EP0637231A1

EP0637231A1 - Prosthesis components and process for producing the same

Info

Publication number: EP0637231A1
Application number: EP93909838A
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: JPH07508189A; EP0637232A1; WO1993021863A1; US5554190A; DE4213597A1; JPH07508190A; DE4213599A1; WO1993021862A1; EP0637230A1; JPH07508191A; WO1993021864A1; DE4213598A1

Abstract

Prosthesis components to be anchored with or without bone cement are disclosed, as well as a process for producing the same. The prosthesis components produced by a computer-assisted design and image analysis process offer the largest possible surface for the transmission of forces, and their mass and stiffness may be adapted to individual bone properties.

Description

Prosthetic component and process for their manufacture

The invention relates to a prosthesis component to be anchored without cement or with bone cement and to a method for its production.

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).

Scientific studies have led to the result that a wide variety of factors are responsible for the loosening of an endoprosthesis component, including infections, inadequate surgical technique, wrong choice of implant and excessive stress. Nevertheless, many easing cases have so far remained without a satisfactory explanation. It was only noticed that certain factors often combined came to bring about a loosening, such as the bones of a rheumatic in connection with a solid cement-free implant. Prosthetic 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.

The problem of anchoring the prosthetic 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 hand, could it be shown that a fragile or soft, but also a normal spongy bone (sponge bone) is stiffened with a PMMA bone cement? ^» Can ground and therefore enormous rock. l.gkei.t (Draener -? t * 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 prosthesis component to be anchored with or without bone cement, which allows good long-term results to be expected 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 shape and an individual strength; 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. Two things are important here: 1. to achieve the greatest possible primary stability of the anchoring and 2. to provide the largest possible surface for the force transmission between the prosthesis and the bones. However, it must be taken into account here that the different compartments of the bone, e.g. Epyphysis, metaphysis and diaphysis, have completely different shapes and strengths. When implanting with bone cement, it is also important to take into account the shape of the bony bed, for example the medullary cavity, because an incomplete cement sheath leads to its early destruction. An early attempt was made to adapt the prosthetic stem to the medullary cavity, cf. e.g. EP-A-0 038 908, but it quickly became apparent that the large number of different shapes of the bones could not be covered with 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.

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 l Bones only showed stable anchorages if

Low mass implants had been inserted.

The invention showed 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-aided calculations, a method could be found which allowed both the morphology of the bone bed of the bone and the strength of the bone to be recorded and taken into account in the design of the prosthesis component there is a design of a prosthesis component which can be ideally fitted into the bony bed and whose mass and / or rigidity can be changed so that the largest possible surface is available for the force transmission between the prosthesis and the bone.

° i ^& prosthesis component according to the invention is adjustable in its mass and / or stiffness to the individual properties of the bone. In the case of a femoral prosthesis component, the bending stiffness of the prosthesis is the decisive factor, for example in the medial, especially the edial-proximal area of the prosthesis. 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. In the case of other prosthesis components, for example shoulder, elbow, knee, hand, finger or ankle joint components, the desired material properties also vary in different areas of the prosthesis components. The material properties discussed above are usually summarized in the present application as "stiffness". According to the invention, the rigidity the prosthesis and / or its mass 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 greatly and e.g. in the spongy region of the femur, only 15 to 20% of the density of the compact bone can be. 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. This allows the stiffness of the prosthesis component to 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 achieved by a suitable choice of the shape of the prosthesis component, in particular the cross section of the prosthesis component. nent in different bone sections. If, for example, the cross-section of a femoral prosthesis component is U-shaped or horseshoe-shaped at least over a substantial part of its length, as suggested in WO 90/02533, the cross-sectional area and thus the prosthesis mass can be selected in various sectional planes by suitable selection of the size and Depth of the gutter or slot between the two. Arms of the U-shaped cross section can be adjusted. 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, ie, for example in the case of a doubling pelung bone density component and the mass or stiffness of the prosthesis in the corresponding area of ^{'Prothesenkompo¬} is amplified percentage. In detail, 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 joint is examined in the computer tomograph, and stacked images of the joint are digitized and stored as cross-sectional images. So-called binary images are generated from the cross-sectional images using image analysis methods, i.e. black and white contrast images that can be captured with their inner and outer contours. The inner contour is put together in a 3-D model. The axis or the axes of the joint is determined and displayed with the aid of the image analysis together with the contour model with the joint surfaces (see figure using the example of a femoral component).

The shaft shape of the prosthetic components can then be adapted to the shape of the bony bed. The areal density of the bone is determined on the binary image in a plurality, preferably a total of six to ten sections, which are evenly distributed over the length of the bone, and is compared to a corresponding section of a correspondingly determined corresponding normal bone. This comparison gives a correlation factor as a measure of the strength of the individual bone. If the specific bone density corresponds to that of normal bone, the contour model of the bony bed is reduced by 20 to 50% for the cemented component, for the cementless component by 1 to 20%, preferably 5 to 10%, specifically, for example, eccentrically and / or concentric in order to fix the cross section of the prosthesis component in the relevant cutting 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 sectional planes. One to be anchored with bone cement The prosthesis cross-section is preferably determined so that the layer thickness of the surrounding bone cement sheath is approximately inversely proportional to the bone density. The data record of the contour model may be passed on to a CAD unit together with the rotation center or the joint axis. 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 prosthesis component can be inserted in the bone cement in a straight line and / or with a slight screwing movement, if necessary, without bumping against the bone . 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 bone is generated, into which the prosthesis component can be fitted. Taking into account and correction of the magnification factor is finally the Pro ⁵ - synthesis component (posterior anterior-ray path) in the ap-Röntcrenbild and projected axial RöntÜjnbild and respectively inserted along its axis of implantation. The mass and / or stiffness of the prosthesis is determined in proportion to the bone density. The CAD data record is then completed with the standard construction data of the prosthesis and the implantation instruments 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 processing the surface, 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 using the example of a cementless femoral prosthesis component, cross-sections of the prosthesis and the inner and outer contour model of the femur being drawn in at 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 shaft cross sections 5 ^{1 are shown} hatched and, in addition, three shaft cross sections are drawn out for clarification. The bone density and the resulting optimum switching cross section are preferably determined in six to ten, for example nine, sectional planes. Reference number 6 shows the outer contour model and reference number 7 the inner contour model of the femur as an example in the individual sectional planes, 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 greatest 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. prosthesis component for anchoring with or without bone cement in the epiphyseal, meta- or diaphyseal bone section, which offers the largest possible surface for force transmission and whose mass and / or stiffness can be adjusted to the individual properties of the bone.

2. The prosthesis component according to claim 1, wherein the shaft shape of the prosthesis is adaptable to the shape of the bone section.

3. The 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 «prosthesis component according to one of claims 1 to 3, wherein the area of the prosthesis cross-section between 30 and 90%, preferably 40 to 80% of the cross-section of the bony bed.

5. prosthesis component according to one of claims 1 to 4, wherein the circumference of the cross section of the prosthesis in the cemented prosthesis constant 60 to 80%, in the cementless prosthesis 70 to 95%, preferably 80 to 90% of the inner contour of the bony bed matters.

Prosthesis component according to one of claims 1 to 5, wherein the specific mass of the prosthesis or its specific stiffness is in each case proportional to the bone density.

Prosthesis component according to one of claims 1 to 6, wherein the material of the prosthesis is a CoCrMo alloy, Titanium or a titanium alloy, steel or plastic or a composite material is used.

8. The method for producing a prosthetic component according to one of claims 1 to 7, wherein the shaft shape of the

The prosthesis is first adapted to the shape of the bony bed, the bony bed being three-dimensionally reconstructed from a series of cuts and a contour model of the bone with outer and inner contours being obtained.

9. The method according to claim 8, wherein the cuts are computer tomographic cuts or magnetic resonance cuts or hiological cuts.

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

11. The method according to any one of claims 8 to 10, wherein bone densities are determined on individual sections, from which specific area ratios of prosthesis shaft cross section and cross section of the bony bed 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 bone, gives a factor which is used as a correlation factor of the strength of the bone in determining the ratio of prosthesis cross section to cross section of the bony bed.

13. The method according to claim 12, wherein the inner contour model corresponds to the correlation factor in the zemen- tated prosthesis by 20 to 50%, in which the cementless is reduced by 1 to 20%, preferably 5 to 10% in cross-section, for example 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 are transmitted to a CAD / CAM unit or a similar construction system and are processed there in such a way that the contour model of the prosthesis in the contour model of the bone is fitted in such a way that it can be implanted in a straight line and / or with a screwing movement along an implantation axis.

15. The method according to any one of claims 9 to 14, wherein from the average density of individual bone sections, the mass and / or the stiffness of the prosthesis is determined in such a way that it is proportional to the density of the bone and that the bone cement layer thickness of Cement sheath is roughly proportional to the bone density.

16. prosthesis component for anchoring with or without bone cement in the epiphyseal, meta- or diaphyseal bone section, producible with a method according to one of claims 8 to 15.