EP0637232A1

EP0637232A1 - Cement-free femur prosthesis component and process for producing the same

Info

Publication number: EP0637232A1
Application number: EP93909839A
Authority: EP
Inventors: Klaus Dr.Med. Draenert
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-04-24
Filing date: 1993-04-26
Publication date: 1995-02-08
Also published as: DE4213599A1; JPH07508191A; JPH07508189A; WO1993021863A1; EP0637231A1; EP0637230A1; WO1993021862A1; US5554190A; WO1993021864A1; DE4213597A1; JPH07508190A; DE4213598A1

Abstract

A femur prosthesis component that can be anchored without cement is disclosed, as well as a process for producing the same. This prosthesis component produced by a computer-assisted design and image analysis process offers the largest possible surface for the transmission of forces and its mass and stiffness may be regulated according to individual bone properties.

Description

Temporary femoral prosthesis component and method for its

Manufacturing

The invention relates to a cemented-in-place female prosthesis component and a method for its manufacture.

Artificial joint replacement has become a standard procedure in surgery and orthopedics of the musculoskeletal system and is one of the most common operations today. The long-term results of replaced joints are very different and range from a few weeks up to the example of the hip joint 27 years (Draenert and Draenert 1992).

# C's scientific investigations led to the result that a wide variety of factors are responsible for the loosening of an endoprosthesis component, including infections, a lack of surgical technique, wrong choice of implant and excessive stress. Nevertheless, so far many easing cases without a satisfactory explanation. It was only noticeable that certain factors frequently came together to bring about relaxation, such as the bones of a rheumatic in connection with a solid cement-free implant. Prosthesis designs that should block in the bone and that were implanted together with bone cement showed (Draenert 1988) that bone cement as a filler material between metal and bone cannot perform an anchoring function, but is ground up.

The problem of anchoring the prosthesis components could ultimately be attributed to the phenomenon of deformability of the bone. From this it became understandable why an easily deformable bone of a rheumatic patient is deformed by a cement-free anchored metal prosthesis so that a quick loosening was the result. On the other

It could be shown that a fragile or soft, but also a normal spongiosa (sponge bone) can be stiffened with a PMMA bone cement and thus gets enormous stiffness (Draenert and Draenert 1992). Such a stiffened bone structure was found in all those implants that had been successfully worn for 10 to 20 years and could be examined histologically. On the other hand, very compact femur bones were supplied with prosthetic components without using bone cement as an anchoring means, and these prosthetic components have been successfully implanted in some cases for about 10 years (Draenert and Draenert 1992). However, the results were not reproducible in these cases either.

The invention has for its object to provide a cementless anchored femoral prosthesis component that can be expected good long-term results during implantation.

This object is achieved by the invention. ^• In the context of the invention, the problem was investigated how the strength of the bone is related to the durability of an implantation. Histological studies were able to clearly demonstrate that the soft, deformable bones showed stable anchors only if implants with a low mass were used.

The invention is based on the following findings relating to anchoring the prosthesis: each bone has an individual

10 individual form and individual firmness; Both factors must therefore be taken into account when choosing the prosthesis. A stable, compact bone is a good indication for metal-bone anchoring without the use of bone cement. It mainly comes up here

1 two things: 1. To achieve the greatest possible primary stability of the anchoring and 2. To provide as large a surface as possible for the force transmission between the prosthesis and the bones. However, it must be taken into account here that the different compartments of the knot

20 chens, e.g. Epiphysis, metaphysis and diaphysis, have completely different shapes and strengths. An early attempt was made to adapt the prosthetic stem to the medullary cavity, cf. e.g. EP-A-0 038 908

25 could not be covered by a single implant design (Noble et al., 1988); there were also no possibilities to record the strength of the bone and to include it in the prosthesis design.

30 In the context of the invention it was found that there is a good correlation between the density of a bone and its strength. The density of the bone can therefore be used according to the invention as a measure of its strength. By combining different image analytical and computer

35 supported calculations a method could be found which allowed both the morphology of the marrow cavity of the bone and the strength of the bone to be increased detect and take into account in the design of the prosthesis component. These experiments resulted in a design of a prosthesis component which can be ideally fitted into the medullary cavity and whose mass and / or rigidity can be changed such that the largest possible surface for the force transmission between the prosthesis and the bone is used Available.

The dimensions and / or stiffness of the femoral prosthesis component according to the invention can be adjusted to the individual properties of the bone. In the medial, especially the medial-proximal area of the prosthesis, the bending stiffness of the prosthesis is the decisive factor. In the lateral area of the prosthesis, tensile stresses occur in the distal as well as in the proximal area, so that the tensile strength of the prosthesis also plays a role there. Tensile strength is particularly important in the distal area. Because of the muscle attachments that leave the femoral neck and femoral neck free, torsional forces also occur. The material properties discussed above are usually summarized in the present application as "rigidity". According to the invention, the stiffness of the prosthesis and / or its mass is adapted to the individual properties of the bone.

The adaptation can be carried out in different ways, for example the material of the prosthesis can be selected according to the individual properties of the bone. In the case of a dense bone, a material with a higher specific mass and higher stiffness can be selected, whereas a material with a lower specific mass and stiffness is selected accordingly for a bone with low density. CoCrMo alloys, Ti, Ti alloys, steel, plastic or composite materials, for example, can be used as prosthesis material. It is also possible to make the material for the prosthetic component inhomogeneous, in the sense that a material with a higher specific mass and / or stiffness is used in areas with higher bone density than in areas with lower bone density. It should be taken into account here that the bone density can vary very greatly and in the cancellous area can only be 15 to 20% of the density of the compact bone. The desired inhomogeneity of the material can be produced, for example, by varying the pore size when using a porous material and setting it smaller in the region of higher bone density. Composite materials can also be used as material for the prosthesis component, the fiber content of the composite material, for example, being able to vary over the length of the prosthesis component. In this way, the rigidity of the prosthesis component in particular can be varied and adapted to the bone density.

The adaptation of the mass and / or stiffness of the prosthesis component to the individual properties of the bone can also be carried out by a suitable choice of the shape of the prosthesis component, in particular the cross section of the prosthesis component in different bone sections. If, for example, the cross section of the femoral prosthesis component is U-shaped or horseshoe-shaped at least over a substantial part of its length, as proposed in WO 90/02533, the. Cross-sectional area and thus the prosthesis mass in different sectional planes can be adjusted by suitable choice of the size and depth of the channel or the slot between the two arms of the U-shaped cross section. Here a transition from a full shaft to a U-shaped cross section with very thin arms is conceivable. The largest possible surface for the power transmission is ensured by the fact that the prosthesis component forms a closed surface in the media, dorsomedial and anteromedial area. Changes in mass and / or rigidity can also be achieved, for example, by making partially through bores, such as blind bores, or completely through bores in the prosthesis socket, in particular in order to reduce the mass and / or rigidity. On the other hand, applied strips and / or reinforcements on the outer and / or inner contour of the prosthesis, for example a U-shaped prosthesis socket, can increase the mass and / or the rigidity of the prosthesis component. This can be carried out either in sections or continuously over the entire length of the prosthesis component.

The adaptation of the mass and / or stiffness of the prosthesis component to the bone density is preferably carried out by providing a linear correlation between the bone density and the mass or stiffness of the prosthesis component, i.e. that, for example, when the bone density is doubled, the mass or stiffness of the prosthesis in the corresponding area of the. Prosthese component is strengthened as a percentage.

In particular, in order to be able to design and manufacture such an individual prosthesis component, the following procedure can be followed:

A patient with a deformed and arthrotically altered hip joint is examined in the computer tomograph, and stacked images of both hip joints are digitized and saved as cross-sectional images. So-called binary images, ie black-and-white contrast images, which can be captured with their inner and outer contours and depict the femur, are generated from the cross-sectional images using image analysis methods. The inner contour is put together in a 3-D model. The center of rotation is determined from the hip joint and, with the help of the image analysis, as a The center point is shown together with the contour model (see figure).

The shape of the stem of the prosthetic component can then be adapted to the shape of the medullary cavity. On several, for example six to ten, preferably a total of nine sections, which are evenly distributed over the length of the proximal femur, the surface density of the bone is determined on the binary image and compared to a corresponding section of a previously determined normal shape. From this comparison, a correlation factor results as a measure of the strength of the individual bone, on the basis of which the ratio of the prosthesis to the medullary cross-section is determined. If the specific bone density corresponds to that of the normal femur, the contour model of the medullary cavity is eccentrically and / or concentrically reduced by 1 to 20%, preferably 5 to 10%, in order to determine the cross section of the prosthesis component in the relevant sectional plane. If the specific bone density is smaller than that of the normal bone, the contour model is correspondingly reduced in size in order to determine the cross section of the prosthesis. The values can be interpolated between the individual cutting planes. The data record of the contour model is passed on to a CAD unit together with the rotation center. The axis of the contour model is determined in the CAD unit and undercuts in the design are corrected in such a way that the prosthetic component can be inserted into the medullary cavity in a straight line and / or with a slight screwing movement. The construction obtained in this way is returned to the image analysis unit, where a double contour model with the outer and inner contour of the femur is generated, into which the prosthetic component can be fitted. Taking into account and correcting the magnification factor, the prosthesis component is finally projected into the ap x-ray image (anterior-posterior beam path) and axial x-ray image and each inserted along its implantation axis. The mass and / or stiffness of the pro- This is determined in proportion to the bone density. Then the CAD data set is completed with the standard construction data of the neck cone for the ball head receptacle and for the implantation instruments (drive in / out of the prosthesis) and passed on to a milling unit. The denture component is milled from a blank in the milling unit, for example in V ₄ A steel. After the surface has been processed, the prosthesis component is washed and sterilized and is then ready for use.

The invention is explained in more detail below with reference to the accompanying figure. The figure shows an embodiment of the prosthesis component according to the invention, cross-sections of the prosthesis and of the inner and outer contour model of the femur being shown in different sectional planes for a more detailed explanation.

In the figure, the prosthesis is shown as a front view (in the implanted state).

The prosthesis shown schematically in the femur bone according to the figure has an attachable spherical head 1, which is seated on the cone 2 of the prosthesis neck 3. The rotation center is designated by the reference number 4. The neck 3 is firmly connected to a shaft 5 of the prosthesis. In the section planes, which are distributed approximately uniformly over the length of the proximal femur and in which the areal density of the bone is determined, the optimal shaft cross sections 5 * resulting in the manner described above are shown. In the figure, eight shank cross sections 5 'are shown hatched and, in addition, three shank cross sections are drawn out for clarification. The bone density and the resulting optimum shaft cross section are preferably determined in six to ten, for example nine, sectional planes. The outer contour model with reference number 6 and the inner contour model of the femur with reference number 7 are examples in the individual Section planes are shown, which each result from the image analysis. The mass and / or rigidity of the prosthesis in the individual sectional planes can be adjusted by suitable design of the shaft cross sections 5 '. If, for example, a low mass is desired, the slot or the recess in the U-shaped cross section of the shaft is correspondingly enlarged, the largest possible surface being available for the force transmission between the prosthesis and bone in the medial region of the prosthesis. When changing the specific mass in a sectional plane, the stiffness of the prosthesis component generally also changes in this area. The reference number 8 designates the construction axis of the prosthesis, which at the same time also represents the medullary cavity axis and the implantation axis.

Literature:

Draenert K. (1988), research and further training in the surgery of the musculoskeletal system 2, on the practice of cement anchoring, Munich, Art and Science.

Draenert K. and Draenert Y. (1992), research and further education in the surgery of the musculoskeletal system 3, the adaptation of the bone to the deformation by implants, Munich, Art and Science.

• Noble PC, Alexander JW, Lindahl LJ, Yew DT, Granberry WM, Tullos HS, Clinical Orthopedics and Related Research, No. 235, October 1988, pp. 148-163.

Claims

Patent claims

1. Femoral prosthesis component for cement-free anchoring in the femoral section near the hip joint, which offers the largest possible surface for force transmission and whose mass and / or rigidity can be adjusted to the individual properties of the bone.

2. The femoral prosthesis component according to claim 1, wherein the shaft shape of the prosthesis is adaptable to the shape of the medullary cavity.

3. The femoral prosthesis component according to claim 1 or 2, wherein the ratio of the prosthesis cross section to the medullary cavity cross section is determined by a correlation factor of the strength of the bone, which is determined on the basis of the specific bone density per unit area.

4. Femoral prosthesis component according to one of claims 1 to

3, the area of the prosthesis cross section being between 30 and 90%, preferably 40 to 80% of the medullary cavity cross section.

5. Femoral prosthesis component according to one of claims 1 to

4, the circumference of the cross section of the prosthesis making up a constant 70 to 95%, preferably 80 to 90%, of the inner medullary cavity contour.

6. Femoral prosthesis component according to one of claims 1 to

5, the specific mass of the prosthesis or the specific stiffness in each case being proportional to the bone density.

7. Femoral prosthesis component according to one of claims 1 to

6, a CoCrMo alloy as the material of the prosthesis tion, titanium or a titanium alloy, steel or plastic or a composite material is used.

8. A method for producing a femoral prosthesis component according to one of claims 1 to 7, wherein the shaft shape of the prosthesis is first adapted to the shape of the medullary cavity, the medullary cavity being reconstructed three-dimensionally from a series of cuts and receiving a contour model of the femur with external and internal contours becomes.

9. The method according to claim 8, wherein the sections are computed tomography sections or nuclear spin sections or histological sections.

10. The method according to claim 8 or 9, wherein in the reconstruction of the marrow cavity, stacked images of the individual sections are digitized and electronically stored and then, e.g. via binary images to which the contour model is processed.

11. The method according to any one of claims 8 to 10, wherein bone densities are determined on the individual sections, from which specific area ratios of prosthesis shaft cross section and medullary cavity cross section are determined.

12. The method according to claim 11, wherein the bone density per unit area, compared to the corresponding specific surface density of a normal femur, gives a factor which is used as a correlation factor of the strength of the bone when determining the ratio of prosthesis cross section to medullary cavity cross section.

13. The method according to claim 12, wherein the inner contour model is reduced in cross-section in accordance with the correlation factor by 1 to 20%, preferably 5 to 10%, either eccentric or concentric, or alternately eccentric and concentric along the construction axis of the prosthesis.

14. The method according to any one of claims 8 to 13, wherein the data of the contour model of the prosthesis to a CAD / CAM unit or a similar construction system are transmitted and processed there in such a way that the contour model of the prosthesis in the contour model of the bone is fitted so that it can be implanted along an implant axis with a slight screwing movement.

15. The method according to any one of claims 9 to 14, wherein from the average density of individual bone sections

Mass and / or stiffness of the prosthesis is determined in such a way that it is proportional to the density of the bone.

L ^ß • Femoral prosthesis component for anchoring by means of bone cement in the femoral section near the hip joint, producible with a method according to one of claims 8 to 15.