Review: Fixed Patellar Dislocation

Congenital patellar dislocation is a rare condition characterized by a fixed, lateral dislocation of the patella that cannot be reduced without surgery.

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Phillip T. Grisdela, MD, orthopaedic resident in the Harvard Combined Orthopaedic Residency Program, Nikolaos Paschos, MD, PhD, pediatric surgeon in the Department of Orthopaedic Surgery at Massachusetts General Hospital, and Miho J. Tanaka, MD, orthopedic surgeon and director of the Women's Sports Medicine Program, recently reviewed the etiology, pathophysiology and management of congenital patellar dislocation in Clinics in Sports Medicine.

A Spectrum of Disease

According to the classical definition, congenital patellar dislocation has four features:

• A permanent lateral patellar dislocation
• Inability to actively extend the knee
• Preserved passive knee range of motion
• Absence of the patella from the trochlea since birth

Another proposed manifestation of congenital patellar dislocation, if present at birth, is habitual or "obligatory" dislocation. In this condition, the patella dislocates and relocates involuntarily as the knee moves from flexion to extension. It does not always require surgery.

In several cases they reported in Acta Orthopaedica Belgica, U.K. physicians observed patients with dysplasia of the patellofemoral joint that led to a fixed patellar dislocation years after birth. They suggest replacing the term "congenital patellar dislocation" with "developmental dysplasia and dislocation of the patella."

Recognition and Diagnosis

Diagnosis of fixed patellar dislocation is frequently delayed. This condition should be suspected in patients with delayed ambulation, genu valgum, flexion contracture of the knee, and external tibial rotation.

Weakened quadriceps function and altered gait are also common in fixed patellar dislocation. That's because the extensor mechanism dislocates posterior to the center of rotation of the knee, causing the quadriceps muscles to act as paradoxical drivers of knee flexion rather than knee extension.

Patients with fixed patellar dislocation may thus ambulate by flexing the hip of the affected leg in the stance phase of the good leg, tensioning the posterior capsule and ligamentous structures of the knee, and pushing their body weight forward to advance. However, as contracture worsens with age, they can ambulate with stability from collateral ligaments and flexion of the adductor musculature.

The review gives detailed information about history-taking, musculoskeletal examination, and imaging. Fixed patellar dislocation has been associated with multiple other congenital syndromes, listed in the paper, that should be considered at the time of evaluation.

Surgery

By definition, fixed patellar dislocation requires surgical management to reduce the patella, and the review mentions multiple procedures. In general, surgery focuses on releasing the contracted lateral structures, medial soft tissue stabilization, quadriceps lengthening, and transfer of the patellar tendon or tibial tubercle, depending on the patient's skeletal maturity.

Study results have been generally favorable; the most common complications are persistent flexion contracture and re-dislocation.

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