Knee Extensor Mechanism Problems
Are You Weak in the Knees?

The knee extensor mechanism allows you to ride a bicycle, kick a football, and run a marathon. It is a complex interaction of muscles, ligaments, and tendons that stabilizes the patellofemoral joint, which is made up of the patella (knee cap) and the end of the femur (thigh bone). This joint must be mobile enough to allow the patella to glide smoothly across the groove in the femur, yet must constrain the patella enough so it does not slide from side to side while maintaining normal tension in the front of the knee. This tension is needed to prevent your knee from giving way when the forward motion of your body decelerates (slows down or stops). Extensor mechanism problems are one of the most common ailments orthopaedists see and may be among the most difficult to diagnose and treat. They are often labeled anterior knee pain or patellar compression syndrome, which are catch-all terms not a true diagnosis. Your doctor must find the true diagnosis, and he or she may have to examine your knee more than once to figure out the cause of your extensor mechanism pain and disability. In the meantime, a proper routine of rehabilitative exercises for the knee may relieve the pain.

The quadriceps, or front thigh, muscles (rectus femoris, vastus medialis including the vastus medialis obliquus vastus intermedius, and vastus lateralis)(see Fig. 1 at right or click here) insert into the patella and the extensor retinaculum. These muscles act together to extend (straighten) the knee and to control the side-to-side movement of the patella. Cartilage on the undersurface of the patella is the thickest of any found in the body. This thick joint cartilage acts as a cushion, absorbing shock in the greatest weightbearing joint in the body during the process of deceleration.

The entire knee joint is covered by a thick, fibrous tissue the capsule. This capsule contains a lining (the synovium) that produces fluid to lubricate the joint and reduce friction and wear. The outer layers of the capsule comprise the extensor retinaculum, which is formed by tendon-like sheets that continue from the quadriceps muscle to the tibia (shin bone) on either side of the patella. This fibrous tissue contains thick, tough bands of tissue or rope-like structures, known as ligaments, that further stabilize the patella and help prevent abnormal movement. The muscles and ligaments must work together in a balanced fashion to maintain normal patellar motion as the knee flexes and extends. If the muscle forces are unbalanced or there is a tear of a ligament or the retinaculum, the patella can become dislocated or slip and move abnormally. The patellar tendon is an extension of the extensor mechanism and connects the patella to the upper end of the tibia. A fat pad located behind the patellar tendon helps reduce friction between the patellar tendon and the tibia.

Extensor mechanism problems can cause pain in the front of your knee. You may hear a snapping sound while walking, or you may feel like your knee is giving way. Repetitive activities, such as cycling, swimming, and running, may cause discomfort. You may also experience pain when descending stairs or when seated for long periods of time (for example, at the movie theater), and you may develop a buildup of fluid in your knee. These symptoms are often the result of plica irritation, fat pad impingement, patellar tendinitis, or patellar instability.

Irritation of the plica
The plica is a fold of the synovium. This tissue can become irritated when an increased force is placed on it by tight hamstring muscles behind the knee. Weak hip abductor muscles can also lead to irritation of this plica. In these situations, the plica rubs on the end of the femur, irritating it and causing pain, popping, snapping, or just aching when you are at rest.

Fat pad impingement
This condition occurs when the bottom of the patella pinches, or impinges on, the fat pad on top of the tibia. If the fat pad is swollen due to irritation or injury, it becomes more susceptible to this condition. Patients who overextend their knee also predispose themselves to this situation. When the impingement occurs, the fat pad becomes irritated and then painful, and scar tissue can form. People with fad pad impingement typically walk with the knee extended; this produces the limp and the stiff-legged gait. Pain is felt in the area beneath the patellar tendon, and the normally soft fat pad may become larger, firmer, and irritated.

Patellar tendinitis
This condition is seen in people whose activities, such as basketball, volleyball, and running, cause repetitive stress on the patellar tendon. The fibers of the tendon that insert into the lower portion of the patella become irritated. In severe cases, actual tearing of these fibers can occur. Once the patellar tendon is irritated, fluid may accumulate in the area under the patella. This area becomes very tender and painful, especially while you are walking or running.

Patellar instability
Rotating your knee while your foot is planted firmly can cause the patella to shift from its normal position in the femoral groove. Sports, such as tennis or baseball, in which rapid changes of direction are required make the knee more susceptible to patellar dislocation or subluxation (partial or temporary dislocation). Differences in knee anatomy also play a role. For example, people who walk with any external (outward) rotation of the tibia or internal (inward) rotation of the femur may be predisposed to patellar instability. The angle between the femur and tibia can also predispose one to an abnormal position of the patella. Individuals who are severely knock-kneed are more likely to experience movement of the patella out of the femoral groove. When the muscles and ligaments of the patella are not strong enough, the knee cap can move out of its normal position and pop to the side of the knee. The knee then commonly gives way, causing the person to fall. The patellar dislocation is generally a traumatic event that is easily diagnosed, but a subluxation is often misdiagnosed or is not even recognized. With patellar subluxation, people often feel that their patella is slipping in and out of place, or simply that the knee is not stable beneath them.

Enough is enough
Don't let a knee extensor mechanism problem get the best of you. If you are troubled by any knee pain, especially if you have had an injury, make an appointment to see your doctor. Once the injury has been diagnosed, the proper treatment can be started, allowing you to return to your normal activities as soon as possible.

Robert J. McAlindon, M.D.
Auburn, Alabama