CA1240101A

CA1240101A - Multi-component prosthesis with increased wall flexibility facilitating component assembly

Info

Publication number: CA1240101A
Application number: CA000453670A
Authority: CA
Inventors: Michael J. Pappas; Frederick F. Buechel
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-05-06
Filing date: 1984-05-07
Publication date: 1988-08-09
Also published as: GB8411711D0; US5376122A; GB2139098A; US5133764A; GB2139098B

Abstract

ABSTRACT OF THE DISCLOSURE

Assembly of the components of a multi-component prosthesic is facilitated by constructing at least one of the components as a non-symmetrical segmented open shell whereby the segmenting of the shell increases the flexibility of the shell wall which flexibility in turn facilitates component assembly; such flexibility also enhances the engagement of retaining means for retaining the components together upon assembly.
Screws utilized to secure a prosthesis component to a bone are recessed within the seat of a screw hole formed in the component for receiving the screw and generally area contact is achieved between the underside of the screw head and the seat of the screw hole by making both the underside of the screw head and the screw hole seat spherical--additionally, a second component engaging the first component may be provided with a recess overlying the screw hole whereby in the event the screw head extends upwardly out of the screw seat the recess accommodates the screw head.

Description

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1 ~ MULTI-COMPONENT PROSTHESIS WITH INCREASED WALL
j' FLEXIBILITY FACILITATING COMPONENT ~SSEI~LY

2'

3 I BACKGROUND OF THE INVENTION
This invention relates generally to a new and improved 5' prosthesis, and more particularly relates to an improved multi-6~¦ component prosthesis, such as for example a multi-component hip 7~l prosthesis, wherein at least one of the components is a shell 8,¦ provided with walls of increased flexibility to facilitate assembly 9'l of the prosthesis components.
- 10 I The conventional prior art total hip prosthesis typically ~ includes a metal femoral head fixtured to the femur and a plastic, i 12`1 generally ultra-high molecular weight (UHMWPe), cup fixtured by 13l, cement to the acetabulum. The prior art also discloses the use ,1 of hip prostheses including metal backed acetabular components 15 ' in which the acetabular component consists of a metal cup with ' 16 1 a plastic bearing insert; examples of such prostheses are 17 I given in U.S. Patent No. 3,840,904 issued October 15, 1979 to ~ 18 l Tronzo and U.S. Patent No. 3,903,549 issued September 9, 1975 j 19 ll to Deyerle. Metal backed acetabular hip prosthesis components have several advantages. The metal or rigid outer acetabular cup 21 produces a much more uniform stress distribution at the inter-22 ! face between the cup and the acetabulum with lower peak forces 23 il thereby improvirg the possibility of long term fixation.
24 I Rigidity of the metal outer acetabular cup also reduces distortion ; 25 1l of the plastic liner improving its sphericity and therefore 26ii contact with the metal femoral head of the hip prosthesis thereby 27 , improving conditions for wear resistance. Further, the use of 23 1 a separate bearing insert allows replacement of the insert if 29 ' the inscrt is damaged intraoperatively or if the insert becomes e~cessively worn as a result of long term use or if as a . .
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1 i result of some problem revision lS necessary which involves 2 a change in the insert bearing. For example, in the event of revision from a surface replacement type hip prosthesis which 4l has a fairly largé diameter head to a stem type femoral ; 5 I prosthesis which includes usually a smaller diameter head one 6 l may simply remove the bearing liner leaving the metal acetabular 7l cup affixed to bone and replace the liner with another liner or 8l insert of appropriate size for the revised unit. Such revision g can therefore be made without disturbing the acetabular fixation thereby preventing dama~e to the acetabular bone.
- 11 ~ There is, however, a potential disadvantage associated 12jl with the use of a rigid or metal acetabular cupO In the event 13; that a load is applied near the rim of the acetabular cup, the 14 Ii elastic properties of the underlying bone combined with the 15l~ rigidity of the cap can produce a situation in which the opposite 16 Il, rim tends to be lifted off of its bony bed. This tends to 17 ' produce tensile loads on the cup and such tensile loads are 18 11 undeslrable in maintaining long term fixation.
19 In the prosthesis disclosed in the Tronzo patent, an 20ljinterlock 26 and interlocking groove 28 as described in FIGS. 2-21 ,8 of Tronzo are used to prevent rotation of the liner relative 22 Ito the cup. This connection, however, is not resistant to a i 23i tensile withdrawal of the cup from the liner. Such withdrawal 24 ! can occur as a result of traction forces due to a layer of liquid 25 '~interposed between the femoral component and the plastic liner j 26 coupled with distraction of the femur from the acetabulum during 27 I,normal activity. Such a situation can, therefore, produce with-28 drawal of the plastic bearing from the acetabular cup producing 29 Idislocation of the prosthetic component.
Il The prosthesis described in the Deyerle patent uses an , .
' :i Il I
ll arcuate ridge 32 which engages an arcuate slot 44 as shown in 2~¦ FIGS. l, 5 and 6 of Deyerle to restrict relative rotation. The 3l Deyerle device uses screws by making use of an annular liner 4l (not identified by number) in conjunction with a retaining screw 5'~ 30 in order to trap the liner in the cup. This design, however, 6, experiences difficulty in liner removal because removal of the 7l liner requires removal of the screws from the bone which may 8 produce damage to the bone. Further, the use of such a connection 9 resistant against tensile loading requires the use of screws lOjand many surgeons would prefer in some applications not to use screws for fixation.
12j~ Both the Tronzo and Deyerle prostheses use screws.
13 INeither, however, provides the ability of the screws to change l~ Itheir angular orientation significantly to facilitate fixation.
15 IFurther, neither provides screws near the lnferior rim to 16jjminimize the possible lifting of the inferior rim as a result 17jl of loads applied near the superior rim. Such loads are normal 18jl in walking and may exceed eight times body weight in stair l9ilclimbing and descent. Since the angular orientation of the screw i 20jl is not adjustable in Tronzo and Deyerle devices, these screw . 211 configurations cannot take maximum advan~age of possible superior i 221 bone stock for screw implantation. Further, Deyerle and 23 Tronzo prostheses both make use of either screws or spikes 1 24 1l for fixation. However, when such acetabular components are 25 1l used with cement, such spikes or screws may not be necessary for 1 26 11 fixation and their use makes the operative procedure more ! 27j difficult and introduces additional damaye to the bone.
28l U.S. Patent No. 3,60~,096 to Link discloses a prosthesis 29l which usesarelieved face on the acetabular shell where a segmentj of the shell is rernoved by means of an oblique cut ~not identificcl I
!l 124~

l by number) as described in Column 2, lines 69-72 and Column 3, 2 lines 1-3 to provide a better approximation to the shape of the 3' natural acetabulum so as to increase clearance reducing possible

4 impingement with the femur during certain kinds of activity as 5jdescribed in Column 3, lines 26-29. Further, the outside 6~, sector 3 as shown in FIG. 1 of the Link patent is eccentric 7 1l to the cavity 2 although the nature and reason for this 8¦eccentricity are not described by Link. Although this oblique ; 911and simple relief provides improvement in fit and clearance, - 101still better fit and clearance can be provided by a somewhat more complex relief of the inferior face of the acetabular 12~,1component.
131 SU~I~RY OF THE INVENTION
141 ~ssembly of the components of a multi-component prosthesis ~ 15 is facili~ated by constructing at least one of the components - 16 1l as a non-symmetrical segmented open shell whereby the segmenting 17 1l of the shell increases the flexibility of the shell wall which 18111 flexibility in turn facilitates component assembly; such l9ji flexibility also enhances the engagement of retaining means 201~ for retaining the components together upon assembly.
21,, Screws utilized to secure a prosthesis component to a 22 1l bone are recessed within the seat of a screw hole forrr,ed in the 231component for receiving the screw and generally area contact 241 is achieved between the underside of ~he screw head and the 25i; seat of -the screw hole by making both the underside of the screw 261head and the screw hole seat spherical--additionally, a second 27lj cornponent engaging the first component may be provided with a 28 recess overlyiny the screw hole whereby in the event the screw 29j hcad extends upwardly out of the screw seat the recess acconmloclate~

thc scrcw head.

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As used in the context of the present invention, and as used in the specification and appended claims, the term "open shell" means a shell segment produced by the cutting of a closed shell (i.e. a shell without openings) by means of a single cutting plane; the term "segmented open shell" means a shell segment produced by cutting an open shell by additional cutting surfaces or by cutting a closed shell by a non-planar surface or by more than one cutting plane. The generation of slots is not considered segmentation in the context of the present invention since the generation of a slot is the equivalent of splitting the shell rather than removal of a shell segment; although slotting normally involves removal of a thin segment, such removal is incidental to function.
In accordance with the present invention there is provided a joint prosthesis comprising first and second components for being assembled together, the assembly of said components requiring the application thereto of assembly forces, said first component for being secured to a bone and said components having means for retaining them together upon assembly, wherein said first component is a non-symmetrical segmented open shell and wherein the segmenting of the shell increases the flexibility of the wall of the shell thereby facilitating assembly of the two components.
There is further provided a joint prosthesis comprising first and second components for being assembled, said components provided with mutually engageable retaining means for retaining said components together upon assembly, at least the first of said components being an asymmetrical segmented open shell wherein the segmentation of the open shell increases flexibility of the shell wall thereby facilitating assembly of the components and enhancing ~z~

engagement of the retaining means during assembly of the components.
There is further provided a joint prosthesis comprising first and second components, said first component for being secured to a bone and said second component for being inserted into said first component and said components provided with mutually engageable retaining means for retaining said second component within said first component upon being inserted therein, at least the first of said components being an asymmetrical segmented open shell wherein the segmentation of the open shell increases the flexibility of the shell wall thereby enhancing insertion of said second component into said first component and the engagement of said mutually engageable retaining means during said insertion of said second component into said first component.

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DESCRIPTIO~ OF TIIE DRAWINGS
2 FIG. 1 is a diagrammatical illustration of a surface 3lreplacement hip joint prosthesis embodying the present invention 4~.and shown diagrammatically as being fi~tured by bone ingrowth 5lto the hip bone and femur;
6', FIG. 2 is a front elevational view of a metal acetabular 7Icup of the present invention, FIG. 2A is a cross-sectional view 8 il taken in FIG. 2 along the line 2A-2A and in the direction of the 911arrows, FIG. 2B is a cross-sectional view taken along the - lOI~line 2B-2B in FIG. 2A and in the direction of the arrows, and FIG. 2C is a partial cross-sectional view ill~strating the 12 !I manner of assembly of the acetabular cup and bearing liner , 131~retaining means of the present invention;
1 14 FIG. 3 is a front elevational view of a bearing liner embodying the present invention; FIG. 3A is a cross-sectional ! 16 1l view taken generally along the line 3A-3A in FIG. 3 and in the 17ildirection of the arrows, and FIG. 3B is a cross-sectional view i 18 11 of an alternate bearing liner embodiment of the present invention, 19 1l similar to FIG. 3A but taken in a direction opposite to the 20 1l arrows 3A-3A in FIG. 3;
2111 FIG. 4 is a side elevational view of a screw of the present 22l invention and FIG. 4A is a left end view of the screw of FIG. 4; .
1 23 1l FIG. 5 is an assembly view of the acetabular cup of the 241jpresent invention and a bearing liner of the present invention, 25~1the bearing liner being an alternate embodiment of the liner jof FIG. 3, and such illustration showing the assembly as it 271 would look in cross-section, but wi-th the cross-sectional lines 2~ 1l h~ing elimirlated for clarity, were such cross-sectional assembly 29.lview to be taken alony lines such as 2A~A in FIG. 2 and 3~-3 30l in F:l(;. 3;

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ll FIG. 6 is a diagrammatical illustration illustrating the 2"assembly of the femoral cap to the resected head of a natural 3~1femur; and 4,i FIGS. 7A-7D are diagrammatical illustrations of a ; 5I teaching of the presen-t invention as to the increase in flexibility 6ilof the wall of a shell to facilitate assembly of prosthesis 7 ! components.
8¦DESCRIPTION OF THE PREFERRE~ EM~ODIMENT
9l Referring now to FIG. l, there is shown a diagrammatical illustration of a natural innominate or hip bone 10 including the ilium ll, the ischium 12 and the pubis 13 and a natural ,femur 16 having a neck 17. Also shown diagrammatically is a ,ihip joint prosthesis embodying the present invention and indica-, 14 ted by general numerical designation 18; the hip joint 15l prosthesis 18 as shown is of the type commonly referred to in the 16 art as a surface replacement hip joint prosthesis but it will 17 1I be understood that the present invention is not limited to the j 18isurface replacement type prosthesis but has wide application to 191 other types of prostheses such as other types of hip and shoulder prostheses; the prosthesis 18 is shown diagrammatically as being ' 21~implanted in or fixtured to the hip bone 10, or more particularly - 22li to the portion of the hip bone providing the acetabulum, or 23l~ acetabular socket and to the femur 16. The hip joint prosthesis 24 11 18 includes a metal acetabular cup 20, a plastic bearing liner 251il or plastic acetabular cup 30 and a metal femoral component or 26 1! cap 40 of the surface replacement type but may be used with 27 !1 other femoral prostheses such as those commonly referred to as 28 ll the femoral stem type prostheses.
2g In the embodiment shown, the metal acetabular cup 20 30~l is prov:ided with a porous outer surface 22 into which the hip ¦ ~ I

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1l bone may grow for permanent fixation. The cup 20 is temporarily 2l fixtured to the hip bone 10 by the use of metal screws 51, 52 3l (not shown) and 53 which screws are used primarily to establish 4'~ temporary fixation of the acetabular cup 20 to the hip bone 10

5 1l during the period of time that is required for bio]ogical bone

6~1 or bony ingrowth to occur into the porous coating 22 to provide

7 ll permanent fixation or to provide augmentation to fixation by use 81 of bone cement. In the preferred embodiment, the metal acetabular - 9I cup 20 is temporarily fixtured to the hip bone by the three metal i lOIlscrews, metal screw 51 being screwed into the ilium 11, metal screw 52 (not shown) ~eing screwed into the ischium 12, 12 ll and metal screw 53 being screwed into the pubis 13. The plastic 13l bearing liner or cup 30 is snapped into the metal acetabular cup I
, 14~ 20, in a manner described in detail below, and its inner surface i 15i provides the acetabular articulation surface. The metal femoral 6 1l cap 40 is provided with a highly polished exterior surface 42 ' . 17 1I providing the femoral articulation surface, a generally hollow 18 1l interior having an interior porous surface indicated by general j l9i~numerical designation 44 into which femur bone may grow to 20lj permanently fixture the femoral cap 40 to the resected head 19 21j~ of the femur 16 as shown in FIG. 6 or which porous surface is 22 used to improve cement fixation, and a relatively smooth or 23l polished metal stem 46 which is used primarily for allgnment 24i purposes and to provide some resistance against fracture of the 25l femoral neck but the relatively smooth surface of the metal stem ¦ 2611 46 is not a fixation surface. Temporary fixation, during the ! 27l time required for permanent bone ingrowth fixation to occur, is 28,l provided ~y a press fit between the interior surface 44 and 29~ the rcsected femur head 19 and a press fit between the stem 46 and a hole 47 (FIG. 6) drilled centrally in the resected femur ~1 1 ! ~ !

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head 19 and femur neck 18. The femoral cap 40 shown in FIG. 1 is the subject of a co-pending Canadian application serial no. 453,671, F. F. Buechel, et al filed May 7, 1984 entitled Prosthesis With Interlocking Fixation and Providing Reduction of Stress Shielding"
Referring now to FIGS. 2, 2A and 2B, it will be noted that the acetabular cup 20 is an asymmetrical, substantially hemispherical cup having a wall 22, a relatively thin wall, defined by concentric inner and outer surfaces 23 and 24, respectively, having a common center 25 from which spherical radii 26 and 27 are struck. The acetabular cup, at its inferior portion indicated by general numerical designation 28, is relieved as may be best seen in FIG. 2A.
One embodiment of the acetabular cup is shown in FIG. 2B.
Cup 20 is a multiply segmented shell of revolution having a generating axis F-F and being of uniform wall thickness and having concentric inner and outer surfaces. The face 60 of the cup is a surface produced by the intersection of a first cut-ting plane A-A perpendicular to a plane of symmetry (such plane of symmetry lying in the plane of the drawing) of the cup where the first cutting plane A-A defining the face 60 of the cup is closer to a parallel plane B-B through the center of the sphere defining the outside and inside surfaces 23 and 24, respectively, of the cup. The cup is segmented by an inferior surface defined by the intersection of a second cutting plane C-C perpendicular to the plane of symmetry and to the first cutting plane B-B defining the face 60 of the cup.
The cup has a transition surface 29 defined by an intersection of the shell of revolution with a segment D-D of a cylindrical surface perpendicular to the plane of symmetry and tangent to the first and second cutting planes defining the face and inferior surfaces, respectively.

-. _ ~ . . _ _ . , _ . . . _ 124C~

1 A ridge 70 protrudes from the inferior aspect oE the cup2 adjacent to the inferior surface 24. An annular groove 64 cut 3;;into the inside surface is employed where this groove lies in 4 a plane parallel to the first cutting plane and adjacent to it.
5I The cup may be provided at its superior portion, FIG. 2, with 6`1a radially inwardly directed key 65 e~tending inwardly a 7j distance from the face of the cup. Additionally, the cup 20 8l may be provided with a pair of generally opposed slots 66-66 9l extending inwardly from the face 60 of the cup, the slots 66-66 1011 being provided, generally, as may be noted in FIG. 2, at the llj lateral or side portions of the cup 20. Further, the cup may i 12! be provided with three apertures or screw holes 66, 67 and 68 13 1l each for receiving a metal screw, such as for example the metal 141~ screw 90 of FIG. 4. The apertures may be provided with a 15 1l recessed spherical seat 69 for accoMmodating any misalignment 1 16 1l with the spherical underside 92 of the screw head 94 of FIG. 4 17jl with the screw head remaining entirely within the recess.
18 il Fur~her, to permit the screw inserted through one of the i 191l apertures to engage the best available bone accessible through the aperture, the spherical seats may be made oblong as shown 211 more clearly with regard to apertures 67 and 68 in FIGS. 2 22j and 2A.
1 23l Referring now to FIGS. 3, 3A and 3B, it will be noted 24jj that the plastic bearing liner 30 is an asymmetrical, eccentric, 25j substantially hemispherical cup defined by eccentric inner 26~ surface 32 and outer surface 34. It will be noted in FIG. 3 27 1l that -the inner and outer spherical surfaces 32 and 34 have 28,l mutually displaced respective spherical centers 36 and 38 with 29j, the spherical radius 80 of the inner surface 32 being struc~
frorn the spherical center 36 and with the spherical radius 82 I !
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1l of the outer spherical surface 34 being struck from the spherical 2' center 38. The liner 30 is further provided with a face indicated 3' by general numerical designation 85, complementary in shape 4I to the face 60 of the acetabular cup 20. The liner 30 used 5l in conjunction with the acetabular cup embodiment shown in FIG.
6~ 2A is provided with a flat region 86 and a curved or cylindrical 71 region 88 and a ~lat region 87 causing the liner 30 to be

8', relieved at its in~erior portion indicated by general numerical

9 designation 89. The outer surface 34 of the plastic bearing liner

10~ 30, as may be best seen in FIG. 3A, is provided with an inter-rupted annular ridge 90 which annular ridge is interrupted by the 12 ll relieved portion 89 of the cup or by the circular or cylindrical 13 1l portion 88 of the face 85. The eccentric inner and outer 141 surfaces 32 and 34, respectively, provide the plastic bearing 151 liner 30 with a wall 92, as may be best seen in FIG. 3A, 161~which wall is thicker at its superior portion 93 and which is 17 1l thinner at its lateral portions 94-94 adjacent the inferior 181 portion 89 of the liner. This is done in order to provide a 191 greater thickness of bearing allowing greater wear in the superior aspect where most wear occurs and to produce a somewhat 21jl thinner sidewall anteriorly and posteriorly thereby increasing 22:' flexibility of the wall in the region where the wall must be 23 1l compressed in order to assemble the bearing insert into the 24jlacetabular cup. Further, as may be best seen in FIG. 3, 25I the plastic bearing liner 30 may be provided at its superior 26 il portion 93 with a radially inwardly extending keyway 97 for 27 1l receiving the key 65 of the acetabular cup 20, FIG. 2.
28'1 The outer surface 3~ of the plastic bearing liner 30 may be 29 provided with a plurality of mutually displaced recesses 101, 30 , 102 and 103 for overlying the apertures 66, 67 and 68 of FIG. 2 31 and their spherical, and oblong spherical, seats 69 which help 1~ ~

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1, prevent contact between the liner and the screw head where the . .
2I screw is not properly seated. The interrupted annular ridge 3 90 is complementary to and closely matches the interrupted 4 annular ridge 64 of the acetabular cup 20, the outer surface 34 5 ll of the plastic bearing liner 30 closely matches the spherical 6'l, inner surface 24 of the acetabular cup 20 and the spherical 7' inner surface 32 of the plastic bearing liner 30 provides the 8 ! acetabular articulation surface.
9I The inferior flat surface 87 of insert 30 engages the lO, ridge 70 on the acetabular cup 20 restraining, in conjunction lllwith key 65 and keyway 97, axial rotation of insert 30 with 12,, respect to acetabular cup 20 about generating axis F-F as shown 13~1 in FIG. 5.
14 1l ~ cross-section of the partially assembled insert and 151¦ acetabular cup in the region of the interconnecting ridge and 161 groove is shown in FIG. 2C. The ridge and groove faces 122 171 and 121, respectively, are substantially parallel to the faces 18!l ~6 and 60 of the insert and cup, respectively. The medial 19l aspect of the ridge is at an angle 120 relative to the face.
20 1l Upon assembly using inward forces with substantial force 21'1 components parallel to the generating axis this medial aspect 22,1 of the ridge 122 engages an edge 123 of the interior surface 24 23,1 of the acetabular cup. Such engagement produces forces with 24j substantial lateral components generally perpendicular to the 25jl generating axis such components tending to compress the insert 26l or expand the acetabular cup. Since the side walls of the 27l insert or cup due to the segmentation described are relatively 2~j~ laterally flexible sufficient deformation of the insert and 29i cup is easily produced to allow the ridge 9O to pass the inside region 124 adjacent the groove 64 and allow the ridge 90 to " ,1 . ;~ - , . J

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l engage the groove 64 thereby relieving the compressive and 2 expanding forces and allowing the components to assume the 3' undeformed states. Removal of inserts from the acetabular cup 4lby use of outward forces with substantial force components 5l parallel to the generating axis is extremely difficult since 6Ij the engaging faces 125 and 126 on the liner and cup respectively ~! are perpendicular to the generating axes and thus fail t~
8ll produce any significant lateral forces compressing the insert 9 ll or expanding the acetabular cup. Rather the forces will be essentially parallel to the generating axis and therefore due to ~ the curved shape of the components will tend to expand the insert i 12 1¦ and compress the cup thereby locking the parts together. Thus, ; 13 disassembly can be carried out only by use of instruments designed ,, 14ll to apply lateral forces compressing the insert or expanding the cup. Due to the nature of the segmentation of these components 6ll described relatively low lateral forces are needed to disassemble 17l~ the components. Such lateral forces cannot be produced in these 181 components in an implanted prosthesis. Even if such forces could, 19; be produced they would be resisted by the head of the femoral . 20 ll component 40 which prevents contraction of the insert and by 21jl the acetabulum; and thus the liner cannot be removed while it is 22 in engagement with the femoral component 40. Hence dislocation i 23~1 resulting from distraction forces as the result of fluid between 2~!l the femoral component and the liner cannot occur. Thus, for 25il the case of a metal acetabular component which is very stiff 2611 compared to plastic and a plastic bearing insert dislocation 1 27 of the hip is necessary to remove the plastic insert.

28 The increase in flexibility produced by the segmentation 29 described can be understood by examining FIGS. 7~-D. It may be secrl that the removal of the inferior segment removes a

11 1 124~

major lateral load supporting segment. Further also in shells 2'l where the wall thickness t is substantially smaller than the 31 half the width or radius of the shell r such segmentation 4ll~ eliminates most of the circumferential compressive stiffness 5 1¦ of the wall thus making bending stiffness dominate. Since for 6 ¦¦ such shells bending stiffness is much less than circumferential 7~1 stiffness flexibility of the wall is greatly increased.
81 With regard to the screw lso, which may be utilized in 9l the present invention, and referring to FIG. 4, it will be noted that in addition to the above-described scr~w structure the end of the screw 196 may be provided with radially disposed

12 1¦ slots, as shown in FIG. 4~, which slots provide self-cutting ~¦action so that a cap need not be used in conjunction with the ¦screw. Preferably, the screw is made of a metal alloy the same 15' or similar to that of the metal acetabular cup 20 and it has been 16 ~¦ found to be preferable to provide several sized screws for 17~different fixation conditions; the threads 198 are preferably large since they are for the purpose of screwing and holding l9 lnto cancellous bone. Further, it has been found that the screw 20 1¦ for fixation into the ischium 12, FIG. 1, is somewhat shorter than the screws used for fixation in the ilium 11 and pubis 13.
22~~ f course, the spherical underside 192 of the screw head 194 ;1matches the spherical seats 69 of the apertures 66, 67, and 68 24!of FIG. 2.
25,1 It may be seen from FIG. 5 that the screw l90 can be 261lmoved through an angle ~ about center 111 with the spherical 27 underside 192 of screw l90 remaining in full contact with 28 spherical seat 69 as observed by the motion of the screw a~is 29~ from line 112 to 114 at which point the shank l99 contacts the 301 edye 115 of hole 66 limiting further motion.

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1' Before describing in detail the manner of assembly and 2'1 disassembly of the metal acetabular cup 20 and the plastic bearing :1 ~
3l11iner 30, and the assembly of the metal femoral cap 40 to the 4~l resected femur 66, a brief description of the surgical implanta-51! tion or flxation technique is presented. The acetabulum, 6l or acetabulum socket or cavity provided by the hip bone 10, 7,ll FIG. 1, is reamed in an ordinary fashion as would be done for ' 8 1I the implantation of any acetabular hip cup, such being well ', gjl known to those skilled in the art, particularly orthopaedic 10~ surgeons skilled in the art. A trlal component is then used to orient the acetabular CUp to the acetabulum and is used to mark 12 1,1 out the location of the holes to be drilled for accepting the 1311 screws 90. The metal acetabuIar cup 20 is provided with a slight-14~ ly larger spherical outer surface 22 than the prepared acetabular cavity and the acetabular cup is then pressed into place and the 161, apertures 66, 67 and 68 are aligned with the prepared drilled 17 1l holes in the acetabulum. The screws 51, 52 and 53 of FIG. 1, 18liwhich screws may be the screw 90 of FIG. 4, are then screwed l9i into the hip bone providing the acetabular cavity to tem,orarily 201! fixture the metal acetabular cup 20 to the hip bone, It will I. , 11 21jlbe understood that the spherical underside of the screw heads, 22 e.g. spherical underside 192 of FIG. 4, engages the spherical 231 seats 69 of the acetabular cup apertures and since the shank 24l 199 of (~IG. 4) of the screw is made to be much smaller than the 25 ! diameter of the apertures, this allows the head of the screws 26'1 to be pivoted about a point 111, FIG. 6, so that the screw may 27 jll be oriented at an angle ~ relative to the axis 112, FIG. 6, 2allof the apertllre. Thus, it will be understood, positioning 2gjvariation is provided by the present invention and this positioning configuration has been found to be important in !

14.

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., 1 seeking out the best available bone for the screw to engage.
2l ~urthermore, this positioning variation is accomplished while 3,~ maintaining excellent contact between the screw head and its 4 I¦ spherical seats. The oblong spherical seats 69 of the 51¦ apertures, FIG. 2, allow for similar misalignment but also 6 11 allow for a further change in the location of the screw within 71~ the confines of the seat thereby again enhancing effective 8¦~utilization of existing bone stock and permitting a greater glchange in location of the screw within the aperture to permit the screw to engage the best available bone accessible ,through the aperture.
121~ The plastic bearing liner 30 is now "snap-fitted" into

13 ¦¦ place in the fixtured metal acetabular cup 20 in accordance 14l~with the teachings of the present invention. Spherically 15jradially inwardly directed forces, such as may be generated 16j digitally and quite easily by the fingers of the operating 17 ! surgeon, are applied to the thinner wall regions 94-94 of the 181 plastic bearing liner 30 to flex the lateral thinner wall portions 1~1 inwardly to permit the interrupted annular ridge 90 of the plastic bearing liner 30 to be inserted into and engage the 211 interrupted annular groove 64 of the metal acetabular cup 20 22 11 and the inwardly flexed bearing liner 20 in combination 23 iI with the respective relieved inferior portions 28 and 89 of 24 he acetabular cup 20 and plastic bearin3 liner 30, respectively, 2al /

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, i 1 readily permit the flexed plastic bearing liner to be inserted, 2 , or "snap-fitted" in-to the metal acetabular cup 20. The 3 key 65 of the acetab-llar cup 20 will be aligned with and received 4 in the key-way 97 at the superior portion of the plastic bearing 5,, liner and the flat end portion of the wall 92 of the plastic 6 1l bearing liner will be aligned with and engaged by the flat - 7 projection or ridge projecting inwardly at the inferior por~ion 8 1 of the metal acetabular cup 20. The key and key-way, and 9Ij engaged flat surfaces, resist relative rotation between the j- 10ll plastic bearing liner 30 and the metal acetabular cup 20 upon 11 I torsional stress being applied thereto during articulation of 12,l the hip joint. Further, lt will be understood, as may be better ~ 13lj seen in FIG. 5, the respective faces 60 and 85 of the acetabular A 14ji cup 20 and plastic bearing liner 30 align, transversely, and 15 1l present a common face for the assembled cup and liner and with 16jj the common relieved inferior portions thereof permitting a 17jl greater range of articulation of the femur bone without impinge-18jl ment with the liner and cup thereby further reducing the 191, possibility of impingement of the cup and liner by the fernur 20 I during joint articulation.
21l With the acetabular cup and plastic bearing liner now 22 I firmly in place in the acetabulum, the head of the femur 16 23 is resected or prepared by suitable instruments to provide ¦, 24;1 a resected head, as shown in FIG. 6, complementary to the interior ~25l~ surface 44 of the metal femoral cap 40; more particularly, the 26l' resected head 19 of the femur 16 is prepared so as to provide 27,l the resected head with a flat portion 121 continuing into a 2 I sphcrical portion 122 continuing into a cylindrical portion 123.
9,' It will be understood that the outer dimensions of the resected fernur head 19 are made somewha-t larger than the inner dimensions ~ ,, ., 1 of the inner surface 44 of the metal femoral cap 40 and the 2 centrally drilled hole provided in the resected femur head 19 3 and femur neck 18 is made somewhat sma]ler in diameter than the 4 diameter of -the femoral cap stem 46 whereby the femoral cap may ; 5 be press fitted on-to the femur with such press fitting providing 1~ i 6 , temporary fixation of the femoral cap to tile femur. The hip 7 joint is then reduced with the outer spheric~l surface 42 o~
8 l the metal femoral cap 40 being received within the spherical g inner surface 32 of the plastic bearing liner 30 for joint 10 articulation. The articulation between the polished outer j 11 , spherical surface 42 of the metal femoral head 40 and the 12 Il spherical inner surface 32 of the plastic bearing liner 30 Z 13~ provide low friction, long wearing articulation and, the ¦,

14 j relatively larger articulation surfaces associated with the 15l surface replacement prosthesis provide greater potential for 16 extended wear resistance than do the smaller articulating 17j~ surfaces associated with the conventional total hip prosthesis 18l known to the prior art.
19l It will be further understood in accordance with the 3~ 20 teachings of the present invention that the screws, described 21 above, providing temporary fixation of the metal acetabular cup 22 to the hip bone providing the acetabulum provide temporary 23 'l fixation during the period of time required for hip bone ingrowth 24 and to the porous outer surface 23 of the acetabular cup which 25l bone ingrowth provides the permanent fixation of the metal ~ 26 ~ ace-tabular cup to the hip bone. Similarly, the press fit between ¦ 27,i the metal femoral cap 40 and the resec-ted femur head 19 provides !
28 temporary fixation of the metal femora] cap to the femur during 29 the tirne required for femur bone ingrowth at the resected head to grow into the porous inner surEace 44 of the femoral cap to 17.

~2~ Q~

1 permanently fixture the femoral cap to the resected femur head.
2 ~ It has been found that such respective temporary fixations 3 are sufficient to maintain the respective components fi~.tured 4 during normal joint articulation and during the time required 5, for bone ingrowth and biological permanent fixation.
6 il In accordance with the further teachings of the present 7,~ invention, the plastic bearing liner 30 may be readily and easily 8ll removed from the metal acetabular cup 20. Referring to FIG. 2, 9,i and as noted above, the lateral portions of the metal acetabular 10ll cup 20 are provided with generally opposed slots 66 into which a tool may be inserted and operated readily by digital force j 12 ll supplied by the operating surgeon to again flex the inner lateral;
'~ 13 , regions of the plastic bearing liner walls 94-94 inwardly to t 14ll flex the plastic bearing liner inwardly to permit the interrupted 15jl annular ridge 90 to be disengaged from the interrupted annular 16 li groove 64 and such engagement, in combination with the respective , 17 ll relieved inferior portions 63 and 89 of the cap and liner, i 18 ll permit the inwardly flexed plastic bearing liner 30 to be ~9,, readily removed from the metal acetabular cup 20. It will be 20l, understood that in accordance with the further teachings of the : 21 1i present invention, the slots 66-66 could be provided on the 1i 22lj outer surfaces of the plastic bearing liner 30, at the same 23,, positions, or mating opposed slots could be provided in both the i 24,l metal acetabular cup and the plastic bearing liner.
25l, Referring again to the recesses 101, 102 and 103, provided 26 1l in the outer spherical surface 34 of the plastic bearing liner 27l, cup 30, FIG. 3, it will be understood that these recesses overlie 2~,~ the screw hcads of the screws 51, 52 and 53 temporarily fixturing 29 " the me~a] acetablllar cup to the hip bone and it will be unclcr-30 ~ stoocl that these recesses accommodate these scrcw heads even "
, I

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1 during any misalignment or relocation of the screws within the 2; apertures as described above. Further, and in accordance with 3~i the teachings of the present invention, the plastic bearing liner 4 , portions providing the recesses engage the metal screw heads and 5 prevent them from becoming unscrewed during joint articulation.
6,l Referring again to FIG. 3B, it will be noted that the 7 ' alternate plastic bearing liner shown herein, smaller than the ' 8i, plastic bearing liner of FIG. 3A, may be used in the event that i 9l the conventional metal femoral stem is used in the femur instead 10,l of the metal femoral cap ~0 of the present invention, the ll,j spherical inner surface 132 of the plastic bearing liner alternate 12 1l embodiment 130 of FIG. 3B being dimensioned to closely match 13,i the exterior surface of such metal femoral stem.
14j Still further in accordance with the teachings of the - 15ll present invention, the bearing liner, instead of being plastic 16lj as described above, may be ceramic and in such alternate 17 il embodiment it will be understood that the relatively thin walls 18j~ of the metal acetabular cup 20 will be flexed outwardly to 19;l permit the ceramic bearing liner to be inserted into and removed j 20 1i from the metal acetabular cup 20; it will be understood that the 21 1i ceramic bearing liner is brittle and less flexible than the 22l, metal acetabular cup but will still undergo some slight inward 23 1l flexing but due to the relieved inferior portions only a very A 24 ,I slight outward flexin-~ of the metal acetabular cup or very 25 1l slight inward flexing of the ceramic liner, in combination with 26 il thc relieved inferior portions, will readily permit the insertion 27 ,l and removal of a ceramic bearing liner from the metal acetabular 1,, cup.
2~ 7~n important advantage of a replaceable bearing lincr 30 is that a surface rcplacement type hip may be revised to a ..

., . , - ~

, ., ~ . ,, ., .. -~Z4(~

1 conventional total hip using a femoral stem without disturbing 2 acetabular fixation by removing the plastic liner shown as 31 embodiment 30 intended for use with a surface replacement 4, femoral component and replacing it with the liner embodiment 5 ll 130 intended for use with a femoral stem prosthesis.
6,l Referring again to FIGS. 7A-D where it is shown that 7~1by open shell segmentation the flexibility of the side walls 92 g;,may be increased it should be observed that this increase in , glflexibility will result in an increase in the amount of lo;lallowable engagement between the groove 64 and ridge 90 for j 111 a given assembly load compared to a non-segmented open shell since the segmented open shell is more easily compressed.
1 13 Il Thus, the strength of this engagement can be increased and/or 3` 141~manufacturing tolerances associated with this engagement can J 15 ll be less critical than those associated with a non-segmented open -t 161shell.

17 1i Further, it will be understood by those skilled in the 18,1art that the prosthesis described above is the subject of many l9i1variations and modifications all within the scope of the present 20j~invention and that the present invention is limited only by the 211lscope of the appended claims.

22 1,l `~ 261, 28 li 30;l !l :

., .

Claims

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

1. A joint prosthesis comprising first and second components for being assembled together, the assembly of said components requiring the application thereto of assembly forces, said first component for being secured to a bone and said components having means for retaining them together upon assembly, wherein said first component is a non-symmetrical segmented open shell and wherein the segmenting of the shell increases the flexibility of the wall of the shell thereby facilitating assembly of the two components.

2. A prosthesis according to Claim 1 wherein at least one of said components is a segmented open shell of revolution.

3. A prosthesis according to Claim 1 wherein said retaining means comprise a groove and a ridge one of which is provided on said segmented open shell and which one is interrupted by a cutting surface causing the segmentation of said open shell.

4. A prosthesis according to Claim 1 wherein at least one of said components has a segmented outside surface of revolution which is eccentric to its inside surface which is likewise a segmented surface of revolution such that the wall of said one component is made thinner in regions which facilitate assembly of the components.

5. A prosthesis according to Claim 4 wherein said eccentricity produces thickening of the component wall in the region which is highly loaded during use of the prosthesis thereby increasing the load resistance of the component.

6. A joint prosthesis comprising first and second components for being assembled, said components provided with mutually engageable retaining means for retaining said components together upon assembly, at least the first of said components being an asymmetrical segmented open shell wherein the segmentation of the open shell increases flexibility of the shell wall thereby facilitating assembly of the components and enhancing engagement of the retaining means during assembly of the components.

7. A joint prosthesis comprising first and second components, said first component for being secured to a bone and said second component for being inserted into said first component and said components provided with mutually engageable retaining means for retaining said second component within said first component upon being inserted therein, at least the first of said components being an asymmetrical segmented open shell wherein the segmentation of the open shell increases the flexibility of the shell wall thereby enhancing insertion of said second component into said first component and the engagement of said mutually engageable retaining means during said insertion of said second component into said first component.