IE42809B1

IE42809B1 - Improvements in or relating to body joint endoprostheses

Info

Publication number: IE42809B1
Application number: IE68776A
Authority: IE
Original assignee: Pfaudler Werke Ag
Priority date: 1975-04-04
Filing date: 1976-04-01
Publication date: 1980-10-22
Also published as: DE2514793B2; DD123056A5; DE2514793C3; JPS51122995A; AT349618B; SE7602440L; BE839461A; DK158176A; LU74699A1; GB1511115A; CA1075851A; NL7602392A; IE42809L; ATA148176A; DE2514793A1; IT1057845B; FR2305961A1; SE410388B; CH611795A5; FR2305961B1

Abstract

The prosthetic articulation comprises a shaft (73) for anchoring in a first bone (30), a first articulation part (113) which is connected to the shaft and forms the articulation together with a second complementary articulation part on a second bone (48), and a support element (107) which is connected to the shaft and bears on a bearing surface of the first bone. In order to ensure that the force introduction into the first bone takes place in a physiologically analogous manner, so that the distribution of stress corresponds as closely as possible to the physiological distribution of stress, the first articulation part (113) and the support element (107) form a pivot part (109) which is connected to the shaft in an articulated or resilient manner. To insert and remove this prosthetic articulation, use is made of a rigid turning instrument which can be applied to coupling means (87) on the shaft of the prosthesis or can be coupled securely to these means.

Description

The invention relates to a body joint endoprosthesis with a shaft for anchorage in a first bone, a first joint member connected to the shaft and cooperable with a second joint member for connection to a second bone, and a support element connected to the shaft for bearing against a seating surface of the first bone.

Wear phenomena in the joints of aged people, the consequence of inherited disease, rheumatic inflammatory ailments and injuries may lead to chronic pain conditions and progressive restriction of joint mobility (arthrosis), which often severely limits the sphere of activity and imposes acute physical stress upon the sufferer.

By way of showing the state of the art, reference is made to the paper Technischer Fortschritt bei Kiinstlichen Huftgelenken in the periodical Technische Rundschau Sulzer, 4/1974 pages 235 to 245. This treatise gives a review of the development of artificial hip joints and describes typical constructional forms known since 1939. From the discussion in the paper of these particular constructional forms, it is apparent that all of the constructions have inherent and more or less serious disadvantages. In addition to wear phenomena in the two members of the joint, namely the ball and the socket, known shaft prostheses are attended by the particular disadvantage that loose-242809 ning of the shaft can be attributed to several different causes. Firstly, the shaft is customarily cemented into the bone. The cement which is used is the self-polymerising synthetic plastics material methyl methacrylate. The heat of polymerisation resulting from the curing of this synthetic plastics material results in temperatures of 80° to 100°C and so gives rise to thermal damage of the surrounding tissue, because the coagulation point of albumen is 56°C.

In addition to these thermonecroses, damaging effects are also caused which result from the mechanical preparation of the bone (rasping or similar operations) which has to be carried out during the preparation of the seating to receive the prosthesis, but at the same time there is formed between the damaged area and the bone a screen of connective tissue, which has a negative influence upon the anchorage in the bone. The screen of connective tissue permits minute relative displacements to take place between the prosthesis and the bone seating. For a fuller treatment of this condition, reference is made to the book Biopolymere und Biomechanik von Bindegewebssystemen 1974 pages 417 to 419 in the article entitled Zur Problematik der Zementverankerung im Knochen by H. G. Willert.

A further cause of the loosening phenomenon is the non-physiological nature of the application of force from the prosthesis to the bone. For the want of any other exposition on this subject in the known art, an explanation thereof will now be given with reference to Figs. 1, 2 and 3 of the accompanying drawings.

In Fig. 1 a shaft 35 of a femur head endoprosthesis 37 is secured in a thigh bone 30 by means of cement 32. The shaft 35 has a collar 39 and a neck 40 terminating in a ball 42 having a centre point M. The ball 42 rests in a ball socket 45, which is secured in a pelvis bone 48 by cement 46. The force Fr resulting from transmission of the person's body weight to, for instance, the floor upon which that person is standing passes through the centre point M of the ball 42 and operates in a direction which encloses an angle a with respect to the longitudinal axis of the neck 40. If this resultant force FR is taken as effectively acting through a point A on the seating surface 50 of -3£2809 the thigh bone 30 cooperating with the collar 39, then a force component FR.cosa will act in a direction normal to the seating surface 50 and a force component FR.sin will act in a direction of the seating surface 50. In addition, due to the parallel displacement of the resultant force FR, i.e. from the centre point M to the effective point A, the force couple FR.a, in accordance with the parallelogram 51 shown hatched in Fig. 1, is also effective. The distance a represents the shortest distance between the point A and the line of action of the resultant force FR.

In Fig. 2 there are indicated qualitatively the surface pressures acting upon the thigh bone 30 and resulting from these two force components and the force couple. It can be seen that the surface pressure p acting normal to the seating surface 50 is practically constant, whilst in the physiological case, represented in Fig. 3, the approximately linear curve of the normal stress σ acting at the right hand or medial edge in Fig. 3 shows a maximum compression stress of tfi)MAX. In the physiological case according to Fig. 3, there will be a neutral position spaced a distance e from this position of maximum compression stress, so that beyond that neutral position, i.e. at the left hand or lateral edge as shown in Fig. 3, there will be a maximum tensile stress σ7ZMAX.

Whilst in the physiological case according to Fig. 3, there is practically no stress normal to, i.e. at right-angles to, the direction of the fibres of the cortical tissue, indicated at 122 in Figs. 1 and 2, nevertheless in the other case the force component FR. sina and the force couple FR.awill give rise to surface pressures q and v, which are indicated qualitatively in Fig. 2 and act at right angles to the inner surface of the cortical tissue. In that case the surface load q at the inner medial margin of the seating surface 50 has a maximum value, <7max, which in the course of the continuous reconstitution of the bone, results in a progressive yielding of the bone.

Along with this yielding of the bone, the bending stress of the prosthesis and the cement increase until, due to further loosening of the shaft 35, cracks appear in the cement 32, and finally a breakage 55 of the shaft 35 causes the prosthesis to fail and leads to immobility of the patient. The relative move-442809 ments between the collar 39 and the seating surface 50 arising from this loosening process prevent the desirable progressive ingrowth of bone cells into pores, cavities or perforations in the surfaces of known prosthesis. The abovementioned cracks in the cement 32 lead to intense corrosion phenomena in the metal (crack corrosion). Sinking of the shaft prosthesis is also possible if the support afforded by the cortical tissue is lost.

The force FR. cos a indicated in Fig. 1 as acting at right-angles to the seating surface 50 is transmitted through the collar 39, and in respect of one part, directly into the thigh bone 30 and, in respect of another part, into to this thigh bone via the prosthesis itself. It is assumed that the case under consideration is a known prosthesis of steel, then the ratio of the modulus of elasticity of the steel to that of bone is about 8:1. Consequently when under load, the bone deformation is relatively greater than that of the steel, whereby there are relative displacements of the contacting surfaces of the prosthesis and the bone. These displacements can lead to shearing off of the osteoblasts building up to form a bridge between the bone and the prosthesis, which otherwise are desirable in order to provide a lasting anchorage of the prosthesis in the bone. As a result, a resilient screen of connective tissue is formed in that region, which permits further relative displacements and therefore a loosening of the prosthesis.

In the book entitled Gesammelte Abhandlungen zur funktionellen Anatomie des Bewegungsapparates by Freidrich Pauwels, Springer-Verlag 1065, in particular at pages 4 to 6 of the chapter Mechanische Faktoren bei der Frakturheilung, a general description is given of the influence of mechanical stimuli upon the final structure of newly formed tissue in the form of connective tissue, cartilage or bone. The force component FR. sin a defined above in connection with Fig. 1 can result in the setting up of a so-called free shearing force in the seating surface preventing the desirable growth of new bone tissue, this force being explained in the above cited book of Pauwels in the chapter Die freie Scherkraft at pages 21 to 24.

Further attempts are also known to anchor shaft prosthesis in thighbones without the use of cement. For this purpose the surface of the shaft is -542809 provided with perforations or macroscopic depressions, in which it is intended that there shall be a newly formed growth of bone tissue resulting in an intimately contacting anchorage of the shaft of the prosthesis in the thigh bone. Nevertheless, because the implanation of hip joint endoprostheses is mostly necessary in elderly patients, great importance must be placed upon early mobilisation of the patient. If the patient is laid up for too long a time until there is a sufficiently firm ingrowth of the shaft of the prosthesis, this delay can, for example, cause the risk of pneumonia, muscular atrophy and damage to the heart and the circulation as well as to the bladder and kidney system. For example, in experiments with animals, the time taken for the formation of load bearing bone tissue has been at least two months. Consequently, the known types of prostheses, whose anchorage in the thigh bone relies exclusively upon the principle of the ingrowth of bone tissue, are therefore very disadvantageous on account of the lack of early mobilisation of the patient. Moreover, because of the difference in the moduli of elasticity and locally high surface pressures, lossening of the prosthesis can occur.

In known hip joint endoprostheses of this type (German Auslegeschrift Specification No. 1,541,246, and French Specification 2,057,418) a metallic shaft is formed integrally with a collar shaped support element and a bearing stud, upon which a ball head is rotatably mounted as the first member of the prosthesis joint. There is no prosthesis joint between the support element and the shaft. In consequence, this construction does not remove the above-explained disadvantages.

In a further known hip joint endoprosthesis of the above mentioned type (French Specification 2,210,909) a shaft is formed integrally with a collarshaped support element, to which can be screwed a first joint member in the form of a ball head to form a rigid unit secured by a clamping device. This construction also lacks a prosthesis joint between the support element and the shaft.

In the Swiss Patent Specification 426,096 there is disclosed a hip joint endoprosthesis, to whose shaft there is integrally connected a first socket. -642808 A freely-rotatable ball, forming the first joint member, is mounted in this socket and the ball is also supported in a second socket in the pelvis bone forming the second joint member. There is thus no provision of a support element connected to the shaft, as there is in the prosthesis joint according to the socket invention. The forces applied to the socket are transmittted exclusively through the shaft into the thigh bone, so that the above-mentioned disadvantages are even more clearly noticeable.

It is already know (German Offenlegungsschrift Specification 2,432,766) to construct an artificial knee joint which during the whole of the movement cycle functions as a crossed quadrangular linkage having the bridge connected to the thigh bone and the couple connected to the tibia.

The present invention takes as its basic purpose gererally to provide a body joint endoprosthesis wherein the transmission of force into the first bone takes place analogously to the physiological condition in such a manner that the stress distribution in the first bone corresponds, at least to a good approximation, to the physiological stress distribution. For the purpose of promoting the growth of new bone on the seating surface, relative movements between the seating surface and the support element Should as far as possible be avoided.

The anchorage in the first bone is to be made without the use of cement. It should be possible to provide for the early mobilisation of the patient.

In accordance with the present invention, a body joint endoprosthesis is provided, which comprises a shaft for anchorage in a first bone, a first joint member connected to the shaft and cooperable with a second joint member for connection to a second bone, and a support element connected to the shaft for bearing against a seating surface of the first bone, wherein the first joint member and the supporting element are components of a pivot member connected to the shaft by way of a prosthesis joint.

By means of the constructional principles of the invention, it is ensured that the transmisssion of force from the support element onto the seating surface is analogous to the physiological condition, whilst affording the stressing conditions which are necessary for the maintenance of the bone tissue. The ingrowth of bone tissue into openings and pores of the prosthesis is consider-742809 ably facilitated and the time taken for this process is shortened. The joint resolves disadvantageous stiffness of the prosthesis, and whilst maintaining the necessary functional stability affords mobility within the prosthesis itself, which removes the above-described disadvantages. As regards its structure, it is advantageous to design the pivot member in one piece.

According to a preferred constructional form of the invention a part of the prosthesis joint which is adjacent the shaft is designed as an extension member releasably connected to the shaft. This arrangement makes possible a preassembly of the pivot member, the prosthesis joint and the extension member before this structural grqup is implanted. This shortens the operating time and also allows, for example, a complete enamelling.of the surface of this part of the entire endoprosthesis to be effected if the part includes a metallic supporting material. The complete part can therefore be subjected, before implantation, to any desired specialised finishing treatment and quality control.

According to a further preferred feature of the invention, the extension member includes a conical socket complementary to and mounted upon a cone of the shaft, which socket is urged against the cone by means of a screw which is screwed into a tapped bore in the shaft. This construction permits continuous rotary adjustment and fixing of the conical socket with respect to the cone of the shaft. Even when a comparatively small axial compression force is exerted by the screw, the cone type of connection affords substantial load bearing frictional forces, which ensure reliable functioning of the prosthesis. Even when the shaft is completely enamelled, the cone type of connection permits satisfactory working of the shaft for the purpose of fitting and fluid tightness, for example by grinding. The tapped bore and the turns of the screw do not require to be enamelled because it is possible by suitable working of the opposing enamelled surfaces to obtain between the screw head and the conical socket a perfect seal for preventing the ingress of body liquids and secretions. Moreover, because of the provision of the prosthesis joint and the quasi'-physiological transmission of force into the bone, the shaft is relieved -842809 of a substantial load as compared with the conditions existing in known shafts. By this means there is achieved, not only primarily an adequate anchorage of the shaft in the bone, but also secondarily a further improvement in the anchorage by growth of the bone into the shaft, which favours an early mobilisation of the patient. Advantageously, for this purpose the shaft may be provided with an external rounded thread for holding the shaft in the first bone. By this means the anchorage of the shaft in the bone can be achieved completely without bone cement, mastic or the like whilst nevertheless achieving an exceptionally good degree of positive and/or non-positive connection between the shaft and the bone.

Preferably the thread of the shaft is conically formed in a manner complementary to the oppositely situated internal wall of the first bone. The maximum degree of care of the bone and a reliable anchorage of the shaft therein are achieved if, before screwing the shaft into the bone, a thread core hole is made therein for the shaft thread by means of a thread core tapping tool or milling tool.

According to one practical form of the invention the prosthesis joint is in the form of a hinge joint. The resulting ability of the pivot member to pivot only in one plane is adequate in most cases to provide a transmission of force into the first bone approximating to a large extent to the physiological condition. However, if universal pivotability of the pivot member is desired with respect to the seating surface, the prosthesis joint may then be designed according to the invention as a ball joint or a Cardan joint.

The prosthesis joint may, however, also be designed as a knife edge type of construction or as a resilient joint. In the last mentioned case the joint may include at lease one resilient member, in particular a leaf spring, clamped between the shaft or the extension member on the one hand and the pivot member on the other hand, said resilient member, or members, being prestressed for the purpose of forcing the support element on to the seating surface. The prestressing may in this case be so chosen that there will act upon the seating p surface a surface pressure of, for example, 0.1 to 0.5 Ν/mm . This surface -9£2809 pressure reinforces the tension of the post-operative weakened muscles and, during the ingrowth period, prevents lifting of the pivot member from the resected bone surface. Moreover this surface pressure exerts an additional stimulus upon the bone structure of the spongiose tissue and the cortical tissue of the seating surface and favours the growth of bone into suitable reception openings or pores in the support element.

For the purpose of guiding the support element in parallel relation to the seating surface, it is possible in accordance with the invention to arrange two resilient members in a pivot plane of the pivot member so that they are spaced from one another in the manner of a parallelogram. The phrase pivot plane, in relation to the pivot member, is used in the normal sense, i.e. so as to mean a plane in which any part of the member moves when pivoting and therefore a plane normal to the axis of pivotation.

According to another preferred feature of the invention there is provided, between the shaft or the extension member on the one hand, and the pivot member on the other hand, a spring which is prestressed so as to force the support element against the seating surface. In this case also it is possible for the 2 surface pressure on the seating surface to amount to 0.1 to 0.5 N/mm . The resilient member may consist, for example, of silicone rubber having a hardness of A 70+5 Shore.

According to another preferred feature of the invention applied to a hip joint endoprosthesis, the first bone is a thigh bone, whilst the first joint member is designed as a ball and the support element is designed as a collar rigidly secured to the ball. This nevertheless results in a robust and simple construction. The axis of the prosthesis joint may be situated at least approximately in a plane containing the seating surface. In this way there is afforded a quasi-physiological transmission of force from the collar to the seating surface.

In one embodiment according to the invention, the resilient member is arranged between the pivot member and a clamping screw, said clamping screw -1042809 being threaded into a female thread of a pivotable stud mounted upon the extension member. The screw permits the surface pressure upon the seating surface to be finely adjusted. This screw may be locked in position by means of a liquid synthetic plastics adhesive, and the screw which connects the extension member to the shaft can be locked into its tapped hole in the same way.

According to another preferred embodiment of the invention, the second bone is a pelvis bone and the second joint member is substantially hemispherical ball socket which is anchored in the pelvis bone by means of button-like studs. The ball socket may have three studs, which are arranged at the corners of a tri10 angle, and in such a manner that the maximum resultant force upon the body joint passes at least approximately through the superficial centroid of gravity of this triangle. One of the studs can be arranged adjacent the apex and the two other studs can be arranged at least approximately at one half the height of the ball socket. The easiest fitting and the most secure anchorage of the ball socket 15 is achieved if the axes of the three studs lie in planes which are at least approximately parallel to each other, in which case the axes of the two other studs enclose, with a normal drawn to a base surface of the ball socket, a larger angle than the axis of the stud which is located adjacent the apex. For the purpose of obtaining an improved anchorage in the pelvis bone, that stud 2o which is located adjacent the apex may be provided at its foot with a circumferential channel, and the two other studs may each have an undercut directed outwardly from the longitudinal axis of the ball socket.

According to a preferred feature of this embodiment of the invention, the edge of the ball socket may be provided with a cavity, which begins at or approximately in the axial plane of one of the two other studs and which extend over an angular range of approximately 120° or more to that side of the ball socket which is remote from the other one on the two studs. This cavity ensures the unimpeded progression of the musculus iliopsoas subsequent to the implantation.

For improving the ingrowth conditions and the rotational stability, according to a preferred feature of the invention the ball socket is provided at its external surface with a number of channels, some of which are concentric and -1142800 some of which extend in radial planes.

According to a further practical and preferred embodiment of the invention, in the case of an elbow joint endoprosthesis, the first bone is an uppper arm bone, whilst the first joint member is designed as a hinge pin, and the support element includes two condylar shells arranged in spaced relation to each other and connected to the ends of the hinge pin. In this case also, there is afforded a quasi-physiological transmission of force from the condylar shells into the oppositely situated seating surface. For the purpose of simplying manufacture and construction, it is possible to design the hinge pin and the condylar shells in one piece.

According to a preferred feature of the invention, the hinge pin extends into each condylar shell by way of a connecting member which is short in relation to the length of the hinge pin itself, whilst in each condylar shell the section which is free of the connecting member is provided with a perforation to receive a bone screw. This requires removal of only a comparatively small amount of bone for the preparation of the seating surface and, moreover, an adequate amount of bone depth is available for secure anchorage of the bone screws. Moreover the musculature can be protected at its ossous connections.

A desirable enlargment of the seating surface is achieved if, according to a preferred feature of the invention, each connecting member is situated only in that peripheral region of the hinge pin which is embraced by the appertaining condylar shell. This peripheral region may amount, for example, to about 180°.

Advantageosuly each condylar shell is provided with a support arm, the two support arms bearing a portion of the prosthesis joint.

According to a preferred feature of the invention, the shaft or the extension member is provided with a stop or abutment for limiting pivotal movement of the pivot member.

According to a further practical form of the invention, the second bone is an ulna and the second joint member is a bearing shell for the hinge pin and is provided with anchorage projections for anchoring the same in the ulna. By -1242809 this means a total endoprosthesis is also provided in this case. The bearing shell may be designed as a cylindrical half shell, whilst an end face of the bearing shell may be provided with an abutment surface for contact with the stop or abutment in the extended position of the humerous and the ulna. By this means the extended position is precisely defined. The side surfaces of the bearing shell can be axially guided by the condylar shells. The anchorage projections may be provided, in the central transverse plane of the bearing shell, in the form of a stud having a peripheral channel at its foot, and a tongue which is spaced from and directed away from the stud. The bearing shell with the tongue may be fitted in a slot previously provided in the ulna, and the stud may be forced or snapped in the maner of a press button into a corresponding bore prepared in the ulna. In this manner, a firm seating of the bearing shell and satisfactory ingrowth conditions in the ulna are ensured.

According to another preferred feature of the invention, at least the external surface of the individual parts of the body joint endoprosthesis consist of metal coated with enamel. The metal and enamel constitute a compound body, in which the components of the compound, i.e. the metal and the enamel, can be selected and adjusted both relatively to each other and relatively to the particular demands of the situation to give the optimum results. This compound material is ideally biocompatible and possesses technical, physical and chemical characteristics which are superior to all known materials used for prostheses.

According to a further preferred feature of the invention, the shaft is provided with an external sawtooth thread, whose steep flanks are those which face towards the proximal end of the shaft, i.e. towards the pivot member.

This sawtooth thread can also be designed to be self-cutting and can therefore be constructed, in known manner, so that its turns are locally interrupted substantially in the axial direction and are provided with cutting edges. That portion of the shaft which is provided with the sawtooth thread may be conically tapered towards its distal end, i.e. the end away from the pivot member, and thus may be suited to the shape of the inner wall of the cortical tissue of -1342809 the receiving bone. The crests or heads and the valleys or roots of the teeth of the sawtooth thread can be rounded off. By this means, firstly, undesirable notch stresses in the receiving bone are prevented and, secondly, external enamelling of the entire shaft including the sawtooth thread is facilitated.

The comparatively steep flanks of the sawteeth allow a kind of barbed hook anchorage of the shaft to take place in the bone, and these steep flanks can be located in planes which are at least approximately normal to the longitudinal axis of the shaft. This asymmetrical shape of the sawtooth is significant because the force Fz> cosp (as defined below in relation to Fig. 7 of the drawings) acts always in one direction, which is in fact the direction toward the pivot member, namely the upward direction in the case of a hip joint endoprosthesis when the person having it is standing, and thus forms a substitute for the physiological tensile stresses. Becuase the steep flanks of the thread are in planes normal to the longitudinal axis of the shaft, there will be no radial forces resulting from the force F^. cose, which could exert upon the bone a non-physiological bursting effect.

An instrument for the insertion and removal of a body joint endoprosthesis according to the above-mentioned features of the invention is advantageously made as a rigid rotary tool which can be coupled to the shaft. This rotary tool allows the shaft to be rotated into a threaded hole previously prepared, for example, by means of self-centering conical boring tool, from which hole the shaft may likewise be withdrawn by rotation.

, The accompanying drawings, apart from the representations of the state of the. art already described in relation to Figs. 1 to 3, comprise the following figures representing preferred practical forms of the body joint endoprosthesis of the invention: Fig. 4 is a femur head-endoprosthesis with a hinge joint, shown in section along the line IV-IV of Fig. 5; Fig. 5 is a side elevation in section along the line V-V of Fig. 4; Fig. 6 is a side elevation as seen from the left of Fig. 5, with the thigh bone shown in cross-section; -1442809 Fig. 7 is a schematic diagram corresponding to Fig. 6, with the conditions operating in practise being indicated; Fig. 8 is a part sectional elevation taken along the line VIII-VII of Fig 9 of a femur head endoprosthesis with a balljoint; Fig. 9 is a part-sectional elevation taken along the line IX-IX of Fig. 8; Fig. 10 is a sectional elevation taken along the line X-X of Fig. 11 of a femur head endoprosthesis with a knife edge bearing construction; Fig. 11 is a part-sectional elevation taken along the line XI-XI of Fig.

; Fig. 12 is a sectional elevation taken along the line XII-XII of Fig. 13 of a femur head endoprosthesis with a resilient joint; Fig. 13 is a part sectional elevation taken along the line XIII-XIII of Fig. 12; Fig. 14 is a sectional elevation taken along the line XIV-XIV of Fig. 12; Fig. 15 is a sectional elevation taken along the line XV-XV of Fig. 16 showing a femur head endoprosthesis with a resilient joint; Fig. 16 is a part-sectional elevation taken along the line XVI-XVI of Fig.

; Fig. 17 is a side elevation as seen from the left of Fig. 16, with the thigh bone shown in longitudinal cross-section; Fig. 18 is a sectional elevation taken along the line XVIII-XVIII of Fig. showing a femur head endoprosthesis with a Cardan joint; Fig. 19 is a part-sectional elevation taken along the line XIX-XIX of Fig. 18; Fig. 20 is a part-sectional elevation taken along the line XX-XX of Fig. showing a ball socket for a total hip joint endoprosthesis; Fig. 21 is a plan view of the ball socket of Fig. 20; Fig. 22 is a longitudinal section through the ball socket of Figs. 21 and 22, together with the corresponding ball; Fig. 23 is a front-elevation of a right-hand elbow joint endoprosthesis (without a bearing shell); -1542809 Fig. 24 is a sectional elevation taken along the line XXIV-XXIV of Figs. 23, but with the bearing shell included; Fig. 25 is a sectional elevation taken along the line XXV-XXV cf Fig. 24; Fig. 26 is a sectional elevation taken along the line XXVI-XXVI of Fig. 24; Fig. 27 is a part-sectional elevation as seen from the right-hand side of Fig. 23; Fig. 28 is a plan view of the bearing shell of Figs. 24, 26 and 27; Fig. 29 is a left side elevation of the bearing shell of Figs. 24 and 26 to 28; Fig. 30 is a longitudinal section through a shaft having a sawtooth thread.

Figs. 4, 5 and 6 represent a femur head endoprosthesis 70. A metallic shaft 73 coated with enamel and provided with an external rounded thread 75 is screwed into an internal thread 77 formed in the thigh bone 30. The internal thread 77 is cut in advance by means of a thread core boring and tapping drill or milling tool, not shown in the drawing.

The metallic shaft 73 is surmounted by a cone 79 (Fig. 5), which is also enamelled and ground to a finish and has a threaded bore 80, which is not enamelled. An extension member 83 is releasably mounted upon the cone 79 and the member 83 has a conical socket 82 complementary to the cone 79. A screw 87 passes with appropriate clearance through a bore 85 in the conical socket 82 and has a non-enamelled thread 89 screwed into the threaded bore 80 for securing the conical socket 82 with respect to the cone 79. The underside 90 of the screw head and the opposing reception surface 92 of the conical socket 82 are enamelled and are ground so that they make a liquid tight connection.

Upwardly from the conical socket 82 extends an arm 95, which in the view shown in Fig. 5 encloses an angle 98 with the longitudinal axis 96 of the shaft 73. The arm 95 carries at its upper end a hinge eye 100 of a hinge joint 102. A hinge pin 103 passes through the hinge eye 100 and also a fork 105, which is formed upon a collar 107 of a pivoting member 109 bearing upon the seating surface 50 of the thigh bone 30. The collar 107 is rigidly connected by means of a neck 110 to a ball 113. From the underside of the collar 107 -1642809 extends a bearing pocket 115 for a spring 117 of silicone rubber, which is supported at its other end upon a support arm 119 of the arm 95.

With the exception of the spring 117, which is also biocompatible, the entire external surface of the pivoting member 109, the hinge joint 102 and the extension member 83 is also enamelled. Those enamelled surfaces of the femur head endoprosthesis 70 which are situated opposite to spongy bone (spongiose tissue) 120 or bone scale (cortical tissue) 122 (Fig. 4) can be suitably prepared by roughening, creation of artificial pores or the like so as to present an optimum condition for the inward growth of bone tissue, and thus to achieve a very desirable secondary anchorage of the prosthesis in the thigh bone 30.

The ingrowth process is reinforced due to the fact that the spring 117 bears with a certain prestress to force the collar 107 on to the seating surface 50 and thereby, whenever the hip joint is not under load due to the attitude of the person fitted with the endoprosthesis, the spring 117 also provides for the establishment of definite load conditions upon the seating surface 50.

In Fig. 7 the operative conditions are illustrated schematically. Analogously to the physiological conditon shown in Fig. 3, the resultant force FR is transmitted to the seating surface 50 as a surface load 125 which progressively decreases from the region of the acetabulum toward the hinge pin 103 and can thus be represented as and regarded as a substantially triangular surface load.

As already explained in the physiological case according to Fig. 3, a tensile stress is effective outwardly, i.e. to the left-hand side of the neutral position as shown in Fig. 7. In a similar way, the presence of the hinge joint 102 results in a resultant tensile force Fz, which, taking into consideration the the angle β can be resolved into its mutually normally directed components Fz.cos β and Fz-sin β. The component Fz.cos β acts as a tractive force upon the shaft 73 and is transmitted through the external round threads 75 thereof into the cortical tissue 122 of the thigh bone 30. In that region thrust stresses τ will arise, as indicated in Fig. 7.

The axis of the hinge joint 102 is laterally removed by a distance b from the plane which contains the longitudinal axis 96 of the shaft 73. This results -1742809 in a force couple b F^.cos β, which acts in the same sense as a moment composed of a frictional force k.Fr and a lever arm d, which corresponds to the perpendicular spacing distance from the seating surface 50 to a centre point p of that portion of the shaft 73 which is provided with the external round thread 75. In the opposite sense, a momemt will act consisting of the force component ^..sine and a lever arm o, the latter corresponding to the perpendicular spacing distance from the centre point P to the line of action of the force component F^.sin β The moment resulting from the above-metnioned force couple and the abovementioned two moments gives rise to surface pressures c at the cortical tissue 122. If, for structural considerations, the distance b is made equal to zero, the above-mentioned force couple b .F^.cos e vanishes. The surface pressures s result then from the resulting moment M, according to: Σ M=F2. sin δ . c~u . F^.d The deformation of the thigh bone 30, resulting from the surface load 125 acting upon the seating surface 50, turns the pivoting member 109 about the axis of the hinge joint 102 and in this way, with any shape of the thigh bone 30 in a manner analogous to the physiological conditions. From the lower margin of the bearing pocket 115 to the upper termination of the external round thread 75 of the shaft 73 there is no positive connection between the femur head endoprosthesis and its bearing in the thigh bone 30. By this means, minute relative movements of the contact surfaces of the prosthesis and the bone are decisively reduced.

Figs. 8 and 9 show a femur head endoprosthesis 130, wherein the joint is designed as a ball joint 131. A ball 133 of the ball joint 131 is formed at the top of the extension member 83, whilst there is a ball socket 135 at the underside of the collar 107, In the bearing pocket 115 is a spring 137, consisting, for example, of silicone rubber, and having an axial aperture. Through this aperture passes a clamping screw 139, which is screwed to a pivot stud 140, which is mounted -1842809 in a bore 141 in the extension member 83. The screw 139 also passes through holes 143 and 144 in the bearing pocket 115 and the extension member 83 whilst allowing sufficient lateral clearance therein to provide for any movement of the pivoting member 109 relative to the shaft 73. Likewise adequate lateral clearance is provided for the head of the screw 139 so that the pivoting member 109 can move universally about the ball joint 131 and can afford particularly uniform power transmission into the thigh bone 30.

In Figs. 10 and 11, a further femur head endoprosthesis 150 is shown, wherein the extension member 83 has its head formed as a hook-shaped knife bearing edge 153, whose free end 155 forms at the same time the upper abutment for the spring 117. At right-angles to the knife edge 153, a counteracting knife edge 157 is provided, which is formed at the base of the collar 107. Moreover the knife edge 153 at its top lies in sliding engagement with the underside of the collar 107. The knife edge bearing 153 and the counteracting knife edge 157 form a knife edge bearing construction 159, which can be fully enamelled, as can also the ball joint 131 in Figs. 8 and 9.

A further femur head endoprosthesis 160 is shown in Figs. 12 and 13. This shows a spring bearing 161 at tbe top of the extension member 83 and a further spring bearing 163 formed upon the collar 107. The spring bearings 161 and 163 are each provided with a slot 165 and 166 respectively, in which a leaf spring 168 is secured, for example by adhesion or pressing in, and this leaf spring 168 is slightly prestressed in such a manner that the collar 107 is applied with the desired surface pressure to the seating surface 50 of the thigh bone 30. The edges of the slots 165 and 166 are rounded off at the places where, otherwise, the deformation of the leaf spring 168 would result in undesirable edge pressures of large dimensions. In this construction, an extension 169 is formed at the base of the collar 107 for positive lateral guidance of the pivoting member with the bone as well as to increase the area of contact with respect to the surrounding bone tissue and thereby to enhance the conditions for ingrowth.

Fig. 14 shows the spring bearings 161 and 163 and the leaf spring 168 inserted therein, as viewed from above as shown in Fig. 12, the resilient joint formed in this way by the bearings 161 and 163 and the leaf spring 168 is indi-192009 cated by the general reference 170.

In Figs. 15, 16 and 17, a further femur head endoprosthesis 173 is shown, wherein there are mounted at the top of the extension member 83 in the following sequence a leaf spring 175, an intermediate ring 177, a further leaf spring 179 and a covering ring 180. The components 175 to 180 are penetrated by a clamping screw 182, which is screwed into the extension member 83 and axially compresses these components. The leaf springs 175 and 179 are thus effectively clamped upon the extension member 83. A similar clamping arrangement of the leaf springs 175 and 179 is provided upon the pivoting member 109, where again the leaf springs 175 and 179 as well as a covering ring 184 and an intermediate ring 185 are penetrated and axially compressed by a clamping screw 187 which is screwed into the collar 107. The clamping screws 182 and 187 can be locked in their threaded position by means of liquid synthetic plastics material.

This construction comprising the two leaf springs guided in parallelogram fashion also constitutes a resilient joint, like the joint 170 of Figs. 12 to 14, the resilient joint of Figs, 15 to 17 being indicated generally at 189.

The leaf springs 175 and 179, like the leaf spring 168 in Figs. 12 and 13, may consist of alloy spring steel, which may be coated, for example, with silicone rubber, to avoid the possibility of any contact of metal with parts of the body. All the other parts of the femur head endoprostheses 160 and 173 may again be fully enamelled.

Figs. 18 and 19 shows a femur head endoprosthesis 190, wherein at the upper end of the extension member 83 there is arranged a fork trunnion bearing 192 within which is rotatably mounted a hinge pin 194 passing through the fork bearing. The pivoting member 109 carries a further fork bearing 195, which, being displaced through 90° with respect to the fork bearing 192, is rotatably mounted upon a hinge pin 197, which is itself penetrated at right angles by the hinge pin 194. Both of the hinge pins 194 and 197 pass through a core 198 and form with the core the cross member of a Cardan joint 199, which, like the ball joint 131 of Fig. 8, permits universal movement of the pivoting member 109 with respect to the thigh bone 30. -2042809 Figs. 20 to 22 show a substantially hemispherical ball socket 250 for a total hip joint endoprosthesis. The external surface of the ball socket 250 is provided with concentric channels 252, 253 and 254 and with channels 256 to 259 extending in radial planes, which are intended to facilitate and promote the macroscopic ingrowth of new bone tissue into the surface of the ball socket and thereby to effect secondary anchorage in the pelvis bone 48 (Fig. 22).

For the primary or temporary anchorage of the ball socket 250 in the pelvis bone 48, three studs 261, 262 and 263 of button shape are provided, which are arranged at the corners of a triangle in such a manner that the maximum resultant force Fr (Fig. 22) is directed at least approximately through the centroid of this triangle. The three studs 261, 262 and 263 are positioned with their longitudinal axes in planes which are at least approximately parallel to each other, whilst the axis of the stud 261 forms an angle 267 (Fig. 20) of 10° with the main axis 265 of the ball socket 250, and the axes of the studs 262 and 263 each form an angle 270 of 25° with respect to a normal drawn to the base surface 269 of the ball socket 250. The stud 261 is provided at its root with a peripheral groove 272, whilst each of the two other studs 262 and 263 is provided with an undercut 274 directed outwardly from the main axis 265 of the ball socket 250.

The ball socket 250 is provided at its lower edge with a cavity 275 at one side thereof proceeding from the base surface 269, the sickle-shaped formation of which cavity, as seen in plan view, is clear from the assumed external contour of the ball socket 250 shown in chain lines in Fig. 21. This cavity 275 is provided for the unimpeded progression of the musculus iliopsoas, and is positioned, for the other hip joint, in the mirror image position with respect to the central plane of the hall socket 250 containing the axis of the stud 261. As shown best in Fig. 21, the cavity 275 extends over an angle 276 of about 125? Fig. 22 shows the ball socket 250 fitted in its assembled position in the pelvis bone 48. In order to prepare the pelvis bone for the implant, the physiological hip joint socket is first of all milled out with a spherical milling tool. In the resultant spherical milled cavity, three bores are made for the studs 261, -2142809 262 and 263 by means of a drilling template. In this operation, the bores are located somewhat closer together than the distance 278 shown in Fig. 20.

Thereafter the ball socket 250 is inserted in such a manner that the undercuts 274 of the studs 262 and 263 are guided over the edge of the cortical tissue 279 of the pelvis bone 48. In this operation the adjacent spongy bone is displaced towards the side. Thereafter the ball socket 250 is gradually inserted into the milled out spherical cavity until the stud 261 snaps into its bore. During this snapping in operation, the cortical tissue 279 between the studs is elastically deformed and, after springing back, locks the ball socket 250 in the inserted position, as illustrated in Fig. 22.

The special positions of the studs 261, 262 and 263 with reference to the resultant force FR has the result that in the studs additional thrust stresses due to elastic deformation of the bone, under the influence of the resultant force Fr when this force assumes its maximum value, do not arise. Thus the possible thrust stresses remain at a minimum value.

The ball socket 250 can be metallic and can be coated with enamel over its entire surface.

In Figs. 23 to 29, a total elbow joint endoprosthesis 290 is shown. As in the case of the foregoing figures, similar parts are indicated by the same reference characters.

In this case the shaft 73 is screwed into a humerous bone 293. The extension member 83 is provided with a downwardly pointing stop 295, shown in Figs. 23, 24 and 27, for a bearing shell 297, which does not appear in Fig. 23 and is shown inserted in an ulna 299 in Figs. 24 to 27. The bearing shell 297 is provided with an abutment surface 300 (Fig. 29) for the purpose of making contact with the stop 295 in the extended position of the humerous 293 and the ulna 299. The side surfaces 302 and 303 (Fig. 28) of the bearing shell 297 are guided by corresponding opposite faces of condylar shells 305 and 306, which are respectively connected to an eye of the fork 105 of the hinge joint 102. A hinge pin 308 rotatable mounted in the bearing shell 297 is made integrally with the condylar shells 305 and 306. The hinge pin 308 extends into each condylar shell 305 and -2242809 306 by means of respective connecting members 310 and 311 only (Figs. 26 and 27) which are comparatively short relative to the hinge pin 308 and of substantially semicircular cross sectional surface. Each condylar shell 305 and 306 further more is provided with a hole 313 for a bone screw 315 (Fig. 27), which, after mounting the condylar shells upon the suitably prepared condyles 317 and 318 (Fig. 25), are screwed into these condyles for the purpose of temporarily anchoring the prosthesis. The connecting members 310 and 311 are comparatively small so that as much as possible of the bone substance of the condyles 317 and 318 can be left during preparation of the site for implantation of the prosthesis.

For the implantation of the elbow endoprosthesis, first of all the distal upper arm bone 293 is milled out in order to make it possible to provide a tapped bore for the external round thread 75 of the shaft 73. Following this, the contact surfaces for the condylar shells are milled upon the condylars 317 and 318. Moreover, the space for the extension member 83 and its stop 295 are milled out. When these milling operations have been completed, the conical socket 82 is mounted upon the cone 79 and secured by the screw 87, the socket 82 having the pivoting member 320 (Figs. 24 and 26) linked to it by the hinge joint 102 and including the condylar shells 305 and 306 as well as the hinge pin 308. Then the two bone screws 315 are driven into the condyles 317 and 318 of the upper arm bone 293. In this manner, the pivoting member 320 is temporarily secured. Its final fixing to the upper arm bone 293 should again follow by reason of ingrowth of the bone cells into the pores of the inner condylar shell surface.

In Figs. 24 and 27, a muscle portion 323 is shown in each case on the ulna 299. This illustration shows that the muscle portion 323 and other body parts which normally stress the implanted body joint endoprosthesis are not affected by the latter and can fully exert their normal functions. The bearing shell 297 is provided at its rear side with anchorage projections in the form of a stud 325 and a tongue 327 arranged in spaced relation to and directed away from the stud 325. The stud 325 is provided at its root with a peripheral channel 329.

In order to effect implantation of this prosthesis first of all the proximal -23ulna 299 is prepared by milling out the cylindrical hollow shell of the physiological joint to the external radius of the bearing shell 297, and by milling off both sides of the elbow protruberance (olecranon) to the width of the bearing shell 297 so as to accept a drilling and milling template. By the aid of this template, a slot for the tongue 327 and a bore for the stud 325 are made.

The bearing shell 297 is next inserted by first locating the tongue 327 in the slot prepared for it and then is snapped into place by pressing the stud 325 in the bore prepared for it. This connection is again temporary and should be completed finally by the later ingrowth of bone cells into the porous bearing shell surface at the contact surfaces of the bone sections.

From the upper edge of the condylar shells 305 and 306 to the lower termination of the external round threads 75 of the shaft 73, there is no positive connection made through bone material between the prosthesis and its bone support, so that relative micro-movements of the contact surfaces between the implant and the bone are decisively reduced.

For the purpose of the elbow joint, the hinge joint 102 has been described as an example with reference to Figs. 23 to 29. However, in place of this hinge joint 102, it is possible to use for the elbow joint other types of prosthesis joint which have been described above in connection with the hip joint.

By analogy with the various hip joints, it is also possible for the total elbow joint endoprosthesis 290 to be made of metal, whose entire surface is enamelled. The bone screws 315 can also be fully enamelled.

Furthermore, it should be noted that it is only for the purpose of the present disclosure that a hip joint has been selected as an example of a prosthesis including a ball joint and the elbow joint has been selected as an example of a prosthesis which includes a hinge joint. The above-disclosed princioles are also applicable generally to all other type of body joint.

Fig. 30 shows a form of shaft 73 which is modified in that it is not provided with a symmetrical external round thread but with a thread 430 of sawtooth shape, which tapers conically downwards towards its distal end. The steep flanks 433 of the sawtooth thread 430 are thus those facing the proximal end of the -2442809 shaft 73 and are preferably located in planes normal to the longitudinal axis of the shaft 73. The crests 435 and the valleys 437 of the teeth of the thread 430 are in each case roundpd. By this means, firstly, the bone which is situated opposite to the sawtooth thread 430 after insertion thereof is carefully treated and, secondly, it facilitates the application of an enamel coating 440, shown in chain-dotted lines in Fig. 30, over the entire shaft 73 with the exception of the threaded bore 80. A bore 443, which is also enamelled, is made in the bottom of the shaft 73, into which a deposit is inserted, for example, an anitbiotic, before fitting of the shaft 73 takes place. For promoting ingrowth of the surrounding bone tissue into the shaft 73, the enamel coating 440 can be externally roughened.

Claims

1. A body joint endoprosthesis, comprising a shaft for anchorage in a first cone, a first joint member connected to the shaft and cooperable with a second joint member for connection to a second bone, and a support element connected to the shaft for bearing against a seating surface of the first bone, wherein the first joint member and the supporting element are comnonents of a pivot member connected to the shaft by way of a prosthesis joint.

2. A body joint endoprosthesis according to claim 1, wherein a part of the prosthesis joint adjacent the shaft comprises an extension member releasably connected to the shaft.

3. A body joint endoprosthesis according to claim 2, wherein the extension member comprises a conical socket mounted upon a complementary cone of the shaft, the socket being urged against the cone by a screw inserted into a tapped bore in the shaft.

4. A body joint endoprosthesis according to claim 1, 2 or 3, wherein the shaft has an external rounded thread for holding the shaft in the first bone. 5. A body joint endoprosthesis according to claim 1, 2, 3 or 4, wherein the prosthesis joint comprises a hinge joint.

5. A body joint endoprosthesis according to claim 1, 2, 3 or 4, wherein the prosthesis joint comprises a ball joint.

6. 7. A body joint endoprosthesis according to claim 1, 2, 3 or 4, wherein the prosthesis joint comprises a Cardan joint.

7. 8. A body joint endoprosthesis according to claim 1, 2, 3 or 4, wherein the prosthesis joint comprises a knife bearing joint.

8. 9. A body joint endprosthesis according to claim 1, 2, 3 or 4, wherein the prosthesis joint comprises a resilient joint.

9. 10. A body joint endoprosthesis according to claim 9, wherein the prosthesis joint includes at least one prestressed resilient member between the shaft or the extension member and the pivot member for urging the supporting element toward the seating surface.

10. 11. A body joint endoprosthesis according to claim 10, wherein the or each -2642809 resilient member comprises a leaf spring.

11. 12. Λ body joint endoprosthesis according to claim 10 or 11, wherein two resilient members are arranged in a pivot plane of the pivot member and are spaced from one another in a parallelogram arrangement.

12. 13. A body joint endoprosthesis according to any of claims 1 to 8, wherein a spring is located between the shaft or the extension member and the pivot member and is prestressed to urge the supporting element, in use, toward the seating surface.

13. 14. A body joint endoprosthesis according to any preceding claim, for use as a hip joint endoprosthesis, wherein the first bone is a thigh bone, the first joint member comprises a ball and the supporting element comprises a collar secured to the ball.

14. 15. A body joint endoprosthesis according to claim 14, wherein the prosthesis joint axis is located at least approximately in a plane containing the seating surface.

15. 16. A body joint endoprosthesis according to claim 13, 14 or 15, whereon the spring is located between the pivot member and a clamping screw located in a thread formed in a pivotable stud mounted on the extension member.

16. 17. A body joint endoprosthesis according to any of claims 14 to 16, wherein the second bone comprises a pelvic bone and the second joint member comprises a substantially hemispherical ball socket having button-like studs for anchorage in the pelvic bone.

17. 18. A body joint endoprosthesis according to claim 17, wherein the ball socket includes three studs arranged at the corners of a triangle so that in use the maximum resultant force upon the body joint passes at least approximately through the centroid of the triangle.

18. 19. A body joint endoprosthesis according to claim 18, wherein one of the studs is arranged adjacent the apex and the two others at least approximately at half the height of the ball socket.

19. 20. A body joint endoprosthesis according to claim 19, wherein the axes of the studs lie in at least approximately mutually parallel planes, the axes of the two -274280® other studs respectively enclosing a greater angle with a normal to a base surface of the ball socket than the axis of the stud adjacent the apex.

20. 21. A body joint endoprosthesis according to claim 19 or 20, wherein the Stud adjacent the apex has a circumferential channel at its foot and the two other studs each have an undercut configuration directed outwardly from the longitudinal axis of the ball socket.

21. 22. A body joint endoprosthesis according to any of claims 19 to 21, wherein the edge of the ball socket includes a cavity which begins at least approximately in the axial plane containing one of the two other studs and extends over an angular range of at least approximately 120° to the side of the ball socket remote from the other of the two studs.

22. 23. A body joint endoprosthesis according to any of claims 17 to 22, wherein the ball socket has a plurality of concentric channels and channels disposed in radial planes in its external surface.

23. 24. A body joint endoprosthesis according to any of claims 1 to 13, for use as an elbow joint endoprosthesis, wherein the first bone is an upper arm bone, the first, joint member comprises a hinge pin and the supporting element comprises two spaced condylar shells rigidly connected to the ends of the hinge pin.

24. 25. A body joint endoprosthesis according to claim 24, wherein the hinge pin and the condylar shells are made in One piece.

25. 26. A body joint endoprosthesis according to claim 24 or 25, wherein the hinge pin extends into each shell by means of a connecting member, the part of each shell free from the connecting piece being provided with a perforation for receiving a bone screw.

26. 27. A body joint endoprosthesis according to claim 26, wherein each connecting piece is located only in the peripheral region of the hinge pin surrounded by the associated condylar shell,

27. 28. A body joint endoprosthesis according to any of claims 24 to 27, wherein each condylar shell includes a support arm bearing a part of the prosthesis joint.

28. 29. A body joint endoprosthesis according to any of claims 24 to 28, wherein the shaft or the extension member includes an abutment for limiting pivotal move-2842809 ment of the pivot member.

29. 30. Λ body joint endoprosthesis according to any of claims 24 to 29, wherein the second bone is an ulna and the second joint member comprises a bearing shell for the hinge pin provided with projections for anchorage to the ulna. 5

30. 31. A body joint endoprosthesis according to claim 30, wherein the bearing shell comprises a cylindrical half shell, an end face of the bearing shell comprising an abutment surface for making contact with the limiting abutment in the extended position of the upper arm bone and the ulna.

31. 32. A body joint endoprosthesis according to claim 30 or 31, wherein end faces 10 of the bearing shell are axially guided by the condylar shells.

32. 33. A body joint endoprosthesis according to any of claims 30 to 32 wherein the anchorage projections are located in the central transverse plane of the bearing shell and comprise a stud having a peripheral channel and a tongue directed away from the stud being spaced from the stud. 15

33. 34. A body joint endoprosthesis according to any preceding claim, wherein at at least the external surfaces of its parts comprise enamel-coated metal.

34. 35. A body joint endoprosthesis according to any preceding claim, wherein the shaft includes an external sawtooth thread having its steeper sides toward the pivot member. 20

35. 36. A body joint endoprosthesis according to claim 35, wherein the feet and heads of the thread are rounded off.

36.

37. A body joint endoprosthesis according to claim 1, substantially as described with reference to any of the accompanying drawings.