Patellar Instability

Patellar Instability

Patella dislocations or patellar instability may be traumatic in nature or secondary to a congenital malalignment. Patellar instability occurs most commonly in the teens and twenties age group, with females more prone to malalignment related causes. Three anatomic characteristics that lead to malalignment or an increased Q angle include: femoral anteversion, genu valgus and external tibial torsion. Other risk factors include disorders with generalized ligamentous laxity, such as Ehlers-Danlos syndrome.

Anatomy and Pathophysiology

Stability of the patella occurs via passive (the MPFL and the patella-trochlea relationship) and dynamic (vastus medialis/VMO) mechanisms. The MPFL inserts on the femur between the medial epicondyle and the adductor tubercle and is the most common site of MPFL avulsion. It is the primary restraint of the patella from straightening to 20 degrees of bending. The patella glides in the trochlear groove, however, trochlea morphology varies, and a shallower groove provides less static stability.

Physical Examination and Work-up

On examination, patients often report instability and anterior knee pain. If there has been a recent patella dislocation, the knee is often swollen and tender medially along the MPFL. Other exam findings may include:

  • Patellar apprehension: guarding and a feeling of apprehension when passively gliding the patella laterally
  • J-sign: excessive lateral gliding of the patella when the knee is in extension

Quadrant testing:

  • Midline patella = 0
  • Normal motion is < 2 quadrants of patella translation
  • Lateral translation of the medial patellar border to lateral edge of trochlear groove = 2


X-rays are utilized to rule out patella fractures or loose bodies, assess alignment, patella position and trochlear dysplasia (groove morphology). CT scans assess the TT-TG distance (the distance between 2 perpendicular lines from the posterior cortex to the tibial tubercle and the trochlear groove). A TT-TG >20mm is considered abnormal. MRI can assess for injury to the cartilage, an MPFL tear and bony bruising.

Management of Patellar Instability

Physical therapy, activity modification, NSAIDs and bracing are the mainstay treatment for a first-time dislocation. Exceptions include the presence of loose bodies or intra-articular damage. Physical therapy focuses on strengthening of the quad, core and hip/gluteal musculature. MPFL repair is performed in the setting of a dislocation with and MPFL avulsion and sizable bony fragment. MPFL reconstruction is performed with autograft (or allograft) in those with repeated instability episodes. A concurrent tibial osteotomy may be recommended to correct significant malalignment based on the TT-TG value. Post-operative rehabilitation is closely monitored and guided with criterion-based advancement based on regular exercise testing intervals at the COSMO Fit Lab.