Patella Dislocations

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Patella Anatomy
Patella Dislocation
Patella X-Ray
Flat Patella
Apprehension Test


The patella (knee cap) is a sesamoid bone. A sesamoid bone is one that is encased in tendon or ligament. The patella is located inside the quadriceps tendon. The patella acts as a fulcrum to increase the strength of the quad muscle. It is held in place by the quadriceps tendon above, the patellar tendon below, and very thin ligaments on either side. The patello-femoral joint is formed by the patella and trochlear groove of the femur.

Due to the twisting nature of sports, the patella can dislocate (come out of joint) with an awkward twist of the femur (thigh) on the tibia (shin). A twisting motion causes the patella to shift to the side. Usually, the patella moves laterally (to the outside). This occurs because the quadriceps muscle contracts to maintain the stability of the body. The shin has shifted so that the line of pull of the quads causes the patella to shift laterally. The patella is pulled laterally because it wants to remain in line with the muscle.

The patella can dislocate more easily in some people than others. Individuals with a greater “Q-angle” are at a greater risk for patellar dislocations. The “Q-angle” is formed by envisioning a circle around the patella, the line of pull of the quad muscle forms the tail of the “Q.” If the tail of the “Q” is more than 25 degrees off of the center of the quad-patella-patellar tendon line of pull, it is considered an abnormally high “Q-angle.”

This places the patella at a greater risk to slide off of the femur. The quad-patella-patellar tendon mechanism wants to form a straight line when the quad muscle contracts, due to this, the patella is pulled laterally. This places a person with a high “Q-angle” at a greater risk for patellar dislocations.

Another risk factor for patellar dislocations is a malformed patella or trochlear groove (the groove located between the two heads of the femur) . The back side of the patella should have a peak, like an inverted mountain top. The trochlear groove should look like the valley between mountains. If either the mountain or the groove are not large enough, the patella is more prone to dislocate.

This is demonstrated by the x-ray. The back side of the patella is flat. This accounts for the sideward lean of the patella. This patella is prone to dislocate and is partially dislocated or subluxed in the x-ray

Females seem to have a greater risk for patellar dislocations than males. This may be due, in part, to the shape of their hips. A female’s hips are shallower and wider to accommodate pregnancy. This tends to cause genu valgum (“knock-kneed” appearance); in other words, their knees are closer together than their ankles. This can result in a greater “Q-angle,” thus making patellar dislocations more probable.

Patella alta (the patella rides too high in the trochlear groove) is also another predicting factor for patellar dislocations. If the patella rides too high in the trochlear groove, it may be prone to dislocating more easily. Often this is present on both knees, so comparing one to the other may not make this symptom easy to find. Most often, this is seen on x-ray by a physician.

If an athlete suffers a patella dislocation that does not spontaneously reduce (go back into place), it is rather obvious to detect. The patella will be laying near the outside of the knee joint. However, it is quite common for the patella to spontaneously reduce. Many times the athlete will straighten his/her leg inadvertently after the injury, causing the patella to reduce.

If the patella has been reduced, the athlete will present with increased pain, swelling, and loss a decrease in range of motion of the knee. The swelling may be great enough as to make the patella “disappear.” Due to the swelling, the patella may also feel, when pressed straight down, as if it is a boat floating in water.

The swelling is due to tearing of the ligaments on the medial side of the patella. This swelling is located inside of the joint, accounting for the patella feeling like a floating boat. Since this injury usually results from a twist, and the swelling is located inside of the joint, an orthopaedic surgeon should be consulted to differentiate between a dislocated patella and an ACL tear.

This picture shows the apprehension test. This test is used to determine if a patellar dislocation has occurred. The Athletic Trainer or physician will push the patella laterally (outside), if this elicits pain or apprehension, it is a positive test.

Any athlete who suffers a dislocated patella should consult with an orthopaedic surgeon for x-rays, protective bracing, and appropriate rehabilitation. X-rays are necessary to rule a fracture of the patella. In some cases the mountain peak of the patella will be "knocked off" when it impacts with the femur. This piece of bone can cause severe damage to the joint if it is not properly addressed. 

Appropriate rehabilitation should concentrate on pain control, swelling reduction, return of full range of motion, and return to normal strength.

The vastus medialis muscle should be targeted during the rehabilitation. This muscle helps to place a medial pull on the patella, reducing the lateral, dislocating force. This is especially important in those individuals with a high “Q-angle” or genu valgum.

Surgery is not indicated unless dislocations are recurrent. The surgical interventions center around correcting improper alignment. They are often viewed as “salvage” or “last resort” procedures. 



©2000 - 2009 David Edell

Information on this site is not a substitute for physician directed care.

Please consult your personal physician for more detailed information

concerning specific injuries or illnesses.

Last Update for 10/24/2009 12:09:35 AM