SELF-HOLDING SCREW HEAD

Abstract

A screw insertion system has a screw having a threaded shaft and a head. The head has a conically tapered outer surface, the taper increasing in distance from a central longitudinal axis of the screw from a free end of the head towards a larger diameter adjacent the connection between the head and the threaded shaft. The conically tapered head outer surface has preferably two helically extending partially rounded threads. The head has a drive element for engaging a driver. A tubular screw holder has an outer surface and internal bore extending between a leading end and a trailing end for receiving the screw. The leading end having a conically tapered inner threaded portion for engaging the conically tapered outer thread of the screw head. The tubular screw holder outer surface having a diameter less than or equal to the maximum diameter of the conically tapered head.

Claims

1. A screw insertion system comprising: a screw having a leading end having a threaded shaft and a head at a trailing end, the head having a first diameter at a free end and a second diameter at an end connected to the threaded shaft, the first diameter being less than the second diameter, the head having an outer surface with a plurality of external helical threads extending from the first to the second end thereof, the head having a drive element located within the first diameter open to the free end of the head; a screw holder having a through bore with a trailing end and a leading end, an interior surface at the leading end having a threaded portion, the thread portion having a first diameter located towards the trailing end of the holder and a second diameter at the leading end of the holder, the second diameter greater than the first diameter, the second diameter of the holder generally equal to the second diameter of the bone screw head; and a driver extending through the tubular screw holder engagable with the drive element on the screw head.

2. The screw insertion system of claim 1 wherein the screw head has a conical outer shape between the first and second diameter of the head, and the threaded portion of the interior surface of the screw holder threaded portion has a matching conical shape.

3. The screw insertion system of claim 1 wherein the screw head has a part-spherical outer shape between the first and second diameters of the head.

4. The screw insertion system of claim 1 wherein the external threads of the head and the interior threads of the screw holder are at least partially rounded threads

5. The screw insertion system of claim 4 wherein the at least partially rounded threads have rounded roots and flattened crests.

6. The screw insertion system of claim 4 wherein the threads on the head and on the screw holder are left-handed threads

7. The screw insertion system of claim 1 wherein the driver has an outer diameter being slidably received within the bore of the screw holder.

8. The screw insertion system of claim 1 wherein the outer diameter of the screw holder does not exceed the second diameter of the screw head.

9. The screw insertion system of claim 1 wherein the drive element of the bone screw head is an internal hex drive.

10. The screw insertion system of claim 2 wherein the conical outer shape has an opening angle of about 20?.

11. A screw insertion system comprising: a screw having a threaded shaft and a head connected to the threaded shaft, the head having a conically tapered outer surface, the taper increasing in distance from a central longitudinal axis of the screw from a free end of the head towards a larger diameter adjacent the connection between the head and the threaded shaft, the conically tapered head outer surface having at least two helically extending partially rounded threads, the screw head having a recessed drive element open to the free end of the head; a tubular screw holder having an outer surface and internal bore extending between a leading end and a trailing end for receiving the screw, the leading end having a conically tapered inner threaded portion for engaging the conically tapered outer thread of the screw head, the tubular screw holder outer surface having a diameter less than or equal to the maximum diameter of the conically tapered head.

12. The screw insertion system of claim 11 further comprising a driver extending through the base in the tubular screw holder for engaging the recessed drive element in the screw head.

13. The screw insertion system of claim 11 wherein the threads on the head and on the screw holder are left-handed threads.

14. The screw insertion system of claim 11 wherein the drive element of the bone screw head is an internal hex drive.

15. The screw insertion system of claim 11 wherein the at least partially rounded threads have rounded roots and flattened crests.

16. The screw insertion system of claim 11 wherein the conical outer shape has an opening angle of about 20?.

17. A bone fastener comprising: a threaded shaft at a leading end and a head connected to the shaft at a trailing end; the head having a first diameter at a free end and a second diameter at an end connected to the shaft, the second diameter being the maximum diameter of the bone fastener; and the first diameter being less than the second diameter, the head having an outer surface extending between the first and second diameter, the head outer surface comprising a plurality of threads extending around the outer surface.

18. The bone fastener of claim 16 wherein each thread on the head has curved flanks and a curved root intermediate the curved flanks, and the threads having a flat crest on the outer surface of the head extending between adjacent curved flanks forming the thread.

19. The bone fastener as claimed in claim 17 wherein the shaft has a threaded portion at the leading end.

20. The bone fastener as claimed in claim 17 wherein the head outer surface has a conical shape.

21. The bone fastener as claimed in claim 20 wherein the conical shape has an opening angle of 18? to 22?.

22. The bone fastener as claimed in claim 17 wherein the threads on the head are left-hand threads and the threaded shaft has right-hand threads.

23. The bone fastener as claimed in claim 22 wherein the threads on the head are double or triple lead threads.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a cross-sectional view of the bone screw insertion system of the presentation invention showing a bone screw, a tissue protection sleeve, and a screw driver;

[0019] FIG. 1A is an enlarged view of the coupling between the tissue protection sleeve, bone screw head, and screw driver of the present invention as shown in FIG. 1;

[0020] FIG. 2 is an isometric view of the bone screw of FIG. 1;

[0021] FIG. 2A is a side view of the screw of FIG. 2;

[0022] FIG. 3 is a view of the threaded head of the bone screw of FIG. 2;

[0023] FIG. 4 is a cross-sectional view of the bone screw of FIG. 2;

[0024] FIG. 5 is an enlarged view of the hex drive of the bone screw of FIG. 2;

[0025] FIG. 6 is a cross-sectional view of the tissue protection sleeve of the present invention;

[0026] FIG. 7 is an enlarged view of the distal end of the tissue protection sleeve of the present invention; and

[0027] FIGS. 8A and 8B (in cross-section) show an alternate distal end for the tissue protection sleeve wherein the distal end of the sleeve can flex with respect to the axis of the bone screw.

DETAILED DESCRIPTION

[0028] Referring to FIG. 1 there is shown the screw insertion system of the present invention generally denoted as 10. The screw insertion system includes a screw 12, a sleeve or holder 14 for connecting to screw 12, and a driver 16 with shaft 17. In the preferred embodiment screw 12 is a bone screw, and sleeve 14 is a tissue protection sleeve used for inserting the threaded screw into a bone plate or bone nail (not shown). Driver 16 can be any standard screw driver having a leading distal end with a standard screw drive 18, such as a hex drive or a Torx? drive. Drive 18 is adapted to engage a socket or recess 20 in a head 22 of screw 12. Preferably screw 12 has a head 22 with threads 23 and a threaded shaft 25. Head 32 is adapted engage the threads 26 of internal surface 24 at a distal end of tissue protection sleeve 14. Threads 23 taper inwardly on moving away in a proximal direction from the bone screw threads on shaft 25.

[0029] Referring to FIG. 1A there is an enlarged view of the engagement between tissue protection sleeve 14, screw head 22, driver 16 and socket 20. It can be seen from FIG. 1A that the distal end 24 of sleeve 14 includes a plurality of threads along a threaded inner diameter 26. At the distal end 24 inner diameter threads 26 taper outwardly from central axis 28 of the bone screw sleeve 14 preferably in a conical manner. The distal threaded distal end inner surface 24 of sleeve 14 preferably includes flat root areas 30 and flat crests 31 along the outwardly tapered surface of inner surface 26 of sleeve 14. Screw head 22 has rounded roots 33 and flat crests 35. The outer diameter 27 at the leading end of the sleeve is substantially equal to the largest thread diameter of head 22 which is located adjacent to shaft 25. While the diameters do not have to be exactly equal a large step is to be avoided so that surrounding tissue is not irritated during insertion.

[0030] Referring to FIGS. 2 and 2A, there is shown bone screw 12 with head 22 and shaft 25 with threads 32. The threads 32 of bone screw 12 include a helical thread adapted to engage bone and may or may not have a self-drilling feature 34 adjacent a distal tip 36. The threads 32 of bone screw 12 may be any typical threads used for bone screws. A transition area 37 extends between the head 22 and shaft 25.

[0031] Sleeve 14 has an inner diameter 52 which receives an outer surface of driver shaft 17. These surfaces may be a close fit to permit sliding engagement.

[0032] Referring to FIG. 3, there is shown an enlarged view of head 22 which tapers outwardly from central axis 28 at an angle ?. Angle ? is preferably 9 to 10? thus making the cone angle of the tapering head 18 to 20?. The head 22 includes helical thread 40 which preferably has an arcuate root-shape 33 separated by a flattened crest 35 as best shown in FIG. 1A. The root 33 may have a circular shape of, for example, a radius of 0.6 to 0.8 mm. The threaded depth from the crest 35 to the bottom of the root 33 may be, for example, 0.14 mm. The threads are preferably a double or triple lead thread, and are left-handed, whereas, the threads of the bone screw shaft are right-handed. As shown in FIG. 3, the head has an annular surface 48 facing the distal end of the screw (i.e. towards tip 36)

[0033] Referring to FIGS. 4 and 5, there is shown a cross-section of bone screw 12 including the bone screw shaft 25 and in particular head 22, which includes a hex socket 20 for receiving the drive end 18 of the screw driver 16. FIG. 5 shows the flat sides 42 of hex socket 20. Socket 20, as shown, is adapted to receive a standard hex drive from an Allen wrench or screw driver. A TORX? drive or other polygonal drive could be used.

[0034] Referring to FIGS. 6 and 7 there is shown tissue protection sleeve 14 including threaded distal end 24 including threaded inner diameter 26, which tapers outwardly from centerline 28. Threaded diameter 26 tapers outwardly at the same angle that screw threads 23 are tapered inwardly towards the head free end i.e., for example, 10?. Thus, the entire distal opening of the sleeve tapers outwardly on moving distally at an included angle of 20?. In order to more easily engage and disengage from head 22 of screw 12, the threads 26, as shown in FIG. 1A, have flat crests and flat roots. The crests are spaced such that they engage the deepest root area of circular or arcuate threads 33. These thread shapes produce less friction so that the tissue protection sleeve may be more easily removed from the head of the screw after implantation.

[0035] Referring to FIG. 3, in the preferred embodiment, head 22 includes a distally facing surface 37 which in FIG. 4 is annular planar surface 48. Surface 48 may be connected to screw shaft 25 by an arcuate portion 50 which, in the preferred embodiment, is concave. However, this connection portion 50 could be convex and have a part-spherical shape around the entire circumference of the junction between shaft 25 and screw head 22. This would allow for polyaxial movement of the screw as, for example, when implanted in a bone plate (not shown).

[0036] In use the surgeon threadably couples the sleeve or screw holder 14 to the head of 22 of screw 52 so that the outer diameter 27 is flush or nearly flush with the largest thread diameter of head 22. The surgeon then inserts the driver 16 through sleeve 14 into the drive 18 of screw 12. Screw 12 is then inserted into the bone. The sleeve 14 can be removed by rotating an exposed end of sleeve 14 in the direction of screw rotation because of the opposite left and right threads on the bone screw head 22 and shaft 25.

[0037] The sleeve may have a smaller outer diameter (OD), for example 7 mm and have markings near the grip which indicate when to stop turning, it may also have slotted structures to allow for gripping even when screws are inserted in a non-axial fashion. In other words, the distal end of sleeve 14 can flex slightly when the screw axis is angled with respect to the sleeve axis. Such structures are shown in U.S. Pat. No. 6,337,142, the disclosure of which is incorporated herein by reference. Such a structure is shown in FIGS. 8A and 8B which has a plurality of slots 200 spaced around the circumference of the tissue protection sleeve.

[0038] The sleeve may have a larger OD, for example 9 mm. The reason for the larger diameter of 9 mm is that especially for axially stable screws such a larger diameter increases the contact area between the tool tip and screw head which respectively improves the tactile sensation for the operator. This is important as the operator needs to know and feel when to stop turning (when the screw head hits the bone surface). With the larger diameter the sleeve distal end will be larger than the screw head diameter by 1 to 2 mm.

[0039] Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.