proximal region of the prosthesis shown in fig1 a the dorsal arm or wing 313 and the ventral arm or wing 314 of the prosthesis cross section are essentially the same in length whereas in the embodiment according to fig1 b the ventral arm or wing 314 is shorter than the dorsal arm or wing 313 this makes it possible to save material without a substantial loss in stability because load transfer mainly takes place in the medial and dorsal region in the embodiment of the cross section according to fig1 c the outside of the dorsal arm 313 comprises a concave wing with a recess 313 39 this gives further improved rotational stability if not specified otherwise the right hand prosthesis shown in fig2 is similar in structure to the left hand prosthesis according to fig1 in the prosthesis shown in fig2 there is a semicircular bowl or shell 11 mounted in a rigid or jointed fashion on the neck 2 which is firmly welded to the stem 3 this semicircular shell is spherical and comprises ventrally and dorsally distended surfaces 111 the medial end 112 of the cap is preferably a retracted blunt edge level with the fovea capitis.