Runner's knee is a condition causing pain in the front of the knee. It is often due to excess wear on the underside of the kneecap. It is also called chondromalacia patellae, or CMP.

What is going on in the body?

When the knee is bent and straightened, the kneecap glides back and forth over the end of the femur, or thighbone. The underside of the kneecap is in contact with the femur. Like the rest of the knee joint, this surface is covered with cartilage. Too much stress between the kneecap and the femur can damage the cartilage. The joint surface becomes softened and roughened. This wear causes joint irritation and pain.

Sometimes the joint swells because it produces extra fluid to improve lubrication. The contact pressure between the kneecap and femur varies according to activity. Deep knee bends exert a force equivalent to five times the body weight. Climbing stairs is equivalent to three times the body weight.

What are the causes and risks of the condition?

Runner's knee is usually caused by overuse when joggers run too long, too fast, or up too many hills. Conditions that cause the knee to be poorly aligned can contribute to extra stress on the kneecap. These include knock-knee, outward rotation of the lower leg, or inward rotation of the thighbone. Tight quadriceps muscles can increase the pressure on the kneecap.