an instrument including a cutter mountable on a cutter 32 spindle and shaped in such a way that it also acts as a test cup 33 engageable by a test head of a femoral prosthesis . the cutter may be attached to the pelvis by means of nails , and the cutter position is detected by sighting means on the cutter spindle . the sighting means are then used to arrange the final cup in the same position as the cutter so that the position of the final cup may be selected in such a way that the risk of joint dislocation is reduced .