The fibularis tertius muscle is located in the lower leg, within the anterior compartment. This compartment houses the structures responsible for lifting the foot. The muscle is notable for its variable presence among individuals and its close structural relationship with the extensor digitorum longus. Understanding its specific connections and function provides a detailed picture of the mechanics that govern ankle and foot movement. This article focuses on the anatomical details of the muscle, including its attachments and its role in locomotion.
Defining the Fibularis Tertius Muscle
The fibularis tertius muscle is situated in the anterior compartment of the leg, alongside the tibialis anterior and the toe extensors. This location differs from the other fibularis muscles (longus and brevis), which are typically found in the lateral compartment. It is sometimes referred to as the peroneus tertius, though fibularis is the preferred anatomical term.
Anatomically, the muscle is often considered a distinct lower slip of the extensor digitorum longus (EDL). Some anatomists describe the fibularis tertius as a “fifth tendon” of the EDL, highlighting their shared origin and close proximity. While it is present in the majority of people, its presence is not universal and varies across different populations.
The Proximal and Distal Attachment Points
The proximal attachment of the fibularis tertius is the distal portion of the lower leg bones. It originates from the distal one-third to one-half of the anterior surface of the fibula bone. Additional muscle fibers arise from the adjacent interosseous membrane, the connective tissue connecting the fibula and the tibia.
The muscle forms a tendon that travels downward, passing beneath the inferior extensor retinaculum of the ankle joint. Its distal attachment is the dorsal surface of the base of the fifth metatarsal bone. This insertion point, located on the bone leading to the little toe, is the defining structural feature separating it from the extensor digitorum longus. The tendon often fans out upon insertion, securing a broad connection to the bone.
Primary Role in Foot Movement
The specific attachments of the fibularis tertius result in two main actions at the ankle and foot. Its position anterior to the ankle joint makes it a primary contributor to dorsiflexion, the action of pulling the foot upward toward the shin. This movement is important for lifting the foot clear of the ground during the swing phase of walking.
The second action is eversion of the foot, which involves turning the sole outward, away from the body’s midline. The distal attachment on the fifth metatarsal allows it to pull the lateral side of the foot upward. The muscle receives its signals from the Deep Fibular Nerve, which innervates all muscles within the anterior compartment of the leg.
Clinical Significance of the Tendon
The fibularis tertius tendon plays a role in the dynamic stabilization of the ankle joint. Its function as a dorsiflexor and evertor helps prepare the foot for ground contact during running and walking. The tendon’s path and insertion point make it susceptible to injury, particularly in severe ankle sprains where it can rupture or sustain damage.
The tendon is sometimes used as a reliable anatomical landmark during ankle arthroscopy procedures due to its consistent anatomy. Surgeons also utilize the tendon as a source of tissue for grafting in reconstructive surgeries of the foot and ankle. The muscle’s insertion site on the fifth metatarsal base is prone to specific stress fractures, such as Jones fractures, which are influenced by the forces exerted by the muscle’s attachment.