CA1169201A - Jersey meniscal bearing knee replacement - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A prosthesis for the surgical replacement of a dysfunctional knee joint is disclosed. The prosthesis includes a tibial platform, one or two tibial bearing inserts, and a femoral component.
In a unicompartmental embodiment of the invention, the tibial platform includes a spike for securing the tibial platform to the tibia. The tibial platform, in the unicompartmental embodiment, includes a track, which may be curved, and which is slidably engaged in dovetail fashion by a tibial bearing insert, typically of high molecular weight polyethylene. The superior surface of the tibial bearing insert is concave spherical, designed to slidably engage the inferior surface of the femoral component. The inferior surface of the femoral component is generally convex spherical, with radius of curvature slightly smaller than the radius of curvature of the tibial bearing insert. In some embodiments the inferior surface of the femoral component may have two or more differing radii of curvature at different points on such surface. Typically the tibial platform and the femoral component are constructed of cobalt-chromium alloy.
In a bicompartmental or tricompartmental embodiment of the invention, the tibial platform includes two tracks, each of which may be curved, and each of which slidably engages in dovetail fashion a tibial bearing insert. The two tibial bearing inserts each engage, via their superior concave spherical surfaces, mating inferior convex sur-faces of the femoral component. The two curved tracks are in general not concentric; rather, the center of each falls on a line normal to the plane of such curved track and passing through the center of curvature of the concave spherical surface of the tibial bearing insert of the other curved track.

Description

.
NEW JEE~SEY ~IENISC~L B~RII~G I~NEE P~EPLACI~ME~lT
TECHNICAL FIELD

2 This invention relates to prosthetic -joints generally,

- 3 and more particularly to a prosthesis for replacement of

4 a dysfunctional knee joint.

6 BACKGI~OUND ART
7 Referring now to prior art knee endoprostheses, 8 and in particular to the prior art knee prostheses with 9 patello-femoral replacement, it has been observed that such prior art prostheses have poorly designed patello-11 femoral interfaces in that they do not provide reason-12 able congruent patello-femoral contact or sliding enyage-13 ment over any appreciable range of knee motion.
14 More particularly, such prior art prostheses typic-all~ produce contact stresses which result in yielding 16 and fatigue of the plastic bearing surface typically 17 present in such prostheses. This result is caused by 18 the fact that the bearing surface of the femoral component, 19 over which the patella prosthesis must pass,generally has several regions or segments of differing shape.
21 For example, there is typically a fairly long, singly 22 curved segment blending into a first doubly curved 23 segment blending again into a second, and dif~erent, 24 doubly curved segment. These varying segments or regions provide ~he femoral portion of thè femoral-tibial 26 articulation, and those segments or regions do not have 27 a common generating curve. Thus, when the patella 28 pros~hesis goes through its excursion over the femoral 29 axticular flange, the patella prosthesis undergoes a variety of contact conditions, namely, substantial 31 portions of line contact, portions of point contact, 32 and perhaps limited portions of area or congruent area 33 contact. As is known, line contact and point contact 34 conditions generally produce high contact stresses which produce yielding and substantial wear of plastic prostheses.
36 Hence, the extended wear life needed for successful .

2 0 3. ~ ~ I

1 prosthetic implantation is not realized.
2 ~eferring next to typical prior art tibio-femoral 3 knee prostheses, it has been observed that those prior 4 art knee prostheses which allow axial rotation and anterior-posterior motion in addition to flexion-6 extension motion have incongruent contact (usually 7 theoretical point-contact) between the femoral and tibial 8 bearing surfaces, producing excessive contact stresses 9 leading to deformation and/or early wear and undesir-ably short prosthetic life. Also, wear products have been 11 shown to produce undesirable tissue reactions which may 12 contribute to loosening of the prosthetic components.
13 Those prior art knee prostheses which do provide 14 congruent or area bearing contact fail to provide the needed axial rotation, or when cruciates are present the 16 needed anterior-posterior motion. This lack of axial 17 rotation and anterior-posterior motion has been shown 18 clinically and experimentally to result in deformation 19 and loosening of the tibial components, and such prosthe-ses now appear to be falling into disuse.
21 Current prostheses of the dislocatable cruciate 22 retaining type,such as the Geomedic knee replacement 23 shown in U.S. Patent No. 3,728,742 issued April 24,1973 24 to Averill et al., that produce area contact provide 25 only one axis of rotation relative to the femur for the 26 flexion-extension motion. Normal flexion-extension 27 is, however, aharacterized by a polycentric flexion-28 extension motion where rotation relative to the femur 29 occurs about many axes. This polycentric motion, which results from the action of the cruciate ligaments and 31 condylar shape, allows for more efficient utilization of 32 muscle forces by providing a posterior shift of the axis 33 when effective quadriceps action is important and an 34 anterior shift when hamstrings effectiveness is important.
Furthermore, in the human knee it is this polycentric 36 action' and the shape of the posterior condyles, which ~ ~ 6~ ~0 1 1 influence this motion so as to allow full ~lexion cap-2 ability for the knee. ~ailure to provide appropriate 3 knee geometry i~bits, when cruciate lig~ments are present, this 4 natural polycentric motion and thus tends to rest~ict muscle effectiveness and inhibit flexion. These restrictions tend to increase 6 both loading on the prosthesis (which increases wear 7 or likelihood of deormation or breakage) and loading 8 hetween prosthesis and bone (whieh inereases the possib-9 ility of component loosening).
Other knee designs, such as the Townley type, 11 a~oid overconstraint by introducing incongruency of the 12 articulating surfaces. The incongruency, while necessary 13 to avoid overconstraint, unfortunately results in in-14 stability and excessive contact stresses.
It is further helieved that loosening problems 16 result from the direct attachment of plastic prosthetic 17 eomponents to bone through the use of relatively brittle 18 cement that is weak in tension, Specifically, it has 19 been demonstrated that even relatively thick plastic eomponents when Ioaded in a normal fashion produce 21 undesirable tensile stresses~in the acrylic cement 22 commonly used to secure such plastic components to bone.
23 Such loading tends to produce bending of the 24 ~plastic component which causes the ends of the plastic ~
component to lift away from the bone, thereby subjecting 26 the bone-cement att~achment to tension. ~s is known, 27 cement has very poor tensile fatigue properties. The 28 bone to which the plastic prosthesis is cemented also 29 appears to be adversely affected~by tensile loads, Accordingly, it is believed that these combined effects 31 contribute substantially to prosthetic loosening problems 32 and, specifically, it has been noted where clinical ~ailure 33 due to loosening occurs in a knee prosthesis that it is almost 34 always the-plastic prosthesis component which loosens.
Another prior art prosthesis problem exists with 36 regard to knee endoprostheses for implantation in thoss ' ' ' ' 11 B 9 ~01 1 cases wherein the cruciate ligaments are functionally 2 absent but where the collateral ligaments are functional 3 or at least reconstructable. In the absence of cruciate 4 ligaments, the prosthetic replacement must provide anterior-posterior knee joint stability so as to replace 6 that stability otherwise provided by the cruciates.
7 Until recently most such cases were treated by a stable 8 hinge-type knee prosthesis which, unfortunately, appears 9 to suffer from the loosening problems described above and furthermore typically produces substantial bone loss 11 as a result of the relatively great bone resection 12 required for implantation. Necrosis of the bone, 13 caused by altered mechanical bone stresses, is also a 14 problem with the hinge-type knee prostheses. More recent attempts have been made to treat such cases with surface 16 replacement prostheses such as the prostheses known as 17 the Total Condylar and similar knee prostheses. However, 18 these knee prostheses have theoretical point-contact 19 bearing surfaces with their abovè-noted attendant problems and, in addition, such prostheses tend to have 21 instability and dislocation problems which result,at 22 least in part, from these point-contact bearing surfaces.
23 Where the cruciate ligaments are present, most 24 surgeons would prefer their retention, since they provide important internal stabilizers and, together with 26 the condylar geometry of the femur and tibia, control the ~7 rotation axis of the knee. Furthermore, these ligaments 28 provide anterior-posterior (A-Pl stability. Thus, it is 29 desirable to preserve the cruciate ligaments, even though reasonable stability can be provided by a properly 31 designed full platform type prosthesis.
32 In addition, the action of the cruciate ligaments 33 produces a shift in the rotation axis of the knee which 34 may result in more efficient muscle utilization. Thus, preservation of these structures may provide better 36 physiological function after knae replacement.

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Still, it is not clear that the physiological advantages gained in retaining the cruciates outweigh the disadvantages of the design compromises, such as increased bearing surface incongruency and reduced tibial prosthesis bearing area, required to retain these ligaments. Thus, the desirability of retaining the cruciate ligaments in the cases of bicompartmental and tricompart-mental replacement is not well established. The design described herein, how-ever, eliminates or compensates for these design compromises, thus allowing the benefits of cruciate retention with minimal or no apparent loss in the ability of the prosthesis to withstand the loads to which it is subjected.
In unicompartmental replacement, the cruciates must be retained in any event since there is insufficient stability in their absence with a uni-condylar replacement. Thus, for such cases a design which accommodates the cruciate ligaments is necessary.-Unicompartmental replacement with a proper bearing design allows sur-gical restoration of a single diseased compartment, rather than the sacrifice of normal structures to replace all three compartments of the knee. Further, reducing the number of compartments replaced has the effect of reducing prosthe-sis wear products. Recent evidence strongly suggests that these wear products produce adverse physiological response to the prosthesis, including an increased tendency for the prosthesis to loosen from its boney attachment.
A recent experimental knee concept, the Oxford knee, appears to pro-vide a partial solution to the problem of overconstraint while maintaining con-gruency by the use of meniscal floating elements. Unfortunately, this knee suffers from several design problems which appear to limit its usefulness. The present invention, the New Jersey Meniscal Bearing Knee Replacement ~NJMBK) utilizes similar concepts in an improved fashion in order to avoid some of the anticipated difficulties of the Oxford design.

~ ~ B9 20 1 .
_IEF DESCRIPTION OF THE DRAWINGS
A complete understanding of the invention may be obtained from the de-tailed description which follows r together with the accompanying drawings, wherein:
FIGURES lA and ls are diagrammatic views of the prior-art Oxford knee.
FIGURES 2A and 2B illustrate the prior-art Oxford knee at 85 and 120 (respectively) flexion, showing the excess ~ posterior displacement of the bearing inserts at 85 flexion.
Two possible dislocation modes of the bearing inserts are shown at 120 flexion.
FIGURES 3A and 3B (on the same sheet as Figures lA and lB) also depict the prior-art Oxford knee. FIGURE 3A shows, in plan view, the position of the bearing inserts at 90 flexlon with - no rotation of the knee. FIGURE 3B shows the positions of the bearing inserts at 90 flexion in~the presence of axial rotations of 15 and 30.
FIGURE 4 illustrates the possibility of dislocation of the bearing inserts, in the prior-art Oxford knee, in the swing ?:~ phase of walking.
FIGURES 5A and 5B compare the anatomical ramp height with the rarnp height provided by the prior-art Oxford knee prosthesis, .
FIGURES 6A through 6D (Figures 6A and 6B being on the same sheet as Figure 8, and Figures 6C and 6D on the same sheet as Figure 4) illustrate some of the disadvantages which result from a design modification to partially constrain the bearing inserts of the prior-art Oxford knee.
FIGURES 7 through 9 show the femoral component of the , 2 ~ 1 present invention, the New Jersey Meniscal Insert Knee~
FIGURES 10 through 12 show the intermediate patella ~earing component according to the present inven-tion.
FIGURES 13 and 14 show the patella fixturing component according to the present inventlon.
FIGURES 15 through 17 show the tibial platform component according to the present invention.
FIGURES 18 through 21 show the intermediate tibial bear-ing component according to the present invention.
FIGURE 22 illustrates the manner in which the surface of the femoral component according to the present invention is gener-ated by a series of segments of surfaces of revolution.
FIGURE 23 illustrates the manner in which the several bearing surfaces of the present invention are generated by rotating a common generating curve about a particular generating axis at pairs of major generating radii.
FIGURE 24 shows the orientation of the patella prosthesis relative to the femoral component at full extension of the knee.
FIGURE 25 illustrates the role of the fixturing fins (of the patella fixturing component) ln resisting tipping loads.
FIGURE 26 (on the same sheet as Figure 9) shows the button portion of the patella fixturing component, which is used to retain the intermediate patella bearing component.
FIGURE 27 shows the manner in which the present invention permits rotation of the patella with respect to the femoral bearing surface.
FIGU~ES 28A and 28s illustrate the relatively low patello-femoral compression force present at full extension of the knee.

2 0 ~

FIGURES 29A and 29B illustra-te the somewhat greater patello-femoral compression force present in the load-bearing stance phase of the normal walking cycle.
FIGURES 30A and 30B illustrate the much greater patello-femoral compression force present in deep knee flexion.
FIGURE 31 (on the same sheet as Figure 27) is an inferior view of the distal femur, showing the femoral anterior articular cartilege involved in patello-femoral articulation, as well as the femoral posterior articular cartilege involved in tibio-femoral articulation.
FIGURES 32A and 32B show the manner in which the inter-mediate tibial bearing components are held in a forward position, on the tibial platform, by virtue of the shape of the bearing surface of the femoral component.
FIGURES 33A and 33B show the manner in which the inter-mediate tibial bearing components move posteriorly with flexion of the knee. FIGURE 33A shows 15 flexion, whil-e FIGURE 33B shows 120 flexion.
FIGURE 34 (on the same sheet as Figure 15) is a cross-sectional view of the curved track of the tibial platform component accordi~g to the present invention.
FIGURES 35A and 35B (on the same sheet as Figures 30A and 30s) illustrate the manner in which the intermediate tiblal bearing components move slightly closer together as they move forward and rearward from a central position in the curved track of the tibial plat~orm component.
FIGURE 36 (on the same sheet as Figures 32A and 32B) illustrates the manner in which the intermediate tibial bearing :~6920~

components move slightly closer together as the femur moves posteriorly.
FIGURES 37A and 37~ show the manner in which the use of an eccentric bearing insert (i.e~ the intermediate tibial bearing component) allows a relatively great inward shift of the bearing insert with little incongruency.
FIGURES 38A through 38C illustrate several advantages of the intermediate tibial bearing component accordiny to the present invention. The larger platform (relative to that of the circular L0 bearing insert of the prior-art oxford knee) is shown in FIGURE
38A. FIGURE 38B illustrates the greater dislocation height of the present invention, and FIGURE 38C illustrates the non-central spherical radius of the present invention.
FIGURES 39A and 39B illustrate the undesirable tensile stresses produced in the prosthesis-bone interface by the MacIntosh type tibial onlays of the prior-art Oxford knee.
FIGURES 40A and 40B show the t~ibial platform of a uni-compartmental version of the present invention.
~ FIGURES 41A and 41B show the manner in which the spike of the tibial platform of the unicompartmental version of the present invention resists both tipping and compressive loads.
FIGURES 42A and 42B compare the tibial platform component of the present invention with a prior-art prosthesis utilizing a flexible platform, which is ineffective in producing any load-sharing across the prosthesis-bone interface.
FIGURES 43 and 44 show the femoral component of a uni-compartmental version of the present invention.
FIGURES 45 and 46 show an implanted bicompartmental - _ 9 _ ~ 1~920 ~

version of the present invention, utilizing a pair of individual femoral components.
FIGURES 47A and 47B (on the same sheet as Figure 43) show an implanted unicompartmental version of the present inven- -tion.
FIGURFS 48, 49 and 50 illustrate an ankle prosthesis according to the present invention. FIGURE 48 is a cross-sectional view of the prosthesis, as indicated in FIGURE 50.
FIGURES 51 and 52 show the implanted ankle prosthesis according to the present invention.
FIGURES 53 and 54 show an anatomical ankle, for compari-son with the implanted ankle prosthesis of FIGURES 51 and 52.
FIGURE 55 shows, in schematic cross-section, an alterna~
tive track (consisting of just a shoulder, rather than a channel) suitable for applications where force loads applied to the prosthetic joint are such as to insure retention of the bearing insert against the shoulder.

- 10 - .;

~ 1~;9 2~ ~

The Oxford knee is shown in FIGURES lA and lB. The femoral components 101 consist of two metal spherical segmentsJ each of constant radius. Bearing inserts 102 are circular in shape with a shallow spherical superior surface and a flat inferior surface. The tibial onlays 103 consist essentially of two flat plates with ixation by means of a fin 10~ at the medial edge of each such flat plate There are several serious problems with the design of the Oxford knee of FIGURES lA and lB. The most basic problem is the potential for dislocation of bearing inserts 102 resulting from the limited flexion range of the device.
As can be seen from FIGURES 2A and 2B, the design provides excellent congruent contact up to about 90 flexion. Beyond that point a surface of constant radius cannot provide proper contact within the geometric constraints imposed by having to fit the prosthesis to the human knee. Flexion substantially beyond 90 produces edge contact and resulting deformation and possible dislocation of bearing inserts 10~. Although 90 of flexion is sa~isfactory from a functlonal standpoint, it is impractical to limit motion to this range, since activities will be encountered ~such as sitting onto a low chair, or returning ~o the standing position after sitting in a low chair) where flexion substantially exceeds 90.
The problem of insert dislocation is made more severe by axial rotation of the knee, as is shown in FIGURES 3A and 3B. In FIGURE 3A, there is shown the position of bearing inserts 102 at 90 flexion, but with no axial rotation of the knee. In FIGURE 3B there is shown the position of bearing inserts 102 at 90 flexion, but with 15 ~solid lines) ~nd 30 ~dashed lines) of axial rotation as well. There is a pronounced overhang of bearing inserts 102, with resultant risk of dislocation, 20~

1 under the combination of 90 flexion and 30 axial 2 rotation of the knee.
3 Normal distraction of one compartment of the knee 4 during the swing phase of walking, as depicted in FIGURE 4, also leaves bearing insert 102 of the prior art 6 Oxford knee free to dislocate.
7 A further disadvantage of the Oxford knee arises 8 ~rom the sha]lowness and placement of the arcs of the 9 contact surfaces, as can be seen from FIGURES 5A and 5B. In FIGURE 5A there is shown a normal knee joint, 11 with the anatomical ramp height designated 105.~ Note, 12 in FIGURE 5B, that the Oxford prosthesis ramp height 106 13 is substantially less than the anatomical ramp height 105, 14 and thexefore the Oxford prosthesis provides less than-normal medial-lateral stability. Thus, when medial-16 lateral shear loads are encountered, additional stress 17 is placed on the cruciate ligaments, which may be already 18 compromised by bone resection. Furthermore, such loading, 19 in conjunction with flexion or extension, will produce undesirable rubbing between the edges 107 o bearing 21 inserts 102 and the cut edges 108 o:E the tibial bone.
22 Other weaknesses of the Oxford design include lack 23 of accommodation for patella replacement, and tibial 24 plateau components with relatively poor load-bearing properties, as will be described iater.
26 An alternate embodimen~ of the Oxford knee which 27 attempts to deal with the problem of dislocation i~
28 depicted in FIGURES 6A-D. Unfortunately, this design has 29 several deficiencies which make it unworkable, at least with materials now commonly used for such components.
31 The anterior-posterior (A-P) travel limit is greatly 32 restricted compared to that of the present invention.
33 There is substantial unsupported area 109 of plastic 34 bearing insert 102, as can be seen from the cross-sectional view o~ FIGURE 6C. Flexure of the plastic 36 bearing insert 102 will occur, transferring load to the ~ l~g20~

remaining areas and thus greatly increasing bearing compressive stresses.
High stress will occur in the inner cavity at the head of retaining pin 110, particularly at the edge of retaining pin 110 and at the contact between the end of retaining pin 110 and the inner cavity, as can be seen from the cross-sectional view of FIG~RE 6D. Furthermore, the use of retaining pin 110 makes installation of the bearing element dlfficult after implantation of femoral and tibial componentsJ since it is neces-sary to separate the knee joint by stretching the ligaments an amount equal to the pin height in addition to the separation normally required to install bearing inserts 102.
SUMMARY OF THE XNVENTION
The present invention is directed to an improved prosthesis for the replacement of all or a portion of a dysfunctional human joint such as a knee joint.
The invention provides an improved prosthetic joint for implantation in an anatomical joint and of the type including:
(a) platform means having a first bearing surface at least a portion of which has no substantial curvature in a predetermined direction, the platform means for being secured to a first bone of an anatomical joint;

1~920~

(b) bearing insert means having a second bearing surface for slidably engaging at least the portion of the first bearing surface of the platform means which has no substantial curvature in the predetermined direction, the bearing insert means having a third bearing surface, the bearing insert means for providing an articulated joint between the platform means and a second bone component means;
(c) second bone component means having a fourth bearing surface for slidably engaging the third bearing surface of the bearing i.nsert means, the second bone component means for being secured to a second bone of the anatomical joint;
(d) the bearing insert mçans experiencing sliding movement relative to at least the portion of the first bearing surface of the platform means which has no substantial curvature in the predetermined direction during articulation of the joint; and ~.
wherein the improvement comprises:
~e) the fourth bearing surface of the second bone component means comprising a plurality of surface segments defined by rotating a common plane generating curve about a plurality of parallel axes of rotation whereby the fourth bearing surface of the second bone component means upon slidably engaging the third bearing surface of the bearing insert means facilitates control of the movement of the bearing insert means relative to at least the portion of the first bearing surface of the platform means which has no substantial curvature in the predetermined direction during articulation of the joint.

92~

-2 Referring now to FIGURES 7-21, there is shown an 3 endoprosthesis embodying the present invention which has 4 been referred to as a tricompartmental knee prosthesis and which includes the femoral component 111 best shown 6 in FIGURES 7, 8, and 9; the patella prosthesis 112 shown in 7 FIGUPE 27 and com~rising ~e intermediate patella bearing ox~nent 113 8 best shown in FIGURES 10, 11, and 12, and the patella g fixturing component 114 shown in FIGURES 13 and 14;
and the tibiaL pros~hesis 115 shot~ in FI~E 27 and comprising 11 the tibial platform com~onent 116 best shown in FIGW~ 15,16, and 12 17 and the intermediate tibial bearing components 117 13 shown in FIGURES 18, 19, 20, and 21.
14 Referring now to FIGURES 7, 8, and 9, there is shown in detail the femoral component 111 which includes, 16 in the counter-clockwise anterior or posterior direction, 17 a flange I18 formed integrally with two condyles 119-119o 18 The femoral component lIl also includes a pair of 19 fixturing posts; only one fixturing post, post 120, ~eing shown. The outside~surface of the.flange 118 21 provides most o~ the bearing surface for patella artic-22 ulation. The condyles ll9 are provided for replacing the 23 condylar surfaces of the human femur. The bearing surfaces 24 of fIange 118 and condyles ll9-119 are referred to gener-ally as the bearing surface 121. In accordance with the 26 teaching of the present invention, bearing ~urface 121 27 in the counterclockwise anterior to posterior direction 28 is a smooth, continuous surface formed by a series of 29 segments of surfaces of revolution the respective shapes of which are genera-ted or defined by rotating a 31 common generating curve (generally ideniified as F) 32 around a plurality of generating axes at respective pairs 33 of major generating radii (or each at a re~pective major 34 generating radius where the radii of each pair are equal) and through respective angles or rotation.
36 This common generating curve F is a smooth continuous .

11~923~ ?
' 1 plane curve and as may be understood from FIGURE 7 the 2 shape of which is defined by (i) two arcs K1 and K2 3 struck, respectively, by two radii Al and A2 from xe-4 spective centers Hl and H2 separated by a distance X;
(ii) two tangent lines 123 and 124 respectively tangent 6 to the arcs Kl and K2 and at angles c~ 1 and ~ 2, 7 respectively, with respect to a line G tangent to arcs 8 Kl and K2; and (ii.i) an arc K3 struck by radius B from 9 center H3 and wherein arc K3 is also tangent to the tangent lines 123 and 124.
11 Referring now to FIGURE 23, where a further under-12 standing of the general teachings of the present invention 13 is illustrated, it will be understood that the shape 14 of the bearing surface 121 (FIGURE 7) is defined or generated by a series of segments of surfaces of revolut-16 ion each of which segments is defined or generated by 17 rotating the common generating curve F around a respective 18 generating axis at respective pairs of major generating 19 radii (or each at a major generating radius where the radii of each pair of major generating radii are equal) 21 and through a respective angle of rotation. In generat- ^.
22 ing each segment of a surface of revolution, the common 23 generating curve F is oriented with respect to a generating 24 axis by a pair of major generating radii Dl and D2 which are the respective distances (shortest distances) 26 from points M1 and ~2 where the common generating 27 curve F contacts tangent line G as shown in FIGURE 23.
28 Réferring now to FIGURE 22, it will be understood 29 that this figure is a diagrammatic illustration showing the manner in which the series of segments of surfaces 31 of revolution Sl, S2, S3 and S4 defining the shape of the 32 bearing surface 121 are generated and where the curve Q
33 represents the trace of points Ml and M2 as viewed along 34 line G (FIGURE 23) resulting from the rotations about the respective generating axes generating the surface 3~ segments. It will be further understood that the shape ~ _j 1 of the bearing surface 121 is defined by a series of 2 segments of surfaces of revolution where each pair of 3 major generating radii Dl and D2 for generating each 4 segment decrease in length respectively as rotation of the generating curve F proceeds about each generating axis 6 in the counterclockwise anterior to posterior direction -7 as viewed in FIGURE 22. In the present embodiment and as 8 illustrated in FIGURE 23, the pairs of major generating 9 radii Dl and D2 are e~ual in each instance and may in each instance be replaced by a single major generating 11 radius R (i.e. Rl, R2~ R3 and R4) às shown in FIGURE 22.
12 In this embodiment, the bearing surface 121 consists of 13 four segments of surfaces of revolution Sl, S2, S3 and S4.
14 Sl is generated by rotating the common generating curve F through an angle 01 about generating axis Cl 16 perpendicular to the plane of FIGURE 22 at a major 17 generating radius Rl. In the present embodiment, Rl is 18 equal to infinity and~since only the i~termediate patella 19 bearing component 113 of FIGURES 1~, 11, and 12 axtic-ulates with segment Sl, it will be xefe-rred to as the 21 patello-femoral bearing surface segment.
22 Segment S2 is generated by rotating the common 23 generating curve F through an angle ~2 about generating 24 axis C2 parallel to Cl at a major generating radius R2 where R2 is equal to radius;Al which is equal to A2 in 26 FXGURE 7; since such radii ~re equal, it will be under-27 stood that se~ment S2 has two spherical surfaces~ ;
28 For continuity and smoothness of bearing 29 surface 121, axis C2 must lie on the ray Ll passing through Cl and defining the end of segment Sl. This segment (S2) 31 is of special importance since both the intermediate 32 patella bearing component 113 and the intermediate tibial 33 bearing component 117 articulate with this seyment and 34 since the greatest loads on these components during normal walking occur when they articulate against this 36 femoral bearing segment. This segment (S2) will, therefore, .

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1 be referred to: as the primary load bearing surface 2 segment.
3 Segment S3 is generated by rotating the common 4 generating curve F through an angle 03 about generating axis C3 parallel to C2 located at major generating radius 6 R3 where R3 is less than R2. Again, for continuity and .
7 smoothness of bearing surface 121, axis C3 must lie on 8 ray L2 passing through C2 and defining the end of ~.
9 segment S2.
Finally, segment S4 is generated by rotating the 11 common generating curve F through an angle 04 about 12 ~generating axis C4 parallel to C2 located at major 13 generating radius R4 which is less than R3. Again for 14 continuity and smoothness of bearing surface 121, axis C4 must lie on ray L3 passing through C3 and defining the 16 end of segment S3. These latter two segments will be 17 referred to, respectively, as the first and second 18 posterior femoral bearing surface segments.
- 19 Referring again to FIGURE 8, lt will be understood that FIGURE 8 is a sectional view of an actual embodi-21 ment of the present invention as shown in FIGURE 7 and 22 that the segments of surfaces of revolution Sl, S2, S3 23 and S4 shown in FIGURE 22 are also shown in FIGURE 8 24 at their respective locations.
~ In one embodiment of the present invention, the 26 respective angles ~ and each respective major generating 27 radius are as follows:

29 SEGMENT - (DEG~EES) ~JOR GENERATING RADIUS

, . . . . .
(inches) 32 Sl 0 cx~(displacement 0.612 inches) 33 S2 107.75 1.388 34 S3 62.25 0.801 S4 62 0.578 36 Referring again to FIGURES 8 and 22, it wi.ll be noted 11~i920~ 7 '' 1 that the generating axes Cl~ C2~ C3 and C4 are parallel 2 with respect to each other and it will be understood 3 that the tangent line G is oriented substantially 4 parallel to the generating axes. ~owever, in accordance ~ 5 with the teachings of the present invention, such need 6 not be the case and the generating axes may be oriented 7 other -than parallel with respect to each other and, as shown in the general case illustrated in FIGURE 23, the g tangent line G may be oriented other than parallel to the generating axes.
11 Referring again to the patella prosthesis and in 12 particular to the intermediate patella bearing component 13 113 of FIGURES 10, 11, and 12, it will be understood that 14 in accordance with the further teachings o the present invention such intermediate patella bearing component 113 16 provides a load-bearing surface indicated by general 17 numerical designation 125 for engaging the bearing surface 18 121 of emoral component.lll and which load bearing 19 surface 125 includes a primary load bearing surface segment `126, a pair of secondary load bearing surface 21 segments 127 and 128 and a pair of transition segments 22 129 and 130 between 126 and 127 and 126 and 128 respect-23 ively- Further, it will be understood in accordance 24 with the teachings of the present invention that the . 25 shape of the load bearing surface`l25 of the intermediate 26 patella bearing component 113 is defined or generated 27 by the common generating curve F used to generate the 28 segments Sl-S4 of the bearing surace 121 of femoral 29 component 111. Referring to FIGURE 11, it will be understood that the common generating curve F is rotated 31 through an angle ~5 (in one embodiment angle ~5 equals 32 20) about generating axis C5 at the pair of major gener 33 ating radii Dl and D2 shown in FIGURE 23, where Dl and 34 D2 are each equal to major generating radius R2 shown in ~GURE 22, to define the shape of the primary load 36 bearing surface segment 126. Therefore, thç patella 1 primary load bearing surface segmen-t 126 congruently 2 matches the primary load bearing surface segment S2 of 3 femoral bearing surface 121 and, upon articulating .
4 therewith, engages the primary femoral bearing surface segment S2 in sliding area contact. The secondary load 6 bearlng surface segments 127 and 128 of the patella 7 load-bearing surface 125 of FIGURE 11 likewise match 8 the patella femoral bearing surface segment Sl of 9 bearing surface 121 (in FIGURE 8) and hence their shapes are defined or generated by rotating the common 11 generatiny curve F about an axis C6 at infinity (and 12 parallel to axis C5) as was done in generating the 13 shape of segment Sl of femoral bearing surface 121.
14 Therefore~ the patella prosthesis secondary load-bearing surface segments 127 and 128 congruently match the 16 patello-femoral bearing surface segment Sl of femoral 17 bearing surface 121 and, upon articulating therewith, 18 engage the femoral bearing surface segment Sl in sliding 19 area contact. The transition segments 129 and 130.
are defined by rotating the common generating curve F
21 through an ang-le ff6 about axes C7 and C8 respectively at 22 a pair of negative generating radii (directed to opposite ;
23 sides of common generating curve F from those shown in 24 FIGURE 23), both about 0.30 inch in onè embodiment.
These transition segments 129 or 130 engage, in line 26 contact, segments S2 and Sl of femoral bearing surface 27 121 near their interface as the contacts shift from 28 segment S2 of the femoral bearing surface 121 with the 29 primary load bearing segment 126 to contact between femoral segment Sl and the secondary load bearing segments 127 31 and 128.

32 In another embodiment of the patella prosthesis of 33 the present invention, secondary load bearing surfaces 34 127 and 128 are inclined downwardly with respect to the horizontal (as viewed in FIGURE 11) to better accommodate 36 the orientation of the patella prosthesis112 with respect c ;~
1 1 6 9 2 ~ 1 !

1 to the femoral component 111 during full extension of 2 the human knee as shown in FIGUR~ 24 and therefore to 3 provide a more uniform load distribution on the secondary 4 load bearing surface seyment 127 or 128.
The intermediate patella bearing component 113 is 6 retained on the remnant of the human patella by use of 7 the patella fixturing component 114 of FIGURES 13 and 14.
8 Patella fixturing componen-t 114 may be suitably affixed 9 to the remnant human patella, using an acrylic grouting agent or cement, by crossed fixturing fins 131 and 132 11 on the dorsal side of the metal plate 133. Such fixturing 12 fins resist tipping loads, as shown in FIGURE 25, and, 13 in addition, provide a reinforcing efe~t which allows 14 the use of a thin metal plate 133, which is desirable, since one wishes to minimize the change in overall 16 patella thickness resulting from prosthetic replacement 17 so as not to adversely affect patella function, skln 18 closure after surgery and cosmesis. The ixturing fins 19 131, 132 and metal plate 133 reinforce and strengthen the patella remnant and minimize the possibility of its 21 fracture. The opposite or ventral side of metal plate 22 133, FIGURE 13, which comprises th bulk of the secondary~
23 fixturing component bearing~surface which mates wlth the 24 secondary bearing sur~ace 134 on the intermediate patella bearing component 113, is provided with a 26 but~on 135 which retains intermediate patella bearing 27 component 113 on the patella fixturing component 114 with 28 a~snap fit. As shown in FIGURES 13 and 26, the outer 29 diameter of the button 135 is formed from a curve with two tangent radii which produce a smooth retaining male 31 surface 136 when mated with correspondingly shaped female 32 surface 137 (FIGURE 10) provided on the intermediate 33 patella bearing component 113. These shapes allow easy 34 entry of the male into the female component without producing the perManent deformation characteristic of 36 conventional snap-fit configurations. The mating conical ( .. (. !
~ 9 ~ 0 1 ~, -2~-1 sections provide additional secondary compressive and 2 thrust bearing surfaces. The button 135 is provided with 3 a generally conical shaped bearing surface 138 for 4 rotatably engaging the correspondingly shaped conical secondary bearing surface 134 (FIGURE lO)provided on the 6 intermediate patella bearing element 113 in congruent or 7 area rotational engagement to permit rotation of the 8 patella with respect to femoral bearing surface 121 and 9 the distal end of the femur about axis A8 ~FIGURE 27).
Further, and referring to FIGURE 13, the patella 11 fixturing component 114 is provided with a pin 139 for 12 engaging a corresponding, curved slot 140 formed in the 13 intermediate patella bearing component 113 (FIGURE 10) 14 to limit the relative rotation between intermediate patella bearing component 113 and the patella fixturing component 16 114 and thereby prevent disorientation between the inter- .
17 mediate patella bearing component 113 and the femoral 18 component 111 during implantation and subsequently during 19 actual use. Furthermore, this limited rotation has been found to be reasonably necessary since effusion (build 21 up of blood) post-operatively may temporarily lift the 22 load-bearing surface 125 of the intermediate patella ~ -23 bearing component 113 free of the restraining effects 24 of the femoral component 111.
It will be further noted, as shown in FIGURES 10-14, 26 that the intermediate patella bearing component 113 and 27 patella fixturing component 114 are made symmetrical 28 about a plane passing through the center of the primary 29 load bearing surface 126 and through the generating axis C5 producing primary load-bearing surface segment 126, 31 so as to allow the use of the same patella prosthesis in 32 either the right or the left knee. It is for this reason 33 that two secondary load bearing segments (127 and 128) 34 are provided on the load bearing surface 125.
Referring now to FIGU~ 2~, 28B, 29A, 29B, 30A, and 30B, there 36 is illustrated diagrammatically the manner in which the patello-femoral 2 0 ~

1 portion of the tricompartmental prosthesis provides 2 area or congruent sliding contac-t be-tween the bearing surface 121 of the femoral component 111 and t'he load 4 bearing surface 125 of the intermediate patella bearing component 113 over the important phases of the range of 6 motion commonly experienced by the human knee, providing 7 line contact between such bearing surfaces only during a 8 brief transitional phase, Referring firstto ~IG~S 28A and 28B, g it will be noted that at full knee extenSioN the quadri-ceps muscle group provides a quadriceps force FQ which in 11 normal activities is quite low at full extension.
12 Because of the orientation of the force FQ the resultant 13 patello-femoral compression force R of FIGURE 28B is only 14 a small fraction of force FQ. During this phase of human lS knee action there is area contact between the bearing 16 surface segments S1 and 127 (or128~ of the femoral 17 and patella components, respectively. See FIGURES 8 18 and 11. ~ , 19 Referring now to FIGURES 29A and~29B wherein the load bearing stance phase experienced during~the normal , 21 walking cycle is illustrated diagrammatically, it will be ~' 22 noted here the quadriceps force FQ is greater and the 23 resultant patello-femoral compression force R is much 24 greater than at the full exténsion illustrated in FIGU~S 28A and 28B- This result is attributable to the greater quad-26 riceps force FQ and the smaller included angle betw,een 27 th,e quadriceps force FQ and the patella ligament orce 28 F!Q. of course, as is known, even greater flexion angles 29 are experienced by the human knee during stair climbing and descent and hence in these activities even greater 31 patella bearing resultant forces R occur.
32 It will be understood that during the short transition 33 phase in moving from segment Sl to segment S2 that 34 transition segments 129 or 130 of the patella load-bearing surface 125 are in sliding line contact with the 36 femoral bearing surface 121. As is further known, during .

g ~ O ~

1 the most common and hence most important human knee 2 activity, namely level walking, there is no substantial 3 quadriceps activity or force present until approximately 4 10 of knee flexion is achieved at which the patella articulation of the prosthesis of the present invention 6 has just entered the primary load bearing surface segment 7 S2 whereln there is sliding area contact between the 8 femoral bearing surface segment S2 and.the patella 9 primary load bearing segment 126. Thus, the above-noted transitional and hence momentary line contact is not of 11 serious concern since at this time the quadriceps force 12 FQ is relatively small and even if it were substantial 13 the resultant compressive force R would still be quite low 14 because of the large .included angle between forces FQ and FQ,. ~rea contact is only needed during the walking load 16 bearing and other activity phases where compression forces . ' 17 R are significant.
18 The regions Sl and S2 on the femoral component 111 19 and correspondins transition segments 129 or 130 and the primary and secondary load bearing surface segments 21 126 and 127 (or 128) are needed to produce anatomical 22 patello-femoral articulation wherei:n at full extension 23 as the superior aspect of the patella lifts off the 24 femur as in FIGURE 28Aand yet allow central area contact engagement at moderate and full fiexion as shown in 26 FIGURES 29A..and 30A.
27~ Referring now :~o FIGURES 30A and 30~ wherein deep ~nee 28 flexion is illustrated diagrammatically, it will be seen 29 that it is during deep knee flexion that the patello-femoral compressive load R is greatest. It will be 31 understood, and as illustrated in FIGURE 30A, the patella 32 load bearing surface 125 (FIGURE 11~ articulates with the 33 same surface segment S2 (FIGURE 8) wherein the tibio-34 femoral articulation occurs during full extension, thus, the primary load bearing surface segment S2 o bearing 36 surface 121 supplies the femoral bearing surface for both 9 2 0 ~. ( ) -25~
1 articulations (patello-femoral and tibio-femoral~artic-2 ulations) a-~ times of yreatest loading during the walking 3 gait cycle, and -this commonality is a si'gnificant feature 4 of the present invention. Of course, as is known to those familiar with the anatomy of the hu~lan knee, this 6 situation (common articula-tion between a portion of the , 7 human condyles and both the patella and tibia]. bearing 8 surfaces) is not present in the anatomical human knee.
9 As shown in FIGURE 31, in the human knee the femoral anterior articular cartilege against which the 11 human patella articulates is dis~inct from that which 12 articulates with the tibiaO Such natural structures adapt 13 during development of the human knee to produce precise 14 mating of the structural and articulation elements of the knee'but such precision of mating is not practical 16 in replacement knee prostheses because of the large 17 individual variations found in different human knees, 18 as well as the manufacturing and surgical difficulties 19 involved in reproducing such precision. ThuS, the use of a common femoral prosthesis primary~load bearing sur-21 face segments S2 for both the patella and tibial artic-22 ulations represents a significant feature in providing 23 the needed sliding area engagement or congruency of art-24 iculation for extended wear life.
Referring again to FIGURE 10, it will be noted that 26 the depth o~ engagement o~'the patella load bearing' sur~
27 face 125 into,the femoral bearing surface 121, distance 28 T in FIGURE 10, is substantial and hence allows substant-29 ial subluxation resistance to side thrust loads. It has been found that in individuals where this dimension is 31 small or excessive knee valgus is present, subluxation 32 of the patella is common. Yet in many known prior art 33 devices, the corresponding depth of engagement is in-34 adequate or non-existent. Further, and referring again to FIGURES 10 and 13, it will be noted that area rotatable 36 mating fit (bearing surfaces 134 and 138) between the l l B 9 2 ~

1 intermediate patella bearing component 113 and the 2 patella fix-turing component 114 allows a rotation there-3 between and this rotation is highly desirable to accom-4 modate possible surgical misalignment during implantation, as well as the small, naturally observed, patella rotation 6 with respect to the human femur during flexion-extension 7 movements.
8 Referring now to FIGURES 18, 19, 20 and 21, and to 9 the intermediate tibial bearing component 117 shown therein, this component provides a primary load bearing 11 surface 141 on its superior side and a second bearing 12 surface 142 on its inferior side. The primary load bear-13 ing surface 141 is also formed as a surface of revolution 14 and its shape is defined or generated by the common generating curve the same as or very similar to curve F
16 used to generate the shape of segments Si-S4 of femoral 17 bearing surface 121 and the shape of patella beariny 18 surface 125.
19 Referring now to FIGURE 19, it will be understood that the shape of the primary load bearing surface 141 21 is defined by rotating the common generating curve su~-22 stantially similar to curve F through an angle ~6 (in 23 one embodiment of the present invention ~6 equals 60 24 degrees3 about generating axis C~ at the same major ~25 generating-radii Dl and D2 shown in FIGURE 23 where Dl and 26 D2 are again each equal to R2 shown in FIGURE 22.
27 Therefore, the tibial primary load bearing surface 141 28 is in substantial area contact with the primary load 29 bearing surface segment S2 of femoral bearing sur~ace 121 and, upon articulating therewith, engages the femoraI
31 primary bearing surface segment S2 in sliding area contact.
32 Therefore, substantially congruent articulation is 33 provided at the tibio-femoral joint interface for 34 approximately 36 degrees of knee flexion wherein the greatest loads during the walking cycle are experienced 36 as indicated in ~IGURES 29A and 29B.

~9201 ~ ~

1 The geometry and particularly the shape of load 2 bearing segment S2 are configured so that, in addition to 3 producing the avorable patello-femoral and tibio-~ femoral articulation described, the intermediate tibial bearing components 117 are held in a forward position 6 on the ~ibial platform 116, as shown in FIGURES 32A and .
7 32B. AS the knee is flexed slightly the ~emur, and thus 8 the intermediate tibial bearing components 117, move 9 rearward relative to the tibia so they then occupy a generally central position on the tibial platform 116, 11 as shown in FIGURE 33A. Additional flexure produces a 12 small additional posterior shift of intermediate tibial 13 bearing components 117 as a result of further anterior 14 displacement of the tibia relative to the femur and as a result of femoral condylar geometry,as shown in FIGURE
16 33~. This posterior shift is reduced at flexion angles 17 above 40 by the use of small major generating radii in 18 segments S3 and S4; shown in FIGURE 8, in the New Jersey 19 Meniscal Insert Knee Replacement. The use of smaller major generating radii in segments ~3 and S4 allows full 21 flexion without excessive shift of intQrmediate tibial 22 bearing components 117, an important feature of the present 23 invention that is not to be found in the prior-art Oxford 24 knee.
The 0 to 90 degree flexion-extension range includes 26 almost all strenuous activi~ies in which an individual 27 wi*h an endoprosthesis is likely to engage. ~rticulation 28 in the 35-95 degree range occurs in the first posterior 29 femoral bearing segment S3 of FIGURE 8 and hence there is line contact as indicated in FIGURE 30A~. Although such 31 line contact or incongruency is less desirable than 32 sliding area contact, it produces acceptably low contact 33 stresses while allowing suf~icient flexion necessary for 34 normal activities since loads during walking in *his phase of flexion are much less than in the 0-36 degree 36 range or area contact phase. Heavy joint loading in this 2 ~

1 range of knee motion occurs much less frequently than in 2 the 0 to 36 degree range and thus higher periodic or 3 transitional stresses can be tolerated without producing 4 fatigue or excessive wear. Flexion from 95 degrees to 140 degrees is accommodated by the second posterior femoral 6 bearing segment S4 of the femoral prosthesis (FIGURE 8) 7 and expected stresses at such flexion angles are such that 8 serious permanent deformation is not anticipated except 9 perhaps during deep knee bend exercises such as deep squats, which should of course be avoided by individuals 11 having an~ knee prosthesis. Fatigue is not of concern 12 here (segment S4) since the expected frequency of occur-13 rence of these stresses is low. Obviously, a patient 14 with such knees should be discouraged from performing deep knee bends or similar exercises. It s`hould be noted 16 that few knee prostheses allow flexion in excess of 90 17 degrees, and those that do, while still allowing reasonable 18 axial rotation, experience far greater contact stress 19 than the present invention. The last region is provided to allow the e~treme flexion range which is often needed 21 during sitting, where small loads on the knee are ex-22 perienced, without producing excessive posterior shift 23 of the intermediate tibial bearing components 117.
24 The two incongruent or line contact phases of contact associated wi~h segments S3 and S4 are tolerated in order 26 to obtain nearly normal flexion and extension motion 27 by providing a reasonable approximation to normal 28 condylar geometry. Incongruency in these phases occurs 29 only in one dimension rather than two dimensions as in most prior art prostheses. Thus, normal knee motion is 31 provided without excessive shift of intermediate tibial 32 bearing components 117 while keeping contact stress 33 within acceptable limits of most normal activit~.
34 The second bearing surface 142, FIGURES 18, 19, 20, and 21, is on the inferior side of the intermediate tibial 36 bearing component 117. This bearing surface is composed ~ 9 2 0 ~ ~
~29-1 of a flat surface 143 and a projecting dovetail surface 2 144. The flat and dovetail bearing surfaces engage the 3 superior surface 145 of the tibial platform component 4 116 shown in FIGURES 15, 16, 17, and 34, and the track surfaces 146 and 154 therein in area contact.
6 This tibial pla-tform 116, as shown in FIGURES 15,-7 16, and 17, consists of a thick plate 147 with a notched 8 area into which fits the section of the proximal tibia to 9 which the cruciate ligaments are attached. Two curved tracks 148 and 153 are provided in thick plate 147.
11 These curved tracks 148 and 153 receive and partially 12 constrain the two identical in~ermediate tibial bearing 13 components 117, which can ke seen in FIGURES 32A and 32B. These 14 bearing inserts are substantially identical to the inte ~ diate tibial bearing com~onent illustrated in FIG~ 18 thru 21.
16 The shape of the thick plate 147 of the tibial plat-17 form component 116 is contoured so as to engage, where 18 practical, the outer cortical bone of the tibia so as to 19 improve load bearing and to allow this component to be~
used for both right and left tibias. ~hree short spikes 21 149, 149, and 172 help distribute joint loads, supply 22 additional load transfer to the cancellous bone, and 23 provide resistance against possible tensile loading.
24 It will be understood that the symmetry of both intermediate tibial bearing component 117 and tikial 26 platform component 116 eliminates the need to designate 27 a right or left knee aspect, and thus eliminates the 28 concern of the implanting surgeon with these matters 29 during implantation.
In FIGURE 16, it can be seen from the shape of 31 curved tracks 148 that as the intermediate tibial bearing 32 components 117 move forward and rearward from the central 33 position tha~ they move somewhat closer together,as 34 shown in FIG~ 35~, 35B, and 36. It may be seen from FIG~RES 37~ and 37Bthat the use of an eccentric bearing insert allows 36 a relatively great inward shift with little incongruency.

f ;f ) f 1~.6~20~

1 For example, a total movement of ~6 mm produces a separ-2 ation change of 0.5 mm. This change of separation is 3 easily accommodated by using a very slightly incongruent 4 surface and/or by pxoviding a slight clearance between the walls 150 and 151 (FIGURE 34) of curved tracks 148, 6 and the mating projecting dovetail surfaces 144 of the 7 intermediate tibial bearing component 117, shown in 8 FIGURE 19. The contact congruency ratio C, when contact 9~ is made with segme~t S2 of the femoral prosthesis, used in one embodiment is approximately 0.99, where C is 11 defined as follows:
12 C- R2/R2' 13 where 14R2= Spherical radius of primary load bearing 15segment S2 of bearing surface 121 on 16femoral component 111 (FIGURES 7,8);
17 and 18R21= Spherical radius of primary load bearing 19surface 141 of the intermediate tibial 20- bearing component 117 tFIGURES 19,20).
21The contact stress is thus kept ~uite low while 22 still allowing the needed change in seperation.
23 In addition to the anterior-posterior shift, axial 24 rotation of the tibia takes place during flexion. This rotation is accommodated by the shape of the conlacting 26 surfaces, and in particular by the spherical radii of the 27 primary load bearing segment S2 of the femoral component 28 111 and primary load bearing surface 141 of intermediate 29 tibial bearing component I17, as well as by the curvature of the curved tracks 148 and 153 of tibial platform 31 component 116. As can best be seen from FIGURE 16, the 32 center 152 of curvature of the left curved track 153 of 33 tibial platform 116 is on a line normal to left track 34 surface 154. This line, on which lies the center 152 of curvature of the left curved track 153, passes through the 36 center 155 (refer to FIGURE 7) of the right spherical .

( 1 ~ 69 20 ~

1 radius of the primar~ load bearing segment S2 of femoral 2 component 111 when the components are all assembled.
3 Thus, if one were to hold the prosthesis so that it could 4 only rotate about this normal line, the motion could be accommodated (even with perfect congruenc~ and rigidity 6 of the plastic) by virtue of the spherical contact on 7 the right side and the track curvature on the left.
8 Similarly, motion about a normal on the left side could 9 also be accommodated. Axial motion about any other normal axis e~pected in the knee produces slight inward 11 motion of the intermediate tibial bearing components 117 12 as shown in FIG~RE 36. This inward motion, as in the 13 case where this motion is produced by anterior-posterior 14 shift, is accommodated with the very slight incongruency used, and/or the slight clearance provided between the 16 projecting dovetail surfaces 144 of intermediate tibial 17 bearing components 117 and curved tracks 148 and 153 of 18 tibial platform component 116.
19 The less constrained prior art Oxford knee also provides for axial rotation and anterior-posterior shift 21 even with perfect congruency. In the present invention, 22 such motion is obtained while allowing the utilization of 23 stabilizing tracks.
24 The method of track engagement utilized in the present invention has several functions:
26 ~ 1. It prevents rotation of the intermediate tibial 27 bearing components 117, and thus:
28 (a) Allows a noncircular and larger bearing 29 insert platform 156 (in FIG~RE 38A), as compared with the smaller, circular platform 31 157 of the prior art Oxford insert. ~he present 32 invention also produces a greater dislocation 33 height 158 as compared with the dislocation 34 height 159 of the prior art Oxford insert as shown in FIGURE 38B. This added height also 36 allows large shifting ~orces for moving the ,.

~ 1 69 20 ~

1 bearing insert anteriorly and posteriorly 2 against the friction generated hy the 3 large compressive load~ ¦
4 (b) Allows use of a noncentral (i.e. noncentral when viewed in the anterior-posterior direction) 6 spherical radius 160, as can be seen from 7 FIGURE 38C, providing additional medial or 8 lateral stability by virtue of the relatively 9 large inside engagement height 161. This is to be contrasted with the central sphsrical 11 radius 162 of the prior-art Oxford knee, with 12 its resultant relakively smaIl inside engage-13 ment height 163. The improved engagement 14 of the present invention is unaffected by axial rotation or anterior-posterior shift. Such is 16 not the case in conventional designs.
17 2. It provides a partially self-retaining feature 18 for the curved tracks 148, 153. This feature, plus the 19 longer intermediate tibial bearing components 117, eliminates the possibility of tipping and dislocation 21 associated wi~h the prior art prostheses 22 discussed earlier.
23 3. The curved tracks 148, 153 provide thrust surfaces 2~ allowing most medial-lateral shear loads to be taken entirely by the prosthesis with no prosthesis-bone 26 rubbing contact as in the Oxford knee.
27 Thus the present invention, the New Jersey Meniscal 28 Insert Knee Replacement (NJMIK) sacrifices a small 29 amount of congruency (and simplicity) to achieve greatly improved stability. The advantages and differences of 31 the NJMIK compared to the prior-art Oxford knee design 32 can be summarized as follows:
33 1. Use of smaller major generating radii for the 34 posterior segments S3 and S4 (FIGURE 8) of femoral component 111, thus allowing ull 1exion and allowing 36 such flexion without excessive shift of the intermediate 11~920~

1 tibial bearing components 117;2 2. Elimination of possible intermediate tibial bear-3 ing component dislocation modes;
4 3. Provision of greater insert shifting forces to overcome friction;
6 4. Provision of greater medial-lateral stability;
7 and, 8 5. Provision o effective patello-femoral articul-g ation coupled with tibio-femoral articulation.
The primary disadvantage of the NJMIX, which also is 11 present in the human knee, is the loss of excellent bear-12 ing congruency beyond about 40 flexion, as previously 13 described. It therefore seems a very advantageous trade-14 off considering the limitations inherent in the prior-art Oxford knee design.
16 Additional benefits result from the tibial fixation 17 methods employed.
18 Loosening and collapse of the tibial component are 19 major problems in knee replacement. This is true of the MacIntosh type onlays used in the prior.-art Oxford knee.
21 The problems with this type~of platform are depicted in 22 FIGURE 39~, which shows posterior load 164 and lateral 23 load 165. Note that posterior load 164 produces high 24 compressive stress at the posterior aspect of the tibia, with tensile stress at the anterior aspect. The anterior 26 portion of the tibial-onlay. tends to lift as a result 27 of the tensile stress, as can be seen from FIGURE 39A.
28 There is also a large stress concentration effect o the 29 fixation fin 166. The tipping of the tibial onlay also produces large posterior or lateral compressive bone 31 stress, thereby increasing the tendency toward bone 32 collapse as shown in FIGURE 39B.
33 In the unicompartmental version of the present 34 invention, tibial platform 167 of rIG~S 40A and 40B for example, tipping loads are resisted by reactive compressive loads 36 on the spike 168. Spike 168 also helps support the .

1 1 6 9 2 o ~ , ~

1 direct compressive loads as well, as can be seen from FIG~ ~ !
2 41A and 41B.In FIGURES 41A and 41B, posterior load 164 3 and lateral load 165 are shown similarlyto FIGURES 39~ and 39B.
4 The combined effects (tipping loads resisted by reactive

5 compressive loads on spike 168, and direct compressive .

6 loads partially supported by spike 168) result in relative- j

7 ly low contact stresses on the bond, in the case of the

8 tibial platform 167 according to the present invention.

9 The tibial platform component 116 according to the

10 present invention resists tipping forces by means of a

11 bridge 169, which can be seen in FIGURE 16. Bridge 169

12 connects the two tibial plateau sections 170 and 171, ~ -

13 and transfers some of the load from one plateau section

14 to the other, as can be seen from FIGUR~i 42A. Shown for comparison in FIGURE 42B is a prior-art prosthesis 16 with a flexible platform, which is ineffective in produc- .
17 ing any load-sharing across the prosthesis-bone inter-18 face. Also, the short anterior spike 172 of the present . 19 invention, shown in FIGURES 15 and 17, serves to resist posterior loads. Furthermore, bridge 169 inhibits the 21 outward splaying fracture of the tibial.condyles depicted 22 in FIGURE 39B.
~3 It will be further understood by those skilled in 24 the art and referring again to the femoral component 111 - 25 and the pa~ella prosthesis 112, that the bearing surfaces 26 173 and 138 of the patella fixturing component 114 27 (FIGURE 13) and bearing surfaces 137 and 134 of the 28 intermediate patella component 113 (FIGURE 10) accommodate 29 both axial surgical misalignment and normal rotation while permitting area contact between the bearing segments Sl 31 and S2 of the femoral component 111 and the load-bearing 32 surface 125 of the intermediate patella bearing component 33 113. Similarly, it will be further understood that the 34 bearing surfaces 143 and 144, respectively, of the intermediate bearing components 117 (FIGURES 18-21) and 36 the mating bearing surfaces of the tibial platform .

1~6~20~

1 component 116 accommodate both axial surgical misalign-2 ment and normal rotation while permittlng sliding sub-3 stantially area contact between the primary load bearing 4 segment S2 of femoral component 111 and the primary load bearing surface 141 of the intermediate tibial 6 bearing component 117. This substantial congruence is 7 provided in the important stance phase of walking illust-8 rated diagrammatically in FIGURE 29A.
9 Referring now to FIGURES 43-46, there is shown a bicompartmental embodiment of the present invention 11 which utilizes a pair of individual femoral components 12 174 and 175 and, as illustrated diagrammatically in 13 FIGURES 45 and 46, omits the use of the patella pros~
14 thesis 112. Referring specifically to FIGURES 43 and 44, there is shown a right individual femoral component 16 174 and it will be understood that the individual 17 femoral component 175-shown in FIGURES 45 and 46 is the 18 mirror image of the right femoral component 174 shown in 19 FIGURES 43 and 44. Tibial prosthesis 115 of this embodi-ment is the same as the tibial prosthesis 115 already 21 described. It will be understood, and referring to 22 FIGURE 46, that the individual femoral components, e.g.
23 175, are provided with a load bearing surface 176 24 which is identical to the segments S4, S3, and a major portion of the primary load bearing segment S2 shown in 26 FIGURE 8. Thus, it will be further understood that 27 segment S2 of these individual femoral components 174 28 and 175 are in area contact with the primary load 29 bearing surface 141 of the intermediate tibial bearing component 117 as taught a~ove, thus providing the same 31 tibio-femoral articulation as described above. For 32 unicompartmental replacement a tibial platform 177, as 33 shown in FIGURES 47A and 47B, is used together with an intenx~-34 iate tibial bearing component 117, as shown in FIGURES
18-21. FIG~ 47A and 47B show the assembly of tibial platform 36 177 and intermediate tibial bearing component 117 to , ~fi920~ ~ I

1 form a unicompartmental knee replacement. k 2 R~ferring again to FIGURES 18-21, it will be still E
3 further understood by those skilled in the art that 4 the in-termediate tibial bearing component 117 may be 5 easily removed intraoperatively to allow replacement of 6 this component with an intermediate tibial bearing comp-7 onent ha~ing a thickness providing proper ligamentous 8 (collateral ligaments) tension.
9 Thus, a number of intermediate tibial bearing 10 components of varying thicknesses may be provided so that 11 the implanting surgeon may shim for proper ligamentous 12 tension or ~or valgus angle without disturbing fixtured 13 components, e.g. tibial platform component 116 and 14 femoral component 111. Further, such s~ructure allows

15 easy replacement of the intermediate tibial bearing

16 component 117 in the event of unusual or unexpected

17 wear or deformation. Similarly, this is true with

18 respect to the patella prosthesis 112 wherein the inter-

19 mediate patella bearing component 113 may be of varying

20 thicknesses and replaceable in the event of unusual or

21 unexpected wear or deformation.

22 It will be ~urther understood that the femoral

23 component lll, the patella fixturing component 114, and

24 the tibial platform component 116 may be made prefer-

25 ably of a surgical metal such as cobalt chromium alloy or

26 titanium or stainless steel but may be made of any

27 relatively rigid material (compared with the grouting

28 agent) that is biocompatible, capable of withstanding

29 the applied loads, and possesses adequate bearing prop-

30 erties against the intermediate bearing inserts, e.g. the

31 intermediate patella bearing component 113 and inter-

32 mediate tibial bearing component 117 may be made of a~y

33 biocompatible material strong enough to withstand loads

34 and adequate in bearing against the material with which

35 it is engaged. Preferably these components are made of

36 a plastic, such as ultra-high molecular weight poly-l ~69201

-37-1 ethylene or copolymer acetal.
2 A prosthetic ankle, an alternate embodiment of the 3 present invention, is shown in FIGURES 48, 49, and 50.
4 Talar platform component 178 is implanted in the talus, and tibial component 179 is implanted in the distal 6 tibia. Intermediate bearing component 1$0 is interposed 7 between talar platform component 1i8 and tibial component 8 179. Talar platform component 178 has a superior bearing 9 surface 181, seen in FIGURE 48, which consists of a segment of a surface of revolution produced by a generat-11 ing curve, as can be seen in FIGURES 48 and 50. The 12 generating curve, in this case, may typically consist 13 of two 0.625 inch radius circular arcs connected by two 1~ 20 tangent lines to a 0.250 inch radius circular arc.
This arrangement is similar in form to the generating 16 curve used for the knee embodiment previously described.
17 The inferior portion of talar platform component 178 18 includes a fixation fin 182, seen in FIGURE 48, with serr-19 ated sides for implantation into the talus. Tibial comp-~20 onent 179 consists of a flat plate 183 with serrated 21 top edge 184 and a fixation fin 185, both of which are 22 used for implantation into the tibia. The plastic inter-23 mediate bearing component 180 has an inferior bearing 24 surface 186 complementary to the superior bearing surface 181 of talar platform component 1i8. Intermediate bear-26 ing component 180 is also provided with a flat superior 27 bearing surface I87 which matches flat inferior bearing 28 surface 188 of tibial component 179.
29 It is important to recognize that the superior bearing surface 181 of talar platform component 178, 31 by virtue of its shape, acts as a track to constrain 32 the motion of intermediate bearing component 180.
33 The ankle prosthesis illustrated in FIGURES 48-50 34 provides flexion-extension motion by rotation of the talar platform component 178 relative to the intermediate 36 bearing component 180. There is sliding engagement of `, _ J

-38-1 the inferior bearing surface 186 of intermediate bearing 2 component 180 with the superior bearing surface 181 of 3 talar platform component 178 as the ankle is flexed or 4 ex-tended, thereby providing fle~ion-extension motion between the tibia and the talus.
6 Sliding engagement oE the flat superior bearing 7 surface 187 of lntermediate bearing component 180 with 8 the flat inferior bearing surface 188 of tibial component 9 179 allows anterior-posterior translation as well as limited medial-lateral translation. The medial-lateral 11 translation is constrained by anatomical features, 12 namely the maleali of the ankle. The anterior-posterior 13 motion is constrained by the action of the ligamen-ts.
14 Thus, the prosthesis of FIGURES 48-50 includes no mechanical constraints against anterior-posterior or 16 medial-lateral translation, a desirable feature because 17 it minimizes force loads on the components of the pros-18 thesis.
19 The prosthetic joint of FIGURES 48-50 also allows axial rotation, that is, rotation about-the axis of the 21 femur, without any restraint other than that pro~ided 22 by natural tissues. In addition, it provides unrestrain-23 ed flexion-extension. The purpose of the track (i.e.
24 the characteristic shape of the generating curve used for the superior bearing suxface i81 of talar platform 26 component 178) i5 to retain the intermediate bearing 27 component so as to prevent its moving outside the medial-28 lateral borders of talar platform component 178. In 29 this way intermediate bearing component 180 is prevented from impinging upon adjacent bone.
31 The prosthetic joint of ~IGU~ES 48-50 differs from 32 one-half of the prior-art Oxford knee by virtue of the 33 track-type of contact between talar platform component 178 34 and intermediate bearing component 180, and also because it affords flexion-extension motion without the possibil-36 ity of eversion-inversion, at least so long as the joint .

2 ~ ~

-39- ..
1 is under compressive force loads (the normal situation~.
2 Axial rotation only is provided by the sliding engagement of the flat superior bearing surface 187 of intermediate 4 bearing component 180 with the flat inferior bearing 5 surface 188 of tibial component 179. The prior-art 6 Oxford knee, on the other hand, incorporates a spherical 7 bearing arrangement allowing three degrees o freedom of 8 rotational motion, rather than two, as provided by the g ankle prosthesi.s according to the present invention.
An implanted prosthetic ankle is shown in 11 FIGURES 51.and 52. Visible in FIGUR~S 51 and 52 are 12 talar platform component 178, intermediate bearing compon- .
13 ent 180, and tibial component 179. For comparison, an 14 anatomical ankle is illustrated in FIGURES 53 and 54.
It will be recognized that the track of the present 16 invention, which serves to constrain motion of a bearing . -17 insert, can take many forms. For example, there is the 18 track with retention, shown in cross-section in FIGURE 34, 19 and there is the track of the ankle prosthesis of .
FIGURE~48.. FIGURE 5S illustrates, in cross-section, 21 still another type of track, suitable for applications 22 where force loads applied to the prosthetic joint are 23 such as to insure retention of bearing insert 189 24 against shoulder 190 of platform component 191.
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1~9201 o SURGICAL IMPLANTATION PROCEDURE FOR ICNl~E EWDOPROSTHESIS
~ _ .
2 The patient is placed in a supine posltion on the 3 operating table. The knee is prepped and draped in a 4 sterile fashion. A thigh tourni~uet previously applied is inflated to 400mm ~{y after elevation of the leg for 6 one minute to allow for venous run-off.
7 The knee is fully extended and a gently curved 8 S-shaped incision is made on the tibial tubercle up 9 towards the medial border of the patella tendon, then curving posteriorly along the medial border of the 11 vastus medialiS.
12 The medial retinàculum, capsule and synovial layer 13 are incised in line with the skin incision. The vastu5 14 medialis muscle belly is elevated free from its attach-ment to the adductor magnus tendon~ The,patella is 16 reflected laterally exposing the entire tibio-femoral 17 joint. If there is excessive tension in the quadriceps 18 mechanism preventing complete lateral displacement of the I9 patella, then shaxp detachment of the medial 1/4 of the 2~ patella tendon from the tibial tubercle-may be necessary.
21 In a similar fashion, fur~her blunt disection of the medial 22 attachment of the vas~tus medialis may be~needed to mobilize 23 the quadriceps mechanism proximally. These maneuvers 24 will allow complete flexion of the knee to 110 degrees with complete anterior exposure of the joint.
26 At this time, excision of hypertrophic synovium 27 and redundant fat pad is performed. Medial and lateral 28 menisectomy will facilitate exposure of the tibial plateau 29 borders and should be performed. Examination of the intercondyler contents will reveal the condition of the 31 cruciates. Redundant synovium should be~excised from this 32 region to prevent possible impinyement or overgrowth 33 onto the tibial component surface 34 With the proximal tibial and distal femur cleared of soft tissue debris, bone guards are slid posteriorly 36 between the collateral ligaments and the posterior 2 ~

1 capsule to protect the posterior neurovascular bun~le 2 duri~g resection of the articular surfaces. A 3/4 3 periosteal elevator may be used to develop the soft tissue planes for the bone yuards, which also serve as ~ 5 knee retractors.
6 The ~nee is flexed to 100 degrees and a drill hole 7 at the intercondyler notch border is made with a 1/4"
8 drill. The drill is taken down to the level of the 9 posterior femoral shaft. Next, a tibial resection jig is placed with a spike located on the posterior aspect 11 f the femoral shaft and a dis-tal limb of the instrument 12 parallel to the tibia. With the collateral ligaments 13 in tension during this flexion phase, a proper resection 14 plane is insured by use of the parallel cutting slots availahle in the ji~. The jig has an automatic 10 16 degree retroversion angle insured when the knee is 17 flexed parallel to the distal limb of the jig. Using 18 an oscillating saw, the tibial preparation is made 19 leaving a ridge of bone to which the cruciate ligaments insert. The resection planes are made at 5, 10, or 15mm, 21 depending upon the amount of bone stock available for 22 perpendicular loading of the tibial component. Once 23 the proper flexion tension has been achieved and the bone 24 resection has ~een made, the tibial alignment jig is removed~from the femoral shaft and the femoral shaper is 26 next replaced into the same channel~ The femoral shaper 27 is situated such that the anterior and posterior cuts are 28 symmetrically parallel to the femoral condyles. Using 29 again an oscillating saw in these cuts, the anterior surface and posterior condyles of the femur are resected.
31 The knee is then brought into full extension after 32 removal of the femoral shaper and an extension femoral 33 alignment jig is placed into the joint. With manual 34 traction on the femur and aligning an adjustable valgus guide into 5 to 10 degrees of physiologic valgus, the 36 horizon-tal cut on the distal femur is made to insure .

! ~ fi 9 2 01 . . I

1 adequate extension tension of the collateral ligaments.
2 Once this cut has been made using the oscillating saw, ,' 3 the extension alignment jig is removed from the knee joint.
4 The knee is again flexed and an oblique osteotomy jig is replaced into the fixturing hole and using a mallet 6 impacted into the distal femoral bone stock. The 7 anterior and posterior oblique cuts are then made in line 8 with the jig surface and a central notch of the oblique 9 osteotomy jig is used to trim away the boney'surface for the anterior femoral flange. The oblique osteotomy jig 11 is removed and the alignment holes made by the jiy are 12 curetted out to accept the fixturing pins of the femoral 13 prosthesis. A trial fit of the femoral component is 14 next made. Excessive bone stock is trimmed to insure proper contact of all surfaces. Next, the tibial prepar-16 ation is completed. A marking template is used to mark 17 out the tibial component spike pos'itions. Following 18 marking with methylene blue, tibial component spike ' 19 channels are fashioned using a curette or gouge. A trial seating of the tibial component is next-made and proper 21 bone resection is performed at this time to insure 22 excellent metal to bone contact of the prosthesis. ~With 23 resections of bo-th bones now finished, the trial reduction 24 of the tibial and femoral components is made as follows:
The metal tibial component is placed on the proximal 26 tibia and the appropriate intermediate bearing components 27 are inserted Lnto place. Next, the emoral component 28 is placed i.n its proper position and the knee joint is 29 tested in both flexion and extension for proper liyament-ous tension. If resection cuts have been made properly,' 31 there should be no gross instabilityO Should mild laxity 32 exist in flexion and extension, then thicker intermediate 33 tibial bearing components may be used to tighten the 34 collateral ligaments. The bearing heights come in 2.5mm increments and may be used to finely adjust the ligamentous 36 tension at this stage. These may also be used to correct 1169201 (`~'7 ~`~

43- !
1 varus-valus alignment. Once the tibial-femoral resect-2 ions have been properly prepared, attention is given to 3 ~he patella replacement, Using a scalpel, the synovial 4 tissue and retinaculum are freed from the periphery of - 5 the patella down to the levei of the patella tendon.
6 A reciprocating saw is then used to remo~e the articular 7 surface. The plane of the cut should parallel the infer-8 ior surface of the patella tendon.
g A patella marking template is now centered over the horizontal and vertical axis of the patella with the long 11 fixturing fin dixected toward the lateral aspect.
12 Methylene blue dye is used to mark the fin channels for 13 the fixturing ~ins of the component. These channels are 14 taken to a depth of 1/~" and undercut for mechanical locking of the cement.
16 The trial patella replacement can now be seated to , 17 assess the fit. Any boney impingement is removed to 18 insure proper seating. The patella is reflected to its 1~ anatomical position to check the alignment in the femoral ~0 track. A range of motion may now be tested with all 21 three components in place. The patella prosthesis should ~2 center in the femoral track and easily glide along the 23 femoral flange without binding. Ilestricting adhesions 24 or boney impingement should be completely corrected at this time.
26 The components are removed after a satisfactory 27 trial fit and the wound is thoroughly irrigated wi-th 28 antibiotic saline solution. The first batch of methyl~
29 methacrylate is mixed and placed on the tibial surface with the knee in the flexed position. The tibial comp-31 onent is gently slid into its fixturlng channels and 32 firmly held in compression until complete polymerization 33 has been obtained. During the setting phase, excess 34 methylmethacrylate may be trimmed using a scalpel and curette from the edges of the tibial component. Next, 36 the bearing components are placed into the tibial component ~1~92~1 ( ') I

1 and the femoral component is cemented in place. Excess 2 methylmethacrylate is removed from around the femoral 3 component to insure that the bearing surface will remain 4 free of this abrasive agent. With a third batch of methylmethacrylate, or else using a portion of that 6 cement used for the femoral component, the cancellous 7 patella bed is covered. The patellar component fixturing 8 fins are firmly pressed into their mating channels and 9 the component is held tightly with a patellar componenk clamp. Excess methylmethacrylate may now be removed 11 from the edges of the patella backplate. Upon complete 12 polymerization of all cement beds, a range of motion is 13 again tested after returning the patella to its anatomical 14 position. Two medium si~ed hemovac drains are now placed in the joint space and brought to exit laterally 16 above the incision line~ A single layer closure of 17 capsule and retinaculum is performed with #2-0 chromic 18 suture with the knee flexed 30 degrees for the first 19 several sutures, then to~0 degrees with the second set of sutures, and finally, to 90 degrees for the remaining 21 closure sutures~ Subcutaneous tissue is closed with #3-0 22~ plain suture, skin in re-approximated in a tension-free 23 fashion with #3-0 nylon su-ture. Hemovac drains are hooked 24 to suction and a Robert-Jones compression dressing is applied. The leg is elevated and the patient is taken to 26 the recovery room where ice packs are placed about the 27 knee.
28 It will be understood by those skilled in the art 29 that many modifications and variations of the present invention may be made without departing from the spirit 31 and the scope thereof.

3~

Claims (6)

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

1. An improved prosthetic knee joint for implantation in the knee of the type including:
(a) tibial platform means having a first superior bear-ing surface at least a portion of which has no substantial curvature in the anterior-posterior direction, the tibial platform means for replacing tibial portions of a knee;
(b) bearing insert means having a first inferior bearing surface for slidably engaging at least the portion of the first superior bearing surface of the tibial platform means which has no substantial curvature in the anterior-posterior direction, the bearing insert means having a second superior bearing surface, the bearing insert means for providing an articulated joint between the tibial platform means and a femoral component means;
(c) femoral component means having a second inferior bearing surface for slidably engaging the second superior bearing surface of the bearing insert means, the femoral component means for replacing femoral portions of the knee;
(d) the bearing insert means experiencing anterior-posterior shift relative to at least the portion of the first superior bearing surface of the tibial platform means which has no substantial curvature in the anterior-posterior direction during flexion and extension of the knee; and wherein the improvement comprises:
(e) the second inferior bearing surface of the femoral component means comprising a plurality of surface segments defined by rotating a common plane generating curve about a plurality of parallel axes of rotation whereby the second inferior bearing sur-face of the femoral component upon slidably engaging the second superior bearing surface of the bearing insert means facilitates control of the anterior-posterior shift of the bearing insert means relative to at least the portion of the first superior bearing sur-face of the tibial platform means which has no substantial curvature in the anterior-posterior direction during flexion and extension of the knee.

2. An improved prosthetic joint as recited in claim 1, wherein:
(a) the second inferior bearing surface of the femoral component means comprises a first surface segment defined by rotating the common plane generating curve about a first axis;
(b) the second inferior bearing surface of the femoral component means also comprises a second surface segment defined by rotating the common plane generating curve about a second axis;
(c) wherein the first surface segment and the second surface segment adjoin to form an intersection; and, (d) wherein the second axis lies in a plane containing the first axis and a point on the intersection producing a smooth continuous bearing surface.

3. An improved prosthetic joint as recited in claim 1, wherein:
(a) a radius of curvature, defined by a distance from a given point on the common plane generating curve to successive axes of the plurality of parallel axes of rotation, is monotonically decreasing for surface segments ranging from anterior to posterior;

(b) whereby full flexion of the prosthetic joint is facilitated.

4. An improved prosthetic joint for implantation in an anatomical joint and of the type including:
(a) platform means having a first bearing surface at least a portion of which has no substantial curvature in a pre-determined direction, the platform means for being secured to a first bone of an anatomical joint;
(b) bearing insert means having a second bearing surface for slidably engaging at least the portion of the first bearing surface of the platform means which has no substantial curvature in the predetermined direction, the bearing insert means having a third bearing surface, the bearing insert means for providing an articulated joint between the platform means and a second bone component means;
(c) second bone component means having a fourth bearing surface for slidably engaging the third bearing surface of the bearing insert means, the second bone component means for being secured to a second bone of the anatomical joint;
(d) the bearing insert means experiencing sliding move-ment relative to at least the portion of the first bearing surface of the platform means which has no substantial curvature in the pre-determined direction during articulation of the joint; and wherein the improvement comprises:
(e) the fourth bearing surface of the second bone com-ponent means comprising a plurality of surface segments defined by rotating a common plane generating curve about a plurality of parallel axes of rotation whereby the fourth bearing surface of the second bone component means upon slidably engaging the third bearing surface of the bearing insert means facilitates control of the movement of the bearing insert means relative to at least the portion of the first bearing surface of the platform means which has no substantial curvature in the predetermined direction during articulation of the joint.

5. An improved prosthetic joint as recited in claim 4, wherein:
(a) the fourth bearing surface of the second bone component means comprises a first surface segment defined by rotating the common plane generating curve about a first axis;
(b) the fourth bearing surface of the second bone com-ponent means also comprises a second surface segment defined by rotating the common plane generating curve about a second axis;
(c) wherein the first surface segment and second surface segment adjoin to form an intersection; and, (d) wherein the second axis lies in a plane containing the first axis and a point on the intersection producing a smooth continuous bearing surface.

6. An improved prosthetic joint as recited in claim 4, wherein:
(a) a radius of curvature, defined as a distance from a given point on the common plane generating curve to successive axes of the plurality of parallel axes of rotation, is monotonically decreasing for surface segments ranging from anterior to posterior;
(b) whereby full flexion of the prosthetic joint is facilitated.