Prosthetic thumb joint and method of manufacture

Claim 1

A prosthesis for a carpometacarpal joint of a thumb which comprises: a trapezial member having a first surface and a second surface, said first surface being a surface of revolution relative to both a first axis and a second axis, said first and second axes being anatomical axes defining the pivoting of a first metacarpal bone of said thumb relative to a trapezium bone of said thumb; a plurality of pegs mounted on said second surface of said trapezial member for engagement with said trapezium bone, to anchor said trapezial member thereon; a metacarpal member for abutment with said trapezial member, said metacarpal member having a first surface cooperating with said first surface of said trapezial member; a stem attached to said metacarpal member, said stem being dimensioned for insertion into a proximal metacarpal canal of said first metacarpal bone of said thumb to anchor said metacarpal member thereon; wherein said first surface of said trapezial member and said first surface of said metacarpal member match a selected portion of a surface of a reference toroid, said reference toroid being mathematically generated by rotating said second axis about said first axis, while rotating a reference point about said second axis, the orbital radius of said reference point being determined by measuring the location, relative to said first and second axes, of anatomical abutment surfaces between said trapezium bone and said first metacarpal bone, and wherein said first axis passes through said trapezium, a naturally occurring location of sad first axis being determined with an axis finder, and said second axis passes through said first metacarpal bone, a naturally occurring location of said second axis being determined with an axis finder, said second axis being in a fixed relationship with said first axis, and further wherein said first axis and said second axis are non-parallel, non-perpendicular and non-intersecting, and the relationship between said first axis and said second axis is characterized by: (a) a compound angle between said axes, measured at a reference plane perpendicular to said first axis, said compound angle consisting of a first angle (∝) between the projection of said second axis in said reference plane and a line tangent to the reference plane orbit of said second axis about said first axis, and a second angle (β) between said second axis and said reference plane; and (b) an orbital distance between said two axes in said reference plane.