The pain inside the right leg knee bending is considered to be related to knee joint injury, knee osteoarthritis, rheumatoid arthritis and so on. 1. Knee injury: Knee injury leads to abnormalities in the corresponding structures, such as meniscus damage caused by trauma, strenuous exercise, or damage to the knee cruciate ligament, joint capsule, etc., all of which can cause pain when bending the knee. 2. Osteoarthritis of the knee joint: mostly seen in middle-aged and old-aged people, joint degeneration, hyperplasia, and aseptic inflammatory reaction, which will cause fluid accumulation in the knee joint and damage to the joint surfaces, and may cause pain when bending the knee. 3. Rheumatoid arthritis: in the acute stage of this disease, the synovial membrane of the knee joint will be congested and edematous, which will cause pain when bending the knee joint. The pain inside the right leg knee bending may also be related to other diseases, it is recommended to consult a doctor in time to clarify the cause of the disease and standardize the treatment under the guidance of the doctor.