Sciatic Nerve Anatomy

Pelvic & Gluteal Course

• Origin: Sacral Plexus (Anterior surface of Piriformis).
• Exit: Leaves pelvis via Greater Sciatic Foramen, usually inferior to the Piriformis muscle.
• Gluteal Region: Descends deep to Gluteus Maximus, resting on the "Bed" of deep external rotators (Gemelli, Obturator Internus, Quadratus Femoris).
• Thigh: Enters posterior thigh deep to the Long Head of Biceps.
• Termination: Divides into Tibial and Common Peroneal nerves at the apex of the Popliteal Fossa (variable).

Structure

• The two components (Tibial and Common Peroneal) are physically separate within the epineurium from the origin.
• Medial: Tibial (Ventral divisions L4-S3).

Critical Relations

• Anterior (Deep): Ischium, Gemelli, Obturator Internus, Quadratus Femoris, Adductor Magnus.
• Posterior (Superficial): Piriformis, Gluteus Maximus, Long Head of Biceps.
• Medial: Inferior Gluteal Artery/Nerve.
• Lateral: Greater Trochanter.

In the posterior approach to the hip, the nerve is identified and protected. The External Rotators are detached to expose the joint, protecting the nerve behind them.

Branches Along the Course

• Articular branch (hip): a fine twig to the posterior hip capsule — the anatomical basis for posterior hip pathology referring pain down the limb (Hilton's law: a nerve crossing a joint supplies it).
• Muscular branches (thigh): all hamstrings and the hamstring (ischiocondylar) part of adductor magnus. Crucially, the long head of biceps, semitendinosus, semimembranosus and adductor magnus are tibial-division branches, whereas the short head of biceps femoris is the only muscle supplied by the common peroneal division — the single most-tested motor discriminator for a high (proximal) lesion.
• No cutaneous branch in the thigh: posterior thigh skin is the posterior cutaneous nerve of thigh (S1-S3), a separate sacral-plexus branch, not the sciatic.
• Terminal branches: the tibial division → muscular branches to the calf, the sural nerve (with a peroneal communicating contribution), and the medial & lateral plantar and medial calcaneal nerves; the common peroneal division → superficial peroneal (evertors + dorsal foot sensation), deep peroneal (dorsiflexors + first web space) and the lateral sural cutaneous nerve.
• Sensory summary: the sciatic supplies the whole leg and foot EXCEPT the medial calf/medial malleolus (saphenous nerve, from the femoral nerve).

Sural nerve = the workhorse autograft. Its predictable course (posterolateral calf, behind the lateral malleolus) and the small, well-tolerated lateral-foot sensory donor deficit make it the standard nerve graft (≈30-40 cm available); this is a classic viva fact.

Variation Overview

The relationship between the Sciatic Nerve and the Piriformis muscle is the most examined anatomical variant in the body. In cadaveric series the undivided nerve passes inferior to piriformis in roughly 90% of limbs (93.6% in Natsis 2013; "typical" pattern). Pooled meta-analysis of over 6,000 cadavers found an anomalous relationship in 16.9% of the population (Smoll 2010).

• High division: the tibial and common peroneal nerves separate within the pelvis or at piriformis rather than at the popliteal fossa. The peroneal part may then pierce or pass above the muscle (Beaton type B/C).
• Clinical weight: importantly, the prevalence of the variant is not significantly higher in piriformis-syndrome patients than in the general population (Smoll 2010) — so an anatomical anomaly alone does not explain symptoms and should not be over-relied upon.
• During deep gluteal decompression, look for a second (doubled) piriformis tendon, between which the peroneal division may run.

Gluteal Region

• PSIS: Posterior Superior Iliac Spine.
• Ischial Tuberosity: Medial landmark (nerve is lateral).
• Greater Trochanter: Lateral landmark (nerve is medial).

The Sciatic Nerve emerges at the midpoint of a line connecting the Ischial Tuberosity to the Greater Trochanter.

Posterior Thigh

• The nerve runs vertically down the midline of the thigh.
• It is covered by the Long Head of Biceps Femoris.
• Surface Marking: Join the midpoint between Ischial Tuberosity/Greater Trochanter to the apex of the Popliteal Fossa.
• Palpation: Difficult due to muscle bulk, but can be tender in the sciatic notch (midway between PSIS and Greater Trochanter).

Palpation is rarely diagnostic.

Beaton & Anson Classification

This classification describes the relationship between the Sciatic Nerve and the Piriformis muscle.

• Type A (85%): Undivided nerve passes below Piriformis.
• Type B (10%): Common Peroneal division passes through Piriformis; Tibial passes below.
• Type C (3%): Common Peroneal passes above Piriformis; Tibial below.
• Type D (less than 1%): Entire nerve passes through Piriformis.
• Type E (less than 1%): Entire nerve passes above Piriformis.
• Type F (less than 1%): Undivided nerve passes below, but there is an accessory piriformis.

Type B is the most clinically significant variant as it predisposes to Piriformis Syndrome (compression of the Common Peroneal division).

MRI

• MRI Lumbar Spine: Mandatory to rule out radiculopathy (disc herniation/stenosis) which is the most common cause of "Sciatica".
• MRI Pelvis: To assess the course of the nerve through the sciatic notch. Look for:
  • Mass lesions (Hematoma, Tumor).
  • Piriformis hypertrophy or asymmetry.
  • Signal change (T2 hyperintensity) within the nerve indicative of neuritis or compression.

MR Neurography

• Specialized fluid-sensitive sequences to track the nerve fascicles.
• Highly sensitive for extraspinal sciatica.

This modality is particularly useful for identifying non-compressive etiologies like neuritis.

EMG / NCS

• Differentiate: Plexus vs Root vs Peripheral Nerve.
• H-Reflex: Prolonged latency indicates S1 radiculopathy (or sciatic).
• F-Wave: Assess proximal conduction.
• Needle EMG:
  • Gluteal sparing: Indicates lesion is distal to the plexus (Sciatic).
  • Short Head Biceps involvement: Indicates lesion is high (thigh/pelvis) rather than at the knee.
  • Paraspinal involvement: Indicates Root level lesion.

NCS shows slowed conduction velocity across the segment of compression (if demyelinating) or reduced amplitude (if axonal loss).

Posterior Exposure (Thigh)

• Incision: Midline posterior thigh.
• Deep Flexia: Split Gluteus Maximus proximally.
• Identification: Find nerve deep to Long Head of Biceps (guides you to it).
• Neurolysis: Release adhesions from hamstring origin or quadratus femoris.

Always identify the nerve in normal tissue (distally or proximally) before dissecting the scarred zone.

Protection in Hip Arthroplasty (Posterior Approach)

• Identify nerve before placing retractors.
• Pearl: Keep the hip extended and knee flexed to relax the nerve.
• Retractor: Place retractors on the bone (acetabular rim), NOT in soft tissue where the nerve lies.
• Palpate nerve tension during reduction.

The "safe zone" for retractor placement or screws is the posterosuperior quadrant of the acetabulum (away from the nerve).

Sciatica and Palsy

• Reduction: In hip dislocation, prompt reduction is the primary treatment for the nerve (ischemia).
• Neurolysis: Indicated for tethering or persistent compression (e.g., scarring after trauma).

Neurolysis should be performed with magnification to avoid disrupting the epineural blood supply.