DE2324865B2 - THIGH PART OF A HIP JOINT PROSTHESIS - Google Patents

Description

The invention relates to the thigh part of a hip joint endoprosthesis for cementless implantation with a curved, conically tapering prosthesis shaft with an outer, the central point of curvature facing away surface and with an inner surface facing the center of curvature, which Spanning surfaces with a square cross-section. It relates in particular to prostheses made from AbOs ceramics and Prostheses from a combination of a ceramic head with a metal shaft, in which the metal shaft with tine glaze, an enamel or a glass ceramic is covered.

With such a hip joint endoprosthesis, it is known (GB-PS 13 06 027) to apply an allegedly biocompatible tissue or a silicone coating to the surface of the prosthesis. With respect to the shaft shape is assumed to be rounded in cross-section rectangle ^R örmigen shape. The curved shaft tapers evenly. In the same context, a triangular cross-sectional shape is also known. The side facing away from the center of curvature of the prosthesis is made relatively narrow, so that there is a risk of pressure necrosis because a torque acts on the prosthesis, which is to be absorbed on this side.

Furthermore, the load situation of the shaft of a thigh part of a prosthesis is already important for both Eementless as well as for the implantation carried out with bone cement except for the one at the beginning mentioned material information has been shown ("The Journal of Bone and Joint Surgery" Vol. 47 B No. 2, May 1965, pp. 354-363). The author, J. C h a r η 1 e y, provides the load on the shaft of a prosthesis for cases with and without load by muscle strength and under the prerequisite that along the shaft of the prosthesis from its lower end to the collar, under which the supporting force acts in a distributed manner, no longitudinal forces, d. H. no tangential forces are effective. This situation also largely corresponds to the case of cementless implantation. In this case, broadcasts also the stem of the prosthesis below the collar Serious longitudinal forces on the femur. The entire load rests on the collar of the prosthesis. Furthermore, Charη 1 ey describes in detail how, im In contrast to the previously described case, the anchoring of such a prosthesis when in use of bone cement result. The still soft cement penetrates when the prosthesis shaft is pressed into it into the irregularly shaped spong.osa on the inside of the bone. Only this interlocking is permitted esTangemial forces on the surface of the prosthesis socket to be transferred over the bone cement to the bone wall, although uncontrollable Pressure peaks in the fine range are possible. This possibility of transferring tangential forces i.e. forces acting parallel to the axis from the in conventionally mostly made of metal prosthesis shaft over the cement on the wall of the Thigh bone, absent in the zemenifreicn Implantation, at least as long as the surface of the sheep is executed smoothly. Since the cement is always deposited tightly around the prosthesis socket, is for this Force introduction the shape of the prosthesis socket is practically meaningless.

As mentioned above and by C h a r η I e y schematically has been expressed, the prosthesis shaft does not serve the purpose of cementless implantation Introduction of tangential forces on its surface. Rather, they must be parallel to the axis of the thigh bone or prosthesis shaft effective forces through the prosthesis collar in the thighbone be initiated. Experience has shown that cement-free fastening does not require any further measures unsatisfactory because the prosthesis parts sink deeper and deeper into the thigh bone when the Bone dissolves under places where the load is too high. The prosthesis can also follow Loosen it for some time, because it usually does not become a firm one due to the unfavorable biocompatibility of the metals Connection of the prosthesis shaft with the bone tissue comes.

It is also known that certain _{types of ceramics, e.g. dense Al 2} O ₃ ceramics with 99% Al ₂ O ₃ gel, as well as certain glass ceramics have excellent tissue compatibility, especially with bone tissue (Journal of Biomed. Material Research, Vol. 6 (1972 ), Pp. 451-461). Under certain conditions, a mechanically strong connection is formed between the surfaces of such implants and your bone tissue. Ceramic or glass ceramic is far superior to plastics such as a silicone layer in terms of biocompatibility. As a result, there is now, in principle, the possibility of anchoring the thigh parts of total hip joint endoprostheses without cement and in a mechanically stable manner in the medullary canal of the thigh bone.

However, the known prosthesis designs are not suitable for cementless implantation, because the load transfer from the implant to the femur does not occur in such a way that bending and Tensile stresses in the interface between implant and bone are avoided as far as possible and high pressure loads are eliminated.

The invention is based on the object, taking full account of the anatomical conditions and their range of variation in humans, to create a prosthesis construction that meets the load

Nissen takes into account and also adapted as possible to the conditions of the aluminum oxide ceramic The solution to this problem is that according to the invention above, just below the collar of the Prosthesis socket the width of the inner surface at least 10 mm and larger than or at least is the same as the width of the outer surface and that the width in the lower area of the prosthesis shaft the inner area is smaller than the width of the outer Area.

The heavily loaded surfaces of this prosthesis are advantageously kept as large as anatomically possible, so that the surface pressure is relatively small and the risk of pressure necrosis is avoided. The shaft is at most on the wider surface and on the four corners of the bone wall. In this way, the remaining parts of the bone wall can also be supplied with blood and nutrients in the area of the prosthesis shaft from the medullary canal. With the design of the prosthesis shaft according to the invention, the situation corresponding to the natural supply of the bone can consequently be maintained with a considerably better approximation than with all prosthesis fastenings with the aid of bone cement and generally with all fastenings that completely fill the medullary canal.

The load conditions are through Charnley clearly described, however, there are no consequences for the cementless implants with regard to the Reduction of the surface pressure in the stationary phase is still the possible optimization of the Supply of the bone with blood and nutrients in the remodeling phase and in the stationary phase in Been considered.

The design of the prosthesis known from GB-PS 13 06 027 for cementless implantation shows in contrast to the thigh endoprosthesis according to the invention that the cross section of the solid part the inside and outside of the prosthesis socket are of the same shape, d. H. both at the center of curvature facing as well as on the side facing away from the center of curvature of the prosthesis shaft. The known prosthesis is essentially form-fitting to the cross-sectional shape of the thigh bone adapted, so that the supply of the bone is difficult and a narrow outer lower surface loaded the same in the stationary phase in an area that is too small.

According to the invention, an all-ceramic hip joint prosthesis is preferably assumed, in which the ratio of the widths of the shaft in the lower region is at most 0.9 to 1.0, the the first-mentioned number is assigned to the width of the inner surface, the second number to the width of the outer surface is.

The invention is explained below with reference to the exemplary embodiment shown in the drawing. It shows

F i g. 1 shows a side view of a thigh part of a hip joint endoprosthesis,

2 shows a view of the same from its center of curvature and

F i g. 3 shows a cross section according to the section line III-III in FIG. 2

1 shows the curved thigh part of a hip joint endoprosthesis for a prosthesis made of dense Al2O3 ceramic in a side view. It consists of a head 1, a femoral neck 2 with a collar 3 and a shaft 4. With an inner surface 5 facing the center of curvature or with a chamfer, the prosthesis rests on the face of the wall of a thigh bone (not shown). This surface 5 is designed as wide as possible in order to avoid any excess pressure when distributing the load from the prosthesis into the femur. The outer upper surface 6, that is to say on the rear side, on the side facing away from the center of curvature of the shaft of the prosthesis, only needs to have the width that is necessary for reasons of strength. The widths of the prosthesis socket at its upper end, the areas marked 5 and 6, are either the same, or the width on the inner surface is greater than the width of surface 6 Prosthesis socket 4 not smaller than 10 mm.

Furthermore, the tapering of the prosthesis shaft takes place downwards in such a way that the Width of the surface on the side facing the center of curvature becomes smaller faster than that on the side facing away from the center of curvature. In the lower part of the prosthesis shaft 4, so about at level 7 of FIG. 2, is the width on the inside, i.e. that facing the center of curvature Side, smaller than on the outer side of the prosthesis shaft facing away from the point of curvature. here can e.g. B. the area widths on the inside 5 to 7 mm, on the outside 8 to 9 mm.

The quotients of the width on the inner surface 5 to the width on the outer surface 6 at the upper end of the prosthesis socket are at least 1.0 to 1.0. These should be in the lower part of the prosthesis shaft Quotients are at most 0.9 to 1.0. These dimensions are favorable for an all-ceramic.

In animal experiments and in experiments where the load transfer conditions are as they are in human Body present, were simulated, it has been shown that the prostheses according to the invention the occurring Can bear total loads and at the same time that no pressures occur at any point that are greater than the Pressure load in natural bone. This means that when the endoprosthesis according to the invention is used no risk of bone degeneration from pressure necrosis.

For this ? Sheet drawings

Claims (2)

Patent claims: 1.Thigh part of a hip joint endoprosthesis for cementless implantation with a curved, conically tapering prosthesis shaft with an outer one facing away from the center of curvature Surface and with an inner surface facing the center of curvature, which Spanning a square cross-section, characterized in that at the top, just below the collar (3) of the prosthesis socket (4) the width of the inner surface (5) at least 10 mm in size and larger than or at least the same size as the width of the outer surface (6) and that in the lower area of the prosthesis shaft the width of the inner surface is smaller than the width the outer surface. 2. Thigh part of an all-ceramic hip joint prosthesis according to claim 1, characterized in that the ratio of the widths of the shaft in the lower region is at most 0.9 to 1.0 where the first number is the width of the inner surface, the second number is the width of the outer surface is assigned.