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Anatomy 3% exam weight

Lower Limb Anatomy

Part of the INI CET (AIIMS PG) study roadmap. Anatomy topic anatom-006 of Anatomy.

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Lower Limb Anatomy — Bones, Joints, Muscles, Nerves and Clinical Correlations

Quick Review

The lower limb is a favourite examination topic for INI CET — questions frequently test knowledge of the hip joint, knee joint, lumbosacral plexus, and nerve injuries. Focus on the femoral triangle, popliteal fossa, ankle mortise, and the nerve supply to lower limb compartments.

High-Yield Facts for INI CET:

  • Femoral triangle: Inguinal ligament (medial), sartorius (lateral), adductor longus (floor); contents: femoral nerve, artery, and lymphatics (“VAN” from lateral to medial)
  • Sciatic nerve (L4-S3): Supplies posterior thigh and entire leg/foot via tibial and common peroneal nerves; enters gluteal region below piriformis
  • Common peroneal nerve: Wraps around fibular neck → foot drop (loss of dorsiflexion = steppage gait); sensory loss over lateral leg and dorsum of foot
  • Tibial nerve: Supplies posterior compartment of leg (plantarflexion) and sole of foot; damage causes loss of plantarflexion

Exam tip: Foot drop (inability to dorsiflex) indicates common peroneal nerve injury. For hip fractures, note that the femoral neck is intracapsular — blood supply to the femoral head enters via the retinacular arteries which are branches of the medial circumflex femoral artery; disruption causes avascular necrosis of the femoral head.

The Hip Joint

Anatomy

The hip joint is a ball and socket synovial joint where the depth of the socket is increased by a fibrocartilaginous labrum. The acetabulum is formed by the ilium, ischium, and pubis with a lunate surface (articular cartilage) that forms an incomplete circle — the acetabular notch (missing inferiorly) leaves the transverse acetabular ligament spanning the gap. The femoral head articulates with the acetabulum and is covered by articular cartilage except for the fovea (attachment of ligament of head — carries artery of ligamentum teres, a minor blood supply).

Hip Joint Capsule

The capsule attaches above along the intertrochanteric line anteriorly and along the femoral neck posteriorly; below along the acetabular margin and transverse ligament. Three ligaments reinforce the capsule:

  • Iliofemoral ligament (of Bigelow): Inverted Y shape on the anterior capsule; the strongest ligament in the body; prevents hyperextension and holds the femoral head in the acetabulum during standing
  • Pubofemoral ligament: Limits abduction and extension
  • Ischiofemoral ligament: Limits internal rotation in extension

Blood Supply to the Femoral Head

  • Primary: Medial circumflex femoral artery (branch of profunda femoris) → retinacular arteries → ascend along femoral neck beneath the synovial membrane → enter femoral head at epiphysis
  • Minor: Artery of ligamentum teres (from obturator artery via acetabular notch) → fovea → head

Clinical significance: Femoral neck fractures interrupt retinacular blood supply → avascular necrosis (AVN) of the femoral head if not treated promptly. Intracapsular neck fractures are at high risk for AVN; extracapsular fractures (intertrochanteric, subtrochanteric) have better prognosis as retinacular blood supply is preserved.

Hip Muscles

Anterior Group (Hip Flexors)

  • Iliopsoas (psoas major + iliacus): Most powerful hip flexor; origin from lumbar vertebrae + iliac fossa; inserts on lesser trochanter; supplied by L2-L4 (ventral rami)
  • Rectus femoris: Also crosses the hip and knee; superficial layer of quadriceps
  • Sartorius: Longest muscle in the body; flexes, abducts, laterally rotates the hip

Posterior Group (Gluteal)

  • Gluteus maximus: Largest, most superficial; extends and laterally rotates the hip; powerful in stair climbing and running; inserts on the IT band + gluteal tuberosity of femur
  • Gluteus medius: Middle; abduction and medial rotation of hip; essential for walking (holds pelvis level when opposite leg is raised — Trendelenburg test)
  • Gluteus minimus: Deep; medial rotation and abduction; smallest gluteal muscle

Lateral Group

  • Tensor fasciae latae (TFL): Flexes, abducts, medially rotates hip; IT band → lateral condyle of tibia

Lumbosacral Plexus (L4-S3)

Formed by ventral rami of L4, L5, S1, S2, S3:

  • Obturator nerve (L2-L4): Emerges medial to psoas → passes through obturator foramen → adductor compartment → adductor longus, adductor brevis, adductor magnus (partial), gracilis; sensory: medial thigh
  • Femoral nerve (L2-L4): Emerges lateral to psoas → passes beneath inguinal ligament lateral to femoral vessels → anterior thigh; motor: quadriceps (hip flexion + knee extension), sartorius; sensory: anterior thigh + medial leg (saphenous)
  • Sciatic nerve (L4-S3): Largest nerve in the body (2cm wide); exits pelvis through greater sciatic foramen below piriformis → posterior thigh; splits into tibial and common peroneal nerves at the lower third of thigh; motor: posterior thigh (hamstrings except short head of biceps femoris) + entire leg and foot
  • Superior gluteal nerve (L4-S1): Exits above piriformis → gluteus medius, gluteus minimus, TFL; injury → Trendelenburg gait (pelvis drops on the opposite side during walking)

The Knee Joint

Anatomy

The knee is a synovial hinge joint (modified condylar) that allows slight rotation when flexed. Three joint spaces exist: patellofemoral (between patella and trochlear groove) + medial tibiofemoral + lateral tibiofemoral.

Articular Surfaces

  • Femur: Medial and lateral femoral condyles (separated by intercondylar notch) + patellar surface (anterior)
  • Tibia: Tibial plateau (medial and lateral condyles; flat surfaces); medial meniscus (C-shaped), lateral meniscus (more circular)
  • Patella: Largest sesamoid bone; embedded in quadriceps tendon; increases lever arm for quadriceps

Menisci

Fibrocartilaginous menisci attach to tibial plateau edges and absorb shock, distribute load, deepen the socket, and lubricate the joint. The outer 1/3 (red zone) is vascularised and can heal; the inner 2/3 (white zone) is avascular and cannot heal without blood supply. The medial meniscus is more C-shaped and attached to the tibial collateral ligament (less mobile than lateral) — more commonly torn and associated with ACL injuries. The lateral meniscus is more O-shaped and not attached to the lateral collateral ligament (more mobile) — less commonly injured in isolation.

Knee Ligaments

Cruciate ligaments (intracapsular but extrasynovial):

  • Anterior cruciate ligament (ACL): Originates from anterior intercondylar area of tibia → attaches to posteromedial femoral condyle; prevents anterior tibial translation; most commonly torn ligament in sports (basketball, football); non-contact pivoting injury; Lachman test is most sensitive clinical test; MRI is gold standard for diagnosis; causes pivot-shift instability
  • Posterior cruciate ligament (PCL): Originates from posterior intercondylar area of tibia → attaches to anterolateral femoral condyle; prevents posterior tibial translation; stronger than ACL; injured in dashboard injury (force hitting flexed knee); posterior drawer test

Collateral ligaments (extracapsular):

  • Medial collateral ligament (MCL): Epicondyle of femur → medial tibia; resists valgus stress; more commonly injured than LCL; joint line tenderness; valgus stress test
  • Lateral collateral ligament (LCL): Lateral epicondyle of femur → head of fibula; resists varus stress; varus stress test; less commonly injured

Posterolateral corner (PLC): Popliteus, lateral gastrocnemius head, popliteofibular ligament, coronary ligament, arcuate ligament; injuries cause posterolateral instability.

Common Knee Injuries

ACL tear (most common sports injury), meniscal tear (twisting injury, McMurray test positive), MCL sprain (valgus stress), collateral ligament injuries, patellar dislocation, quadriceps tendon rupture (in older patients), patellar tendon rupture (in younger patients).

The Ankle and Foot

Talocrural (Ankle) Joint

A hinge joint between the tibia + fibula (forming mortise) and talus. It allows plantarflexion (pointing toes down) and dorsiflexion (pulling toes up). The medial malleolus (tibia) and lateral malleolus (fibula) hold the talus in place.

Ankle Mortise

The talus fits between the medial and lateral malleoli. Any fracture of both malleoli creates an unstable joint requiring surgical fixation.

Ligaments

  • Deltoid ligament: Strong medial ligament complex; prevents eversion; attached to medial malleolus → talus, navicular, calcaneus; eversion injury can damage this ligament
  • Lateral ligaments of ankle: Anterior talofibular ligament (ATFL) — most commonly sprained; calcaneofibular ligament (CFL); posterior talofibular ligament (PTFL); inversion injury (rolling ankle)

Arches of the Foot

  • Medial longitudinal arch: Calcaneus, navicular, cuneiforms, metatarsals 1-3; highest arch; spring ligament (plantar calcaneonavicular) supports the head of the talus
  • Lateral longitudinal arch: Lower arch; formed by calcaneus, cuboid, metatarsals 4-5
  • Transverse arch: Metatarsal heads; supported by peroneus longus tendon (everts foot, raises lateral arch)

Nerve Supply to Lower Limb

  • Femoral nerve (L2-L4): Quadriceps (knee extension), sartorius; saphenous nerve (medial leg and foot)
  • Obturator nerve (L2-L4): Adductors (adduction of hip); medial thigh sensation
  • Sciatic nerve (L4-S3): Posterior thigh (hamstrings), leg, foot
  • Common peroneal nerve (L4-S2): Wraps around the fibular neck; superficial peroneal (lateral compartment — foot eversion, ankle dorsiflexion; sensory: lateral leg, dorsum of foot); deep peroneal (anterior compartment — foot dorsiflexion, toe extension; sensory: first web space); injury → foot drop + sensory loss
  • Tibial nerve (L4-S3): Posterior compartment of leg (plantarflexion); medial and lateral plantar nerves (sole of foot); sensory: sole + heel; sural nerve (lateral foot)
  • Sural nerve (S1-S2): Posterior lower leg to lateral foot; commonly used for nerve biopsy

Lower Limb Nerve Injuries — Clinical Summary

  • Femoral nerve injury: Cannot extend knee; knee reflex lost; sensory loss anterior thigh + medial leg
  • Obturator nerve injury: Cannot adduct hip; sensory loss medial thigh
  • Sciatic nerve injury: Posterior thigh sensation loss; below-knee motor and sensory loss
  • Common peroneal nerve injury: Foot drop (loss of dorsiflexion and eversion); sensory loss lateral leg + dorsum of foot
  • Tibial nerve injury: Loss of plantarflexion (cannot walk on toes); sensory loss sole of foot

Gait Analysis

  • Stance phase (60%): Foot on ground
  • Swing phase (40%): Foot in air
  • Normal gait: Heel strike → foot flat → push off (toe off) → swing
  • Trendelenburg gait: Hip abductor (gluteus medius) weakness → pelvis drops on the opposite side during stance
  • Foot drop (steppage gait): Cannot dorsiflex foot → high-stepping gait to clear ground; seen in common peroneal nerve palsy or L5 radiculopathy