Plantar tendon sheath of fibularis longus muscle

The plantar tendon sheath of the fibularis longus muscle refers to the synovial sheath and associated fascial structures that envelop the distal portion of the fibularis (peroneus) longus tendon as it courses through the plantar aspect of the foot. After passing posterior to the lateral malleolus and traversing the lateral side of the calcaneus, the tendon enters the cuboid tunnel—a fibro-osseous canal formed by the groove for tendon of fibularis longus muscle of cuboid bone on the plantar surface of the cuboid bone and the long plantar ligament. Within this tunnel, the tendon is surrounded by a synovial sheath that facilitates gliding and reduces friction as the tendon changes direction sharply to cross the sole of the foot and insert onto the base of the first metatarsal and medial cuneiform.

The sheath is composed of an inner synovial layer and an outer fibrous layer. The synovial sheath extends from just proximal to the cuboid tunnel to the region of the tendon’s insertion, providing lubrication. The outer fibrous sheath is continuous with the long plantar ligament and the deep fascia of the foot, anchoring the tendon and maintaining its position during foot movements.