Arthrometer used to measure angle of extension. The right femur is fixed in a grooved metal clamp (F). The lateral condyle is positioned at the center of rotation (C). A motor-driven movable arm with two uprights posts pushes the posterior leg (L) into extension at a fixed distance from the center of rotation and at a predetermined speed. At torque = 12.5 N-cm a picture is taken of the knee (left upper corner insert shows the camera mounted above the arthrometer). The femorotibial angle is drawn using the femoral line from lateral condyle (C) to the middle of the femur clamp (F) (femoral diaphysis) and the tibial line from the lateral condyle (C) to lateral malleolus (M). The femorotibial angle corresponds to the angle of extension reached by the knee, with full extension defined at 180°. Once the rat leg is positioned, joint angle measurement is investigator-independent.