Intrasheath Instability of the Peroneal Tendons: Dynamic Ultrasound Imaging.

Peroneal intrasheath instability is a painful snapping condition of the lateral ankle. It consists of a transient retromalleolar subluxation of the peroneal tendons without disruption of the superior peroneal retinaculum or evidence of a previous definite trauma. In type A intrasheath instability, the peroneus longus and brevis tendons are intact, and there is an intertendinous intrasheath switch. In type B intrasheath instability, the peroneus brevis tendon has a longitudinal split tear through which the peroneus longus subluxates. Both types can be missed on a physical examination because there is no displacement of the peroneal tendons over the lateral malleolus. Dynamic ultrasound is the imaging modality of choice for evaluating retromalleolar subluxation of the peroneal tendons. This review article aims to provide an overview of the anatomic basis for peroneal intrasheath instability and provide physicians with guidelines for its ultrasound assessment.