CA1075851A

CA1075851A - Body joint endoprosthesis and instrument therefor

Info

Publication number: CA1075851A
Application number: CA249,497A
Authority: CA
Inventors: Helmut Grell; Achim Engelhardt
Original assignee: Pfaudler Werke GmbH
Current assignee: Pfaudler GmbH
Priority date: 1975-04-04
Filing date: 1976-04-02
Publication date: 1980-04-22
Also published as: LU74699A1; IE42809B1; GB1511115A; IT1057845B; NL7602392A; FR2305961B1; SE410388B; DK158176A; SE7602440L; AT349618B; DE2514793B2; DD123056A5; DE2514793A1; IE42809L; JPS51122995A; BE839461A; FR2305961A1; DE2514793C3; CH611795A5; ATA148176A

Abstract

BODE JOINT ENDOPROSTHESIS AND
INSTRUMENT THEREFOR

ABSTRACT
A body joint endoprosthesis includes an anchoring member having a shaft anchored in a first bone and a pivot member connected to the anchoring member by a pivot joint. The pivot member includes a first body joint member and a support element that bears against a seating surface of the first bone. The first body joint member and a second body joint member, connected to the second bone, form the body implant joint.

Description

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This invention relates to a body ~oint endoprosthesis, an~ more particularly to an endoprosthesis that transmits forces in a manner more closely approximating normal physio-lo~ical s~ress aistribution.
Wear phenomena in the ~oints of aged people, the conse-quenca of inherited disease, rheumatic inflammatory ailments and in~uries may leaa to chronic pain conditions and pro-gressive restriction of ~oint mobility (arthrosis), which often se~erly limits the sphere of activity and imposes acute physical stress upon the suffererO
By way of showing the state of the art, reference is made to the paper "Technischer Fortschritt bei kunstlichen Huftgelenken" in the periodical Technische Rundschau Su-læer, 4/1974, pages 235 to 245. This treatise gives a review o~
the development of artificial hip ~oints and describes typical constructional forms known since 1939. From the discussion in the paper o~ these particular structural ~oxms, it is apparent that all of the constructions have more of less serious inherent disadvantages. In addition to wear p~enomena in the tw~ parts of the joint, the ball and the socket, the known shaft prosthesis are accompanied by the particular disad~antage that loosenin~ of the sockets and the sbafts occurs, accompanied in certain cases by the subsequent breakage of the shaft~ In the known artificial hip joints, this loosening o~ the shaft can be attributed to several di~erent causes. Firstly, the shaft is preferably cemented into the bone. The cement which is used is the sei~-poly~erizing synthetic plastic material methyl methacrylat~. The heat of polymerization resulting from the curin~ of this material results In temperatures o~ 80 to 100C and can cause thermal damage of the surrolmding tissue, because the coagulatlon point of albumen amDunts to 56C, In addition to these thermonecroses, damaging ef~ects are also revived which resulted from the mechanical preparation of the bone ~rasping or similar operations~ carried out during the preparation of the seating to receive the implant, and at khe 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 micro relative displacements to take place between the implant and the bone seating. For a fuller treatment of thi~ condition, re~erence is made to the book Biopolymere und Biomechanik von_Bihdegewebssystemen, Springer-Verlag 1974, pages 417 to 419, in the article "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 implant to the bone. For the want of any other exposition on this sub~ect in the known art, an explanation thereo now be given with reference to Figs 1, 2 and 3 of the accom-panying drawings~
In Fig. 1 a sha~t 35 o~ 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 extension termin-ating in a ball 42 having a center point M. The ball 42 rests in a ball socket 45~ which is secure~ in a pelvis bone 48 by cement 46. The resulting force F~ passes throu~h the center point M o~ the b~ll 42 and in Fig~ 1 is shown in the direction in which this resultant force has its ~-ximum value~
In that case, the direction o~ the force with respect to .

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the longitudinal axis o~ the neck 40 encloses an angleo~ .
If this resultant ~orce F is red~tced to a point A o~ the seating sur~ace 50 o~ ~he thigh bone 30 cooperating with the collar 39, then there will act in a direction normal to the seating sur~ace 50 the force F .coso~ and in the direction of the seating surface 50 the ~orce component F~.sin ~ . In addition, due to the parallel displacement of the resultant ~orce FR there is also e~ective the force couple FR.a in accordance with the parallelogram surface 51 shown hatched in Fig. 1, The distance a repre~ents the shortest spacing o~ the point A from the line of action of the resultant ~orce 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 ~orce couple~ It is seen that the surface pressure p acting normal to the seating surface 50 is practically constant, while in the physiological case, represented in Fig. 3, the approximately linear aurve o~ the normal stress ~ acting at the right hand or medial edge in Fig. 3 shows a maximum compression stress o~ Gr D max~
In the physiological case according to Fig 3 there will he a neutral fiber at a position spaced by a distance e from this position of maximum compression s~ress so that remote from it at the left hand or lateral edge in Fig~ 3 there will be a tensile stress ~Z max.
In the physiological case according to Fig. 3 practically no normal stxess arises at right angles to the direction of the fibres o~ the cortical tissue 122 indicated in Fig.
1 and 2, but in the othe~ case the ~orce component FR.sin and the force coupie FR~a will give rise to the surface pressures 1~'7~
q and r, which act at right an~les to the inner surface of the cortical tissue. The surface load q at the inner medlal mar~in of the seating surface 5n has a maximum qmax' which, in the course of the continuous reconstitution of the bone, results in a progressive yieldLng of the bone.
Along with this yielding o the bone, the bending stress of the prosthesis and the cement increases until, due to further loosening of the shaft 35, cracks appear in the cement cladding 32, and finally a breakage 55 of the shaft 35 causes the prosthesis to fail and leads to immobility of the patient~
The relative movements between the collar 39 and the seating surface 50 arising from this loosening process prevent the desirable pro~ressive ingrowth of the bone ceiis into porasr cavities or perforations in the surface of ~he known prostheses.
The above mentioned cracks in the cement cladding lead to intense corrosion phenomena in the metal (crack corrosion)~
Sinking of the shaft prosthesis is also possihle if the support afforded by the cortical tissue is lost.
The force F~.cos~C indicated in Fig~ 1 as acting at right angles to the seating sur~ace 50 is transmitted through collar 39, ana, in respect of one part, over the seating surface 50 into the thigh bone 30 and, in respect of another part, is transmitted onto this thigh bone by virtue o~ the positive connection between the implant and the thigh bone 30. If it is assumed that ~he case under consideration is a known impiant of steei, then the relationship of the modulus of elasticity o~ the steei implant to that o~ ~one is about 8al~ Conse~lently, under load the bone deformation is relatively greater than that o~ the steel, and there is a relative displacement of the contacting surfaces o~ the implant and .
. . : :

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the bone. These displacements can lead to shearing o~ o~
the osteoblasts building up to form a bridge between the bone and the implant, which otherwise are desirable for a lasting anchorage of the implant in the bone, As a resul~, there is formed in that regioll a resilient screen of connective tissue, which permlts further relative displacements and therefore a loosening of the prosthesis.
In the book entitled Gesammelte ~bhan~lungen zur ~unktion~
ellen Anatomie des Bewegungsapparates by Friedrich Pauwels~
Springer-Verlag 1965, in particular at page~ 4 to 6 o~ the chapter "Mechanische Faktoren bei der Frakturheilung", a general description is given of tha 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~ defined above in connection with Fig, 1 can result in the setting up o~ a so-called free shearing force in the sea~ing surface, preventing the desirable growth of new hone tissue, this force being explained in the abo~e cited book of Pauwels in the ~hapter "Die freie Scherkraft"
at pagas 21 to 24.
Furthermore, attempts are also known to anchor shaft prosthesis in thigh bone without the use of cenent. Fox this purpose the surface of the shaft is provided with per~or-ations or macroscopic depressions, in which it is intended that there shall be a n~wly formed growth of ~one tissue resulting in an intimately contacting anchorage of the shaPt of the prosthesis in the thigh bone. Nevertheiessr because the implantation of hip joint endoprostheses is mostly necessary in elderl~ patients~ great importance must be placed upon early mobilization of the patient. If the patien~ is laid :

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up ~or too lon~ a time until there is a sufficiently firm in~rowth of the sha~t o~ the prosthesis, thls clelay can, for example, cause the risk of pneumonia, mNscular atrophy and damage to the heart and circulation as well as to the bladder and kidney system, For example, in experiments with animals, the time taken for the formation of load bearln~
bone tissue has been at least two months, Consequently, the kno~n types of prostheses whose anchorage in the thi~h bone relies exclusively upon the principle of the ingrowth of bone tissue are therefore very disadvantageous on account of the lack o~ early mohilization o the patient. Moreover, because of the difference in the moduli of elasticity anfl locally high sur~ace pressures, loosening phenomena can appear at the implant, In known hip ~oint endoprostheses of this type ~German Auslegeschrift Speci~ication No~ 1,54i,246, and French Specifica-tion 2,057,418) a metallic shaft ls formed integrally with a collar shaped suppor~ slement and a bearin~ stud, upon wh~ch a ball head is rotatably mounted as the ~irst part of the joint.
There is no joint between the support eiement and the shaft.
In consequence this construction does not remove the aho~e explained disadvantages.
In a further known hip joint endoprosthesis of the above mentioned type ~French Specification Wo. 2,210,909) a sha~t is formed integrally with a collar shaped support element, to which can be screwed a ~irst joint member in the form of a ball head to form a rigid unit secured by a clampin~ device~
This construction also lacks a joint between the support element and the s~ft~
In the Swiss Patent Specification 426,096 there is dis-closed a hip joint endopros~hesis, to whose shaft there is ... .
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integrally connected a ~irst socket. In this socket there is mounte~l a freely rotatable ball forming the first ~oint member, which latter is also supported in a socket ln the pelvis bone ~orming the second ~olnt member. There is thus the lack of a support element and accordingly also a lack of the joint according to the present invention. The forces applied to the first socket are transmitted exclusively through the sha~t into the thigh bone, so that the abovementioned disadvantages are even more clearly noticeable.
It is already known (German Offenlegungsschrift Speciflca-tion 2,432,766~ to construct an artificial knee joint whlch 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.
; An ob~ect of the present invention is to provide a body joint endoprosthesis w~erein the transmission of force into a 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 in the seating sur~ace, relative movements between the seating surface and the support element should as far as possible be avoided, The anchor~ge in the first bone should take place without the use of cement, It should be possible to provide for the early mobilization of the patient~
`; The invention provides a body joint endoprostheses including an anchoring palrt ha~ing a shaft adapted to be anchored in a first bone, and a pivot member connected to the anchoring part by a pivot jointO The pivot member includes a first body -~75~S~

joint member and a support element adapted to beax against a seatin~ surface o~ the first bone. The ~irst ~oint member, and a second joint member adapted to be connected to a second bone, ~orm the body implant jOillt. The transmission of force ~rom the supporting element into ~he seating sur~ace is analogous to the physiological condition, while a~fordin~ the stxessing conditions which are necessary for the maintenance of the bone tissue, The ingrowth of bone tissue into the openings and pores of the prosthesis is considerably acilitated and the time taken for this process is shortened, The joint resolves disadvantageous stiffness of the prosthesis, and while maintaining the necessary functional stability affords mobillty within the prosthesis itself, which removes the above described dis-advantage s O
~ ccoxding to one practical ~orm o~ the in~ention, thepivot ~oint is in the form of a hinge ~oint. The resulting ability of the pivot member to pivot only in one plane is adequa~e in most cases to provide a transmission o~ ~orce into the first bone approximating to a large extent to the physio-logical condition. However, if universal pivotability o~ the pivot member is desired with respect to the seating sur~ace, the joint may ba a ball ~oint or a Cardan joint.
The ~oint may also be a knife edge type of construction or an elastic ~oint~ ~n the last mentioned case, the joint may include at least one xesilient member, preferably a lea~
spring, clamped between the anchoring part and the pivot member, w1th the resilient member or members being prestressed to ~orce the suppoxting element onto the seating sur~ace, The prestress may produce a suxface pressure of, for example, 0.1 to 0.5 Newtons per square millimete~ ~N~mm2) on the seating surface~

~75~351 This surface pressure rein~orces the tension of the postoperative weakened muscles and, during the ingrowth period, prevents lifting of the pivot memker from the bone section surface~
Moreover, this surface pressure exerts an addltional stimulus upon the bone structure of the spongiose tissue and the cortical tissue o~ the seating surface and favors ~he growth o~ bone into suitable reception openings or pores o the supporting element.
For the purpose of guiding the supporting 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 o~ the pivot member and spaced ~rom each other in the mann~r of a parallelogram.
According to one practical form of the invention applied to a hip joint endoprosthesis, the first bone is a thigh bone, the first joint member is a ball and the support element is a collar rigidly secured to the ball. This produces a robust and simple construction. The axis of the joint may be situated at least approximately in a plane containing the seating surface.
This produces a quasi-physiological transmission o~ ~orce ~rom the collar to the seating surface.
In this form o~ the invention the second bone is a pelvis bone and the second joint member is a ball socket anchored in the pelvis bone. The socket is preferably anchored ln ~he bone by three studs arranged at the corners of a triangle 50 that the maxLmum resultant force upon the body joint passes at least approximately through the surface center of gravity of the triangLev In another practical form of the invention, an elhow joint endoprosthesis, the ~irst bona is an upper arm bone, the ~irst !

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joint member is a hinge pin, and the support el~ment 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 to the oppositely situated seating surface.
According to a further feature of the lnvention the shaft is provided with an external sawtooth thread, whose compara-tively steep flanks are directecl towards the plvot member.
The compara~ively steap flanks of the sawtee~h allow a kind of barhed hook anchorage of the shaft to take place in the bone, and can be directed at least approximately normal to the longitudinal axis of the shaft~ This asymmetrical shape of the sawtooth is signi~icant because the force F~.cos acts always in one direction, which is in fact the upward direction, and thus substitutes the physiological tensile stresses. Upon the ~lanks of the thread directed normal to the longitudinal axis o~ the shaft there will be no radial force resulting from the force Fz.cos ~ , which could exert upon the bone a non-physiological bursting effec~, `!
Another object of this invention is to provide an instrument for the insertion and removal o~ body joint endopros~heses described above~ According to the invention, a rotary tool is adapted to be rigidly connected to the shaft of the anchorlng part. This facilitates screwing the sha~t into or out of a tapped hole in a bone without producing any bending movements.
Undesired stresses upon the bone are thus a~oided~
A holder having a guide device ~or a m~lling tool is preferably ad~ustably secured to this rotary tool. By virtue of the stiffness o~ the rotary tool and its coupling to the ::,. "- .
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shaft it is possible to perform the necessary milling operations upon the bone which is to receive the shaft, and to do this in perfectly determinate geometric relationship with reference to the shaft. During this operation, the shaft there~ore opera~es as a reference body anchored in the bone which is to be milled in the same way as later, after the removal of the instrument, it will be used for the remaining operation of mounting the endoprosthesis upon the shaft. In this manner, precise cooperation o~ the respective prosthesis with the bone parts to be mllled is ensured.
Other objects and advantages of this inventlon will be seen from the following detailed descrlption~
Fig. 1 is a cross-sectional view iliustratlng the resolution of forces in prior axt hip joint endopros~heses.
Fig. 2 is a sectional view illustrating how the prior art hip ~oint endoprosthesis illustrated in Fig. 1 transmits forces to the femur.
Fig~ 3 is a schematic view illustrating stress distribution in the physiological ~emur.
Fig, 4 is a femNr head endoprosthesis with a hinge joint in section along the line IV-IV o~ FigO 5, Fig, 5 is a side elevation in section along the line V-V
of Pig~ 4~
Fig~ 6 is a side elevation from the left of the represent-ation in Fig. 5 with the thigh bone in section.
Fig. 7 is a schema~ic diagram corresponding to ~hat of Fig~ 6 with practical operating quantities insertedD
Fig. 8 is substantially a sectioned ele~ation along the line VI~I-VIII of Fig~ 9 o~ a femur head endoprosthesis with a ball joint, 1~758~1 Flg, 9 is the paxtially sectioned elevation accordin~
to line IX-IX in Fig. 8, Fig, 10 is essentially a sectional elevation along the line X-X of Fig. 11 of a femur head endoprosthesis with a knife edge bearing construction.
Fig. 11 is a partly sactioned elevation along the line XI-XI of Fig. 10.
Fig. 12 is essentially the sectional elevation along the line XII-XII of Fig, 13 of a femur head endoprosthesis with an elastic joint.
Fig, 13 is a partially sectioned elevation along the line XIII-XIII of Fig~ 12~
Fig, 14 is the sectional elevation along the line XIV-XIV
of Fig. 12.
Fig, 15 is essentially the sectional elevation along the line XV-XV of Fig, 16 showing another femur head endoprosthesis with an elastic ~oint.
Fig. 16 is the partial sectional ele~ation along the line XVI-XVI of Fig. 15.
Fig. 17 is the side elevation from the left of the view shown in Fig. 16 with a longitudinally sectioned thigh bone.
Fig. 18 is essentially the sectional elevation along the line XVIII-XVIII of Fig, 19 showing a femur head endoprosthesis with a Cardan joint, Fig. 19 is the partially sectioned eleYation along the line XIX-XIX in Fig~ 18~
Fig~ 20 is the partially sect~oned eievation alon~ the line XX-XX of Fig. 21 showin~ a ball socket.
Fig, 21 is a plan ~iew of the ball socket accor~ing to Fig~ 20, ~17585~
Fi~, 22 is a lon~itudinal section through the ball socket ~ccordlng to Fig, 2Q and 22 with the corresponding ball, ~ ig, 23 is a front elevation of a right hanfl elbow ~olnt endoprosthesls (without bearing shell).
Fig. 24 is a sectioned elevation along the line XXIV-XXIV
of Fig, 23, Fig. 25 is the sectioned elevation along the lines XXV-XXV
of Fig, 24.
Fig, 26 is the sectioned e:Levation along the line XXVI XXVI
of Fig, 24, Fig, 27 is the partially sectioned elevation from the riyht hand side of the view in Fig, 23, Fig, 28 is a plan vlew of the bearing shell in Figs, 24, 26 and 27, Fig, 29 is a side elevation from the left hand side o the bearing sheli according to Fig, 28, Fig, 30 is the sectional elevation along the line XXX-XXX
in Fig. 31 of an instrument for the insertion and removal o~
a body joint endoprosthesis, Fig, 31 is the sectional elevation along the line XXXI-XXXI
in Fig. 30.
Fig. 32 is a side elevation of the instrument with a lon-gitudinally sectioned rotary tool, Fig. 33 is t~e sectional elevation along the line XXXIII-XXXIII of Fig, 32, Fig, 34 ls a longitudinal section through a shat having a sawtooth thread~
Figs, 4, 5 and 6 illustrate a femur head endoprosthests 70 embodying this inventionO Preferably, at least the external surfaces of the endoprosthesis consist of metal coated with .. ''' ~:

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enam~el, i.~., with a vitreous or partially devltxifLed inorganic coatin~ bonded to the metal at a temperature above 800F~
The metal and enamel constitute a compound body, in which the components o~ the compound, i.ec, the metal and the enamel, can be selected and adjusted both with respect to each other and wi~h respect to the particular demands o~ the situation to give the optimum results, This compound ma~erial is ideally biocompatible and possess technical, physical and chemical characteristics which are superior to all known materials used for prostheses.
A shaft 73 coated with enamel and provided with an external rounded thread 75 is screwed into the internal thread 77 of the thigh bone 30. The external rounded thread 75 provides complete anchorage of the sha~t in the bone without bone cement, mastic or the like while still achieving an exceptionaLly good degree o~ positive and/or non-positive connection between the shaft and the bone.
Prefexably the thread 75 of the shaft is conically formed in a manner complementary to the oppositely situated internal wall of ~h~ thigh bone 30. The maximum degree of care o~ the bone and a relLable anchorage of the shaft therein are achieved if 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, before the shaft is screwed into the bone.
The metallic shaft 73 is surmounted by a cone 79 which is also enameLled and ground to a ~inish and has a threaded bore 80, whic~ is ~ot enamelledO Upon the cone 7~ there is releasably mounted an extension member 83 having a conical socke~ 82 complementary to the cone 79. A screw 87 penetrates with appropriate clearance a bore 85 in the conical socket :~.

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82 and has its non-enamelled thread 89 screwed ~nto the tapped bore 80 for ~ecurin~ the conical socket 82 with respect to the cone 79. The underside 90 o~ the screw head and the oppos~ng reception surface 92 of the conical socket 82 are enamelled and are ground to make a li~uid tight connection. Screw 87 may be locked in place with liquid synthetic plastic adhesive~
This construction permits a continuous rotary adjustment and fixing of the conical socket 82 with respect to the cone 79 of the shaft. Even when a comparat~vely small axial com-pression force is exerted by the screw 87, the cone type o~
connection a~fords substantial load bearing frictional forces which insure a reliable functioning of the prosthesis. Even when the shaft 73 is completely enamelled, the cone type o~
connection permits satisfactory working o~ the shaft for the purpose o~ ~itting and fluid tightness, for example by grinding.
The tapped bore 80 and the threads 89 of the screw do not have to be enamelled, because it is possible by suitable working of the opposing enamelled surfaces to obtain a per~ect seal between the screw head and the conical sockek, which prevents the ingress of body liquids and secretions.
An arm 95 extends upwardiy from the conical socket and, as mQy be seen in Fig. 5, encloses an angle 98 with the longitud-inal axis 96 o~ the shaft 73~ The upper end of the arm 95 carries a hinge eye 100 of a hinge ~oint 102. A hinge pin 103 penetrates ~oth the hinge eye 100 and a fork 105, formed upon the collar 107 of a pivot member 109 that bears upon the seating sur~ace 50 of the thigh bone 30 ? The collar 107 is rigidly connected through a neck 110 to a bali 113, From the underside of the collar 107 extends a bearing pocket 115 for a spring 117 o~ silicone rubber~ which supports itsel~ at the other side upon a support arm 119 o~ the arm 95, The use of the extension membex 83 releasably connected to the shaft 73 makes it possible to preassemhle the pivot member 10, the ~oint 102 and the extension member 83 before this structural group is implanted. This shortens the operating time and also allows, for example, a complete enamelling of the surface to be effected. The complete structural group can also be sub~ected, before implantation, to any desired specialized finishing treatment and quality control, With the exception of the spring 117, which is also bio-compatible, the entire external surface of the pivot member 109, the hinge joint 102 and the extension member 83 is enamelle~.
Those enamelled surf~ces of the femur head endoprosthesis 70 which are situated opposite to spongy bone (spongiose tissue) 120 or bone scale (cortical t~ssue) 122 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 by the fact that the spring 117 urges the collar 107 against the seating surface 50 and maintains definite load conditions on seating sur~ace 50 when the load on the hip joint is relieved, Preferably, the collar 107 is pressed against the seating surface 50 with a sur~ace pressure of 0.1 to 0 5 N/mm2.
The pivot joint 102 and the quasi-physiological transmission of ~orce into the bone that it provides relieve the shaft 73 of a substantial load as compared with the conditions existing in known shafts, Thus there is achieved, not only primarily an adequate anchorage of the shaft in the bone, but also sec ~L0~85~

ondarily a further improvement in the anchorage by growth of the bone into the shaft, which favors an early mobiliæation of the patient.
In Fig. 7 the operatin~ quantities are plotted schematically, Analogously ~o the physiological condition shown in Fig. 3, the resultant force FR is transmitted as a substantially tri-angular surface load 125 onto the seating surface 50. As already explained in the physiological case according to Fig~
3, a tensile stress is ef~ective outwardly or to the left hand side of the neutral fiber. In a similar way there will result from the presence of the hinge ~oint 102 a resultant tensile force Fz, which, taking into consideration the angle ~ , can ~e resolved into its mutually normally directed components Fz.cos ~ and Pz.sin ~ , The component Fz,cos~ acts as a tractive force upon the shaft 73 and is transmitted through the outer r~und threads 75 thereof into the cortical tisæue 122 of the thigh hone 30. In that region khere will arise the thrust stresses ~ indicated in Fig. 7, The axis of the hinge joint 102 is laterally removed by a distance b from the plane in which is situated the longit-udinal axis 96 of the shaft 73. This results in a force couple b.Fz.cos ~ , which acts in the sam~ sense as a moment composed o~ a frictional force ~,FR and a lever arm d, which corxesponds to the perpendicular spacing distance from the seating surface 50 to a center point P o that portion o~ the shaft 73 which is provided with the externally rounded thread 75~ In the opposite sense there will act a moment consist1ng of the force component Fz sin~ and a lever arm c, the latter corresponding to the normal spacing distance rom the center polnt P to ~he line of action of the force component Fz,sin~ . The moment ~[)7~

resultinq ~rom the above mentione~ ~orce couple and the above mentioned two moments ~ives rise to the sur~ace pressure s at the cortical tissue 122 0 If, for structural considerations, the spacing distance b is made equal to zero, the above mentioned force couple b.Fz.cos~ vanishes. The surface pressure s results then from the resulting moment M = Fz.sin ~ .c - ~ .PR.d The deformation o~ the thigh bone 30 resulting from the surface load 125 acting upon the seating surface 50 turns the pivot member 109 about the axis of the hinge joint 102 and can in this way, in any occurring shape condition of the thigh kone 30, transmit the force into the thigh bone 30 in a manner analogous to the physiological conditions. ~rom the lower margin of the bearing pocket 115 to the upper termlnation of the externally rounde~ thread 75 of the shaft 73 thexe wil1 exist no positive connection between the ~emur head endoprosthesis 70 and its bearing in the thigh bone 30. By this means relative micro 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, while there is a ball socket 135 at the underside of the collar 107.
- In the hearin~ pocket 115 is a spring i37~ consi~ting, for example, of sili~one rubber having a hardness of A 70~
5 Shore, and having an axial aperature. Through this aperature leads a ciamping screw 139, ~hich is screwed into a pi~ot stud ; 140 mounted in a bore 141 in the extension member 83. The i85~

clamping screw 139 permits the surf~ce pressure exerted by the collar 107 on the seating surface 50 to be ~inely ad~usted.
This screw may he locked in position by means o~ liquid syn-thetic plastic adhesive.
The screw 139 also leads through holes 143 and 144 in the bearing pocket 115 and the extension member 83, while allowing suficient lateral clearance therein to provide for any swinging movement. ~ikewise adequate latexal clearance is provided ~or the head of the screw 139 so that the pivot memher 109 can have motion universally about the ball ~oint 131 and can afford particularly uniorm ~orce transmission into the thigh bone 30.
Figs, 10 and 11 show a further ~emur head endnprosthesis 150, wherein the extension member 83 has its head formed as a hook shapPd knife edge bearing 153, whose free end 155 ~orms the upper abutment for the spring 117~ At r~ght angles to the knife edge 153 proceeds a counteracting knife edge 157, which is formed at the base of the collar 107~ Moreover, the top of the kni~e edge 153 is in sliding engagement with the underside of the collar 107~ The knife e~ge bearing 153 and the counter-acting kni~e 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 ~urther femur head endoprosthesis 160 is shown in Figs~
12 and 130 They show a spring bearing 161 at the top of the extension member 83 and a further spring bearing 163 ormed upon the collax 107, The spring bearin~ me~bers 161 and 163 are each provided with a slo~ 165 and 166 respectively~ in which a lea spring 168 is secured, ~or example by adhesion or pressing in. Spring 168 is slightly prestressed in such 107S~3S~

a manner that the collar 107 is applied with the desired sur~ace pressure to ~he seating surface 50 of the thigh bone 30. The edges o~ the slots 165 and 166 are strongly rounded of~ at the places where, otherwise, the de~oxmation of the leaf spring 168 would result in undesirable edge pressures of large dimen-sions, In this construc~ion, an extension 169 is formed at the base of the collar 107 for positive latexal guidance of the pivoting member with the bone as well as to increase the area of contact with respect to the surrounding bone tlssue and thereby to enhance the conditions for ingrowth.
Fig. 14 shows the spring supports 161 and 163 and the leaf spring 168 inserted therein as viewed from a direction other than that shown in Fig. 12. The elastic joint is shown at 170.
- Figs. 15, 16 and 17 show a further femur head endoprosthesis 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 msmber 83. A similar clamping arrangement of the leaf springs 175 and 179 is provided upon the pivot 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 plastic material. This construc~ion comprising the two leaf springs guided in parallelogram~
fashion also represents an elastic joint 189 ~7~

The leaf spring 175 and 179, like the leaf spring 168 in Figs. 12 and 13, may consist of alloy spring steel, which may be coated, ~or example, with silicone rubber, to avoid the possibility o any metallic contact with parts o~ the body.
All other parts of the femur head endoprosthesis 160 and 173 may again be fully en~melled.
Figs, 18 and 19 show a fe~r head endoprosthesis 190, wherein at the upper end OI the extension member 83 there is arrangea a fork trunnion bearinq 192 within which is rotatably mounted a hinqe pin 194 passing through said ~ork. The pivot member 109 carries a further fork bearing 195, which is displaced through 90 with respect to the fork bearing 192 and is rotatably mounted upon a hinge pin 197, which is itsel penetrated at right angles by the hinge pin 194. Both of the hinge pins 194 and 197 penetrate a core 198 and form with the core the cross member of a Cardan joint 199, i.e., a universal joint consistin~ of a cross like piece, opposite ends o~ which rotate within the forked end of bearings 192 or 195. The Cardan joint 199, like the ball joint 131 of Fig. 8, permits a universal movement of the pivoting member 109 with respect to the thigh bone 30.
Figs. 20 and 21 show a ball socket 250 for a total hip joint endoprosthesis. ~rhe external surface of the ball socket 250 is provided with concentric channels 252, 253 and 254 and with channels 256 to 259 located in planes extending through the axis 265 oi~ the socket, These channels faciliate and promote the macroscopic ingrowth o~ new bone tissue into the surface of the ball socket and thereby effect secondary anchorage in the pelv~s bone 48 ~Fig. 22~ .
For the primary or ten~porary anchorage orC the ball socket 250 in the pelvis bone 48 there are provided three studs 261, 262 and 263 of button shape, which are arranged at the corners 1~7585i of a triangle in such a manner that the maximum xesultant ~orce F~ (Fig. 22) is directed at least a~proximatel~ through the s~face center of gravity or centroid of this trlanglel i.e,, at least approximately through the point where three lines, each of which extends from a corner of this triangle to the mid-point of the opposite side o~ the triangle, intersect.
Stud 261 is positioned near the apex of the socket 250 ana the other two studs 262 and 263 are positioned at approximately one-half the heighth of khe socket. The easiest fitting and most secure anchorage of the ball socket is achieved if the axis of the three studs 261, 262 and 263 are positioned with their lon~itudinal axes in planes which are at least approximately parallel to each other, with the axis of studs 262 and 263 and lines drawn normal to the base surface of the ball socket defining an~les larger than the angle defined by the axis of stud 251 and the radial axis 265 of the ball socket. In the socket illustrated in the fiqures, the axis of the stud 261 forms an angle 267 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 normals drawn to the base surface 269 of the ball socket 250. The stud 261 is provided at its root with a peripheral groove 272, while each o~ the two other studs 262 and 263 is provided with an undercut 274 outwardiy directed rom the main axis ~65 of the ball socket ~50~ These undercuts provide an improved anchorage in the pelvis bone, The ball socket 250 is provided at its lower edge with a cavity 275 at one side thereo~ proceeding from the base surface 269, The sickel shaped formation o~ this cavity, as seen in plan view, is clear from the assumed external contour i ....... . .

~l07585~
of the ball socket 250 shown in dotted lines in Fig~ 21. This cavity 275 is provided for the unimpeded progression o~ the musculous iliopsoas subse~lent to the implantatlon, and ~s positionedt ~or the other hip ~oint, in the mirror image position with respect to the central pla,ne of the ball socket 250 con-taining the axis o~ the stud 261, As may be seen in Pig. 21, the cavity 275 begins at least i~pproximately in a plane that extends through the radlal axis 265 of the socket and through stud 262, and extends over an annual range o~ at least approx-imately 120 to that side of the ball socket xemote from stud 263. In the illustrated socket, cavity 275 extends over an angle 276 o~ about 125 degrees.
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 ~oint socket is first of all milled out with a spherical milling tool. In this spherical milled cavity three bores are made for the studs 261, 262 and 263 by means o~ a drilling templateG In this operation the bores are concentrated somewhat closer together than a 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 o~ the cor~ical tissue 279 o~ 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~ Durlng this snapping in operation, the cortical tissue 279 between the studs is elastically de~ormed and? after springing back, locks the bali socket 250 in the inserted position according to Fig, 22~
The special position o~ the studs 261, 262 and 263 with ~23-107~

refer~nce to the resultant ~orce FR has the result that in the studs additional thrust s~resses do not arise by the elastic deformation of the bone under the in~luence of the resultant force F~ when this force assumes its maximum value, Thus, the possihle thrust stresses remain at a minimum value.
The ball socket 250 can be metallic and can he coated wi~h enamel over its entire surface, Figs. 23 ~o 29 show a total elbow joint endoprosthesis 290, As in the case of the foregoing figures, similar parts are indicated by the same reference characters.
In this case7 the sha~t 73 is screwed into a humerus bone 293, The extension member 83 is provided with a do~nwardly pointing stop 295, shown in Figs~ ~3, 24, 26 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, 26 and 27, The bearing sheli 297 is provided with an abu~ment surface 300 (Fig~ 29~ ~or the purpose of making contact with the stop 295, which precisely defines the extended position o~ ~he humerus 293 and the ulna 299~ The side surfaces 302 and 303 (Fig~
28) of the bearing shell 297 are axially guided by corresponding opposite faces of condylar shells 305 and 306, each of which is provided with a support arm or hinge fork 105 that contains an eye for ~he hinge joint 102.
A hin~e pin 308, which forms the first ~ody ~oint member, is rotatably mounted in the bearing shell 297, which forms the second body joint member~ The hinge pin 308 is made integrally with the condylar shells 305 and 306 by means of only a compara-tivaly short connecting member 310 and 311 (Figs, 26 and 27~
of substantially semicircular cross sectional area. Each con-necting membar 310, 311 is situated only in that peripheral :~75 !35~

rec~ion o~ the hinge pin 308 which is emhxaced by the appertaining condylar shell 305, 306, ~his provides an enlarged seating surface.
Each condylar shell 3n5 and 306 furthermore is providecl with a hole 313 for a bone screw 315 (Fig. 27) in the section of the shell which is free of the connecting piece 310, 311.
A~ter the mounting o~ the condylar shells upon the suitably prepared condyles 317 and 318 lFig. 25), the screws 315 are screwed into these condyles for the purpose of temporarily anchoring the prosthesis. The connecting members 310 and 311 are compara~i~ely small so that as much as possible of the bone substance of the con~yles 317 and 318 can be left standing during the preparation of the bed of the implant for anchoring the bone screws 315, Moreover, with this construction the musculature can be protected at its osseous connections.
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 rounded thread 75 of the shaft 73. Following this the contact surfaces ~or the condylar shells are millea. Moreover, the space for the extension member 83 and its stop 295 are milled out. When these milling operations have been completed, the conical sleeve 82 is mounted upon the cone 79 and secured by the screw 87~ said conical sleeve having the pivot member 320 which includes the condylar shells 305 and 306 as well as the hlnge pin 308, linked to it by the hinge joint 102~ Then ~he two bone screws 315 are driven into the condyles 317 and 318 of the upper arm bone ~93. In this manner the pi~oting member 320 is ~emporarily secured~ Its final fixing to the upper æ m bone 293 should again be followed by ingrowth of the bone ~7~8~

cells into the pores of the inner condyLar shell surface.
In Fig. 24 and 27 a start of muscle 323 is shown in each case at the ulna 299r The bearing shell 297 is provided at its rear side with anchorage projections in the form of a stud 325 and a tongue 327 arran~ed in spaced relation to an~ direcked away ~rom the stud 325. The stud 325 is provi~ed at its root with a peripheral channel 329.
For making the i~plantation, first of all the proxlmal ulna 299 is prepared by milling out the cylindrical hollow shell of the physiological joint to the external radius of the bParing shell 297, and by milling off both sides of the elbow protruberance (olecranon) to the ~idth of the bearing shell 297 so as to accept a drilling and milllng template, By the aid of this template there are made a slot for the tongue 327 and a ~ore for the stud 325. The bearing shell 297 is first o~ all insexted with the tongue in the appertaining slo~, and then is "snapped" with the stud 325 in the appertaining bore. This provides a flrm seating of the shell in the ulna.
The connection is completed by the later ingrowth of bone cells into the porous bearing shell surface at the contact surfa¢es of the bone sections.
From the upper edge of the condylar shells 305 and 306 to the lower termination of the externally rounded threads 75 of the shaft 73 there will exist no positive connection made through bone material between the prosthesis and its bone support, so that micro relative m~vements of t~e contact surfaces between the implant and the bone are decisivel~ reduced~ In correspondence with the various hip joints it is also possible for the total elbow joint endoprosthesis 290 to he nade of . . .

~0758~

metal, whose entire surface is enamelled, The bone screws 315 can also he fully enamelled~
For the purpose of the elbow joint implant, a hin~e ~oint 102 has been described as an example with reference to Figs.
23 ~o 29~ However, in place of t:he illustrated hinge joint 102, it is possible to use for the elbow joint implant other types of pivot ~oints which have been described above in connection with the hip joint implant, Also~ while a hip ~oint has been selected as an example of a body ball joint, and an elbow ~oint has been selected as an example of a body hinge ~oint suitable ~or the fitting of a prosthesis, it should be noted that the above disclosed principles are also basically appl~cable to ali other body joints.
Figs 30-33 illustrate an instrument for inserting the shaft 73 for the hip joint prostheses illustrated above in a threaded hole in the thigh b~ne previously prepared, for example, with a self-centering conical boring tOolq The instrument may also be used to preciseiy mill the femur for insertion of the pivot me~ber 107 of these prostheses, and to remove the shaft 73~
Fig, 30 shows a rotary tool 350 having two tubes 353 and 354 rigidly connected together. The tube 353 carries at its upper end a key surface 356 and a screw plug 357 threaded into the tu~e 353. At the lower end o~ the tube 354 there is secured a conical socket 359 complementary to the cone 79 at the top of shaft 73~
As shown in Fig, 32 a threaded screw 360 passes through the intexior of the tubes 353 and 354, and is threaded into the tapped bore 80 in the cone 79 of the shaft 73. By means of the screw 360 and the complementary conical socket 359, ~t7S8Sl the rotary tool 350 can be coupled to the shaft 73 to ~orm a rigid unit. ~hen the sha~t is being screwed into or out o~
the tapped hole in the thigh bone, the longitudinal axis of this unit coincides with the axis of the tapped hole. Thus, the shaft can be screwed in or out without any bending moments, and undesirecl stresses on the bone are avoided~
A head 363 of the screw 360 bears against a shoulder 365 of the tuhe 353. The screw plug 357 is provicled with a central bore 367 and its lower sur~ace maintains a certain clearance from the upper surface of the screw head 363, Through the bore 367 or the screw plug 357, a hexagon socket key can be inserted into the hexagon socket o the head 363 for the purpose of rotating the screw 360. When the screw 360 is rotated back out of the position shown in Fig. 32, the screw 360 moves axially upwards relative to the tubes 353 and 354 untll the upper surface o~ the head 363 bears against the lo~er surface of the screw plug 357 which forms a stop 370. From this instant the described relative motion ceases and upon continuin~ the rotation of the screw 360 the conical socket 359 is withdrawn from the cone 79. To prevent the screw plug 357 w~rking loose, it is provided with a left hand thread 371.
With the shaft 73 scre~ed into the thi~h bone 30, when milling operations are to be carried out upon this bone, a holder 373 is externally mounted upon the tube 353, as shown in Figs. 30 to 33, and is clamped in the desired angular position with reference to the tube 353 by means of a clamp 375 and clamping screws 377.
In two cantile~ers 379 and 380 of the holder 373 there are secured two guide ~olts 383 and 384 screwed into the cantilever 379. A carrler 386 is mounted on the guide bolts and ls axially displaceable alonq the bolts through a length of feed path 387. Between the carrier 386 and the lower cantilever 379 of the holder 373, a tubular spacin~ member 389 is flttecl upon the guide bolt 383 to determine the ma~nitude of the feed path 387, Spacing members 389 of different axial length may therefore be ~itted for varying the length o~ the feed path 387.
In an aperture 390 (Fig. 30) of the carrier 386 a milling tool 395 is mounted upon combined axial anfl radial bearings 392 and 393. The milling tool 395 is provided with a continuous shaft 397, which carries at its lower end a key surface 39g and a quick change clamping chuck 400, and at its upper encl a threaded section 402. Upon the threaded section 402 there are mounted two counteracting fluted nuts 404 and 405 which are tightened up with respect to the key surface 399 so as to apply to the bearings 392 and 393 the correct axial stress.
Upon the threaded section 402 there is also applied a union nut 407 of a flexible shaft 408 driven by a motor, not shown in the drawing~
In the quick change clamping chuck 400 there is clamped a combination milling tool 410, comprising an end miller 412 and a cylindrical milling cutter 413 projecting from the base of the end milling cutter 412.
At the beginning of the milling operation the milling tool 395 is in a starting position, in which the carrier 386 is supported with its lower pro~ecting arm 415 (Fig. 30) bearing against the lower side of the upper cantilever 380 of the holder 373, The flexible shaft 408 is then driven so that the combin-ation milling tool 410 rotates at the desired speefl~, The milling tool 395 is then ~ed by hand downwardly along the two :

~07~3~35~

guide bolts 383 and 38~, whereby initially the cylindrical millin~ tool 413 ~ills out a hole inside the thigh bone 30, whereafter the end millin~ tool 412 comes more and more into en~a~ement with the thigh kone 30 until, in the end position shown in Fig. 30 the seatinq surface 50 containing spongiose and cortical tis~ue is completed for the endoprosthesis~ The plane of the seating surface 50 is thus precisely clefined with reference to the lon~itudinal axis of the shaft 73, so that during the entire further course of the operation any further fitting of the remaining part of the prosthesis is unnecessary.
This method shortens the operation time, protects the patient and affords a precisely defined seating for the endoprosthesis.
The angle between the axis of the rotary tool and the milling tool can be suited to all kinds of anatomical requiremen~s by suitable design of the holder 373, As was mentioned above, this adaptation to anatomical requirements may be affected ~y insertin~ spacing members 389 of varying length between the carrier 386 and the holder 373. Thus, the depth of the milling may he reliably limited~ -Fig, 34 shows a modified form of shaft 73 with a thread 430 of sawtooth shape, which tapers conically downwards towards its distal end to suit the shape of the inner wall of the cortical tissue of the receiving bone, instead of the symmetrical external round threa~ shown in the previous figures. The steep flanks 433 of the sawtooth thread 430 point towards the p~oximal end of the shaft 73 and are directed normal to the longitudinal axis of the shaft 73. Since the flanks 433 are normal to the axis of the shaft~ and to the tractive ~orce Fz~cos~ that . .

~17S~S~

acts upwardly along the axis o the sha~t, the sawtooth thread eliminates radial forces on the bone resulting from this tractive force, which could exert a non-physiological bursting effect on the bone.
The crests 435 of the teeth and the valleys A37 of the teeth are in each case rounded. This ensures that the bone which is situated opposite to the sawtooth thread 430 after insertion thereof is carefully treated and facilitates the application of an enamel coating 440, shown in chain and dotted lines in Fig. 34, over the entire shaft 75 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 antibotic, be~ore the fitting of the shaf~ 73, For promoting the ingrowth of the surrounding bone tissue into the shaft 73, the enamel coating can be exter-nally roughened~
This sawtooth thread can also be designed to be sel~-cutting by locally interrupting the thread in the axial direction and pro~iding it with cutting edges.

:, ~

., :

Claims

THE EMBOIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE: IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A body joint endoprosthesis comprising;
an anchoring part comprising a shaft adapted to be anchored in a first bone;
a pivot member connectlecl to said anchoring part by a pivot joint, said pivot member comprising a first joint member and a support element adapted to bear against a seating surface of the first bone and a second joint member adapted to he connected to a second bone, with said first joint member and said second joint member forming a body implant joint.

2, A body joint endoprosthesis according to claim 1 wherein said anchoring part comprises an extension member releasably connected to said shaft.

3. A body joint endoprosthesis according to claim 2 wherein there is a cone, having a tapped bore, at an end of said shaft and said extension member comprises a complementary conical socket mounted upon said cone and a screw, inserted into said tapped bore, that urges said socket against said cone.

A body joint endoprosthesis according to claim 1 wherein the shaft has an external rounded thread for anchoring the shaft in the first bone.

5, A body joint endoprosthesis according to claim 1 wherein said pivot joint is a hinge joint.

6. A body joint endoprosthesis according to claim 1 wherein said pivot joint is a ball joint,

7, A body joint endoprosthesis according to claim 1 wherein the pivot joint is a Cardan joint,

8. A body joint endoprosthesis according to claim 1 wherein the pivot joint is a knife bearing joint.

9. A body joint endoprosthesis according to claim 1 wherein the pivot joint is an elastic joint.

10. A body joint endoprosthesis according to claim 9, wherein the pivot joint comprises at least one prestressed resilient member that connects said anchoring part to said pivot member and urges said support element toward said seating surface,

11. A body joint endoprosthesis according to claim 10 wherein said resilient member is a leaf spring,

12. A body joint endoprosthesis according to claim 10, wherein two resilient members are arranged for pivotal movement in a common plane and are spaced from one another in a parallelo-gram arrangement.

13. A body joint endoprosthesis according to claim 1 wherein a spring, located between said anchoring part and said pivot member, urges said support element toward said seating surface.

14. A body joint endoprosthesis according to claim 13 wherein said spring is located between the pivot member and a clamping screw screwed into a pivotable stud mounted on said anchoring part.

15. A body joint endoprosthesis according to claim 1 for use as a hip joint endoprosthesis wherein the first bone is a thigh bone, the first joint member comprises a ball, and the support element comprises a collar secured to said ball.

16. A body joint endoprosthesis according to claim is wherein said pivot member pivots about a point or an axis located at least approximately in a plane containing said seating surface.

17. A body joint endoprosthesis according to claim 15 wherein said second bone comprises a pelvic bone and said second joint member comprises a ball socket having button-like studs for anchorage in the pelvic bone.

- 18. A body joint endoprosthesis according to claim 17 wherein said ball socket includes first, second and third studs arranged at the corners of a triangle so that the maximum resultant force upon the body joint passes substance tially through the surface center of gravity of said triangle.

19. A body joint endoprosthesis according to claim 18 wherein said first stud is located near the apex of said socket and said second and third studs are located at sub-stantially one-half the height of said socket.

20. A body joint endoprosthesis according to claim 19 wherein the axes of said first, second and third studs lie in substantially mutually parallel planes, and the axes of said second and third studs enclose greater angles than the axis of said first stud with normals to a base surface of said ball socket.

21. A body joint endoprosthesis according to claim 20 wherein said first stud has a circumferential channel at its foot and said second and third studs each have an undercut configuration directed outwardly from the axis of said ball socket.

22. A body joint endoprosthesis according to claim 19 wherein the edge of said ball socket includes a cavity, said cavity beginning at least approximate in a plane that extends through the axis of the socket and through said second stud, said cavity extending over an annular range of at least about 120° to the side of the ball socket remote from said third stud.

23. A body joint endoprosthesis according to claim 17 wherein said ball socket has a plurality of concentric channels and a plurality of channels lying in planes passing through the axis of the socket.

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

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

26. A body joint endoprosthesis according to claim 24 wherein the hinge pin is connected to each shell by a short connecting piece and the part of each shell free from the connecting piece contains a perforation for a bone screw.

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.

28. A body joint endoprosthesis according to claim 24 wherein each condylar shell includes a support arm bearing a part of the pivot joint.

29. A body joint endoprosthesis according to claim 24 wherein the anchoring part includes a stop for limiting pivotal movement of the pivot member.

30. A body joint endoprosthesis according to claim 24 wherein the second bone is an ulna, the second joint member comprises a bearing shell for the hinge pin, and the bearing shell is provided with projections for anchorage to the ulna.

31. A body joint endoprosthesis according to claim wherein the bearing shell comprises a cylindrical half shell, and an end face of the bearing shell comprises an abut-ment surface for making contact with said stop in the extended position of the upper arm bone and the ulna.

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

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

34. A body joint endoprosthesis according to claim 1 wherein at least the external surfaces of the endoprosthesis comprise enamel coated metal.

35. A body joint endoprosthesis according to claim 1 wherein the shaft has an external sawtooth thread having its steeper sides towards the pivot member.

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