A prosthetic flexible bearing element cup or cup bearing liner is adapted for use with a part-spherical bearing head. The flexible bearing element includes an inner bearing surface which has a first portion which is substantially part-spherical about a hemispherical center located on a main loading axis, and a second portion at least part of which is relieved with respect to the first portion. The relieved part of the second portion has a radial distance (d) from the hemispherical center which increases from an inner end towards an outer end as the angle of rotation (θ) about the hemispherical center increases towards the rim of the cup.