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A new metacaprophalangeal joint prosthesis
A N e w Metacarpophalangeal Joint Prosthesis--J. Schetrumpf
A NEW
METACARPOPHALANGEAL
JOINT PROSTHESIS
J. S C H E T R U M P F , London This prosthesis is designed to improve the prehension of the chronic rheumatoid hand, and to withstand the stresses of rehabilitation. It is a hinge with two components, the joint consisting of a roller and socket (Fig. 1) which although large enough to spread the load, and reduce wear and stress particles, does not produce an unsightly knuckle. Both the polyacetal* roller and polypropylene socket components wore well in the bench test summarised in Fig. 2. Both substances can be sterilised by autoclave without damage. The components lock with each other when projections on the inside of the lateral stabilising plates of the socket locate into depressions at the end of the roller (Fig. 1). Axial laxity within the joint allows fluid to collect between the bearing surfaces and this, apart f r o m ensuring lubrication, acts by inertia under impact and limited escape to dampen the jarring of compression forces and should, on theoretical grounds, reduce any erosion of the bony stabilisation.
Fig. 1. The prosthesis is separated to show the projections on the lateral stabilising plates of the socket which locate into and lock with the depressions at the ends of the roller.
2 x 107 excursions in isoviscous glucose Fig. 2.
With a polyacetal roller in a polypropylene socket wear was 0.125 mm in this test.
The H a n d - - V o l . 7
No. 1
1975
75
A New Metacarpophalangeal Joint Prosthesis--J. Sehetrump[
DORSUM
Metacarpal
Proximal phalanx
Fig. 3. The three fins of each component are orientated to conform to the medullary cavity of the bone into which they will be inserted.
Fig. 4. Above Right. A single reamer is suitable for all sizes. Fig. 5. Below Right. The general lines of the prosthesis are shown: a. A fine extension which, after cutting to length as necessary, serves to "centre" the stem in the medullary cavity. b. This indicates the axis offset provided to give some extensor advantage. The tapered peg of each component has three fins, orientated to conform to the general lines of the medullary cavity of each bone (Fig. 3). It is hoped this conformity and the disposition of the large surface area provided by the fins will provide good anchorage and prevent rotation. The fine extension to each peg (Fig. 4a) is trimmed to size at operation. Its purpose is to centre the peg in the axis of the cavity and, by its suppleness, to present a curved tail to the cortex rather than a rigid point and thus reduce erosion when the joint is stressed. The volarwards offset of the axis of the roller component seen in Fig. 4b is to provide extensor advantage, At insertion a radial incision is made in the expansion, the metacarpal head alone is excised and the bone end is undercut to seat and stabilise the socket component. The medullary cavity of each bone is prepared with the reamer (Fig. 5a), which is suitable for all the joint sizes (Fig. 5b). The components of the joint, inserted separately, are locked together in situ and seated into the bone ends by extension and longitudinal compression. Skin closure is effected after ulnar release of the expansion and imbrication on the radial side. The proximal interphalangeal joints are excluded f r o m the splintage after operation and are exercised as pain allows. At the third week this splintage is replaced by a Presswell lively splint which is maintained for a further six weeks (Nicolle and Presswell, 1975). 76
The Hand--Vol. 7
No. 1
1975
A New Metacarpophalangeal Joint Prosthesis--J. Schetrumpf
Fig. 6. Fig. 7.
Of two similarly afflicted hands the left had improved power and movement and was pain-free following joint replacement in four fingers. The size range is 8, 10, 12, and 16 mm in width. They have common peg sizes which are pared down as necessary with a scalpel.
I n t h i r t e e n joint r e p l a c e m e n t s in five patients there have been no complications a n d there has b e e n i m p r o v e m e n t in power (both flexor a n d extensor), p r e h e n s i o n , a p p e a r a n c e a n d f r e e d o m f r o m pain (Fig. 6). A l t h o u g h insignificant in n u m b e r the results are e n c o u r a g i n g . I n t e r e s t e d surgeons are invited to extend the trial a n d a set of prostheses a n d a r e a m e r are available on request. * " K e m e t a l " b r a n d polyacetal was used. REFERENCE
NICOLLE, F. V. and PRESSWELL, D. R. (1975) A Valuable Splint for the Rheumatoid Hand. The Hand, 7:
The t l a n d - - V o l . 7
No. 1
1975
77
A NEW
METACARPOPHALANGEAL
JOINT PROSTHESIS
J. S C H E T R U M P F , London This prosthesis is designed to improve the prehension of the chronic rheumatoid hand, and to withstand the stresses of rehabilitation. It is a hinge with two components, the joint consisting of a roller and socket (Fig. 1) which although large enough to spread the load, and reduce wear and stress particles, does not produce an unsightly knuckle. Both the polyacetal* roller and polypropylene socket components wore well in the bench test summarised in Fig. 2. Both substances can be sterilised by autoclave without damage. The components lock with each other when projections on the inside of the lateral stabilising plates of the socket locate into depressions at the end of the roller (Fig. 1). Axial laxity within the joint allows fluid to collect between the bearing surfaces and this, apart f r o m ensuring lubrication, acts by inertia under impact and limited escape to dampen the jarring of compression forces and should, on theoretical grounds, reduce any erosion of the bony stabilisation.
Fig. 1. The prosthesis is separated to show the projections on the lateral stabilising plates of the socket which locate into and lock with the depressions at the ends of the roller.
2 x 107 excursions in isoviscous glucose Fig. 2.
With a polyacetal roller in a polypropylene socket wear was 0.125 mm in this test.
The H a n d - - V o l . 7
No. 1
1975
75
A New Metacarpophalangeal Joint Prosthesis--J. Sehetrump[
DORSUM
Metacarpal
Proximal phalanx
Fig. 3. The three fins of each component are orientated to conform to the medullary cavity of the bone into which they will be inserted.
Fig. 4. Above Right. A single reamer is suitable for all sizes. Fig. 5. Below Right. The general lines of the prosthesis are shown: a. A fine extension which, after cutting to length as necessary, serves to "centre" the stem in the medullary cavity. b. This indicates the axis offset provided to give some extensor advantage. The tapered peg of each component has three fins, orientated to conform to the general lines of the medullary cavity of each bone (Fig. 3). It is hoped this conformity and the disposition of the large surface area provided by the fins will provide good anchorage and prevent rotation. The fine extension to each peg (Fig. 4a) is trimmed to size at operation. Its purpose is to centre the peg in the axis of the cavity and, by its suppleness, to present a curved tail to the cortex rather than a rigid point and thus reduce erosion when the joint is stressed. The volarwards offset of the axis of the roller component seen in Fig. 4b is to provide extensor advantage, At insertion a radial incision is made in the expansion, the metacarpal head alone is excised and the bone end is undercut to seat and stabilise the socket component. The medullary cavity of each bone is prepared with the reamer (Fig. 5a), which is suitable for all the joint sizes (Fig. 5b). The components of the joint, inserted separately, are locked together in situ and seated into the bone ends by extension and longitudinal compression. Skin closure is effected after ulnar release of the expansion and imbrication on the radial side. The proximal interphalangeal joints are excluded f r o m the splintage after operation and are exercised as pain allows. At the third week this splintage is replaced by a Presswell lively splint which is maintained for a further six weeks (Nicolle and Presswell, 1975). 76
The Hand--Vol. 7
No. 1
1975
A New Metacarpophalangeal Joint Prosthesis--J. Schetrumpf
Fig. 6. Fig. 7.
Of two similarly afflicted hands the left had improved power and movement and was pain-free following joint replacement in four fingers. The size range is 8, 10, 12, and 16 mm in width. They have common peg sizes which are pared down as necessary with a scalpel.
I n t h i r t e e n joint r e p l a c e m e n t s in five patients there have been no complications a n d there has b e e n i m p r o v e m e n t in power (both flexor a n d extensor), p r e h e n s i o n , a p p e a r a n c e a n d f r e e d o m f r o m pain (Fig. 6). A l t h o u g h insignificant in n u m b e r the results are e n c o u r a g i n g . I n t e r e s t e d surgeons are invited to extend the trial a n d a set of prostheses a n d a r e a m e r are available on request. * " K e m e t a l " b r a n d polyacetal was used. REFERENCE
NICOLLE, F. V. and PRESSWELL, D. R. (1975) A Valuable Splint for the Rheumatoid Hand. The Hand, 7:
The t l a n d - - V o l . 7
No. 1
1975
77