Patient: Pain in both knees when walking up and down stairs was only noticed in early 2007, and the pain increased after running and playing basketball. There was a rubbing sound when extending and flexing the legs. Laboratory tests, a rheumatoid factor test was done in May 2008 and was negative. No medication has been used. What is the possible cause of the disease? Is it serious? How to treat? Zhang Yadong, Department of Orthopedics, The First Affiliated Hospital of the Chinese People’s Liberation Army General Hospital Zhang Yadong, Department of Orthopedics, PLA 304 Hospital: According to your age and description of your condition, you may be suffering from “patellar malalignment (patellar instability)” or “chondromalacia patellae”. The main cause of this disease is excessive wear of the joint surface due to stress concentration in the patellofemoral joint, which eventually leads to damage or even loss of articular cartilage and unevenness of the joint surface. The disease progresses slowly, but it is difficult to cure except for joint surface replacement surgery. General treatment for patients with mild symptoms includes exercise of the quadriceps, appropriate reduction of weight-bearing activities in the knee, oral glucosamine sulfate and non-steroidal anti-inflammatory pain medications, intra-articular injections of hyaluronic acid, etc. Those with severe symptoms may require arthroscopic or other surgical treatment.