CA1134104A

CA1134104A - Metacarpophalangeal joint prosthesis

Info

Publication number: CA1134104A
Application number: CA347,066A
Authority: CA
Inventors: Peter S. Walker; David Greene
Original assignee: Howmedica Inc
Current assignee: MTG Divestitures LLC
Priority date: 1979-03-05
Filing date: 1980-03-05
Publication date: 1982-10-26
Also published as: IE49185B1; IE800431L; GB2045085A; GB2045085B; DE3008292A1; FR2450600A1; FR2450600B1; JPS5841857B2; JPS55120863A

Abstract

ABSTRACT
A novel joint prosthesis, particularly suitable as a metacarpophalan-geal joint prosthesis, is disclosed comprising first and second intramedullary plugs adapted to be secured to the walls of the intramedullary canals of the two bones forming the joint, and first and second components each comprising a bear-ing portion and a stem slidably received in a longitudinal bore provided in each of said intramedullary plugs. Transmission of stresses to the bone-prosthesis fixation interfaces is substantially reduced as a result of the freedom of motion of the stems within these longitudinal bores. In a preferred embodiment, axial rotation of one, but only one, of said stems with respect to the intramedullary plug receiving it is prevented. Use of preferred configurations for the bearing portions of the two components provides an accurate reproduction of the movement and degrees of freedom of the natural metacarpophalangeal joint.

Description

~i34104 JOINT PROSTHF.SIS
This invention relates to joint prostheses, in particular finger joint prostheses and most particularly prostheses for the metacarpophalangeal joint.
A variety of prostheses for use in the surgical repair of diseased or damaged finger joints are known. One type of prior art device (see, e.g., United States Patents 3,462,765 and 3,593,342) consists of a unitary flexible rubber body hav-ing stems for fixation within the intramedullary canals of the two bones forming the joint. Although widely used, this type of prosthesis has certain disadvant-ages. For example, rubber with adequate flexibility has less than the desired level of mechanical strength, and some mechanical degradation and even breakage of this type of prosthesis under the conditions of actual use have been observed clinically. Also, these prostheses can have a highly unnatural feel in the body because they do not always flex at the desired point or adequately control radio-ulnar rotations, and because they provide an insufficient moment arm to the ex-tensor tendon to overcome extensor lag.
A second type of prior art finger joint prosthesis (see, e.g., United States Patents 3,506,982; 4,011,603 and 4,059,854) comprises two components, with the metal protuberant head portion of one component snapping into 1 - ~b X r4~

:

-`' 1~34~n4 and retained by ~he plastic housing of the other componelt.
This type ~f prosthesis generally provides a more accurate reproduction oi the movement of the natural finyer joints than the unitary rubber i~plants, but n~vertheless is basically a linked clevice and thus can feel awkward and overly corlstrained in the body of a patient. Ihe ~orces inherent in the mechanical constraint of this type of prosthesis are transmitted to the plastic housing, which can fail by creep or fracture.
~n always potential problem wi~h the use of joint prostheses generally is the 3ooseni}lg of the fixatiorl ~etween an intra~edullary stem of the prosthesis and the wall of the intrameduilary cana~ of the host hone under the influence of the stresses e~perienced in us~ at the bone-prosthesis fixation nterface. rl'his problem may develop whether the fixatio~. is by cement~ bone tissue ingrow-h, or alternative method. To~sional, shear, tensile and compressive forces upon the prosthesis are generally transmitted to the bone-prosthesis fixation intexfaces, which may then be weakene~ by the constantly r~curriny action of these forces.
Therefore, it would be hi~hly beneficial to the ar, to provide a class o~ joint prosthesis wherein the transmission of forces acting upon the prostnesis to the bone-prosthesis fixa'ion interfaces is minimized, thus assuriny the integrity of the fixation obtained after in~-plantation and, e.g. tissue ingrowth, and also reducing i~e risk of subsequent loosening of the fixation dur;ll~ us~ of the prosthesis. Also, in the case o~ the metacarpophalangeal joint, it ~70uld be highly beneficial to provide a ne-~Y
prosthesis reproducillg the motiorl and desrees of freedom of the natural joint, without, o~ course, sacrificing the mechanical stability of the devise.

.. . . .
.' ` -; -3-A no~el prosthesi.s for a joint of a first hone with a second ~one has now been invented~ com~rlsing a first component~ a second compGnent..i.n articulatory bearing relat.ionship with said first component, said first com-ponent being operatively connected to said first bone,said second ccmponent being operatively connected to sai~. second bone, and the connect.icn of at least one of said components to its bone being such as to provide for xela~ive longitudinal movement .between said componer:t 10 and its bone. Althouyh of particular interest as a finger joint prvskhesis, especially a metacarpophalangeal joint prosthesis, tne novel prcsthesis may also be a~a~';.ed for use in other joint.s, ~ the elbow or the knee.
Of par-ticular interest is a novel pro~thesis fo~ a joint of a ~i.rst bone with a second bone comprising a first intrâmedul]ary plug adapted to ~e secured to th{~ wall of the intramedullary canal of said first bone and provided wi~h a longitlldinal bore therein, a second intramedullaxy plug adapted to be secured to the wali of the intramedullary 20 canal o said second bone and provided with a longitud-inal borè therein, â first component comprising a bearing portion and a stem extending therefrom and a sPcond com-ponent comprising a bearing portion and a stem extendin~
therefrom, with said stems of said first and second . components ~eing longitudinally slidably received in sai~
bores in said first and second intramedullary plugs, respectively, ând with said bearing portions of said first and second components ~eing in mutual articulâtory engagement.
Because the stel~s of th~ first and second components are fIee to longitudinally slide within the bores receiving them, the transmission of tensile forces exerted upon the prostilesis to the plug-bone f.ixation interfaces is virtually eliminated.

; . ::~ , 113~104 Also, the transmission of compressive forces as shear UpO!I
the plug-bone fi~a~ion interfaces can be greatly reduced.
The intra~edullary ~lugs are preferably made of plastic with the slidable ste~ns of the two components preferably made Or metal or metal ailoy, thereby reducing wear and friction between the plugs and stems. The beari~g portion of one of said components (tha distal component ir. the case of a metacarpophalarlgeal joint prostnesis) prefer-ably preser.ts a convex metal or metal alloy bearing surface to the other component, and the bearing por~ion ~f said other component preferably includes a remo~able plastic bearing insert which pr~s~nts a concave bearing surface to, and contacts in use, said convex bearing surface. Vse of such a removAble plastic bearing insert will, of course~ greatly reduce wear and friction between the bearing portions of the two components.
In a preferred metacarpophalangeal joint prosthesis embodiment, the prosthesis is provided with means to prevent ~ither rotation of the proximal component relative to the proxi~al intramedullary plug about the longitudinal axis of the metacarpus, or rotation of the distal cornponent relative to the distal intramedullary plug about the longi~
tudinal axis of the proximal phalanx, but not both. As a result of this f~ature, which can of course he incorporat~d into prostheses for other joints as well, the transmission o~ most torsional forces exerted upon the prosthesis to the plug-bone fi~ation interfaces as shear is also virtually eliminated or substantially reduced, without detracting from the inheren ~echanical stability of the prosthesis.
Another aspect of a novel rnetacarpophalangeal joint prosthesis of the invention concerns the design and interaction of the bearing portions of the two components in the prosthesis, which afford a movement and degrees of freedom similar to those of the natural metacarpophalanyeal joint.

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" 1.~3~104 The invention also relates to a finger joint prosthesis comprising a proximal intramedullary plug adapted to be secured to the wall of the intra-medullary canal of the proximal bone and provided with a longitudinal bore therein; a distal intramedullary plug adapted to be secured to the wall of the intramedullary canal of the distal bone and provided with a longitudinal bore therein; a proximal component comprising a bearing portion and a stem extending therefrom; and a distal component comprising a bearing portion and a stem extending therefrom, with said stems of said proximal and distal components being longitudinally slidably received in said bores in said proximal and distal intramedullary plugs, respectively so as to provide for relative movement between each of said components and the corresponding bone longitudinally of said bone, and with said bearing portions of said proximal and distal compon-ents being in mutual articulatory engagement.

- 4a ,.' ., ':- ~

.. , i 113~34 . 5--~ 'he i.n~ention w~ e d~scribed in detail w;th r~erence to C! preferred ernbodime1lt thereof, which is a metacarpophala!.lgeal joint prosthesis. Reference to this embodiment does not lim.i.t the scope of the ~vention.
which is li~lited only by the sco~e of the c~aims~
In the drawi.n~s:
FIG. 1 is an e~ploded perspective view OL a metacarpQ-phalangeal jcint prosthesis of the invention, FIGc 1~ is an enlarged view of a portion of the phalan~eal intramedullarv pl~lg of the prosthesis of FIG, '~
showing the bone t.ssue ingrowth surfzce in greater ~etail; FIG~ 2 is a top plan view of the components of the prosthesis of FIG. 1 in mutual engagement at extension;
FICT. 3 is a, side elevation of the components of ~he prosthesis of FIG. 1 .in mutual engayement at e.~tension~ witn the components at flexion ~phantom outline) and the.
direction of articulation of the prosthesis shown;
FIG. 4 is a bottom plan view of the components of the pxostheses of FIG. I in mutual engagement at extensio~ 7 with the axial rotation of the stem of the phalangeal com~
ponent within the longitudinal bore in the phalangeal intra~Tedullary plug shown, FlG. S is a view similar to FIG. 2, illustratin~
radio-ulnar rocking of the phalangeal component wi~.h respect to the metacarpal component at extension;
FIG. 6 is a bottom view of the components of the prosthesis of FIG. 1 in mutual enqagement at extension with the phalangeal co~ponent substantially rotated ~ith resp~ct to the metacarpal component via radio-ulnar rockin~, in which the vo~ar half of the socket porti.on of th~ metacarpal component is cut-away and the moment arm availa~le to the collateral ligament for restoration of the prosthesis shGwn;

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.,, ,. ' . ~

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~i3~04 . --6 FIG. 7 is a side elevation of the prosth.esis of FIG~ l secured withLn the intramedullary canals of the ~etacarpus and proximal phalanx at. nearly full flexion, with parts o.
the intramedullary plugs cut-away, FIG. 8 is a perspective view o~ a ~eft humall hand with the thumb press ny against the ~ip of the index fi.nger in a type of pinching action against whlch the prosthesis of FIG. 1 is resistant; and FIG. 9 is a frontal v.iew of a modified version of the plastic bea~ing insert of the. metacarpal component of the prosthesis of YIG~ 1.
A prefe~red embodiment of the invention, i.e a metacarpophalangeal joint prosthesis 1, is shown in FIGS. 1 to 7. Prosthesis 1 consists of a plastic metacarpa]. intramedullary plus 3, a metaca.rpal c~mponent S cc>nsisting of a metal housing memher 6 and a plastic bearing ir,sert 7, a metal phalangeal component 9 and a plastic ph~.langeal intramedullary plug 11~ The metacarpus and proximal phalanx o~ the surgically repaired metacarpophalangeal joint are shown in FIG. 7. The various ligaments and tendons participating in the natural joint, e.~. the collateral ligaments, are r~ot sho~n in FIG. 7 but will generally b~ viable and remain in place.
Intramedullary plugs 3 and 11 are preferably made of ultra high molacular weight polyethylene. The~7 can be provided in various different sizes so that their dimensions can be matched to the size of the bones of the recipient. Metacarpal plug 3 is generally lar~er than phalanyeal plug 11. Both plugs are tapered Wit-Q
decreasing cross-sectional area towards the e.~tremities of the prc)sthesis. Prior to insertion of an intramedullary plug, the intramedullary canal is reamed tv a diameter slightly less ~by about Q.02 inches) than the diameter .,, ,., ... . . . . . .
"

, 1~3~

of the plu~j? so thclt the plug can be press--fltted agair,s~
the'wall of the callal. The exteriors of plu~s 3 a:~d ll are provide~ ~see FIG. lA) with '~terally ex'ending ri.~ges, e.~. 13, defining between them la'erally e~tending gro~lves, e~ 15. The ~alls of the rid~?es facins away from the contact area of the tWc? ~omponents, e ~. wall l7f ~re sloped so as tG minimize resistance to insertion of the plugs within the intramedullary canals. Withdrawal of the insert~d pl~gs, however, is rendered very difficult ky the biting action of the ~lat tips, e.g. l9, of the rid~es a~inst the walls of the canals. Permanenr fi~.ation is ohtained by bone tissue ingrowth within the space of the grooves, e g D 15. Permanent fixation may, of course, be achieved by other kno~ln me~hods, such as b~ the use of an orthopedic cement.
Each of intramedullary plugs 3 and ll is pro~ided wLth a longitudinal bore 21 and 23 aligned along the cer.tral ~xis oE ? he plug and open t-o both ends of the plu~. Each of p'ugs 3 and ll terminates at its end facins to~ard the contact area of the two components with a circular flange 25 and ~7, the back faces 2~ and 31 of which seat respectively against the resected extremi~ies of the metacarpus and the proximal phalanx. As can be seen from E'IG. 7, only minimal resection of natural bone i5 required for the surgical insertion of intra-medullary plu~s 3 and ll.
Phalangeal component 9 may be a unitary cast surgical implant alloy ar~icle. A highly suitable alloy is Vitallium (Howmedica Inc.; New York, New York).
Componer!t 9 consists of a stem 33, a f ange 3~ at t~e proximal end of stem 33, a neck portion 37 extending from the proximal face of flange 35 and a protuberant head portion 3~, also referred to herein as ~he bearing portion, carried by neck portion 37. Stem 33 is slid~bly li3~10~

and r~~t.ably received within lon~itu~inal bore 23 i~-the phalangeal intramedullary pluy 11. A limit on the extent of insextion is reached when the distal face 41 of ~lanye 35 seats ayainst the front face 43 of f~allge ~7 on plug 11.
Flan~e 35 is shaped as a ci.rcle wil:h t~lo egual and opposed lateral se~ments removed (.see FIGo 1 ) sO as not to preve substantial radio-uinar roc~in~ at extension.
Metacal~pal component 5 consists of metal housinq member 5 and plastic bearing insert 7. Housing mem..ber 0 6 May ~e a unitary cast sur~ical implant alloy article, preferably of VitalliumR (Howmedica Inc., New York, New Yor~).
Housinq mem~er 6 consists of female head portion 45 and stem 47 extending therefrom i~ the proximal direction.
Head porti.on 45 and plastic ~earing .insert 7 taken togethex comprise the bearing portion ! also referred to herein as the soc~et portion, of the metacarpal component 5. Re-movable ~lastic bearing insert 7 is retained within head portion 45 by frictional fit~ In the unusual event that sub s'antiai wear occurs in an irlsert 7 during use of the prosthesis, the insert may be re~oved surgicall~ and replaced with a new one, ~ith minimal disruption of the remainder of the surgically repaired metacarpophalangeal joint. Insert 7 is preferably made of ultra hi~h molecular weight polyethylene.
Stem 47 is slidably recei.ved within longitudinal bore 21 in the metacarpal intramedullary plug 3. A
limit on the extent of insertion is reached when the rear face 49 of head portion 45 seats against the front face 51 of flan~e 25 on plu~ 3. One or more longitudinal fins (a pair of such fir,s, e.~ 53, a~e shown in the drawin~s) projectinS from the exterior of stem 47 are received within correspondin~ l~nqitudinal slots, e_~

, li3'~1 04 ~9-55, in the wall of bore 21, thereby preventing rotatic,n of component 5 relative to in~ramedullary plug 3 ahout the longitudinal axis of the metacarpus.
Since axial rotation of _ly one of the 'cwo componerlts of prosth~sis 1 with respect to the plug receiving it is allowed, th. situation can never arise wherein both co~lponents 5 and 9 rotate with respect to the two bones fo~ming the joint while r~maining fixed relative to eac.-other.
Means to pre~ent rotation of metacarpal compo~lent 5 with respect to plug 3 about the long-,tudillal ~xis of the metacarpus other than those shown in the drawings may be employed. Thus, for example, front face 51 of ~lange 25 ~ay have a concave cylindrical surface which interacts with a totally or partially conforming con~ex cylindrical surface o' rear face 49 of head portion 45 so as to stahilize plug 3 and component 5 against relative rotation. Use of this alternate means to prevent rotaticn el:iminates the need for the fins, e.g. 53~ and slots, e.g.
55, shown in the drawings~
As a result of the configurations of plugs, stems and flanges illustrated in the drawings, the transmission of forces exerted upon the prosthesis to the bone-plug ` fixation interfaces is greatly reduced. The transmission of tensile stresses to the fixation interfaces is eliminated because stems 47 and 33 are free to slide longitudinally within bores 21 and 23 in plu~s 3 and 11. Compres3ive stress~s are transmitted by head portion 45 and flange 35 to plug flanges 25 and 27, and then disiributed primarily 3~ as compression upon the resected cortical rims against which the plug flanges are seated and along the substantial surface area of the tapered walls of he intrameduliary canals. Thus, trans~ission of compressive stxesses as shear upon t~e fixation interfaceæ is effecti~ely minil.li2ed.
The transmission of torsional stresses of a kind creating 113~04 -10~

a tors.ional motnent about the longitudinal axis of t~le proximal phalanx is also ~irtually eliminated because of the freedom of stem 33 to ro~ate within bore 23 in plug 1'. The proportion of torsi.onal stresses of a kind cre~tin~
a torsional ~oment about the longtudinal axis of the metacarpus ~hat is transmitted as shear upon the metacarpal bone-plug fi~ation interface is given approximately by the sine of the angle of flexion (or hyper-extension). Shear forces acting upon components 5 and 9 of prosthesis 1 do exert small bending moments upon plugs 3 and 11 which are d.istributed along the lengths of said plugs. '~he stresses along the fixation interfaces resulting from these small bending moments are more evenly distributed because of th~
softness of the plastic plugs and have little effect on lS fixation integrity~ Thus, in conclusion, the risk of motion of the pluys with respect to the walls of the intrameduliary canals du.ring the tissue ingrowth period and the risk of loosening of the fixations during the subsequent use of th~ prosthesis are both greatly reduced.
~ead portion 39 of phala~geal component 9 is cylindrical with rounded ends 57 and 5~. Preferably, ends 57 and 59 are hemispheroidal as shown in the drawin.~s. The ionsitudinal axis of cylindrical head portion 39 is perpendicular to the plane of articulation of the prosthesis (see FIG. 3). The inner walls of plastic bearinq insert 7 define a socket cavity 61 by which cylindrical head portion 39 is slidably received when components 5 and 9 are mutually engaged. The concave proximal i.nner wall 63 of plastic bearing insert 7 serves as the bearing surfacQ of metacarpal component 5. Surface 63 is half cylindrical in shape with rounded ends and conforms closely in shape with cylindrical head portion 39, so that bear.ing surface 63 and head portion 3~ fit together snugly when the prosthesis 1 is under ., i ... .

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~.13~

compression. Socket cavity 61 exterlds dis-taily from ~e~ring surface 6~ to aperture 65 in the distal end ol :Lnsert 7 Aperture 6~ has approximat21~ the samè dimensions as a cross-section taken throuyh t~e longitudinal axis of h~ad portion 39. Therefore, head por-tion 39 can freely slide through aperture 65 to enter soc~et cavity 61. A slo-t 67 in the volar wall of insert 7 extends distal'y from bel~Lr.g surfac2 6~ untii it communicates with aperture 65. hs prosthesis 1 is fle~red (see ~IG. 3), neck portion 37 of phalangeal component 9 passes throuyh volar slot 67 ir insert 7 and a registered slot ~ in head portion 43.
Sufficient clearance is provided so that neck portion 37 does not contact the side walls defining either of slots 67 and 69. ~rticulation is limited by abutment of neck portion 37 of phaiangeal component 9 against surface 71 of plastic bearir~g insert 7 and surface 73 of metal housing i~ember 6. A range of articulation of from about 20 of hyper-e~ension to about 90 of flexion is permit~ed~
Prosthesis 1 accurately reproduces the ranges of movement and degrees of freedom of the natural metacarpophalan~eal joint, and ~ilUS will havè a natural and realistic feel when implanted within the body of a patient. Since head portion 39 can slide within socXet cavity 61, components 5 and 9 of prosthesis 1 are con-siderably less constrained and possess greater freedomfor relative play than the components of the linked prostheses of the prior art. The freedom of movement of head portion 39 within socket cavity 61 provides an additional means of preventing the transmission OL tensile stresses exerted upon the prosthesis to the bone-plug fixation interfaces. The mechahical stability of prosthesis 1 is assisted by the self-stabilizing actions of the various natural ligaments and tendons, e.g. the collateral ligaments and the extensor and fle~or tendonsO

The extensor tendon is retained during flexion in track 75 in the dorsal face of head portion 45. Track 75 has a disto-volar curve. As is shown in Figure 3, the axis of articulation of prosthesis 1, i.e. the longitudinal axis of head portion 39, is offset volarly from the line defined by the longitudinal axes of stems 33 and 47 at extension, in order to increase the moment arm available to the weaker extensor tendon while still providing an adequate moment arm to the stronger flexor tendon.
The natural metacarpophalangeal joint possesses substantial freedom at extension for relative lateral rocking of the proximal phalanx with respect to the metacarpus, but is highly resistant at flexion to the type of relative rock-ing movement wherein the proximal phalanx would be rotated about the longitudinal axis of the metacarpus. Thus, for example, the natural flexed metacarpophalan-geal joint is highly stable against the type of pinching action illustrated in Figure 8. Prosthesis 1 reproduces both of these qualities of the natural meta-carpophalangeal joint. At extension (see Figures 5 and 6), component 9 is me-chanically free to laterally rock to a substantial extent (about 25 to either side of the center line) with respect to component 5. A mechanical restraint upon rocking at extension is finally reached by the impingement of component 9 against insert 7. However, the natural collateral ligaments will stabilize the joint prosthesis by themselves in normal use. Note that a 25 lateral rocking displacement at extension of the proximal phalanx with respect to the metacarpus requires an extremely large distention (about 4 to 5 mm) of the collateral lig-ament opposing such displacement. Also, the configurations of head portion 39 and bearing surface 63 of the two components impart a maximum possible moment arm (given anatomical dimensions) to the collateral .:
', ~ '' '.

113~iO4 ligament or intrinsic muscles acting to restore the prosthesis. The moment arm available for sucl- restoration is shown a~proximately as length L drawn from the center o~ xotation of the prosthesi in FIG. 6. The forces exerted upon component 9 by the collateral ligaments in bringing about radio-ulnar rocking at extension and restorinS the prosthesis ~rom such rocking are transmitt:ed as compressive stresses to component 5 and then to flanye 25, without exerting shear forces on component 5 and thus without placing bending moments upon ste~s 33 and 47.
At flexion, forces upon the prosthesis which would tend to cause rotation of component 9 about the longitudinal axis of the metacarpus (see, _.g., FIG. R) are resisted by tlle contact of head portion 39 against the dorsal ~nd volar walls 77 and 79 of plastic bearing insert 7 and the cooperation of fins, e.g. 53, with slots~
- e.y. 55. Addit;onally, as already indicated, torsional stresses upon the prosthesis of a kind tending to rotate cylindrical head portion 39 about the longitudillal a~is of the proximal phalanx are in fact ineffective to cause sucll rotation because of the freedom of stem 33 to rotate within longi-tudinal bore 23 in intramedullary plug 11.
In a modification o~ metacarpal component 5 shown in FIG. 9 and within the scope of the present invention, lips 101 are provided at the distal end of plastic bearing insert 107 to reduce the size of aperture 165. The purpose of lips 101 is to prevent dislocations resulting from ex~treme r~lative dis~lacement of the components under tension or extreme relative radio-ulnar rocking of the co,nponents ~greater than about 30 to either side of center).
Head portion 39 i8 engaged within socket cavity 161 by pressure fit. Relative radio-ulnar rocking at extension and relative componellt separation longitudlnally of the ' ;

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1`13~ 4 metacaJpal c-mponent are meshani_ally un~estra~.ned in normal use, i~e., controlled and limited under non-e~treme conditions only by natural anatomical means, e.~. ligaments and tendons. Since lips 101 would only beicontacted in use ~y head portion 39 in an unusual extre~e si~u~tion, head portion 39 is not retained or embraced firmly by said lips within the socket ca~ity in the same manner ax are the head portions of the embodiments described in U.SO Patents 3,506,982; 4,011,603 and 4,059,854.
It îs to be understood that the aspec-t of the invention descri'oed herein rela~ing to the detailed configuration~ of metacarpophalan~eal joint prosthesis bearing portions may be practicea without employin~ the other aspect of the invention involvin~ motion of component stems within int~a-medullary p].ugs. Thust for example~ the metacarpophalangeal joint prosthe.sis component stems may be secured directly to the walls of the intramedullary canals by tissue in~row~h, cement, or other fixation means. L~kewise, said other aspect of the invention relating tO the motion of component stems within intramedullary pluys may be practiced while employing bea_ing port.ion designs, such as those described in U.S. Patents 3,506,982; 4,011,603 and 4rO59~85~ distinct from those described herein. As was stated earl.ier, said other aspect of the invention is not restricted to ~5 metacarpophalangeal joint prostheses, but has general -applicabilit~ to other types of joint prostheses as well.
It is preferred, however, to employ both aspects of the in-~ention together to obtain the optimum metacarpophalan~eal joint prosthesis.

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Claims

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

1. A finger joint prosthesis comprising a proximal intramedullary plug adapted to be secured to the wall of the intramedullary canal of the proximal bone and provided with a longitudinal bore therein; a distal intramedullary plug adapted to be secured to the wall of the intramedullary canal of the distal bone and provided with a longitudinal bore therein; a proximal component comprising a bearing portion and a stem extending therefrom; and a distal component comprising a bearing portion and a stem extending therefrom, with said stems of said proximal and distal components being longitudinally slidably received in said bores in said proximal and distal intramedullary plugs, respectively so as to provide for relative movement between each of said components and the corresponding bone longitudinally of said bone, and with said bearing portions of said proximal and distal components being in mutual articulatory engagement.

2. A prosthesis of claim 1 wherein said joint is the metacarpophalangeal joint, said proximal bone is the metacarpus and said distal bone is the prox-imal phalanx.

3. A prosthesis of claim 2 wherein said prosthesis contains means to prevent rotation of at least one of said components relative to the intramedu-llary plug receiving it about the longitudinal axis of the bone to which said plug is secured.

4. A prosthesis of claim 3 wherein rotation of only one of said compon-ents relative to the intramedullary plug receiving it about the longitudinal axis of the bone to which said plug is secured is prevented.

5. A prosthesis of claim 4 wherein said rotation preventing means comprises one or more longitudinal fins projecting from the exterior of the stem of the component whose rotation is prevented and corresponding longitudinal slots to receive said fins in the wall of the longitudinal bore in the intramedullary plug with which said component is slidingly engaged.

6. A prosthesis of claim 4 or 5 wherein rotation of said proximal com-ponent relative to said proximal intramedullary plug about the longitudinal axis of the metacarpus is prevented.

7. A prosthesis of claim 2 wherein said proximal and distal intramedu-llary plugs both terminate, at their distal and proximal ends, respectively, with flanges adapted to seat against the cortical surfaces of said proximal and distal bones, respectively, and to distribute upon said surfaces a sub-stantial portion of compressive forces exerted in use upon said prosthesis.

8. A prosthesis of claim 7 wherein each of said intramedullary plugs is tapered so as to have decreasing cross-sectional area with increasing distance from the flange-terminated end of said plug.

9. A prosthesis of claim 1 wherein said intramedullary plugs are made of plastic and said stems of said proximal and distal components are made of metal or metal alloy.

10. A prosthesis of claim 9 wherein the bearing portion of one of said components presents a convex metal or metal alloy bearing surface to the other component, and the bearing portion of said other component includes a remov-able plastic bearing insert which presents a concave bearing surface to, and contacts in use, said convex bearing surface.

11. A prosthesis of claim 10 wherein said joint is the metacarpophalan-geal joint, said proximal bone is the metacarpus, said distal bone is the proximal phalanx, and said convex bearing surface is presented by the bearing portion of said distal component.