Below the knee is often medically called the patellofemoral joint, and there are several main causes of pain: 1, degeneration of the knee joint. It is often seen in older, over 30 years old, especially in older women, including inconvenient squatting and unfavorable up and down stairs activities. 2, patellar cartilage. It is common in younger patients, mostly between the ages of 20-40, and is generally considered to be caused by degeneration or wear and tear of the patellofemoral articular cartilage. 3, synovitis. It is often caused by overexertion, incorrect posture, trauma, cold, etc. 4. Meniscal injury. It can occur in the lateral or medial side of the knee, and can even become embedded when extending or flexing the knee joint. 5. Intra-articular free bodies, osteophytes, synovial polyps, patellar subluxation, ligament injuries, etc. can all be caused.