Lateral Compartment

Introduction

Introduction to the Lateral Compartment of the Leg

The lateral compartment of the leg, also known as the peroneal or evertor compartment, is one of the four osteofascial compartments of the leg. It is located on the lateral aspect of the fibula, bounded by the anterior and posterior intermuscular septa and the deep fascia. It contains two muscles: fibularis (peroneus) longus and fibularis (peroneus) brevis. These muscles are the primary evertors of the foot and weak plantar flexors. The compartment is innervated by the superficial fibular (peroneal) nerve (L5, S1, S2) and receives its blood supply from branches of the fibular artery and the anterior tibial artery. Due to its tight fascial boundaries, it is also susceptible to compartment syndrome, though less commonly than the anterior compartment. The superficial fibular nerve is at risk during surgical approaches to the fibula, and entrapment can occur where it pierces the deep fascia.

Study

1. Boundaries

The lateral compartment is bounded medially by the lateral surface of the fibula. Anteriorly, it is separated from the anterior compartment by the anterior intermuscular septum. Posteriorly, it is separated from the superficial posterior compartment by the posterior intermuscular septum. Laterally, it is enclosed by the deep (crural) fascia of the leg. Superiorly, the compartment is closed by the attachment of fibularis longus to the fibular head, and inferiorly, it continues behind the lateral malleolus under the superior and inferior peroneal retinacula.

2. Muscles

Two muscles are located in the lateral compartment, both innervated by the superficial fibular nerve (L5, S1, S2). Fibularis longus: Origin – head and upper two-thirds of lateral surface of fibula, and occasionally the lateral tibial condyle. Its tendon passes posterior to the lateral malleolus, grooves the cuboid bone, and crosses the sole obliquely to insert into the base of the first metatarsal and the medial cuneiform. Action – eversion and weak plantar flexion of the foot; supports the transverse and lateral longitudinal arches. Fibularis brevis: Origin – lower two-thirds of lateral surface of fibula. Its tendon passes posterior to the lateral malleolus with fibularis longus and inserts into the tuberosity at the base of the fifth metatarsal. Action – eversion and weak plantar flexion. It lies deep to fibularis longus.

Both muscles act as evertors, working in synergy with the tibialis anterior and tibialis posterior to maintain balanced inversion-eversion and support the foot arches. They are active during the stance phase of gait to stabilise the foot.

3. Neurovascular Supply

The superficial fibular nerve (L5, S1, S2) arises from the common fibular nerve in the lateral compartment. It descends between fibularis longus and brevis, supplying both muscles. In the distal third of the leg, it pierces the deep fascia to become subcutaneous and divides into medial and intermediate dorsal cutaneous nerves, supplying the dorsum of the foot (except the first web space, which is supplied by the deep fibular nerve).

The blood supply to the compartment comes mainly from muscular branches of the fibular artery (a branch of the posterior tibial artery) that pierce the posterior intermuscular septum. The anterior tibial artery also contributes via branches. There is no major named artery running within the lateral compartment itself.

4. Clinical Relevance

Superficial fibular nerve entrapment: As the nerve pierces the deep fascia in the distal leg, it can become compressed by fascial defects, direct trauma, or ankle sprains. This causes pain, paraesthesia, and numbness over the dorsum of the foot (excluding the first web space). Surgical decompression may be required.

Fibularis tendon subluxation/dislocation: The tendons of fibularis longus and brevis can dislocate from behind the lateral malleolus, usually due to rupture of the superior peroneal retinaculum during an inversion injury. Patients feel a snapping sensation and pain posterior to the lateral malleolus. Surgical repair of the retinaculum is often necessary.

Lateral compartment syndrome: Acute compartment syndrome can occur due to fractures, crush injuries, or vigorous exercise. Pain is worsened by passive inversion of the foot (stretching the evertors). Sensory loss over the dorsum of the foot is an early sign due to superficial fibular nerve ischemia. Fasciotomy is required.

Common fibular nerve injury: Since the superficial fibular nerve is a branch, a common fibular nerve lesion at the fibular neck will also paralyse the lateral compartment muscles, resulting in loss of eversion in addition to foot drop. This leads to an inverted and plantar-flexed foot (equinovarus deformity).

Avulsion fracture of the fifth metatarsal base: Sudden forceful inversion of the foot can avulse the insertion of fibularis brevis, resulting in a fracture at the tuberosity of the fifth metatarsal. This is known as a pseudo-Jones fracture if at the metaphyseal-diaphyseal junction.

Summary

Summary of the Lateral Compartment of the Leg

The lateral compartment contains two muscles, fibularis longus and brevis, innervated by the superficial fibular nerve (L5–S2). They are the primary evertors of the foot and assist in plantar flexion. The compartment is bounded by the fibula, anterior and posterior intermuscular septa, and crural fascia. Blood supply is from perforating branches of the fibular artery. Clinically, the superficial fibular nerve can be entrapped, tendons can dislocate, and the compartment is susceptible to syndrome. Avulsion of the fifth metatarsal base is a common injury.