The following are possible causes of knee pain when the leg is straightened: first, osteoarthritis, when osteoarthritis is severe, it can involve the bone and synovial membrane, then there will be obvious pain in the knee when the leg is straightened, and the diagnosis and treatment should be carried out by X-ray and MRI. Second, patellofemoral arthritis, patellofemoral arthritis patients can also appear knee pain when straightening the knee, this time to carry out nuclear magnetic examination to see the extent of patellofemoral joint cartilage damage, to early treatment. Third, meniscal injury, if the anterior horn of the meniscus is damaged, there will be significant pain when the knee is straightened, which is due to the meniscus being squeezed and pain, to carry out the relevant magnetic examination to see the degree of meniscal injury, for early treatment.