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.

Variation Overview

The anatomical relationship between the Sciatic Nerve and the Piriformis muscle is highly variable. The standard anatomy (nerve improper passing inferior to muscle) is present in 85% of cases.

Other key variations include high divisions, where the Tibial and Common Peroneal nerves exit the pelvis separately.

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).

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.

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).

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.

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).