Cause

A combination of factors is usually responsible for patellar tracking disorder. These factors can include:

• How your body is formed. The patellar tendon connects the bottom edge of the patella to the front of the shinbone. An excessively long patellar tendon, which allows the patella to slide too high on the knee joint (patella alta), is a developmental cause of patellar tracking disorder. Other physical traits that increase your risk of developing knee problems include a patellar shape that is unusually small or flat, flat feet, hips set more to the outside of the knees than usual, a shallow femoral groove for the patella to glide along, or a knock-knee position.
• Weak thigh muscles. Strong thigh muscles (quadriceps) anchor the kneecap (patella) in place. Weak quadriceps allow the patella to move off track. See a picture of the muscles and tendons related to patellar tracking disorder.
• An imbalance in the contraction of the thigh muscles. Your quadriceps are a group of four muscles. If the outer muscle is stronger than the inner muscle, or if it tends to contract quicker than the inner muscle, your kneecap can be pulled toward the outside as your leg straightens.
• Tendons and muscles in the leg, foot, or hip areas that are too tight or too loose. For example, weak quadriceps combined with tight hamstrings and a tight iliotibial (IT) band can pull the patella toward the outside of the knee.
• A blow to the kneecap, causing it to shift (partially dislocate or sublux) or dislocate. Dislocation can also be caused by twisting the thigh inward while the foot is firmly planted, pointing outward. After an initial dislocation, the patella is at increased risk of dislocating more easily.
• A previous injury that has healed improperly, causing an imbalance in how the leg functions.
• Generalized ligamentous laxity, or "loose-jointedness." Joint instability results from ligaments that are not as tight as they should be.