external convex surface s u2032 2 which is substantially complementary to surface s 2 as well as a surface s u2032 1 which is also convex and substantially complementary to surface s 1 the surface s u2032 2 of the intermediate component 3 is preferably unconstrained relative to the surface s 2 in the embodiment described the surfaces s 1 s 2 s u2032 1 and s u2032 2 are sections of a sphere the radius r 1 common to surfaces s 1 and s u2032 1 being smaller than the radius r 2 common to surfaces s 2 and s u2032 2 however surfaces s 1 s 2 s u2032 1 and s u2032 2 are not necessarily sections of a sphere the relative position of surfaces s 1 s 2 s u2032 1 and s u2032 2 is in accordance with the technical disclosure of fr a 2 841 768 note that c 1 is the instantaneous centre of rotation of surface s 1 in relation to surface s u2032 1 it is a centre common to the spheres defining the surfaces s 1 and s u2032 1 the geometry of elements 2 3 and 12 is such that the centre c 1 is situated in the glenoid bone.