Patent Publication Number: US-6984235-B2

Title: System for fusing joints

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 now abandoned. In turn. U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 is a continuation-in-part of U.S. patent application Ser. No. 09/305,841, filed May 5, 1999, issued as U.S. Pat. No. 6,017,347 on Jan. 25, 2000, which is a continuation-in-part of U.S. patent application Ser. No. 09/157,783, filed Sep. 21, 1998, issued as U.S. Pat. No. 6,120,505 on Sep. 19, 2000, which is a continuation-in-part of U.S. patent application Ser. No. 08/457,624, filed Jun. 1, 1995, issued as U.S. Pat. No. 5,810,825 on Sep. 22, 1998. U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 is also a continuation-in-part of U.S. patent application Ser. No. 09/375,306, filed Aug. 16, 1999, issued as U.S. Pat. No. 6,299,615 on Oct. 9, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/019,135, filed Feb. 5, 1998, issued as U.S. Pat. No. 5,976,134 on Nov. 2, 1999, Which is a continuation-in-part of U.S. patent application Ser. No. 08/847,820, filed Apr. 28, 1997 now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 08/587,210, filed Jan. 11, 1996, now issued as U.S. Pat. No. 5,624,440 on Apr. 29, 1997, and also a continuation-in-part of U.S. patent application Ser. No. 08/622,368, filed Mar. 26, 1996, now issued as U.S. Pat. No. 5,665,087 on Sep. 9, 1997, and a continuation-in-part of U.S. patent application Ser. No. 09/318,437, filed May 25, 1999, issued as U.S. Pat. No. 6,162,224 on Dec. 19, 2000 and U.S. patent application Ser. No. 09/318,669, filed May 25, 1999, issued as U.S. Pat. No. 6,171,309 on Jan. 9, 2001. The &#39;224 and &#39;309 patents are continuations-in-part of U.S. patent application Ser. No. 08/636,326, filed Apr. 22, 1996, now issued as U.S. Pat. No. 5,662,649 on Sep. 2, 1997. U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 is also a continuation-in-part of U.S. patent application Ser. No. 08/715,017, filed Sep. 17, 1996, now issued as U.S. Pat. No. 5,658,283 on Aug. 19, 1997, and a continuation-in-part of U.S. patent application Ser. No. 08/759,075, filed Dec. 2, 1996, now issued as U.S. Pat. No. 5,697,934 on Dec. 16, 1997, and also a continuation-in-part of U.S. Des. patent application Ser. No. 29/063,695 now U.S. Pat. No. D404,128, filed Dec. 13, 1996, and a continuation-in-part of U.S. patent Application No. 08/773,968, filed Dec. 26, 1996, now issued as U.S. Pat. No. 5,702,472 on Dec. 30, 1997, and also a continuation-in-part of U.S. patent application Ser. No. 09/034,046, filed Mar. 3, 1998, issued as U.S. Pat. No. 5,964,768 on Oct. 12, 1999, which is a continuation-in-part of U.S. patent application Ser. No. 08/781,471, filed Jan. 10, 1997 now U.S. Pat. No. 5,871,486, and also a continuation-in-part of U.S. patent application Ser. No. 08/792,988, filed Feb. 3, 1997 now U.S. Pat. No. 5,868,789. U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 is also a continuation-in-part of 09/157,783, filed Sep. 21, 1998 now U.S. Pat. No. 6,120,505, which is a continuation-in-part of U.S. patent application Ser. No. 08/457,624, filed Jun. 1, 1995, now issued as U.S. Pat. No. 5,810,825 on Sep. 22, 1998. U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 is also a continuation-in-part of U.S. patent application Ser. No. 08/986,717, filed Dec. 8, 1997, issued as U.S. Pat. No. 5,994,721 on Aug. 31, 1999, and also a continuation-in-part of U.S. patent application Ser. No. 09/093,415, filed Jun. 8, 1998, issued as U.S. Pat. No. 6,001,099 on Dec. 14, 1999. U.S. patent application Ser. No. 09/483,076, filed Jan. 14, 2000 is also a continuation-in-part of U.S. patent application Ser. No. 09/216,316, filed Dec. 18, 1998, issued as U.S. Pat. No. 6,030,162 on Feb. 29, 2000, and U.S. patent application Ser. No. 09/263,141, filed Mar. 5, 1999, issued as U.S. Pat. No. 6,077,271 on Jun. 20, 2000 which claims priority from U.S. Provisional Patent Application Ser. No. 60/077,168, filed Mar. 6, 1998. All of the above patents and applications are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to a bone screw for drawing together bone fragments separated by a fracture and more particularly to such a screw which draws the bone fragments together as a result of different-pitched threads on the screw. 
   BACKGROUND OF THE INVENTION 
   In healing bone fractures it is desirable to compress the fractures so that the fractured surfaces are pressed against one another. In the prior art, bone screws have been used to draw the fractured surfaces together and thereby optimize the healing process. 
   A number of prior art bone screws have been constructed in a fashion resembling wood screws. For example, some prior art bone screws include a threaded distal portion and a head with a relatively long unthreaded shank disposed between the head and the distal portion. A drill is used to create a bore through the fracture and the screw is threaded into the remote bone fragment with the head of the screw compressing the near fragment tightly against the remote bone fragment. 
   Other bone screws are threaded along the length thereof, thus requiring a first drill bit to create a bore in both bone fragments extending across the fracture and a second bit to drill a larger bore in the near bone fragment so that the screw threads do not engage the near bone fragment. Thereafter, the screw is tightened in the same manner as described above in connection with the screw having an unthreaded shank, thereby compressing the fragments together. 
   The operation of two prior art headed lag screws is illustrated in  FIGS. 8A–10D . The operation of a lag screw A 1  with a head B 1  and a shank C 1  is shown in  FIG. 8A–D . Shank C 1  of screw A 1  includes threads D 1  at the distal end and an unthreaded region E 1  proximal to head B 1 . The pitch of threads D 1  is constant.  FIG. 8A  shows screw A 1  partially engaged in a bore F 1  in a near bone fragment G 1 . The diameter of bore F 1 , is less than the diameter of threads D 1  and therefore the threads engage the walls of the bore as the screw is twisted in.  FIG. 8B  shows screw A 1  as it starts threading into a bore H 1  in a remote bone fragment I 1 . At this point threads D 1  are engaged in both bores and moving forward at the same speed in both fragments so no compression between the fragments is achieved. Head B 1  has reached the top of fragment G 1  in  FIG. 8C , as indicated schematically by the radiating “force” lines. Since threads D 1  are no longer engaged in fragment G 1 , screw A 1  rotates freely in the fragment without being drawn forward therein. Subsequent rotation of screw A 1  draws fragment I 1  further up the screw. Because head B 1  prevents fragment G 1  from moving further up screw A 1 , fragment I 1  is drawn up against fragment G 1  and compression between the fragments is achieved as shown in  FIG. 8D , with the head pulling down on the near fragment and the threads pulling up on the remote fragment. 
   The importance of the unthreaded region of screw A 1  is illustrated in FIGS.  9 A– d . A lag screw A 2  including a head B 2  and a shank C 2  is shown partially engaged in a bore F 2  in a near fragment G 2  in  FIG. 9A . Shank C 2  includes threads D 2  running the entire length with no unthreaded region such as E 1  on screw A 1 . Rotating screw A 2  causes it to be drawn through fragment G 2  and pass into a bore H 2  in a remote fragment I 2 , as shown in FIG.  9 B. Further rotation of screw A 2  brings head B 2  down against the upper surface of fragment G 2 . See  FIG. 9C . At this point, threads D 2  are still engaged in bore F 2  of fragment G 2  and the interaction of the head on the surface of fragment G 2  impedes the further rotation of screw A 2 . To have additional rotation, head B 2  would have to be drawn down into fragment G 2  or the portion of threads D 2  in fragment G 2  would have to strip out. Therefore a fully threaded screw, such as screw A 2 , would not be preferred for use in the fragment and bore configuration of  FIGS. 9A–D . 
   The proper bore configuration for using screw A 2  is illustrated in FIGS  10 A–D. As shown in  FIG. 10A , bore F 2  in fragment G 2  is enlarged to allow threads D 2  of screw A 2  to pass freely through the bore. Screw A 2  therefore slips into bore F 2  until it reaches fragment I 2 . At that point, threads D 2  engage the walls of bore H 2  and draw screw A 2  down into fragment I 2 . See  FIGS. 10B–C . When head B 2  reaches the upper surface of fragment G 2 , further rotation causes fragment I 2  to be drawn up into contact with fragment G 2  as shown in  FIGS. 10C–D . No binding occurs between head B 2  and threads D 2  in the near fragment because of the large bore in fragment G 2 , and the screw functions as intended to draw the two fragments together. 
     FIGS. 11A–12D  illustrate the effect of substituting headless screws in the place of lag screws A 1  and A 2 .  FIG. 11A , in particular, shows a headless screw A 3  partially installed in a bore F 3  in a near fragment G 3 . Screw A 3  includes threads D 3  extending along its entire length. The pitch of threads D 3  is constant.  FIG. 11B  shows screw A 3  extending through fragment G 3  and just entering a bore H 3  in a remote fragment I 3 .  FIG. 11C  shows screw A 3  advanced further into fragment I 3 . It should be noted that, since the pitch of threads D 3  is constant, screw A 3  moves forward in fragments G 3  and I 3  by the same amount with each rotation. As shown in  FIG. 11D , screw A 3  will pass through both fragments without altering their relative spacing or compressing them together. Thus, a headless screw such as screw A 3  will not work to draw the fragments together in the same way as lag screws A 1  and A 2 . 
   A variation of screw A 3  is shown at A 4  in  FIG. 12A . Screw A 4  includes threads D 4  of constant pitch extending along its entire length and differs from screw A 3  in that it tapers from a smaller outside diameter at the leading end to a larger outside diameter at the trailing end. Screw A 4  is shown because it incorporates tapering, which is one of the features of the present invention, however, it is unknown whether such a screw is found in the prior art. Screw A 4  is shown partially installed in a bore F 4  in a near fragment G 4  in  FIG. 12A . As screw A 4  is rotated, it moves through fragment G 4  and into a bore H 4  in a remote fragment I 4 , as shown in  FIG. 12B . Subsequent rotation simply carries screw A 4  further into and through fragment I 4  without any effect on the spacing between the fragments. See  FIGS. 12C–D . With a constant pitch thread, such as found on thread D 4 , the taper does not facilitate compression. Taper may, however, make a screw easier to start in a small pilot hole or even without a pilot hole. The threaded portion of many wood screws follows this general format, tapering to a sharp point, to allow installation without a pilot hole. 
   It can be seen from the above discussion that a headless screw of constant pitch does not achieve the desired compressive effect between the two fragments as will a lag screw with a head. It is, however, possible to draw two fragments together with a headless screw if it has varying pitch.  FIG. 13A  shows a headless screw A 5  with threads D 5  formed along its entire length. Such a screw is shown in U.S. Pat. No. 146,023 to Russell. The pitch of threads D 5  varies from a maximum at the leading end to a minimum at the trailing end. It is expected that such a screw moves forward upon rotation in a fragment according to the approximate average pitch of the threads engaged in the fragment. Screw A 5  is shown in  FIG. 13A  with the leading threads engaged in a bore F 5  in a near fragment G 5 . Rotation of screw A 5  causes it to move forward into and through fragment G 5  and into a bore H 5  in a remote fragment I 5 , as shown in  FIG. 13B . Additional rotation after the leading threads engage fragment I 5  causes the two fragments to be drawn together. See  FIGS. 13C–D  This is because the average pitch of the threads in fragment I 5  is greater than the average pitch of threads in fragment G 5 . Since the screw moves forward in each fragment with each 360° rotation by an amount roughly equal to the average pitch of the threads in that fragment, each rotation will move the screw forward further in fragment I 5  than in fragment G 5 . This effect will gradually draw the fragments together as the screw moves forward. Depending on the initial spacing between the fragments, they can make contact either before or after the trailing end of the screw has entered fragment G 5 . It should be noted that screw A 5 , in contrast to constant pitch screws such as screws A 1  and A 2 , can be used to separate fragments G 5  and I 5  by simply reversing the rotation. 
   One drawback of a screw such as shown in Russell is the stripping or reaming of the female threads created in the bore by the leading threads as the trailing threads follow. Because the pitch changes along the length of the screw, no thread exactly follows the thread directly in front of it. Rather, each thread tends to cut its own new path which only partially overlaps the path of the thread ahead of it. Thus, the trailing threads tend to ream out the female threads in the bore made by the leading threads. This effect reduces the grip of the trailing threads and therefore the overall compressive force available to urge the fragments together. 
     FIG. 14A  shows a headless screw A 6 , such as disclosed in U.S. Pat. No. 4,175,555 to Herbert, that offers one solution to the problem of reaming of threads. As noted in the Herbert patent, bone screws having heads suffer from several disadvantages A including concentrated loads beneath the screw head and the protrusion of the screw head itself after the screw is installed. Several other shortcomings of the standard type of bone screw are detailed in the Herbert patent. 
   Screw A 6 , as per Herbert, includes a shank C 6  with leading threads J 6  at the leading end, trailing threads K 6  at the trailing end and an unthreaded region E 6  separating the leading and trailing threads. Threads J 6  and K 6  each have fixed pitch, but leading threads J 6  have a larger pitch and smaller outside diameter than trailing threads K 6 .  FIG. 14A  shows leading threads J 6  of screw A 6  installed in a bore F 6  of a near fragment G 6 . It should be noted that threads J 6  do not engage the walls of bore F 6 , the bore having been bored large enough to allow leading threads J 6  to pass freely. As the screw moves forward, the leading threads engage a bore H 6  in a remote fragment I 6 . See  FIG. 14B . The diameter of bore H 6  is adapted so that leading threads J 6  engage the walls. Meanwhile, at the trailing end of the screw, trailing threads K 6  start to engage the walls of bore F 6 , which has been bored to an appropriate diameter therefor. 
   As soon as trailing threads K 6  are engaged in bore F 6  and leading threads J 6  are engaged in bore H 6 , the two fragments start drawing together. See  FIG. 14C . Further rotation of screw A 6  completes the process of moving the two fragments together as shown in  FIG. 14D . Screw A 6  operates on the same general principle as screw A 5 , except that the average pitch of the threads in the remote and near fragments is simply the pitch of the leading and trailing threads, respectively. For instance, if the pitch of the leading threads is 0.2 inches and the pitch of the trailing threads is 0.1 inches, each rotation of screw A 6  will move it 0.2 inches further into fragment H 6 , but only 0.1 inches further into fragment I 6 , thus moving the fragments 0.1 inches closer together. 
   The Herbert screw overcomes at least one of the drawbacks of the Russell screw, the reaming of female threads by subsequent threads on the screw, but at the same time suffers from a number of other disadvantages. In the Herbert screw, the leading threads have a smaller diameter than the trailing threads. This is necessary to permit the leading threads to pass through the relatively large bore in the near bone fragment and engage the smaller bore in the remote bone fragment. The larger trailing threads then engage the larger bore in the near bone fragment. As a result of this arrangement, any stripping of the threads cut into the bones during installation of the screw occurs in the remote bone. If the stripping occurred in the bore in the near bone fragment, a screw having a head thereon could still be used to compress the fracture even though the near bore was stripped; however, when stripping occurs in the bore in the remote bone, a standard screw with the head thereon cannot be used and another bore must be drilled. 
   Further, the Herbert screw must be correctly positioned, i.e., it is imperative that the fracture intersect the unthreaded central portion of the Herbert bone screw when the same is installed. Thus, the Herbert screw is not suitable for treating fractures that are very near the surface of the bone where the hole is to be drilled. In addition, because the Herbert screw is not threaded entirely along the length thereof, the purchase obtained by the screw in the bone is not as good as with a screw threaded along the entire length. Also, two bores of different sizes must be drilled to install the Herbert screw rather than a single bore. 
   Yet another problem with the Herbert screw is the stripping that can occur if additional tightening occurs after the screw has drawn the bone fragments together. While the bone fragments are being drawn together, trailing threads K 6  all follow a single path through the near fragment. Similarly, leading threads J 6  all follow a single path through the remote fragment. When, however, the bone fragments make contact, the two sets of threads can no longer move independently. Further rotation of the Herbert screw after contact between the fragments can cause the leading threads to strip out as they attempt to move forward through the distal bone fragment faster than the trailing threads will allow. See  The Herbert Bone Screw and Its Applications in Foot Surgery The Journal of Foot and Ankle Surgery , No. 33, Vol 4., 1994, pages 346–354 at page 346, which reports on a study that found compression of 10 kg. after only two complete turns of the trailing threads engaged in the near bone fragment. Each subsequent revolution lead to a decrease in compressive force. Thus, care must be taken not to over-tighten the Herbert screw. 
   In addition to drawing two bone fragments together to repair fractures, it is sometimes desirable to draw together two bones for fusing the same together in connection with arthrodesis of the interphalangeal joints. This procedure is sometimes indicated with symptoms of pain or instability in the finger joints. The purpose is to immobilize and draw together adjacent bones across a joint to cause them to fuse together thereby preventing further movement at the joint. 
   In one prior art procedure for immobilizing the distal interphalangeal joint (DIP), axial bores are drilled in the articular surfaces of the distal and proximal phalanges. The bore in the distal bone is sufficiently large to receive without threading a screw which is inserted therein via an incision in the tip of the finger. The screw threadably engages the bore in the proximal bone and when the screw head is tightened against the distal end of the distal bone, the two bones are compressed together. After several weeks, the bones fuse together. A second procedure to remove the screw must be performed because the head of the screw will cause discomfort in the finger pad if the screw is not removed. 
   This procedure is undesirable because it requires two separate surgeries. Katzman, et al.,  Use of a Herbert Screw for Interphalangeal Joint Arthrodesis, Clinical Orthopedics and Related Research , No. 296 pages 127–132 (November 1993), describes use of the screw disclosed in the Herbert patent in procedures for interphalangeal joint arthrodesis. 
   Many of the above-discussed disadvantages associated with using a Herbert screw to compress a fracture are also present when the Herbert screw is used for interphalangeal joint arthrodesis. 
   It would be desirable to provide a headless bone screw which overcomes the disadvantages associated with the Herbert bone screw, as well as other prior art bone screws. 
   SUMMARY OF THE INVENTION 
   A bone screw for drawing together bone fragments separated by a fracture includes a root portion having a leading end and a trailing end. The leading end has a smaller diameter than the trailing end. A screw thread is formed on the root portion between the leading and trailing ends and has a pitch which varies along the length thereof, having a larger pitch near the leading end and a smaller pitch near the trailing end. The thread is adapted to thread in the cancellous material of the respective bone fragments to be joined by the screw. Means are provided on the trailing end of the root portion to accommodate a tool for driving the screw. The present invention also contemplates a method for drawing together bone fragments separated by a fracture. 
   The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment which proceeds with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an enlarged side elevation view of a bone screw constructed in accordance with the present invention; 
       FIG. 1A  is a view of the screw of  FIG. 1  shown partially in cross section; 
       FIG. 2  is an end view of the bone screw of  FIG. 1 ; 
       FIG. 3  is a drawing illustrating the outside diameter of the screw; 
       FIG. 4  is a drawing illustrating the diameter of the root portion of the screw; 
       FIG. 5  is a cross-sectional view of a bone screw constructed in accordance with the present invention installed in a bone to draw a fracture together; 
       FIG. 6  is a side elevation view of a bone screw constructed in accordance with the present invention which may be used for interphalangeal joint arthrodesis; 
       FIG. 7  is a view of the bone screw of  FIG. 5  installed in a distal interphalangeal joint with the bones forming the joint as shown in cross-section; 
       FIGS. 8A–14D  show the operation of various screws to compress two bone fragments together; 
       FIGS. 15A–D  show the operation of a screw constructed according to an alternative embodiment of the present invention to compress two bone fragments together; 
       FIGS. 16A–B  are detailed views of the screw shown in  FIGS. 15C and 15D , respectively; 
       FIG. 17A  is a side elevation view of a bone screw constructed according to an alternative embodiment of the present invention; 
       FIG. 17B  is a representation of the side profile of a root portion of the screw of  FIG. 17A ; 
       FIG. 17C  is a representation of the outside diameter of the screw of  FIG. 7A ; 
       FIG. 18A  is a side elevation view of a bone screw constructed according to a fourth embodiment of the present invention; 
       FIG. 18B  is a representation of the side profile of a root portion of the screw of  FIG. 18A ; 
       FIG. 18C  is a representation of the outside diameter of the screw of  FIG. 8A ; 
       FIG. 19  is an enlarged side elevation view of a bone screw constructed in accordance with an alternative embodiment of the present invention; 
       FIG. 19A  is a view of the screw of  FIG. 19  shown partially in cross section; 
       FIG. 20  is an end view of the bone screw of  FIG. 19 ; 
       FIG. 21   a  illustrates the outside diameter and root profile of an alternative embodiment of the present invention; and 
       FIG. 21   b  is an elevational view of the screw of  FIG. 21   a.    
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Indicated generally at  10  in  FIGS. 1 and 1A  is a bone screw constructed in accordance with the present invention. Bone screw  10  is centered on a longitudinal axis  11 . The length of screw  10  as measured along axis  11  is 0.394 inches in the present embodiment of the invention. The bone screw includes a root portion  12  having a continuous screw thread  14  formed thereon. 
   Root portion  12  includes a leading end  16  and a trailing end  18 . As can best be seen in  FIG. 4 , the diameter of leading end  16  is less than the diameter of trailing end  18 . Also in  FIG. 4 , it can be seen that root portion  12  tapers between trailing end  18  and leading end  16 . A 45° bevel  20 , in  FIGS. 1 and 1A , is formed on trailing end  18 . In the present embodiment of the invention, trailing end  18  has a diameter of approximately 0.092 inches. A frusto-conical nose portion  22  is formed on leading end  16  of root portion  12 . 
   Screw thread  14  extends continuously between nose portion  22  and bevel  20 . As can be seen in  FIGS. 2 and 3 , a trailing thread  24  has a crest height, i.e., the distance between axis  11  and a crest  26  of trailing thread  24 , which varies so as to form a substantially 45° angle, illustrated as angle  28  in  FIG. 3 , between the outside diameter of crest  24  and axis  11 . 
   A similarly tapering leading thread  30  also has a crest  32  which varies in height over a first partial turn of screw thread  14  so as to form an angle of substantially 45° with axis  11  as illustrated in  FIG. 3 . 
   The crest of screw thread  14  between trailing and leading threads  24 ,  30  respectively, varies in height along the length of thread  14 . In the present embodiment of the invention, the outside diameter defined by the crest of thread  14  between the leading and trailing threads forms an angle  34 , in  FIG. 3 , of approximately 1.43° with respect to an axis  35  extending from the radially outermost portion of thread  14  parallel to axis  11 . In the present embodiment of the invention, the diameter of the radially outermost portion of thread  14  is approximately 0.138 inches. 
   The pitch of thread  14 , i.e., the distance from one point on the thread to the corresponding point on an adjacent thread measured parallel to axis  11 , decreases between the leading and trailing ends of the screw. It should be noted that the term pitch is also sometimes used to refer to the number of threads per unit length, i.e., 20 threads per inch. This alternative definition is simply the inverse of the definition chosen for use in this application. The distinction is important to remember for proper understanding of the subsequent description because the screw of the present invention relies on varying pitch to achieve its function. 
   In the embodiment of the invention shown in  FIGS. 1 and 1A , the distance between the uppermost portion of crest  32  and a corresponding crest portion  36  is 0.04964 inches. The distance between the uppermost portion of crest  26  and a corresponding crest portion  38  is 0.04748 inches. In the present embodiment of the invention, the pitch change per revolution is approximately 0.00036 inches. 
   The pitch depth, i.e., the distance between the crest and the radially outer surface of root portion  12  similarly varies along the length of the screw. In the present embodiment of the invention, the pitch depth where leading thread  30  joins the remainder of screw thread  14  is approximately 0.0302 inches. The pitch depth where trailing thread  24  joins the remainder of thread  14  is approximately 0.0240 inches. 
   The decrease in pitch depth between the leading end and trailing end of the screw can be seen by comparing  FIG. 3  and  FIG. 4  wherein root portion  12  tapers more sharply from the trailing to the leading end of the screw than does the change in crest height as shown in  FIG. 3 . In the present embodiment of the invention, the outside diameter of root portion  12  between leading and trailing ends,  16 ,  18 , respectively, forms an angle  40 , in  FIG. 4 , of approximately 2.5° with respect to an axis  42  extending from the radially outermost portion of trailing end  18  parallel to axis  11 . 
   A hex socket  44  is formed on the trailing end of screw  10  to accommodate a driver as will be hereinafter further explained in connection with a description of the procedure in which the screw is used to draw opposing fragments of a fractured bone together. 
   An alternative embodiment of the screw of the present invention is shown generally at  410  in  FIGS. 19 and 19A . Screw  410  includes a root portion  412  on which is formed a thread  414 . Thread  414  extends from a leading end  416  to a trailing end  418  and includes a land  474 . The pitch of thread  414  at the leading end is 0.055 inches and the pitch at the trailing end is 0.035 inches. The land varies from 0.010 inches to 0.004 inches overt the same range. Thread  414  includes a cutting flute  415  near the leading end to facilitate the cutting of female threads as the screw is installed. Both the outside diameter of thread  414  and root  412  taper from a smaller value at the leading end to a larger value at the trailing end. See  FIGS. 21–22 . The root diameter tapers from 0.062 inches to 0.122 inches, while the outside diameter tapers from 0.130 inches to 0.156 inches. The length of screw  410  is 0.689 inches. 
   Screw  410  also includes an axial bore  425  which extends from the leading end to the trailing end. Bore  425  is adapted to receive a stiff guide wire, not shown, which facilitates installation of screw  410 . A hex socket  444  is formed at the trailing end to allow the screw to be driven by an hex wrench. See  FIG. 20 . 
   Turning now to  FIG. 5 , illustrated therein is a fracture  46  which separates adjacent bone fragments  48 ,  50 . Screw  10  is illustrated installed in a bore  52  which extends through bone fragments  48 ,  50  across fracture  46 . 
   In installing screw  10 , a surgeon first drills bore  52  across bone fragments  48 ,  50  as shown. The bit may be a conventional cylindrical bone bit or may comprise a bit having a slight taper from the leading to the trailing end thereof. Thereafter, the surgeon inserts a tool (not shown) having a hex driver extending therefrom which is connectable to hex socket  44  for screwing screw  10  into bore  52 . Bore  52  is of a size to just receive leading end  16  of screw  10 . As soon as nose portion  22  is received within the bore, torque is applied using the tool inserted into hex socket  44  thereby causing leading thread  30  to cut into the bone adjacent bore  52 . 
   In the view of  FIG. 5 , screw  10  is hatched to show the path cut by leading thread  30  after screw  10  is installed in the position illustrated in  FIG. 5 . The path of thread  30  is depicted using hatching, like hatching  54 ,  56 ,  58  which indicates the position of the path cut by leading thread  30  relative to succeeding threads of the screw. Hatching  60  depicts the actual position of the thread on screw  10  and root  12 . It is to be appreciated that hatching  54 ,  60  are not used in  FIG. 5  to depict different structure, which is unitary as illustrated in  FIG. 1 , but to depict relative positions of the path cut by leading thread  30  in the actual position of subsequent threads in the installed screw. 
   Because of the decreasing pitch along the length of the screw, each successive thread received in the path cut by thread  30  exerts pressure against the right side (as viewed in  FIG. 5 ) of the path cut by thread  30  thereby tending to compress the bone along the length of the screw. As can be seen in  FIG. 5 , by the time the screw is fully installed, trailing thread  24  compresses a substantial amount of bone when it is received in the path cut by thread  30 . This tends to draw bone fragments  48 ,  50  tightly together across fracture  46  thereby promoting healing of the fracture. 
   As can be appreciated from the view of  FIG. 5 , the thread taper is important for two reasons. First, each succeeding portion of the thread is spaced further radially outwardly as a result of the taper and therefore the outer portion of each thread (that portion closely adjacent the crest) cuts into new bone which was not cut by the preceding thread. This provides a much better purchase than would a thread having a continuously varying pitch with constant diameter. In such a configuration, each succeeding thread cuts additional bone within the generally cylindrical volume defined by the outside diameter of the threads. The outer portion of each thread (that portion closely adjacent the crest) therefore cuts into bone uncut by the preceding thread. 
   The tapered root is also advantageous in that the radially outer surface of the root, i.e., that portion between adjacent threads, is tightly urged against uncut bone defining the wall of bore  52 . It is desirable to maximize the surface area of screw  10  urged against adjacent bone, rather than a space cut by a thread, to increase purchase of the screw. 
   The details of the operation of the screw of the present invention, as currently understood, may be better appreciated by examination of  FIGS. 15A–D  and  FIGS. 16A–B  and the following description.  FIGS. 15A–D  illustrate the operation a screw  310  to draw together and join bone fragments  348  and  350 .  FIG. 15A  shows screw  310  partially installed in a bore  349  in bone fragment  348 . Screw  310  is shown just entering a bore  351  in bone fragment  350  in  FIG. 15B . Subsequent rotation of screw  310  starts the process of drawing the bone fragments together as shown in  FIGS. 15C–D . 
     FIG. 16A  shows the interaction of a thread  314  in bores  349  and  351  when screw  310  is positioned therein as shown in  FIG. 15C . In  FIG. 16A  a leading end  316  of screw  310  is engaged in bore  349 . Each revolution of the thread  314  is labelled for reference in the subsequent discussion, from thread T 1  at the leading end to thread T 23  at the trailing end. 
   As the screw moves through bone fragments  348  and  350 , thread  314  will cut a mating female thread  353 . However, because the pitch of thread  314  changes along the length of the screw, female thread  353  will not precisely match thread  314  of screw  310  along its entire length. In particular, since subsequent threads will not track in the same path as the preceding threads, a pattern of leading gaps  355  and trailing gaps  357  will evolve between female thread  353  and screw thread  314  as the screw moves forward in the bores. 
   The screw will move forward in the bone fragment with rotation at a rate that is a function of the competing forces from all of the threads engaged in the bore. The rate will correspond to an effective pitch of the threads in the bore and will be equal to the pitch of the screw at an effective pitch point  359  along the portion of the screw engaged in the fragment. As more of the screw enters the bore, the effective pitch point will move back along the screw and further into the bone fragment. Once the screw extends completely through the bone fragment, the location of the effective pitch point will stabilize at a relatively constant location in the bone fragment, simply moving back along the screw at the rate the screw moves forward in the bore. The threads ahead of the effective pitch point, which will be referred to as the pulling threads  371 , will have greater pitch than the effective pitch. Similarly, the threads behind the effective pitch point, or dragging threads  373 , will have a pitch that is smaller than the effective pitch. In  FIG. 16A  the pulling threads in fragment  348  are T 1 –T 4  and the dragging threads are T 5  and T 6 . 
   Each rotation of the screw will move it forward in fragment  348  by an amount corresponding to the present value of the effective pitch. In  FIG. 16A  the effective pitch will be equal to the pitch of thread  314  between threads T 4  and T 5 . Starting at the leading end, thread T 1 , will always be cutting a new thread path in the fragment, so no gap will form around it. Thread T 2 , however, will attempt to follow the track of thread T 1 , in fragment  348 , which would carry it forward by an amount equal to the pitch between thread T 1 , and T 2 . Since, however, the screw will only move forward by the effective pitch, i.e., the pitch between threads T 4  and T 5 , thread T 2  can only move forward by the same amount. This causes thread T 2  to pull back against the surrounding bone and creates a leading gap in front that thread. Similarly, thread T 3  will attempt to move into the position of thread T 2 , but will be held back from moving as far forward as its pitch would indicate, thus creating a leading gap as thread T 3  is pulled back against the surrounding bone. Behind the effective pitch point, thread T 6  will attempt to move into the prior position of thread T 5 , but will be dragged forward somewhat, leaving a trailing gap. 
   The pattern of leading and trailing gaps created by screw  310  in bone fragment  350  is also shown in  FIG. 16A . Bone fragment  350  includes leading gaps  361  and trailing gaps  363  similar to those found in bone fragment  348 . However, because more of the screw has moved through bone fragment  350 , the gaps have evolved to a greater extent. The earlier position of screw  310  in fragment  350  is shown in dotted lines in  FIG. 16A  to illustrate the evolution of the threads as the screw moves forward. 
   In the earlier position of screw  310 , the effective pitch point falls at approximately thread T 8 . With the screw positioned as shown, the effective pitch point is at approximately thread T 16 , the screw having completed approximately 8 revolutions between the two positions. The current and prior screw positions are aligned at effective pitch point  367  in fragment  350  based on the assumption that thread  314  will track through this point uniformly. The evolution of the position of threads behind and ahead of the effective pitch point can thus be seen by comparing the prior position with the current position. 
   Leading gaps  361  have a sloping upper surface  365 , which is a result of the gradual expansion of the outside diameter of thread  314  toward the trailing end of the screw. Upper surface  365  represents a line from the prior position of the thread to the position as shown. As thread  314  at a given point in the bone fragment is held back, it simultaneously expands in diameter. This effect prevents thread  314  from completely reaming out the female thread in the bone fragment, as discussed above. Without the taper, sloping upper surface  365  would be flat and as soon as the width of the gap grew to equal the spacing between the threads, there would be no purchase left for subsequent threads along a portion of the bore. 
   Once the leading end of screw  310  has passed through bone fragment  351  the effective pitch point remains at a relatively constant position along the bore for the remainder of the screw. If the pitch change per revolution is dP and the effective pitch points are separated by N threads, then the bone fragments will be drawn together by a distance N times dP for every revolution of the screw. In screw  310 , dP=0.0008 inches and the effective pitch points are separated by approximately 11 threads, therefore the gap between the bone fragments will close by about 0.009 inches per revolution. 
   It is thought that the effective pitch point will be somewhat behind the geometric middle of the portion of the screw engaged in the bore as shown in  FIG. 16A . Because bone becomes less dense near the center in the cancellous portion, the threads nearer to the surface in the cortex are expected to have greater effect. Also, the threads nearer the surface are of larger diameter because of the taper in the outside diameter of the thread. 
   The other factor tending to cause the pitch point to be closer to the surface of the bone relates to balancing the amount of bone displaced as the leading and trailing gaps are formed. As shown in  FIG. 16A , the pulling threads  371 , which have pitch greater than the effective pitch, are held back from moving as far forward with each rotation as their pitch would indicate. Likewise, dragging threads  373  are drawn forward faster than their pitch would dictate. This effect creates leading gaps  355  in front of pulling threads  371  as they pull against the surrounding bone. Similarly, trailing gaps  357  form behind dragging threads  373  as they are dragged forward through the surrounding bone. 
   Since the leading and trailing gaps are formed in opposition to one another, it is reasonable to assume that they will evolve at a relatively balanced rate. Combining this assumption with the fact that the effective pitch point is constantly moving forward in the bone fragment as the screw enters, suggests that the effective pitch point will be behind the geometric middle of the portion of the screw in the bone fragment. Because the effective pitch point is moving forward in the bone fragment by approximately one-half the pitch change per revolution, the dragging threads will be dragged forward by approximately an extra one-half the pitch change per revolution for each revolution of the screw. The fact that the effective pitch point is moving forward means that the pulling threads are not held back as much as would be the case if the effective pitch point remained constant. If the movement of the two thread regions through the bone are balanced, then the effective pitch point will not move forward in the bone fragment as rapidly as would otherwise be expected and the effective pitch point will lie behind the geometric middle. 
     FIG. 16B  shows how the pattern of gaps changes once the two bone fragments have been drawn together. After the bone fragments meet, the pattern of gaps starts to evolve toward that found in a single fragment. In particular, gaps form or increase on the leading side of all of the pulling threads ahead of an effective combined pitch point  369 , and on the trailing side of all the dragging threads behind the effective combined pitch point. Near the joint between the fragments, the gaps will generally transition from leading to trailing and vice versa, because the dragging threads in fragment  348  near the joint are converted to pulling threads after the joint closes. The pulling threads in fragment  350  likewise become dragging threads after the fragments meet. 
   Rotation of screw  310  after the bone fragments have come together tends to increase the pressure in the joint between them. Additional rotation can be used to set the depth of the screw as desired. Since the outside diameter of the thread tapers, as described above, the screw can be driven in until the trailing end is below the surface of the bone without danger of stripping the female thread formed by the preceding threads, even if the bone fragments first meet with the trailing end protruding substantially. This is because subsequent threads expand and cut into some new bone even as they partially ream the female threads left by preceding threads on the screw. This is in contrast to the Herbert screw, where, as discussed above, additional tightening after the fragments have come together can strip out the threads in the distal fragment and reduce compression. Since it is important in the preferred application of the present invention to have the trailing end of the screw below the surface of the bone, this is an important feature and advantage over prior art screws. 
   The tolerance in the screw of the present invention to further tightening after the fragments have come together is also important because it simplifies the installation process by eliminating the danger of over-tightening that must be guarded against when using the Herbert screw. 
   Turning now to  FIG. 6 , indicated generally at  62  is a second embodiment of a bone screw constructed in accordance with the present invention. Bone screw  62  is sized and constructed for use in connection with interphalangeal joint arthrodesis. Screw  62  includes a tapered root  64  having a thread  65  formed thereon from a leading end  63  to a trailing end  67 , a substantially cylindrical leading extension  66  joined to the leading end and a substantially cylindrical trailing extension  68  joined to the trailing end. The diameter of leading extension  66  is slightly larger than root  64  at leading end  63 , while the diameter of trailing extension  68  is slightly smaller than root  64  at trailing end  67 . The trailing extension  68  includes a hex socket (not visible), like hex socket  44  in  FIG. 1A , formed on an end surface  70  thereof. Leading extension  66  includes a tapered nose  72  formed on the forward end thereof. In the present embodiment of the invention, screw  62  is 1.259 inches in length with the threaded portion being 0.630 inches long and the diameter of leading extension  66  being 0.05 inches. The trailing extension diameter is 0.100 inches. As is the case with the previously described embodiment, the pitch of thread  65  decreases between the leading and trailing ends. In the embodiment of  FIG. 6 , a land  74  is formed in the crest of thread  65  and decreases in width between the leading and trailing ends of the screw. 
   Turning now to  FIG. 7 , a distal phalanx  76  comprises the outermost bone of one of the four fingers. A proximal phalanx  78  is adjacent thereto with a distal interphalangeal (DIP) joint  80  being formed therebetween. 
   The joint includes a pair of articular surfaces  82 ,  84  which have been flattened in accordance with a known technique for immobilizing DIP joint  80 . Bores  86 ,  88  are drilled into each of phalanxes  76 ,  78  from articular surfaces  82 ,  84 , respectively. Thereafter the bones are repositioned as shown in  FIG. 7  and screw  62  is driven into the distal end of the bore in phalanx  76  until the screw is positioned as shown in  FIG. 7 . 
   Screw  62  thus compresses across joint  80  even though it has a relatively small diameter, which is critical in DIP joint arthrodesis because of the small diameter of the bones involved. Screw  62  also has sufficient length, due to the leading and trailing extensions  66 ,  68 , to provide stability while the bones are fusing. Because the screw is entirely received within the bones, i.e., there is no protrusion from the screw, it can remain implanted and thus a second procedure to remove the bone is not necessary. 
   A third embodiment of a screw constructed according to the present invention is shown generally at  110  in  FIG. 17A . Screw  110  includes a root portion  112  on which is formed a continuous screw thread  114  and associated land  174 . Screw  110  includes a leading end  116  and a trailing end  118 . Leading cutting flutes  115  are formed in thread  114  near leading end  116  to help the thread self tap into the bone. A series of trailing cutting flutes  117  are formed in thread  114  along the sides of the screw toward the trailing end. Trailing cutting flutes  117  facilitate installation and removal of the screw by helping to cut a thread path in the bone. Screw  110  may be formed with two sets of trailing cutting flutes, one oriented to cut female threads upon insertion and another oriented to cut female threads upon removal of the screw, thus easing both installation and extraction. A hex socket  144  is formed in the trailing end of screw  110  to receive a drive tool. 
   Screw  110  is formed with a variable pitch portion  119  and a constant pitch portion  121 . Variable pitch portion  119  extends from leading end  116  back toward trailing end  118  for about 70 percent of the length the of the screw. The length of the screw is 0.961 inches. It should be noted that screw  110  does not include a bevel at the trailing end as formed on screw  10  and shown at  20  in  FIG. 1A . The bevel was eliminated in screw  110  to provide additional structural support around hex socket  144  which is used for driving the screw. 
   Variable pitch portion  119  of screw  110  is formed according to the previously described construction of screw  10 . In particular, the pitch of thread  114  is largest at leading end  116  and decreases over variable pitch portion  119  back toward trailing end  118 . The pitch starts at 0.050 inches and decreases to 0.0365 inches at the trailing end of the variable pitch portion. As shown in  FIG. 17B , root portion  112  tapers outward from leading end toward trailing end over variable pitch portion  119  with an angle  140  of 1.93° relative to the longitudinal axis of the screw. The diameter of the root portion is 0.032 inches at the leading end and 0.091 inches at the trailing end. The outside diameter of thread increases over the same region at an angle  134  of 1.0°. See  FIG. 17C . The outside diameter of the thread at the leading end is 0.077 inches and 0.1 inches at the trailing end. 
   The construction of constant pitch portion  121  is considerably different from that of variable pitch portion  119 . The pitch and outside diameter of thread  114  are constant over the section of the screw forming constant pitch portion  121 . Root portion  112  continues to taper outward relative to the axis of the screw but at a lesser angle  127  of 1.57° over the constant pitch portion. The width of land  174 , i.e., the flat at the crest of the thread, which decreases from the leading end over the variable pitch portion, increases over the length of the constant pitch portion toward the trailing end. Land  174  starts at the leading end at 0.008 inches and decreases to 0.002 inches at the end of the variable pitch region. Land  174  starts to increase again moving back over the constant pitch portion, reaching a value of 0.006–0.007 inches at the trailing end. 
   The constant pitch portion at the rear of screw  110  allows construction of a longer screw without the commensurate increase in diameter that would occur by extending the structure of the variable pitch portion. This is important where the screw is to be used in small bones that cannot accept a larger bore, but which require a longer screw. A longer screw may be required to reach deeper fractures or for use in fusing two bones together. Screw  110  is particularly suitable for use in distal interphalangeal fusions in the hand as described above. 
   A fourth embodiment of a screw constructed according to the present invention is shown at  210  in  FIG. 18A . Screw  210  is generally similar to screw  110  of  FIG. 17A , and includes a root portion  212 , a thread  214 , a leading end  216  and a trailing end  218 . Screw  210  also includes a variable pitch portion  219  and a constant pitch portion  221 . See  FIG. 18B . The diameter of root portion  212  tapers at an angle  240  of 2.29° from 0.050 inches at the leading end to 0.106 inches at the trailing end. The outside diameter of thread  214  tapers at an angle  234  of 1.2° from 0.110 inches to 0.140 inches over the same range. The overall length of screw  210  is 0.787 inches. 
   The principal difference between screws  110  and  210  is found in the constant pitch portions. In screw  210 , neither the root portion nor the outside diameter of the thread is tapered in the constant pitch region. See  FIG. 18B–C . Screw  210  is designed, like screw  110 , to have additional length without additional thickness. If additional length is desired, it is possible to form screw  210 , or screw  110 , with leading and/or trailing extensions such as found on screw  62  in  FIG. 6 . 
   Thread  214  on screw  210  includes a land  274 . Land  274  starts at a maximum of 0.007 inches at the leading end and decreases to 0.003 inches at the trailing end. In contrast to screw  110 , land  274  does not increase over the constant pitch portion. Thread  214  also includes leading cutting flutes  215  and trailing cutting flutes  217  to facilitate installation and removal. 
   Screw  210  also varies from screw  110  in that it includes an axial bore  225 . Axial bore  225  permits screw  210  to be guided into the bone on a stiff wire to facilitate positioning and prevent the screw from wandering off axis as it is driven in. 
   A screw according to the present invention particularly adapted for use in ankle fusions is shown generally at  410  in  FIGS. 21   a–b . Screw  410  includes a root portion  412  that tapers at a constant rate from a leading end  414  to a trailing end  416 . In the preferred embodiment the root has a length of 2.383-inches and tapers from a radius of 0.184-inches near the trailing end to a radius of 0.098-inches near the leading end. 
   A screw thread  418  is formed on root portion  412  and extends from the leading end to the trailing end thereof. Thread  418  has a thread crest  420  at its radial outermost edge. As with the previously described embodiments, the thread is terminated at the leading end and trailing end with a 45-degree taper. Thread  418  has a pitch measured between consecutive thread crests which varies between a larger value near the leading end to a smaller value near the trailing end. Preferably, the pitch changes uniformly between the ends from a value of 0.097-inches at the leading end to a value of 0.066-inches at the trailing end. 
   In contrast to the previously described screws, screw  410  has a guide taper  422  at the leading end of the root portion. The guide taper has a taper angle of approximately 15-degrees and serves to help maintain the leading end of the screw centered in the pilot hole in the bone in which it is installed. The guide taper extends along the root portion back from the 45-degree taper for a distance of 0.129-inches. 
   Screw  410  has a region  424  of constant outside diameter that extends back from the guide taper for a length of 0.090-inches with a diameter of 0.205-inches. A second region  426  of constant diameter is disposed adjacent the trailing end of the screw with a diameter of 0.256-inches for a length of 0.197-inches. Provision of regions  424  and  426  allows screw  410  to have a long length while reducing the amount of taper that would otherwise be required. It is important to maintain the radius as large as possible near the lead end to obtain adequate grip in this region. This is particularly important in the preferred application for screw  410  of ankle fusions because the amount of screw  410  engaged in the tibia may be limited. It is likewise important not to make the radius at the trailing end any larger than necessary to minimize the size of the hole required. The region of constant diameter at the trailing end is also important because it provides a region for gripping the screw during manufacture. Between the regions of constant diameter is a central region  428  in which the pitch and diameter of the screw change together. The central region has a length of 1.870-inches in the preferred embodiment. 
   A significant difference between screw  410  and the previously described embodiment lies in the formation of the threads. In particular, in the previously described embodiments, the screw thread is cut with a tool with a flat face and outwardly sloping sides. In the previous embodiments, the width of the face determines the spacing between the threads on the root portion, which was therefore constant along the length of the screw. By pulling the tool back from the axis of the screw and adjusting the pitch properly, the thread can be cut with a varying pitch and depth. However, with each pass of the tool along the screw, the tool follows the same longitudinal path in the thread but simply cuts closer to the root portion. The land at the crest is also increased near the leading end to allow for additional pitch gain near the leading end while maintaining a decreasing outside radius. 
   In screw  410 , in contrast, the longitudinal position of the tool along the root portion is changed from pass to pass as the screw is being turned. In particular, in one pass down the screw thread, the tool follows a first path. In a subsequent pass the tool is shifted longitudinally along the screw slightly at the same depth to increase the width of the inter-thread distance  428  on the root toward the leading end. Cutting the thread in this fashion allows a sharper thread to be produced while still obtaining the desired outside diameter taper and pitch variation. Sharper thread is beneficial because it leaves a smaller track in the bone which leaves more bone for subsequent threads to grip and makes the screw easier to drive in during installation. As with previously described embodiments, it is important that the radius and depth of the threads near the leading end be sufficient to provide a grip on the bone which is comparable to the grip of the threads near the trailing end of the screw. 
   It should be understood that screw  410  could be manufactured in a variety of lengths to accommodate different size patients. Moreover, for shorter screws, the region of constant outside diameter near the leading end may be eliminated without unduly compromising the grip of the leading threads. It should also be noted that shorter screws will typically taper at a greater angle. 
   In the actual fusion, a hole is drilled up from the heel through the calcaneous and talus and into the distal end of the tibia. The screw is then driven into the hole to draw the three bones together. With time, the pressure generated by the screw leads to fusion of the bones. The present screw is advantageous for this operation because it can be mounted sub-surface since it does not have a head. Furthermore, the screw offers excellent grip and controllable compression when compared with standard lag screws. 
   Although not shown in  FIGS. 21   a–b , screw  410  preferably is cannulated to provide improved stability during installation. 
   It should be noted that the length, number of threads, pitch, pitch change per revolution and the various diameters are not critical to the present invention and can be varied without departing from the spirit of the invention. Such parameters are chosen to suit the particular use to which the screw is applied. 
   While the invention has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense because numerous variations are possible. Applicant regards the subject matter of his invention to include all novel and non-obvious combinations and subcombinations of the various elements, features, functions, and/or properties disclosed herein. No single feature, function, element, or property of the disclosed embodiments is essential. The following claims define certain combinations and subcombinations which are regarded as novel and non-obvious. Other combinations and subcombinations of features, functions, elements, and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such claims, whether they are broader, narrower, or equal in scope to the original claims, also are regarded as included within the subject matter of applicant&#39;s invention.