An instrument for removing tissue from a bone to create a cavity consists of an approximately spherical body having cutting formations in its external surface for cutting a bone. The body has a connector by which it can be connected to a drive by which it can be rotated about an axis, and a recess at the pole, opposite the connector. When the instrument is applied to the surface of a bone and is rotated about its axis, bone is cut as a result of the action of the formations against the bone, and a circular region of bone adjacent to the polar recess in the body remains generally intact.
