Origin and Course
The deep fibular nerve arises from the division of the common fibular nerve (n. fibularis communis). This bifurcation occurs within the superior musculofibular canal, located 6–7 cm distal to the apex of the fibular head.
Course of the nerve:
- From the lateral compartment, it passes into the anterior compartment by piercing the anterior intermuscular septum of the leg (septum intermusculare anterius cruris) in the upper third of the leg.
- It descends along the anterior surface of the interosseous membrane.
- Initially, the nerve lies lateral to the anterior tibial vessels; in the lower third of the leg, it shifts to lie anterior and medial to them.
- As it crosses onto the dorsum of the foot, the nerve lies medial to the anterior tibial artery (dorsalis pedis artery).
Muscle Innervation
The deep fibular nerve provides motor innervation to the anterior compartment muscles of the leg and the intrinsic short extensors on the dorsal foot:
- Tibialis anterior (m. tibialis anterior) — originates from the lateral condyle of the tibia, interosseous membrane, and crural fascia; inserts into the medial cuneiform and base of the first metatarsal bone. It performs dorsiflexion and inversion of the foot, and supports the medial longitudinal arch. Innervated by L4–L5.
- Extensor digitorum longus (m. extensor digitorum longus) — originates from the lateral condyle of the tibia, fibula, and interosseous membrane; inserts into the middle and distal phalanges of digits II–V. Extends digits II–V and, together with the fibularis tertius, everts the foot (L4–S1).
- Extensor hallucis longus (m. extensor hallucis longus) — originates from the middle third of the fibula and interosseous membrane; inserts into the distal phalanx of the hallux. Extends the great toe, and assists with dorsiflexion and inversion of the foot (L4–S1).
- Fibularis tertius (m. peroneus tertius) — often considered part of the extensor digitorum longus. Originates from the distal fibula and inserts into the base of the fifth metatarsal bone. Dorsiflexes and everts the foot (L4–S1).
- Extensor digitorum brevis — receives its motor branch on the dorsum of the foot at the level of the malleoli. This branch travels alongside the lateral tarsal artery (a. tarsalis lateralis).
Cutaneous Innervation
The sensory distribution of the deep fibular nerve is very restricted. On the dorsum of the foot, it supplies sensation exclusively to the narrow skin cleft between the first and second toes (the first web space).
The rest of the skin on the dorsum of the foot is innervated by the superficial fibular nerve (n. fibularis superficialis), while the borders of the foot are supplied by the saphenous (n. saphenus) and sural (n. sural) nerves.
Clinical Significance
Damage or entrapment of the deep fibular nerve leads to functional deficits in its target musculature:
- Paresis (weakness) of the foot and toe extensors.
- Development of the characteristic clinical presentation of "foot drop".
- In mononeuropathies of the fibular nerve not related to direct trauma or traction, sensory loss may also be detected on the dorsal surface of the foot and the anterolateral leg.