Sciatic Nerve - Short Note

Overview

Course

1. In the Pelvis

Variant: Occasionally, the common fibular division (or the entire nerve) passes through or above the piriformis muscle - clinically relevant in piriformis syndrome.

2. In the Gluteal Region

• Passes deep to the gluteus maximus
• Descends in the plane between the superficial and deep muscle groups, crossing the posterior surfaces of the obturator internus, gemelli, and quadratus femoris
• Lies at the midpoint between the ischial tuberosity and the greater trochanter (the classic intramuscular injection landmark to avoid)

3. In the Posterior Thigh

• Lies on the adductor magnus, crossed by the long head of biceps femoris
• Gives off branches to all posterior thigh muscles before dividing

4. Division

• Tibial nerve - descends vertically through the popliteal fossa
• Common fibular (peroneal) nerve - winds around the neck of the fibula

Cross-sectional anatomy (Imaging Anatomy Atlas)

Motor Supply

Sensory Supply

• Lateral side of the leg
• Lateral side and entire sole of the foot
• Whole of the foot (except medial leg/foot supplied by the saphenous nerve)

Applied Anatomy / Clinical Importance

1. Intramuscular Injection

• Misplaced injections into the buttock (especially in thin/elderly patients with reduced gluteal muscle mass) can injure the sciatic nerve
• Safe injection site: upper outer quadrant of the gluteal region
• (Andrews' Diseases of the Skin)

2. Complete Sciatic Nerve Lesion

• Weakness of knee flexors (hamstrings)
• Complete paralysis of all muscles below the knee
• Sensory loss over the entire foot and leg below the knee (except medial leg - saphenous nerve territory)
• Loss of ankle jerk and plantar reflexes

3. Partial Sciatic Nerve Lesion (Common Pattern)

• Fewer fascicles with less supportive connective tissue
• More taut and secured at the sciatic notch and fibular neck

4. Causes of Sciatic Nerve Injury

• Hip surgery - most common cause (hip replacement)
• Posterior dislocation of the hip
• Subtrochanteric femur fracture
• Bullet/stab wounds
• Hematoma in the posterior thigh
• Prolonged sitting on hard surface ("toilet seat palsy")
• Baker cyst compression in popliteal fossa
• Endometriosis involving the nerve (causes recurrent sciatic mononeuropathy)

5. Piriformis Syndrome

• History of buttock trauma
• Pain maximal during prolonged sitting (driving, biking)
• Pain reproduced by hip adduction, internal rotation, and flexion (AIF maneuver)
• Usually normal neurological and nerve conduction studies
• MRI may show hypertrophied piriformis

6. Terminal Branch Injuries

Summary Table

Branches of the Sciatic Nerve

A. Branches in the Thigh (Before Division)

The short head of biceps femoris is the only posterior thigh muscle supplied by the common fibular division - this is the key EMG landmark for localizing proximal sciatic lesions.

B. Terminal Divisions

1. Tibial nerve (L4-S3, anterior divisions)
2. Common fibular (peroneal) nerve (L4-S2, posterior divisions)

I. TIBIAL NERVE

1. In the Popliteal Fossa

• Muscular branches to superficial posterior leg muscles:
  • Gastrocnemius (both heads)
  • Plantaris
  • Soleus
  • Popliteus
• Sural nerve (cutaneous) - arises high between the two heads of gastrocnemius, descends superficially, is joined by the sural communicating branch of the common fibular nerve, then supplies skin of the lower posterolateral leg, lateral ankle, lateral foot, and little toe

2. In the Posterior Leg (Deep to Soleus)

• Muscular branches to deep posterior compartment:
  • Tibialis posterior
  • Flexor digitorum longus
  • Flexor hallucis longus
• Medial calcaneal nerve - pierces the flexor retinaculum to supply skin of the heel

3. In the Foot (via Tarsal Tunnel, Behind Medial Malleolus)

a) Medial Plantar Nerve (larger - analogous to median nerve in hand)

• Motor: Abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, 1st lumbrical
• Sensory: Medial two-thirds of the sole + plantar surfaces of medial 3½ toes (including great toe)

b) Lateral Plantar Nerve (smaller - analogous to ulnar nerve in hand)

• Motor: All remaining intrinsic foot muscles - quadratus plantae, abductor digiti minimi, flexor digiti minimi, lumbricals 2-4, all interossei, adductor hallucis
• Sensory: Lateral one-third of sole + plantar surfaces of lateral 1½ toes
• Divides into deep branch (motor to interossei) and superficial branch (digital nerves to little toe)

II. COMMON FIBULAR (PERONEAL) NERVE

Branches Before Division (at fibular neck)

• Lateral cutaneous nerve of the calf - skin over upper third of lateral leg
• Fibular (peroneal) communicating nerve - joins the sural nerve

Terminal Branches

a) Deep Fibular (Anterior Tibial) Nerve

• Pierces the anterior intermuscular septum into the anterior compartment
• Motor: Tibialis anterior, extensor hallucis longus, extensor digitorum longus, fibularis (peroneus) tertius, extensor digitorum brevis (on dorsum of foot)
• Sensory: First web space between great toe and 2nd toe only (very small area)
• Runs with the anterior tibial vessels on the interosseous membrane

b) Superficial Fibular (Musculocutaneous) Nerve

• Remains in the lateral compartment
• Motor: Fibularis (peroneus) longus and brevis
• Sensory: Lower two-thirds of lateral leg + entire dorsum of foot and toes (except 1st web space and lateral border supplied by sural nerve)

Accessory deep fibular nerve: present in up to 28% of individuals - arises from the superficial fibular nerve, passes behind the lateral malleolus to supply the lateral part of extensor digitorum brevis.

Summary Tree

SCIATIC NERVE (L4-S3)
│
├── Muscular branches in thigh (tibial division)
│   ├── Semitendinosus
│   ├── Semimembranosus
│   ├── Biceps femoris (long head)
│   └── Adductor magnus (ischial part)
├── Short head of biceps femoris (fibular division)
│
├── TIBIAL NERVE
│   ├── Popliteal fossa: gastrocnemius, plantaris, soleus, popliteus
│   ├── Sural nerve (cutaneous - lateral leg & foot)
│   ├── Leg: tibialis posterior, FDL, FHL
│   ├── Medial calcaneal nerve (heel skin)
│   └── Tarsal tunnel → Medial plantar nerve + Lateral plantar nerve
│
└── COMMON FIBULAR NERVE
    ├── Lateral cutaneous nerve of calf
    ├── Fibular communicating nerve (→ sural)
    ├── Deep fibular nerve (anterior compartment + 1st web space)
    └── Superficial fibular nerve (lateral compartment + dorsum of foot)