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Chondromalacia Patellae (CMP) - Runner's Knee


Overview, Causes, & Risk Factors

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.


Symptoms & Signs

What are the signs and symptoms of the condition?

Runner's knee causes pain in the front of the knee, around the kneecap. The pain can happen with activity or after prolonged sitting with the knees bent. Sometimes the knee will seem to give way. The knee may seem to lock when the rough surfaces of the kneecap and femur catch as they rub over each other.

Sometimes a rubbing or clicking of the kneecap against the femur can be heard when the knee is bent and straightened. The edge of the kneecap may be tender. The knee may contain extra fluid, called an effusion. In some cases, the kneecap tends to slide toward the outside of the knee.

When the pain lasts a long time, the quadriceps muscle in the thigh can atrophy, or become smaller.


Diagnosis & Tests

How is the condition diagnosed?

Diagnosis of runner's knee begins with a history and physical exam. Joint X-rays are usually normal. The healthcare provider may do an arthroscopy. The provider inserts a small, lighted tube into the joint and examines it.


Prevention & Expectations

What can be done to prevent the condition?

Stretching exercises are good for prevention of runner's knee, particularly those exercises designed for the quadriceps muscles. A moderate training program should avoid stress on the knee. Excessive distance, speed, and hills should be avoided. Orthotics, or arch supports, may help to reduce pronation of the foot.

What are the long-term effects of the condition?

Long-term effects of runner's knee are rare. If overuse continues and symptoms are ignored, arthritis is a possibility.

What are the risks to others?

Runner's knee is not contagious, and poses no risk to others.


Treatment & Monitoring

What are the treatments for the condition?

Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or naproxen, may be taken. The healthcare provider will also outline some activity limitations.

What are the side effects of the treatments?

NSAIDs may cause stomach upset or allergic reaction.

What happens after treatment for the condition?

Proper training, preparatory stretching, and activity limitations should be continued. Orthotics, such as arch supports, may be recommended.

How is the condition monitored?

The effects of the running program on the knee are monitored. Any new or worsening symptoms should be reported to the healthcare provider.


Attribution

Author:John A.K. Davies, MD
Date Written:
Editor:Crist, Gayle P., MS, BA
Edit Date:06/01/01
Reviewer:Warren Katz, MD
Date Reviewed:06/01/01


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