Common tendon sheath of fibularis muscles

The common tendon sheath of the fibularis muscles (Peroneal tendon sheath) is a synovial-lined structure that envelops the tendons of the fibularis longus and fibularis brevis as they pass posterior to the lateral malleolus. The sheath begins proximally at the level of the lateral malleolus and extends distally along the retromalleolar groove of the fibula, which is consistently concave and forms the osseous floor of the sheath.

The sheath is divided into two compartments by a septum: the fibularis brevis tendon lies anteriorly and medially, while the fibularis longus tendon is posterior and lateral. The sheath provides a lubricated environment for tendon gliding and is reinforced by the superior peroneal retinaculum, which is closely associated with the aponeurotic fibers of the posterior fibulotalocalcaneal ligament complex. Notably, the origin of this ligament complex forms the floor of the peroneal tendon sheath, contributing to its structural integrity.

Variants such as accessory muscles (e.g., peroneus quartus) or split tendons may also be present within the sheath, which is clinically relevant for imaging and surgical approaches. The sheath is essential for tendon stability and function, and its disruption can predispose to peroneal tendon subluxation or tendinopathy.