1. If the arthritic knee cannot be straightened and is in semi-flexion, joint replacement is required. If not, in order to walk the opposite knee must also be in semi-flexion, which will eventually lead to the replacement of 2 knees, and in order to save the other good joint, the knee that cannot be straightened needs to be replaced. 2. If the joint space is gone on the knee radiograph, or close to it, and the joint flexion and extension activities are limited and cannot be squatted, and if the age is greater than 60, especially greater than 65, knee replacement is also suitable. 3, if the knee joint pain is unbearable, although there is a joint gap, walking pain, rest pain, upstairs pain, downstairs pain, squatting pain, age greater than 65 years old, generally not suitable for arthroscopy and other minimally invasive treatment, suitable for joint replacement. 4, if the knee is a serious rotundity, or valgus leg, also suitable for joint replacement, otherwise it will lead to compensatory changes in the lumbar spine.