Abstract: A bone prosthesis (10) which is hydraulically operated is disclosed. It comprises a cylinder (12), which is closed at one end, and piston (28), which slides inside the cylinder (12). The relative locations of the cylinder (12) and piston (28) are controlled by a liquid (26) filled bladder (24) which is located inside the cylinder (12). Liquid (26) is introduced into the bladder (24) via a valve (22) which fits through the side wall (21) of the cylinder (12). The valve (22) is constructed so that liquid (26) can be introduced to or removed from the bladder (24). One end of the prosthesis (10) is shaped to mate with the remaining segment of bone. The other end of the prosthesis (10) is shaped to replace the other end of the surgically removed bone segment. In addition, the internal surface (18) of the cylinder (12) and the external surface (30) of the piston (28) are grooved or keyed so as to prevent rotation of the piston (28) inside the cylinder (12).