An implantable assembly (21) for a knee joint includes a force absorber assembly (33) connected to a first attachment structure (25) for a tibia and a second attachment structure (29) for a femur. The absorber assembly includes a cylinder body (47) connected to the first attachment structure and a piston (41) connected to the second attachment structure. The piston includes a first portion (39) connected to the second attachment structure and a second portion receivable in a cylinder body opening (43) of the cylinder body, the second portion of the piston being of greater flexibility than the first portion of the piston.