Knee pain down stairs may be due to osteoarthritis, chondromalacia patella, meniscus injury. 1. Osteoarthritis: Osteoarthritis, also known as degenerative arthritis, is a chronic joint disease characterized by degenerative changes in articular cartilage and secondary osteophytes, and the knee joint is prone to the disease because of its heavy load. 2. Chondromalacia patella: chondromalacia patella is a disease caused by injury to the femoral patellar joint cartilage degeneration, the disease occurs in the knee activities more people, such as track and field, mountaineering athletes and dancers. 3. Meniscus injury: meniscus injury is a kind of meniscus injury caused by knee rotation and extrusion, patients can have symptoms of knee pain down stairs, also can be accompanied by severe knee pain, swelling, can not straighten and other symptoms. If the symptoms of knee down stairs pain persist without relief, please consult a doctor in time to find out the cause, so as not to delay the condition.