The most common causes of pain inside the knee are meniscus injury, ligament injury, rheumatoid arthritis, osteoarthritis, synovitis, etc. If the symptoms persist, it is recommended to seek medical treatment promptly. 1. Meniscus injury: if the patient’s meniscus is worn or torn, pain inside the knee will occur. 2. Ligament injury: excessive internal or external rotation of the knee joint may cause ligament injury or rupture, which may result in joint pain and difficulty in movement. 3. Rheumatoid arthritis: This is an autoimmune disease that is prone to redness, swelling and pain in the knee joint. 4. Osteoarthritis: mostly related to degenerative changes in articular cartilage, it occurs in middle-aged and old-aged people, and may be followed by osteomalacia, causing pain in the knee. 5. Synovitis: When synovitis occurs in the knee joint, a large amount of fluid may form, causing pain due to increased pressure in the joint. Causes of pain inside the knee are more reasons, may also be related to knee bursitis, gouty arthritis, osteoporosis, herniated disc compression of the nerve and other diseases, if rest does not relieve, it is recommended to go to the hospital in a timely manner to find out the cause of the disease, by the doctor targeted treatment.