Trochanteric nailing systems and related methods

Abstract

To provide trochanteric nailing systems and related methods.SOLUTION: A trochanteric nailing system makes use of a lag screw operatively connectable to a trochanteric nail. The lag screw has a helical thread disposed over a suitable surgical length on the outer surface of the shaft of the lag screw. The helical thread includes undercuts formed therein to improve resistance to cutout or lag screw migration. The lag screw may also include a variable-width thread to improve fixation and resistance to migration or cut out. Helical flutes reduce torque forces exerted on bone during insertion, while maintaining the improved fixation of other features about the entire circumference of the lag screw. The lag screw may also be equipped with tapered set screw grooves which engages opposing portions of a set screw of the trochanteric nail when received therein to resist migration or cutout.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application is a non-provisional application claiming the benefit of priority to Provisional Patent Application No. 63 / 551,305, filed February 8, 2024, the entire contents of which are incorporated herein by reference for all purposes.

[0002] The present disclosure relates generally to the field of orthopedics, and more particularly to a trochanteric nailing system. [Background technology]

[0003] The important long bones of the extremities are the humerus, radius, and ulna of the upper limb, and the femur and tibia of the lower limb. Following injuries to long bones, and particularly injuries resulting in one or more fractures of a long bone, one or more fixation devices can be used to immobilize the fracture fragments and stabilize the long bone. Bone fractures can be treated using screws or other fixation devices inserted into or through the bone to stabilize the bone once the fractured portions are properly aligned. For example, femoral neck fixation can be used to treat hip fractures by inserting an intramedullary nail into the medullary cavity of the fractured femur, followed by inserting a locking screw into the femoral neck / head at an angle relative to the intramedullary nail. Similarly, fractures of other long bones can be treated by inserting an intramedullary nail into the intramedullary canal of the bone and providing appropriate proximal and / or distal fixation. Trochanteric (or hip) nailing systems traditionally use a single large-diameter (approximately 10 mm) lag screw placed through the intramedullary nail into the femoral head to fixate hip fractures. Such systems often suffer from various drawbacks and disadvantages. One potential problem is referred to as cutout, which can be described as protrusion of the lag screw from the femoral head as a result of intracranial migration of the screw and varus collapse of the neck-shaft angle. It would be desirable to address these drawbacks and promote potential improvements in trochanteric nailing systems. Summary of the Invention

[0004] In one possible implementation, the trochanteric nailing system includes a trochanteric nail and an associated trochanteric nail fixation device. The trochanteric nail fixation device has a lag screw designed to be rotatably advanced into a femur that is the subject of a procedure associated with the trochanteric nailing system. A helical thread extends on the shaft of the lag screw at its anterior end. The thread has a thread form with at least one undercut formed therein. The presence of the undercut allows the lag screw to hold, grasp, or otherwise engage bone material within or adjacent to the undercut. Due to such bone material engagement, when the lag screw is subjected to a lateral force, for example, during postoperative patient activity, the bone material held within the undercut on whichever side of the lag screw is subjected to tension from the lateral force acts to increase resistance to such lateral force. In this way, the undercut portion under tension from the lateral force receives bone material and helps hold the lag screw against undesired movement.

[0005] In another implementation, the undercuts are formed on both the leading and trailing edges of the threads, and the undercuts extend over a predetermined surgical length associated with the threads.

[0006] In yet another implementation, the thread form of the thread may be designed with a variable cross-sectional width over adjacent paths of the thread, with the width of the thread increasing from the tip of the shaft toward the head. This configuration results in increased compression of the bone material when the lag screw is inserted.

[0007] A particular implementation of the trochanteric nailing system has at least one flute defined in the lag screw. The threads of the lag screw have a first pitch, while the flutes extend helically at a second pitch. The second pitch of the flutes is coarser, i.e., less fine, than the first pitch of the threads, meaning that the flutes intersect the threads to define channels extending through the sides of the threads at different corresponding radial positions.

[0008] In one particular implementation, there are three such helical flutes with starting locations at different angularly spaced locations around the shaft of the lag screw.

[0009] Yet another implementation of the trochanteric nailing system includes a bore extending through the trochanteric nail at an angle appropriate for the associated hip joint procedure. The bore is sized to receive a lag screw therethrough during the hip joint procedure. The lag screw is dynamically or statically locked to the trochanteric nail by locking the set screw of the trochanteric nail into opposing set screw grooves defined in the lag screw shaft. The lag screw has one or more of these set screw grooves designed with a distal taper, thereby increasing engagement of the lag screw with the set screw of the trochanteric nail.

[0010] A method for manufacturing a lag screw is provided in certain implementations of the present disclosure. The method creates a thread form that differs from the more typical V-shaped thread form of a lag screw, resulting in a thread form that includes recesses to define undercuts on both the leading and trailing edges of the thread. The lag screw is further manufactured to have variable-width threads while maintaining a constant average pitch. The manufacturing process may also involve increasing the width of the continuous path of the thread from the distal end toward the proximal end to increase compression when the lag screw is rotatably inserted into bone material during a hip joint procedure. [Brief explanation of the drawings]

[0011] A more complete understanding of the present invention and its attendant advantages and features will be more readily appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings. [Figure 1] FIG. 1 is a cross-sectional view of one implementation of a trochanteric nailing system according to the present disclosure. [Figure 2] FIG. 10 is an enlarged cross-sectional partial view of a lag screw of a trochanteric nailing system according to one possible implementation of the present disclosure. [Figure 3] FIG. 3 is a partial schematic cross-sectional view of the lag screw of FIG. 2 of the present disclosure. [Figure 4]FIG. 4 is a side view of the lag screw of FIGS. 2 and 3 showing bone material after insertion into the femur. [Figure 5] FIG. 10 is a cross-sectional schematic diagram of another possible implementation of the threads of the lag screw of the present disclosure. [Figure 6] FIG. 10 is an enlarged side view of a tip of a lag screw according to an implementation of the present disclosure. [Figure 7] FIG. 7 is an enlarged end view of the tip of the lag screw shown in FIG. 6. [Figure 8] FIG. 2 is an enlarged cross-sectional view of the proximal portion of the lag screw shown in FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] 1-6 , a trochanteric nailing system 21 utilizes a trochanteric nail 23 configured for intramedullary insertion into a patient's femur to address a hip fracture, dislocation, or other condition during an associated surgical procedure. The nailing system 21 similarly includes a trochanteric nail fixation device 25 designed to suitably connect to the trochanteric nail 23. In the illustrated implementation, the trochanteric nail fixation device 25 includes a lag screw 27 configured to rotatably advance into the femur and engage bone material adjacent the lag screw 27 in response to torque applied to the lag screw 27. As shown, the lag screw 27 passes through a suitable angled bore 29 extending diametrically through the shaft of the trochanteric nail 23.

[0013] Lag screw 27 has a head 31 and a tip 33 located at opposite ends of lag screw 27, and a shaft 35 extending between heads 31 and 33. Helical threads 37 are disposed on the outer surface of shaft 35 at the distal end of lag screw 27. Threads 37 have a thread start at tip 33 and wrap proximally from tip 33 partially up shaft 35 toward head 31. The relative longitudinal extent of threads 37 with respect to shaft 35 may be varied to accommodate any number of operating parameters within the scope of the present disclosure.

[0014] 2, thread 37 has its thread form 39 shown in cross section with recesses 41 defined in one or both of leading edge 43 and trailing edge 45. In the illustrated embodiment, recesses 41 are defined in both the leading edge and the trailing edge, and such recesses are referred to herein as undercuts 47, respectively.

[0015] In this implementation, the thread 37 is designed to have a predetermined pitch selected relative to the bone material it passes through, such that the bone material M ( FIG. 4 ) can be received and engaged within the undercuts 47. By way of further illustration with reference to FIGS. 2 and 4 , the bone material M ( FIG. 4 ) is received between opposing pairs of undercuts 47 on the leading edge 43 and trailing edge 45 of adjacent, longitudinally spaced thread portions 48 ( FIG. 2 ). The bone material M is thus engaged by the undercuts 47, which extend longitudinally along the lag screw 27 for a surgical length 53 suitable for the intended procedure. Two of such adjacent thread portions 48 are shown by way of example in FIG. 2 by reference numerals 49, 51. Each 360° arc of the thread 37 shall be referred to herein as a respective thread path.

[0016] 3 and 4, after the trochanteric nailing system 21 is secured to the femur, the system is subjected to cyclic lateral forces, as indicated by arrow A (FIG. 3), during patient activity. In response to such lateral forces on the lag screw 27, bone material M located in the portion of the undercut 41 opposing such forces resists such lateral forces, similar to how the bone material and the corresponding undercut resist separation from one another, thereby exerting a holding or tensioning force on the lag screw 27 along that side or half of the shaft 35 under tension. Thus, a tensile force is exerted by the aforementioned portion of the undercut 41, and such tensile force acts to counteract the lateral forces induced by patient activity on the nailing system 21. Such tensile force increases the resistance caused by compression of the bone material on the side of the lag screw 37 that is being subjected to the lateral force.

[0017] In addition to the tension forces discussed above, the formation of undercuts 47 along the surgical length 53 of the lag screw 27 results in a lateral profile 37 and its corresponding surface area along the surgical length 53 (FIG. 3). Such a lateral profile is greater than a comparable thread form without undercuts 47, for example, a thread form having a simple pointed crest and V-shape between adjacent lateral flanks of the thread. Stated another way, the addition of undercuts 47 and their corresponding recesses 41 increases the effective surface area of ​​the surgical length 53 by adding arcuate surfaces associated with the recesses 41 in place of flat surfaces that might otherwise be associated with the thread form.

[0018] 2, by way of example only, the recess of the undercut 47 may define an angle α of at least 45 degrees as measured from an associated reference triangular thread form indicated by the reference letter B. Other values ​​of the angle α are similarly suitable, as are other shapes or configurations of the undercut 47, which would result in an increased surface area of ​​the transverse profile 55 over the surgical length 53. The increased transverse profile surface area acts to reduce stress concentrations, reduce force per unit area, and otherwise improve distribution of forces that may be applied to one or both sides of the shaft 35 post-operatively, thereby potentially resisting undesirable lag screw migration or cutout.

[0019] Referring now more specifically to FIG. 5 , the thread form 39 is characterized by a centerline indicated by reference arrow C. In one possible implementation, the thread 37 and its thread form 39 are characterized by a variable cross-sectional width indicated by reference line W across adjacent passages of the thread 37. The width W of the thread 37 may gradually increase from the tip 33 as the thread 37 wraps toward the head 31. Such a configuration creates a biomechanical effect of increasing compression on the bone material exerted by the lag screw 27 during its insertion. Defining pitch only for purposes of FIG. 5 as being the center points labeled x and y between the undercuts 47 of adjacent passages, the distance between center points x and y, i.e., the pitch defined for this implementation, remains constant, even though the width W of the thread 37 changes as it moves from the tip 33 to the head 31.

[0020] Additionally or alternatively, a variable width profile, such as that described above for thread 37, may be achieved by assigning different pitches to the leading and trailing edges, the pitch being selected to increase the distance between the leading and trailing edges from one another as the thread winds from the tip toward the head. The resulting variable width thread form similarly provides the desired biomechanical increased compression during surgical insertion.

[0021] The thread 37 may be further defined as having a valley 56 located radially proximal to the outer surface of the shaft 35. Extending from the valley 56 are two laterally extending thread flanks 58 that terminate at a crest 60 of the thread 37. The valley 56 and crest 60 correspond to the inner and outer diameters characteristic of the thread 37. The thread flanks 58 correspond to the leading edge 43 or the trailing edge 45 of the thread 37, respectively.

[0022] 6 and 7 , at least one flute 57 is defined in the lag screw 27 and is similarly arranged to extend helically over the same surgical insertion length 53 as the threads 37. The flutes 57 generally contribute to reducing the amount of torque required for lag screw insertion, particularly for self-tapping or similar applications where reduced insertion torque is desirable. The flutes 57 extend using a second pitch that is coarser than the first pitch of the threads 37. As a result, the arrangement of the flutes 57 and their coarser pitch define channels 59 extending through each thread flank 58 at different angular positions between adjacent passes of the threads 37. Stated differently, the channels 59 alternate at different positions around the circumference of the lag screw 27 over several adjacent passes of the threads 37, as opposed to a non-helical arrangement of the flutes 57.

[0023] Although the channels 59 eliminate portions of the undercut 47 and the associated potential benefit of increased fixation, any such potential loss of fixation is distributed to different angular locations around the circumference of the shaft 35. Thus, the flutes 57 do not reduce resistance to lateral movement and potential cutout from any one angular location to another, and maintain the same resistance to movement regardless of the orientation of the lag screw 27 or the direction of the lateral force on the lag screw 27.

[0024] In the illustrated embodiment, as best seen in FIG. 6 , there are three flutes 57 with three corresponding starting positions angularly spaced about the shaft 35, in this case evenly distributed around a 360-degree arc. The channels 59 created by the flutes 57 in this embodiment do not extend to the inward depth of the valleys 56. As a result, at least 300 degrees of undercuts 47 remain located in each 360-degree arc of the threads 37 to engage adjacent bone material and enhance fixation. Other dimensions for the channels 59 are possible. The width of the channels 59 can be selected to balance the desired increase in resistance to lag screw movement from the undercuts 47 with the reduced torque required due to the flutes 57.

[0025] 8 in combination with FIG. 1, the shaft 35 of the lag screw 27 may include four set screw grooves 61. The grooves 61 have respective pairs of opposing groove sidewalls 63. The grooves 61 in the illustrated implementation are defined at equally spaced angular positions around the cylindrical surface of the trochanteric nail 23, in this case at approximately 90° intervals. Other configurations may be suitable for other applications as well.

[0026] The undercut 47 in the illustrated implementation extends in a single concave arc radially outward from a location proximal to the valley 56 to a portion adjacent the apex 60. The undercut 47 has an outer undercut tip 65 ( FIG. 2 ) formed at or adjacent the apex 60. This geometry, and any other geometry for receiving and retaining bone material in the undercut 47, is suitable for other applications.

[0027] Further modifications to the disclosed implementation are within the scope of this disclosure. Although the threads 37 in the illustrated embodiment are shown as single-start threads, it should be understood that the components and features of the present disclosure are equally applicable to double-start threads or other thread configurations.

[0028] The threads 37 in the illustrated implementation are male, but may be female in other potential applications.

[0029] The threads 37 may be configured as either right-handed or left-handed for appropriate use on the right or left side of the patient to minimize potential loss of fracture reduction during insertion.

[0030] Thus, from the foregoing disclosure, improved lag screws can be manufactured to increase resistance to cutout after a trochanteric hip procedure. Manufacturing involves creating a thread form with an increased surface area compared to a standard V-shaped thread form and also manufacturing the thread form to include an undercut on one or both of the leading and trailing edges of the thread. Additionally, lag screws can be manufactured using threads with a variable profile width that increases proximally, while creating such lag screws with a constant average pitch despite the increased width of the thread. The lag screw manufactured in this manner increases compression as it is rotatably inserted into bone material during a hip procedure to increase fixation.

[0031] The operation and advantages of the trochanteric nailing system 21 are apparent from the foregoing disclosure.

[0032] The disclosed system has improved single lag screw fixation to better resist cutout, retard intracranial migration, and extend the time available for healing to occur.

[0033] The hip nailing device incorporates undercuts on the leading and trailing edges of the lag screw threads, causing both edges to wrap around the bone upon insertion.

[0034] These undercuts grip the bone radially, meaning that when a lateral force is applied to the device, the bone around the device opposite the direction of movement is in tension to resist lateral movement in addition to the bone being in compression.

[0035] Bone stresses resulting from a given load are effectively spread around the device compared to conventional lag screws, reducing the volume of bone subjected to loads beyond its ultimate stress. In essence, conventional screws push against the bone above the screw to resist cutout. In contrast, the disclosed trochanteric nail system 21 pushes against the bone above the screw and pulls against the bone below the screw to resist cutout.

[0036] Additionally, as disclosed, the disclosed thread forms and bone trapped inside the undercuts result in an increased effective lateral profile of the device over that of the device body alone. This additional lateral profile distributes compressive forces over a greater area to resist cutout.

[0037] In certain implementations, the geometry of the threads 37 themselves is modified to improve performance in bone in tension by, in combination with the undercuts 47, applying additional compression to bone trapped within the undercuts during insertion. This additional compression can be achieved via a variable thread profile, where the threads widen as they move down the body of the device while maintaining a constant pitch at the center of the profile. Additionally or alternatively, a variable width profile is created by using different pitches for the leading and trailing edges of the thread profile, where the average pitch between the two edges is the pitch of the thread.

[0038] The spiral flutes disclosed herein equalize biomechanical performance in all directions of lateral movement while still providing the benefits of traditional flutes.

[0039] The addition of a taper to the set screw groove increases resistance to lateral sliding via mechanical interference between the nail's set screw and the disclosed fixation device and its lag screw.

[0040] Additionally, the disclosed features reduce the risk of complications from potentially high insertion torque (which can result in femoral head rotation and loss of reduction during insertion) through device options that allow for clockwise or counterclockwise insertion. By having both left-handed and right-handed versions, the soft tissue, muscle anatomy, and common fracture patterns better resist clockwise insertion in right hips and counterclockwise insertion in left hips. The likelihood of loss of reduction during insertion can be reduced through appropriate clockwise or counterclockwise device selection.

[0041] Furthermore, it should be understood that various changes in the details, materials, and arrangements of parts described and illustrated to explain the nature of the invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the claims. Those skilled in the art will appreciate that the embodiments discussed above are non-limiting. It will also be understood that one or more features of one embodiment may be incorporated, in part or in whole, into one or more other embodiments described herein.

Claims

1. 1. A trochanteric nailing system comprising: a trochanteric nail configured for intramedullary insertion into a patient's femur to address a hip fracture; a trochanteric nail fixation device operably connectable to the trochanteric nail, the trochanteric nail fixation device including a lag screw configured to be rotatably advanced into the femur and to engage bone material adjacent the lag screw in response to an applied torque, the lag screw having a head and a tip formed on opposite ends of the lag screw and a shaft extending at least partially between the head and the tip; a helical thread disposed on an outer surface of the shaft and extending longitudinally distally from the head to terminate at the tip, the thread having a valley located radially proximal to the outer surface of the shaft, the thread further having thread flanks extending laterally from the valley to terminate at a crest, whereby the valley and the crest correspond to the minor and major diameters of the thread, and the thread flanks correspond to the leading and trailing edges of the thread, respectively; the thread comprises a thread form having a recess defined in at least one of the leading edge and the trailing edge to form at least one corresponding undercut; the threads are characterized by a predetermined pitch selected for the bone material such that the undercuts enable engagement of the bone material adjacent to the lag screw, whereby in response to a lateral force in a given force direction on the lag screw after insertion into the femur, the bone material engages with a portion of the undercut located on the opposite side of the given force direction to resist the lateral force.

2. 2. The system of claim 1, wherein the thread form comprises two recesses, one on each lateral side of the thread form, to create two respective undercuts on each of the leading and trailing edges.

3. 3. The system of claim 2, wherein the thread is positioned to extend longitudinally on the shaft for a predetermined surgical insertion length that is less than the length of the shaft between the head and the tip, and the undercut extends on the helical thread for the predetermined surgical length.

4. 4. The system of claim 3, wherein the surgical length and the threads extending across the surgical length define a lateral profile, the lateral profile oriented to oppose the lateral force when applied to the lateral profile.

5. 5. The system of claim 4, wherein the thread form comprises a centerline and a variable cross-sectional width across adjacent paths of the thread, the width increasing from the tip of the shaft toward the head to increase compression of the bone material during insertion of the lag screw.

6. 5. The system of claim 4, wherein the thread form is shaped to have a first pitch for the leading edge and a second pitch for the trailing edge, the first and second pitches being selected to be different from one another and to cause increased compression on the bone material upon insertion of the lag screw.

7. 2. The system of claim 1, wherein the thread form is characterized by a first pitch, and the lag screw further comprises at least one flute extending helically at a second pitch, the second pitch being coarser than the first pitch of the thread so as to define channels extending through each thread flank at corresponding radial positions that differ between adjacent passes of the thread.

8. 8. The system of claim 7, further comprising three helical flutes having three corresponding flute initiations at angularly spaced locations around the shaft.

9. 9. The system of claim 8, wherein the three flutes are defined at three corresponding arcuately spaced positions in a 360 degree rotation of the thread.

10. the trochanteric nail further comprises an outer cylindrical surface characterized by a longitudinal axis and a cross-sectional diameter, the nail having a portion defining a bore extending transversely and diametrically through the shaft at an angle to the longitudinal axis, the bore being sized to receive the lag screw therethrough, the system further comprising a set screw interconnectable between the nail and the screw, the lag screw being operably connectable to the trochanteric nail by locking the set screw to the lag screw after insertion of the lag screw through the bore of the trochanteric nail; the lag screw further comprises at least one set screw groove defined in the lag screw shaft by a pair of radially inwardly extending opposed groove sidewalls extending longitudinally at a location on the outer surface of the shaft proximal to the threads when the lag screw is oriented for insertion. The system of claim 1 .

11. The system of claim 10 further comprising four of the set screw grooves defined at spaced angular locations on the cylindrical surface of the nail.

12. The system of claim 1 , wherein the threads include a single start thread.

13. 2. The system of claim 1, wherein the major and minor diameters define a thread height, and the undercut is configured to have a radius of curvature selected to extend the undercut over a distance of at least 75% of the thread height.

14. The system of claim 13 , wherein the undercut has an outer undercut tip located proximate the crest of the thread.

15. The system of claim 1 , wherein the threads are male.

16. The system of claim 1 , wherein the threads are selected from at least one of a right-handed thread and a left-handed thread.

17. 1. A trochanteric nailing system comprising: a trochanteric nail configured for intramedullary insertion into a patient's femur to address a hip fracture; a trochanteric nail fixation device operably connectable to the trochanteric nail, the trochanteric nail fixation device including a lag screw, the lug configured to rotatably advance into the femur and engage bone material adjacent the lag screw in response to an applied torque, the lag screw having a head and a tip formed on opposite ends of the lag screw and a shaft extending at least partially between the head and the tip; a helical thread disposed on an outer surface of the shaft and extending longitudinally distally from the head to terminate at the tip; the trochanteric nail further comprises an outer cylindrical surface characterized by a longitudinal axis and a cross-sectional diameter, the nail having a portion defining a bore extending transversely and diametrically through the shaft at an angle to the longitudinal axis, the bore being sized to receive the lag screw therethrough, the system further comprising a set screw interconnectable between the nail and the screw, the lag screw being operably connectable to the trochanteric nail by locking the set screw to the lag screw after insertion of the lag screw through the bore of the trochanteric nail; the lag screw further comprises four set screw grooves defined in the lag screw shaft by respective pairs of radially inwardly extending, opposing groove sidewalls that extend longitudinally at locations on the outer surface of the shaft proximal to the threads when the lag screw is oriented for insertion; the pair of groove sidewalls are tapered distally to increase engagement with opposing portions of the set screw when received in a selected one of the set screw grooves.

18. 1. A trochanteric nailing system comprising: a trochanteric nail configured for intramedullary insertion into a patient's femur to address a hip fracture; a trochanteric nail fixation device operably connectable to the trochanteric nail, the trochanteric nail fixation device including a lag screw configured to be rotatably advanced into the femur and to engage bone material adjacent the lag screw in response to an applied torque, the lag screw having a head and a tip formed on opposite ends of the lag screw and a shaft extending at least partially between the head and the tip; a helical thread disposed on an outer surface of the shaft and extending longitudinally distally from the head to terminate at the tip; the threads comprise a thread form having a centerline and a variable cross-sectional width relative to the centerline across adjacent paths of the threads, the width increasing from the tip of the shaft toward the head to increase compression of the bone material during insertion of the lag screw.

19. the thread has a root located radially proximal to the outer surface of the shaft, the thread further having thread flanks extending laterally from the root to terminate at a thread crest, whereby the root and the thread crest correspond to the minor and major diameters of the thread, and the thread flanks correspond to the leading and trailing edges of the thread, respectively; the thread comprises a thread form having two recesses defined therein, one on each of the lateral sides of the thread form, to create two respective undercuts on each of the leading edge and the trailing edge; the threads are characterized by a predetermined pitch selected for the bone material such that the undercuts allow engagement of the bone material adjacent to the lag screw, whereby in response to a lateral force in a given force direction on the lag screw after insertion into the femur, the bone material engages with a portion of the undercut located opposite the given force direction to resist the lateral force; the threads are positioned to extend longitudinally on the shaft for a predetermined surgical insertion length that is less than the length of the shaft between the head and the tip of the shaft, and the undercut extends on the helical thread for the predetermined surgical length; the surgical length and the threads extending across the surgical length define a lateral profile, the lateral profile being oriented to oppose the lateral force when applied to the lateral profile; the recess of the undercut defines an angle of at least 45 degrees as measured from an associated reference triangular thread form so as to increase the surface area of ​​the lateral profile across which the lateral force is distributed; the thread form is characterized by a first pitch, and the lag screw further comprises at least one flute extending helically across the surgical length of the thread, the flute having a second pitch, the second pitch being coarser than the first pitch of the thread so as to define channels extending through each thread flank at corresponding radial positions that differ between adjacent passes of the thread; 20. The system of claim 18.

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