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Anatomy of the Knee Joint

The patella (kneecap) is a bone attached to the quadriceps muscles of the thigh by quadriceps tendon. In turn, it is attached to the patellar tendon below it. The patellar tendon attaches to the tibia (leg bone). It articulates with the femur bone to form the patellofemoral joint. The patella provides mechanical advantage to the knee which means it makes it easier for the quadriceps muscle to straighten the knee. It is held in place by ligaments on either side to include the medial patellofemoral ligament (MPFL) as well as the trochlea, or trochlear groove, of the femur.

What is Patellar Dislocation?

Patellar dislocation occurs when the patella moves out of the trochlear groove. If the kneecap partially comes out of the groove, it is called subluxation; if the kneecap completely comes out, it is called dislocation. Sometimes, a piece of cartilage can be fractured off the knee. This typically requires timely surgery to fix it back in place.

Causes of Patellar Dislocation

Patella dislocation is most common in people under the age of 25 with the highest incidence in adolescent patients. Almost all patellar dislocations are due to some sort of underlying anatomic abnormality. The most common abnormality that leads to patellar instability is a shallow trochlea, also called trochlear dysplasia. Other causes include an abnormally long patellar tendon which causes the knee cap to sit higher than it should (called patella alta), knocked knee alignment (called genu valgum), excessive in-toeing (called femoral anteversion) or general ligamentous laxity. When the patella dislocates, the MPFL is torn and can become stretched out and incompetent.

Symptoms of Patellar Instability

The common symptoms include a sense of the knee giving way or buckling as well as a general sense of apprehension. There can also be pain and swelling. In addition, there can be painful dislocation events that may require a trip to the emergency department to get the kneecap put back in place.

Treatment of Patellar Dislocation/Patellofemoral Dislocation

Your doctor will examine your knee and suggest diagnostic tests such as X-ray, CT scan, and MRI scan to confirm the condition and provide treatment. There are non-surgical and surgical ways of treating patellofemoral dislocation. 

Non-surgical or conservative treatment includes:

  • PRICE (protection, rest, ice, compression and elevation)
  • Non-steroidal anti-inflammatory drugs and analgesics to treat pain and swelling
  • Braces that can stabilize the kneecap and allow the MPF ligament to heal

Physical therapy is recommended to enhance strength around the knee to help keep the kneecap in place. In addition, strengthening exercises of the hip muscles can assist with overall control of the knee. Physical therapy will not correct the underlying problem, but it may be able to compensate for it, if the underlying problem is mild.

Surgical treatment is recommended for recurrent patella instability or when a dislocation is accompanied with a surgical osteochondral fracture. Some of these surgical options include:

  • Trochleoplasty: This deepens the shallow trochlear groove and is indicated when trochlear dysplasia is high grade.
  • Patellar tendon imbrication: this shortens the long patellar tendon and pulls the kneecap into a better position with respect to the trochlear groove.
  • Femoral osteotomy: this can be done to correct valgus or anteversion.
  • Medial patellofemoral ligament reconstruction: In any case of patellar instability the MPFL is stretched out and incompetent. In addition to retensioning the MPFL, a tendon graft, typically from a donor, is used to augment the ligament and make it stronger that what is was before. This can be done when the underlying problem (trochlear dysplasia, for example) is more mild and doesn’t necessarily require correction. It is also done in conjunction with other procedures such as trochleoplasty to rebalance the incompetent ligament.
  • Lateral retinacular lengthening: In distinction from the MPFL, the lateral retinaculum becomes tight in the setting of patellar instability. This collection of ligaments is lengthened to provide balance on both sides of the patella. This is always done in conjunction with a MPFL reconstruction.

  • American Orthopaedic Society for Sports Medicine
  • Arthroscopic Association of North America
  • posna
  • prism
  • Andrew Inst Child Health