WO1994020040A1

WO1994020040A1 - Compression fastener having varied screw thread pitch

Info

Publication number: WO1994020040A1
Application number: PCT/US1994/002209
Authority: WO
Inventors: Henry Fletcher
Original assignee: Synvasive Technology, Inc.
Priority date: 1993-03-01
Filing date: 1994-03-01
Publication date: 1994-09-15

Abstract

A compression fastener (10) in which the thread pitch (18) extends the length of the fastener and increases gradually from the screw tip (14) to the screw head (16). The shaft (12) of the fastener (10) is generally of a uniform diameter, although it may be slightly tapered toward the tip (14). The fastener (10) may be further provided with a sharply tapered tip for ease of insertion.

Description

COMPRESSION FASTENER HAVING VARIED SCREW THREAD PITCH

Field of the Invention 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 repai 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 object 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.

An early, but unsatisfactory, solution to achieving compressive engagemen using fixed pitch screws was the use of a bolt at the tip end of the screw. As th bolt is tightened, the length of screw between the screw head and the bolt is shortened, thereby achieving compression. The use of a bolt requires extra steps in the fastening process and is not entirely successful in that bolt can loosen and compression can be lost over time. Further, the use of bolts in bone surgery is generally impractical because the screws are inserted in such a way as to leave the screw tip buried in bone. Also, exposure of the screw head as an attachment point for a bolt is problematic in that it creates a solid projection from the bone that may abrade surrounding tissue, particularly where repair takes place within or adjacent to joints. Recently, there have been several attempts to provide a suitable compression bone screw that overcomes the problems of the prior art, all with limited results. One type of compression screw, the Herbert bone screw, is disclosed in United States Patent 4,175,555. This screw has like-handed threads at its distal and proximal ends separated by an unthreaded intermediate section. The screw achieves compression by having a thread pitch at its proximal end that is less than the thread pitch at its distal end. As the screw is advanced in the bone tissue, the large pitch distal end advances further in the remote bone fragment, per screw turn, than the distance advanced by the proximal end of the screw in the near bone fragment, resulting in a compressive coupling of the bone fragments.

Although able to achieve compressive coupling, the Herbert screw suffers from certain serious drawbacks that affect its practical utilization. For instance, the diameter of the unthreaded intermediate section is smaller than the major diameter of the threaded distal end, and smaller than the minor diameter of the threaded proximal end. As a consequence, the unthreaded intermediate section resides in a bore section that must be large enough to accommodate the minor diameter of the proximal end thread, and therefore does not fully fill the bore. Th result is a lateral instability that sacrifices the integrity of the fragment interface during the healing process. The Herbert screw further suffers from the requirement that its unthreaded intermediate section be longer than either threaded section, thereby reducing the number of screw turns possible, along with the possible compressive force that can be delivered to the bone. in an attempt to avoid these problems, United States Patent 5,019,079, to Ross, discloses a bone screw in which the distal threaded section is longer than the unthreaded intermediate section, allowing more screw turns and thus more compressive force. Further, Ross adjusts the diameter of the central threaded portion to permit the central portion to fill the bore and thereby increase lateral stability.

Notwithstanding these improvements, both the Herbert screw and the Ross screw suffer significant drawbacks. The provision of a central, unthreaded region results in a screw that is depth intolerant. In other words, it is necessary to insert the screw just enough to ensure that the non-threaded region actually bridges the interface between the bone fragments. Furthermore, the tripartite division of the screw into a central unthreaded region bound on both sides by threaded regions is impractical for use in very narrow gauge fasteners such as surgical wire.

In view of the above, there is a clear need for a depth tolerant compressive fastener that does not require precise alignment between bone fragments, that is capable of controlled compression and that is suitably embodied both as a bone screw and as surgical wire.

Summary of the Invention It is an object of the invention to provide a depth tolerant, surgical bone fastener that is capable of creating a compressive engagement between two bone fragments.

It is another object of the present invention to provide a surgical bone fastener that is capable of fastening two bone fragments without the possibility of torque about the axis of the fastener.

Yet another object of the invention is to provide a means of compressive engagement that is suitable both to broad gauge screws as well as narrow gauge wire.

The invention meets these objects by providing a depth-tolerant, surgical bone fastener in which the thread pitch extends the length of the fastener and in which the thread pitch increases gradually from the screw tip to the screw head.

The shaft of the screw is generally of a uniform diameter, although in one embodiment, the screw shaft is slightly tapered toward the screw tip. The screw may be further provided with a sharply tapered tip for easier initial insertion. In one embodiment, the invention comprises a surgical screw having a shaft diameter greater than 0.062 inch and generally of at least 0.10 inch. Such screws are suitable in surgery involving large bones, such as the hip and femur, as well as in the fastening of larger bone fragments. In another embodiment, the invention comprises surgical wire, much like Kirschner wire, except that the screw threads are configured in the progressively tightening pitch described above. Typical diameter for this embodiment is 0.062 inch or less, and generally within the range of 0.028 to 0.062 inch. Such an embodiment is particularly suited to the repair of small bones, such as those found in the hands and feet.

The fastener of the invention is clearly advantageous over the prior art in that it is depth insensitive. That is, it allows bone fragments to be successfully joined in compressive engagement without the need to align precisely the position of the fastener relative to the bone fragments.

Another advantage of the invention is the ability to precisely adjust the amount of compression desired. Because the pitch of the fastener is gradually increased over the length of the fastener, it is possible to apply compression more precisely than is possible with existing compression screws. Where the pitch of the screw thread is fixed within a particular region.

A further advantage of the invention is its application to surgical wire. Because of the very narrow diameter of most surgical wire, the Herbert/Ross compression screw configuration is unsuitable. The use of a fastener-long screw thread with variable pitch permits the incorporation of compression engagement capabilities into surgical wire that is lacking in the prior art.

These and other objects and advantages of the invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.

Brief Description of the Drawings

FIG. 1 shows a side view of a fastener according to one embodiment of the invention.

FIG. 2 shows a side view of a fastener according to another embodiment of the invention. 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 n 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.

The fastener can also be provided with a canula 20 as shown by ghost lines in FIG.1. Canula 20 extends the entire length of shaft 12, with a leading opening 21 and a trailing end opening 23. Provision of a canula permits a guide wire to be threaded through the fastener, thereby allowing the fastener to be accurately guided into place in situations where the surgeon's view is partially obstructed or where the bone to be repaired is difficult or awkward to reach.

Use of a large diameter embodiment of the fastener to couple bone fragments follows methods that are standard and well known in the art. In general, a narrow bore is formed using a drill or similar tool through both a the proximate and remote bone fragments, the drilling into the remote bone fragment being at least sufficient to accept enough of the leading end of the fastener to secure the remote bone fragment. The bore is generally about the diameter of or slightly smaller than the diameter of the shaft of the fastener. Once the bore is formed, the fastener is screwed through the bore of the near fragment, followed by the remote fragment. Although the bone fragments are separate after initial insertion of the fastener into the remote fragment, successive screw turns bring the fragments into contact as the fastener advances into the remote bone fragment at a slightly faster rate than the near bone fragment. As the fastener is turned more, compressive engagement arises. The fastener is then turned until the desired amount of compressive force is achieved.

According to another embodiment of the invention, illustrated in FIG. 2, shaft 12 is provided with a sharply tapered tip region 20 for facilitating initial insertion of the fastener. In another embodiment of the invention, shaft 12 may be slightly tapered from trailing edge 16 down to leading edge 14. Such tapering functions similarly to tapered end 22 in that it provides for greater ease in insertion of the fastener. According to yet another embodiment of the invention, an improved surgical wire or Kirschner wire is provided. It differs from the fasteners just described in that the shaft has an extremely narrow gauge, generally ranging from 0.028 to 0.062 inch in diameter.

Although similar to the larger diameter surgical screws of the previous invention, the surgical wire embodiment differs significantly in several respects. First, the surgical wire embodiment is considered self-threading, that is, there is no need to form or pre-drill a hole into which the surgical wire is inserted. Rather, the surgical wire is of sufficient strength that, when inserted with a standard wire driver, the wire acts as its own drill. Further, the narrow diameter of the shaft forecloses the possibility of a canula, which would be redundant in any event in as much as this embodiment acts both as its own guide wire and fastener. Narrow gauge fasteners such as this are particularly well suited to coupling very small bones and bone fragments, such as those found in the hands and feet. The fastener of the invention can be fabricated from any suitable non- rusting, biocompatible alloy, such as surgical steel and the like, although it is preferred to fabricate the fastener from a titanium alloy or having a titanium plating.

It will of course be appreciated that although the embodiments described above have been directed to surgical applications, the fastener of the present invention also has wide application in the construction and carpentry industry. Screws employing the variable pitch screw threads described herein can be used to join everything from furniture components to concrete slabs together when compressive engagement is needed. The invention obviates the need to use bolts or similar securing devices and reduces the number of screws need to finish a particu'βr piece because problems with torquing and screw loosening are avoided. From a consideration of the foregoing, it will now be clear that the invention is advantageous over the prior art in that it permits controlled compressive coupling of objects without resort to bolts or similar tightening elements. The fastener of the invention is also depth insensitive in that it does not require precise alignment within and between the objects to be joined. The fastener is also advantageous in that it can be used singly without the fear of loosing the compressive grip or having the objects torque about the axis of the fastener.

Although the fastener has been described with respect to specific embodiments, it will be appreciated that various modifications of the fastener are possible without departing from invention, which is defined by the claims set forth below.

Claims

I claim:

1. A compression bone screw for connecting two objects, said bone screw comprising a generally cylindrical shaft having a leading end, a trailing end and a diameter greater than 0.062 inch, said shaft provided with a screw thread helically extending substantially from the leading end to the trailing end of said shaft, the screw thread configured to increase in pitch toward the trailing end of said shaft.

2. The bone screw of claim 1 wherein said shaft is tapered toward the lead end.

3. The bone screw of claim 1 wherein the lead end is provided with a sharply tapered tip for facilitating initial insertion of the fastener into the object.

4. The bone screw of claim 1 wherein the shaft is provided with an axially disposed canula.

5. A compression bone screw for connecting two objects, said bone screw comprising a generally cylindrical shaft having a leading end, a trailing end, a diameter equal to or greater than 0.10 inch, and an axially disposed canula, said shaft provided with a screw thread helically extending substantially from the leading end to the trailing end of said shaft, the screw thread configured to increase in pitch toward the trailing end of said shaft, said shaft further provided with a sharply tapered tip for facilitating initial insertion of the fastener into the object.

6. A compression surgical wire for connecting two objects, said surgical wire comprising a generally cylindrical shaft having a leading end, a trailing end and a diameter equal to or less than 0.062 inch, said shaft provided with a screw thread helically extending substantially from the leading end to the trailing end of said shaft, the screw thread configured to increase in pitch toward the trailing end of said shaft.

7. The bone screw of claim 6 wherein said shaft is tapered toward the lead end.

8. The bone screw of claim 6 wherein the lead end is provided with a sharply tapered tip for facilitating initial insertion of the fastener into the object.

9. A compression surgical wire screw for connecting two objects, said bone screw comprising a generally cylindrical shaft having a leading end, a trailing end, and a diameter equal to or less than 0.062 inch, said shaft provided with a screw thread helically extending substantially from the leading end to the trailing end of said shaft, the screw thread configured to increase in pitch toward the trailing end of said shaft, said shaft further provided with a sharply tapered tip for facilitating initial insertion of the fastener into the object.