Variable pitch bone screw

A bone screw having a continuously varying pitch includes a tapered root portion having a relatively small diameter on a leading end of the screw and a larger trailing diameter. The pitch of the screw decreases between the leading and trailing ends thus causing the bone fragments in a fracture to be drawn together when the screw is installed across the fragments. The radially outer diameter of the threads increases between the leading and trailing ends thus causing each successive thread portion to cut into bone radially outwardly from the preceding thread portion thereby providing uncut bone in which the succeeding threads can gain purchase.

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.sub.1 with a head B.sub.1 and a 
shank C.sub.1 is shown in FIG. 8A-D. Shank C.sub.1 of screw A.sub.1 
includes threads D.sub.1 at the distal end and an unthreaded region 
E.sub.1 proximal to head B.sub.1. The pitch of threads D.sub.1 is 
constant. FIG. 8A shows screw A.sub.1 partially engaged in a bore F.sub.1 
in a near bone fragment G.sub.1. The diameter of bore F.sub.1 is less than 
the diameter of threads D.sub.1 and therefore the threads engage the walls 
of the bore as the screw is twisted in. FIG. 8B shows screw A.sub.1 as it 
starts threading into a bore H.sub.1 in a remote bone fragment I.sub.1. At 
this point threads D.sub.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.sub.1 has reached the top of fragment G.sub.1 in FIG. 
8C, as indicated schematically by the radiating "force" lines. Since 
threads D.sub.1 are no longer engaged in fragment G.sub.1, screw A.sub.1 
rotates freely in the fragment without being drawn forward therein. 
Subsequent rotation of screw A.sub.1 draws fragment I.sub.1 further up the 
screw. Because head B.sub.1 prevents fragment G.sub.1 from moving further 
up screw A.sub.1, fragment I.sub.1 is drawn up against fragment G.sub.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.sub.1 is illustrated in 
FIGS. 9A-d. A lag screw A.sub.2 including a head B.sub.2 and a shank 
C.sub.2 is shown partially engaged in a bore F.sub.2 in a near fragment 
G.sub.2 in FIG. 9A. Shank C.sub.2 includes threads D.sub.2 running the 
entire length with no unthreaded region such as E.sub.1 on screw A.sub.1. 
Rotating screw A.sub.2 causes it to be drawn through fragment G.sub.2 and 
pass into a bore H.sub.2 in a remote fragment I.sub.2, as shown in FIG. 
9B. Further rotation of screw A.sub.2 brings head B.sub.2 down against the 
upper surface of fragment G.sub.2. See FIG. 9C. At this point, threads 
D.sub.2 are still engaged in bore F.sub.2 of fragment G.sub.2 and the 
interaction of the head on the surface of fragment G.sub.2 impedes the 
further rotation of screw A.sub.2. To have additional rotation, head 
B.sub.2 would have to be drawn down into fragment G.sub.2 or the portion 
of threads D.sub.2 in fragment G.sub.2 would have to strip out. Therefore 
a fully threaded screw, such as screw A.sub.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.sub.2 is illustrated in 
FIGS. 10A-D. As shown in FIG. 10A, bore F.sub.2 in fragment G.sub.2 is 
enlarged to allow threads D.sub.2 of screw A.sub.2 to pass freely through 
the bore. Screw A.sub.2 therefore slips into bore F.sub.2 until it reaches 
fragment I.sub.2. At that point, threads D.sub.2 engage the walls of bore 
H.sub.2 and draw screw A.sub.2 down into fragment I.sub.2. See FIGS. 
10B-C. When head B.sub.2 reaches the upper surface of fragment G.sub.2, 
further rotation causes fragment I.sub.2 to be drawn up into contact with 
fragment G.sub.2 as shown in FIGS. 10C-D. No binding occurs between head 
B.sub.2 and threads D.sub.2 in the near fragment because of the large bore 
in fragment G.sub.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.sub.1 and A.sub.2. FIG. 11A, in particular, shows a 
headless screw A.sub.3 partially installed in a bore F.sub.3 in a near 
fragment G.sub.3. Screw A.sub.3 includes threads D.sub.3 extending along 
its entire length. The pitch of threads D.sub.3 is constant. FIG. 11B 
shows screw A.sub.3 extending through fragment G.sub.3 and just entering a 
bore H.sub.3 in a remote fragment I.sub.3. FIG. 11C shows screw A.sub.3 
advanced further into fragment I.sub.3. It should be noted that, since the 
pitch of threads D.sub.3 is constant, screw A.sub.3 moves forward in 
fragments G.sub.3 and I.sub.3 by the same amount with each rotation. As 
shown in FIG. 11D, screw A.sub.3 will pass through both fragments without 
altering their relative spacing or compressing them together. Thus, a 
headless screw such as screw A.sub.3 will not work to draw the fragments 
together in the same way as lag screws A.sub.1 and A.sub.2. 
A variation of screw A.sub.3 is shown at A.sub.4 in FIG. 12A. Screw A.sub.4 
includes threads D.sub.4 of constant pitch extending along its entire 
length and differs from screw A.sub.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.sub.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.sub.4 is 
shown partially installed in a bore F.sub.4 in a near fragment G.sub.4 in 
FIG. 12A. As screw A.sub.4 is rotated, it moves through fragment G.sub.4 
and into a bore H.sub.4 in a remote fragment I.sub.4, as shown in FIG. 
12B. Subsequent rotation simply carries screw A.sub.4 further into and 
through fragment I.sub.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.sub.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 As with threads D.sub.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.sub.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 As is shown in FIG. 
13A with the leading threads engaged in a bore F.sub.5 in a near fragment 
G.sub.5. Rotation of screw As causes it to move forward into and through 
fragment G.sub.5 and into a bore H.sub.5 in a remote fragment I.sub.5, as 
shown in FIG. 13B. Additional rotation after the leading threads engage 
fragment I.sub.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.sub.5 is greater than the average pitch of threads in fragment G.sub.5. 
Since the screw moves forward in each fragment with each 360.degree. 
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.sub.5 than in fragment G.sub.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.sub.5. It 
should be noted that screw A.sub.5, in contrast to constant pitch screws 
such as screws A.sub.1 and A.sub.2, can be used to separate fragments 
G.sub.5 and I.sub.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.sub.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 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.sub.6, as per Herbert, includes a shank C.sub.6 with leading 
threads J.sub.6 at the leading end, trailing threads K.sub.6 at the 
trailing end and an unthreaded region E.sub.6 separating the leading and 
trailing threads. Threads J.sub.6 and K.sub.6 each have fixed pitch, but 
leading threads J.sub.6 have a larger pitch and smaller outside diameter 
than trailing threads K.sub.6. FIG. 14A shows leading threads J.sub.6 of 
screw A.sub.6 installed in a bore F.sub.6 of a near fragment G.sub.6. It 
should be noted that threads J.sub.6 do not engage the walls of bore 
F.sub.6, the bore having been bored large enough to allow leading threads 
J.sub.6 to pass freely. As the screw moves forward, the leading threads 
engage a bore H.sub.6 in a remote fragment I.sub.6. See FIG. 14B. The 
diameter of bore H.sub.6 is adapted so that leading threads J.sub.6 engage 
the walls. Meanwhile, at the trailing end of the screw, trailing threads 
K.sub.6 start to engage the walls of bore F.sub.6, which has been bored to 
an appropriate diameter therefor. 
As soon as trailing threads K.sub.6 are engaged in bore F.sub.6 and leading 
threads J.sub.6 are engaged in bore H.sub.6, the two fragments start 
drawing together. See FIG. 14C. Further rotation of screw A.sub.6 
completes the process of moving the two fragments together as shown in 
FIG. 14D. Screw A.sub.6 operates on the same general principle as screw 
A.sub.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.sub.6 will move it 0.2 inches further into fragment H.sub.6, 
but only 0.1 inches further into fragment I.sub.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.sub.6 all follow a single path through the near 
fragment. Similarly, leading threads J.sub.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 (Nov. 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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.degree. 
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.degree. 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.degree. 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.degree. 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.degree. 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 T1 at the leading end to thread T23 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.sub.1 -T.sub.4 and the dragging threads are T.sub.5 and 
T.sub.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.sub.4 and T.sub.5. Starting at the leading end, thread T.sub.1 
will always be cutting a new thread path in the fragment, so no gap will 
form around it. Thread T.sub.2, however, will attempt to follow the track 
of thread T.sub.1 in fragment 348, which would carry it forward by an 
amount equal to the pitch between thread T.sub.1 and T.sub.2. Since, 
however, the screw will only move forward by the effective pitch, i.e., 
the pitch between threads T.sub.4 and T.sub.5, thread T.sub.2 can only 
move forward by the same amount. This causes thread T.sub.2 to pull back 
against the surrounding bone and creates a leading gap in front that 
thread. Similarly, thread T.sub.3 will attempt to move into the position 
of thread T.sub.2, but will be held back from moving as far forward as its 
pitch would indicate, thus creating a leading gap as thread T.sub.3 is 
pulled back against the surrounding bone. Behind the effective pitch 
point, thread T.sub.6 will attempt to move into the prior position of 
thread T.sub.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.sub.8. With the screw positioned as shown, the 
effective pitch point is at approximately thread T.sub.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.degree. 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.degree.. 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.degree. 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.degree. 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.degree. 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. 
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. 
Having illustrated and described the principles of our invention in a 
preferred embodiment thereof, it should be readily apparent to those 
skilled in the art that the invention can be modified in arrangement and 
detail without departing from such principles. We claim all modifications 
coming within the spirit and scope of the accompanying claims. 
A screw according to the present invention particularly adapted for use in 
ankle fusions is shown generally at 410 in FIGS. 21a-b. Screw 410 includes 
a root port 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 allow 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 heal 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. 21a-b, screw 410 preferably is cannulated to 
provide improved stability during installation.