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What is Patellar (Kneecap) Instability?

Patellar (kneecap) instability is a very common pathology, especially in younger female athletes. The knee joint is made up of three bones; the patella, the femur (thigh bone) and the tibia (shin bone). The patellofemoral joint refers to the joint between the kneecap (patella) and the femur. During normal knee mechanics and movement, the patella glides smoothly in the center of the trochlea or femoral groove (a groove located on the lower end of the femur). In cases of patellar instability, due to the groove being too shallow or uneven, the kneecap may begin to move abnormally and may even slide out of the groove. Moving out of the groove is called dislocation. The kneecap normally will dislocate laterally, or towards the outside of the knee. When these occur, they are often associated with significant pain and swelling.

Though complete recovery is possible, associated injuries and secondary problems are common and should be looked for before returning to sports. In a patellar dislocation, the ligaments on the inside of the knee are stretched and injured as the kneecap slides laterally. While stretching and tearing of these ligaments is unfortunate, with appropriate care they do have the potential to heal. With these dislocations, it is possible that small fragments of cartilage and bone may be knocked off of the kneecap or the part of the femur (thigh bone). These fragments become loose bodies and usually require surgical removal. It is possible for the patella to only partially dislocate; this is called a patellar subluxation. This is very disconcerting and often gives patients a sense of giving way or buckling. Oftentimes in athletes who have patellar instability, they will experience a number of these subluxation episodes before going to a physician.


  • Uneven femoral groove; shallow femoral groove
  • Direct blow or fall onto the kneecap
  • Fall directly onto the kneecap
  • Twisting on a planted foot during activity

Signs & Symptoms

  • Knee dislocations or subluxations:
    – These episodes may cause knee swelling
    – Pain on the inside of the knee, where the ligaments were stretched
    – Knee buckles and can no longer support your weight
  • Feelings of catching or locking during activity
  • Pain in the front of the knee, increased with physical activity and walking up and down stairs
  • Pain when sitting
  • Feelings of creaking or cracking sounds during movements (i.e., bending and extending the knee)


Following a patellar subluxation or dislocation, the patella should be reduced back to its anatomical position (lying flush in the femoral groove), if it does not relocate on its own. This can usually be done by simply straightening the knee. Patellar instability, acutely, should initially be treated with physical therapy to strengthen the lower extremity musculature Use of a knee brace to help stabilize the patella during activities of daily living and physical therapy exercises are recommended. Physical therapy as well as home exercises that focus on strengthening the thigh, hip and core musculature is important to provide dynamic stability and control to the kneecap during movement.

Normally following a first time dislocation, rest and a strengthening program will be enough to correct the instability.  Unfortunately, if continued episodes of instability or subluxation/dislocation events occur, surgical intervention to manually stabilize the patella and keep it tracking normally within the femoral groove may be necessary if conservative treatment fails. Surgical stabilization can include reconstructing the ligaments that help tether the patella in place, releasing tight tissue on the outside of the knee, or repositioning the attachment of the patellar tendon so as to create less force on the patella. The type of surgery is dependent on a multitude of factors and discussed on a case-by-case basis with the orthopedic surgeon. Following surgery, physical therapy to regain normal range of motion and strengthening is required.

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