Bone Fixation System for Spinal Stabilization

Abstract

A bone fixation system where an alignment rod is engaged by pedicle screws. The alignment rod has sections of buttress thread. Each pedicle screw has a threaded shaft that supports a slotted receptacle. A set screw threads into the slotted receptacle. A locking plate is interposed between the set screw and the alignment rod. The locking plate contains protrusions that engage the sections of buttress thread on the alignment rod. The threaded shaft of the pedicle screw has an upper section and a lower section. A threaded connecting rod interconnects the sections. The upper section, lower section and threaded connecting rod all have external threads of the same thread pitch.

Claims

1. A bone fixation system, comprising: an alignment rod having a top surface, wherein at least part of said alignment rod has sections of buttress thread disposed along said top surface; a pedicle screw having a threaded shaft that supports a slotted receptacle, wherein said slotted receptacle defines a slot sized to receive said alignment rod therethrough, said slot having opposing walls, wherein said opposing walls are internally threaded; a set screw that threads into said opposing walls, wherein said set screw can be selectively advanced into said slot to an adjustment position and to a locked position; a locking plate having a top surface, a bottom surface and a plurality of parallel buttress protrusions that extend from said bottom surface, said locking plate being sized to fit within said slot of said slotted receptacle, wherein said locking plate is interposed between said set screw and said alignment rod, wherein said plurality of parallel buttress protrusions engage said sections of buttress thread when said locking plate is biased against said alignment rod by said set screw, wherein said locking plate limits said alignment rod to movement in only one direction through said slot when said set screw is in said adjustment position, and wherein said locking plate prevents said alignment rod from moving through said slot in any direction when said set screw is in said locked position.

2. The system according to claim 1, wherein said threaded shaft of said pedicle screw has a length that is selectively adjustable.

3. The system according to claim 2, wherein said threaded shaft has an upper section and a lower section that thread together, wherein said length is adjusted by turning said upper section and said low section relative to each other.

4. The system according to claim 3, wherein a threaded connecting rod interconnects said upper section and said lower section, and wherein said threaded connecting rod is exposed in different amounts when said lower section is turned relative to said upper section, and wherein said upper section, said lower section and said threaded connecting rod all have external threads of the same thread pitch.

5. The system according to claim 4, wherein said threaded connecting rod is rigidly affixed to said lower section of said threaded shaft.

6. The system according to claim 5, wherein said slotted receptacle is rigidly affixed to said upper section of said threaded shaft.

7. The system according to claim 6, wherein said upper section of said threaded shaft is hollow and said threaded connecting rod is accessible through said upper section.

8. The system according to claim 1, wherein said alignment rod has a smooth bottom surface that rests in said slotted receptacle.

9. A bone fixation system, comprising: an alignment rod having a top surface; a pedicle screw having a threaded shaft that supports a slotted receptacle for receiving said alignment rod, wherein said threaded shaft has an upper section with a first external thread pitch, a lower section with a second external thread pitch and a threaded connecting rod with a third external thread pitch that interconnects said upper section and said lower section, wherein said first external thread pitch, said second external thread pitch and said third external thread pitch are all equal, therein providing all of said threaded shaft with a single external thread pitch; wherein said threaded shaft is selectively adjustable in length by rotating said lower section relative to said upper section, and wherein said threaded connecting rod is exposed to different degrees when said lower section is turned relative to said upper section.

10. The system according to claim 9, wherein said threaded connecting rod is rigidly affixed to said lower section of said threaded shaft.

11. The system according to claim 9, wherein said slotted receptacle is rigidly affixed to said upper section of said thread shaft.

12. The system according to claim 11, wherein at least part of said alignment rod has sections of buttress thread disposed along said top surface.

13. The system according to claim 12, wherein said slotted receptacle defines a slot sized to receive said alignment rod therethrough, said slot having opposing walls, wherein said opposing walls are internally threaded.

14. The system according to claim 13, further including a set screw that threads into said opposing walls, wherein said set screw can be selectively advanced into said slot to an adjustment position and to a locked position.

15. The system according to claim 14, further including a locking plate that is interposed within said slot between said set screw and said alignment rod, wherein said locking plate contains protrusions that engage said sections of buttress thread when biased against said alignment rod by said set screw, wherein said locking plate limits said alignment rod to movement in only one direction through said slot when said set screw is in said adjustment position, and wherein said locking plate prevents said alignment rod from moving through said slot in any direction when said set screw is in said locked position.

16. A pedicle screw system, comprising; a slotted receptacle; a length adjustable threaded shaft that supports said slotted receptacle, wherein said length adjustable threaded shaft has an upper section and a lower section that are interconnected by a threaded connecting rod, wherein said threaded connecting rod is exposed to different degrees when said lower section is turned relative to said upper section, and wherein said upper section, said lower section and said threaded connecting rod all have external threads of the same thread pitch, therein providing said length adjustable threaded shaft with a single, consistent external thread pitch regardless of adjusted length.

17. The system according to claim 16, wherein said upper section of said threaded shaft is hollow and said threaded connecting rod is accessible through said upper section.

18. The system according to claim 16, wherein said slotted receptacle is rigidly affixed to said upper section of said threaded shaft.

19. The system according to claim 18, wherein said threaded connecting rod is rigidly affixed to said lower section of said threaded shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a better understanding of the present invention, reference is made to the following description of an exemplary embodiment thereof, considered in conjunction with the accompanying drawings, in which:

[0012] FIG. 1 is a perspective view of a segment of a bone fixation system that utilizes adjustment rods and pedicle screws;

[0013] FIG. 2 is a side view of the adjustment rod shown in FIG. 1;

[0014] FIG. 3 is a cross-sectional view of the alignment rod shown in FIG. 2 viewed along section line 3-3;

[0015] FIG. 4 is an exploded view of the exemplary embodiment of FIG. 2;

[0016] FIG. 5 is a cross-sectional view of the exemplary embodiment of FIG. 1;

[0017] FIG. 6 is a cross-sectional view of a vertebral body being engaged by the pedicle screw of the present invention bone fixation system.

[0018] FIG. 7 is a cross-sectional view of an anchor head assembly of the pedicle screw in a locked position engaging an alignment rod; and

[0019] FIG. 8 is a cross-sectional view of an anchor head assembly of the pedicle screw in an adjustment position partially engaging an alignment rod.

DETAILED DESCRIPTION OF THE DRAWINGS

[0020] Although the present invention system can be embodied in many ways, only one exemplary embodiment is illustrated. The exemplary embodiment is being shown for the purposes of explanation and description. The exemplary embodiment is selected in order to set forth one of the best modes contemplated for the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the claims.

[0021] Referring to FIG. 1, a bone fixation system 10 is shown. The bone fixation system 10 consists primarily of a specialized pedicle screw 12 and a specialized alignment rod 14. The pedicle screw 12 has a shaft assembly 16 that terminates at one end with an anchor head assembly 18. The anchor head assembly 18 is designed to selectively received and engage the alignment rod 14. As will be explained, the anchor head assembly 18 works in concert with the alignment rod 14 so that the alignment rod 14 can be pulled in one direction while the anchor head assembly 18 automatically prevents the tensioned alignment rod 14 from moving back in the opposite direction.

[0022] Referring to FIG. 2 and FIG. 3 in conjunction with FIG. 1, it can be seen that the alignment rod 14 can be any length as needed by a surgeon. The alignment rod 14 can be either straight or curved. The alignment rod 14 preferably has a rounded profile. However, polygonal shapes can also be used. Regardless of the profile shape, the alignment rod 14 has a top surface 20 and an opposite bottom surface 22. The bottom surface 22 is smooth and faces the shaft assembly 16 of the pedicle screw 12. The opposite top surface 20 is machined with segments of buttress thread 24. Each segment of buttress thread 24 has a vertical surface 26 and a sloped surface 28, wherein all the sloped surfaces 28 slope in a common direction at the same pitch angle. The segments of buttress thread 24 extend through arc angles 21 on the top surface 20 of the alignment rod 14. Preferably, the arc angles 21 extends between ten degrees and thirty degrees from the top dead center of the alignment rod 14.

[0023] Referring to FIG. 4, FIG. 5, in conjunction with FIG. 1, it can be seen that the anchor head assembly 18 of the pedicle screw 12 includes a slotted receptacle 30. The slotted receptacle 30 has an open top 32 and two opposing walls 34, 36 that are spaced apart by a slot 38. The slot 38 is accessible through the open top 32. The slot 38 is slightly wider than the width of the alignment rod 14. The opposing walls 34, 36 are internally threaded. That is, the opposing walls 34, 36 have threads 35 on interior surfaces that face the slot 38. The slotted receptacle 30 has a bottom surface 40 that is contoured to match the shape of the bottom surface 22 of the alignment rod 14. In this manner, the alignment rod 14 can be cradled in the slotted receptacle 30.

[0024] A locking plate 42 is provided as part of the anchor head assembly 18. The locking plate 42 is sized to fit into the slot 38 between the opposing walls 34, 36 atop the alignment rod 14. The locking plate 42 has a top surface 41 and an underlying bottom surface 43. Buttress protrusions 44 are disposed on the bottom surface 43. The buttress protrusions 44 are sized and spaced to engage the buttress threads 24 on the top surface 20 of the alignment rod 14. A set screw 46 is also provided. The set screw 46 screws into the threading 35 on the opposing walls 34, 36. As the set screw 46 is tightened, the set screw 46 presses against the top surface 41 of the locking plate 42. This biases the locking plate 42 against the alignment rod 14.

[0025] Referring to FIG. 7 and FIG. 8 in conjunction with FIG. 4 it can be seen that locking plate 42 is biased against the alignment rod 14 by the set screw 46, the buttress protrusions 44 on the locking plate 42 engage the buttress threads 24 on the alignment rod 14 to some degree. If the set screw 46 is firmly tightened, then the buttress protrusions 44 on the locking plate 42 fully engage the buttress threads 24 on the alignment rod 14. The alignment rod 14 is therefore firmly locked in place and cannot move independently in relation to the pedicle screw 12. See FIG. 5. However, as can be seen from FIG. 8, if the set screw 46 is not firmly tightened, a gap space 48 is created between the locking plate 42 and the alignment rod 14. If there is a gap space 48, the locking plate 42 has the ability to move in the gap space 48. If the gap space 48 is correctly sized, the buttress protrusions 44 on the locking plate 42 will engage the buttress threads 24 on the alignment rod 14 only if the alignment rod 14 is moved in the direction of arrow 50. If the alignment rod 14 is moved in the opposite direction of arrow 50, then the locking plate 42 floats atop the alignment rod 14 without engaging the alignment rod 14. It will therefore be understood that if the gap space 48 is properly sized using the set screw 46, then the alignment rod 14 can be pulled in one direction using one hand. This can place the alignment rod 14 in tension. Once properly positioned, the alignment rod 14 can be released and it will not move backward and release the tension. A surgeon can then lock the alignment rod 14 in place by tightening the set screw 46 to eliminate the gap space 48.

[0026] Referring back to FIG. 4 and FIG. 5, it can be seen that the pedicle screw 12 has a shaft assembly 16. The shaft assembly 16 has a first upper section 52, a second lower section 54 and a threaded connection rod 70 that are all vertically aligned. The threaded connection rod 70 is affixed to the lower section 54. The upper section 52 has a first open end 56 and an opposite second open end 58. The upper section 52 is hollow between the first open end 56 and the second open end 58. As such, the upper section 52 is tubular with both an interior surface 60 and an exterior surface 62. Both the interior surface 60 and the exterior surface 62 are threaded. As such, there are interior threads 64 and exterior threads 66. The exterior threads 66 have a certain thread pitch. The interior threads 64 have the same thread pitch, albeit with a smaller major diameter.

[0027] The first open end 56 of the upper section 52 is affixed to the slotted receptacle 30. The first open end 56 is accessible through the bottom 40 of the slotted receptacle 30. In this manner, an Allen key or similar tool can be inserted into the interior of the upper section 52 though the bottom of the slotted receptacle 30.

[0028] The second lower section 54 of the shaft assembly 16 is solid. The lower section 54 has a first end 74 and an opposite second end 76. The threaded connection rod 70 extends from the first end 74 of the lower section 54. The threaded connection rod 70 has a smaller diameter than does the lower section 54. As such, there is a stepped transition 75 at the point of attachment. The threaded connection rod 70 has external threads 72. The external threads 72 match the pitch, thread angle and diameter of the interior threads 64 on the interior surface 60 of the upper section 52. In this manner, the upper section 52 of the shaft assembly 16 threads onto the threaded connection rod 70. A keyed depression 80 is formed in the threaded connection rod 70. This keyed depression 80 can be accessed through the slotted receptacle 30. It will therefore be understood that a surgeon can advance a tool through the slotted receptacle 30 and the upper section 52. The tool can be used to turn the threaded connection rod 70 and the lower section 54 relative the upper section 52. As the lower section 54 turns relative to the upper section 52, the external threads 72 on the threaded connection rod 70 engage the interior threads 64 within the upper section 52. Accordingly, the turning of the threaded connection rod 70 and the lower section 54 will cause the lower section 54 and the upper section 52 to either move together or spread apart, depending upon the direction of rotation. As the lower section 54 and the upper section 52 move apart, the threaded connection rod 70 becomes more exposed and the overall length of the shaft assembly 16 increases. Conversely, as the lower section 54 and the upper section 52 thread together, the exposed length of the threaded connection rod 70 decreases as does the overall length of the shaft assembly 16.

[0029] The lower section 54 of the shaft assembly 16 has exterior threads 82. The exterior threads 82 match the pitch, thread angle and diameter of the exterior threads 66 on the upper section 52. Furthermore, when the lower section 54 is fully seated into the upper section 52, the exterior threads 82 of the lower section 54 seamlessly align with the exterior threads 66 on the upper section 52, as though the two sections 52, 54 were threaded as a single piece.

[0030] Referring to FIG. 6 in conjunction with FIG. 4 and FIG. 5, it will be understood that to utilize the bone fixation system 10, a surgeon sets the pedicle screw 12 into a vertebral body 55. The length of the pedicle screw 12 can be selectively adjusted by turning the lower section 54 of the shaft assembly 16 relative to the upper section 52. Fine adjustments can be made after initial implantation. The pedicle screw 12 is adjusted so that both the threaded upper section 52 and the threaded lower section 54 engage the cortical bone 57 on opposite sides of the vertebral body 55. The pedicle screw 12 passes through the cancellous bone 59 in the center of the vertebral body 55, however, most of the strength of the mechanical connection is provided by the denser cortical bone 57.

[0031] As the length of the pedicel screw 12 is increased, the threaded connection rod 70 becomes increasingly exposed within the vertebral body 55. However, the threaded connection rod 70 has the same thread pitch as is used on the exteriors of both the lower section 54 and the upper section 52. Accordingly, all exposed threading follows a single thread path through the vertebral body 55. Over time, bone grows against the pedicle screw 12. By utilizing a single pitch thread across all exposed threading, the pedicle screw 12 can be removed from the bone with far less damage to the bone since all threading follows a single thread path.

[0032] Once a pedicle screw 12 is implanted, the slotted receptacle 30 is oriented and adjusted to the proper elevation, via length adjustments to the pedicle screw 12. The surgeon selects the proper alignment rod 14 and sets the alignment rod 14 into the slotted receptacle 30 of the pedicle screw 12. The locking plate 42 and set screw 46 are installed above the alignment rod 14. The set screw 46 is advanced to an adjustment position where the locking plate 42 only engages the alignment rod 14 in one direction. The surgeon can therefore pull and tension the alignment rod 14 with one hand. Once the alignment rod 14 is properly tensioned, the alignment rod 14 can be released by the surgeon. The surgeon can then fully advance the set screw 46 form the adjustment position to a locked position, wherein the alignment rod 14 is locked in place.

[0033] It will be understood that the embodiment of the present invention that is illustrated and described is merely exemplary and that a person skilled in the art can make many variations to that embodiment. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.