Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1993-03-15
1995-05-09
Willse, David H.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
623 18, A61F 242
Patent
active
054136091
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a joint prosthesis for the human or animal body. More particularly, the invention relates to a prothesis for the metacarpal-phalangeal (MCP) joint of the index finger of the human hand.
There have been many proposals for joint prostheses (including MCP joint prostheses), for example as disclosed in U.S. Pat. Nos. 4,231,121, 4,725,280, 4,731,087, UK 1304837 and EP 310483, but most of these have failed to mimic the natural joint and have resulted in unnatural joint movements and appearance. Natural joints are mechanically complex and provide complex joint movements arising from the control imposed by the natural ligaments, tendons and muscles in the vicinity of the joint.
It is an object of the present invention to provide a new and improved joint prosthesis particularly, but not exclusively, for metacarpal-phalangeal joint replacement.
According to the present invention there is provided a joint prosthesis for a human or animal body, comprising first and second components each made of bio-compatible material and adapted for secural to respective bones of the body in the vicinity of a natural joint after surgical excision of the natural joint, the components of the prothesis being unattached to each other and, in use, being held together to form the joint by the natural ligaments, tendons and muscles of the body in the vicinity of the natural joint.
The first component comprises a stem for reception by a bone cavity and a bulbous head presenting a three-dimensionally curved convex articulation surface which in at least one plane has a radius of curvature which changes within the arc length of the surface and in that plane the length of said arc being at least 90.degree., and the second component comprises a stem for reception by a bone cavity and a head presenting a three-dimensionally curved concave articulation surface, said concave surface being dimensioned to slide over said convex surface substantially along said 90.degree. arc length to provide flexing movement of the prosthetic joint with the instantaneous centre of rotation of the joint changing during said flexing movement due to said changes of radius of curvature so as to change the distance between the respective bones and thereby allow natural collateral ligament function around the prosthetic joint to aid flexor muscle effect when the joint is extended and extensor muscle effect when the joint is flexed.
Preferably, the concave articulation surface is also capable of sliding movements across the convex articulation surface in directions orchogonal to said plane, said movements being limited in extent by the collateral ligaments of the joint and being substantially less in the joint-flexed condition than in the joint-extended condition due to ligament tension in the flexed condition arising from the aforesaid change of radius of curvature.
Preferably also, said bulbous head of said first component comprises a single lateral protrusion for differentially controlling ligament tension when the joint is in the flexed condition.
Preferably, the convex articulation surface forms part, of an ellipsoid or substantially an ellipsoid whereby said radius of curvature changes occur progressively or substantially progressively along said arc length, and the concave articulation surface forms part of a hollow spheroid or substantially a hollow spheroid, the latter having a curvature which substantially matches that of the former when the joint is midway between its extended and flexed conditions, providing peripheral contact when the joint is in its extended condition and central contact when the joint is in its flexed condition.
The convex articulation surface may have said arc in said one plane formed by two conjoined arc portions having constant but different radii of curvature, the arc portions being arranged with minimal disconformity at their intersection.
An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which
FIG. 1 schematically illustrates a human
REFERENCES:
patent: 4340978 (1982-07-01), Buechel et al.
patent: 4725280 (1988-02-01), Laure
patent: 4784661 (1988-11-01), Beckenbaugh et al.
patent: 5037440 (1991-08-01), Koenig
Marsden Donald C.
Nicol Alexander C.
Souter William A.
University of Strathclyde
Willse David H.
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