Bottom loaded bone anchor assembly with resistance retaining ring and related method - Patents.com

Abstract

A bottom loaded bone anchor assembly and components thereof are disclosed that have a reduced profile and can be utilized with bone screws of various sizes. The bone anchor assembly (200) of the present disclosure includes a receiver member (206), a resistance retaining ring (204), and a shank (202). The retaining ring and shank can be proximally inserted into a bore in the receiver member. The resistance ring can have a base and a wall extending proximally therefrom. The resistance retaining ring can be disposed in a groove in the receiver member, with the base forming a seat for a shank head that can retain the shank in the receiver member. The wall of the resistance retaining ring can apply a resistance force to the shank head seated in the base to prevent unintended movement between the shank and the receiver member.

Description

[Technical field]

[0001] Disclosed herein is a bottom loaded bone anchor assembly having a resistance retention ring and associated methods. [Background technology]

[0002] Bone anchor assemblies can be used in orthopedic surgery to secure bones during healing, fusion, or other processes. For example, in spinal surgery, bone anchor assemblies can be used to secure rods or other spinal fixation elements to one or more vertebrae to provide rigid or dynamic stabilization of the spine.

[0003] A bone anchor assembly may typically include a threaded shank portion configured to be secured to a bone, and a head or receiver member attached to the shank portion and configured to receive a rod or other fixation element therein. The shank and receiver member may be assembled such that the head of the shank is retained within the receiver member and the threaded portion of the shank extends distally therefrom. In some configurations, the shank and receiver member may be provided as a polyaxial assembly, whereby the receiver member has free angular movement relative to the shank. While this freedom of movement may be useful in aligning multiple components of the bone anchor assembly, it may also pose challenges. For example, it may be difficult to maintain a desired angular orientation between the receiver member and the shank prior to locking the assembly. Thus, during provisional positioning of the assembly embedded in the bone, the receiver member may have a tendency to "float" or fall off, requiring subsequent repositioning by the user to achieve the desired alignment, or requiring the user or an assistant to hold the receiver member in the desired position during introduction of the rod or other fixation element. This may be cumbersome for the user and may add unnecessary length to the surgical procedure.

[0004] Many bone anchor assemblies can be "top loaded" in that the bone screw is inserted through a proximal opening in the receiver member, moved distally to seat the head of the screw in the receiver member, and the threaded shank is passed distally through the distal opening. However, such top loading assemblies are of limited use because the diameter of the bone screw shank is limited by the diameter and size of the receiver member. Thus, when a larger diameter shank is desired, the bone anchor assembly is often assembled in a "bottom loaded" configuration in which the shank head is loaded by passing proximally into the opening at the distal end of the receiver member. A bottom loaded bone anchor assembly can have an increased size or profile of the receiver member, for example, to accommodate a larger shank and additional components required to hold the shank in the receiver member, which can determine the placement of the spinal fixation element relative to the bone. Summary of the Invention [Problem to be solved by the invention]

[0005] There is a need for an improved bone anchor assembly that addresses the shortcomings of conventional designs, for example, a bone anchor assembly having a reduced profile and improved pre-positioning that can be assembled and used regardless of the diameter of the bone engaging component. [Means for solving the problem]

[0006] The present disclosure provides a bone anchor assembly and associated method that has a reduced overall assembly size and can be utilized across a wide variety of surgical procedures. The bone anchor assembly disclosed herein can allow bottom loading of various bone anchor components into the receiver member, thereby providing a common assembly procedure regardless of the maximum outer diameter of the bone engaging component, e.g., the bone shank. The bone anchor assembly disclosed herein can include a resistance retaining ring that can be advanced proximally into the receiver member during assembly and retained within its distal portion. The resistance retaining ring can retain the head of the bone shank within the receiver member to allow desired polyaxial movement between the receiver member and the bone shank, but provide resistance to unintended or accidental movement therebetween. More specifically, the resistance retaining ring can include a base configured to seat the head of the bone shank and a wall extending proximally from the base. The wall can contact the head of the shank and exert a radially inward resistance or frictional force on the shank head in resistance to unintended movement. In this manner, the bone anchor assemblies of the present disclosure can be configured, for example, by a surgeon or other user to position the receiving member in a desired position relative to the shank and maintained in the desired position by frictional forces exerted on the head of the shank by the walls of the retaining ring.

[0007] In one aspect, a bone anchor assembly is disclosed that includes a receiver member having a proximal end and a distal end, a central longitudinal axis extending therebetween, a longitudinal bore formed therein, and an opening at a distal end, a retaining ring, and a shank. The retaining ring is disposed within a groove formed in the receiver member and includes a base and a wall extending proximally from the base, the base of the retaining ring having a maximum outer diameter greater than a maximum outer diameter of the wall. The shank has a head portion seated within the base of the retaining ring and a bone-engaging portion extending distally from the receiver member. The wall of the retaining ring is configured to exert a drag force on the head of the shank to resist rotation thereof.

[0008] The instruments and methods described herein can have many additional features and / or variations, all of which are within the scope of this disclosure. For example, the retaining ring and shank can be configured for proximal insertion through the distal end of the receiver member. The groove formed in the receiver member can be formed on an inner surface of the distal end of the receiver member. In some embodiments, the bone anchor assembly can further include a saddle disposed within the bore of the receiver member proximal to the retaining ring, the saddle configured to apply a distal force to the shank head seated within the retaining ring.

[0009] The wall of the retaining ring can be configured to contact the head of the shank at its maximum diameter. In some embodiments, the proximal end of the wall can extend proximally beyond the maximum diameter of the head of the shank when the shank is seated within the base of the retaining ring. In other embodiments, the proximal end of the wall can be configured to contact the head of the shank at its maximum diameter. The wall of the retaining ring can optionally include a first wall segment and a second wall segment circumferentially separated from the first wall segment. The retaining ring can include a split extending through the base and the wall configured to allow selective radial expansion and compression of the retaining ring.

[0010] The base of the retaining ring can have a maximum outer diameter that is greater than the diameter of the opening at the distal end of the receiving member. The distal portion of the base of the retaining ring can have an inner diameter that is less than the maximum diameter of the head of the shank. In some embodiments, the inner diameter of the distal portion of the base of the retaining ring can be less than the maximum outer thread diameter of the bone-engaging portion of the shank.

[0011] In another aspect, a method for assembling a bone anchor assembly is disclosed, the method including radially compressing a retaining ring disposed about a shank, the shank including a head and a bone-engaging portion, the retaining ring including a base and a wall extending proximally from the base. The wall of the base has a maximum outer diameter less than a maximum outer diameter of the base. The method further includes passing the shank and the compressed retaining ring proximally through an opening in a distal end of a receiver member and advancing the retaining ring proximally within the receiver member such that at least a portion of the base aligns with a first annular recess of a groove formed in the receiver member. The method also includes expanding the retaining ring to retain at least a portion of the base within the first annular recess of the groove and seating the head of the shank against the base of the retaining ring such that the maximum diameter of the head of the shank contacts the wall of the retaining ring and the bone-engaging portion of the shank extends distally from the retaining ring.

[0012] As discussed above, any of a variety of additional steps and / or variations are possible and within the scope of the present disclosure. For example, in some embodiments, the method can further include positioning the receiver member in a desired position relative to the shank and holding the receiver member in the desired position by a counter force exerted by a wall of the retaining ring against a head of the shank. Positioning the receiver member in the desired position can optionally include moving the receiver member in multiple directions relative to the shank. In some embodiments, the counter force can be exerted by the retaining ring at an interface between a base and a wall of the retaining ring. In other embodiments, the counter force can be exerted by a proximal end of the wall of the retaining ring. In certain embodiments, the method can further include embedding the shank in bone and applying a closure mechanism to lock the receiver member in the desired position relative to the shank.

[0013] In some embodiments, the method can further include advancing a saddle proximally through a distal opening in the receiver member and urging the shank distally with a distal force exerted by the saddle against the head of the shank. In some embodiments, the above-mentioned method step of radially compressing a retaining ring disposed about the shank can further include radially compressing the retaining ring about the neck of the shank, which can aid in assembling the retaining ring to the receiver member. In some embodiments, the distal face of the retaining ring can be flush with the distal face of the receiver with at least a portion of the base of the retaining ring retained within the first annular recess of the groove.

[0014] Any of the features or variations described above may be applied to any particular aspect or embodiment of the present disclosure in several different combinations, and there is no explicit mention of any particular combination, but this is simply to avoid redundancy in this summary. [Brief description of the drawings]

[0015] The present invention will be more fully understood from the following detailed description considered in conjunction with the accompanying drawings. [Figure 1A] FIG. 2 is a perspective exploded view of a bottom loaded bone anchor assembly. [Figure 1B] FIG. 1B is a cross-sectional view of the bone anchor assembly of FIG. [Diagram 2] FIG. 1 is an exploded perspective view of a bone anchor assembly having a multi-element bone anchor. [Diagram 3] 1 is a front cross-sectional view of one embodiment of a bone anchor assembly of the present disclosure; [Figure 4] FIG. 4 is a perspective exploded view of the bone anchor assembly of FIG. 3. [Diagram 5] 4 is a perspective view of one embodiment of a resistance retaining ring of the bone anchor assembly of FIG. 3. [Figure 6] FIG. 6 is a front view of the drag retaining ring of FIG. [Figure 7] 6A is a front cross-sectional view of the drag retaining ring of FIG. 5 taken along line AA of FIG. 6. [Figure 8] 4 is a perspective view of one embodiment of a receiver member of the bone anchor assembly of FIG. 3. [Figure 9] 9 is a front cross-sectional view of the receiving member of FIG. 8 taken along line BB of FIG. 8. [Figure 10] 13 is a front cross-sectional view of another embodiment of a bone anchor assembly of the present disclosure; FIG. [Figure 11] FIG. 11 is a perspective exploded view of the bone anchor assembly of FIG. [Figure 12] 11 is a perspective view of one embodiment of a resistance retaining ring of the bone anchor assembly of FIG. 10. FIG. [Figure 13] 11 is a perspective view of one embodiment of a receiver member of the bone anchor assembly of FIG. 10. [Figure 14] 11A-11C show steps of an embodiment of a method for assembling the bone anchor assembly of FIG. 10. [Figure 15] FIG. 11 is a side view of the bone anchor assembly of FIG. 10 coupled to a spinal fixation element. [Figure 16] FIG. 1 is a side view of two embodiments of the bone anchor assemblies of the present disclosure and one conventional bone anchor assembly coupled to a rod. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Disclosed herein is a bottom-loaded bone anchor assembly that provides a reduced bone anchor assembly profile and can be utilized with a variety of bone shanks regardless of thread diameter. Thus, the present disclosure provides a general bone anchor assembly method and configuration suitable for use across a wide range of surgical procedures. More specifically, the disclosed bone anchor assembly includes a drag retaining ring that (i) prevents separation between the bone-engaging shank and the receiver member, and (ii) resists multi-axial movement between the shank and the receiver head to exert a drag force on the head of the shank, for example, allowing a surgeon to position the receiver member in a desired alignment relative to the screw. The drag retaining ring (also referred to herein as a "retaining ring") can include a base configured to seat or retain the shank head, and a wall extending proximally from the base that can exert a radially inward frictional drag force on the shank head. The resistance force exerted by the retaining ring can help maintain the relative position between the receiver member and the shank prior to locking the bone anchor, which can prevent unintended movement while still allowing free movement when intended by the user. The retaining ring and shank can be bottom loaded into the receiver member, i.e., inserted proximally through a distal opening in the receiver member, such that the bone anchor assemblies of the present disclosure are adapted for use with small, medium, or large diameter bone shanks.

[0017] Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices, systems, and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. The devices, systems, and methods specifically described herein and illustrated in the accompanying drawings are non-limiting embodiments. Features illustrated or described in connection with one embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present disclosure.

[0018] Additionally, to the extent that linear or circular dimensions are used in describing the disclosed devices and methods, such dimensions are not intended to limit the types of shapes that may be used with such devices and methods. Equivalents to such linear and circular dimensions may be determined for different geometric shapes. Furthermore, like-numbered components of the embodiments may generally have similar characteristics. Still further, the size and shape of the device and its components may depend at least on the anatomy of the subject with whom the device is used, the size and shape of the object with which the device is used, and the method and procedure with which the device is used.

[0019] 1A and 1B show a bone anchor 100 including an anchor portion or shank 102, a head or receiving member 104 for receiving a spinal fixation element coupled to the shank 102, such as a spinal rod 106, and a fastener or closure mechanism 108 for capturing the spinal fixation element within the receiving member and securing the spinal fixation element relative to the receiving member. The shank 102 includes a proximal head 110 and a distal shaft 112 configured to engage bone. The receiving member 104 has a proximal end having a pair of spaced apart arms 114A, 114B defining a recess or channel 116 therebetween, and a distal end having a distal end face defining an opening through which at least a portion of the shank 102 extends. A closure mechanism 108 is positionable between the arms 114A, 114B and can engage the arms 114A, 114B to capture a spinal fixation element (e.g., spinal rod 106) within the receiving member 104 and secure the spinal fixation element relative to the receiving member.

[0020] The proximal head 110 of the shank 102 is generally in the shape of a truncated sphere having a planar proximal surface and a generally spherical distal surface. The illustrated bone anchor 100 is a polyaxial bone screw designed for posterior implantation into the pedicle or lateral mass of a vertebra. The proximal head 110 of the shank 102 engages the distal end of the receiver member 104 in a ball and socket-like arrangement that allows the proximal head and distal shaft 112 to pivot relative to the receiver member. The distal surface of the proximal head 110 of the shank 102 and the mating surface within the distal end of the receiver member 104 can have any shape that facilitates this arrangement, including, for example, spherical (as shown), toroidal, conical, frusto-conical, and any combination of these shapes.

[0021] The distal shaft 112 of the shank 102 may be configured to engage bone, and in the illustrated embodiment includes external threads for engaging bone. The thread form of the distal shaft 112, including the number of threads, pitch, major and minor diameters, and thread shape, may be selected to facilitate connection with bone. Exemplary thread forms are disclosed in U.S. Patent Application Publication No. 2011 / 0288599, filed May 18, 2011, and U.S. Patent Application Publication No. 2013 / 0053901, filed August 22, 2012, both of which are incorporated herein by reference. The distal shaft 112 may also include other structures for engaging bone, such as hooks. For example, to facilitate delivery of the shank over a guidewire in a minimally invasive procedure, the distal shaft 112 of the shank 102 may be cannulated, having a central passage or cannula extending the length of the shank. Other components of the bone anchor 100, including, for example, the closure mechanism 108, the receiver member 104, and the compression cap or saddle 118 (discussed below), may be cannulated or have openings to allow delivery over a guidewire. To allow bone ingrowth or to allow distribution of bone cement or other materials through the shank 102, the distal shaft 112 may also include one or more sidewall openings or fenestrations in communication with a cannula. The sidewall openings may extend radially from the cannula through the sidewall of the distal shaft 112. Exemplary systems for delivering bone cement to the bone anchor 100, as well as alternative configurations of the bone anchor to facilitate cement delivery, are described in U.S. Patent Application Publication No. 2010 / 0114174, filed October 29, 2009, the disclosure of which is incorporated herein by reference. The distal shaft 112 of the shank 102 can also be coated with a material that allows for bone ingrowth, such as, for example, hydroxyapatite, and the bone anchor 100 can be partially or completely coated with an anti-infective material, such as, for example, triclosan.

[0022] The proximal end of the receiver member 104 includes a pair of spaced apart arms 114A, 114B defining a U-shaped recess 116 therebetween for receiving a spinal fixation element (e.g., spinal rod 106). Each of the arms 114A, 114B can extend from the distal end to a free end of the receiver member 104. The outer surface of each of the arms 114A, 114B can include features, such as, for example, recesses, indentations, notches, protrusions, etc., to facilitate connection of the receiver member 104 to an instrument. For example, the outer surface of each arm 114A, 114B can include an arcuate groove at the arm's respective free end. Such grooves are described in more detail in U.S. Patent No. 7,179,261, issued Feb. 20, 2007, the disclosure of which is incorporated herein by reference.

[0023] The distal end of the receiving member 104 includes a distal end face that is generally annular in shape defining a circular opening through which at least a portion of the shank 102 extends. For example, the distal shaft 112 of the shank 102 can extend through the opening.

[0024] The shank 102 can be selectively fixed relative to the receiver member 104. Prior to fixation, the shank 102 is movable relative to the receiver member 104 within a cone of angulation generally defined by the geometry of the distal end of the receiver member and the proximal head 110 of the shank 102. The bone anchor 100 can be a favored angle screw, for example, as disclosed in U.S. Patent Nos. 6,974,460, issued December 13, 2005, and 6,736,820, issued May 18, 2004, the disclosures of both of which are incorporated herein by reference. Alternatively, the bone anchor 100 can be a conventional (non-biased) polyaxial screw in which the shank 102 pivots the same amount in all directions.

[0025] A spinal fixation element (e.g., spinal rod 106) can directly contact the proximal head 110 of the shank 102 or can contact an intermediate element, such as a compression member or saddle 118. The saddle 118 can be positioned within the receiving member 104 and interposed between the spinal rod 106 and the proximal head 110 of the shank 102 to compress a distal outer surface of the proximal head into direct fixed engagement with a distal inner surface of the receiving member 104. The saddle 118 can include a pair of spaced apart arms 120A, 120B that define a U-shaped seat 122 for receiving the spinal rod 106 and a distal face for engaging the proximal head 110 of the shank 102.

[0026] The proximal end of the receiver member 104 can be configured to receive a closure mechanism 108 positionable between and engaging the arms 114A, 114B of the receiver member. The closure mechanism 108 can be configured to capture a spinal fixation element, such as a spinal rod 106, within the receiver member 104, secure the spinal rod relative to the receiver member, and secure the shank 102 relative to the receiver member. The closure mechanism 108 can be a single set screw that is externally threaded for engaging with the internal threads on the arms 114A, 114B of the receiver member 104. However, in the illustrated embodiment, the closure mechanism 108 includes an outer set screw 124 operable to act on the saddle 118 and an inner set screw 126 operable to act on the rod 106. Alternatively, or in addition, various other closure mechanisms 108 can be used, such as a nut that extends around the outer periphery of the receiving member 104, a cap or fastener that slides onto the receiving member from the side, or a cap or fastener that locks onto the receiving member with a quarter turn.

[0027] The bone anchor 100 may be used in conjunction with a spinal fixation element, such as a rigid spinal rod 106. In other embodiments, the spinal fixation element may be a dynamic stabilization member that allows controlled mobility between the instrumented vertebrae.

[0028] In use, a bone can be prepared to receive the bone anchor assembly 100, generally by drilling a hole in the bone that is appropriately sized to receive the shank 102. If not already completed, the bone anchor 100 can be assembled such that the distal shaft 112 extends through an opening in the distal end of the receiver member 104 and the proximal head 110 of the shank 102 is received in the distal end of the receiver member 104. A driver tool can be attached to the shank 102 for driving the shank into the bone. The saddle 118 can be positioned within the receiver member 104 such that the arms 120A, 120B of the saddle are aligned with the arms 114A, 114B of the receiver member 104 and the underside of the saddle 118 contacts the proximal head 110 of the shank 102. A spinal fixation element (e.g., a spinal rod 106) can be positioned in the recess 116 of the receiver member 104. The closure mechanism 108 may be engaged with internal threads provided on the arms 114A, 114B of the receiver member 104. A torsional force may be applied to the outer set screw 124, moving it within the recess 116 to press the saddle 118 against the proximal head 110 of the shank 102, thereby locking the angular position of the shank 102 relative to the receiver member 104. A torsional force may be applied to the inner set screw 126, forcing the spinal rod 106 into engagement with the saddle 118, thereby securing the spinal rod 106 relative to the receiver member 104.

[0029] Further information regarding bone anchor 100 can be found in U.S. Patent Application No. 2018 / 0325569, filed May 10, 2017, the disclosure of which is incorporated herein by reference.

[0030] 2 shows a bone anchor assembly 100' similar to the bone anchor assembly 100 of FIGS. 1A and 1B, but including a multi-element bone anchor 101' with a shank 112' advanced proximally into a receiving member 104' to engage a spherical head or ball 110'. A clip 111' locks the head 110' to the shank 112'. The spherical head 110 engages a distal end of the receiving member 104' in a ball and socket like arrangement, allowing a proximal head and a distal shaft 112 coupled thereto to pivot relative to the receiving member. The bone anchor assembly 100' can include a closure mechanism 108' and a compression member 118' as described above. The closure mechanism 108' can include an outer set screw 124' and an inner set screw 126' and can be configured to capture a spinal fixation element, such as a spinal rod 106', within the receiving member 104' and secure the spinal fixation element relative to the receiving member.

[0031] Further details of the bone anchor 100' can be found in U.S. Patent Application No. 9,775,660, filed March 14, 2013, which is incorporated herein by reference.

[0032] 3-9 illustrate one embodiment of a bone anchor assembly 200 of the present disclosure. The structure and configuration of the bone anchor assembly 200 and its various components can result in a reduced size of the bone anchor assembly and allow for a common assembly procedure regardless of the bone anchor thread size. To this end, the components of the bone anchor assembly 200 can be bottom-loaded into a receiver member including a drag retaining ring that can serve to retain the bone shank within the receiver member of the bone anchor and apply a drag thereto (described in more detail below). As shown in FIGS. 3 and 4, the bone anchor assembly 200 can include a shank 202, a drag retaining ring 204, and a receiver member 206. The bone anchor 200 can also include a compression cap or saddle 208, and a fastener or closure mechanism 210 (see FIG. 8). Except as described herein or otherwise apparent from this disclosure, the shank 202, receiver member 205, compression cap 208, and closure mechanism 210 can include any of the features of the corresponding components of the bone anchor 100 described above. For example, the shank 202 can have a head 202a and a distal shank or bone-engaging portion 202b. The bone-engaging portion 202b can include an external bone-engaging thread. The receiver member 206 can include a pair of spaced apart arms 207a, 207b that define a recess 209 therebetween. The closure mechanism 210 can be positionable between the arms 214A, 214B and engage the arms 207a, 207b to capture a spinal fixation element (e.g., spinal rod 206) within the receiver member 206 and secure the spinal fixation element to the receiver member and secure the receiver member to the shank 202. The receiver member 106 can have a proximal end 206p, a distal end 206d, and a central longitudinal axis A1 extending therebetween. The receiving member 206 may include a longitudinal bore 211 capable of receiving at least a portion of the shank 202, the retaining ring 204, the compression cap 208, and the closure mechanism 210. The shank 202 may include a central longitudinal axis A2. The shank 202 may rotate and pivot relative to the receiving member 206 about the receiving member's central longitudinal axis A1.

[0033] 3, the retaining ring 204 may be disposed between the head 202a of the shank 202 and the distal portion 206d of the receiver member 206 to (i) retain the shank head within the receiver member and (ii) exert a drag force on the shank head. More specifically, the retaining ring 204 may be at least partially retained within a groove 312 of the receiver member 206 (FIG. 9), and the shank head 202a may be polyaxially seated within the retaining ring in a ball and socket-like configuration such that the shank 202 may pivot and rotate relative to the retaining ring and receiver member. In the collinear orientation of FIG. 3, i.e., with the longitudinal axis A2 of the shank 202 aligned with the longitudinal axis A1 of the receiver member 206, the retaining ring 204 may exert a radially inward frictional force on the shank head 202a along its equator or maximum diameter D. This force exerted by the retaining ring 204 may resist relative movement between the shank head 202a and the receiving member 206, thereby positioning and temporarily holding the shank 202 in a desired position relative to the receiving member without the receiving member falling off or "floating." As described in more detail below, the resisting force exerted by the retaining ring 204 may result from an interference fit between the wall 402 (FIG. 5) of the retaining ring and the outer diameter of the shank head. The various components of the bone anchor assembly 200 will now be described in more detail with reference to FIGS. 5-9.

[0034] One embodiment of the retaining ring 204 is shown in more detail in FIGS. 5-7. The retaining ring 204 may be generally cylindrical having a base 300 configured to seat the shank head 202a and a wall 302 extending proximally from the base. The retaining ring 204 may be configured to insert the shank 202 proximally into a bore 211 in the receiving member 206 and may be retained in a groove 212 formed in a distal end 206d of the receiving member. A lumen 301 may extend through the retaining ring 204 having a distal opening 301d through a distal face 300d of the base 300 (FIG. 7). The distal opening 301d of the base has a diameter D that is smaller than the maximum diameter D of the head 202a of the shank 202. L, so that at least a portion of the shank can be retained within the retaining ring 204. In some embodiments, the diameter D of the distal opening 301d of the base can be L may be less than the maximum external thread diameter of the bone engaging portion 202b of the shank 202.

[0035] The proximal portion of the base 300 can be formed from an annular boss 304 having an outer diameter D1 that is greater than an outer diameter D2 of the distal portion 306 of the base. The wall 302 can extend proximally from the annular boss 304. The outer diameter D1 of the boss 304 can be greater than the outer diameter D3 of the wall 302 such that the outer diameter of the boss represents the maximum outer diameter of the retaining ring 204. The retaining ring 204 can include radial splits 308 that allow for selective radial expansion and compression of the retaining ring. Although the retaining ring 204 is shown with radial splits 308 extending through the base 300 and the wall 302, the retaining ring can include additional or alternative features to allow for radial expansion and compression, such as slits or notches. In this manner, the diameter of the retaining ring 204 can be selectively adjusted, for example, to reduce the outer diameter of the retaining ring for proximal insertion into the receiving member 206 and then expand the diameter to seat the retaining ring within the receiving member groove 412.

[0036] The inner surface 300i of the base 300 can form a seat for the shank head 202a. More specifically, the inner surface 300i of the base 300 can be sized and shaped to retain at least a portion of the shank head 202a therein to allow multi-axial movement of the shank head relative to the base. In some embodiments, the inner surface 300i can conform to the shape of the outer surface of the shank head 202a. For example, at least a portion of the inner surface 300i can have a spherical shape that can correspond to the outer spherical surface of the shank head 202a. Although the illustrated embodiment shows the shank head 202a having a spherical shape and the inner surface 300i of the retaining ring 204 having a complementary spherical surface, other shapes of the shank head and / or inner surface are within the scope of the present disclosure. For example, in some embodiments, the inner surface 300i of the base 300 can be a chamfered surface (see 300i′ in FIG. 11 ) that can seat or receive the spherical shank head 202a. In other embodiments, the shank head 202a can have a different shape and the shape of the inner surface 400i of the base can be formed such that the shank head 202a can be received or seated therein.

[0037] As introduced above, the wall 302 can provide a frictional or drag force to the shank head 202a when the shank head is seated within the base 300. The drag force can resist multi-axial movement of the shank 202 relative to the retaining ring 204. The inner surface 302i of the wall 302 can have a geometry configured to apply a drag force to the head 202a of the shank 202. For example, the inner surface 302i of the wall 302 can have a cylindrical shape, and the interface 310 between the spherical inner surface 300i of the base 300 and the cylindrical inner surface 302i of the wall 302 can have a rest diameter that is smaller than the maximum diameter D of the shank head 202a. In this manner, the wall 302 can apply a radially inward drag force along a single line of contact with the shank head 202a at the interface 310. When the shank head is seated within the retaining ring base 300 and the interface 310 is aligned with the maximum diameter of the shank head, the wall 302 can have a height H such that the proximal end 302p of the wall extends proximally beyond the equator or maximum diameter D of the shank head 202a. The wall height H can be measured from the wall's distal end 304d to the wall's proximal end 302p at the proximally facing surface 302p of the annular boss 304. The relative dimensions of the retaining ring 204 and other bone anchor assembly 200 components can be selected, at least in part, to achieve a desired drag and contact between the retaining ring and the shank head 202a. As a non-limiting example, in some embodiments, the wall 402 height H can be about 0.5 mm. Thus, once assembled, i.e., when the retaining ring 204 is received within the groove 412 of the receiving member and the shank head 202a is seated therein, the walls 302 of the retaining ring can exert a radially inward frictional drag force against the head 202a of the shank 202 at the interface 310, resisting multi-axial movement between the shank head and the receiving member 206.

[0038] In some embodiments, the wall 302 can have two wall segments 312a, 312b that can be circumferentially separated by a recess or gap 314. The recess 314 between the wall segments 312a, 312b can aid in smooth retraction when the retaining ring 204 is advanced proximally into the receiver member 206. For example, in some embodiments, the recess 414 can have a substantially "U" shape, with the recess extending from the proximal end 302p of the wall segments 312a, 312b to the distal end 302d of the wall segments. As a further non-limiting example, the gap 314 can have a substantially truncated "V" shape (FIG. 11). The wall segments 312a, 312b can pivot or bend about a point opposite the slot 308 of the retaining ring 304. In some embodiments, a scalloped notch 311 (FIG. 3) may be provided along the outer surface 302o of the wall 302 or wall segments 312a, 312b at its distal end 302d. The scalloped notch 311 may provide additional flexibility to the wall 302 or wall segments 312a, 312b.

[0039] 8 and 9 show the receiver member 206 in more detail. FIG. 8 is a perspective view of the receiver member 206, and FIG. 9 is a front cross-sectional view of the receiver member along axis AA of FIG. 8. Referring to FIG. 9, the distal end 206d of the receiver member 206 can include a groove 212 formed on an inner surface thereof. The groove 212 can be configured to receive and at least partially seat the retaining ring 204. The groove 212 can have distal, intermediate, and proximal annular recesses or annular portions 212a, 212b, 212c, each of which can be sized at least in part to correspond to corresponding outer dimensions of the distal portion 306 of the base 300, the annular boss 304, and the wall 306 of the retaining ring 204, respectively. For example, the inner diameter D4 of the distal annular recess 212a can be equal to or slightly larger than the outer diameter D2 of the distal portion 306 of the base 300. Similarly, the inner diameter D5 of the intermediate annular recess 212b and the inner diameter D6 of the proximal annular recess 212c may be equal to or slightly larger than the outer diameters D1, D3 of the annular boss 304 and the wall 302 of the retaining ring 204, respectively.

[0040] The distal annular recess 212a may be the most distal portion of the groove 212 and may extend proximally from the distal surface 212 of the receiver member 206, thereby forming a distal opening 216 in the receiver member. The distal annular recess 212a may form a distal opening to the receiver member 206d such that the inner diameter D4 of the distal annular recess is equal to the distal opening of the receiver member. The intermediate annular recess 212b may be proximally adjacent to the distal annular recess 212a, and the inner diameter D5 of the intermediate annular recess may be greater than the inner diameter D4 of the distal annular recess. The proximal annular recess 212c may be proximally adjacent to the intermediate annular recess 212b. The inner diameter D6 of the proximal annular recess 212c may be less than the inner diameter D5 of the intermediate annular recess 212b, and in some embodiments, may be greater than the inner diameter D4 of the distal annular recess 212a. The longitudinal bore 211 can extend proximally from the proximal annular recess 212c with an inner diameter D7, which in some embodiments can be smaller than the inner diameter D6 of the proximal annular recess. In this manner, the retaining ring 204 received within the groove 312 can be prevented from sliding proximally past the groove 212.

[0041] 3 and 4, another embodiment of a bone anchor 200 will be described. Although the method is illustrated in connection with the bone anchor assembly 200 of FIGS. 3-9, the method can be used with any of the bone anchor assemblies or variations thereof disclosed herein. The bone anchor assembly 200 can be assembled during manufacturing, prior to surgery, or prior to embedding the shank 202 into bone during surgery. If used, the compression cap 208 can be advanced proximally through the distal opening 216 of the receiver member and into the hole 211. Prior to inserting the shank 202 into the receiver member 206, the retaining ring 204 can be snapped or otherwise positioned around a portion of the shank. In some embodiments, the retaining ring 204 can be snapped or otherwise positioned around the unthreaded portion of the shank 202 between the shank head 202a and the threads of the bone-engaging portion 202b, i.e., the neck 202c of the shank. For example, the neck 202c of the shank can be inserted into the retaining ring 204, e.g., by passing the retaining ring slot 308 transversely relative to the shank neck 202c such that the shank neck is located within the lumen 301 of the retaining ring. The retaining ring can be compressed, e.g., by applying a radial compressive force Fc (see FIG. 14), thereby temporarily reducing the maximum diameter D1 of the retaining ring. The retaining ring 204 can be compressed such that the maximum diameter of the retaining ring, i.e., the outer diameter D1 of the annular boss 304, can be smaller than the diameter D4 of the distal annular recess 214a in the groove 212 and the distal opening 216 of the receiving member 206.

[0042] With the retaining ring 204 in a compressed state and disposed about the shank 202, the shank and retaining ring can be advanced proximally through the distal opening 216 of the receiver member 206, i.e., bottom loaded into the receiver member. As the retaining ring 204 advances proximally, the annular boss 304 of the retaining ring can align with the intermediate annular recess 212b of the groove 212 formed in the receiver member 206. As discussed above, the inner diameter D5 of the intermediate annular portion 212b of the groove 212 can be substantially equal to or slightly larger than the outer diameter D1 of the annular boss. Thus, once the boss 304 of the retaining ring 204 is aligned with the intermediate annular recess 212b of the groove 212, the retaining ring 204 can expand from its compressed state to a back or rest state such that the retaining ring is seated within the groove (FIG. 3). More specifically, the distal portion 306 of the base 300, the annular boss 304, and the wall 302 of the retaining ring 204 may extend into and be retained within the distal, intermediate, and proximal annular recesses 212a, 212b, and 212c, respectively, of the groove 212. In some embodiments, with the retaining ring 204 seated in the groove 212, the distal facing surface 300d of the retaining ring may be flush with the distal facing surface 214 of the receiver member 206.

[0043] With the retaining ring 204 seated within the groove 212, i.e., with a portion of the retaining ring held within a corresponding recess in the groove (as described above), the shank 202 can be moved distally within the longitudinal bore 211 of the receiver member 206 to seat the head 202a of the shank within the base 300 of the retaining ring. More specifically, the shank 202 can be moved distally such that the equator D of the shank head 202a contacts the inner surface 302i of the wall 302 of the retaining ring 204 and a distal opposing portion of the shank head contacts the inner surface 300i of the base 300. The neck 202c and bone-engaging portion 202b of the shank 202 can extend distally from the retaining ring 204 and the receiver member 206. A portion of the shank head 202a can also extend distally beyond the retaining ring 204 and the receiver member 206. The retaining ring 204 is sized to prevent the shank head 202 a from passing through the distal opening 301 d , such that the head of the shank is maintained within the retaining ring 204 , and therefore within the receiving member 206 .

[0044] The bone anchor assembly 200 can be embedded in a bone (not shown) and driven using known techniques, for example, using a driver tool to screw the bone-engaging portion 202b of the shank 202 into the bone, to a desired depth along a desired trajectory. Once embedded, the receiver member 206 can be positioned in a desired orientation relative to the shank 202. For example, the receiver member 206 can be multi-axially rotated about the head 202a of the shank 202. Prior to attaching and / or tightening the closure mechanism 210 to the bone anchor 200, the receiver member 206 can be maintained in a desired orientation, for example, via a force between the retaining ring 204 and the shank head 202a, and more specifically, via a force between the wall 302 of the retaining ring 204 and the shank head. A spinal fixation element, for example, a spinal rod (FIG. 15), can be positioned in the recess 209 of the receiver member 206. When the spinal rod is in the desired position, the closure mechanism 210 can be tightened to urge the rod and compression cap 208 (if used) distally against the receiver member 206, thereby locking the bone anchor 200. Specifically, application of the closure mechanism 210 can be effective to lock movement of the receiver member 206 relative to the shank 202. For example, the rod and compression cap 208 can apply a distal force to the shank 202 such that the shank contacts the inner surface 300i of the retaining ring base 300, causing the retaining ring 204 to radially expand until any of the outer diameters D1, D2, or D3 of the retaining ring base 300 contacts one of the inner diameters D4, D5, or D6 of the receiver. During this radial expansion of the retaining ring base, the shank head 202a can be retained within the wall 302 of the retaining ring 204, for example, by a compression fit. Application of the closure mechanism 210 may also be effective to lock the movement of the rod 206 relative to the receiving member 206 .

[0045] In this manner, the bone anchor assembly of the present disclosure allows for assembly using bottom loading techniques. This may be particularly advantageous with large diameter shanks that are not sized to be advanced distally through the proximal end of the receiver because, for example, sizing the receiver member to accommodate such large diameter shanks would require a prohibitively large receiver member. Furthermore, a single component, the retaining ring, can maintain the connection between the receiver member and the bone shank, allowing for multi-axial adjustment of the bone shank to a desired position relative to the receiver member while preventing unintended movement between the two components.

[0046] 10-14 illustrate another embodiment of the bone anchor assembly 400 of the present disclosure. The bone anchor assembly 400 can include a shank 402, a drag retaining ring 404, a receiving member 406, a compression cap 408, and a closure mechanism (not shown). Except as noted below, the structure, operation, and use of this embodiment are similar or identical to those of the bone anchor assembly 200, and like-numbered components generally have similar features. Accordingly, a description of the structure, operation, and use of such features is omitted herein for the sake of brevity. As with the above-described embodiment, the drag retaining ring 404 can apply a friction or drag force to the head 402a of the shank 402 to resist multi-axial movement therebetween. 10, i.e., with the shank 402 extending coaxially with the central axis B1 of the receiving member 406, the retaining ring 404 can exert a frictional force along the equator or maximum diameter D' of the shank head 402a, similar to the bone anchor assemblies described above. However, the retaining ring 402 can have an alternative configuration compared to the retaining ring 204 described above, whereby the proximal end 502p (FIG. 12) of the wall 502 of the retaining ring 402 can exert a drag force in a radially inward direction against the shank head 402a.

[0047] FIG. 12 shows the retaining ring 404 of the bone anchor assembly 400 in more detail. The retaining ring 404 may be generally cylindrical with a base 500 configured to seat the shank head 402a and a wall 502 extending proximally from the base. The proximal portion of the base 500 may be formed from an annular boss 504 having an outer diameter D1' (see FIG. 10) that is greater than the outer diameter D2' (see FIG. 10) of the distal portion 506 of the base. The wall 502 may extend proximally from the annular boss 504 at an outer diameter D3' (see FIG. 10) that is less than the outer diameter D1' of the boss. In such a construction, the retaining ring may include three "stepped" portions, i.e., portions of varying outer diameter: the distal base 506, the annular boss 504, and the wall 502. In an assembled configuration of the bone anchor assembly 400 (e.g., FIG. 10 ), the retaining ring 404 may be received within a groove 412 formed in an inner surface of the receiver member 406 at its distal end 406d. As described above, the groove 412 may include three annular recesses, namely, the distal annular recess 412a, the intermediate annular recess 412b, and the proximal annular recess 412c, which are configured to receive the distal portion 506 of the base 500, the annular boss 504, and the wall portion 502 of the retaining ring 404, respectively. Thus, the retaining ring 404 may be securely held within the groove 412 at the distal end 406d of the receiver member 406.

[0048] Returning to the retaining ring 404 and FIG. 12, the retaining ring can have a radial split 508 or other slots, notches, notches, etc., that can allow for selective radial expansion and compression of the retaining ring. In some embodiments, the split 508 can be sized to allow a portion of the shank 402, such as the neck 402c of the shank, to pass through the split 508 and into the retaining ring 404. The inner surface 500i of the base 500 can have a chamfered edge to form a seat for the shank head 402a. The chamfered edge can extend from a point along the inner surface 500i of the distal portion 506 of the base 500 to a point along the inner surface at the proximal end 504p of the annular boss 504. The wall 502 can extend cylindrically from the free-standing proximal end 502p to the distal end 502d, terminating at the proximal end 504p of the annular boss. As shown in FIG. 10, the proximal end 502p of the wall 502 can be configured to apply a resistance force at the equator of the shank head 402a when the shank is seated coaxially within the retaining ring base 500. For example, the proximal end 502p of the wall 502 can have an inner diameter smaller than the maximum diameter D' of the shank head. In some embodiments, the inner surface 502i of the wall 502 can have a chamfered edge at its proximal end 502p that can contact the shank head 402a and apply a frictional force thereto to resist multi-axial movement between the shank head and the retaining ring. The wall 502 can extend a height H' such that the proximal end 502p of the wall 502 can contact the shank head 402a at the maximum diameter D' when seated collinearly or coaxially within the base 500 (e.g., the orientation shown in FIG. 14).

[0049] As discussed above, in some embodiments, the wall 502 can include a first wall 512a segment and a second wall segment 512b circumferentially separated from the first wall segment by a gap or recess 514. In some embodiments, the recess 514 can have a substantially truncated "V" shape. A notch 511, groove, or other similar cut-out feature can extend along an outer surface of the wall 502 or wall segments 512a, 512b at the distal end 502d.

[0050] FIG. 13 illustrates receiver member 406 in more detail. Receiver member 406 can have a "closed" configuration, as compared to receiver member 206 described above, which has a pair of spaced apart arms 207a, 207b at its proximal end 206p. The closed configuration can provide sufficient strength to receiver member 406 for use in high load applications, e.g., for implantation within the iliac spine, while allowing for a reduction in size of the receiver member. The proximal end 406p of receiver member 406 can circumferentially surround a longitudinal bore 411 that extends distally from the proximal end of the receiver member into the receiver body. Notches 413 can be formed on either side of the receiver body to form rod receiving recesses 409 therethrough. Each of the notches 413 can have a closed periphery formed by the receiver body. Such a structure is in contrast to the rod receiving recess 209 of the receiver member 206 shown in, for example, Figures 3 and 4, which is formed between the spaced apart arms 207a, 207b and is open in a proximal direction. Returning to Figure 13, the notch 413 can be sized and shaped to laterally receive a spinal fixation element, such as a spinal rod, therethrough such that the spinal fixation element can extend through the receiver member 406 laterally relative to the longitudinal bore 411. For example, in the illustrated embodiment of Figure 13, the notch 413 has a generally oval shape to receive a spinal rod therethrough. The longitudinal bore 411 can receive a set screw or other fastening mechanism (not shown) to secure the spinal fixation element within the rod receiving recess 409 relative to the receiver member 406. The groove 412 can be formed in an inner surface of the distal end 406d of the receiving member 406 having an opening 416 through the distal face 414 (FIG. 10) of the receiving member such that the shank 402 and retaining ring 404 can be advanced proximally into the receiving member 406 and retained therein, as described above with respect to FIGS. 3-9.

[0051] The bone anchor assembly 400 can be assembled in the same or similar assembly procedures as described above with respect to the bone anchor 200 of FIGS. 3-9. For example, with reference to FIG. 14, the compression cap 408 can be advanced proximally through the distal opening 416 of the receiving member 406 and into the hole 411. Alternatively, the compression cap 408 can be loaded by advancing it distally from the proximal end of the receiving member 406 through the hole 411. In some embodiments, the compression cap can be retained in the receiving member 406 by swaging. The retaining ring 404 can be snapped or otherwise disposed around the shank 402, for example, around the neck 406c of the shank. The retaining ring 404 can be compressed, for example, by applying a radial compressive force Fc to the retaining ring such that the outer diameter of the annular boss 504 can be reduced below the diameter of the distal opening 516 of the receiving member 506. The shank head 402a and the retaining ring 404 in a compressed state can be advanced proximally into the bore 411 through the distal opening 516 of the receiver member 506. The retaining ring 404 can be advanced proximally within the bore 411 until the annular boss 504 is aligned with the intermediate annular recess 412b of the groove 412. With the annular boss 504 aligned with the intermediate annular recess 412b, the retaining ring 404 can expand radially into the groove 412 and be retained therein. The shank 402 can be moved distally relative to the receiver member 406 such that the shank head 402a seats within the base 400 of the retaining ring 204 and contacts the wall 502 at the maximum diameter D' (FIG. 10) of the shank head. The bone anchor can be embedded in a bone (not shown) and the receiver member 406 can be manipulated to position the receiver member in a desired orientation relative to the shank 402. In some embodiments, the receiving member 406 can be moved multi-axially relative to the shank 402. With reference to FIG. 15 , the spinal rod 420 can be inserted laterally into the receiving member 406 through the rod receiving recess 409. The closure mechanism 410 can be tightened to lock the spinal rod 420 relative to the receiving member 406. The closure mechanism 410 can also lock the receiving member 406 relative to the shank 402.

[0052] The bone anchor assemblies 200, 400 of the present disclosure can have a reduced overall size and profile due, at least in part, to the incorporation of the drag retaining rings 204, 404 disclosed herein. Figure 16 shows two embodiments 500, 510 of the bone anchor assemblies of the present disclosure and one known bone anchor assembly 600 coupled to a spinal rod 520. Three dimensions X, Y, and Z of the various bone anchor assemblies 500, 510, 600 are illustrated and described in detail below, where "X" represents the overall height of each bone anchor assembly measured from the proximal-most opposing surface of the receiver member to the distal-most opposing surface of the receiver member. "Y" represents the distance that each receiver member extends distally under the spinal rod, and "Z" represents the maximum lateral width of each receiver member measured in a direction parallel to the longitudinal axis of the spinal rod when the spinal rod is received within the bone anchor assembly.

[0053] The known bone anchor assembly 600 can be a bottom loading assembly for use with large bone screws. The first bone anchor assembly 500 of the present disclosure can be sized and configured for use with small and medium sized bone screws (e.g., bone screws having a maximum outer thread diameter of about 4 mm to about 8 mm). The second bone anchor assembly 502 of the present disclosure can be sized and configured for use with large sized bone screws (e.g., bone screws having a maximum outer thread diameter of greater than about 7.5 mm). In particular, the bone anchor assembly 510 of the present disclosure configured for use with large sized bone screws can have the same overall height X'' and can extend the same distance Y'' distally below the spinal rod 520 as the bone anchor assembly 500 configured for use with small or medium sized bone screws. By way of non-limiting example, in some embodiments, the overall height X', X'' of the bone anchor assembly 500, 510 can be less than about 16 mm, less than about 15.5 mm, or about 15 mm. In some embodiments, the distance Y', Y'' between the distal end of the receiving member of the bone anchor assembly 500, 510 and the distal surface of the spinal rod 510 can be less than about 6.5 mm, less than about 6 mm, or between about 5.5 mm and about 6 mm. In contrast, the known bone anchor assembly 600 can have a height X''' of about 16.2 mm or more and can extend a distance Y'''' below the spinal rod of about 6.9 mm or more. The bone anchor assemblies 500, 510 of the present disclosure can also have a reduced width Z', Z'' as compared to the width Z''' of the known bone anchor 600. As a non-limiting example, the overall width Z', Z' of the bone anchor assembly 500, 510 can be less than about 12 mm in some embodiments and less than about 11 mm in some embodiments. The known bone anchor 600 can have a Z'''' greater than about 12 mm. The reduced size of the bone anchor assemblies 500, 510 of the present disclosure can allow the bone fixation element, e.g., spinal rod 520, to be secured closer to the bone into which the bone anchor assembly is implanted. Additionally, the smaller size can improve ease of placement and manipulation, for example, by minimizing the portion of the surgical site occupied by spinal instrumentation.

[0054] The assemblies and components disclosed herein may be constructed from any of a variety of known materials. Exemplary materials include materials suitable for use in surgical applications, including metals such as stainless steel, titanium, cobalt chrome, or alloys and combinations thereof, polymers such as PEEK, ceramics, carbon fiber, and the like. The various components disclosed herein may have various degrees of stiffness or flexibility appropriate for their use. The size of the assemblies and / or components may also vary widely depending on the intended use and the anatomy of the surgical site. Additionally, certain components may be formed from different materials than other components. One or more components or portions of the instrument may be formed from radiopaque materials to facilitate visualization under fluoroscopy and other imaging techniques, or may be formed from radiolucent materials such as carbon fiber and / or high strength polymers so as not to interfere with visualization of other structures.

[0055] The devices and methods disclosed herein can be used in minimally invasive and / or open surgery. Although the devices and methods disclosed herein are generally described in the context of surgery on a human patient, it will be understood that the methods and devices disclosed herein can be used in any of a variety of surgical procedures or in non-surgical procedures with any human or animal subject.

[0056] Although specific embodiments are described above, modifications may be made within the spirit and scope of the described concepts. For example, a receiver member having a "closed" configuration as described with respect to the bone anchor assembly 400 of Figs. 10-14 may be utilized with the elements of the bone anchor assembly 200 of Figs. 3-9. A receiver member having an "open" configuration as described with respect to the bone anchor assembly 200 of Figs. 3-9 may be utilized with the elements of the bone anchor assembly 400 of Figs. 10-14. Similarly, the various embodiments of the resistance retaining rings 204, 404 described herein may be utilized with the components of the various embodiments of the bone anchor assemblies 200, 400 described herein and are within the scope of the present disclosure. Additional features and advantages based on the embodiments described above are possible and are within the scope of the present disclosure. Thus, the present disclosure is not limited by what has been specifically shown and described. All publications and references cited herein are incorporated herein by reference in their entirety, except for any definitions, disclaimers, or disclaimers of subject matter, and except to the extent that the incorporated documents contradict the disclosure expressly set forth herein, in which case the language within the present disclosure will control.

[0057] Examples of the embodiments described above may include the following. 1. A bone anchor assembly comprising: a receiving member having a proximal end and a distal end, a central longitudinal axis extending between the proximal end and the distal end, the receiving member having a longitudinal bore with an opening at the distal end of the receiving member; a retaining ring disposed in a groove formed in the receiver member, the retaining ring having a base and a wall extending proximally from the base, the base having a maximum outer diameter greater than a maximum outer diameter of the wall; a shank having a head portion seated within the base of the retaining ring and a bone engaging portion extending distally from the receiver member; A bone anchor assembly, wherein the walls of the retaining ring are configured to exert a resistance on the head of the shank to resist rotation thereof.

[0058] 2. The assembly of claim 1, wherein the retaining ring and shank are configured to be inserted proximally through a distal end of the receiving member.

[0059] 3. An assembly as described in claim 1 or 2, wherein the groove is formed in an inner surface of the distal end of the receiving member.

[0060] 4. An assembly as claimed in any one of claims 1 to 3, wherein the maximum outer diameter of the base of the retaining ring is greater than the diameter of the opening at the distal end of the receiving member.

[0061] 5. An assembly as claimed in any one of claims 1 to 4, wherein the inner diameter of the distal portion of the base of the retaining ring is less than the maximum diameter of the head of the shank.

[0062] 6. The assembly of any one of claims 1-5, wherein the inner diameter of the distal portion of the base of the retaining ring is less than the maximum outer thread diameter of the bone-engaging portion of the shank.

[0063] 7. An assembly as claimed in any one of claims 1 to 6, wherein the wall of the retaining ring is configured to contact the head of the shank at its maximum diameter.

[0064] 8. The assembly of claim 7, wherein the proximal end of the wall of the retaining ring extends proximally beyond the maximum diameter of the head of the shank when the shank is seated within the base of the retaining ring.

[0065] 9. The assembly of any one of claims 1-8, wherein the wall of the retaining ring includes a first wall segment and a second wall segment circumferentially separated from the first wall segment.

[0066] 10. The assembly of any one of claims 1-9, wherein the retaining ring includes a split extending through the base and wall configured to allow selective radial expansion and compression of the retaining ring.

[0067] 11. The assembly of any one of claims 1-10, further comprising a saddle disposed within the bore proximal to the retaining ring and configured to apply a distal force to the shank head.

[0068] 12. A method of assembling a bone anchor assembly, comprising: radially compressing a retaining ring disposed about a shank, the shank including a head and a bone engaging portion, the retaining ring including a base and a wall extending proximally from the base, the wall having a maximum outer diameter less than a maximum outer diameter of the base; passing the compressed shank and retaining ring proximally through an opening in a distal end of the receiver member; advancing the retaining ring proximally within the receiver member such that at least a portion of the base is aligned with a first annular recess of a groove formed in the receiver member; expanding the retaining ring to retain at least a portion of the base within the first annular recess of the groove; and seating the head of the shank within the base of the retaining ring such that the maximum diameter of the head of the shank contacts the wall of the retaining ring and the bone engaging portion of the shank extends distally from the retaining ring.

[0069] 13. Positioning the receiving member at a desired location relative to the shank; and retaining the receiver member in the desired position by a counterforce exerted by a wall of the retaining ring against the head of the shank.

[0070] 14. The method of claim 13, wherein positioning the receiver member at the desired location includes moving the receiver member multi-axially relative to the shank.

[0071] 15. The method of claim 13 or 14, wherein the drag force is exerted by the retaining ring at the interface between the base and the wall of the retaining ring.

[0072] 16. The method of claim 13 or 14, wherein the resisting force is applied by a proximal end of the wall of the retaining ring.

[0073] 17. Embedding the shank into the bone; 17. The method of any one of claims 13 to 16, further comprising tightening a closure mechanism to lock the receiver member in a desired position relative to the shank.

[0074] 18. Advancing the saddle proximally through the distal opening of the receiving member; 18. The method of claim 12, further comprising biasing the shank distally with a distal force applied by the saddle against the head of the shank.

[0075] 19. The method of any one of claims 12-18, further comprising compressing a retaining ring around the neck of the shank.

[0076] 20. The method of any one of claims 12-19, wherein a distal surface of the retaining ring is flush with a distal surface of the receiving member with at least a portion of the base of the retaining ring retained within the first annular recess of the groove.

[0077] [Embodiment] (1) A bone anchor assembly, comprising: a receiving member having a proximal end and a distal end, a central longitudinal axis extending between said proximal end and said distal end, said receiving member having a longitudinal bore with an opening at said distal end; a retaining ring disposed in a groove formed in the receiver member, the retaining ring having a base and a wall extending proximally from the base, the base having a maximum outer diameter greater than a maximum outer diameter of the wall; a shank having a head portion seated within the base portion of the retaining ring and a bone engaging portion extending distally from the receiver member; The wall of the retaining ring is configured to exert a resistance on the head of the shank to resist rotation thereof. (2) The assembly of embodiment 1, wherein the retaining ring and the shank are configured to be inserted proximally through the distal end of the receiving member. (3) The assembly of embodiment 1, wherein the groove is formed on an inner surface of the distal end of the receiving member. (4) The assembly of claim 1, wherein the maximum outer diameter of the base of the retaining ring is greater than the diameter of the opening at the distal end of the receiving member. (5) The assembly of claim 1, wherein an inner diameter of the distal portion of the base of the retaining ring is smaller than a maximum diameter of the head of the shank.

[0078] (6) The assembly of claim 1, wherein an inner diameter of the distal portion of the base of the retaining ring is smaller than a maximum outer thread diameter of the bone engaging portion of the shank. (7) The assembly of claim 1, wherein the wall of the retaining ring is configured to contact the head of the shank at its maximum diameter. (8) The assembly of embodiment 7, wherein a proximal end of the wall of the retaining ring extends proximally beyond the maximum diameter of the head of the shank when the shank is seated within the base of the retaining ring. (9) The assembly of claim 1, wherein the wall of the retaining ring includes a first wall segment and a second wall segment circumferentially separated from the first wall segment. (10) The assembly of claim 1, wherein the retaining ring includes a split extending through the base and the wall configured to allow selective radial expansion and compression of the retaining ring.

[0079] (11) The assembly of embodiment 1, further comprising a saddle disposed within the bore proximal to the retaining ring and configured to apply a distal force to the shank head. (12) A method of assembling a bone anchor assembly, comprising: radially compressing a retaining ring disposed about a shank, the shank including a head and a bone engaging portion, the retaining ring including a base and a wall extending proximally from the base, the wall having a maximum outer diameter less than a maximum outer diameter of the base; passing the shank and the retaining ring in the compressed state proximally through an opening in a distal end of a receiver member; advancing the retaining ring proximally within the receiver member such that at least a portion of the base is aligned with a first annular recess of a groove formed in the receiver member; expanding the retaining ring to retain at least a portion of the base within the first annular recess of the groove; and seating the head of the shank within the base of the retaining ring such that a maximum diameter of the head of the shank contacts the wall of the retaining ring and the bone engaging portion of the shank extends distally from the retaining ring. (13) positioning the receiving member at a desired position relative to the shank; 13. The method of claim 12, further comprising: retaining the receiver member in the desired position by a force exerted by the wall of the retaining ring against the head of the shank. (14) The method of claim 13, wherein positioning the receiving member at the desired position includes moving the receiving member in a multi-axial direction relative to the shank. 15. The method of claim 13, wherein the resisting force is exerted by the retaining ring at an interface between the base and the wall of the retaining ring.

[0080] (16) The method of claim 13, wherein the resisting force is applied by a proximal end of the wall of the retaining ring. (17) embedding the shank into a bone; 14. The method of claim 13, further comprising tightening a closure mechanism to lock the receiver member in the desired position relative to the shank. (18) advancing a saddle proximally through the distal opening of the receiver member; 13. The method of claim 12, further comprising biasing the shank in a distal direction with a distal force applied by the saddle against the head of the shank. 19. The method of claim 12, further comprising compressing the retaining ring about the neck of the shank. (20) The method of claim 12, wherein a distal surface of the retaining ring is flush with a distal surface of the receiving member with at least a portion of the base of the retaining ring retained within the first annular recess of the groove.

Claims

1. 1. A bone anchor assembly comprising: a receiving member having a proximal end and a distal end, a central longitudinal axis extending between said proximal end and said distal end, said receiving member having a longitudinal bore with an opening at said distal end; a retaining ring disposed in a groove formed in the receiver member, the retaining ring having a base and a wall extending proximally from the base, the base having a maximum outer diameter greater than a maximum outer diameter of the wall; a shank having a head portion seated within the base portion of the retaining ring and a bone engaging portion extending distally from the receiver member; The wall of the retaining ring is configured to exert a resistance on the head of the shank to resist rotation thereof.

2. The assembly of claim 1 , wherein the retaining ring and the shank are configured to be inserted proximally through the distal end of the receiver member.

3. The assembly of claim 1 , wherein the groove is formed in an inner surface of the distal end of the receiver member.

4. The assembly of claim 1 , wherein the maximum outer diameter of the base of the retaining ring is greater than a diameter of the opening at the distal end of the receiver member.

5. The assembly of claim 1 , wherein an inner diameter of the distal portion of the base of the retaining ring is less than a maximum diameter of the head portion of the shank.

6. The assembly of claim 1 , wherein an inner diameter of the distal portion of the base of the retaining ring is less than a maximum outer thread diameter of the bone engaging portion of the shank.

7. The assembly of claim 1 , wherein the wall of the retaining ring is configured to contact the head portion of the shank at its maximum diameter.

8. 8. The assembly of claim 7, wherein a proximal end of the wall of the retaining ring extends proximally beyond the maximum diameter of the head portion of the shank when the shank is seated within the base of the retaining ring.

9. The assembly of claim 1 , wherein the wall of the retaining ring includes a first wall segment and a second wall segment circumferentially separated from the first wall segment.

10. The assembly of claim 1 , wherein the retaining ring includes a split extending through the base and the wall configured to allow selective radial expansion and compression of the retaining ring.

11. The assembly of claim 1 , further comprising a saddle disposed within the bore proximal to the retaining ring and configured to apply a distal force to the head portion of the shank.

12. 1. A method of assembling a bone anchor assembly, comprising: radially compressing a retaining ring disposed about a shank, the shank including a head and a bone engaging portion, the retaining ring including a base and a wall extending proximally from the base, the wall having a maximum outer diameter less than a maximum outer diameter of the base; passing the shank and the retaining ring in a compressed state proximally through an opening in a distal end of a receiver member; advancing the retaining ring proximally within the receiver member such that at least a portion of the base is aligned with a first annular recess of a groove formed in the receiver member; expanding the retaining ring to retain at least a portion of the base within the first annular recess of the groove; and seating the head of the shank within the base of the retaining ring such that a maximum diameter of the head of the shank contacts the wall of the retaining ring and the bone engaging portion of the shank extends distally from the retaining ring.

13. positioning the receiving member at a desired location relative to the shank; 13. The method of claim 12, further comprising: retaining the receiver member in the desired position by a counterforce exerted by the wall of the retaining ring against the head of the shank.

14. The method of claim 13 , wherein positioning the receiver member at the desired location comprises multi-axially moving the receiver member relative to the shank.

15. The method of claim 13 , wherein the resisting force is exerted by the retaining ring at an interface between the base and the wall of the retaining ring.

16. The method of claim 13 , wherein the resisting force is exerted by a proximal end of the wall of the retaining ring.

17. advancing a saddle proximally through a distal opening in the receiver member; 13. The method of claim 12, further comprising biasing the shank distally with a distal force exerted by the saddle against the head of the shank.

18. The method of claim 12 further comprising compressing the retaining ring about a neck of the shank.

19. The method of claim 12 , wherein a distal surface of the retaining ring is flush with a distal surface of the receiver member with at least a portion of the base of the retaining ring retained within the first annular recess of the groove.

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