The invention solves the problem of anchoring a prosthetic component into compliant, cancellous bone by a thin, preferably pure titanium, or a titanium-based alloy, perforated, hence hydraulically open, shell (1, 13), spaced apart from the second, non-perforated shell (2, 14). In most applications, dictated by the anatomical site of insertion, shells are approximately hemispherical in form, connected to each other at the equator The solid shell may contain a further, articulating component of the joint, e.g. a polymer or a ceramic insert, or it may serve that function itself. One or both components of a joint prosthesis can be anchored according to the invention.