term "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. While various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0064] The following examples are included to provide guidance to those skilled in the art for carrying out representative embodiments of the subject matter of the present disclosure. In light of the present disclosure and the general level of skill of those skilled in the art, it will be understood that the following examples are intended to be merely illustrative, and that numerous changes, modifications, and alterations can be made without departing from the scope of the subject matter of the present disclosure.

[0065] Fixation of bone segments with bone screws can be utilized in a variety of surgical procedures, including but not limited to trauma fixation, arthrodesis, osteotomy, and the like. For example, trauma fixation procedures may be necessary when a high energy event causes a bone to break and fragment. Bone screws can be used to fix bone fragments in the correct anatomical position while the bone heals. Arthrodesis can treat degenerative bone joints that may cause pain and loss of joint function by removing deteriorated articular cartilage from the bone joint and holding the bone joint in compression with bone screws while the bone fuses across the joint. Osteotomy can realign a bone to a more favorable position by first cutting the bone and then using bone screws to hold the cut bone segments in the new desired position while the bone heals.

[0066] Exemplary applications / procedures that may utilize any of the fixation devices described or contemplated herein, in any configuration, with any of the features described herein, may include, but are not limited to, trauma treatments (e.g., fracture fixation, etc.), post-traumatic reconstruction (pelvis or joint fusion), spinal procedures (e.g., SI fusion, small section fixation, etc.), joint reconstruction procedures (total hip arthroplasty, total knee arthroplasty), sports related procedures, extremity procedures, craniomaxillofacial procedures, ribmate procedures, veterinary procedures, bone plating procedures (e.g., femur plates, humerus plates, tibia plates, etc.), intramedullary nailing procedures, amputee connection procedures, sarcoma treatments, shoulder / knee joint fixation, small bone fixation, correction, or fusion (e.g., foot / ankle, hand / wrist, etc.), joint fusions, procedures involving osteoporosis or fractures, and the like.

[0067] The following disclosure presents various bone fixation devices, systems, and methods for use in bone and other tissues as implantable devices (e.g., orthopedic implants, spinal implants, sports medicine implants, trauma implants, reconstructive implants, extremity implants, veterinary implants, etc.) It will be understood that any feature of any bone fixation assembly described or contemplated herein may be combined with any other bone fixation assembly described or contemplated herein without departing from the spirit or scope of the present disclosure.

[0068] According to some embodiments, a bone screw 100 may be provided. The bone screw may have a longitudinal axis 102, a proximal member 104, a distal member 106, and a tension member 108. The bone screw 100 is shown and described with reference to FIGS. 1A-5. FIGS. 1A, 1B, 1C, and 1D are perspective, side, front, and rear views of the bone screw 100. FIG. 2 is an exploded perspective view of the bone screw 100. FIGS. 3A, 3B, 3C, and 3D are side views of the bone screw 100, the proximal member 104, the distal member 106, and the tension member 108, respectively. FIG. 4 is a front elevational cross-sectional view of the bone screw 100. FIG. 5 is another front elevational cross-sectional view of the bone screw 100.

[0069] As shown, the longitudinal axis 102 of the bone screw 100 may be an axis that extends along the longest length of the bone screw 100, along the geometric center and / or axis of radial symmetry of the bone screw 100. Although the terms "proximal" and "distal" are generally used with reference to displacement along the longitudinal axis 102, they may be used as adjectives to connect features to proximal or distal members, such as proximal member 104 and distal member 106.

[0070] The proximal member 104 may have a proximal shank 110 at a distal end of the proximal member 104, a head 112 at a proximal end of the proximal member 104, and a proximal inner surface 114 that cooperates with an inner surface 115 of the proximal member 104 to define a proximal portion 132 of a variable length cavity 130. The head 112 may have a width 126 (i.e., a maximum dimension perpendicular to the longitudinal axis 102) that is greater than a width 128 of the proximal shank 110. Thus, when inserted into a bone, the head 112 may protrude across the longitudinal axis 102 to engage the outer surface of the bone against which the bone screw 100 is driven, as will be shown later. The proximal inner surface 114 and the inner surface 115 may face inwardly toward the longitudinal axis 102. The head 112 may have a driver engagement mechanism 230 that receives torque from a driver (not shown). For example, the driver engagement feature 230 may be a socket having a radially symmetrical pattern that receives a matching shaped boss on the distal end of the driver. The driver engagement feature 230 may be a hexagonal socket as shown.

[0071] The distal member 106 may have a distal shank 120 at a proximal end of the distal member 106, bone engaging threads 122 at a distal end of the distal member 106, and a distal inner surface 124 that defines a distal portion 134 of the variable length cavity 130. The bone engaging threads 122 may be designed to engage bone and may be shaped to function optimally upon insertion into a pilot hole previously formed in the bone. Alternatively, the bone engaging threads 122 may be self-tapping, allowing the bone screw 100 to form its own pilot hole in the bone into which it is inserted.

[0072] The tension member 108 may have a proximal end 140, a distal end 142, and a shank 144 extending along the longitudinal axis 102 connecting the proximal end 140 to the distal end 142. The proximal end 140 may have proximal threads 146 that facilitate coupling of the tension member 108 to the proximal member 104 via threaded engagement with internal threads 147 in the proximal member 104. The distal end 142 may have a distal flange 148 that facilitates coupling of the tension member 108 to the distal member 106 via abutment of the distal flange 148 on a corresponding surface 149 of the distal member 106 distal to the bone engaging threads 122. The distal end 142 may further have a distal tip 150 that is sufficiently sharp for bone penetration. The distal tip 150 may have one or more features, such as a channel or groove, that aid in removing bone cuts from in front of the distal tip 150. Additionally or alternatively, the distal tip 150 may have a slot 152 that facilitates rotation of the tension member 108 with a driver, such as a flat head screwdriver.

[0073] 2 and 3A, the bone screw 100 may be assembled by first inserting the proximal end 140 of the tension member 108 through the open distal end of the distal member 106 along a proximal direction until the distal flange 148 of the distal end 142 of the tension member 108 abuts a corresponding surface 149 of the distal member 106. The distal shank 120 of the distal member 106 and the proximal end 140 of the tension member 108 may then be inserted into the proximal portion 132 of the variable length cavity 130 within the proximal member 104. The tension member 108 may be rotated relative to the proximal and distal members 104 and 106 (e.g., by rotating the distal tip 150 of the tension member 108 with a flat head driver or other driver) such that the proximal threads 146 of the proximal end 140 of the tension member 108 engage the internal threads 147 of the proximal member 104.

[0074] 3A , in which the tension member 108 generally resides within a variable length cavity 130 defined by the proximal member 104 and the distal member 106. The variable length cavity 130 can include a proximal portion 132 within the proximal member 104 and a distal portion 134 within the distal member 106. A portion of the distal portion 134 may also reside within the proximal member 104, as the distal shank 120 resides within the proximal portion 132 of the variable length cavity 130.

[0075] Bone screw 100 may be used for a variety of purposes, including, but not limited to, fracture fixation, joint fixation, and implant fixation. In some implementations, bone screw 100 may be inserted through one bone portion or implant into a separate bone portion to which bone engaging threads 122 are secured. As previously mentioned, in some embodiments, bone screw 100 may be inserted into a pilot hole pre-formed in bone. In alternative embodiments, bone screw 100 may be driven against bone to form and pierce its own hole.

[0076] In either case, the bone screw 100 may be rotated (e.g., via a driver) such that the bone engaging threads 122 draw the bone screw 100 forward until the head 112 abuts the outer surface of the proximal bone portion or implant. The bone screw 100 may then be further advanced such that the distal member 106 is pushed distally relative to the proximal member 104 by the action of the bone engaging threads 122. The tension member 108 may be dimensioned such that the shank 144 of the tension member 108 elongates in response to this force, allowing the distal member 106 to move distally while the proximal member 104 remains generally in place.

[0077] Tension on the shank 144 can cause the tension member 108 to apply a compressive force, pulling the distal member 106 proximally relative to the proximal member 104. This compressive force can act across the interface between the bone portions (or between the bone and the implant, in the case of implant fixation) and can beneficially promote osseointegration, fracture healing, sustained fracture fixation, and the like.

[0078] This further advancement of the distal member 106 may continue until the bone screw 100 reaches its desired length. The length may be selected such that the tension member 108 continues to exert a compressive force even after some of the strain in the tension member 108 has been relieved due to, for example, bone subsidence, patient movement, and / or other factors.

[0079] In some embodiments, the proximal and distal members 104, 106 may be formed of a relatively high strength biocompatible material, such as titanium and / or a titanium alloy. The tension member 108 may be advantageously formed of a biocompatible superelastic material, such as Nitinol. A superelastic material may beneficially undergo significant strain while exerting a generally constant compressive force on the proximal and distal members 104, 106. Thus, the tension member 108 may maintain compression even after strained relaxation has occurred.

[0080] The bone screw 100 is likely to be subject to significant stresses when it is driven into bone and / or when a patient engages in activities with the bone screw 100 in place. With larger screws (such as most wood screws), these stresses may not be a concern. However, the bone screw 100 is desirably relatively small in diameter (e.g., 3.5 mm to 7.0 mm) as measured at the diameter of the proximal shank 110 of the proximal member 104. Thus, the bone screw 100 may have some mechanism to help distribute, transfer, and / or otherwise manage stresses within the proximal member 104, distal member 106, and / or tension member 108 to avoid failure (e.g., fracture or plastic deformation) of the proximal member 104, distal member 106, and / or tension member 108. In particular, a hollow screw of this size lacking such stress distribution mechanisms may fail during insertion and / or healing.

[0081] More specifically, bone screw 100 may have torque transmission features, length limiting features, bend transmission features, and intentionally selected outer diameters, each of which may serve to control some aspect of the stress experienced by proximal member 104, distal member 106, and / or tension member 108, as described below.

[0082] The bone screw 100 may have a torque transmission mechanism that transmits torque from the proximal member 104 to the distal member 106. The torque transmission mechanism may be configured to control stresses in the proximal member 104 and the distal member 106 associated with the application of torque when the bone screw 100 is driven into bone. It has been observed that torque transmission mechanisms having circumferentially or nearly circumferentially oriented torque transmission surfaces may be subject to high hoop stresses as these surfaces transmit torque. Traditional polyhedral interfaces (such as hexagonal holes and drivers) are subject to high stresses during torque transmission. Similarly, interfaces in which one or more flat surfaces on a cylindrical member are disposed within a hole with one or more matching flat surfaces will also be subject to high stresses.

[0083] Thus, the torque transmission mechanism employed by bone screw 100 may have a design in which the torque transmission surfaces are angled significantly from the circumferential direction. This angle may be greater than 20°, greater than 30°, greater than 40°, greater than 50°, greater than 60°, greater than 70°, or greater than 80°. In some embodiments, the angle may be 90°. Additionally, in other embodiments, the angle may be even greater than 90°. While larger angles may be beneficial for hoop stress reduction, in some embodiments sufficient hoop stress reduction may be obtained with torque transmission surfaces angled to be oriented closer to the radial direction than the circumferential direction.

[0084] 1A-5 and shown most clearly in FIG. 4, the torque transmission mechanism of the bone screw 100 may be a spline 200. The spline 200 may include an outer spline component 202 formed on the proximal inner surface 114 of the proximal member 104 and an inner spline component 206 formed on the distal shank 120 of the distal member 106. The outer spline component 202 may mesh with the inner spline component 206 such that the outer spline component 202 transmits torque to the inner spline component 206. Thus, when the proximal member 104 is rotated by the surgeon (e.g., via a driver engaging the head 112), the distal member 106 may also be rotated to drive the bone-engaging threads 122 into the bone.

[0085] 4, the outer spline component 202 may have torque transmission surfaces 203 at the tips of the outer teeth 204 as the proximal member 104 rotates about the longitudinal axis 102. The outer teeth 204 may extend along a portion of the length of the proximal portion 132 of the variable length cavity 130 parallel to the longitudinal axis 102. The outer teeth 204 may be separated from one another by outer grooves 205 that extend parallel to the longitudinal axis 102.

[0086] Although the external spline component 202 is shown with ten external teeth 204, one skilled in the art will recognize that any number of teeth may be present. In some embodiments, there may only be a single tooth. The presence of multiple teeth may help to spread the loads induced by torque transfer over additional surfaces and across multiple sectors of the proximal and distal members 104, 106.

[0087] The internal splined component 206 may have a torque receiving surface 207 that resides on the rear surface of the internal teeth 209 as the distal member 106 rotates about the longitudinal axis 102 along the rotational direction R. The internal teeth 209 may extend along a portion of the length of the distal portion 134 of the variable length cavity 130 that is parallel to the longitudinal axis 102. The internal teeth 209 may be separated from one another by internal grooves 208 that extend parallel to the longitudinal axis 102.

[0088] The number of internal grooves 208 on the internal spline component 206 may be equal to the number of external teeth 204 on the external spline component 202. In FIG. 4, there are ten external teeth 204 residing in ten internal grooves 208. Similarly, there are ten internal teeth 209 residing in ten external grooves 205. Thus, the external spline component 202 meshes with the internal spline component 206. As previously mentioned, there may be more or fewer teeth or grooves on either component. The number of teeth on one component may be equal to the number of grooves on the other, but this is not necessarily the case, and in some embodiments, unequal numbers of teeth and / or grooves may be present between the internal and external spline components.

[0089] The torque transmission surfaces 203 of the outer spline component 202 may advantageously be angularly displaced from the circumferential direction C by an angle Φ as shown in FIG. 4. Line L represents one of the torque transmission surfaces 203 of the outer spline component 202. As discussed above, the angle Φ may be significant to reduce hoop stresses in the proximal member 104 and / or the distal member 106. In some embodiments, the angle Φ may be greater than the angle Θ between the line L and the radial direction R. Thus, the angle Φ may be greater than 45°.

[0090] Spline 200 represents only one of several different types of torque transmission mechanisms that may be used within the scope of this disclosure. A variety of other torque transmission mechanisms may be used, including but not limited to polyhedral and curvilinear shapes. Polyhedral torque transmission mechanisms may include star shapes, rectangular, and / or other shapes with torque transmission surfaces that are angularly displaced from the circumferential direction. Curvilinear torque transmission mechanisms may similarly have such angled torque transmission surfaces and may include curved and / or straight line segments. In some embodiments, more organically shaped rounded splines may be used. In other embodiments, oval, elliptical, or other curvilinear torque transmission mechanisms may be present.

[0091] The bone screw 100 may also have a length limiting mechanism 210 that helps control extension of the bone screw 100. Unlimited extension of the bone screw 100 may cause the tension member 108 to fail in tension as the loads (static and / or fatigue loads) experienced by the tension member 108 may cause the tension member 108 to break or plastically deform. In some embodiments, the length limiting mechanism 210 may operate to limit the displacement of the distal member 106 relative to the proximal member 104 such that the stress on the tension member 108 remains within its superelastic zone, as shown and described below. Additionally, in some embodiments, the bone screw 100 may be designed for an infinite lifespan. Thus, the length limiting mechanism 210 may be designed to limit the displacement of the distal member 106 relative to the proximal member 104 so as not to exceed the strength limits of the tension member 108.

[0092] As shown, the length limiting mechanism 210 may include a proximal stop mechanism on the proximal member 104 and a distal stop mechanism on the distal member 106. The proximal and distal stop mechanisms may contact one another when a maximum length of the bone screw 100 is reached, preventing further distal movement of the distal member 106 relative to the proximal member 104. The proximal and distal stop mechanisms may each take many forms. One or more of each of the proximal and distal stop mechanisms may be present in a bone screw according to the present disclosure.

[0093] As shown in FIGS. 3A, 3B, and 5, the proximal member 104 may have a plurality of proximal stop holes, each of which is a protrusion 212 on the proximal inner surface 114. Each protrusion 212 may protrude inwardly from the remainder of the proximal inner surface 114 (i.e., toward the longitudinal axis 102 and distal member 106 nested within the proximal portion 132 of the variable length cavity 130). Each protrusion 212 may be formed, for example, through the use of an opening 214 formed in the outer surface of the proximal member 104. In some embodiments, the opening 214 may be formed as a blind hole separated from the proximal portion 132 of the variable length cavity 130 by a relatively thin wall. After the distal shank 120 of the distal member 106 is inserted into the proximal portion 132 of the variable length cavity 130, a pin or other protruding member may be inserted into the opening 214 and forced inward to bend the thin wall inward, thus forming each protrusion 212.

[0094] The distal member 106 may have a single distal stop feature that contacts all of the protrusions 212. As shown in Figures 3A, 3C, and 5, the distal stop feature of the distal member 106 may be a shoulder 216 that defines one end of a relief 218 formed on the distal shank 120 of the distal member 106. Specifically, the relief 218 may be formed as a reduced diameter section of the distal shank 120 on an extension of the distal shank 120 that extends proximally from the internal spline component 206. The relief 218 may be adjacent the proximal end of the distal shank 120 and may define the shoulder 216.

[0095] After fully inserting the distal shank 120 into the proximal portion 132 of the variable length cavity 130, the relief 218 may be aligned with the opening 214 of the proximal member 104. Thus, when the protrusion 212 is formed as described above, the protrusion 212 may extend inwardly into the relief 218. The protrusion 212 may protrude sufficiently into the relief 218 such that the shoulder 216 cannot move distally beyond the protrusion 212. Thus, the abutment of the shoulder 216 against the protrusion 212 may limit the extent to which the distal member 106 can move distally relative to the proximal member 104.

[0096] When the distal shank 120 is fully inserted into the proximal portion 132 of the variable length cavity 130 (such that the entire distal shank 120 is received within the proximal portion 132), the protrusion 212 may reside near the distal end of the relief 218. Thus, the length of the relief 218 may define the extent to which the distal member 106 can move distally relative to the proximal member 104.

[0097] Advantageously, length limiting mechanism 210 may be moved proximally of spline 200. Thus, length limiting mechanism 210 may operate without interfering with the operation of spline 200 and without requiring additional complexity in the torque transmission mechanism of bone screw 100.

[0098] The "length limiting mechanism" may include any of a wide variety of devices capable of limiting the elongation of a bone screw. Similarly, the "proximal motion stop" and the "distal motion stop" may each include any mechanism capable of physically impeding such elongation. Thus, the proximal and distal motion stops may include any known combination of protruding elements, including, but not limited to, flanges, bumps, tabs, detents, shoulders, and the like. Such elements may protrude inwardly, outwardly, and / or circumferentially.

[0099] The bone screw 100 may also have a bending transfer mechanism 220 that serves to transfer bending loads between the proximal member 104 and the distal member 106 at a location displaced from the location where such loads are applied. For example, when the bone screw 100 is inserted through a first bone portion and the bone engaging threads 122 are secured to a second bone portion, a user's movement (or attempted movement) may urge the second bone portion to move relative to the first bone portion. This force may be shear (i.e., urging relative motion parallel to the interface or fracture between the first and second bone portions), tension (urging relative motion or fracture perpendicular to the interface), bending (urging relative motion along an axis offset from the longitudinal axis 102 of the bone screw 100), and / or torsion (urging rotation of the second bone portion relative to the first bone portion about the longitudinal axis 102 of the bone screw 100).

[0100] These forces may result in a bending moment on the bone screw 100, which may be greatest at the interface between the first and second bone portions. The bending transfer mechanism 220 may advantageously be displaced proximally or distally from this interface, thus dispersing some of these forces away from the interface. Additionally, the bending transfer mechanism 220 may transfer some bending loads from the distal member 106 to the proximal member 104. The distal member 106 may have a smaller cross-sectional shape proximate the interface between the first and second bone portions, and thus be subject to higher bending stresses. Thus, shifting some of this bending stress to the proximal member 104 may increase the overall bending load that may be tolerated by the bone screw 100.

[0101] 3A, 3B, and 3C, the bend transfer mechanism 220 can include a proximal engagement surface 222 on the proximal member 104 and a distal engagement surface 226 on the distal member 106. In the embodiment of FIGS. 3A, 3B, and 3C, the distal engagement surface 226 can be an outwardly facing surface proximal to the shoulder 216 at the proximal end of the distal shank 120. The proximal engagement surface 222 can be disposed on a proximal portion of the inner surface 115, which faces inwardly and faces the distal engagement surface 226.

[0102] The distal engagement surface 226 may be sized to have a diameter that approximates the diameter of the proximal engagement surface 222. Advantageously, the distal engagement surface 226 may be slightly smaller than the proximal engagement surface 222 such that the distal engagement surface 226 may be received within the proximal engagement surface 222 with a clearance during assembly of the proximal and distal members 104, 106. This clearance may be relatively small such that in response to slight bending of the bone screw 100, the distal engagement surface 226 abuts the proximal engagement surface 222 to transfer a portion of the bending load from the distal member 106 to the proximal member 104.

[0103] For example, a bending load on the bone screw 100 can shift the distal member 106 such that its axis is no longer collinear with the axis of the proximal member 104. This movement of the distal member 106 can cause the distal shank 120 of the distal member 106 to move toward one side of the inner surface 115. The abutment of the distal engagement surface 226 with the proximal engagement surface 222 can limit this bending such that material near the maximum bending stress (e.g., near the interface between the bone portions) is not stressed at a level that would cause it to plastically deform or fail.

[0104] The bending transfer feature 220 is only one of many possible structures that may be used to transfer bending loads from the site of maximum stress. In particular, the bone screw 100 may include other features in addition to or as an alternative to the proximal and distal engagement surfaces 222, 226 that may also function as bending transfer features. Any surfaces of the proximal and distal members 104, 106 that abut one another in response to the application of a bending load to the bone screw 100 may be considered a bending transfer feature. In particular, the entire length of the distal shank 120 (excluding the relief 218) adjacent the internal spline component 206, and the corresponding inwardly facing region of the inner surface 115 of the proximal member 104 may also function as a bending transfer feature since they abut one another and may transfer bending loads from the distal member 106 to the proximal member 104 as the bone screw 100 is loaded in bending.

[0105] Additionally, the phrase "bend transfer feature" includes any combination of surfaces, whatever shape, adjacent to one another to transfer such loads. By way of example and not limitation, such surfaces may have cylindrical, splined, polygonal, irregular, and / or other cross-sectional shapes. Such surfaces may be parallel to the longitudinal axis 102, or in alternative embodiments, may be angled non-parallel to the longitudinal axis 102. Thus, the bend transfer feature need not be a linear extrusion, but instead may have a conical, hemispherical, or other shape that varies toward and / or away from the longitudinal axis 102.

[0106] The bone screw 100 may further have other features and / or dimensions that serve to provide the bone screw 100 with enhanced strength and / or stiffness over prior art variable length screws. For example, many known screws have a shank that is as large as the minor diameter of the thread. Such a design has the advantage of easy preparation, since a corresponding pilot hole can be formed with a drill bit (not shown) having a constant diameter. In contrast, the bone engaging thread 122 of the bone screw 100 may be sized such that the portion of the bone screw 100 adjacent the bone engaging thread 122 (excluding the head 112) is larger than the minor diameter of the bone engaging thread 122.

[0107] 3C, the bone engaging threads 122 may have a minor diameter 242, a major diameter 244, and a pitch 246. The distal shank 120 may have an outer diameter 248 adjacent the bone engaging threads 122 that is larger than the minor diameter 242 of the bone engaging threads 122. This may provide the distal shank 120, and particularly the portion of the distal shank 120 adjacent the bone engaging threads 122, with additional bending strength over that which would be present in a bone screw having a shank that is limited in size to the short diameter of the threads.

[0108] The distal shank 120 may also have an outer diameter 250 that moves proximally to the bone engaging threads 122 and also moves proximally to the internal spline component 206. The outer diameter 250 may also be larger than the minor diameter 242 of the bone engaging threads 122. In some embodiments, the outer diameter 248 and / or the outer diameter 250 may be equal to and / or larger than the major diameter 244. In yet other embodiments, the outer diameter 248 and / or the outer diameter 250 may be sized equal to, larger than, or smaller than the average of the minor diameter 242 and major diameter 244 of the bone engaging threads 122.

[0109] As a result of these deliberate sizings, the weakest cross-section of the bone screw 100 may be displaced from the location of maximum bending stress, with respect to bending. More precisely, the maximum bending stress may be experienced at the interface between items being fastened together by the bone screw 100 (e.g., between two bone fragments or portions being fastened together). A screw (not shown) having a distal thread with a small diameter equal to the major diameter of the shank of the screw may have its weakest cross-section, with respect to bending, moving significantly proximally of the thread. Thus, the location of maximum stress may be aligned with the most flexible portion of the screw, which may unfortunately be near the center of the screw.

[0110] Conversely, bone screw 100 may have its weakest cross-section, in bending, distal to the center of the thread, and therefore likely to move distal to the location of maximum bending stress. For example, the weakest cross-section of bone screw 100, in bending, may be immediately proximate bone-engaging thread 122. This location may be moved distal to the interface between the bone portions when bone screw 100 is fully inserted, because bone-engaging thread 122 may advantageously be driven fully into the distal bone portion or fragment and then further driven to cause elongation of bone screw 100, as described below.

[0111] Additionally, as shown in FIG. 3B, the proximal shank 110 may have an outer diameter 252 that is larger than the minor diameter 242 of the bone-engaging threads 122. Thus, the distal member 106 as well as the proximal member 104 may have enhanced strength and / or stiffness. As further shown in FIG. 3A, the outer diameter 252 may be as large as the major diameter 244 of the bone-engaging threads 122, providing additional strength and / or stiffness to the bone screw 100. In an alternative embodiment, the outer diameter 252 may be larger than the minor diameter 242 but smaller than the major diameter 244 of the bone-engaging threads 122.

[0112] The outer diameter 252 may conform to standard or otherwise known sizes for orthopedic screws. This may facilitate the use of the bone screw 100 in place of conventional screws for a wide variety of orthopedic applications. In some embodiments, the outer diameter 252 may be in the range of 1 mm to 10 mm. Even more specifically, the outer diameter 252 may be in the range of 2 mm to 8 mm. Even more specifically, the outer diameter 252 may be in the range of 3.5 mm to 7 mm. In some embodiments, the outer diameter 252 may be in the range of 4 mm to 6 mm. Even more specifically, the outer diameter 252 may be approximately 5 mm. FIGS. 1A through 5 are shown to scale for certain embodiments of the bone screw 100. Thus, other dimensions of the bone screw 100 may be derived from these possible values ​​of the outer diameter 252.

[0113] The bone screw 100 may be applied to the bone and / or implant structure in a wide variety of ways. In some embodiments, a pilot hole may first be formed in the bone where the bone-engaging thread 122 is to be engaged. A stepped pilot drill bit (not shown) may be advantageously used for this purpose. The stepped pilot drill bit may have a distal portion having a smaller diameter and a proximal portion having a larger diameter. The distal portion may have a length and position that corresponds to the position of the bone-engaging thread 122 when the bone screw 100 is fully inserted and / or when the bone-engaging thread 122 reaches its final position in the bone.

[0114] 6A and 6B are side cross-sectional views of the bone screw 100 of FIG. 1 upon initial insertion into a bone and upon further insertion to tension the bone screw, respectively. As shown, the bone screw 100, in this particular embodiment, may be used to secure a first bone portion 160 to a second bone portion 162. The first bone portion 160 and the second bone portion 162 may be fragments of a single fractured bone, or they may be previously separate bone structures that are to be fused together and / or otherwise secured to one another.

[0115] As shown, the first bone portion 160 and the second bone portion 162 may initially be separated from one another by an interface 164 where a gap exists between the first bone portion 160 and the second bone portion 162. The first bone portion 160 may be closer to the surgeon and may have an outer shell 166.

[0116] The bone screw 100 may be inserted first into the pilot hole. To reach the position shown in FIG. 6A, torque may be applied to the head 112 of the bone screw 100 such that the bone engaging threads 122 engage the second bone portion 162. Torque may continue to be applied until the head 112 is secured to the outer shell 166 of the first bone portion 160, as shown.

[0117] With the head 112 secured against the outer shell 166, further torque may be applied to the bone screw 100 to drive the bone engaging threads 122 further into the second bone portion 162, drawing the second bone portion 162 towards the first bone portion 160 and closing the gap at the interface 164. Leaving the bone screw 100 in this configuration may hold the second bone portion 162 and the first bone portion 160 together for a period of time, but as stress is applied to the first bone portion 160 and the second bone portion 162, or as the bone proximate the interface 164 begins to settle, a gap may again form at the interface 164.

[0118] Thus, further torque can be applied to the bone screw 100 to elongate the bone screw 100 and apply compression between the first bone portion 160 and the second bone portion 162 even after such movement and / or settling has occurred. More specifically, elongation of the bone screw 100 can occur as the shank 144 of the tension member 108 elongates, resulting in the configuration shown in FIG. 6B. The tension member 108 can then pull the distal member 106 back towards the proximal member 104, compressing the second bone portion 162 against the first bone portion 160. Such compression can continue as long as the tension member 108 elongates. The use of a superelastic material to form the tension member 108 may help provide a continuous level of compression between the first and second bone portions 160, 162 such that the bone screw 100 does not provide a high level of compression at maximum elongation, but rather continues to provide less compression as the bone screw 100 shortens due to movement of the first and second bone portions 160, 162 or as the bone screw 100 is allowed to shorten due to settling.

[0119] As previously mentioned, the length of the bone screw 100, and thus the compression applied by and to the tension member 108, may be limited by operation of the length limiting mechanism 210. Thus, when the bone screw 100 reaches the length shown in FIG. 6B, the protrusion 212 of the proximal member 104 may abut against the shoulder 216 of the distal member 106 to prevent further extension of the bone screw 100. This limit may help ensure that the tension member 108 does not fail in tension and apply excessive compression across the interface 164.

[0120] The configuration of the bone screw 100 can help control how torque is applied to the bone screw 100 to insert and then elongate the bone screw 100. A threshold torque may be required to elongate the bone screw 100, which may generally be higher than the torque level required to drive the bone screw 100 into the first bone portion 160 and the second bone portion 162 until the head 112 contacts the outer shell 166 of the first bone portion 160.

[0121] 7 is a flow chart illustrating a method 300 of inserting a bone screw 100 into a bone, according to one embodiment. As shown, the method 300 may begin at step 310, where an insertion torque is applied to the bone screw 100. As discussed above, this insertion torque may be less than a threshold torque required to cause extension of the bone screw 100. Step 310 may continue until the head 112 is secured against the outer shell 166.

[0122] The method 300 may then proceed to step 320 where additional torque is applied at a level equal to the threshold torque required to elongate the bone screw 100 until it is fully elongated. The use of a superelastic material (such as Nitinol) in the tension member 108 may allow the threshold torque to remain generally constant as the bone screw 100 elongates. This is unlike conventional materials, which may require increasing levels of tension (and therefore increasing torque) as the strain in the material increases.

[0123] Once the bone screw 100 is fully elongated, the method 300 may proceed to step 330 where the insertion torque is again applied at a level above the threshold torque as the bone screw 100 is further tightened at the first bone portion 160 and the second bone portion 162. This tightening may not further elongate the bone screw 100, and the bone screw 100 may be at its maximum length. However, for example, the head 112 may be secured deeper into the outer shell 166 to apply additional compression across the interface 164 between the first bone portion 160 and the second bone portion 162.

[0124] The bone screw 100 may not be designed for insertion over a guidewire. Full assembly of the bone screw 100 prior to insertion may preclude the use of a guidewire since the tension member 108 may occupy the variable length cavity 130 that would receive the guidewire. However, in an alternative embodiment, the bone screw may be designed for insertion over a guidewire, followed by assembly and tensioning in situ. One such embodiment is shown and described below in connection with Figures 11 through 17.

[0125] Figure 8 is a flow chart illustrating a method 400 of inserting a bone screw, such as the bone screw 100 of Figures 1A-5, into a bone, according to one embodiment. The method 400 assumes that the bone screw 100 is used to secure two bone portions (e.g., two fragments of a single bone to be repaired, or two bones to be locked together). A similar method can be envisioned for using the bone screw 100 to secure an implant to a bone.

[0126] As shown, the method 400 may begin with step 410, where bone holes are formed in the first and second bone portions. As previously described, this may be done with a stepped pilot drill bit (not shown) that forms a small hole in the second bone portion 162, and a stepped pilot drill bit (not shown) that forms a larger hole in the first bone portion 160. In an alternative embodiment, the bone screw 100 may be self-tapping and step 410 may be omitted.

[0127] The method 400 may proceed to step 420 where the bone screw 100, fully assembled (including the tension member 108), is inserted into holes formed in the first and second bone portions 162. This may be done without the use of a guide wire. Insertion of the bone screw 100 may be performed by rotating the bone screw 100 with a driver (not shown) until the head 112 of the bone screw 100 abuts the outer shell 166 (proximal bone portion) of the first bone portion 160.

[0128] In step 430, the bone screw 100 may be further advanced to extend the bone screw 100 from a first length to a second length. The first length may be the base (unextended) length of the bone screw 100, as shown in FIG. 6A. The second length may be the fully extended length of the bone screw 100, as shown in FIG. 6B. Alternatively, the second length may be less than the maximum length of the bone screw 100. The extension of the bone screw 100 according to step 430 may be performed by further rotating the bone screw 100 with a driver such that the distal member 106 moves distally relative to the proximal member 104. This rotation may be performed until the bone screw 100 reaches the second length.

[0129] After performance of step 430, the tensioning member 108 may be tensioned and may exert a compressive force urging the distal member 106 to return proximally toward the proximal member 104. This compressive force may be propagated to the interface 164 between the first bone portion 160 and the second bone portion 162 to accelerate healing and / or fusion.

[0130] Those skilled in the art will recognize that method 300 of Figure 7 and method 400 of Figure 8 may be performed with other extendable bone screws other than bone screw 100 of Figures 1A-5. Additionally, bone screw 100 may be used in connection with surgical methods other than method 300 and method 400.

[0131] Returning to the bone screw 100 of Figures 1A through 5, as previously mentioned, the tension member 108 may be formed of a superelastic material, such as Nitinol. The manner in which this affects the performance of the bone screw 100 is further shown and explained in connection with Figure 9A.

[0132] 9A is a diagram 500 illustrating stress versus strain for an exemplary hyperelastic material. Diagram 500 is idealized and is only meant to illustrate general properties of hyperelastic materials.

[0133] As shown, diagram 500 may have a strain curve 510 showing stress versus strain under increasing stress and a recovery curve 520 showing stress versus strain as stress is decreased. Strain curve 510 may have a horizontal portion 530, which indicates that the material undergoes steadily increasing strain as an upper plateau stress is applied. The strain range represented by horizontal portion 530 is referred to herein as the superelastic strain. The strain at the left end of horizontal portion 530 represents the beginning of the transition of the superelastic material from a first crystal structure phase, such as austenite, to a second crystal structure phase, such as martensite. The right end of horizontal portion 530 represents a complete transformation to the second crystal structure phase, which represents the limit of the superelastic strain. The span between the left and right ends of horizontal portion 530 is referred to herein as the superelastic zone. This upper plateau stress may be the stress experienced by tension member 108 when a threshold level of torque is applied. Horizontal portion 530 may be generally horizontal, indicating that application of torque at a threshold level (as opposed to an increasing level) continues to elongate tension member 108.

[0134] The recovery curve 520 may also have a horizontal portion 540, showing that the material undergoes a steadily decreasing strain while a constant level of stress is maintained. This shows the performance of the bone screw 100 when it is allowed to shorten again due to a shift of the first bone portion 160 and the second bone portion 162 and / or subsidence of the first bone portion 160 and / or the second bone portion 162. The strain level experienced by the tension member 108 may be equal to the compressive force applied by the bone screw 100, which forces the first bone portion 160 and the second bone portion 162 together. The length and horizontal direction of the horizontal portion 540 shows how a relatively stable compression at the lower plateau stress level can be maintained even as significant shortening of the bone screw 100 occurs. Thus, the bone screw 100 can maintain compression to help the first bone portion 160 and the second bone portion 162 heal and / or fuse, even while the bone screw 100 is shorter than its maximum length during surgery.

[0135] FIG. 9B is a diagram 600 showing the change in insertion torque and thread length during screw insertion. Torque curve 610 shows the torque required to advance bone screw 100. Length curve 620 shows the length of bone screw 100. Torque curve 610 may gradually rise and then level off at a threshold torque where bone screw 100 is fully inserted and begins to elongate. A horizontal portion 630 of torque curve 610 may show how the insertion torque remains relatively constant during elongation of bone screw 100. In the length curve, this elongation is shown by a flat portion 640 with a constant upward slope. After bone screw 100 reaches its maximum length, prior to application of the threshold torque, length curve 620 is horizontal, reflecting no change in the length of bone screw 100.

[0136] Figure 10A, like Figure 9B, is a diagram 700 showing the change in screw compression during insertion of bone screw 100. Torque curve 610 is compared to torque curve 710 for "PRIOR ART #1," a standard bone screw with no extension, and torque curve 720 for "PRIOR ART #2," a bone screw that is not made of a superelastic material and has extension provided by a more traditional coil spring.

[0137] As shown, the torque curve 710 has a relatively steep constant slope, reflecting the fact that conventional bone screws do not stretch significantly. Thus, the insertion depth of the screw is limited by the excessively high torque required to deepen the insertion of the screw and the corresponding high tension applied to the screw (and compression applied across the interface 164). Excessive torque may cause the screw to fail during insertion, and excessive compression may cause the bone to fail. Thus, the insertion depth is limited, and any settling or movement in the first bone portion 160 and / or second bone portion 162 may be expected to negate the compression across the interface 164.

[0138] Torque curve 720 also has a constant slope that is less steep than the slope of torque curve 710. This reflects the extension provided by a coil or other conventional spring, which provides greater elongation than a conventional screw, but still requires increased torque to obtain greater insertion depth. Again, any relative movement and / or settling within the first bone portion 160 and / or second bone portion 162 may reduce the elongation within the spring, reducing the compression applied across the interface between the bone portions in proportion to the slope of torque curve 720. This concept is further illustrated in FIG. 10B.

[0139] 10B is a diagram 800 illustrating the change in screw decompression during relative shortening between the bone portions. Shown is a decompression curve 810 for the bone screw 100, as well as a decompression curve 820 for Prior Art #1, and a decompression curve 830 for Prior Art #2. Decompression curves 820 and 830 show how compression applied across the interface 164 between the first bone portion 160 and the second bone portion 162 decreases in response to shortening of the bone screw. Decompression curve 810 may have a horizontal portion 840.

[0140] In a conventional bone screw, only a minimal amount of strain needs to be relaxed in the screw before all compression across the interface 164 is lost. In a conventional bone screw with a spring, strain relaxation reduces the compression across the interface 164 in relation to the slope of the decompression curve 830, which may be the inverse of the slope of the torque curve 720 in FIG. 700. Thus, the only way to maintain an optimal compression level across the interface 164 with this screw (e.g., a compression level approaching the "threshold compression" in FIG. 10B) is to apply excessive compression during surgery, so that strain relaxation only reduces the compression by the screw from the excessive compression level to a healthy compression level. As previously mentioned, applying excessive compression (i.e., by applying high torque to the screw) can cause failure of the screw and / or the surrounding bone.

[0141] In contrast, decompression curve 810 illustrates how a wide range of strain relaxation within bone screw 100 can occur without significantly altering the level of compression applied across interface 164. To accomplish this, bone screw 100 does not need to be over-torqued. Rather, bone screw 100 may be advantageously inserted just enough to remain within horizontal portion 840 of decompression curve 810. In some embodiments, horizontal portion 840 may extend over a length of 0-5 mm. More precisely, horizontal portion 840 may extend over a length of 1-4 mm. Even more precisely, horizontal portion 840 may extend over a length of 1.5-3 mm. Even more precisely, horizontal portion 840 may extend over a length of 2 mm.

[0142] This concept may be applied throughout Figures 9A, 9B, 10A, and 10B. For example, referring to Figure 9A, the tension member 108 of the bone screw 100 may be tensioned sufficiently such that the stress and strain applied by the tension member 108 remains in the horizontal portion 530 of the strain curve 510. This limit may be provided by the length limiting mechanism 210, which may limit the extension of the tension member 108 during insertion of the bone screw 100 to prevent the stress of the tension member 108 from moving beyond (i.e., to the right of) the horizontal portion 530. Thus, relaxation of the stress in the tension member 108 may only traverse the horizontal portion 540 of the recovery curve 520. This is reflected in Figure 9B, where further torque applied to the bone screw 100 after the bone screw 100 has traversed the horizontal portion 630 of the torque curve 610 may not cause further extension of the bone screw 100. All of this may be transparent to the surgeon, who may simply drive the screw in a generally conventional manner.

[0143] The bone screw 100 of FIGS. 1A-5 is only one of many embodiments of the present disclosure. Those skilled in the art will recognize that many variations are possible. For example, in some embodiments, the bone engaging threads 122 may be adapted to the type of bone being penetrated. This may involve the use of more or fewer bone engaging threads 122, or bone engaging threads 122 having different shapes and / or sizes, than those shown in FIGS. 1A-5. In other embodiments, the proximal member 104 and the distal member 106 may be reconfigured such that the proximal member (not shown) has a distal end that resides within the proximal end of the distal member (not shown). Furthermore, as discussed above, a wide variety of torque transmission mechanisms, length limiting mechanisms, bend transmission mechanisms, driver engagement mechanisms, and / or similar mechanisms may be used in addition to or in place of the torque transmission mechanisms of the bone screw 100.

[0144] In some embodiments, it may be desirable to use a variable length bone screw in conjunction with a guidewire, as described in method 1300 of Figure 17. Figures 11A-15 show a bone screw 1100 configured to facilitate use with a guidewire.

[0145] Bone screw 1100 may have a longitudinal axis 102, a proximal member 1104, a distal member 1106, and a tension member 1108. FIGS. 11A, 11B, 11C, and 11D are perspective, side, front, and rear views, respectively, of bone screw 1100. FIG. 12 is an exploded perspective view of bone screw 1100. FIGS. 13A, 13B, 13C, and 13D are side views, respectively, of bone screw 1100, proximal member 1104, distal member 1106, and tension member 1108. FIG. 14 is a front elevational cross-sectional view of bone screw 1100. FIG. 15 is another front elevational cross-sectional view of bone screw 1100. Various portions of bone screw 1100 may be identical to or similar to their corresponding portions on bone screw 100, and they will not be described again here. All descriptions relating to bone screw 100 apply to bone screw 1100 unless they are contradicted by differences between the two.

[0146] The bone screw 1100 may be configured to allow the tension member 1108 to be inserted into the proximal member 1104 and the distal member 1106 after the remainder of the bone screw 1100 (i.e., the proximal member 1104 and the distal member 1106) have been assembled together and inserted into the bone. The tension member 1108 may be designed to be inserted into and coupled to the proximal member 1104 and the distal member 1106 after the proximal member 1104 and the distal member 1106 have been implanted into the bone.

[0147] 1A-5, the tension member 1108 may have a proximal end 1140, a distal end 1142, and a shank 1144 extending along the longitudinal axis 102 connecting the proximal end 1140 to the distal end 1142. However, instead of proximal threads 146 of the tension member 108, the proximal end 1140 of the tension member 1108 may have a head 1146 that is wider than the shank 1144. The proximal member 104 may have a head 1112 with an opening 1113 leading to the interior of the proximal member 1104, which defines a surrounding shoulder 1116 against which the head 1146 of the tension member 1108 may rest.

[0148] The distal end 1142 may have distal threads 1148 that facilitate coupling of the tension member 1108 to the distal member 1106, which engage internal threads 1149 of the distal member 1106. The distal end 142 may further have a distal tip (not shown) that is sharp enough for bone penetration, or alternatively, the distal tip 1150 may be blunt, as shown, since the distal tip 1150 does not need to penetrate bone because the tension member 1108 does not need to be inserted into bone until already performed through the creation of pilot holes and / or insertion of the distal member 1106 into the bone.

[0149] The head 1146 can have one or more driver engagement features that are operable independently of the driver engagement feature 230 of the head 1112. For example, the head 1146 can have a slot 1152 (shown in FIG. 11C ) that facilitates rotation of the tension member 1108 with a driver, such as a flat head screwdriver.

[0150] In use, the proximal and distal members 1104 and 1106 may be assembled and driven into the bone over a guidewire, as described in method 1300 of FIG. 17. The guidewire may then be removed, and the tension member 1108 may be inserted into the proximal and distal members 1104 and 1106 by inserting the distal end 1142 through the opening 1113, through the proximal member 1104, and into the distal member 1106, such that the distal threads 1148 of the distal end 1142 reach the internal threads 1149 of the distal member 1106. A driver may be used to rotate the tension member 1108, causing the distal threads 1148 to engage with the internal threads 1149. When the distal threads 1148 are fully received into the internal threads 1149, the head 1146 of the proximal end 1140 of the tension member 1108 may rest on the shoulder 1116 of the head 1112 of the proximal member 1104, preventing further distal movement of the head 1146.

[0151] With the tension member 1108 in place within the proximal and distal members 1104, 1106, the bone screw 100 can be driven further into the bone to move the distal member 1106 distally relative to the proximal member 1104 and elongate the tension member 1108. The head 1146 of the tension member 1108 can continue to press against the shoulder 1116 of the proximal member 1104 as the internal threads 1149 of the distal member 1106 move distally.

[0152] Similar to bone screw 100, bone screw 1100 may have a length limiting mechanism 1210 that controls the degree to which bone screw 1100 can increase in length. Length limiting mechanism 1210 may function in a manner similar to length limiting mechanism 210 of bone screw 100.

[0153] Specifically, the length limiting mechanism 1210 may have a distal stop mechanism and a proximal stop mechanism that engage one another when the distal member 1106 reaches a maximum displacement relative to the proximal member 1104 to prevent further distal movement of the distal member 1106 relative to the proximal member 1104. The distal stop mechanism may be a protrusion 1212 on a distal shank 1120 of the distal member 1106. The protrusion 1212 may extend radially outward, away from the longitudinal axis 102 of the bone screw 1100. The proximal stop mechanism may be a shoulder 1216 defined at a distal end of a relief 1218 formed in the proximal inner surface 1114 of the proximal member 1104. The protrusion 1212 may extend radially into the relief 1218 such that distal movement of the distal member 1106 causes the protrusion 1212 to abut against the shoulder 1216 , preventing further distal movement of the distal member 1106 relative to the proximal member 1104 .

[0154] 16A and 16B are side cross-sectional views of the bone screw 1100 of FIG. 11 upon initial insertion into a bone and upon further insertion to tension the bone screw, respectively. FIG. 16A may show the bone screw 1100 immediately after performance of step 1350 of the method 1300 of FIG. 17, i.e., immediately after the tension member 1108 has been inserted and coupled to the proximal member 1104 and the distal member 1106, but before further torque is applied to the bone screw 1100 to further drive the distal member 1106 into the second bone portion 162. FIG. 16B may show the bone screw 1100 after performance of step 1360 of the method 1300 of FIG. 17, after the bone screw 1100 has been extended to its maximum length. Again, the interface 164 has been closed by compression applied between the first bone portion 160 and the second bone portion 162 by the bone screw 1100 in its elongated form.

[0155] In particular, bone screws 100 and 1100 are not always attenuated to their maximum length, and in some embodiments, it may be beneficial to stop applying torque to bone screws 100 and / or 1100 before the maximum length is reached.

[0156] 17 is a flow chart illustrating a method 1300 of inserting a bone screw into a bone along a guidewire, according to one embodiment. As shown, method 1300 may begin at step 1310, where a guidewire is inserted into two bone portions, such as first bone portion 160 and second bone portion 162 of FIGS. 6A and 6B, at a desired location for the bone screw. Method 1300 may then proceed to step 1320, where a cannulated drill (e.g., having a step diameter as described above) is inserted over the guidewire and used to form a pilot hole at the desired location.

[0157] With the pilot hole formed, method 1300 may proceed to step 1330 where the bone screw is inserted over the guide wire without the tension member. At this stage, with the guide wire in place, the bone screw can be partially or fully inserted into the bone. Step 1330 may be similar to step 310 of method 300. The guide wire may then be removed at step 1340, leaving the variable length cavity of the bone screw open.

[0158] In step 1350, a tension member may be inserted into the variable length cavity of the bone screw and connected to the proximal and distal members of the bone screw so as to be under tension as the screw elongates. Then, in step 1360, further torque may be applied to the fully assembled bone screw such that the bone screw elongates and places the tension member under tension, as in step 320 of method 300. Optionally, in a further step (not shown), further torque may be applied to the bone screw, as in step 330 of method 300.

[0159] In particular, in some alternative embodiments, the elongated bone screw may have a head designed to be partially or completely embedded in the proximal bone portion. In such embodiments, the screw threads may be provided on the screw head. One such example is shown and described in connection with FIG.

[0160] Figure 18 is a side view of a bone screw 1400 according to another embodiment of the present disclosure. Similar to the bone screw 1100 of Figures 11A-15, the bone screw 1400 may be insertable into a bone via a guide wire 1402. The bone screw 1400 may also have a proximal member 1404, a distal member 1406, and a tension member (not shown) that is functionally similar to the tension member 1108 of Figures 11A-15.

[0161] The proximal member 1404 may have a generally conically shaped head 1412 that tapers such that the head 1412 has a decreasing diameter along a distal direction. The head 1412 may have proximal threads 1414. The proximal threads 1414 may also have a tapered major diameter and a tapered minor diameter and may have a pitch that is smaller than the pitch of the bone engaging threads 1422 of the distal member 1406.

[0162] Thus, when the proximal threads 1414 engage the proximal bone portion and the bone engaging threads 1422 of the distal member 1406, the proximal member 1404 may advance more slowly than the distal member 1406. This difference in advancement speed can elongate the bone screw 1400 even when the head 1412 is embedded in the proximal bone portion.

[0163] Such an embodiment may help distribute compressive stresses from the head 1412 over a larger volume of bone and may also avoid any portion of the head 1412 protruding proximally from the proximal bone. More precisely, if desired, the head 1412 may be fully embedded in the proximal bone portion. If desired, the differential pitch between the proximal threads 1414 and the bone engaging threads 1422 may be selected such that the bone screw 1400 reaches a maximum length as the proximal surface of the head 1412 becomes flush with the outer surface of the proximal bone portion.

[0164] Throughout this specification, a reference to "one embodiment" or "the embodiment" means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, references or variations thereof recited throughout this specification do not necessarily all refer to the same embodiment.

[0165] Similarly, in the above description of the embodiments, it should be understood that various features may be grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure. However, this method of disclosure should not be interpreted as reflecting an intention that any embodiment requires more features than are expressly recited in that embodiment. Rather, inventive aspects lie in the combination of fewer than all features of a single disclosed embodiment described above.

[0166] As used herein, the term "proximal" means a location relatively closer to the user (i.e., surgeon) when the user is installing the implant. The term "distal" means a location relatively farther from the user. For example, when a user uses a driver to install a bone screw into material, the end of the bone screw that engages the driver is the proximal end and the tip of the bone screw that first engages the material is the distal end. The term "cannula" means having a central bore that runs along the longitudinal axis of the part between the proximal and distal ends of the part.

[0167] The term "first" with respect to a feature or element does not necessarily imply the presence of a second or additional such feature or element. Elements described in a means-and-function form are intended to be interpreted pursuant to 35 USC § 112, section 6. It will be apparent to those skilled in the art that changes can be made to the details of the above-described embodiments without departing from the underlying principles described herein.

[0168] The phrases "connected to," "coupled to," and "in communication with" refer to any form of interaction between two or more entities, including mechanical, electrical, magnetic, electromagnetic, fluid, and thermal interactions. Two components may be functionally coupled to one another even if they are not in direct contact with one another. The term "coupled" can include components coupled to one another via a single piece of material, as well as components that are detachably and / or non-detachably coupled to one another. The term "adjacent" refers to things that may be in direct physical contact with one another, but are not necessarily attached together. The phrase "fluid communication" refers to two or more features that are connected such that fluid in one feature can pass to another feature. As defined herein, the term "substantially" means within + / - 20% of a target value, measurement, or desired property.

[0169] While particular embodiments and applications of the present disclosure have been illustrated and described, it is to be understood that the scope of the disclosure is not limited to the precise configuration and components disclosed herein. Various modifications, changes, and variations may be made in the configuration, operation, and details of the devices and systems disclosed herein, as will be apparent to those skilled in the art.

Claims

1. A bone screw insertable into bone, comprising: a proximal member; a distal member having a bone-engaging thread, the distal member configured to slidably engage with the proximal member such that a variable-length cavity is defined within the proximal member and the distal member; a tension member at least partially present within the variable-length cavity; wherein the tension member has: a proximal end coupled to the proximal member; and a distal end coupled to the distal member such that, in response to movement of the distal member away from the proximal member, the tension member tapers and urges the distal member to move toward the proximal member; wherein at least one of the proximal member and the distal member has a bending transmission mechanism that moves proximally or distally from a torque transmission mechanism that transmits torque from the proximal member to the distal member while allowing slidable engagement of the distal member with respect to the proximal member; the proximal member and the distal member are interconnected to share a bending load by the bending transmission mechanism, the bone screw.

2. The bone screw according to claim 1, wherein the torque transmission mechanism has a torque transmission surface oriented in a direction closer to the radial direction than the circumferential direction with respect to a cross-section perpendicular to the longitudinal axis of the bone screw.

3. The bone screw according to claim 1, wherein a proximal portion of the distal member, proximal to the bone-engaging thread, has an outer diameter greater than a minor diameter of the bone-engaging thread.

4. The proximal member has a proximal member stop mechanism; the distal member has a distal member stop mechanism configured to abut against the proximal member stop mechanism to prevent further extension of the tension member when the tension member is at its maximum length; the tension member is at least partially formed from a superelastic material; the maximum length is selected such that it does not exceed the superelastic strain level of the superelastic material, the bone screw according to claim 1.

5. The bone screw according to claim 1, wherein the distal end has a distal member coupling interface that is inserted through a proximal end of the distal member and then coupled to the distal member within the variable-length cavity.

6. The bone screw according to claim 1, wherein the proximal member and the distal member cooperate to define a weakest cross-section with respect to a bending stress distal to the center of the bone screw.

7. The bone screw according to claim 6, wherein the weakest cross-section is disposed closest to the bone-engaging thread.

8. The proximal member has a proximal inner surface that defines a proximal portion of the variable-length cavity. The distal member has an extension portion that extends proximally within the proximal portion and proximal to the torque transmission mechanism. The bone screw according to claim 1, wherein the extension portion includes a bending transmission mechanism having an engagement surface that presses against the proximal inner surface in response to a bending load applied between the proximal member and the distal member. **Claim 9** The proximal member has a proximal shank, and a head wider than the proximal shank, The bone screw according to claim 8, wherein the engagement surface is closer to the head than the torque transmission mechanism. **Claim 10** The extension portion has a distal end having the engagement surface, and has an outer diameter smaller than that of the engagement surface and a relief distal to the engagement surface, The proximal inner surface has a protrusion that projects into the relief toward the longitudinal axis of the bone screw, The protrusion functions as a proximal member stop mechanism, The bone screw according to claim 9, wherein the relief defines a shoulder that functions as a distal member movement stop mechanism configured to abut the proximal member stop mechanism to prevent further extension of the tension member when the tension member is at its maximum length. **Claim 11** A bone screw insertable into bone, comprising a proximal member, a distal member having a bone-engaging thread, the distal member being configured to slidably engage the proximal member such that a variable-length cavity is defined within the proximal member and the distal member, a tension member at least partially present within the variable-length cavity, The tension member has a proximal end coupled to the proximal member, and a distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member, At least one of the proximal member and the distal member has a torque transmission mechanism that transmits torque from the proximal member to the distal member while allowing slidable engagement of the distal member with respect to the proximal member, the torque transmission mechanism having a torque transmission surface oriented in a direction closer to the radial direction than the circumferential direction with respect to a cross-section perpendicular to the longitudinal axis of the bone screw. **Claim 12** The bone screw according to claim 11, wherein the distal member has a proximal end that extends proximally to a proximal portion formed in the proximal member of the variable-length cavity. **Claim 13** The proximal portion of the distal member has an extension that extends proximally to the torque transmission mechanism within the proximal portion of the variable length cavity, the bone screw according to claim 12.

14. The torque transmission mechanism has a spline defined by the proximal member and the distal member, The torque transmission mechanism is, A plurality of proximal member teeth extending toward the distal member, the proximal member teeth having a torque transmission surface, and A plurality of distal member teeth, the distal member teeth having a plurality of distal member teeth extending toward the proximal member so that the distal member teeth intersect the proximal member teeth, The distal member teeth have a torque receiving surface that receives torque from the torque transmission surface, the bone screw according to claim 11.

15. The torque receiving surface is also oriented in a direction closer to the radial direction than the circumferential direction, the bone screw according to claim 14.

16. A bone screw insertable into bone, A proximal member, A distal member having a bone engaging thread, the distal member configured to slidably engage with the proximal member such that a variable length cavity is defined within the proximal member and the distal member, A tension member at least partially present within the variable length cavity, The tension member is, A proximal end coupled to the proximal member, and A distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member, The proximal portion of the distal member, proximal to the bone engaging thread, has an outer diameter greater than the minor diameter of the bone engaging thread, the bone screw.

17. The distal member has a torque transmission mechanism that receives torque from the proximal member while allowing slidable engagement of the distal member with respect to the proximal member, The torque transmission mechanism is disposed proximal and adjacent to the bone engaging thread, the bone screw according to claim 16.

18. The proximal member is, A proximal shank, and Has a head wider than the proximal shank, The distal member has a distal shank proximal to the bone engaging thread, At least one of the proximal shank and the distal shank has an outer diameter not less than the major diameter of the bone engaging thread, the bone screw according to claim 16.

19. A bone screw insertable into bone, A proximal member, A distal member having a bone engaging thread, the distal member configured to slidably engage with the proximal member such that a variable length cavity is defined within the proximal member and the distal member; A tension member at least partially present within the variable length cavity, The tension member, A proximal end coupled to the proximal member, A distal end coupled to the distal member such that in response to movement of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member; The tension member is at least partially formed from a superelastic material, The proximal member has a proximal member stop mechanism, The distal member has a distal member stop mechanism configured to abut against the proximal member stop mechanism to prevent further extension of the tension member when the tension member is at its maximum length, The maximum length is selected such that it does not exceed the superelastic strain level of the superelastic material, bone screw.

20. The bone screw according to claim 19, wherein the maximum length is further selected such that it does not exceed the fatigue limit of the superelastic material.

21. The bone screw according to claim 19, wherein the proximal member stop mechanism is disposed proximal to a torque transmission mechanism that transmits torque from the proximal member to the distal member while allowing slidable engagement of the distal member with respect to the proximal member.

22. The proximal member, A proximal shank, and A head wider than the proximal shank, The bone screw according to claim 21, wherein the proximal member stop mechanism is closer to the head than the torque transmission mechanism.

23. The proximal member has a proximal inner surface defining a proximal portion of the variable length cavity, The distal member has an extension extending proximally within the proximal portion, The extension, A distal end, and A relief distal to the distal end and having an outer diameter smaller than the distal end, The proximal inner surface has a proximal member stop mechanism, the proximal member stop mechanism having a protrusion protruding into the relief toward the longitudinal axis of the bone screw, The bone screw according to claim 19, wherein the relief functions as a distal member stop mechanism and defines a shoulder configured to abut against the protrusion when the tension member is at its maximum length.

24. A bone screw insertable into bone, A proximal member, and A distal member having a bone engagement thread, the distal member configured to slidably engage with the proximal member such that a variable length cavity is defined within the proximal member and the distal member; A tension member at least partially present within the variable length cavity; The tension member; A proximal end coupled to the proximal member; A distal end coupled to the distal member, wherein in response to movement of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member; The distal end is inserted through a proximal end of the distal member and then has a distal member coupling interface that couples to the distal member from within the variable length cavity, a bone screw. **Claim 25** The distal member has a distal inner surface that defines a distal portion of the variable length cavity; The distal inner surface defines an internal thread; The distal member coupling interface has an external thread engageable with the internal thread, the bone screw according to claim 24. **Claim 26** The proximal end of the tension member has an enlarged portion; The proximal member; A proximal shank; A head wider than the proximal shank; A proximal inner surface that defines a proximal portion of the variable length cavity; and A proximal opening that provides access to the proximal portion of the variable length cavity through the head; The proximal opening is sized to allow passage of a distal end of the tension member therethrough; The proximal opening defines a shoulder on which the enlarged portion rests when the distal member coupling interface is coupled to the distal member, the bone screw according to claim 25. **Claim 27** The proximal member; A proximal shank; A head wider than the proximal shank; A proximal inner surface that defines a proximal portion of the variable length cavity; and A proximal opening that provides access to the proximal portion of the variable length cavity through the head; The distal member; A distal inner surface that defines a distal portion of the variable length cavity; and A distal opening that provides access to the distal portion of the variable length cavity; The proximal opening and the distal opening are sized to allow a K-wire to pass therethrough, the bone screw according to claim 24. **Claim 28** A method for fixing a first bone portion to a second bone portion, the method comprising: Forming holes in the first bone portion and the second bone portion; Inserting a bone screw into the holes, the bone screw comprising: A proximal member; A distal member; and A tension member having a proximal end coupled to the proximal member and a distal end coupled to the distal member, and a step of having the tension member. Applying torque to the bone screw to move the distal member away from the proximal member so that the tension member elongates and urges the distal member to move toward the proximal member. The step of applying torque to the bone screw includes: Transmitting from the proximal member to the distal member via a torque transmission mechanism; A bending transmission mechanism that moves proximally or distally from the torque transmission mechanism is arranged to share a bending load between the proximal member and the distal member. A method.

29. The method according to claim 28, wherein the torque transmission mechanism has a torque transmission surface oriented in a direction closer to the radial direction than the circumferential direction with respect to a cross section perpendicular to the longitudinal axis of the bone screw.

30. The distal member has bone-engaging threads, A portion of the distal member proximal to the bone-engaging threads has a distal outer diameter that is at least as large as the proximal outer diameter of the most distal end of the proximal member. The method according to claim 28.

31. The proximal member has a proximal member stop mechanism, The distal member has a distal member stop mechanism, The tension member is at least partially formed from a superelastic material, Applying the torque causes the distal member stop mechanism to abut against the proximal member stop mechanism to prevent further extension of the tension member at the maximum length of the tension member, The method according to claim 28, wherein the maximum length is selected so as not to exceed the superelastic strain level of the superelastic material.

32. The step of inserting the bone screw into the hole includes: Inserting the proximal member and the distal member into the hole; After inserting the proximal member and the distal member into the hole, inserting the tension member into the hole such that the proximal end of the tension member is coupled to the proximal member and the distal end of the tension member is coupled to the distal member. The method according to claim 28.

33. The distal member is configured to slidably engage with the proximal member such that a variable-length cavity is defined within the proximal member and the distal member, The proximal member has a proximal inner surface that defines a proximal portion of the variable-length cavity, The distal member has an extension portion that extends proximally to the torque transmission mechanism within the proximal portion. The extension part is a bending transmission mechanism having a mating surface. The step of arranging the bending transmission mechanism to share a bending load is the method according to claim 28, which is arranged to press the mating surface against the proximal inner surface in response to a bending load applied between the proximal member and the distal member.

34. The extension part has a distal end with the mating surface, has an outer diameter smaller than that of the mating surface and a relief distal to the mating surface. The proximal inner surface has a protrusion protruding into the relief toward the longitudinal axis of the bone screw. The protrusion functions as a proximal member stop mechanism. The relief defines a shoulder that functions as a distal member stop mechanism. The step of applying the torque is the method according to claim 33, which abuts the distal member stop mechanism against the proximal member stop mechanism to prevent further extension of the tension member at the maximum length of the tension member.

35. A method for fixing a first bone portion to a second bone portion, comprising the steps of forming holes in the first bone portion and the second bone portion, inserting a bone screw into the holes, the bone screw having a proximal member, a distal member, and a tension member having a proximal end coupled to the proximal member and a distal end coupled to the distal member, and applying torque to the bone screw to move the distal member away from the proximal member such that the tension member elongates and the distal member moves toward the proximal member. The step of applying torque to the bone screw includes transmitting torque from the proximal member to the distal member through a torque transmission mechanism having a torque transmission surface oriented in a direction closer to the radial direction than the circumferential direction with respect to a cross-section perpendicular to the longitudinal axis of the bone screw.

36. The distal member has an extension proximal to the torque transmission mechanism that extends proximally into the proximal member, according to the method of claim 35.

37. The torque transmission mechanism includes a plurality of proximal member teeth of the proximal member extending toward the distal member, the plurality of proximal member teeth having a torque transmission surface, and a plurality of distal member teeth of the distal member extending toward the proximal member such that the distal member teeth intersect the proximal member teeth, the plurality of distal member teeth having a torque receiving surface. ​ ​ The method according to claim 35, wherein the step of transmitting torque from the proximal member to the distal member includes receiving torque from the torque transmission surface at the torque receiving surface.

38. The method according to claim 37, wherein the torque receiving surface is also oriented in a direction closer to the radial direction than the circumferential direction.

39. A method for fixing a first bone portion to a second bone portion, comprising: forming holes in the first bone portion and the second bone portion; inserting a bone screw into the holes, the bone screw comprising: a proximal member having a proximal stop mechanism; a distal member having a distal stop mechanism; and a tension member formed at least partially of a superelastic material, wherein the tension member: has a proximal end coupled to the proximal member; and has a distal end coupled to the distal member; applying a torque to move the distal member away from the proximal member such that the tension member elongates and urges the distal member to move toward the proximal member until the distal stop mechanism abuts the proximal stop mechanism to prevent further elongation of the tension member at its maximum length; The method, wherein the maximum length is selected so as not to exceed the superelastic strain level of the superelastic material.

40. The method according to claim 39, wherein the maximum length is further selected so as not to exceed the strength limit of the superelastic material.

41. The method according to claim 39, wherein the step of applying the torque includes transmitting torque from the proximal member to the distal member via a torque transmission mechanism distal to the proximal stop mechanism.

42. The proximal member: has a proximal shank; and has a head wider than the proximal shank, The method according to claim 41, wherein the proximal stop mechanism is closer to the head than the torque transmission mechanism.

43. The proximal member and the distal member cooperate to define a variable-length cavity, the proximal member has a proximal inner surface within a proximal portion of the variable-length cavity, the distal member has an extension that extends proximally within the proximal portion of the variable-length cavity, the extension having a relief, the proximal inner surface has a protrusion that projects into the relief toward the longitudinal axis of the bone screw, the protrusion functions as a proximal member movement stop mechanism, the relief defines a shoulder that functions as a distal member movement stop mechanism. The step of applying the torque includes abutting the distal member stop mechanism against the proximal member stop mechanism to prevent further extension of the tension member at the maximum length of the tension member, the method according to claim 39.

44. A method for fixing a first bone portion to a second bone portion, comprising: forming holes in the first bone portion and the second bone portion; inserting a proximal member and a distal member of a bone screw into the holes; after inserting the proximal member and the distal member into the holes, inserting the tension member of the bone screw into the holes such that a proximal end of the tension member is coupled to the proximal member and a distal end of the tension member is coupled to the distal member; after the tension member is inserted into the holes, applying torque to the bone screw to move the distal member away from the proximal member so as to urge the tension member to elongate and the distal member to move toward the proximal member.

45. The proximal member and the distal member cooperate to define a variable-length cavity, The method according to claim 44, wherein inserting the tension member into the holes includes inserting the tension member into the variable-length cavity.

46. The distal member has a distal inner surface defining an inner thread, The distal inner surface has an outer thread, The method according to claim 45, wherein the step of inserting the tension member into the variable-length cavity includes engaging the inner thread with the outer thread.

47. The proximal end of the tension member has an enlarged portion, The proximal member includes: a proximal shank, a head wider than the proximal shank, a proximal inner surface defining a proximal portion of the variable-length cavity, and a proximal opening providing access to the proximal portion of the variable-length cavity through the head, The proximal opening defines a shoulder, The method according to claim 45, wherein the step of inserting the tension member into the variable-length cavity further includes inserting the distal end of the tension member through the proximal opening and disposing the enlarged portion on the shoulder of the proximal opening.

48. The proximal member has a proximal opening providing proximal access to the variable-length cavity, The distal member has a distal opening providing distal access to the variable-length cavity, The method further includes disposing a K-wire in the holes before inserting the bone screw into the holes. Inserting the proximal member and the distal member into the hole includes passing the proximal end of the K-wire through the distal opening and the proximal opening, the method of claim 45.

49. A method for fixing a first bone portion to a second bone portion, forming holes in the first bone portion and the second bone portion; inserting a bone screw into the hole, the bone screw comprising: a proximal member having a proximal member stop mechanism, a distal member having a distal member stop mechanism, and a tension member formed at least partially of a superelastic material, the tension member including: a proximal end coupled to the proximal member, and a distal end coupled to the distal member; inserting the bone screw to a first insertion depth that generates a first compressive load; inserting the bone screw to a second insertion depth that generates a second compressive load similar to the first compressive load, the second insertion depth being greater than the first insertion depth; inserting the bone screw to a third insertion depth that generates a third compressive load greater than the second compressive load, the third insertion depth being greater than the second insertion depth; A method comprising:

50. The difference between the first insertion depth and the second insertion depth is between 1 mm and 4 mm, the method of claim 49.

51. A method of accommodating shortening between a first bone portion and a second bone portion fixed together by a bone screw, forming holes in the first bone portion and the second bone portion; inserting the bone screw into the hole such that a first compressive load is created, the bone screw comprising: a proximal member having a proximal member stop mechanism, a distal member having a distal member stop mechanism, and a tension member formed at least partially of a superelastic material, the tension member having: a proximal end coupled to the proximal member, and a distal end coupled to the distal member; the bone screw generates a second compressive load that is less than the first compressive load at a first total shortening between the first bone portion and the second bone portion; the bone screw generates a third compressive load similar to the second compressive load at a second total shortening between the first bone portion and the second bone portion that is greater than the first total shortening; The method, wherein the bone screw generates a fourth compressive load that is smaller than the third compressive load at a third total shortening between the first bone portion and the second bone portion, which is greater than the second total shortening. **Claim 52** The method according to claim 51, wherein a difference between the second total shortening and the third total shortening is between 1 mm and 4 mm.

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