The inability to curl the leg at the knee 40 days after the second hip replacement surgery is generally an abnormality and requires active medical attention. Hip surgery theoretically does not affect the function of the knee, and it is also recommended that functional exercises (including the knee) be carried out early after hip surgery. If there are no special circumstances, 40 days after hip replacement surgery, you can already walk independently with the help of a walker. So it is not normal that the knee cannot curl the leg at this time. The main reason for not being able to bend the knee is probably a fracture or injury of the knee joint (not very likely given that you have already had 2 hip replacements and should have had your knee examined). There is also the reluctance to exercise because of the pain of the wound (adhesion and stiffness of the knee joint due to time). In this case, the knee joint should be examined first, and if organic damage is ruled out, knee flexion and extension exercises should be carried out actively and passively under the guidance of the doctor. For orthopedic patients, rehabilitation exercises are an important part of treatment. Second hip replacement surgery 40 days knee can not curl the leg, need to actively consult the doctor, clear cause for targeted treatment.