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Patella/Knee Cap Instability

Kneecap Anatomy

The patellofemoral joint is made of the kneecap and the femur, the thigh bone. The kneecap is a triangular shaped bone located on the front of the knee joint and performs an important role in stabilizing the knee during bending and straightening. It also protects the front of the joint from physical trauma.

The patella fits in a groove on the femur and slides up and down in the groove to allow the knee to bend and straighten (tracking). It is held in place by the strong ligaments that keep it centered over the joint and assist in knee movement. When the kneecap slips out of its track it can cause pain, stiffness, and joint instability.

Patellar instability due to subluxation or dislocation is a commonly recurring condition. It is common in both adolescent females ages 10-16 and some adults. It is often caused by a fall or sports-related injury like a noncontact twisting injury that forces the patella out of its track either partially (subluxation) or completely (dislocation).

When the kneecap comes out of its track sometimes it can resume its normal position on its own. When it dislocates this may require repositioning by an orthopaedic surgeon. When painful dislocations occur repeatedly this is called chronic patellar instability. Chronic instability is a disabling condition that causes significant discomfort and impacts the ability to participate in physical activities. Eventually, pain and functional decline can lead to osteoarthritis.

Acute dislocation is often caused by trauma from a sport, a fall, or a direct impact to the knee that forces the kneecap out of its groove. The risk of another dislocation after initial injury or dislocation is high.

Patellar instability is caused by repeated dislocations and subluxations, not trauma. The cause is a preexisting condition such as chronic ligament laxity caused by lax muscles and ligaments, a shallow femoral groove, weak quadriceps muscles and tight hip flexors or a combination.

Acute patellar dislocation from trauma produces pain, swelling, stiffness, and joint deformity.

Chronic kneecap instability symptoms include:

  • knee pain with activity
  • swelling and bruising
  • joint stiffness
  • pain when standing from a seated position
  • knee buckling and catching
  • a creaking sensation with movement
  • trouble walking
  • repeated episodes of instability including subluxation and dislocation. 

During your orthopaedic evaluation Dr. Petrigliano will review your medical history to differentiate between acute and chronic instability and rule out other causes of knee pain including osteoarthritis and tendonitis, bone and cartilage abnormalities or fracture.

A physical exam will include assessment of bone alignment, patella alignment and tracking, and ligament looseness.  He will order x-rays and a CT scan to detect abnormal tracking, subluxation, and anatomical problems. An MRI can provide good images of the soft tissues and cartilage defects.

Conservative management

Conservative management is usually reserved for a first-time dislocation, but recurrence of instability is high particularly in young athletic individuals.  Patients with a history of two or more dislocations have a 50% chance of recurrent dislocations and persistent patellar instability for life.


Surgical options depend on the cause of instability and the severity of the condition. Knee arthroscopy can help diagnose the causes, which may be able to be treated during arthroscopy. Repeated dislocations can damage the cartilage at the ends of the bones. Arthroscopic debridement is a procedure to remove loose cartilage fragments. Lateral release is an arthroscopic procedure to release tight ligaments that pull the kneecap out of its track to correct subluxations and eliminate pain.

When a ligament tear or laxity is the problem, the treatment may involve ligament reconstruction to restore stability. If the bones are malaligned a procedure called an osteotomy may be necessary to restore normal anatomy and function to prevent further dislocations. In all cases, rehabilitation is vital to strengthen the muscles that support the joint and restore normal function.

Why choose Dr. Petrigliano?

Dr. Frank Petrigliano is an orthopaedic surgeon who was fellowship trained in sports medicine and shoulder surgery at the prestigious Hospital for Special Surgery where he provided care to athletes of all ages. He currently serves as the head team physician for the LA Kings hockey team and associate team physician for USC Athletics. Dr. Petrigliano is a renowned orthopaedic surgeon and researcher who employs state of the art treatments and procedures to get you back to your active life and back to sport. He always treats his patients with compassion and respect. Dr. Petrigliano is located in El Segundo California, and serves greater Los Angeles, Beverly Hills, the South Bay, and the Santa Clarita Valley. Contact Dr. Petrigliano to schedule a consultation today.

At a Glance

Dr. Frank Petrigliano

  • Associate Professor of Orthopaedic Surgery at USC
  • Chief of the Epstein Family Center for Sports Medicine
  • Team physician for the LA Kings and USC Athletics
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