The inability to bend the leg and the pain when bending are mainly due to the following reasons: 1. Arthritis, such as rheumatoid arthritis, traumatic arthritis and osteoarthritis, which induces pain when bending and the inability to bend. 2. It is due to the meniscus lesion, such as meniscus damage or tear, joint compression, joint movement limitation, which will result in the inability to bend the leg and even the narrowing of the joint gap when bending, resulting in pain. Even when bending, the joint gap becomes narrower, which will compress the meniscus and lead to pain. It is recommended that patients go to the hospital in time to check which cause is due to, and then give targeted treatment. If it is caused by meniscus injury, it is best to give timely surgical treatment to completely release the joint compression before flexion.