Note: Descriptions are shown in the official language in which they were submitted.
~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
--4
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.
, ...
:-
--:
" 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;
.
.,, ,. ' . ~
' '
.
~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~
--7--
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
... .
.
~.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
04
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
-13
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
' ;
,~ .
1`13~ 4
-14-
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.
': ~ ' '''
