For patients who are doing their own post-operative rehabilitation at home, the method of determining the angle of knee flexion is simple (it is only a rough judgment and differs somewhat from the professionally measured angle, but the difference is not significant): when the knee is flexed less than 90°, the knee is taken as zero degrees when the knee is straight, and the mid-thigh and mid-calf axes are considered to be in a straight line, at which point the angle of knee flexion is how many degrees the lower leg is bent downward. When the knee is bent more than 90° (the angle between the mid-thigh and mid-calf axes is a right angle), the knee bending angle = 180° – the angle between the mid-thigh and mid-calf axes.