Cannulated bone screw

• US 20040006345A1
• Filed: 02/12/2003
• Published: 01/08/2004
• Est. Priority Date: 02/12/2002
• Status: Active Grant

• Current Assignee: RTI Surgical LLC (Surgalign Holdings, Inc.)
• Original Assignee: Pioneer Laboratories Incorporated
• Inventors: Songer, Matthew N., Vlahos, Jeff, Davenport, Kenneth
• Granted Patent: US 7,207,994 B2
• Time in Patent Office: Days
• US Class Current: 606/73
• CPC Class Codes: A61B 17/8605 Heads, i.e. proximal ends projecting from bone
• CPC Class Codes: A61B 17/8625 Shanks, i.e. parts contacting bone tissue
• CPC Class Codes: A61B 17/864 hollow, e.g. with socket or cannulated
• CPC Class Codes: A61B 17/866 Material or manufacture
• CPC Class Codes: A61B 2090/031 torque limiting

Abstract

A small cannulated bone screw is provided having an optimized combination of strength, cannula size and thread depth. A set of such strong and small size screws is also provided where the smallest sizes can have one performance ratio emphasized over another, and the larger sizes can have the emphasis reversed. In particular, the smallest screws optimize the combination of their strength and cannula size with the larger screws optimizing the combination of their strength and thread depth. In any event, each of the screws in the set has an optimized combination of strength, cannula size and thread depth throughout the full range of screw sizes in the set, e.g. 3.0 mm to 7.5 mm major diameters.

Claims

1. A bone screw apparatus comprising:

a small bone screw having an elongate shank of predetermined material and including proximal and distal ends thereof;

external threads along the shank having predetermined dimensions that maximize the purchase for the small screw threaded into bone material;

an outer diameter of the threads having a range of predetermined thread diameter sizes between a minimum and a maximum thereof;

an axial through bore in the shank having a predetermined diameter that is maximized in size relative to the thread outer diameter to permit a relatively large and stiff guide wire to be inserted therethrough for accurate guided implantation of the screw in the bone material; and

a screw head at the proximal end of the shaft integrally formed therewith to resist a predetermined high level of applied torsional forces to allow turning of the small screw about the large guide wire to thread the screw into the bone material.

2. The bone screw apparatus of claim 1 wherein the predetermined thread diameter is approximately 0.185 inch at its maximum and 0.118 inch at its minimum, and the axial bore diameter is in the range of approximately 0.053 inch at the minimum thread diameter to approximately 0.098 at the maximum thread diameter.

3. The bone screw apparatus of claim 1 wherein the shank has a predetermined wall thickness that is minimized for providing the maximized diameter of the throughbore, and the predetermined high level of torsional forces is between approximately 5.86 in/lbs at thread outer diameters of approximately 0.118 inch and between approximately 23.86 in/lbs and approximately 32.23 in/lbs at thread outer diameters of approximately 0.185 inch.

5. The bone screw apparatus of claim 1 wherein the predetermined thread diameter and the predetermined bore diameter provide a range of predetermined ratios of thread-to-bore diameter that are minimized through the full range of thread diameters to allow small screws with good purchase to be utilized with large guide wires for accurate implantation thereof in bone material.

6. The bone screw apparatus of claim 5 wherein the range of predetermined thread diameters is limited to the smaller sizes in the full range of approximately 3.0 mm to approximately 4.5 mm.

7. The bone screw apparatus of claim 1 in combination with the guide wires, the guide wires having a range of predetermined diameters of approximately 1.25 mm at the minimum thread diameter of approximately 3 mm to approximately 2.39 mm at the maximum thread diameter of 4.5 mm.

8. The bone screw apparatus of claim 1 wherein the predetermined screw material comprises an essentially nickle-free austenitic stainless steel alloy including a high nitrogen content.

9. The bone screw apparatus of claim 1 wherein the screw head includes driving surfaces recessed into the head, and the predetermined screw material is a high strength alloy so that stripping deformation of the driving surfaces occurs prior to screw shearing along the shank including the head with high applied torque via a driver for the screws.

10. The bone screw apparatus of claim 9 wherein the screw head driving surfaces are recessed to a predetermined depth that is kept to a maximum with the high strength screw material allowing applied torque via the driver engaged with drive surfaces to rotatively drive the screw into bone material.

4. The bone screw apparatus of claim 4 wherein the predetermined wall thickness is between approximately 0.0105 inch at the minimum thread diameter and approximately 0.035 inch at the maximum thread diameter.

11. A set of small cannulated bone screws with the screws sized by major diameters of screw threads thereon, the screw set comprising:

a plurality of screws of different major diameter sizes ranging between approximately 3.0 mm and approximately 7.5 mm;

a screw wall of each screw in the set having predetermined high strength levels;

an axial through bore of each screw in the set having an optimized diameter thereof for receipt of a guide wire therethrough;

a first screw performance ratio for each of the screws defined by dividing the major diameter by the through bore diameter with the first screw performance ratio being minimized for each screw in the set;

a thread depth of each screw in the set that is optimized in size to provide the screws with pull-out resistance; and

a second screw performance ratio for each of the screws defined by dividing the major diameter by the size of the thread depth with the second screw performance ratio being minimized for each screw in the set.

12. The screw set of claim 11 wherein the first and second performance ratios include a predetermined priority based on screw sizes with the first ratio emphasized over the second ratio with the smallest screw sizes between approximately 3.0 mm and approximately 4.5 mm, and the second ratio emphasized over the first ratio with the larger screw sizes between approximately 5.5 mm and 7.5 mm.

13. The screw set of claim 11 wherein the plurality of screws include smallest screws in the set between approximately 3.0 mm and approximately 4.5 mm, and the optimized through bore diameters for the smallest screws is maximized in size to permit a maximum diameter guide wire to be received therethrough.

14. The screw set of claim 13 wherein the optimized diameters of the smallest screws are approximately 1.35 mm for the 3.0 mm screw and approximately 2.49 mm for the 4.5 mm screw.

15. The screw set of claim 14 in combination with the guide wires with the maximum diameters thereof being approximately 1.25 mm for the 3.0 mm screw and approximately 2.39 mm for the 4.5 mm screw.

16. The screw set of claim 11 wherein the plurality of screws include larger screws in the set between approximately 5.5 mm and approximately 7.5 mm, and the optimized size of the thread depth is maximized in size to provide the larger screws with maximized pull-out resistance.

17. The screw set of claim 16 wherein the optimized thread depth sizes of the larger screws are approximately 0.0355 inch for the 5.5 mm screw and approximately 0.0545 inch for the 7.5 mm screw.

18. The screw set of claim 11 wherein the strengths of the walls of the screws are approximately 5.86 in/lbs for the 3.0 mm screw and approximately 92.3 in/lbs for the 7.5 mm screw.

19. The screw set of claim 18 wherein the screw wall is formed of a predetermined high strength material comprising an essentially nickle-free austenitic stainless steel alloy including a high nitrogen content.

20. The screw set of claim 18 wherein the screws each include a head at one end thereof including recessed drive surfaces configured such that application of excessive torque levels thereto causes deformation of the drive surfaces prior to breakage of the high strength screw walls.