Bone compression plate

A bone compression plate is provided with several sliding slits for countersunk bone screws having ball heads in the longitudinal dimension of the plate. Each sliding slit is countersunk in its end area opposite the fracture, the depth of which countersinking in the plate determines the final axial position of the screwhead. That depth is greater than that of the rest of the sliding slit. The bearing surface of the countersinking for the screwhead extends over an area which is included within an angle of more than 180.degree.. It is thus possible, on the one hand, to move the plate with respect to the pieces of fractured bone to obtain a pressing together of the two pieces of bone when the bone screw has not been screwed tight. On the other hand, it is possible to fasten the plate immovably to one piece of the fractured bone by inserting a bone screw into the countersink slit. Pressing of the pieces of bone fragment together can be accomplished automatically by means of chamfering provided on the ball head of the countersunk bone screw in the area at the opposite end of the sliding slit. Deeper countersinking provided in the end area containing the original countersunk depression makes possible the positioning of the countersunk bone screw at an oblique angle.

BACKGROUND OF THE INVENTION 
The invention is concerned with a bone compression plate. Such artificial 
bone compression or pressure plates used in osteosynthesis have either 
round holes formed with a spherical seat corresponding to the ball head of 
a screw or holes shaped like slits, which extend longitudinally along the 
plate and make a sliding movement of the plate possible. Screw holes which 
are countersunk for bone screws have been known for a long time and are 
used to ensure an osseous contact in the fracture area under compression. 
For the initial generation of pressure, that is, to press the two pieces 
of bone of the fracture together, a special clamping device, which is 
applied to the compression plate from the outside is used. This clamping 
plate is removed after the compression plate in contact with the pieces of 
fractured bone, has been screwed tight. In another embodiment, compression 
plates, in accordance with Swiss Pat. No. 462 375, are used. In this case, 
the countersunk holes (which correspond to the ball head of the screw as a 
horizontal half-cylinder and constitutes the seat for the screw) are 
provided in every sliding slit at the end of the slit, which is opposite 
the fracture. Chamfering is formed in the slit by a beveled half-cylinder 
which is inclined in such a way that when the screw head is sunk at that 
end of the sliding slit, the plate is, of necessity, pushed away from the 
location of the fracture in the longitudinal direction. If the plate is 
fastened by screws to the second of the two pieces of bone which are 
separated by the fracture, a longitudinal pushing of the fragment of the 
fracture, and consequently an automatic pressing of the two pieces of 
fractured bone against each other results from this longitudinal pushing 
of the plate. For the purpose of fastening the plate to the fractured 
bone, screws are sunk in a so-called neutral position in other slit-shaped 
holes in the plate to prevent a pulling apart of the fracture, that is, 
they are sunk at the place in the slit-shaped hole where the said inclined 
and horizontal half-cylinders run together. 
In practice, both compression plates having round holes and those with 
slit-shaped holes, particularly those formed with chamfering to obtain the 
self-tightening effect which has been described above, are used. Depending 
upon the nature of the fracture, the surgeon will normally prefer a 
round-hole fastening having the advantage of a rigid connection between 
the screw, the bone and the plate and little necessary pressure of the 
plate on the bone. In other cases, he will use a sliding-slit fastening 
having the advantage of an automatic compression and the avoidance of any 
interference with the closing of the gap of the fracture. This choice will 
accept either the disadvantage of an interference effect in the case of 
round-hole fastening and a discontinuation of the compression, or even a 
pulling apart if the screw is placed eccentrically in the hole in the 
plate, in contrast with the disadvantage of a pulling apart in the case of 
a sliding-slit fastening if the screw is not placed precisely in the 
neutral position which could result in a loosening of the screw in the 
course of time. 
To be able to use either screw holes of round or slit-shaped design, as 
desired, for optimum immobilization of the area of the fracture, 
compression plates of both kinds must therefore be available. Aside from 
the increased expense connected with this method, the application of the 
compression plate is unnecessarily made more difficult in many cases if 
the surgeon can only decide the most advantageous configuration of holes 
at the time of the insertion of the screws or, respectively, the 
pre-drilling of the bone part after bending the plate conformingly. 
Making some of the screw holes in the compression plate slitlike in shape 
and others round, so that the compression plate is immovably fastened to 
the pieces of fractured bone after the final process of screwing the 
countersunk bone screws into the round holes is, in fact, known from Swiss 
Pat. No. 515 032. However, such compression plates incur the disadvantage 
that they are not universally usable any more because, depending upon the 
nature of the pieces of bone and the fracture, the slitlike holes and 
likewise the round holes have to be located at different places along the 
compression plate. 
It is the object of the present invention to provide a compression plate of 
the type mentioned above, which contains exclusively slit-like holes for 
the corresponding countersunk ball-headed bone screws, but which makes 
possible an immovable fixing of the plate in the manner of screws of the 
round-hole type. 
As explained below by means of exemplary embodiments of the present 
invention, with the aid of the drawings, the compression plate, according 
to the present invention, combines the advantages of plates with round as 
well as slit-like holes, avoids their disadvantages to a large degree and, 
additionally, has the important advantage of universal application with 
the possibility of being manufactured in a single model with no additional 
cost, with the exception that differing dimensions in length and width of 
the plate, necessarily and obviously, will be required.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 illustrates in perspective, a sliding or longitudinal slit 1 of a 
section cut along with middle line of a bone compression plate 2. Plate 2 
is provided with several sliding slits running, in a known manner, 
longitudinally through the plate on both sides of the middle of it or 
offset alternately with respect to its middle line. The sliding slit, 
which is shown, is intended to receive a bone screw having a spherical 
bearing surface and, also in a known manner (cf Swiss Pat. No. 462 375), 
has a chamfering 3 at one end, that is, at the end away from the fracture. 
The chamfering 3 serves, as was described above, to tighten the bone screw 
which is partially screwed into one of the two pieces of bone which are to 
be pressed against each other. This displaces the plate 2 which is 
fastened to the other piece of bone in the longitudinal direction with 
respect to the piece of bone in question and, in that way, functions to 
press the two pieces of bone against each other without the use of any 
external clamping devices. In FIG. 2, sliding slit 1 of FIG. 1 is 
represented in plan view. 
At its other end, which is the one opposite the fracture, the sliding slit 
1 has a tapered bearing and limit-stop surface 4 which, in contrast with 
the limit-stop surfaces of the sliding slits known from the Swiss Pat. No. 
462 375, for example, is provided with tapered countersinking 5 which is 
lower than the area of contact for the bone screw in the middle part of 
the sliding slit 1. Consequently, the bearing surface of the bone screw 
contacts an area which is included within an angle of more than 
180.degree. around the circumference. In FIG. 1, the edge of the slit 1 
together with its middle part forms the tapered countersinking 5 which is 
represented as the line 6. Thus, the bearing surface 4 extends over an 
area b included within an angle of more than 180.degree. between the two 
lines or edges 6, as can be seen in FIG. 2. 
The producing and shaping of the sliding slit, depicted in FIG. 1, and 
particularly its countersinking 5, with the bearing surface 4, can be 
explained by reference to the longitudinal section shown in FIG. 3. The 
oblong hole opening of the sliding slit is first milled out using a 
cylinder milling cutter. Then the chamfering 3, which is straight in cross 
section, is milled out of the plate 2 to a certain depth 8, utilizing a 
slanting thrust of the tool, by means of a spherical cutter, which is 
indicated by a dot-dash line 7. Next, the spherical cutter is thrust 
horizontally over a short stretch, shown to the right in FIG. 3, and is 
then removed. Finally, the countersinking 5 of FIG. 1, provided with the 
bearing surface 4, is produced using a countersinking milling cutter, 
which is indicated by a dot-dash line 9. As can be seen from the boundary 
lines of the chamfering 3, which are shown in perspective in FIG. 1 and 
from the front in FIG. 3, the middle part of the sliding slit, along the 
line 10, which follows them and the bearing surface 4 of the 
countersinking 5, which closes off the sliding slit, the bearing surface 
of the countersinking 5 is deeper than the bearing surface in the middle 
part of the sliding slit by a distance d. A lower countersinking 11 is 
provided to make possible the positioning of a bone screw in the 
countersinking 5 at an oblique angle, as is explained in greater detail in 
the illustration of FIG. 8. 
In FIG. 4, a bone screw 13 provided with a spherical head 14, which is 
completely countersunk in the countersinking 5 and screwed into the piece 
of bone 12, is shown. Good immobilization of the fracture with a miminum 
amount of pressure exacted by the plate 2 on the piece of bone 12 is 
obtained using this embodiment when compared with that attained using 
plates provided with round holes. Other possible uses and advantages of 
plates possessing the sliding slit, as shown in FIGS. 1 through 4, will be 
described below, referring to the illustrations in FIGS. 5 through 8. 
In FIG. 5, another embodiment of the sliding or longitudinal, slit 1 of the 
compression plate 2 is shown in perspective, in which the end opposite the 
fracture has a spherical bearing and limit-stop surface 4' which is formed 
with tapered countersinking 5'. The countersinking 5' also is formed 
deeper than the areas of contact for the bone screw in the middle part of 
the sliding slit 1'; consequently it embraces the bearing surface of the 
bone screw over an area included within an angle of more than 180.degree. 
around the circumference, similar to that shown in FIG. 2 for the tapered 
countersinking 5. In FIG. 5, the edge which forms the tapered 
countersinking 5' with the middle part of the sliding slit 1' is 
represented as the line 6'. 
The producing and shaping of the sliding slit 1' of FIG. 5 is shown in FIG. 
6 in the same way shown in FIG. 3, the opening of the sliding slit first 
being milled out using a cylinder milling cutter. Then chamfering 3', 
which is curved concavely in cross section, is milled out of the plate 2 
to a certain depth 8, again using a variably slanting thrust of the tool, 
by means of a spherical cutter which is indicated by a dot-dash line 7. 
Next the spherical cutter is thrust horizontally over a short stretch, to 
the right in FIG. 5, and is then removed. Finally, the countersinking 5' 
is milled out by exercising a vertical thrust with the same spherical 
cutter, which is indicated by a dot-dash line 15 at a point which is 
displaced horizontally. The bearing surface 4' is produced by that means 
and in this manner. As can be seen from the boundary line 10 of the 
bearing surface, in the middle part of the sliding slit 1', and the 
horizontal section of the bearing surface 4' in FIG. 6, the bearing 
surface 4' of the countersinking 5' is deeper than the bearing surface 
for the bone screw, which is indicated by the line 10 in the middle part 
by a certain distance. Once again, a lower chamfering of the 
countersinking 5' is designated by the numeral 11. 
The curved chamfering 3', shown in FIGS. 5 and 6, results in a change in 
the longitudinal force along the line of compression, that is, the line 
along which the plate 2 moves as a result of the countersinking of the 
bone screw, which is required for a specific pull on the part of the plate 
2. This pull is naturally constant in the case of the straight chamfering 
3 of FIGS. 1 and 3. Because of the concave chamfering 3' of FIGS. 5 and 6, 
the required longitudinal force of the bone screw is at first greater than 
that of the embodiment in FIGS. 1 and 3, but this force becomes smaller 
than that of FIGS. 1 and 3 as the compression travel increases. However, 
since the plate 2 must exercise less pull when compression begins than at 
the end of compression, the embodiment possessing the chamfering 3' shown 
in FIGS. 5 and 6 in which a high pulling power of the plate 2 can be 
produced with relatively little longitudinal force being exerted by the 
bone screw. 
In FIG. 7, the bone screw 13 having a spherical head 14, which is 
completely countersunk in the countersinking 5' and screwed into the piece 
of bone 12, is shown in the same way as is shown in FIG. 4. 
Other details for using the disclosed compression plate embodiments are as 
follows. It has been mentioned above that the bone screw 13 can be sunk 
into the piece of fractured bone 12 through the sliding slit 1 or 1', in a 
known manner, in such a way that, when the screw is screwed in, its ball 
head 14 slides along the half-cylindrical or curved chamfering 3 or 3' and 
then rests on the half-cylindrical bearing surface indicated by the line 
10 (FIGS. 3 and 6). As a result, the plate 2 is moved to the left with 
respect to the piece of bone 12 and by that means pulls the other piece of 
bone (not shown) to the piece of bone 12 to close the gap of the fracture. 
If that movement to close the gap of the fracture, which is known to be 
approximately 1 mm, is not enough, an aftercompression can be produced in 
another sliding slit in the same way. For the final fastening of the plate 
2 to the piece of bone 12, a bone screw is not put into the so-called 
neutral position, that is, at the place where the half-cylinder of the 
chamfering 3 or 3' and the horizontal half-cylinder (Line 10) run 
together, in another sliding slit 1 or 1', but is placed in the 
countersinking 5 or 5' as shown in FIGS. 4 and 7. As a result, a good 
immobilization of the pieces of fractured bone is obtained with the 
exertion of minimum pressure. 
This same compression plate 2 can be used by the surgeon during the 
operation if it seems desirable in view of the conditions of the fracture. 
However, a round-hole plate can also be used in the same way, using an 
external clamping device, which is subsequently removed. This clamping 
device is again used to press the pieces of bone together, the screws 
being inserted into the countersinkings 5 or 5' to fasten the plate. 
Furthermore, the compression plate described permits the practice of a 
sliding fastening process taking advantage of the function of the sliding 
slit 1 or 1'. Final fastening is accomplished by means of screws which are 
introduced into the countersinkings 5 or 5', even if an external clamping 
device is preliminarily used until the pieces of bone are definitively 
secured in an immobile position. 
When a bone screw 13 is inserted in the tapered countersinking 5' near the 
fracture, a drawing apart of the fracture can be prevented automatically 
by taking into consideration normal manufacturing tolerances for the 
spherical screw head 14 and the tapered countersinking 5'. For this 
purpose, the screw is inserted eccentrically into the tapered 
countersinking 5' of FIGS. 5 and 6, in the sliding slit heading in a 
direction away from the fracture, utilizing a drilled jig which fits into 
the countersinking 5'. As a result, minimal compression in the direction 
of the fracture, and consequently maintenance of the static compression 
stress of the bone corticalis, can be achieved. 
As can be seen from FIG. 8, there is the additional possibility of 
obtaining a universal positioning of the screw at an oblique angle by 
introducing the screw 13 centrally into the spherical countersinking 5' of 
the plate 2 which is provided with the lower chamfering 11. The advantage 
of this procedure is that a supporting or self-tightening functioning is 
obtained and that the point of the screw avoids an area of a fracture 
involving the bone fragments. 
Thus, the present invention provides a compression plate which is uniformly 
furnished with holes of the same kind, that is, the sliding slits 1 or 1' 
which have been described above. This leaves the surgeon free to chose 
whether or not to use every hole as a sliding slit and/or fastening hole 
in the same way as is done with a countersunk round hole. Furthermore, the 
construction of the compression plate, in accordance with the invention, 
does not result in an increase in the cost of production since the 
countersinking feature which has been described can be produced in the 
same working operation, as heretofore, i.e., on a numerically-controlled 
milling machine, for example.