Anatomy Lower Limb Lecture for NRE Step 1 ||PMC Pakistan Medical Commission || Dr Sadiq Ali

Anatomy of the Lower Limb: Overview & Key Concepts ​The lower limb is specialized for weight-bearing, locomotion, and maintaining posture. It is anatomically divided into the gluteal region, thigh, leg (crus), and foot. For medical licensing examinations like the NRE and USMLE Step 1, a strong command of compartmental organization, nerve lesions, and vascular supply is essential. ​1. Compartments of the Lower Limb ​Muscles, nerves, and vessels within the thigh and leg are enclosed by deep fascia and divided into functional compartments separated by intermuscular septa: ​Thigh Compartments: ​Anterior (Extensor) Compartment: Contains the quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) and sartorius. Innervation: Femoral nerve (L2-L4). Action: Knee extension and hip flexion. ​Medial (Adductor) Compartment: Contains the adductor longus, adductor brevis, adductor magnus, gracilis, and obturator externus. Innervation: Obturator nerve (L2-L4) (with the hamstring part of adductor magnus receiving tibial nerve innervation). Action: Adduction of the thigh. ​Posterior (Flexor / Hamstring) Compartment: Contains the semitendinosus, semimembranosus, and biceps femoris. Innervation: Tibial division of the sciatic nerve (L5-S3) (except the short head of biceps femoris, which is supplied by the common fibular division). Action: Knee flexion and hip extension. ​Leg (Crus) Compartments: ​Anterior (Extensor) Compartment: Contains the tibialis anterior, extensor digitorum longus, extensor hallucis longus, and fibularis tertius. Innervation: Deep fibular (peroneal) nerve. Action: Dorsiflexion of the ankle and toe extension. ​Lateral (Evertor) Compartment: Contains the fibularis (peroneal) longus and brevis. Innervation: Superficial fibular (peroneal) nerve. Action: Eversion of the foot. ​Posterior Compartment: Divided into superficial and deep layers by the transverse intermuscular septum. ​Superficial Layer: Gastrocnemius, soleus, and plantaris. Innervation: Tibial nerve. Action: Plantarflexion of the ankle. ​Deep Layer: Popliteus, flexor digitorum longus, flexor hallucis longus, and tibialis posterior. Innervation: Tibial nerve. Action: Toe flexion and ankle plantarflexion. ​2. Major Nerves & Clinical Correlates (Step 1 Focus) ​Sciatic Nerve Injury: Can be injured by posterior hip dislocations or improper gluteal intramuscular injections. Results in deficits in knee flexion and all movements below the knee (foot drop and sensory loss in the leg and foot). ​Common Fibular (Peroneal) Nerve Injury: Highly vulnerable to injury as it winds around the fibular neck (e.g., from a tight cast, leg crossing, or fibular neck fracture). Results in foot drop (inability to dorsiflex due to loss of the anterior compartment) and loss of eversion (lateral compartment), along with a high-stepping gait. ​Superior Gluteal Nerve Injury: Innervates the gluteus medius and minimus (hip abductors). Injury (often during pelvic surgery or intragluteal injections in the wrong quadrant) leads to a Trendelenburg sign: when the patient stands on the affected limb, the contralateral pelvis drops because the weakened abductors cannot stabilize the hip. ​Femoral Nerve Injury: Can be caused by pelvic surgery, retroperitoneal hematoma, or compression. Results in impaired knee extension and loss of the patellar reflex, alongside sensory loss over the anterior thigh and medial leg (saphenous nerve distribution).