Kit of parts for a modular femur head prosthesis, in particular, a reoperation prosthesis, and a femur head prosthesis from such a kit of parts

The kit of parts comprises a distal shaft part (2) and a neck part (3) which can be equipped with a joint ball (10), with the neck part being supported on the shaft part (2) by approximately rectangular shaped supporting surfaces (2c) and (3c) positioned transverse to the longitudinal axis (L) of the shaft part (2). The shaft part (2) and the neck part (3) are provided with prismatic guide surfaces (2d) and (3d) brought together in a form-fit and orientated parallel to the longitudinal axis (L), and are rigidly connected to each other by an extensible shaft screw (11) which can be elastically prestressed to carry a predetermined bending moment. The guide surfaces (2d) and (3d) are formed at a spigot extension (2b) of the shaft part (2) and at a complementary recess (3b) of the neck part (3), respectively. This design allows a form-fitted centering and accurate positioning of the neck part (3) on the shaft part (2) by the guide surfaces (2d, 3d) and, independent therefrom, a force-locked connection between the neck part (3) and the shaft part (2) by way of the supporting surfaces (2c, 3c) which can be clamped against one another with the bias force of the extensible shaft screw (11).

BACKGROUND OF THE INVENTION 
The invention relates to a kit of parts for a modular femur head 
prosthesis, in particular a reoperation prosthesis comprising a distal 
shaft part to be anchored in a femur bone and a neck part which can be 
fixed to the proximal end portion of the shaft part by means of a screw 
connection, and can be equipped with an artificial ball. 
Moreover, the invention relates to a femur head prosthesis assembled from 
such a construction kit. 
A femur head prosthesis of the named kind is known from EP 0 399 920 A1 and 
comprises a shaft part having an outside thread and a proximal extension 
in the shape of a truncated cone and a neck part which can be mounted on 
the shaft part and which has a slip-on mandrel for the joint ball, with 
the mandrel being inclined with respect to the longitudinal axis of the 
shaft part. The neck part is held so that it can be clamped against the 
extension of the shaft part by a securing screw which can be screwed into 
an axial bore of the shaft part or of an insert part connected to it. The 
shaft part of the known endoprosthesis is provided with tongue-like, 
outwardly deployable segments in a longitudinal section of its axial bore. 
The segments can be clamped radially outwardly against the piece of femur 
surrounding the shaft part, in each case relative to two neighbouring 
fixed segments of the shaft part, by an adjusting means which can be 
introduced into the bore in the axial direction and which has wedge-shaped 
supporting surfaces. The main loading forces to be transferred to the 
femur are in each case respectively introduced into the femur shaft 
prosthesis by way of the joint ball which is laterally displaced with 
respect to the longitudinal axis of the shaft part. Accordingly, femur 
shaft prostheses of the type mentioned are stressed in each case by 
relatively large dynamic forces and bending moments. With the previous 
endoprostheses of the type mentioned, dynamic stresses or loads of this 
kind can lead to a loosening of the screw connection between the neck part 
and the shaft part and/or to a loosening of the anchoring of the shaft 
part in the femur bone and can, as a result, lead to high local stresses 
of the bone tissue, for example, in the region of a dowel-like spreading 
of the shaft part. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a kit of parts for a femur shaft 
prosthesis of the named kind which is improved in this respect and has a 
simple and robust method of construction, the kit of parts comprising a 
few favourably stressed components and ensuring a permanent connection 
between the neck part and the shaft part, as well as between the neck part 
and the femur, which remains unaffected by the dynamic stresses or loads 
in the implantation region. 
According to the invention, this object is achieved in that the proximal 
end piece of the shaft part and of the neck part are provided with 
supporting surfaces which can be braced against one another and are 
positioned transverse to the longitudinal axis of the shaft part, and also 
with guide surfaces orientated in the direction of the longitudinal axis 
which can be brought together in a form-fitted manner in a defined 
coupling position of the two parts; and in that the screw connection 
includes an extensible shaft screw which can be elastically prestressed 
with a predetermined bias force. 
The kit of parts formed according to the invention allows, on the one hand, 
a functional separation of the form-fitted, correct angle positioning of 
the neck part on the shaft part and, on the other hand, a force locked 
connection between the neck part and the shaft part inserted in the femur. 
Accordingly, the supporting surfaces of the neck part and the shaft part 
can be dimensioned for transmitting the clamping force of the extensible 
shaft screw corresponding to a predetermined greatest bending moment, and 
the guide surfaces can be dimensioned for transmitting relatively smaller 
transverse forces. A further advantage of the embodiment according to the 
invention is based on the fact that because the supporting surfaces of the 
neck part and the shaft part contact one another with a large surface 
pressure, the emergence of abraded particles arising between these parts 
can be prevented. The kit of parts according to the invention may comprise 
relatively few components which can be taken from a store comprising a 
plurality of shaft parts with different cross-sections and/or varying 
longitudinal dimensions, e.g., graded in a centimetre fashion, and a 
plurality of neck parts and extensible shaft screws, e.g. realized in two 
different dimensions, such that the kit of parts can be put together as a 
femur head prosthesis according to the given anatomical circumstances.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The femur head prosthesis according to FIGS. 1 to 5 is part of a 
reoperation prosthesis which is intended for use with patients as a second 
or third prosthesis, by whom, for some reason, a corresponding, earlier 
implanted endoprosthesis must be reoperated and replaced, for example, due 
to loosening of the embedding of the prosthesis in the bone tissue. The 
femur head prosthesis as represented, includes a distal shaft part 2 to be 
implanted in a femur bone 1 and a neck part 3. The distal shaft part 2 is 
to be embedded in the femur by way of bone cement, or as shown, without 
cement. The neck part 3 is made of a metal well-tolerated by the body, 
e.g. titanium, and can be mounted on the proximal end piece 2a of the 
distal shaft part. The neck part 3 is provided as a replacement-for a 
missing piece of bone which, in the example as shown, is the proximal end 
piece of the femur bone 1 which, especially in reoperation cases, must 
often be partly or, as shown, fully removed surgically, as a result of 
decomposition manifestations and demineralisation of the bone tissue. The 
shaft part 2 comprises a shaft body, conically tapered from its end piece 
2a towards its distal end piece (not shown), which is to be inserted in 
the femur bone 1 in the direction of its longitudinal axis and a 
spigot-like extension 2b formed on the end piece which projects into a 
complementary recess 3b formed in the neck part 3. The shaft body 2 is 
provided over at least a part of its longitudinal extent with a plurality 
of longitudinal grooves 5 and between them a plurality of knife-like 
longitudinal ribs 6 which penetrate the bone tissue. On account of this 
the shaft body is held secure against turning in the femur bone 1. 
The neck part 3 includes a main body corresponding to the surgically 
removed end of the femur bone 1 having a connecting part 3a to be attached 
onto the end piece 2a of the shaft part 2, and a mandrel 7 formed at the 
side and positionable so that it projects laterally with respect to the 
longitudinal axis L of the shaft part 2, with the longitudinal axis H of 
the mandrel or spigot 7 being inclined at an angle .alpha. with respect to 
the longitudinal axis L. The slip-on mandrel 7 ends in a cone 8 onto 
which, in known manner, a plastic ball 10 of a joint, represented by the 
chain-dotted line, can be mounted or plugged into position, with the 
plastic ball being laterally displaced with respect to the longitudinal 
axis. The end piece 2a of the shaft part 2, and at least the connecting 
part 3a of the neck part or, as shown, the whole main body of the neck 
part are each provided with a cross-section which is at least 
approximately rectangular in shape and with corresponding rectangular 
shaped supporting surfaces 2c and 3c which can be braced against each 
other and are positioned transverse to the longitudinal axis L. Each 
supporting surface 2c and 3c is arranged with a greater length dimension A 
directed in the lateral direction and a smaller breadth dimension B 
directed in the transverse direction. Likewise, the extension 2b of the 
shaft part 2 and the recess 3b of the neck part 3 are each provided with a 
corresponding approximately rectangular shaped cross-section and with side 
surfaces 2d and 3d which can be brought together in a form-fitted manner 
in the direction of the longitudinal axis L. In this way the shaft part 2 
and the neck part 3 can be coupled with each other at a position defined 
with respect to the location of the joint ball 10. 
The neck part 3 is fixed on the shaft part 2 by means of a screw 11 with an 
extensible shaft. The screw 11 comprises a head 12 to be supported 
countersunk in a recess 9 of the neck part 3, and a screw shaft 13 which 
extends through a bore 14 of the neck part 3 and an axial bore 15 of the 
shaft part 2. The screw shaft 13 includes a threaded section 13a to be 
screwed into a threaded bore of the shaft part 2 and two guide portions 
13b and 13d thicker than said threaded screw section arranged at a 
distance from each other, as well as, an elastically prestressable, 
extensible section 13c connecting these two guide portions and having a 
cross-section, which, in known manner, is smaller than the core 
cross-section of the threaded section 13a. Cylindrical locating surfaces 
are formed on the guide portions 13b and 13d, and are intended to 
cooperate with corresponding fitting surfaces formed in the bores 14 and 
15. According to FIG. 2, the guide portion 13d is formed close to the head 
13 and cooperates with a bore section 14d of the neck part 3 while the 
guide portion 13b extends over the separating line between the neck part 3 
and the shaft part 2 and cooperates with a bore section 14b of the neck 
part 3 and a bore section 15b of the shaft part 2. 
By tightening the extensible shaft screw 11, the neck part 3 and the shaft 
part 2 are tensioned against one another with a bias clamping force which 
can be chosen as the peak force which corresponds to a predetermined 
greatest bending moment, introduced by a main load force acting by way of 
the joint ball 10 of a joint and applied at the neck part 3 in a laterally 
offset position with respect to the longitudinal axis L. The respective 
bias force is determined by the tightening torque for turning the 
extensible shaft screw 11 which can be defined, in a known manner, as a 
predetermined upper value on a torque wrench. The supporting force 
corresponding to the bias force is transmitted exclusively by the 
rectangular shaped supporting surfaces 2c and 3c surrounding the extension 
2b and the recess 3b which are appropriately dimensioned to carry the 
dynamic forces and bending moments resulting from the main load. 
When placing the neck part 3 onto the shaft part 2 these parts are centered 
and secured against turning by means of the guide surfaces 2d and 3d of 
the extension 2b and recess 3b respectively. An additional fine centering 
of the neck part 3 and of the shaft part 2 is achieved by the extensible 
shaft screw 11 by way of the cooperating locating surfaces of the guide 
portions 13b and 13d and the bore sections 14b, 15b and 14d respectively 
which, at the same time, ensures a guidance of the extensible section 13c 
of the reduced shaft screw 11 which is free of bending stresses. With the 
described arrangement, which enables a functional separation of the parts 
transmitting the longitudinal forces and the parts transmitting the 
transverse forces, a dimensionally stable connection which is rigid in 
bending is obtained between the rigid neck part 3 and the rigid end 
section of the shaft part 2 widened for the introduction of the forces to 
be transmitted in the lateral direction. With this embodiment a safe 
transmission of the dynamic forces and bending moments acting between the 
joint ball 10 and the shaft part 2 is ensured, even with large cyclical 
stresses. 
The guide surfaces 2d and 3d brought together in a form-fit within the 
recess 3b are sealed outwardly by the supporting surfaces 2c and 3c 
surrounding them which are pressed together under a bias load such that, 
should the situation arise, abraded particles can be safely prevented from 
entering the surrounding tissue. On account of the supporting surfaces 2c 
and 3c of the shaft part 2 and the neck part 3 formed with a rectangular 
shaped hollow cross-section optimised to carry the greatest bending 
moment, together with the surface portions in the lateral edge parts which 
determine the moment of resistance of the cross-section, the loading which 
occurs on transmission of the pressure forces can, at the same time, be 
kept to surface pressure within predetermined limits. In order to obtain a 
design suitable for varying anatomical circumstances and which can be used 
as universally as possible, the respective supporting surfaces 2c and 3c 
can each have a length dimension A equal to around 20 mm to 30 mm and a 
breadth dimension B equal to around 10 to 15 mm. The extension 2b and the 
recess 3b each can have a corresponding measurement, e.g. a length 
measurement C equal to around 10 to 20 mm and a breadth measurement D 
equal to around 10 to 15 mm. It goes without saying that other dimensions 
for the end piece 2a and the connecting part 3a are possible which are 
matched to the respective anatomical circumstances. 
As a substitute for a missing, surgically removed piece of a bone, front 
and back jaw-like filling bodies 17 and 18 can be fitted to the side walls 
of the neck piece 3 in accordance with the representation. These bodies 
can, as shown, be realised with bevelled edge parts 19 and varying 
decreasing thicknesses with respect to the neck piece 3, and can also be 
provided with means, not shown, for attaching muscles and ligaments 
thereon. The filling bodies 17 and 18 which can consist of metal or, as 
shown, of a synthetic material compatible with the body, are each equipped 
with a semicircularly shaped collar portion 20 which can be attached on 
the proximal end piece of the neck part 3 and introduced into its recess 
9. Moreover, the filling bodies 17, 18 can be equipped with a groove-like 
guideway 21, with which they can be pushed onto a guide section, formed on 
the side wall of the neck piece 3 in the form of a rail-like cam or track 
16, and slid onto the relevant side wall in the direction of the 
longitudinal axis L until the collar portion 20 latches into the recess 9. 
The cam 16 and the guideway 21 can each be realised with a rectangular 
cross-section or, as shown, with a cross-section in the form of a 
dovetail. According to another embodiment, the guideways 21 can be formed 
on the neck piece 3 and the corresponding guide sections formed on the 
filling bodies 17, 18. 
For the axial securing of the filling bodies 17 and 18 which are centered 
by the shoulder part 20 and guided sideways by the cams 16, a cap 22 can 
be provided which covers the recess 9. The cap has a threaded stem 23 
which can be screwed into the screw head 12 and is tensioned against the 
collar portion 20. The cap 22 prevents at the same time the growth of 
tissue into the recess 9. The design of the filling bodies 17 and 18 and 
of the cap 22 as described, permits the operating surgeon to select the 
filling bodies 17, 18, corresponding to the respectively prevailing 
anatomical circumstances, from a supply of filling bodies made available 
in varying designs. It also permits the fixing of the filling bodies 17, 
18 onto the already implanted neck piece 3 during an advantageous later 
stage in the operation. 
An embodiment is also possible in which only one of the filling bodies 17 
and 18 is fixed on the neck piece 3. Moreover, a suitably formed filling 
body can also be fixed to the lateral side wall of the neck piece 3. The 
embodiment according to FIG. 6 substantially corresponds to the embodiment 
described above, with the neck piece 3 having a smaller overall height and 
the extensible shaft screw 11 having a correspondingly shorter screw shaft 
13' than in the example of FIGS. 1 and 2. The screw shaft 13' has two 
elastic prestressable extensible portions 13c and two guide portions 13e 
and 13f, with the guide portion 13e connected to the threaded portion 13a 
cooperating exclusively with the bore section 15b of the shaft part 2 and 
with the guide portion 13f positioned between the two extensible portions 
13c extending over the partition line separating the neck piece 3 and the 
shaft piece 2 and cooperating with the upper end of the bore section 15b 
of the shaft part 2 and the lower end of the bore section 14b of the neck 
piece 3. In this embodiment, a fine centering of the shaft part 2 and the 
neck part 3 is achieved by the guide portion 13f as well as a stressing of 
the extensible shaft screw 11 which is substantially free from bending. 
Numerous modified embodiments of the invention are possible. Instead of the 
illustrated embodiment having approximately rectangular support surfaces 
2c and 3c, which are especially suited for the absorption of bending 
stresses, an embodiment is also possible having corresponding or, e.g., 
approximately circular or oval supporting surfaces. Similarly, instead of 
the illustrated rectangularly shaped projection 2b and the complementary 
recess 3b, a corresponding projection, e.g., a cylindrical or spherical 
projection and a complementary recess can be provided, as well as a 
centering element arranged outside of this recess. Such a centering 
element permits a corresponding correct angle positioning of the neck part 
3 on the shaft part 2. 
To summarise, the invention can be described as follows: 
The kit of parts comprises a distal shaft part 2 and a neck part 3 which 
can be equipped with a joint ball 10, with the neck part being supported 
on the shaft part 2 by approximately rectangular shaped supporting 
surfaces 2c and 3c positioned transverse to the longitudinal axis L of the 
shaft part 2. The shaft part 2 and the neck part 3 are provided with 
prismatic guide surfaces 2d and 3d brought together in a form-fit and 
orientated parallel to the longitudinal axis L, and are rigidly connected 
to each other by an extensible shaft screw 11 which can be elastically 
prestressed to carry a predetermined bending moment. The guide surfaces 2d 
and 3d are formed at a spigot extension 2b of the shaft part 2 and at a 
complementary recess 3b of the neck part 3, respectively. This design 
allows a form-fitted centering and accurate positioning of the neck part 3 
on the shaft part 2 by the guide surfaces 2d, 3d and, independent 
therefrom, a force-locked connection between the neck part 3 and the shaft 
part 2 by way of the supporting surfaces 2c, 3c which can be clamped 
against one another with the bias force of the extensible shaft screw 11.