Hip joint prosthesis

In patients with hypoplastic hip joints, hip joint prostheses with relatively thin and narrow shank blades must be used. For this reason, due to the torque effect of the force acting on the joint head, the danger exists that deformations of the shank blades may occur. This danger is counteracted in that the collar type projection forming the transition between the shank and the prosthesis neck extends in a direction parallel to the prosthesis neck, preferably in a prolongation of the lower edge of the prosthesis neck.

This invention relates to a hip joint prosthesis. 
Heretofore, various types of prostheses have been known for implanting in 
femurs in order to form a hip joint. For example, hip joint prostheses 
have been formed with a straight blade type shank for anchoring as by a 
wedging action and through a cement bed in a femur. In some cases, the 
shank first widens conically from a free distal end symmetrically of a 
longitudinal median axis of the shank and then, at a point about 
three-quarters along the shank, bevels inwardly along the narrow lateral 
side towards the shank axis. The opposite narrow medial side, in these 
cases, passes along a smooth curve in stepless manner into a neck of the 
prosthesis. In addition, a collar-type projection is formed between the 
shank and the neck and a joint head is carried at the end of the neck. 
Hip joint prostheses of a similar kind are known, for example from the 
journal "Orthopadie" 8, (1979), page 73, 74, and in particular FIG. 1. The 
so-called straight shank of this prosthesis is intended to be wedged in a 
hollow space of a medullary cavity, which has been surgically matched to 
the shank and filled with a bed of bone cement in such a way that the 
cement bed or cement quiver is largely relieved from carrying loads. The 
carrying support of this prosthesis takes place primarily by jamming along 
the narrow medial and lateral sides of the shank. 
In the usual prosthesis constructions, the collar which separates the neck 
from the shank of the prosthesis extends substantially perpendicular to 
the axis of the prosthesis neck. The collar is thus essentially parallel 
to the sectional plane through the neck of the femur. As such, the collar 
is additionally supported on the edge of the cement quiver within the bone 
and prevents the shank from sinking into the bone cement. 
Because the prosthesis neck acts as a "lever" the load acting on the joint 
head causes a torque effect by which the relatively thin blade of the 
prosthesis is placed under a relatively large bending and torsional 
stress. In patients with hypoplastic hip joints, the femur is generally 
malformed and crippled in the region of the neck and of the joint head. 
Also, the medullary space is often extremely narrow. Thus, prostheses of 
the above mentioned type for these diseases are therefore relatively thin 
and provided with a relatively narrow blade. However, because of the 
torque referred to, plastic deformations may result in the relatively thin 
and narrow blade of these prostheses. 
Accordingly, it is an object of the invention to prevent plastic 
deformations in the shank of an implanted hip joint prosthesis. 
It is another object of the invention to prevent twisting of a hip joint 
prosthesis in a femur due to loads applied on a head of the prosthesis. 
It is another object of the invention to provide a hip joint prosthesis 
which can be firmly anchored in a femur. 
Briefly, the invention provides a hip joint prosthesis which includes a 
shank for anchoring in a cement bed in a surgically prepared femur, a neck 
and a collar-like projection between the shank and neck. 
The shank has a longitudinal median axis, a narrow blade-like portion 
extending from a distal end with a conical taper symmetrically of the 
longitudinal axis to define a narrow lateral side and a narrow medial 
side, a beveled side extending from the lateral side at an angle inwardly 
toward the longitudinal axis and an arcuate side extending from the medial 
side on a radius of a circle. 
The neck is angularly disposed relative to the axis of the shank and has a 
lower edge connected to the arcuate side of the shank. 
The projection extends at least approximately in the direction of the neck 
relative to the shank. 
The prosthesis is constructed so that in the region of critical loads, the 
shank is reinforced. Thus, even in the case of thin narrow shanks for 
dysplasia patients, plastic deformation need not be feared. Further, the 
position of the neck allows the neck to be braced directly in the bone in 
the region of the medial calcarine arc. Thus, there is a further reduction 
in the torque exerted by a load on the prosthesis. Moreover--as is the 
case with a collar of a conventional prosthesis--good support of the 
prosthesis in the cement is made possible. 
The desired effect of the prosthesis can be improved if the collar-like 
projection extends at least approximately in alignment with the lower edge 
of the neck of the prosthesis. Thus, the prosthesis neck is understood 
functionally to be a part of the prosthesis shank, because--in contrast to 
the usual prosthesis--the neck takes over a part of the bracing in the 
bone and cement.

Referring to FIGS. 1 and 2, the hip joint prosthesis has a shank 2 for 
anchoring in a cement bed (not shown) in a surgically prepared femur. As 
shown, the shank 2 has a longitudinal medial axis 3 with a narrow 
blade-like portion extending from a distal end with a conical taper 
symmetrically of the axis 3. This portion defines a narrow lateral side 4 
and a narrow medial side 13. At a level about 3/4 up the shank 2, the 
lateral side 4 has a discontinuity from which a beveled side extends at an 
angle inwardly towards the axis 3 to terminate in an, at least, nearly 
horizontal shoulder 5 (i.e. the shoulder 5 is perpendicular to the axis 
3). At a level slightly above this discontinuity, the medial side 13 of 
the shank has an arcuate side extending on a radius from the conical taper 
upwards. 
The prosthesis also has a neck 6 angularly disposed relative to the axis 3 
of the shank 2. This neck 6 includes a lower edge 8 which is connected to 
the arcuate side of the shank portion (see FIG. 1). Thus, the medial side 
13 of the shank 2 is connected in a smooth stepless manner to the lower 
edge 8 of the neck 6, while the shoulder 5 forms a transition from the 
shank 2 to the neck 6. In addition, the neck 6 carries a joint head 11 on 
the free end to receive a main load in the direction indicated by the 
arrow 9. 
Referring to FIGS. 1 and 3, a collar-like projection 12 is disposed between 
the shank 2 and the neck 6 and extends at least approximately in the 
direction of the neck 6 relative to the shank 2. The projection 12 extends 
from the relatively thin and narrow shank portion and forms a transition 
to the thicker neck 6. The height of the projection 12 is chosen so that 
the projection 12 is a prolongation of the lower edge 8 of the neck 6. 
With the prosthesis correctly inserted in a cement bed, the collar 12 and 
the prolonged neck 6 extending from the collar 12 to the shoulder 5 are 
practically completely submerged in the cement bed or at least largely so, 
as is indicated in FIG. 1 by the broken line 10, representing 
approximately the upper limit of the cement bed. Regardless of where the 
pivot point for the torque occurring due to the eccentric load must be 
assumed on the collar 12, a secure anchoring of the shank in the cement 
and bone bearing is thereby obtained. In addition, the chance of a plastic 
deformation of the shank 2 occurring is diminished. 
As shown in FIGS. 1 and 2, longitudinal grooves 16 are formed in side walls 
7 of the shank portion. In addition, these side walls 7 terminate at the 
distal end in a circular arc which extends from the lateral side 4 to the 
medial side 13 while coming to a point with a relatively large radius 
(FIG. 2). This latter curvature is chosen so that, to the extent possible, 
there is smooth transition of the loading forces from the shank 2 and then 
to the spongy bone tissue which may be compacted upon driving in of the 
prosthesis. 
The invention thus provides a hip joint prosthesis which can be used for 
patients with hypoplastic hip joints without plastic deformation of a thin 
blade-like shank.