Miho J. Tanaka
Medial patellar instability is an uncommon form of patellar instability. The traditional surgical approaches to patellar instability aim to address lateral instability through medialization of the patella or reconstruction of the medial patellofemoral ligament. In the setting of prior lateral release, however, the patient can complain of medial instability, or, in some cases, both medial and lateral instability. Careful history taking and clinical examination can often elicit findings of this uncommon entity.
Anatomy and biomechanics of the lateral retinaculum
The lateral retinaculum consists of the lateral patellofemoral ligament, also termed the epicondylopatellar ligament, which originates on the lateral epicondyle and inserts on the lateral patella. The patellotibial ligament is distal to this, and arises from the lateral tibia and attaches to the lateral aspect of the patella and patellar tendon. Interpretations vary on whether these ligaments arise from the quadriceps aponeurosis, iliotibial band fibers, or the capsule. The meniscopatellar ligament is another lateral structure that is often described as a condensation of capsular tissue from the inferolateral patella to the anterolateral meniscus, although this not always consistently identifiable.
Merican and colleagues noted in a biomechanical study that progressive lateral release can cause an increase in medial patellar translation, as well as a smaller increase in lateral translation. In this study, a capsular release led to a 16% reduction in medial stability at 0° and 20° of knee flexion. Marumoto noted increased medial translation with distal extension of the release, and several studies have noted symptomatic medial instability as a consequence of extension of the release proximally into the vastus lateralis.
Christoforakis and colleagues noted that the lateral retinaculum has a posteriorizing vector, which can aid in patellar stability and be compromised after release. In this study, the authors found lateral release increased lateral translation as well, with 16% to 19% decrease in force required to displace the patella 10 mm laterally at 0°, 10° and 20° of knee flexion.
Evaluation
When evaluating for patellar instability, the clinician should note the direction of the instability that the patient describes, as well as the location of the pain. As always, patellofemoral pain should be differentiated from instability. Any history of previous knee surgery, and particularly a history of lateral release, should be noted. As in cases of lateral instability, a history of recurrent effusions should raise the suspicion of a chondral lesion.
