An instrument for controlling the distribution of bone cement on the surface of the head of a bone to prepare the bone to receive a re-surfacing prosthesis comprises a hollow shell which can be fitted on to the head of the bone to define a space around the head. The shell is rotationally symmetrical and has a spike like projection extending axially from the internal surface of the shell. At least one rib is located on the internal surface of the shell, extending from the spike like projection or from close to it, towards the open face of the shell, which contacts cement on the head of the bone in a sweeping action when the shell is rotated around its axis on the head of the bone.