Universal joint prosthesis with cap

Claim 1

A universal joint prosthesis for engaging the head portion of a bone end while retaining the major portion thereof, comprising a cap having a calotte-shaped outer surface terminating in a free edge and an inner surface having a reference osculating surface which is symmetrical about a central longitudinal axis, extending from said free edge defining a concavity in which a bone or the like may be received, the concavity being sufficiently large to contain a major portion of the cancellous bone end of the joint, said inner surface having a plurality of spaced projections, said projections being spaced to define depressions therebetween, said projections and depressions being successively arranged along lines extending from said free edge toward the innermost portion of said inner surface, said lines extending along planes laying in said longitudinal axis, an open ended sleeve attached to said inner surface and arranged coaxially to said axis and extending at least partly beyond the free edge of said concavity and rigidly connected to said cap, said sleeve including a plurality of macroscopic holes extending therethrough, said concavity as well as said projections and depressions extending from said free edge toward a depth where said inner surface converges toward said axis in a direction away from said free edge, wherein said projections and depressions increase the inner cap surface which can bear on a bone after the prosthesis has been inserted at least 50% with respect to the reference osculating surface that would limit the concavity if the projections and depressions and the sleeve are absent.