Abstract: A device for measuring a varus rotation angle in a knee comprises a femoral reference member configured to be positioned medially of the knee, a first arm hingedly engaged to the femoral reference member, and a first potentiometer to output an angular relationship between the femoral reference member and the first arm. The device also includes a second arm hingedly engaged to the first arm, and a second potentiometer to output an angular relationship between the first and second arms, wherein the varus rotation angle is determinable from the outputs of the first and second potentiometers. Furthermore, a joint-space opening of the knee joint may be determined using the outputs of the first and second potentiometers.