🦴 Lumbosacral Plexus
The lumbosacral plexus is a network of spinal nerve roots (L2–S3) that provides all motor and sensory innervation to the lower limb. It is formed by the ventral rami of L2–L4 (lumbar plexus) and L4–S3 (sacral plexus), with L4 contributing to both divisions.
🔹 Major nerve roots & divisions
- Femoral (L2–L4, posterior divisions) → anterior thigh, hip flexors, knee extensors.
- Obturator (L2–L4, anterior divisions) → medial thigh adductors.
- Tibial (L4–S3, anterior divisions) → posterior thigh & leg, plantar flexion.
- Common fibular (peroneal) (L4–S2, posterior divisions) → divides into superficial & deep fibular nerves.
- Superior gluteal (L4–S1) & Inferior gluteal (L5–S2) → pelvic stabilizers, hip abduction/extension.
High yield The sciatic nerve is a large trunk formed by the tibial and common fibular nerves, enclosed in a common sheath.
Terminal & Collateral Nerves
Femoral nerve (L2–L4)
- Innervates: quadriceps, sartorius, pectineus
- Actions: knee extension, hip flexion
Obturator nerve (L2–L4)
- Innervates: adductors (longus, brevis, magnus anterior part)
- Actions: thigh adduction, medial rotation
Tibial nerve (L4–S3)
- Posterior thigh (hamstrings, adductor magnus posterior)
- Posterior leg (gastrocnemius, soleus, tibialis posterior, flexors)
- Actions: knee flexion, plantar flexion, inversion, digit flexion
Common fibular nerve (L4–S2)
- Short head of biceps femoris (knee flexion)
- Divides into superficial (eversion) and deep (dorsiflexion, extension of digits) fibular nerves.
Collateral nerves (gluteal)
- Superior gluteal (L4–S1): gluteus medius, minimus, tensor fasciae latae → pelvic stability, hip abduction.
- Inferior gluteal (L5–S2): gluteus maximus → hip extension, lateral rotation.
Segmental Innervation Gradient
Proximal → Distal gradient: higher segments (L2–L3) innervate proximal muscles, lower segments (S1–S2) innervate distal muscles.
- Anterior hip (flexors): L2–L3
- Anterior knee (extensors): L3–L4
- Anterior ankle (dorsiflexion): L4–L5
- Posterior hip (extensors): L4–L5
- Posterior knee (flexors): L5–S1
- Posterior ankle (plantar flexion): S1–S2
Nerve Injuries & Gait Abnormalities
Superior gluteal nerve
Weak hip abduction → Trendelenburg gait (pelvis drops on contralateral side during stance).
Inferior gluteal nerve
Weak hip extension → difficulty rising from chair, climbing stairs.
Femoral nerve
Weak hip flexion & knee extension; sensory loss over anterior thigh, medial leg & foot.
Obturator nerve
Loss of thigh adduction; sensory loss on medial thigh.
Sciatic nerve
Complete lesion: weak knee flexion, loss of all below‑knee functions, sensory loss over posterior thigh, leg (except medial) and foot. Often damaged in posterior hip dislocation.
Common fibular nerve (most frequently injured)
- Foot drop (loss of dorsiflexion), loss of eversion.
- Sensory loss: lateral leg & dorsum of foot (except first web space).
- Piriformis syndrome: compression by piriformis muscle → motor/sensory deficits in anterior & lateral compartments.
Deep fibular nerve
- Weak inversion, loss of digit extension, foot drop.
- Sensory loss: first web space (between great & second toes).
Superficial fibular nerve
- Loss of eversion, foot drop (less severe).
- Sensory loss: dorsum of foot (except first web space).
Sensory Innervation · Lower Leg & Foot
- Superficial fibular → lateral leg & dorsum (except first web space).
- Deep fibular → first dorsal web space.
- Tibial (medial & lateral plantar) → sole of foot.
- Sural (tibial + fibular branches) → posterior leg & lateral foot.
- Saphenous (femoral branch) → medial leg & medial foot.
Arterial Supply
External iliac → femoral artery → profunda femoris (medial & lateral circumflex, perforating arteries).
- Medial circumflex femoral → main supply to head of femur; risk of avascular necrosis in femoral neck fractures.
- Popliteal artery (knee joint) → divides into anterior & posterior tibial arteries.
- Anterior tibial → dorsalis pedis (pulse on dorsum of foot).
- Posterior tibial → passes posterior to medial malleolus; supplies plantar arch.
- Fibular artery → lateral compartment.
Femoral Triangle
Borders: inguinal ligament (superior), sartorius (lateral), adductor longus (medial).
Contents (lateral → medial): Femoral Nerve, Artery, Vein, Empty space (femoral canal), Lymph nodes (NAVEL).
The femoral nerve lies outside the femoral sheath. The femoral canal is the site of femoral hernias.
Hip Joint
Articulation: head of femur and acetabulum. Capsule reinforced by iliofemoral, ischiofemoral, and pubofemoral ligaments.
Knee Joint
Synovial joint: femoral & tibial condyles, patella. Stability depends on quadriceps, hamstrings, and ligaments.
Collateral ligaments
- Tibial (medial) collateral: medial epicondyle → medial tibia; attached to medial meniscus. Prevents lateral displacement (valgus). Most frequently torn.
- Fibular (lateral) collateral: lateral femur → fibular head; not attached to lateral meniscus. Prevents medial displacement (varus).
Cruciate ligaments
- ACL: tibial anterior → lateral femoral condyle; prevents anterior tibial displacement. Taut in extension.
- PCL: tibial posterior → medial femoral condyle; prevents posterior tibial displacement. Taut in flexion.
Menisci
- Medial (C‑shaped, attached to tibial collateral) → less mobile, more frequently injured.
- Lateral (circular, more mobile) → not attached to fibular collateral.
Ankle Joint
Most common injury: inversion sprain – anterior talofibular ligament is frequently damaged.
📌 Clinical pearls
- Foot drop → consider common fibular nerve or L5 radiculopathy.
- Trendelenburg sign → superior gluteal nerve or L5 lesion.
- Piriformis syndrome → sciatica‑like symptoms, but from muscle compression.
- Femoral neck fracture → always evaluate for avascular necrosis.
- Anterior drawer test → assess ACL integrity.