 Description
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+ COMPRESSION FASTENER HAVING VARIED SCREW THREAD PITCH
Field of the Inventijn The invention relates generally to surgical fasteners for piercing and fixedly binding together bone, and more particularly to surgical wire and the like holding together in compressive engagement small portions of fractured bone.
+ Background of the Invention In certain medical arts, such as bone surgery, it is necessary to fasten objects in compressive engagement, that is, to bring the surfaces of the objects together in compressive conformation. Bone fragments will not knit properly unless the surfaces of the fragments are brought into compressive contact, thereby stimulating the formation of vascular tissue and the general growth of bone tissue. Standard prior art screws are generally unsatisfactory for bone repair because the use of such screws generally results in the creation of narrow gaps between bone fragments. This is because the screw tip necessarily pushes the second bone fragment slightly away from the first as it penetrates the second bone. This gap is maintained, notwithstanding further screw turns, because the pitch of the screw thread is uniform throughout the screw length. Thus the separated positions of the bone fragments are maintained as they are carried up the screw shaft with each successive screw rotation.
Another problem associated with the use of prior art screws in bone surgery is the possibility of torque, that is a shifting of the positions of the objects relative to one another about the screw axis. Such torquing necessitates the use of at least two, and most often three screws to secure the objects fixedly.
+ Detailed Description of the Invention Referring now to the figures, the fastener of the invention will now be described. The fastener 10 comprises a shaft 12 that has gn insertion or leading end 14 and a head or trailing end 16. The shaft may be uniform in thickness and roughly cylindrical, as shown in FIG. 1, or it may be slightly tapered toward leading end 14, as shown in FIG. 2. Generally speaking, the shaft will have a diameter of 0.10 inch or greater, and in no case shall the shaft be 0.062 inch or less. Trailing end 16 may be flat-topped, but is preferentially provided with a depression for receipt of a screw driver or similar device, as shown in FIGS. 1B-C.
Shaft 12 is also provide with a screw thread 18. Screw thread 18 extends substantially the entire length of shaft 12, although it is acceptable for screw thread 18 to stop somewhat short of trailing end 16. The pitch of screw thread 18 is not uniform, but rather is variable, increasing from leading end 14 to trailing end 16. The form of screw thread 18 is conventionally symmetric, forming a substantially isosceles-shaped triangle in cross-section. However, other screw thread configurations may be suitable for particular applications.
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