Sciatica Anatomy

Courtesy: Prof Nabil Ebraheim
University of Toledo, Ohio, USA

Anatomy of sciatica

• The sciatic nerve is a large nerve that originates from the lumbosacral plexus of lower spine with a root value of L4, L5, S1, S2& S3 .
• The nerve initially emerges from the pelvis and leaves the greater sciatic notch, anterior and deep to the piriformis muscle .
• Thus exiting the notch below the piriformis muscle.
• However in 10% people, the muscle or a part of it, tends to separate the nerve in the sciatic notch.
• The sciatic nerve then descends downwards to the thigh by passing in between the ischial tuberosity and the greater trochanter of femur and further, emerging beneath the lower border of gluteus maximus to enter the posterior aspect of thigh .
• Within the posterior aspect , the sciatic nerve gives out multiple sensory and motor branches which innervate specific regions of leg and foot.
• The two main branches are the posterior tibial nerve and the common peroneal nerve. Hence, sciatica can be caused due to irritation of the nerve at multiple sites.
• In the spine, sciatica is usually caused due to herniation of the lumbar disc, also known as “lumbar radiculopathy” or “true sciatica”!
• Another site of sciatica is at the piriformis muscle where the nerve gets compressed in between the muscle, leading to a condition known as “piriformis syndrome”.
• However the syndrome is often a diagnosis of exclusion where MRI spine is found to be negative, proving that sciatica wasn’t caused by a problem of the disc, hence it could be due to piriformis syndrome.