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Medial Patellar Instability: Primary and Iatrogenic
Introduction
Acute patellar dislocation is a common knee injury in adolescents and young adults, with primary patellar dislocations occurring at a rate of 42 per 100,000 person-years. However, the vast majority of primary and recurrent dislocations are in a lateral direction, as a result of the resultant lateral force vector acting on the patella at the time of injury, as well as predisposing anatomical risk factors such as a lateralised tibial tuberosity, trochlear dysplasia and medial patellar restraint (medial patellofemoral ligament and medial quadriceps tendon femoral ligament) insufficiency. Non-iatrogenic medial patellar dislocations are rare when collagen and congenital neuromuscular disorders are excluded; there are very few reports in the literature. Medial patellar instability, however, is a recognised complication of a lateral retinacular release. With the exception of congenital neuromuscular cases, no reports in the literature have included definitive evidence of a locked medial dislocation and few have radiographic evidence of dislocation. As a result, it is difficult to distinguish between subluxation and dislocation, defined as partial or complete escape of the patella from the femoral trochlea . In this chapter we highlight what is understood about the unusual pathological condition of medial patellar instability, both in the noniatrogenic and iatrogenic settings.
Evaluation
Because medial instability is a rare and challenging diagnosis to make, a thorough history and physical examination are crucial. Patients may present with a history of instability, swelling and pain with squatting or stairs. The condition is disabling, presenting with kinesiophobia, depression, anxiety and pain. Adding to the confusion is the not uncommon report by patients that they saw their ‘knee cap on the inside of the knee’. In fact they were seeing the prominence of the medial femoral condyle as the patella was dislocated laterally. Documenting a history of prior knee procedures, especially lateral retinacular release, is crucial. Although patients with lateral instability typically have tenderness over the medial patellofemoral ligament and lateral trochlear ridge–lateral femoral condyle margin, patients with medial instability present with tenderness over the inferomedial patella and anteromedial femoral condyle. The patient may have medial patellar apprehension or increased medial patellar glide beyond one to two quadrants. Special tests such as the Fulkerson medial subluxation test may be positive in patients with medial instability; this test involves reproducing the patient’s symptoms by pressing the patellar medially, flexing the patient’s knee and releasing the patella. A gravity subluxation test as described by Nonweiler may also be positive. This test demonstrates medial patellar subluxation to gravity that persists with quadriceps contraction when the patient lies in the lateral decubitus position.
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