Anterior Muscular Compartment of the Lower Leg

Front of the Leg Anatomy

• The tibia's subcutaneous surface is located on the medial side of the leg.
• The extensor muscular compartment is situated on the anterolateral side.
• The femoral nerve provides cutaneous nerves over the tibia.
• The common peroneal nerve provides cutaneous nerves over the extensor compartment.
• The saphenous nerve's infrapatellar branch supplies the upper tibia's periosteum and overlying skin.
• The saphenous nerve descends behind the great saphenous vein and passes in front of the medial malleolus, typically bifurcating 5-7 cm above it.
• The main nerve (anterior branch) of the saphenous nerve ends at the metatarsophalangeal joint (bunion region) on the medial side of the foot.
• The lateral cutaneous nerve of the calf, a branch of the common peroneal nerve, supplies deep fascia and skin over the upper extensor and peroneal compartments.
• The superficial peroneal nerve replaces the lateral cutaneous nerve over the rest of these surfaces.
• Subcutaneous fat attaches to the periosteum of the tibia's subcutaneous surface, which lacks deep fascia.
• The great saphenous vein and nerve lie in this fat, accompanied by lymphatic vessels ascending to the superficial inguinal nodes.
• The great saphenous vein has deep connections in this area.
• Perforating veins connect the great saphenous vein to deep veins of the calf along the medial side of the calf behind the medial border of the tibia.
• Perforating veins are usually located just below, just above, and about 10 cm above the medial malleolus, with another near the middle of the leg, and sometimes one below knee level.
• A constant perforator in the lower thigh joins the great saphenous vein (or its tributaries) to the femoral vein in the adductor canal.
• Lower perforators are often connected by the posterior arch vein, which typically joins the great saphenous vein below the knee.
• Some perforating veins in the leg join the venae comitantes of the posterior tibial artery, while others join the venous plexus deep to soleus.
• Valves in the perforating veins are directed inwards, found where they pierce the deep fascia and join the deep veins.
• Saphenous blood is largely transferred from superficial to deep veins through the perforators.
• Muscle contractions in the calf pump blood upwards in the deep veins.
• Incompetent valves in the perforators increase pressure in the saphenous system, leading to varicose veins.
• Some smaller perforators lack valves, allowing bidirectional blood flow.

Tendons at the Tibia

• The upper end of the tibia's subcutaneous surface receives tendons from three muscles: sartorius, gracilis, and semitendinosus.
• Sartorius is supplied by the femoral nerve.
• Gracilis is supplied by the obturator nerve.
• Semitendinosus is supplied by the sciatic nerve.
• The sartorius tendon is lateral, the gracilis is medial and the semitendonosus is posterior
• The tendons are separated by a bursa, the bursa anserina, located deep to the sartorius tendon.

Role of Muscles

• The text proposes that the sartorius, gracilis, and semitendonosus muscles act as 'guy ropes' to stabalize the bony pelvis.
• Sartorius flexes and rotates the hip and knee, but weakly.
• Gracilis weakly adducts the thigh.

Patellar Ligament and Bursae

• The patellar ligament inserts into a smooth area on the tibial tuberosity, located entirely on the epiphysis of the upper tibia.
• Patellar retinacula, fibrous expansions of the quadriceps femoris tendon, insert into the patella's edges and the tibial condyles' inferior borders.
• The prepatellar bursa lies in front of the patella.
• The superficial infrapatellar bursa lies in front of the patellar ligament.
• The deep infrapatellar bursa lies between the patellar ligament and the upper part of the tibia head.

Deep Fascia of the Leg

• The deep fascia of the leg covers muscles and attaches to periosteum where bone is subcutaneous.
• Below the tibial condyles, it encloses leg muscles and attaches to the tibia's anterior and posterior borders.
• Above the ankle, it attaches to the lateral malleolus and fibula's triangular subcutaneous area.
• Two intermuscular septa extend from its deep surface to the fibula, enclosing the peroneal compartment.
• The extensor compartment lies between the anterior intermuscular septum and the tibia.
• The flexor compartment (calf of the leg) lies between the posterior intermuscular septum and the tibia posteriorly.

Extensor Compartment

• The extensor compartment sits between the deep fascia and the interosseous membrane, bounded by the tibia and fibula's extensor surfaces and the anterior intermuscular septum.
• It contains tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius muscles, along with the deep peroneal nerve and anterior tibial vessels.
• The deep fascia attaches to the subcutaneous border of the fibula above the lateral malleolus.
• The superior extensor retinaculum is a thickening of the fascia in this region.
• The tibialis anterior tendon perforates the superior extensor retinaculum, creating a double attachment to the tibia.
• The tibialis anterior tendon has a synovial sheath that extends down to its insertion, while other extensor tendons lack synovial sheaths at the superior retinaculum level.

Tibialis Anterior

• This muscle has a spindle-shaped belly that moulds the anterior subcutaneous border of the tibia into a concavity.
• It originates from the upper two-thirds of the tibia's extensor surface, the interosseous membrane, and the overlying deep fascia.
• The anterior tibial recurrent artery pierces it.
• Fibers converge into a central tendon that pierces the superior extensor retinaculum, where it gains a synovial sheath.
• The tendon and sheath are secured by the inferior extensor retinaculum.
• The tendon inserts into a facet at the anteroinferior angle of the medial cuneiform and the adjacent first metatarsal bone.
• A small bursa separates the tendon from the facet's upper part.
• Tibialis anterior and peroneus longus have symmetrical insertions into opposite sides of the adjoining medial cuneiform and first metatarsal base.
• Tibialis anterior inserts mainly into the cuneiform, while peroneus longus inserts mainly into the metatarsal.
• Innervation is by the deep peroneal and recurrent genicular nerves (L4).
• Its action is combined dorsiflexion of the ankle joint and inversion of the foot.
• The muscle can be tested by dorsiflexing the foot against resistance.

Extensor Hallucis Longus

• This muscle arises from the middle two-fourths of the fibula and the adjacent interosseous membrane.
• It lies deep at its origin but emerges between tibialis anterior and extensor digitorum longus in the lower leg.
• It passes beneath the superior extensor retinaculum and is slung by the inferior extensor retinaculum, where it receives a separate synovial sheath.
• It inserts into the base of the terminal phalanx of the great toe along the medial side of the dorsum of the foot.
• It is innervated by the deep peroneal nerve (L5, S1).
• Its action is to dorsiflex (extend) the great toe, and secondarily dorsiflex the ankle.
• Dorsiflexion of the great toe 'winds up' the plantar aponeurosis around the metatarsal head, increasing the concavity of the medial longitudinal arch of the foot.
• The muscle can be tested by dorsiflexing the big toe against resistance.

Extensor Digitorum Longus

• This muscle arises from the upper three-quarters of the fibula's extensor surface and a small area of the tibia across the superior tibiofibular joint.
• It originates from the anterior intermuscular septum and the overlying deep fascia.
• It forms four tendons superior to the lower part of the tibia beneath the superior extensor retinaculum
• These tendons are slung by the inferior extensor retinaculum and enclosed with the peroneus tertius tendon in a shared synovial sheath.
• The four tendons diverge superficial to extensor digitorum brevis, just beneath the deep fascia on the dorsum of the foot.
• The tendons insert into the lateral four toes.
• The tendon divides into three slips over the proximal phalanx, with the central slip inserting into the base of the middle phalanx.
• The two side slips reunite after being joined by the tendons of the interossei and lumbricals and insert into the base of the distal phalanx.
• It is innervated by the deep peroneal nerve (L5, S1).
• Its action is to extend (dorsiflex) the lateral four toes.
• Testing involves dorsiflexing the four lateral toes against resistance.

Peroneus Tertius

• This muscle arises from the lower third of the fibula, below extensor digitorum longus.
• It is unipennate, with the tendon forming anteriorly, in contact with the superior extensor retinaculum.
• The tendon passes through the inferior retinaculum, sharing the synovial sheath of extensor digitorum longus.
• It inserts into the dorsum of the base of the fifth metatarsal bone and, by a falciform extension, into the superior surface of that bone.
• Innervation is by the deep peroneal nerve (L5, S1).
• Its action is to dorsiflex and evert the foot.
• It has good mechanical advantage in dorsiflexion because its tendon passes in front of the ankle's axis of movement and its insertion is far forward on the foot.

Deep Peroneal Nerve

• This nerve arises within peroneus longus, over the neck of the fibula, at the bifurcation of the common peroneal nerve.
• It spirals around the fibula's neck deep to extensor digitorum longus, reaching the interosseous membrane lateral to the anterior tibial vessels.
• It runs with the anterior tibial vessels between extensor digitorum longus and tibialis anterior in the upper extensor compartment.
• In the middle of the leg, the neurovascular bundle lies on the interosseous membrane between tibialis anterior and extensor hallucis longus.
• Extensor hallucis longus crosses the bundle, so the tendons of tibialis anterior and extensor hallucis longus are medial, and extensor digitorum longus and peroneus tertius are lateral.
• The deep peroneal nerve regains its position lateral to the vessels near the lower end of the tibia.
• The deep peroneal nerve supplies the four extensor compartment muscles and provides sensory innervation to the periosteum of the tibia and fibula's extensor surfaces.

Anterior Tibial Artery

• This artery forms at the popliteal artery's bifurcation in the calf and passes above the interosseous membrane to reach the extensor compartment.
• It lies closer to the fibula than the tibia, with a companion vein on each side.
• The fibular companion vein may leave a notch in the fibula visible on a radiograph.
• The artery runs vertically downwards on the interosseous membrane and crosses the lower tibia at the front of the ankle joint, midway between the malleoli, becoming the dorsalis pedis artery.
• It gives off a recurrent branch, which pierces tibialis anterior, to the arterial anastomosis around the upper end of the tibia.
• It supplies the extensor compartment muscles and gives malleolar branches to both malleolar regions.
• Tibialis anterior lies medial to the artery throughout its course.
• Extensor digitorum longus and peroneus tertius lie lateral to it throughout.
• Extensor hallucis longus crosses it from the fibular to tibial sides.
• The deep peroneal nerve reaches it from the lateral side, runs in front of it in the middle of the leg, and returns to its lateral side below.
• The anterior tibial veins run on each side of the artery, anastomosing by cross channels.

Tibiofibular Joints

• The fibula articulates with the femur in the embryo, but differential growth results in a separate tibiofibular articulation.
• The superior tibiofibular joint is a synovial joint with a capsule and synovial membrane.
• The inferior ends of the tibia and fibula are strongly bound by ligaments, forming a fibrous joint.
• The interosseous membrane connects the interosseous borders of the tibia and fibula.
• The fibular malleolus maintains contact with the talus in all ankle joint positions.
• Ankle joint dorsiflexion and plantar flexion produce fibula rotation around its axis due to the talus' lateral surface shape.
• The superior tibiofibular joint surfaces are nearly horizontal to allow rotation. Other times the facets are vertical.
• The superior tibiofibular joint capsule's fibers thicken anteriorly and posteriorly, sloping downwards from the tibia to the fibula.
• The joint cavity may communicate posteriorly with the bursa under the popliteus tendon and the knee joint.
• Interosseous membrane fibers slope steeply from the tibia down to the fibula.
• The interosseus membrane resists downward movement of the fibula caused by powerful fibular muscles.
• Biceps femoris is the only muscle exerting an upward pull on the fibula.
• The inferior tibiofibular joint is a fibrous joint, with the bones strongly bound by the interosseous tibiofibular ligament.

Overview of the anterior muscular compartment located on the anterolateral side of the lower leg. Innervation details include the infrapatellar branch of the saphenous nerve and the lateral cutaneous nerve of the calf. Also covered are perforating veins and their role in venous pressure.