Resorbable metal screw with increased torsional strength for osteopathy

• US 9,962,210 B2
• Filed: 03/03/2016
• Issued: 05/08/2018
• Est. Priority Date: 03/24/2015
• Status: Active Grant

• Current Assignee: Biotronik AG (Biotronik SE & Company KG)
• Original Assignee: Biotronik AG (Biotronik SE & Company KG)
• Inventors: Bayer, Ullrich, Anopuo, Okechukwu, Lootz, Daniel
• Primary Examiner(s): Sevilla, Christian
• Application Number:
  US15/059,571
• Publication Number:
  US 20160278834A1
• Time in Patent Office: 796 Days
• CPC Class Codes: A61B 17/1655 for tapping
• CPC Class Codes: A61B 17/863 with thread interrupted or changing its form along shank, other than constant taper
• CPC Class Codes: A61B 17/8635 Tips of screws
• 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 2017/00526 Methods of manufacturing
• CPC Class Codes: A61B 2017/561 Implants with special means for releasing a drug
• CPC Class Codes: A61B 2017/8655 with special features for locking in the bone
• CPC Class Codes: A61L 2430/02 for reconstruction of bones; weight-bearing implants
• CPC Class Codes: A61L 31/022 Metals or alloys
• CPC Class Codes: A61L 31/14 Materials characterised by their function or physical properties , e.g. injectable or lubricating compositions, shape-memory materials, surface modified materials
• CPC Class Codes: A61L 31/148 Materials at least partially resorbable by the body

Abstract

A bone screw formed primarily of a magnesium alloy has a self-tapping thread, and is prestressed with a retained torsional stress oriented oppositely to the direction in which the screw is screwed into bone. The pre-stressing opposes the torsional stresses arising from screwing in the screw, thereby raising the effective strength of the screw and/or allowing for a reduction in its size/mass.

Claims

1. A bone screw:

a. formed primarily of a magnesium alloy,

b. having a screw length wherein at least a portion of the screw length bears a self-tapping thread, and

c. being pre-stressed with an inherent torsional stress oriented opposite the screw-in direction of the thread.

2. The bone screw of claim 1 wherein the inherent torsional stress is constant over at least the portion of the screw length bearing the self-tapping thread.

3. The bone screw of claim 1 wherein the bone screw is cannulated along at least a portion of the screw length.

4. The bone screw of claim 3 wherein the bone screw has:

a. an outer diameter in the range between 1.5 and 5 mm, and

b. an inner diameter in the range between 0.5 and 2.5 mm.

5. The bone screw of claim 3 wherein the bone screw has:

a. an outer diameter in the range between 2.5 and 3.5 mm, and

b. an inner diameter in the range between 0.8 and 1.3 mm.

6. The bone screw of claim 1 further including a surface coating on at least a portion of the thread, wherein the surface coating includes one or more of:

a. micro-abrasives, and

b. microcapsules containing at least one of;

(1) a lubricant, and

(2) a bioactive substance configured to promote bone growth,

wherein the microcapsules are configured to rupture as the thread penetrates bone.

7. The bone screw of claim 6 wherein the micro-abrasives include crystalline hydroxyapatite.

8. The bone screw of claim 6 wherein the micro-abrasives are needle-shaped.

9. A method for producing the bone screw of claim 1, the method including the steps of:

a. providing a member formed primarily of a magnesium alloy;

b. forming a self-tapping thread in an outer wall of the member, whereby the threaded member defines the bone screw;

c. pre-stressing the member with an inherent torsional stress oriented opposite the screw-in direction of the thread.

10. The method of claim 9 wherein the step of pre-stressing the member occurs at least partially simultaneously with the step of forming the self-tapping thread.

11. The method of claim 9 wherein the step of forming the self-tapping thread includes urging the member through a die while rotating at least one of the member and the die.

12. The method of claim 11 wherein the step of forming the self-tapping thread further includes cutting and/or grinding the member.

13. The method of claim 12 wherein cutting and/or grinding the member removes less than 0.2 mm from the diameter of the member.

14. The method of claim 11 wherein the step of forming the self-tapping thread occurs at a temperature above 100°

C.

15. The method of claim 11 wherein the step of forming the self-tapping thread occurs at a strain rate between 0.05 s⁻

1 and 25 s<sup>−

1</sup>.

16. The method of claim 11 wherein the step of forming the self-tapping thread includes rotating the member with respect to the die by 1.0 to 8 revolutions per centimeter of advancement of the member through the die.

17. The method of claim 9 wherein the member is at least partially defined by a tube.

18. The method of claim 9 further including the steps of:

a. forming a surface coating on at least a portion of the thread; and

b. providing one or more of;

a. micro-abrasives, and

b. microcapsules containing at least one of;

(1) a lubricant, and

(2) a bioactive substance configured to promote bone growth, to the surface coating.

19. The method of claim 18 wherein the micro-abrasives and/or the microcapsules are provided to the surface coating prior to forming the surface coating on at least a portion of the thread.

20. A method for producing the bone screw of claim 1, the method including the steps of:

a. providing a member formed primarily of a magnesium alloy;

b. forming a self-tapping thread in the member via rotary extrusion, whereby the threaded member defines the bone screw.