CA2231715A1

CA2231715A1 - Prosthesis for knee replacement

Info

Publication number: CA2231715A1
Application number: CA002231715A
Authority: CA
Inventors: John Nevil Insall; Michael Wadcock; Jonathan Blamey; Peter Stanley Walker
Original assignee: John Nevil Insall; Michael Wadcock; Jonathan Blamey; Zimmer Ltd.; Peter Stanley Walker
Current assignee: Zimmer Ltd
Priority date: 1997-03-13
Filing date: 1998-03-11
Publication date: 1998-09-13
Also published as: AU5833898A; EP0864306A2; GB2323034B; EP0864306A3; AU747825B2; JP4026919B2; GB2323034A; US6068658A; GB9705161D0; JPH10314196A

Abstract

The prosthesis comprises a femoral component (2) having at least one substantially spheroidal condylar bearing surface (10,11), a tibial component (6) including a tibial platform (20), and a plastics meniscal component (7) located between the condylar bearing surface (10,11) and the tibial component (6) and being slidable on the platform (20). The meniscal component (7) is retained on a guide (23) upstanding from the platform (20).
The guide (23) includes a post (24) and a cap (25) which is a snap-fit in a recess (30) in the meniscal component (7). The guide (23) is slidable in a track (32) within the meniscal component (7) so that movement of the meniscal component (7) in the anterior/posterior direction (A-P) is limited by impingement of the meniscal component (7) on the post (24) and/or on a rail member (21) projecting from the surface of the tibial platform (20). To install the prosthesis, a femur (1) is resected to provide a base for receiving the femoral component (2), which is retained in the resected femur by pins (3). The tibia (4) is resected to provide a flat base into which the tibial component (6) is fixed by a stem (5).

Description

I'ROSrrlll~SIS 1~0R KNIIII l~l~l'I,ACI~MEN'I' This invention relates to prostheses for knee replacement, particularly those intended for total knee replacement.
UK l'atcnt Application No. ')414247.X (I'ublication No. 22X()375) describes a prosthesis of the condylar replacement type, comprisinL~ a femoral component, a tibial base plate and a plastic meniscal compollcnt arran~ed to support the femoral component and to slide on the base plate. In the above UK application, the meniscal component is guided on a rail or stud upstanding from the base plate, so as to constrain movement of the meniscal component substantially in an anterior/posterior direction, whilc allowing rotational i'rccdom of at least about ~- 15". The L~uide member is received in a slot in the meniscal component and limits on the displacement of the mcniscal component in the antcrior/posterior direction are provided by upstandinL~ metal stops on the posterior sicle Or the base plate and abutment of the ~,uide on th~ end ol'thc slot in the plastics componellt in the anterior direction.
In one embodiment described in the above application, the stud takes the form of a post having a circular metal cap.
The present invention provides f'urther improvements in the design described in the above application.
'I'here are several considerations in the desi~n of prostheses of this kind. One of these is to provide a system where dislocation of the plastics component is avoided at all lin~cs. Anothcr r equirelllcnt is Il~ hc p]clS~iCS componcnt should not jam on the guide in its movements in the anterior/posterior direction and, thirdly that the plastics component should not be capable of bein~ assembled incorrectly when in$talling the prosthesis in thc patient.
It is ~owards the solution of the above problems that the present invention is primarily ~lirectcd.
According to one aspect of the present invention there is provided a prosthesis f'or knee replacement which compriscs:-(a) the femoral component having at least one condylar bearing surface(preferably spheroidal), (b) a tibial component including a tibial platform, (c) a plastics mcniscal conlpollenl located between the condylar bearing surface and the tibial component, and being mobile on the platforrn, said mcniscal componcnl bcin~ rcklillcd on a ~ui(le upstanding from the platform, said guide including a post and a cap, which cap is a snap-fit in a rcccss in lllc nlClliSC.II COItll~OIlCIlt alld iS mobilc in a track wilhin the meniscal con-ponent, wherein sliding movement of said meniscal component in the anterior/posterior direction (A-P) is limited by impinL!,ement of the meniscal component on said post and/or on a rail mermber projectin~ from thc surlàce of the tibial platf'orm.
Pre~erably, the displacement of the mcniscal component in the anterior directiorl is limited by a rail forming an abutmenl al or close to the anterior edge of the tibial platform. The rail generally shoukl providc an upstanclin~ vertical surface, substantially at ri~ht-allgles to tlle A-l' slidin~ direc~ioll of the meniscal component.

Posterior displacement of the meniscal component is controlled by the guide slidin~ witllin the track in tlle meniscal component. In order to prevent j~mmin~
occurring at any time during relative slidin~ movement or rotational movement of the meniscal componellt on lhe tibial platl'orm, tlle dhllellsions of tl-e cap are selected so that there is a clearance between lhe periphely of'the cap and tlle corresponding si~e of lhc openin~, in llle meniscal track. 'I'he Iell~,tll of' the track is also preferably selected so that the periphery of the cap does not impin~e on the inner wall of the eavity within the meniscal component at any point in ils movement Thus, t'or example, anterior movement of the meniscal component is limited by abutmen~ of ~he anlerior fàce of the rnelliscal con-ponent on the anterior rail, forming a limit stop in the anterior direction. In the posterior direction, movement is arrested by abutment of the meniscal component on the post supporting the cap. Thus, at the limits of movement in the A-P direction, the periphery of the cap eome into contact with the end wall Or ~lle lrack wi~hin llle meniscal component lt will be appreeiated that because the diameter of the post will be smaller than the hole (at least at some points), in the meniscal component whicll gives access to the recess, the cap can limit medialllateral movement of the meniscal component in some positions, by contactin~
the side walls of the recess 'I'he dimensions of the cap and of tl-e recess in the underside of the meniseal component comprising the traek are selecled so lhat the plasties eompollent ean be snapped over the s~ucl l'his is genelally assured by manufaeturing the reeess so as to be of the same plan~r maxinllltn dimellsioll as lhe cap, or up to ().5 mm smaller.

Preferably, the recess is manufactured to be between about 0.2 and 0.3 blm smaller than the cap rhe cap is preferably manufactured to be asymmetrical about at least one plane passing axially throllL~,h the post, and to provide a ~,enerally correspondin~, shaped opening in the plane of the underside of' the meniscal component. ~n this way, the meniscal component is capable o I assembly in the tibial platform in only one orientation. This avoids the dan~,er that the meniscal component may be accidentally assembled the wron~, way round on the tibial .~uide slud.
In a preferred construction, the cap on the tibial stud has a flat face which is parallel wilh the rail on the tibial plalform and the rail is positioned as an anterior stop.
Rotation of the meniscal component on the tibial base plate is permitted within limils detcrmined by the abutmcnt ol'the anlerior race of the meniscal component with the anterior rail on the tibial platform. In order to reduce the chance of accidental dislocation of tlle plastics component Irom the tibial platform, the recess and the tibial guide stud are orientated so that the snap-fit occurs at a point in the orientation of the plastics component on the base plate, which is the least likely point at which dislocation of the meniscal component f'rom the tibial tray may occur.
Dislocation of the femoral component fro-n the meniscal component is most likely where the meniscal component is at or near the posterior limit, and the femoral component is hyper-extended. At this point, there is a maximum moment tending to lill thc Inc,.iscal componcl-t Olr the hase platc. As a prolection a~,ainst hyper-extension of the prosthesis, the meniscal compollellt is formed with a saddle positioned ~cnerally posleriorly ol' the centre point. 'I'his saddle has a substantially vertical face on the anlerior side, w}lich is engagcd with an intercondylar brid~e portion between the two condylar spheroidal surfaces. 'I'his contact between the bridge portion of the femoral component and the sa~lclle provides a positive stop afler the femoral component passes throu~h the zero degree position.
On one side at least ol'tlle saddle projection, a vertical face is provided in order to provide mediaVlateral stability to the fèmoral component. I-{owever, it is preferred that at the medial side of the proslhesis~ tl-c lateral face of the saddle projection slopes into the spheroidal cavity in order to avoid point loading of the plastics member should the femoral component be tilted internally.
I urther f'eatures and advalltages ol' tlle present invention will become apparent from the t'ollowing descl-iplion in conneclion wilh the accompanying drawings, in which:-Figure I A is a perspective view of the femoral component,ure I B is a schematic saL~ittal view of the prosthesis in place, l~igure 2 is a plan view of the meniscal component, l~igure 3 is a section along the linc Ci-G in l;i~ure 2 liigure 4 is a section alon~ tlle line l~-F in FiL~ure 2, Figure S is an underside view ot' the meniscal component showing the recess t'or receivin~ the guide stud, Fiuure 6 is a perspective view showing tl~e tibial component and the guide stud, Figure 7 is a perspective view of the meniscal component, and Figures ~A to 8D show vàrious relative positions of the meniscal component on the tibial base plate.
Refcrrin~ ~o ~ ure 113 Or lhe drawill~s, a femur I is resected to provide a base for receiving the femoral component 2, which is retained in the resected femur by pins 3. '1'he tibia 4 is resected to provide a flal base inlo which the tibial component is fixed by a stem 5, to provide a generally flat tibial base plate 6 with a 5-7~ posterior slope. A meniscal plastics component 7 is interposed between the tibial base plate 6 and Ihe fèmoral component 2, ancl the condylar ~lrticulating surfàces 1() and 11 of the f'cmoral colllponellt 2 arc pre1crably sr)llcroidally shaped and preferably are closing conformin~. l'he femoral component 2 is shaped with notches 12 and 13, designed to be engaged by meniscal anterior upsweep portions 14 to provide a whole or partial hyper-extcnsive slop.
'I'he tibial component is shown in perspective view in Figure 6, from which it can be seen that it comprises a generally f]at pkltform 2() supported on the stem 5.
Upstanding f'rom the anterior face of'thc tibial platfolm is a rail 21 which, as will be explained later, provides an anterior positive stop f'or the anterior displacement of the m~niscal component.
'I'he poslcrior siclc of Ihc libi.ll plallor1ll is cu~ away at 22 to allow for retention ot'lhe posterior cruciate ligam~nt, and an upstanding guide 23 is provided for retaining lhe meniscal component on the tibial platform and l'or guiding the plastics component f'or limited slidi--g movement on the plalform. The L~uide 23 comprises a post 24 and a cap 25 'I'he cap 25 is fixed to tlle lop of' the post and has a flat fa~e 26 in the posterior direction, which is substanlially parallel to the face 27 of the anterior stop Although the cap is shown to be D-shaped, other designs are possible The cap should preferably be asymmetrical about at least one plane passing axially through the guide post, but preferably extends in at least two clirections in the plane parallel to the tibial platf'orm, so that it will provide resistance to the meniscal component lifting up, when the meniseal component is in or elose to its maximum posterior position and is, therefore, subjected to the greatest tipping movement Various shapes sueh as trian~ular, quadrilateral or polyL~onal may be employed, althou~,h we currently prefer to employ one flat side while the rernainder of the cap has a curved perimeter 'I'he recess wilhin the meniscal component is shown in l iL~ure 5, and it will be seen lhat the recess 3() is of a re~ular lorlll and is desi~ned so that tlle eap ean be snapped into place at only one relative orienlation belween tlle meniscal eomponent and the tibial base plate 'rhis is achieved by selectin~ a maximum dimension 31 across the recess whieh corresponds precisely with or is within 0 5 mm of the correspondinL~ maximum dimension across the eap of the ~uide cap This ensures that it is not possible to assemble the meniscal component the wron~ way round As clearly seen in l~igure 5, the recess 3() ~ives access to a slot 32 which is closed at both ends but when ~lle cap 2S is received witllin llle slol, the cap extends ~hereill allcl preven~s tl-e cap b~in~ lillc(l ol~'. In ordel- to facili~ale assembly, a ~uide noteh 33 may be formed in the anterior face of the plastics component. When this notcll is orientated so that il is pressed hard a~,ainst one ed~,e of the rail rnember 21, the sur~eon knows that thc cap is ali~ned f'ol- en~ry into the recess.
Movement of' the plastics component on the tibial base plate is illustrated in Figures XA to 8D. As can be seen, this permits internal rotation up to about 17~ and external rotation up to about l ()~. Rotation is limited by impingement of the cut-away portion 34 of the anterior face of' the plastics component, with the anterior rail 21 and by the abutment of the post 24 a~ainst tl-e f'orward or rearward end of the recess 35 or 3~. The notch 33 allows f'urtller rotation in the internal direction. It is to be noted that the overall climension of the cap and the internal dimension of the slot 32 is selected so that there is some freedom of movement of the cap within the slot 32. In general, there is always a L~ap of at least about 0.2 rnm. This reduces the chances of jamming of the meniscal component on thc guide.
The laxity in this respect in various positions of the meniscal component on the libial basc plate is illuslratcd in 1~ ures XA to X13.
The meniscal component itself is shown in perspective view in FiL~ure 7. It will be noted that the posterior surlàce has a slopin~ face 40 which permits the posterior cruciate ligament to pass alonL,side the posterior of the prosthesis without abrasion. Moulded ~,enerally posteriorly bctween the dished bearin~ surfaces 41 and 42 is a saddle 43. The saddle has an anteriorly facing ~enerally vertical wall 44, which is intended to engage in the fork 45 (see l~i~ure I A) of the femoral component, and to limit the rotation and translation of the fèmoral component on the meniscal component in hyper-extension.

At the same time, the saddl~ 44 incorporates a lateral, substantially vertical wall 46 alongside the inner face 47 of the correspondin~g side of the femoral component during flexion. This provides for enhanced stability of the femoral component on the meniscal component in a medial/lateral direction. On the medial side, the saddle has a sloped face 4X which merges into the spheroidal-shaped depressioll 42. I l~is ensllles tlli~l sl~oukl tl~e Ietnoral component tilt and pivot inwardly on the meniscal component, the articulating s~rfaces 42, 10 and 11 would not be subjecled to point loadill~. I'llis kind ol' movement occasionally occurs if the collateral ligaments are not balanced and, in such cases, there is a tendency for the femur to be tipped inwardly.
In order to render the meniscal component radioopaque, the meniscal component can be provided with sealcd holc~ containin~ a heavy metal such as tantalum in a part of the plastic whicll will not contact the metal femur. This may, Ior example, be points S() and 51 as shown in l~i~ure 2"and this will enable the surgeon to determine correct location and movement of the component after installation of the prosthesis.

Claims

1. A prosthesis for knee replacement which comprises:-(a) a femoral component having at least one substantially spheroidal condylar bearing surface, (b) a tibial component including a tibial platform, (c) a plastics meniscal component located between the condylar bearing surface and the tibial component and being slidable on the platform, said meniscal component being retained on a guide upstanding from the platform, said guide including a post and a cap which cap is a snap-fit in a recess in the meniscal component and is slidable in a track within the meniscal component, wherein sliding movement of said meniscal component in the anterior/posterior direction (A-P) is limited by impingement of the meniscal component on said post and/or on a rail member projecting from the surface of the tibial platform.

2. A prosthesis according to claim 1 wherein the cap is asymmetrical about at least one plane passing axially through the post and said recess has a correspondingly shaped opening in the plane of the underside of the meniscal component, whereby the meniscal component is capable of assembly on the tibial platform in only one orientation.

3. A prosthesis according to claim 1 or claim 2 wherein the cap when viewed in plan view, has a flat side on one face which is parallel with a rail projecting from the tibial platform and positioned as an anterior stop.

4. A prosthesis according to claim 3 wherein the cap is substantially 'D'- shaped in plan, the flat side being substantially parallel with said rail.

5. A prosthesis for knee replacement which comprises:
(a) a femoral component having at least one substantially speroidal condylar surface, (b) a tibial component including a tibial platform, (c) a plastics meniscal component located between the condylar bearing surface and the tibial component and being moveable on the platform, said meniscal component being retained on a guide upstanding from the platform, said guide including a post and a cap, the cap being received in a recess in the meniscal component and slidable in a track therein, the cap being asymmetrical about at least one plane passing axially through the post and the recess having an opening which is so shaped that the meniscal component can be assembled onto the guide in a limited number of relative orientations of the meniscal component and the platform.

6. A prosthesis as claimed in claim 5 wherein the meniscal component can be assembled onto the guide in only one relative orientation of the meniscal component and the platform.

7. A prosthesis as claimed in claim 5 or 6 wherein the cap, when viewed in plan view, has one flat side which is parallel with an abutment projecting from the platform and positioned as an anterior stop for the meniscal component.

8. A prosthesis as claimed in any one of the preceding claims wherein the meniscal component includes a saddle, one lateral face of which is substantially upright, said lateral face being positioned to interact with a surface on the femoral component to enhance medial/lateral stability of the prosthesis.