Abstract: There is disclosed a surgical instrument for use in preparing a bone cavity for the subsequent implantation of a joint replacement prosthesis, and especially for the enlargement of a medullary cavity of a femur prior to the implantation of a femoral component of a hip prosthesis. The instrument comprises a shank (2) having serrations (4) on at least a part of its outer surface and a handle (3) mounted at one end of the shank, in which the shank is divided into a main body member (5) and at least one sliding member (6) which is engageable with the main body member in such a way as to allow relative sliding motion between the sliding member and the main body member during use of the instrument, so that the serrations can shape the interior surface of the bone cavity.