WO1995015727A1

WO1995015727A1 - Variable pitch bone screw

Info

Publication number: WO1995015727A1
Application number: PCT/US1994/014071
Authority: WO
Inventors: Dennis W. Burke
Original assignee: Burke Dennis W
Priority date: 1993-12-08
Filing date: 1994-12-07
Publication date: 1995-06-15
Also published as: AU1266695A

Abstract

A bone screw (10, 40, 66, 80, 94) for applying compression to a fracture site. The bone screw (10) of this invention includes an elongated shaft (16) having a cutting tip (12) and a rear end (14) adapted to be inserted into the chuck of a drill. The central shaft (16) contains a continuous thread about its outer surface from the front end to the rear end. The pitch of this thread is greater adjacent the front end than adjacent the rear end of the shaft (16). In one embodiment (fig. 3, 4, 5), the pitch of the threads (52) changes continuously along the shaft (46). In one version of this embodiment (fig. 5), the outer diameter (D) of the shaft measured at the outer edges (54) of the screw threads (52) increases continuously at a uniform rate from the front end to the rear end, while the rood diameter (d) or the diameter of the shaft measured at the base (56) of the screw threads (52) remains generally constant throughout the length of the shaft (46). In another embodiment (fig. 7), the pitch of the threads is uniform along portions (86) adjacent the front end and changes abruptly to a different, lesser pitch at a point (89) spaced from the front end of the shaft (84) of the screw (80).

Description

VARIABLE PITCH BONE SCREW

FIELD OF THE INVENTION

This invention relates generally to bone screws, and more particularly to a variable pitch bone screw for providing compression at a fracture site for better healing.

BACKGROUND OF THE INVENTION

When treating a bone fracture, the surgeon often inserts a pin or bone screw to hold the bone fragments in place and in contact with one another during the healing process. In almost all instances, it is essential for the fractured surfaces of the bone fragments to be brought into closely contacting mutual confrontation, and to be held in well fitting contact throughout the entire healing process. The degree of contact is usually referred to as "compression" . Ideally, a compressive loading should be applied to the bone fragments in a direction generally normal to the fractured sur aces.

The most commonly used form of bone screw includes a constant diameter shaft which is threaded along its entire length with threads of a uniform pitch. Such bone screws are provided in overly long shafts which can be cut to the desired length after insertion. This type of bone screw is commonly referred to as a Kirshner wire or a K-wire or a Compere wire. Typically, such a threaded bone screw is driven into the bone using a drill without predriIling. The screw is introduced through a first bone fragment and passes across the fracture site, in a direction generally normal thereto, and thereafter passes into a second bone fragment to hold the two bone fragments together. Since the shaft is threadably advanced through both bone fragments, a separation occurs at the fracture site, because during the period of time that the tip of the screw is gaining a foothold in the second fragment, it is continuing to advance through the first bone fragment. Once formed, this separation is difficult to close.

This separation problem may be avoided by predrilling the hole and by providing a sufficiently large hole in the first fragment so that no threadable coupling occurs, while drilling a hole in the second fragment which is sufficiently small to allow threadable engagement between the hole and the screw. If an enlarged hole is predrilled, it is necessary to use a screw with an enlarged head and possibly even a washer or other metallic insert disposed between the screw head and the adjacent cortical surface of the bone to apply adequate compression to the fracture site. One type of bone screw which is used with this technique includes that which has threads only on the end of the shaft spaced from the head. Between the threads and the screw head, the shaft is smooth and generally cylindrical.

Although this predrilling technique does provide some compression at the fracture site, many problems can result. In the first place, the extra hardware required may protrude from the bone, and be particularly intrusive. In addition, the compression produced causes the bone immediately below the screw head or washer to be subjected to concentrated bearing loads. Since the bone is rarely flat, the bearing stresses produced by the head or washer remain highly concentrated over a small area, and could cause crumbling or other failure of the adjacent bone. Such failure is particularly likely if the cortical bone layer directly under the screw head or washer provides inadequate support either during insertion or during healing. Bone failure is highly undesirable, since the only thing holding the bone fragments in position relative to each other is the compressive forces which act along the line of the screw. Also, the screw head merely abuts the cortical surface of the bone near the fracture, rather than being firmly and securely anchored to the fragment. Thus, if the cortical bone layer fractures, then compression is lost, and the first bone fragment is free to move relative to the second bone fragment.

Another problem that may occur is bone resorption. Such resorption can occur in response to localized pressure, either directly under the screw head or at the fracture site. Where such resorption occurs, the screw will loosen, thus permitting movement of the bone fragments relative to one another.

It is an object of this invention to provide a bone screw which produces the desired compression at a fracture site; and which is anchored in the bone and does not exert excessive bearing loads on the cortical surface of the bone.

It is a further object of this invention to provide a bone screw which produces the desired compression at a fracture site, and which is overly long and can be cut at any point along its length, so that one screw can be used in a wide variety of applications.

It is another further object of this invention to provide a bone screw which can be easily installed by the surgeon utilizing conventional tools.

SUMMARY OF THE INVENTION In accordance with the above and other objects of the invention, a bone screw is provided having a shaft which is continuously threaded over its entire length from a point adjacent a sharpened cutting tip to a second point spaced from the tip toward its rear end, and in which the pitch of the threads at the point adjacent the tip of the bone screw is greater than the pitch of the threads adjacent the second point. There are several embodiments of this invention which achieve the desired result. In one preferred embodiment, the pitch of the threads varies gradually continuously, and uniformly from a very coarse thread at the point adjacent the tip to a much finer thread at the second point. In a preferred variation of this embodiment, the outer diameter of the screw measured between the outer tips of opposed screw threads increases gradually and uniformly from the point adjacent the tip to the second point adjacent the rear end. In this embodiment, typically, the root diameter of the screw measured between the bases of opposed threads remains relatively constant along the entire threaded length of the shaft. In this manner, the taper of the screw is provided by increasing the depth of the threads. In another variation of this embodiment, the outer diameter of the screw measured between the outer tips of opposed screw threads remains relatively constant along the entire threaded length of the screw shaft, as does the root diameter of the screw.

In another embodiment of this invention, the pitch does not vary continuously along the length of the screw shaft. Rather, a coarse thread is provided adjacent the tip, while a finer thread is provided along the remainder of the threaded length of the shaft, beginning at a point spaced from the tip. The change from a coarse to a fine thread is abrupt or discontinuous. The coarse thread does not vary in pitch along the length of the shaft on which it is found, and similarly, the pitch of the fine thread remains constant along the length of the shaft on which it is disposed. There are two variations of this embodiment. In one variation, the outer diameter of the shaft remains relatively constant throughout its length, so that the portion of the shaft having a coarse thread has roughly the same outer diameter as the portion of the shaft having the fine thread. In another embodiment, the portion of the shaft bearing the coarse thread is tapered from the tip to the point at which the thread abruptly changes over to the fine thread. In these embodiments, typically the root diameter increases as the thread pitch decreases. In other words, the root diameter is less in the vicinity of the coarse thread and greater in the vicinity of the fine thread. However, the root diameter could also remain constant if desired.

The screw of the present invention can be installed in the same manner as a conventional Kirshner wire. The tip of the screw is sharpened to cut through bone, and the rear end of the screw, which is smooth, is connected to the chuck of a drill. The tip can cut through skin and muscle tissue, so that percutaneous insertion is possible. The orientation of the screw can be controlled by controlling the alignment of the drill. The screw is driven by the drill through one bone fragment, past the fracture site and into the other bone fragment. The decreasing pitch of the screw causes the bone fragments to be drawn together tightly at the fracture site and eventually closes the gap that forms as the tip first penetrates into the second bone fragment. Once the desired penetration has been achieved and sufficient compression exists at the fracture site, the screw can be cut adjacent the cortical surface of the first fragment so that no portion of the screw projects from the bone.

One advantage of this invention is that the bone screw can be installed percutaneously using the same tools and techniques as are presently used for installation of a Kirshner wire. Another advantage is that the amount of compression can be carefully controlled. A further advantage is that the installation procedure is simpler than many prior art devices, since no predrilling is required. A further advantage is that no screw heads, washers or other features are required, so nothing projects from the bone, and there is no danger of bone resorption or crumbling of the bone or the abutting cortical surface. Also, one length of screw is suitable for most uses, and it is unnecessary to first select an appropriate screw length.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic representation of an ulna fracture at the olecranon showing use of a prior art screw;

Fig. 2 is a fragmentary side view of the prior art screw of Fig. 1;

Fig. 3 is a fragmentary side view of the prior art screw of Fig. 2;

Fig. 4 is a fragmentary side view of the bone screw of the present invention;

Fig. 5 is a fragmentary side view of the bone screw of Fig. 4;

Fig 6 is a fragmentary side view of another embodiment of the bone screw of the present invention;

Fig 7 is a fragmentary side view of yet another embodiment of the bone screw of the present invention;

Fig 8 is a fragmentary side view of yet another further embodiment of the bone screw of the present invention; and

Fig. 9 is a diagrammatic representation of an ulna fracture at the olecranon showing use of the bone screw of Fig. 4.

DETAILED DESCRIPTION OF THE INVENTION With reference now to Figs. 1-3, a prior art bone screw 10 will be described. The prior art bone screw illustrated by Figs. 1-3 is commonly referred to as a Kirshner wire or K-wire or Compere wire. Bone screw 10 includes a sharpened cutting tip 12, a rear end 14 adapted to be placed in a chuck of a drill, and an intermediate threaded shaft 16. Tip 12 includes a wedge-shaped portion 20, and a tapered point 18. Tip 12 is provided with a sharpened cutting edge which will cut through bone. Wedge shaped portion 20 has smooth, lateral faces 22 flanked by rounded side edges 24 which are threaded to a point adjacent tip 18. Rear end 14 can be either smooth or threaded and it is a generally cylindrical shank. Shaft 16 is continuously threaded along its entire length with threads of the same pitch and root diameter. Screw 10 is available in lengths of about 9 inches, and after insertion, it can be cut to the desired length.

Use of such prior art screws 10 will now be described with particular reference to Fig. 1. Fig. 1 shows, for illustration purposes only, an ulna bone 30 fractured in the vicinity of the olecranon. Such bone screws 10 are commonly used in conjunction with fractures of the ulna. In Fig. 1, the ulna 30 is shown fractured at fracture site 32 into two fragments 26 and 28. Screw 10 is shown extending across fracture site 32 generally normal thereto and through the medullary canal 34 of ulna 30. Screw 10 is driven into the position shown in Fig. 1 without predrilling by attaching the chuck of a drill (not shown) to end 14, and applying tip 18 to the desired location either directly on fragment 28 or on muscle or skin tissue covering fragment 28. Screw 10 is aligned in a direction generally normal to the line of the fracture site 32. The drill is then actuated, and tip 18 cuts through adjoining tissue and into fragment 28, and the screw threads of shaft 16 cut into and engage fragment 28 along the interior surface of a hole cut by tip 18.

As tip 18 crosses the fracture site 32 and begins to cut into fragment 26, there is a delay while tip 18 begins cutting a hole and before the screw threads engage the bone of fragment 26. During this delay, screw 10 continues to advance through fragment 28 as screw 10 rotates. As a reεult, by the time screw 10 is threadably engaged with fragment 26, screw 10 has advanced through fragment 28 a certain distance without a comparable advance through fragment 26, thus forcing fragments 26 and 28 apart at the fracture site 32. Thereafter, since screw 10 advances through fragments 26 and 28 at the same pace, the gap formed at the fracture site remains constant. When tip 18 cuts into fragment 26 to the point where screw 10 is firmly anchored in both fragments, the surgeon will sever screw 10 at a point adjacent the cortical surface 36 of fragment 28. It is difficult, if not impossible, to close the gap across the fracture site 32 created by screw 10. If tip 18 is sharp and the surgeon is skillful, the gap is not great, but a gap will generally be present. Such a gap inhibits the healing of the bone, since proper healing requires that bone cells extend across the gap and join the two fragments 26 and 28 together. The larger the gap, the longer it takes for the bone to heal. Conversely, if the gap is very small, or if compression exists at the fracture site, healing is enhanced.

One embodiment of the bone screw 40 of the present invention will now be described with particular reference to Figs. 4 and 5. Bone screw 40 includes a sharpened cutting tip 42, a rear end 44 and an intermediate, threaded shaft 46. Rear end 44 is virtually identical to end 14 of screw 10, and is typically a substantially cylindrical shank which can either be smooth on its outer surface, or which can contain screw threads. In either event, end 44 is adapted to be inserted into a chuck of a drill (not shown) . Tip 42 includes a wedge-shaped portion 43 and a tapered point 45. Wedge-shaped portion 43 includes flat, smooth, lateral faces 48 flanked by rounded side edges 50 which contain screw threads extending adjacent to point 45. Faces 48 contain no threads whatsoever, and taper outwardly away from point 45 to merge into shaft 46. The threads on edges 50 preferably are of a uniform pitch, and are continuous, and these threads merge with the threads in shaft 46. However, the threads on edges 50 can also be of a continually decreasing pitch.

Shaft 46 is continuously threaded from a first point 53 at its juncture with edges 50 to a second point 55 at its juncture with end 44. The pitch of threads 52 on shaft 46 decreases from point 53 to point 55. The pitch of the threads is defined for purposes of this application as the distance from any point on a thread of the screw to the corresponding point on an adjacent thread measured parallel to the axis of the screw. This distance is shown in Fig. 5 by the designation "p" , while the longitudinal axis of the screw is indicated by the designation A-A. The pitch of the threads of shaft 46 preferably continuously decreases at a constant or uniform rate and at a gradual rate without any discontinuities throughout the entire length of shaft 46.

Another feature of bone screw 40 of the present invention will now be described with particular reference to Fig. 5. The outer diameter of shaft 46 which is defined as the distance normal to the axis A-A of the screw between the outer edges 54 of opposing threads on shaft 46, gradually increases from tip 42 to end 44. Preferably, there are no discontinuities and the increase in diameter occurs gradually and continuously at a constant rate. A typical increase in outer diameter from point 53 to point 55 is such that the diameter at point 55 is in the range of 10-25% greater than the diameter of shaft 46 at point 53. In a further aspect of the invention, preferably this increase in outer diameter is produced by increasing the depth of the threads, or the distance between base 50 and outer edge 54 of a given thread. The depth of the threads increases at a uniform rate continuously along the entire length of shaft 46 from point 53 to point 55. The "root diameter" d of shaft 46 remains relatively constant along the entire length of shaft 46. The root diameter d is defined as the distance normal to the axis A-A of the screw between the bases 56 of opposing threads on shaft 46. However, the root diameter also could increase or decrease along with an increase in the depth of the threads, so long as the outer diameter of shaft 46 increased along the threaded portion of shaft 46 from point 53 to point 55.

Since the threads of bone screw 40 are intended principally to engage the cancellous or spongy inner tissue of a bone, threads 52 preferably are of a form known as an acute backed buttress type. In such a type of thread, the thread helices have pressure faces 60 which are substantially normal to the longitudinal axis A-A of the screw, and backing surfaces 62 which form a relatively acute angle with respect to the longitudinal axis A-A of the screw.

Another embodiment of the bone screw of the present invention will now be described with particular reference to Fig. 6. Bone screw 66 is similar to bone screw 40 and includes a sharpened cutting tip 68, a rear end (not shown) and an intermediate threaded shaft 70 with threads 74. The rear end is virtually identical to rear end 44 of bone screw 40. Tip 68 can be similar to tip 42 of bone screw 40, or it can be as shown in Fig. 6 and include a point 71 flanked by flat, smooth, lateral faces 72. Shaft 70 is continuously threaded from a point closely adjacent tip 68 until its juncture with the rear end. As in bone screw 40, the pitch of threads 74 on shaft 70 decreases from a point adjacent tip 68 to the rear end of the shaft. The pitch of threads 74 is greatest adjacent tip 68 and is the least adjacent the rear end. In this embodiment, it is not necessary that the pitch of threads 74 continue to decrease after about the midpoint of shaft 70, and if desired, depending on the application, from about the midpoint to the rear end of the shaft, the pitch of threads 74 could be maintained substantially uniform. This embodiment differs primarily from that of screw 40 in that the outer diameter of shaft 70 is substantially uniform along its entire length. There is no taper as measured by the distance between the outer edges of opposing threads 74.

Fig. 6 is illustrated with a steadily increasing root diameter. The root diameter of shaft 70 is smallest adjacent tip 68 and increases gradually and continuously along the length of shaft 70 until a point adjacent the rear end. However, in this embodiment the root diameter may also be substantially constant along the entire length of shaft 70.

Another embodiment of the bone screw in the present invention will now be described with particular reference to . Fig. 7. Bone screw 80 of Fig. 7 also includes a sharpened cutting tip 82, a rear end (not shown) and an intermediate threaded shaft 84 with two sets of threads 86 and 88. The rear end is virtually identical to rear end 44 of bone screw 40, while cutting tip 82 is substantially identical to cutting tip 68 of bone screw 66 of Fig. 6. As shown in Fig. 7, all of threads 86 have the same pitch, while all of threads 88 have the same pitch. The pitch of threads 86 is substantially greater than the pitch of threads 88. Threads 86 are disposed between threads 88 and cutting tip 82 such that threads 86 always engage a bone surface before threads 88. In this embodiment, the outer diameter of shaft 84 is substantially uniform along its entire length, except at cutting tip 82, so that the outer diameter of shaft 84 is substantially the same for threads 86 as it is for threads 88. There is a discontinuity or abrupt change in pitch at point 89 where threads 86 meet threads 88. In contrast to the embodiment of Fig. 6, there is no smooth transition from threads of greater pitch to threads of lesser pitch. However, the transition from threads 86 to threads 88 is continuous . While the bone screw 80 is illustrated in Fig. 7 with threads 86 having a smaller root diameter than threads 88, it is to be understood that the root diameter of threads 86 could be the same as the root diameter of threads 88, and bone screw 80 would still perform in a satisfactory manner. The axial length of threads 86, or the distance between point 89 and cutting tip 82 would vary from application to application. However, it typically would only comprise less than 20% of the total length of bone screw 80.

Yet another embodiment of the bone screw of the present invention will now be described with particular reference to Fig. 8. The embodiment of Fig. 8 is substantially identical to the embodiment of Fig. 7. Bone screw 90 includes sharpened cutting tip 92, a rear end (not shown), and an intermediate threaded shaft 94 containing two sets of screw threads, threads 96 and threads 98. The rear end is virtually identical to end 44 of bone screw 40, and cutting tip 92 is identical to cutting tip 82 of Fig. 7. As with the embodiment of Fig. 7, threads 96 have a greater pitch than threads 98. Also, all threads 96 have the same pitch, and all of threads 98 have the same pitch. Threads 96 and threads 98 meet at a point 99 which represents a discontinuity in the pitch or a point where the pitch abruptly changes. Threads 96 also merge with threads 98 to form a continuous thread. The only difference of significance between the embodiment of Fig. 7 and that of Fig. 8 is that the outer diameter of threads 96 tapers from point 99 to cutting tip 92. The outer diameter of threads 96 is greatest adjacent point 99 and is smallest adjacent cutting tip 92. This taper is typically gradual but continuous, such that each thread has a slightly and uniformly greater outer diameter than the previous thread as one moves from cutting tip 92 to point 99. The root diameter of threads 96 typically is smaller than the root diameter of threads 98 to accommodate this taper and still provide the necessary depth to the threads. As shown in Fig. 8, the depth of threads 96 increases as one moves from cutting tip 92 to point 99 such that the threads adjacent point 99 have a substantially greater depth than those adjacent tip 92. While the root diameter of all threads 96 is shown as being constant in Fig. 8 to create this change in the depth of the threads, it may be desirable, in some applications, to vary the root diameter of threads 96 such that the root diameter is less adjacent tip 92 than adjacent point 99. In this way, the depth of the threads 96 could be maintained relatively uniform if desired.

The operation of bone screw 40 of this invention will now be described with particular reference to Fig. 9. In the discussion regarding Fig. 9, only the use of bone screw 40 is illustrated. However, the uses of bone screws 66, 80 and 90 would be substantially identical to that of bone screw 40, except where noted hereinafter. Also, use of bone screw 40 is described in conjunction with a fracture of the ulna in the region of the olecranon as in Fig. 1, and like numbers are used for like parts, where applicable. However, Fig. 6 is intended only to illustrate an exemplary application for the bone screw of this invention, and is not intended in any way to limit the use or applicability of this invention. The bone screw of this invention also can be used to join fractured fragments of other bones of the body, such as the femur, tibia, fibula, radius, humerus and other like bones. In addition, if made with a smaller diameter, the bone screw of the present invention can also be used to join broken bones of the wrist, ankle, foot and other like bones.

As with bone screw 10, bone screws 40, 66, 80 and 90 can be installed percutaneously without the need for removal of any skin or muscle tissue. Furthermore, a hole need not be predrilled for insertion of the screws. In fact, bone screws 40, 66, 80 and 90 will provide a stronger, tighter and more long lasting bond if they are not inserted into a predrilled hole. Once the fracture site 32 has been located by X-ray, or otherwise, the desired angle and path of insertion of bone screw 40 can be determined. Preferably, bone screw 40 should pass through the fracture site 32 in a direction generally normal thereto. In the example of Fig. 9, the fracture site is generally normal to the medullary canal 34 of ulna 30 and screw 40 is inserted generally along medullary canal 34. End 44 of screw 40 is placed in the chuck of a drill (not shown) . This drill can be any one of the sort commonly used by orthopedic surgeons for installation of the prior art screws such as that shown in of Figs. 1-3. Once the chuck has been tightened on end 44, tip 42 is placed adjacent the desired entry site 33. The drill is activated and tip 42 first pierces the skin, and then engages the cortical surface of bone fragment 28 adjacent entry site 33. Further activation of the drill causes tip 42 to cut through fragment 28 axially until it reaches the fracture site 32.

As tip 42 cuts through fragment 28, the threads on shaft 46 engage the bone on the inner surface of the channel cut by tip 42, and continued rotation of screw 40 by the drill threadably advances screw 40 axially through bone fragment 28. During the advance of screw 40, the continuously decreasing pitch of the threads 52 causes the portions of the shaft 46 nearest tip 42 to advance somewhat more rapidly than portions of shaft 46 therebehind. As a consequence, gradual stripping of the threads on shaft 46 will occur, in which threads 52 are pulled in a direction somewhat parallel to axis A-A and normal to their faces 60. This longitudinal pull exerted on faces 60 causes threads 52 to break free from the side walls of the channel cut by tip 42, breaking away pieces of bone tissue in the process. However, at the same time the threads are being stripped from the side walls of the channel, the corresponding increasing diameter of shaft 46 causes these same threads 52 to continually dig anew into the inner surfaces of the channel at a greater depth into the bone and at a different point along the inner surface of the channel as screw 40 continues to advance axially. This process in which the threads are stripped from the bone surface and subsequently reengage that surface at a different point and depth will occur over and over again as the screw advances axially through fragments 26 and 28.

As tip 42 reaches fracture site 32, there is a delay in the axial advance of screw 40 into fragment 26 as tip 42 begins to cut a hole into bone fragment 26. As a consequence, since screw 40 continues to advance axially through fragment 28 during this delay, fragments 26 and 28 are forced apart momentarily at fracture site 32. However, once tip 42 has gained a foothold in fragment 26 and begins cutting a channel therethrough, threads 52 also engage the inner surface of this channel and fragments 26 and 28 are no longer forced apart. Thereafter, because the pitch of threads 52 adjacent tip 42 is greater than the pitch of the threads at points spaced rearwardly thereof toward end 44, the more forwardly portions of shaft 46 advance axially through bone fragment 26 more rapidly than the more rearwardly portions of shaft 46 advance axially through bone fragment 28. Therefore, the gap initially created at fracture site 32 soon closes, as shaft 46 advances farther into fragment 26. Fragments 26 and 28 are drawn together at fracture site 32 until the desired degree of compression is achieved. Thereafter, once bone screw 40 is tightly and securely engaged in both fragments 26 and 28, the surgeon ceases the drilling operation and removes the drill from end 44. Screw 40 is then severed closely adjacent the cortical surface of fragment 28 at a level beneath the skin, and the wound is closed. Once the bone has healed, screw 40 can be removed by reversing the direction of rotation and unscrewing it. Because the outer diameter of screw 40 increases from the front towards the rear of the screw, removal of the screw is facilitated.

Since screw 40 can be severed closely adjacent the cortical surface of the bone, no portion of bone screw 40 projects above the bone surface. As a result, screw 40 can be inserted through the cartilage of a joint surface and buried beneath that surface so that the joint when used is unaffected by the presence of the bone screw. Thus, the normal joint relationship is not impaired, and there is no break or hindrance to a full reestablishment of the cartilage over the rubbing surfaces of the joint. Also, no intrusive hardware projects above the skin.

Since, as previously described, the steadily increasing outer diameter D of shaft 46 compensates for the continually decreasing pitch of threads 52, adequate compression is achieved at the fracture site, while threads 52 are at all times firmly anchored to the bone fragments. The degree of compression at the fracture site can be controlled to suit the surgeon's needs by the degree to which screw 40 extends into fragment 26 and by the selection of a screw 40 having a particular rate of change of pitch and diameter. Since the compressive forces are applied directly at the fracture site by the screw threads themselves, and not by a screw head or washer bearing upon the cortical surface of the bone fragment, there is little danger that the compressive forces would fail due to fracture or failure of the cortical bone surface. There is also no danger of bone resorption on the cortical surface of the bone, or at the fracture site, since there is no pressure applied on the cortical surface of the bone, and since the compression at the fracture site can be accurately adjusted to the optimal level. Further, since the threads of bone screw 40 are firmly anchored in both segments 26 and 28, such segments are not free to move with respect to one another, so that the desired compression is maintained by screw 40 throughout the entire healing process.

The operation of bone screw 66 is substantially identical to the operation of bone screw 40, except that bone screw 66 typically is used in applications where bone fragment 28 is relatively thin, and where threads 74 would not be substantially stripped from the side walls of the channel through which screw 66 passes before threads 74 extend through the fracture site 32 and engage bone fragment 26. Once bone screw 66 engages bone fragment 26, screw 66 is advanced just far enough to draw fragments 26 and 28 together at the fracture site 32 to prevent significant stripping of the threads away from the interior surface of the channel formed by bone screw 66 through fragment 28.

The operation of bone screw 80 of Fig. 7 is also very similar to the operation of bone screw 66. Bone screw 80 also typically is used in situations where bone fragment 28 is relatively thin where it is pierced by bone screw 80, so that there is very little stripping of the threads from the walls of the channel in fragment 28 prior to the entry of threads 86 into bone fragment 26. However, the embodiment of Fig. 7 is used with somewhat thicker bone fragments 26 and 28 than the embodiment of Fig. 6, since the threads of bone screw 80 do not change continuously over the length of the entire bone screw. Threads 86 have a uniform pitch, as do threads 88, so that once threads 86 have entered fragment 26, the bone screw is advanced until point 99 is adjacent the fracture site. Thereafter, no further stripping of the threads occurs. While there may be some initial stripping of threads 88 within the channel already created by threads 86, since threads 88 have a different pitch, they dig anew into the inner surface of the channel at locations different from threads 86 and therefore threads 88 subsequently reengage that inner surface to create the desired engagement of threads 88 with fragment 28.

The operation of bone screw 90 of Fig. 8 closely parallels the operation of bone screw 40. The outer diameter of threads 96 on shaft 94 is at all points less than the outer diameter of threads 98, which are not tapered. Therefore, while there may be some stripping of the threads initially in the channel in bone fragment 28 as bone screw 90 is advanced therethrough, once threads 96 pass fracture site 32 and engage bone fragment 26, the differential advance of threads 96 and 98 will be used entirely to close the gap between bone fragments 26 and 28 at fracture site 32. At this point, further advance of threads 98 through bone fragment 28 will not result in any further stripping of the threads and will allow threads 98 to reengage the inner surface of the channel cut by bone screw 90, particularly in bone fragment 28 adjacent fracture site 32. Once the gap at the fracture site has been closed and the desired level of compression has been applied, the surgeon will cease advancing bone screw 90. Thereafter, bone screw 90 is severed closely adjacent to the cortical surface of fragment 28, as previously described.

In an alternative method of operation, to avoid any possibility of stripping of the threads with the embodiment of Fig. 8, using a conventional drill, the surgeon may initially drill a hole through bone fragment 28 up to but not past fracture site 32. The diameter of this hole or channel would be somewhat less than the outer diameter of threads 98, but somewhat greater than the outer diameter of threads 96. Thereafter, bone screw 90 may be advanced through bone fragment 28. Since the size of the drilled channel is greater than the outer diameter of threads 96, no threads 96 would engage the interior surface of this channel until point 99 was reached. Thereafter, as bone screw 90 is advanced, threads 98 would cut into the interior surface of the channel in fragment 28, causing bone screw 90 to advance in a normal fashion. As tip 92 reaches bone fragment 26, it begins cutting a new hole into fragment 26. Thereafter, the differential axial advance of threads 96 and 98 has no other effect except to draw fragments 26 and 28 together at fracture site 32 and to apply the desired compression. No stripping of the threads occurs in either fragments 26 or 28. The length of the portion of shaft 94 carrying threads 96 is selected, depending upon the bone separation at fracture site 32 and the amount of compression desired. The greater the separation at fracture site 32, and the greater the desired level of compression, the greater would be the axial length along shaft 94 of the portion thereof carrying threads 96.

The lengths of bone screws 40, 66, 80 and 90 typically are about the same as a prior art bone screw 10. Moreover, the outer diameters of the threads at the middle of bone screw 40, of the threads 74 of bone screw 66 in Fig. 6, of the outer diameter of threads 88 of screw 80 in Fig. 7, and of the outer diameter of threads 98 of screw 90 in Fig. 8 all are about the same as the outer diameter of a conventional prior art bone screw 10. The pitch of threads 88 and 98 of bone screws 80 and 90 respectively is about the same as the pitch of the threads on a conventional prior art bone screw 10. The lengths of the shafts 84 and 94 respectively of bone screws 80 and 90 bearing respective threads 86 and 96 varies, depending upon the particular application, the thickness of bone fragment 28 and the separation at fracture site 32. The pitch of threads 86 and 98 of respective bone screws 80 and 90 is greater than the pitch of threads 88 and 98 of respective bone screw 80 and 90 by an amount depending upon the particular application. An exemplary range is that pitches for threads 86 and 96 are twice to four times the pitches of respective threads 88 and 98. However, the foregoing dimensions are exemplary only and are not intended to limit the scope of the invention.

In view of the above description, it is likely that modifications and improvements will occur to those skilled in the art which are within the scope of this invention. The above description is intended to be exemplary only, the scope of the invention being defined by the following claims and their equivalents.

What is claimed is:

Claims

1. A bone screw for coupling together two fragments of a bone at a fracture site, said bone screw comprising: an elongated shaft having a central longitudinal axis extending parallel to a direction of elongation, a front end and a rear end; a sharpened tip disposed on said front end of said shaft and adapted to cut bone tissue; and a screw thread disposed on an outer surface of said shaft and extending from a first point adjacent said tip at said front end of said shaft to a second point spaced from said front end of said shaft toward said rear end of said shaft, the pitch of said screw thread being greater adjacent said first point than adjacent said second point.

2. A bone screw as recited in Claim 1 wherein said thread has outer edges and wherein the outer diameter of said shaft measured across said shaft normal to said axis between outer edges of opposed portions of said thread increases from said first point to said second point.

3. A bone screw as recited in Claim 2 wherein said thread has a base and wherein the root diameter of said shaft measured across said shaft normal to said axis between bases of opposed portions of said thread remains generally constant from said first point to said second point.

4. A bone screw as recited in Claim 1 wherein said pitch decreases continuously at a substantially uniform rate.

5. A bone screw as recited in Claim 2 wherein the outer diameter of said shaft increases continuously at a substantially uniform rate.

6. A bone screw as recited in Claim l wherein said shaft comprises: a first portion extending axially from said tip to a third point on said shaft spaced from said tip intermediate said first and second points, said screw thread on said first portion having a first substantially constant pitch; and a second portion extending axially from said third point towards said second point of said shaft, said screw thread on said second portion having a second substantially constant pitch, said second pitch being less than said first pitch, the pitch of said screw thread changing abruptly from said first pitch to said second pitch at said third point.

7. A bone screw as recited in Claim 6 wherein said screw thread on said first portion of said shaft has an outer diameter which increases from said tip to said third point, and wherein the outer diameter of said screw thread on said first portion of said shaft adjacent said tip is less than the outer diameter of said screw thread on said second portion of said shaft.

8. A bone screw as recited in Claim 6 wherein a root diameter of said screw thread on said first portion of said shaft is less than a root diameter of said screw thread on said second portion of said shaft.

9. A bone screw as recited in Claim 6 wherein a root diameter of said screw thread on said first portion of said shaft is substantially equal to a root diameter of said screw thread on said second portion of said shaft.

10. A bone screw as recited in Claim 1 wherein a root diameter of said screw thread is less adjacent said tip than a root diameter of said screw thread adjacent said rear end of said shaft.

11. A bone screw for coupling together two fragments of a bone at a fracture site, said bone screw comprising: an elongated shaft having a central longitudinal axis extending parallel to the direction of elongation, a front end and a rear end; a sharpened tip disposed on said front end of said shaft and adapted to cut bone tissue; a screw thread disposed on an outer surface of said shaft and extending continuously from said front end toward said rear end; a first section of said shaft extending axially from adjacent said tip to a point on said shaft intermediate said front end and said rear end, said screw thread on said first section having a first pitch; and a second section of said shaft extending from said point toward said rear end of said shaft, said screw thread on said second section having a second pitch less than the first pitch of said thread on said first section of said shaft, the pitch of said screw thread changing abruptly from said first pitch to said second pitch at said point.

12. A bone screw for coupling together two fragments of a bone at a fracture site comprising: an elongated shaft having a central longitudinal axis extending parallel to the direction of elongation, a front end and a rear end; a sharpened tip disposed on said front end of said shaft adapted to cut bone tissue; and a continuous screw thread disposed on an outer surface of said shaft and extending from a first point adjacent said front end to a second point adjacent said rear end of said shaft, the pitch of said screw thread decreasing continuously at a substantially uniform rate from said first point to said second point, the outer diameter of said shaft as measured normal to said axis between outer edges of opposed portions of said thread increasing continuously at a substantially uniform rate from said first point to said second point.