Anatomy of the Lower Limb

The lower limb is designed for weight-bearing, locomotion, and postural stability. Its bones are more massive, its joints more constrained, and its muscles larger than the upper limb equivalents — reflecting the demands of supporting the entire body weight against gravity. Lower limb anatomy is critical for orthopedic surgeons, physiotherapists, sports medicine physicians, and vascular surgeons. This guide is for educational purposes only.

## The Hip Joint

The hip joint (coxofemoral joint) is a ball-and-socket joint between the spherical femoral head and the cup-shaped acetabulum of the hip bone. The acetabulum is formed by contributions from the ilium (superior), ischium (posteroinferior), and pubis (anteroinferior), which fuse at the triradiate cartilage during adolescence. The acetabular labrum — a fibrocartilaginous rim deepening the socket — increases joint stability and contact area.

The hip is intrinsically more stable than the glenohumeral joint due to deeper bony congruence, stronger ligamentous support (the iliofemoral, pubofemoral, and ischiofemoral ligaments), and powerful surrounding musculature. The femoral head receives blood supply primarily via the medial circumflex femoral artery (a branch of the profunda femoris). This artery is at risk in femoral neck fractures, potentially causing avascular necrosis (osteonecrosis) of the femoral head — a devastating complication that may necessitate total hip arthroplasty.

The acetabular notch at the inferior margin of the acetabulum is bridged by the transverse acetabular ligament; the ligamentum teres (ligament of the head of femur) passes through this notch to attach to the fovea capitis of the femoral head and carries the artery to the head of the femur (a minor contributor to femoral head blood supply in adults but more important in children).

## Thigh Compartments

The thigh is divided into three compartments by fascia. The anterior compartment contains the quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius — innervated by the femoral nerve; the primary knee extensor) and the sartorius (innervated by the femoral nerve; flexes and laterally rotates the hip, flexes the knee). The medial compartment contains the hip adductors (adductor longus, brevis, and magnus; gracilis; pectineus; obturator externus), innervated mainly by the obturator nerve (the adductor hiatus in adductor magnus transmits the femoral artery to the popliteal fossa). The posterior compartment contains the hamstrings (biceps femoris, semitendinosus, semimembranosus — innervated by the tibial division of the sciatic nerve, except the short head of biceps which is innervated by the common fibular division).

The femoral triangle — bounded by the inguinal ligament superiorly, the medial border of sartorius laterally, and the medial border of adductor longus medially — contains from lateral to medial: the femoral nerve, femoral artery, femoral vein, and the femoral canal (containing lymphatics; the site of femoral hernias). The femoral canal lies medial to the femoral vein within the femoral sheath.

## Leg Muscles

The leg (between knee and ankle) contains four compartments. The anterior compartment (extensors and dorsiflexors — tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis/peroneus tertius) is innervated by the deep fibular (peroneal) nerve; compartment syndrome here causes foot drop. The lateral compartment (fibularis longus and brevis — evertors of the foot) is innervated by the superficial fibular nerve. The superficial posterior compartment (gastrocnemius and soleus, united as the triceps surae — the primary plantarflexors — and plantaris) inserts via the Achilles tendon into the calcaneus and is innervated by the tibial nerve. The deep posterior compartment (tibialis posterior, flexor digitorum longus, flexor hallucis longus) is also innervated by the tibial nerve.

The Achilles tendon is the strongest tendon in the body; rupture typically occurs in the "watershed" zone 2–6 cm above the calcaneal insertion, in middle-aged recreational athletes, and is detected by the Thompson test (absence of plantarflexion when the calf is squeezed).

## Foot Arches

The foot has three arches maintained by bony congruence, ligaments, and muscle activity. The medial longitudinal arch — the highest, running from the calcaneus through the talus, navicular, cuneiforms, and medial three metatarsals — is supported principally by the plantar calcaneonavicular (spring) ligament, the plantar fascia (plantar aponeurosis), and the tibialis posterior tendon. The lateral longitudinal arch is lower, comprising the calcaneus, cuboid, and lateral two metatarsals. The transverse arch runs across the midfoot and forefoot. Pes planus (flat foot) results from weakness or laxity of arch-supporting structures; pes cavus (high arch) is often neurogenic.

## Lumbosacral Plexus

The lower limb is innervated by the lumbar plexus (L1–L4, formed within the psoas major) and the sacral plexus (L4–S3, formed on the piriformis muscle on the posterior pelvic wall).

Key lumbar plexus branches: femoral nerve (L2–L4; anterior thigh, knee extension, sensation to medial leg and foot via saphenous nerve), obturator nerve (L2–L4; medial thigh adductors), lateral cutaneous nerve of the thigh (L2–L3; sensation to lateral thigh; compressed in meralgia paresthetica). Key sacral plexus branches: the sciatic nerve (L4–S3; the largest nerve in the body, exiting the pelvis through the greater sciatic foramen below the piriformis) divides in the posterior thigh into the tibial nerve (L4–S3; posterior compartments of leg, sole of foot) and common fibular (peroneal) nerve (L4–S2; anterior and lateral compartments, dorsum of foot). The common fibular nerve winds around the fibular neck — the most common site of lower limb nerve injury (from fibular fractures or prolonged compression against a hard surface).

## Arterial Supply

The femoral artery (continuation of the external iliac below the inguinal ligament) is the main artery of the lower limb. It gives off the profunda femoris (deep femoral artery), which supplies most of the thigh via perforating arteries. The femoral artery passes through the adductor canal and then the adductor hiatus to become the popliteal artery behind the knee, which gives off the superior and inferior genicular arteries (forming the genicular anastomosis around the knee) before dividing at the soleal arch into the anterior tibial artery (continuing as the dorsalis pedis artery on the dorsum of the foot) and the tibioperoneal trunk, which divides into the posterior tibial artery (palpable posterior to the medial malleolus) and the fibular (peroneal) artery.

## Clinical Correlations

Deep vein thrombosis (DVT) most commonly occurs in the calf veins and may propagate to the popliteal, femoral, and iliac veins, with risk of pulmonary embolism. Virchow's triad (venous stasis, hypercoagulability, endothelial injury) underlies DVT formation. Compartment syndrome of the leg is a surgical emergency — intracompartmental pressure above 30 mmHg (or within 30 mmHg of diastolic pressure) requires fasciotomy of all four compartments. Hip fractures in the elderly (typically femoral neck and intertrochanteric fractures) carry significant mortality and are a leading cause of disability; most require surgical fixation or arthroplasty.