Apparatus for producing a conical surface on a bone

An apparatus for producing a conical, outwardly tapering surface on a bone, especially on a cylindrically pre-worked femur head of a human hip joint. The apparatus includes a drive shaft having a free end. A knife-type cutting device is arranged at the free end of the drive shaft, wherein the cutting device has an inner edge. An adjusting device is included for moving the inner cutting edge of the cutting device from an initial position essentially parallel to the longitudinal axis of the drive shaft to a position forming an acute angle relative to the longitudinal axis. The cutting edge of the cutting device is radially outwardly offset relative to the longitudinal axis of the drive shaft. In the initial position, the extent of radial offsetting is at least as great as half the diameter(r) of the bone section to be treated.

FIELD OF THE INVENTION 
The invention relates to an apparatus for producing a conical surface which 
tapers from the outside, on a bone, especially on a cylindrically 
pre-worked femur head of a human hip joint. 
BACKGROUND OF THE INVENTION 
When endoprostheses are implanted, especially shell-shaped prostheses or 
parts of prostheses, respectively, it is necessary, especially in hip 
joints, for creating suitable fitting areas, to prepare the existing 
natural shape of the bone section by a chip-removing process. This is done 
so that the bone or bone section, respectively, which is provided with the 
prosthesis or the part of the prosthesis, is geometrically adapted to the 
shape of the prosthesis or the part of the prosthesis. 
To attach a shell prosthesis on the femur head of a hip joint, first it is 
necessary to provide a cut, which is usually circular, on the front side 
of the femur head, which cut is adapted to the inner side of the 
prosthesis. 
Afterwards, the femur head has to be worked at its outer side, which is 
adjacent to the front side, in order to create a suitable fitting area for 
the jacket of the shell prosthesis. 
For shell prostheses which are known per se, it is sufficient when the 
above-mentioned outer side of the bone is cylindrically cut. However, to 
improve these known shell prostheses having cylindrical jackets, there 
have been developed shell prostheses whose jacket surfaces are not 
cylindrical, but conically tapered towards the free edge. Accordingly, for 
mounting such a shell prosthesis, it is necessary to provide a conical 
area which tapers from the outside on the bone or bone section to be 
treated. In the case of a human hip joint, this is the femur head which 
can be pre-worked cylindrically. 
Such a treatment obviously results in considerable problems because the 
fitting areas, which had been created in advance, may not be damaged 
during this treatment. Moreover, the basic principle has always to be 
adhered to that only as little tissue as possible should be removed in 
order to reduce the extent or seriousness of the operation and not to lose 
the advantages which are achieved with shell-shaped prostheses as compared 
to total-endoprostheses. 
A primary object of the present invention is to create an apparatus of the 
type described with which a conical area which tapers from the outside, is 
produced on a bone or bone section, especially the femur head of a human 
hip joint, without damaging the rounded fitting area (usually prepared in 
advance) on the front side of the bone. The conical area which is to be 
produced must reliably be axis symmetrical in order to guarantee an 
optimum adaptation to the geometric form of the existing shell prosthesis 
and, thus, to use only as little amount of bone cement as possible. 
Removal of portions of tissue, whose removal is not necessary, must be 
avoided. Further, it should be possible to treat the bone as quickly as 
possible, in order to keep the time of the operation as short as possible. 
SUMMARY OF THE INVENTION 
According to the invention, a solution of this object resides in the 
provision of an apparatus which is characterized by a drive shaft at whose 
free end there is arranged a knife-type cutting means. The cutting means 
has an inner cutting edge which is radially outwardly offset to the 
longitudinal axis of the drive shaft, and which, by means of an adjusting 
device must be moved from an essentially parallel initial position to a 
position forming an acute angle relative to the longitudinal axis. The 
extent of the radial offsetting of the cutting edge in the initial 
position (and, preferably, also in the end position) is at least as great 
as half the diameter of the bone section which is to be treated. 
In a preferred further development of the present invention, the knife-type 
cutting means is mounted in an articulated manner on a supporting body 
which is arranged on the lower end section of the drive shaft. In this 
arrangement, the adjusting device acts on the end section of the cutting 
means which is provided above the hinge point. 
For a better understanding of the present invention, reference is made to 
the following description and accompanying drawings, while the scope of 
the present invention will be pointed out in the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 of the drawings shows, in solid lines, an apparatus 60 for producing 
a conical surface 61 on a bone or bone section 2, respectively, which, in 
this case, is the femur head of a human hip joint. 
Before using the apparatus 60, the femur head 2 has already been 
preliminarily worked by means of a suitable device, i.e., the head has on 
its front side a circularly rounded surface 62 and on its section, which 
is adjacent to the front side 62, a cylindrical area 63, as shown in FIG. 
1 with dash-dotted lines. 
Moreover, concentrically to the femur head 2, a blind end bore 64 with a 
diameter of 8 mm had been made by means of a shank cutter or twist drill, 
preferably guided by a Kirschner wire. 
The apparatus 60 has a drive shaft 67 which can be driven about its 
longitudinal axis 66 and on whose free end there are arranged two 
knife-type cutting means 68 whose radially inwardly arranged cutting edges 
69 are moved by an adjusting device, from an essentially parallel initial 
position, which is radially offset outwardly to the longitudinal axis 66, 
to a position forming an acute angle relative to the longitudinal axis 66. 
The extent of the radial offsetting a of the cutting edges 69, in the 
initial position, is at least the same size as and, preferably, a little 
larger than half the diameter r of the bone section 2 which is to be 
treated. This adjusting device is subsequently described in detail. 
Since the two knife-type cutting means 68, which are arranged diametrically 
opposed to each other and mirror-symmetrically with respect to the 
longitudinal axis 66, do not differ in principle from each other, only one 
of the cutting means 68 will be described. 
The cutting means 68 is attached in an articulated manner to a pot-shaped 
supporting body 71 which is arranged on the lower end section of the drive 
shaft 67 in such a manner that the adjusting device, which is to be 
described, can act on the end section 73 of the cutting means 68 arranged 
above the hinge 72. The attachment to the disk-shaped or pot-shaped 
supporting body 71 is accomplished by joint bolts, wherein the supporting 
body 71 is provided with respective recesses in which the cutting means 68 
are arranged. 
On the guiding end of the cutting means 68, there is an outer guide body 74 
which is circularly constructed and which is rotationally movably and 
axially immovably supported by a roller bearing on the upper end of 
portion 73. 
The guide body 74 cooperates with the adjusting device which is described 
in the following. 
The adjusting device consists, essentially, of an adjusting bash 77 which 
is arranged concentrically to the drive shaft 67 by means of a fine thread 
76. The outer diameter of the adjusting bush 77 increases continually from 
its lower end through a first axial longitudinal portion 78 which is 
conically constructed and, then, diminishes again through a following 
second axial longitudinal portion 79 which is also conically constructed. 
The lowest end section 81 of the adjusting bush 77 is cylindrically formed 
and has a diameter which is of such size that the cutting means 68 assumes 
an essentially parallel position when its upper guide end, which faces 
away from the cutting edge 69, i.e., its guide body 74, fits on the lowest 
end section 81 of the adjusting bush 77. This condition is shown in FIG. 1 
with dotted lines; however, in FIG. 1, the supporting body 71 with its 
cutting means 68, is not in the initial position, which is further up, but 
in the final position of the treatment, for clarity. This is, however, 
insignificant with respect to the position of the cutting means 68 
relative to the longitudinal axis 66. 
The diameter of the adjusting bush 77 on the upper end of the second axial 
longitudinal portion 79, or directly behind it, respectively, is of such 
size that the smallest distance of the cutting means 68 to the 
longitudinal axis 66 is larger than the largest radius r of the femur head 
2 which is to be treated or was treated. 
Between the second axial longitudinal section 79 of the adjusting bush 77 
and its end constructed as adjusting flange 82, there is arranged a 
section 83 which--slightly conically--ascends upwardly. 
The jacket surface of the adjusting flange 82 is roughened by edging or 
beading. 
The supporting body 71, which is pot-shaped and is open outwardly and whose 
inner front side corresponds with the form of the inner side of the 
prosthesis shell which is to be applied, is provided with a centering pin 
84 which protrudes downwardly. 
The manner of operation of the apparatus, according to the invention, is as 
follows: 
At the beginning of the treatment, the position of the adjusting bush 77 
relative to the drive shaft 67 is such that the guide bodies 74 of the 
knife-type cutting means 68 rest on the lowest end section 81 of the 
adjusting bush 77 which is cylindrically formed, as shown in FIG. 1 in 
dash-dotted lines. The adjusting flange 82 is arranged in the initial 
position, and is identified in FIG. 1 by 82', so that, in contrast to the 
simplified representation according to FIG. 1, the supporting body 71 is 
also actually shown in a higher position for reasons of clarity and is 
identified by 71'. Thus, the cutting means 68 are also arranged in a 
higher position by about a distance h, than is shown in FIG. 1. On the 
other hand, their position relative to the longitudinal axis 66 coincides 
with the positions shown, which is very important. That is, the cutting 
edges 69 are nearly parallel to the longitudinal axis 66, each radially 
offset by about a distance a. 
In this position, the centering pin 84 is then inserted in the already 
mentioned central bore 86, wherein the hand of the operating surgeon holds 
the apparatus 60 on the drive shaft 67 (and not yet on the adjusting 
flange 82). 
When the hand of the operating surgeon acts on the adjusting flange 82, 
there is accomplished a downwardly, radially directed adjustment of the 
adjusting bush 77, as a result of the fine thread 76 which is constructed 
as a left-hand thread and which connects the drive shaft 67 with the 
adjusting bush 77. Thus, the guide bodies 74 start to move upwardly along 
the conically-constructed, first axial longitudinal portion 78 of the 
adjusting bush 77. The upper end sections 73 of the cutting means 68 are, 
therefore, tilted about the hinge 72 outwardly and, accordingly, the lower 
lever section of the cutting means 68 (which include the cutting edges 69) 
are tilted inwardly. This leads to an engagement of the upper end sections 
73 with the cylindrical surface 63 of the femur head 2 and, therefore, 
they start to remove the tissue shown in FIG. 1 in dotted lines, i.e., the 
tissue between the cylindrical area 63 and the conical area 61 of the 
femur head 2. 
When the guide bodies 74 have reached the upper end of the conical first 
axial longitudinal portion 78 of the adjusting bush 77, as shown in FIG. 1 
with solid lines, the production of the conical surface on the femur head 
2 is ended; the surface tapers from the outside or widens outwardly. 
The apparatus 60, however, can obviously not be moved upwardly, because, 
otherwise, the cutting edges 69 would, then, obviously damage the tissue 
of the femur head 2 which is to be treated. 
One possibility for removing would be to stop the actuation of the drive 
shaft 67 and to upwardly screw the adjusting bush 77 in a direction 
reverse to that of the drive drive shaft 67. However, this would be 
relatively time-consuming. 
With the apparatus according to the invention, the releasing of the cutting 
means 68 is accomplished automatically, because the guide bodies 74 now 
operate, with further holding of the adjusting bush 77 on the adjusting 
flange 82 and the still operating drive shaft 67, over the second axial 
longitudinal section 79, which is conically constructed in the opposite 
direction. The cutting means 68, again, are swivelled around their hinge 
72, until the guide bodies 74 have reached the lower end section of the 
section 83 of the adjusting bush 77, i.e., a position in which the cutting 
edges 69 are, again, arranged almost parallel to the longitudinal axis 66 
and, in any case, are swivelled outwardly a sufficient distance so that 
the apparatus 60 can be moved upwardly without engaging the cutting edges 
69 with the femur head 2. 
FIG. 2 of the drawing, shows an embodiment wherein a bone which has been 
treated in the described way, is provided with a shell prosthesis. The 
shell prosthesis for the femur head 2 consists in the present case of a 
double shell, including an inner femur head shell 6 of plastic and an 
outer femur head shell 4 of metal. A guide pin 24 of the outer femur head 
shell 4 projects through a respective bore into the inner femur head shell 
6 and extends into the bore 86 of the femur head 2. 
The curvature of the inner femur head shell 6 on its inside is equal to the 
curvature of the rounded area 62 on the front side of the femur head 2 
and, therefore, is also equal to the curvature on the inner front side of 
the supporting body 71. 
It can be seen that the inner femur head shell 6 has a jacket 12 which runs 
corresponding to the conical area 61 created at the femur head 2 by means 
of the apparatus 60 and, therefore, guarantees an exceptional holding 
action. The possibility of attaching the inner femur head shell 6 to the 
femur head 2, created in this manner, is guaranteed by sufficient 
elasticity which, inter alia, can be created by a radial slot which does 
not negatively influence the positive locking after attachment. 
For the sake of completeness, it shall be mentioned that in the Acetabulum 
1 there is arranged, as counter-sliding surface for the outer femur head 
shell 4, an Acetabulum pan 3 of plastic. 
As described above, various features of the invention are most 
advantageous. In this regard, the adjusting device has, preferably, an 
adjusting bush which is arranged concentrically to the drive shaft and is 
axially movable relative to this drive shaft, wherein the outer diameter 
of the adjusting bush increases from its lower end section to a first 
axial longitudinal portion and diminishes to an adjacent second 
longitudinal portion, both axial longitudinal portions, preferably, being 
constructed conically. 
The lowest end section of the adjusting bush can have a diameter of such a 
size that the cutting means or, more exactly, that section which is 
provided with the cutting edge, assumes an essentially parallel position 
(initial position), when the upper guide end of the cutting means, which 
faces away from the cutting section, fits on the lowest end section of the 
adjusting bush. In this manner, first, the apparatus is attached to the 
bone without damaging the bone. 
In order to be able to remove the apparatus, after finished treatment, 
without further damaging the bone, the diameter of the adjusting bush is 
on the upper end of the second axial longitudinal section, preferably, 
having such a size that the smallest distance of the cutting means to the 
longitudinal axis is larger than the largest radius of the bone section to 
be treated. 
It is especially suitable when the apparatus has two cutting means which 
are located opposite each other, preferably diametrically opposite each 
other. In this manner, not only is a quicker treatment achieved but, also, 
a state of equilibrium, of the forces during treatment is obtained. 
For adjusting the cutting means during the treatment, the adjusting bush, 
preferably, engages the drive shaft by means of a fine thread, i.e., a 
thread with smaller thread pitch. It is, furthermore, preferred that the 
thread pitch is oppositely directed to the rotational direction of the 
drive shaft. 
Furthermore, the adjusting bush can be provided at its upper end section 
with an adjusting flange or the like, by means of which the adjustment is 
ultimately effected during the treatment. 
The supporting body is provided, in one embodiment, on its outer side with 
a centering pin which protrudes downwardly and which is otherwise suitably 
constructed in a pot-shape, i.e., partially hollowed form wherein the 
shape of its inner front side, directed outwardly, is suitably constructed 
in the shape of the inner side of the shell prosthesis which is to be 
attached, or of the outer side of the bone which is suitably already 
prepared. 
It can be seen that, with the apparatus 60 according to the invention, 
there can be made, in quick and safe manner, a tapered conical surface on 
the bone from without which surface is exactly axis symmetrical, so that, 
after treatment, there can be achieved extraordinarily exact fitting 
surfaces which are the prerequisite for a physiologically correct 
construction of the implant. 
Because of the automatic ending of the cutting process and of the centering 
of the apparatus, there is no removal of tissue which does not have to be 
removed per se, as is the case when there is free exposed treatment. The 
operating surgeon can do the adjusting extremely sensitively and can 
interrupt it at any time, which is accomplished by releasing an adjusting 
flange. In spite of these numerous advantages which include, in addition, 
a very short operation time, the apparatus according to the invention is 
extremely simply constructed and, therefore, is very inexpensive. Finally, 
it can be extremely simply disassembled into its individual parts when the 
apparatus must be cleaned. 
While the foregoing description and drawings represent the preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the true spirit and scope of the present invention. 
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LIST OF REFERENCE NUMERALS 
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1 Acetabulum 
2 Femur head 
3 Acetabulum pan 
4 Outer femur head shell 
6 Inner femur head shell 
12 Jacket (of 6) 
24 Guiding pin (of 4) 
60 Apparatus 
61 Conical surface (of 2) 
62 Sounded surface (of 2) 
63 Cylindrical surface (of 2) 
64 Blind-end bore (in 2) 
66 Longitudinal axis (of 67) 
67 Drive shaft 
68 Knife-type cutting means 
69 Cutting edges (of 68) 
71, 71' Supporting body 
72 Hinge point (from 68 to 71) 
73 Upper end section (of 68) 
74 Guide body (to 73) 
76 Fine thread 
77 Adjusting bush 
78 First axial longitudinal section (of 77) 
79 Second axial longitudinal section (of 77) 
81 Lower end section (of 77) 
82, 82' Adjusting flange 
83 Section (of 77) 
84 Centering pin (of 71) 
86 Central bore (in 2; for 84) 
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