Lateral Compartment Muscles (Fibular Muscles)
The lateral compartment contains the fibular (peroneal) muscles, which perform several vital biomechanical tasks. Their primary functions include plantarflexion and eversion of the foot. Additionally, these muscles are responsible for lowering the medial border of the foot and play a critical role in supporting its transverse arch.
Innervation of the lateral compartment is provided by the superficial fibular nerve (n. peroneus superficialis or n. fibularis superficialis), with fibers originating from spinal segments L₄–S₁.
Blood supply is delivered by branches of the fibular artery (a. fibularis or a. peronea), as well as the anterior tibial artery (a. tibialis anterior).
Fibularis Tertius Muscle
The fibularis tertius muscle (m. fibularis tertius or m. peroneus tertius) is an important anatomical structure that classical anatomy often considers part of the extensor digitorum longus (m. extensor digitorum longus) rather than a completely independent unit.
- Origin: The muscle originates from the distal (inferior) part of the fibula and the interosseous membrane of the leg.
- Insertion: The tendon securely attaches to the base of the fifth metatarsal bone.
- Function: Unlike the main lateral compartment muscles, this muscle dorsiflexes the foot while everting it and elevating the lateral border.
- Innervation: Provided by the deep fibular nerve (n. fibularis profundus or n. peroneus profundus; segments L₄–S₁).
- Blood supply: Derived from the fibular and anterior tibial arterial basins (aa. fibulares, tibiales anteriores).
Fascial Structures and Tendon Retinacula
Proper function of the leg and foot muscles requires secure anchoring of their long tendons. This biomechanical role is fulfilled by specialized fascial thickenings known as retinacula (retinacula).
- Anterior surface of the ankle: Contains the superior and inferior extensor retinacula (retinacula mm. extensorum superius et inferius). They hold the extensor tendons in place, preventing bowstringing during contraction.
- Lateral surface: The tendons of the fibular muscles are held by the superior and inferior fibular retinacula (retinacula mm. fibularum/peroneorum superius et inferius).
- Medial surface: Contains the flexor retinaculum (retinaculum mm. flexorum), which stabilizes the flexor tendons.
Topographical Anatomy and Clinical Correlations
Understanding leg topography is critical for mastering clinical syndromes and physical examinations.
Key anatomical landmarks:
- Crural canal (cruropopliteal canal, canalis cruropopliteus): Located between the gastrocnemius (m. gastrocnemius) and soleus (m. soleus) muscles. Within this canal run the posterior tibial artery and vein (a. et v. tibialis posterior) and the tibial nerve (n. tibialis).
- Nerve branching: In the space between the tibialis posterior (m. tibialis posterior) and flexor digitorum longus (m. flexor digitorum longus), the tibial nerve divides, giving off branches that form the medial and lateral plantar nerves (n. plantaris medialis et lateralis).
- Pulse points: The popliteal artery pulse (a. poplitea) can be palpated in the popliteal fossa, and the dorsalis pedis artery pulse (a. dorsalis pedis) on the dorsum of the foot.
Clinical correlations:
- Foot drop: This characteristic symptom occurs due to deep fibular nerve injury (n. fibularis profundus), resulting in a loss of dorsiflexion.
- Tarsal tunnel syndrome: Develops when the posterior tibial artery (a. tibialis posterior) and tibial nerve are compressed within the tight space posterior to the medial malleolus.
- Tendon injuries: The Achilles tendon is the strongest structure in this region, yet it remains vulnerable to ruptures. Injury most commonly occurs during sudden jumps or forceful heel impacts on hard surfaces.