geometry of a bearing element of a prosthesis not shown the elongate portion 17 is flat and is used to mount the stylus 16 to the retaining member 14 the stylus 16 has a curved protrusion 64 which comprises the proximal end of the bowl portion 19 the curved protrusion 64 acts as an indicator which identifies to the surgeon the correct position for the primary peg of the femoral implant and the flexion angle achieved the c shaped body 12 has a first arm 22 which is the upper arm as viewed in the drawings a second lower arm 24 and a central portion 26 connecting the first and second arms 22 24 together the inner surfaces 28 30 of the arms 22 24 i e the surfaces facing one another define a space 23 therebetween for receiving part of the surgical tool guide 50 the retaining member 14 may be a pin or screw or other suitable fixing which extends at right angles through the first arm 22 of the body 12 and has an enlarged head 34 the elongate portion 17 of the stylus 16 has a substantially planar lower surface 13 and a substantially planar upper surface 15 such that the elongate portion 17 has the form of a flat plate the elongate portion 17 has a longitudinal slot 36 running therethrough which has an enlarged portion 37 at one end sized to accommodate the head 34 of the retaining member 14 the retaining member 14 is located within the slot 36 as a clearance fit thus enabling translation and rotation of the stylus 16 about the pin 14 the stylus 16 is retained on the retaining member 14 by the enlarged head.