Lower Leg — Surgical Anatomy and Approaches

Introduction

The lower leg, extending from the knee to the ankle, contains the tibia and fibula and the four muscular compartments that house the muscles of dorsiflexion, plantar flexion, inversion, and eversion of the foot. The surgical anatomy is dominated by the four compartments (anterior, lateral, superficial posterior, deep posterior), the relatively superficial position of the anteromedial tibial cortex with its limited soft-tissue coverage, and the regional vascular and neural structures (anterior tibial artery and deep peroneal nerve anteriorly, posterior tibial artery and tibial nerve posteriorly, superficial peroneal nerve in the lateral compartment). The principal surgical approaches — anteromedial for the tibial shaft, posterolateral for the fibula, and the fasciotomy approaches for compartment syndrome — together provide access to the bony and neurovascular structures of the leg. This chapter draws on Orthopaedic Surgical Approaches, Netter’s Concise Orthopaedic Anatomy, and Gray’s Anatomy.

Bony Anatomy

The tibia is the principal weight-bearing bone of the lower leg, with the anatomical features detailed in Topic Trauma-27 (tibial shaft) and Topic Trauma-28 (ankle): triangular cross-section with anteromedial subcutaneous surface, isthmus in mid-shaft, and characteristic anterior bow. The fibula is the slender lateral bone, articulating with the tibia at the proximal tibiofibular joint and at the distal tibiofibular syndesmosis, forming the lateral wall of the ankle mortise distally.

Muscular Compartments

The leg compartments are four: Anterior compartment: Contains the tibialis anterior, extensor hallucis longus (EHL), extensor digitorum longus (EDL), and peroneus tertius. Functions: ankle dorsiflexion and toe extension. Innervated by the deep peroneal nerve. Vascular supply: anterior tibial artery. Lateral compartment: Contains the peroneus longus and peroneus brevis. Functions: foot eversion and assistance with plantar flexion. Innervated by the superficial peroneal nerve. Vascular supply: branches of the peroneal artery. Superficial posterior compartment: Contains the gastrocnemius, soleus, and plantaris. Functions: ankle plantar flexion. Innervated by the tibial nerve. Vascular supply: branches of the posterior tibial artery. Deep posterior compartment: Contains the tibialis posterior, flexor digitorum longus (FDL), and flexor hallucis longus (FHL). Functions: foot inversion, toe flexion, ankle plantar flexion. Innervated by the tibial nerve. Vascular supply: posterior tibial artery.

The compartments are separated by intermuscular septa attached to the bones and the interosseous membrane.

Neurovascular Anatomy

Anterior tibial artery and deep peroneal nerve: Enter the anterior compartment by passing through the proximal interosseous membrane (the artery is a branch of the popliteal artery; the nerve is a branch of the common peroneal nerve). Descend along the anterior interosseous membrane to the ankle, where they emerge anterior to the ankle to enter the foot as the dorsalis pedis vessels and the deep peroneal nerve (supplying the first webspace sensation). Superficial peroneal nerve: Branches from the common peroneal nerve and descends in the lateral compartment, supplying the peroneal muscles, then becomes subcutaneous at the junction of the middle and distal thirds of the leg, providing sensation to most of the dorsum of the foot. Posterior tibial artery and tibial nerve: Enter the deep posterior compartment by passing through the proximal tibial-fibular interosseous space. The nerve and artery descend along the posterior aspect of the deep posterior compartment to the medial malleolus, where they enter the foot deep to the flexor retinaculum (in the tarsal tunnel). Sural nerve: Forms in the popliteal fossa from contributions of the tibial nerve (medial sural cutaneous) and the common peroneal nerve (lateral sural cutaneous). The sural nerve runs subcutaneously down the posterolateral leg, providing sensation to the lateral foot. The sural nerve is the classical donor nerve for nerve grafting (Topic Trauma-31) because of its minimal sensory deficit when harvested. Saphenous vein and nerve: The great saphenous vein runs subcutaneously up the medial aspect of the leg from the medial malleolus to the femoral triangle. The saphenous nerve (a branch of the femoral nerve) accompanies the vein in the distal leg, providing sensation to the medial leg.

Tibial Shaft Approaches

The anteromedial approach to the tibial shaft uses the subcutaneous medial surface of the tibia. An incision approximately 1 cm lateral to the anterior tibial crest provides access; the skin and subcutaneous tissue are incised, and the periosteum is opened to expose the bone. The approach is straightforward but has the disadvantage of a thin soft-tissue envelope over the tibia, with high rates of wound healing problems and infection. The approach is used for plate fixation of tibial shaft fractures (with caution about soft-tissue tension), biopsy, and bone grafting. The anterolateral approach through the interval between the anterior compartment muscles and the lateral compartment provides access to the lateral aspect of the tibia and the upper fibula. The superficial peroneal nerve is encountered in the lateral compartment and must be protected.

The posterolateral approach through the interval between the lateral and posterior compartments provides access to the posterior aspect of the tibia and the fibula. The peroneal artery is at risk. The suprapatellar approach to tibial intramedullary nailing (Topic Trauma-27) has gained popularity for ease of fracture reduction and reduced anterior knee pain compared with the traditional infrapatellar approach.

Fibular Approaches

The lateral approach to the distal fibula is the standard for ORIF of ankle fractures (Topic Trauma-28). A longitudinal lateral incision over the fibula provides direct access. The superficial peroneal nerve crosses the operative field at the junction of the middle and distal thirds of the leg, becoming subcutaneous; it must be identified and protected. The lateral approach to the fibular shaft continues the same approach proximally. The fibula is exposed through subperiosteal dissection. The proximal fibular approach (for fibular head and neck pathology, including the proximal fibular fracture in the Maisonneuve variant of ankle fracture) uses a posterolateral incision behind the fibular head. The common peroneal nerve wraps around the fibular neck and is at substantial risk; its identification and protection are critical.

Fasciotomy of the Leg

The leg is the most common site of compartment syndrome, and leg fasciotomy is one of the most commonly performed orthopedic emergency procedures. The double-incision technique (Mubarak) is the standard: Lateral Incision A longitudinal incision along the lateral leg (approximately 2 cm anterior to the fibular shaft) provides access to the anterior and lateral compartments. The fascia of each compartment is opened along its length. The anterior compartment is released through the lateral aspect of the incision; the lateral compartment is released through the medial aspect. The superficial peroneal nerve crossing the operative field at the junction of the middle and distal thirds is identified and protected. Medial Incision A longitudinal incision along the medial leg (approximately 2 cm posterior to the medial tibial border) provides access to the superficial and deep posterior compartments. The fascia of the superficial posterior compartment is opened. The soleus is detached from the medial tibia to provide access to the deep posterior compartment, and its fascia is opened. The saphenous vein and nerve along the medial leg are identified and protected.

Post-Fasciotomy Management The wounds are left open with sterile dressings (or VAC therapy), with delayed primary closure or skin grafting at 5 to 10 days as discussed in Topic Trauma-32.

Approach to the Tibial Nerve and Posterior Tibial Artery

The tibial nerve and posterior tibial artery in the leg are accessed through a posteromedial approach. A longitudinal incision along the posteromedial leg provides access; the fascia is opened, and the soleus is reflected to expose the nerve and artery in the deep posterior compartment. The approach is used for vascular injury repair, decompression of the tibial nerve, and selected exposure of the deep posterior compartment. The tibial nerve at the ankle in the tarsal tunnel is approached through a curved incision posterior to the medial malleolus. The flexor retinaculum is incised, decompressing the nerve in the tarsal tunnel (tarsal tunnel release for tarsal tunnel syndrome).

Approach to the Common Peroneal Nerve

The common peroneal nerve around the fibular neck is approached through a longitudinal incision posterior to the fibular head, providing access to the nerve as it wraps around the bone. The approach is used for common peroneal nerve decompression, exploration of nerve injury, and operative management of peroneal nerve palsy. The nerve is identified and traced from the popliteal fossa around the fibular neck and into its bifurcation into the deep and superficial peroneal branches.

Summary and Take-Home Points

The lower leg contains four muscular compartments (anterior with dorsiflexors and deep peroneal nerve, lateral with peroneal muscles and superficial peroneal nerve, superficial posterior with gastrocsoleus and plantaris, deep posterior with deep flexors and tibialis posterior with the tibial nerve and posterior tibial artery). The anteromedial subcutaneous surface of the tibia is the principal landmark and is used for the anteromedial approach to the tibial shaft, with the caveat of thin soft-tissue coverage and substantial wound healing concerns. The fibular approaches use a lateral incision for the shaft (with the superficial peroneal nerve crossing the field) and a posterolateral incision for the proximal fibula (with the common peroneal nerve at substantial risk around the fibular neck). Leg fasciotomy using the Mubarak double-incision technique (lateral incision for anterior and lateral compartments; medial incision for superficial and deep posterior compartments) is the standard treatment for leg compartment syndrome — the most common compartment syndrome in the body. The principal neurovascular structures of the leg include the anterior tibial artery and deep peroneal nerve (anterior compartment), the peroneal artery (lateral and deep posterior compartments), the posterior tibial artery and tibial nerve (deep posterior

compartment), the superficial peroneal nerve (lateral compartment, becoming subcutaneous in the distal third), the sural nerve (subcutaneous posterolateral leg, the classical donor for nerve grafting), and the saphenous vein and nerve (subcutaneous medial leg). The chapter that follows turns to the ankle and foot, completing the lower extremity anatomical sequence.