Lumbosacral plexus and nerve supply of lower limb

• A spinal nerve arises as rootlets from the anterior and posterior horns of the spinal cord grey matter. Each of the posterior and anterior rootlets converge to form corresponding nerve roots.
• Posterior (dorsal) spinal root - sensory
• Anterior (ventral) spinal root - motor
• The posterior and anterior roots of the spinal nerve unite within or just proximal to the intervertebral foramen to form a spinal nerve - functionally a mixed nerve
• Each spinal nerve divides into posterior and anterior rami
• The posterior and anterior roots and rootlets are the only sites where the motor and sensory fibres of a spinal nerve are segregated. The nerve roots could selectively be sectioned for the relief of intractable pain or spastic paralysis.
• Femoral nerve (L2-L4)
• Obturator nerve (L2-L4)
• Lateral femoral cutaneous nerve (L2-L3)
• Genitofemoral nerve (L1-L2)
• Ilioinguinal nerve (L1)
• Iliohypogastric nerve (L1)
• The lumbar plexus forms in the substance of the psoas major muscle anterior to its attachment to the transverse processes of the lumbar vertebrae in the posterior abdominal wall. Relative to the psoas major muscle, its branches emerge:

  • anterior - genitofemoral
  • medial - obturator
  • lateral
  • femoral
  • lateral femoral cutaneous
  • ilioinguinal
  • iliohypogastric
• Femoral nerve supplies muscles of the anterior compartment of thigh - flexors of hip and extensors of knee
• Obturator nerve supplies muscles of medial (adductor) compartment of thigh
• Lumbosacral trunk formed by the union of the descending branch of L4 and the anterior ramus of L5. Joins lumbar plexus with sacral pleuxs
• Sacral plexus is formed by the anterior rami of S1 to S4 and the lumbosacral trunk in the posterolateral wall of the lesser pelvis, in relation to the anterior surface of the piriformis muscle.
• Each anterior ramus of L4-S3, has ventral and dorsal divisions that combine with similar divisions from other levels to form terminal nerves.
• Most branches of the sacral plexus leave the pelvis through the greater sciatic foramen.
• Sciatic nerve (L4-S3)
• Superior gluteal nerve (L4-S1)
• Inferior gluteal nerve (L5-S2)
• Pudendal nerve (S2-S4)
• Sciatic nerve is the largest nerve of the body. Supplies muscles of the posterior compartment of the thigh and muscles of the entire leg and foot. Carries sensory fibres from the skin of the foot and lateral leg.
• Superior gluteal nerve

  • passes through the greater sciatic foramen above the piriformis muscle
  • supplies gluteus medius and gluteus minimus muscles
• Inferior gluteal nerve

  • passes through the greater sciatic foramen below the piriformis muscle
  • supplies gluteus maximus muscle
• Pudendal nerve

  • main nerve supply of perineum
  • main sensory nerve of external genitali
• Injuries to the lumbosacral plexus will cause symptoms in the territories of the sensory and motor distributions of their branches in the lower limb
• The lumbosacral plexus may be damaged by injury such as tumour, blood clots (haematoma) or autoimmune reactions, resulting in pain, weakness and loss of sensation in all or part of the lower limb
• Lumbosacral plexus neuropathy is not very common and idiopathic.
• Acute lumbosacral plexitis presents as rapid onset of pain involving the hip and the thigh, leg muscle weakness and atrophy.
• Sciatica (lumbar radiculopathy): compression of the spinal nerve root of the sacral plexus often due to degeneration of an intervertebral disc, or disc herniation, resulting in pain affecting the back, hip and outer side of leg.
• During childbirth, fetal head may compress the nerves of the mother's sacral plexus producing pain in the lower limbs.
• Obturator nerve is vulnerable to injury during surgery for removal of cancerous lymph nodes from the lateral pelvic wall, leading to painful spasm of the adductor muscles of the thigh and sensory deficits in the medial thigh region.
• Spasm of the adductor muscles of the thigh in spastic paraplegia can be relieved by division of the obturator nerve - obturator nerve neurectomy.
• Piriformis syndrome is a neuromuscular disorder due to spasms of the piriformis muscle and compression of the sciatic nerve causing pain, numbness and tingling along the back of the leg and foot - similar to sciatica
• Piriformis syndrome is seen in individuals involved in sports that require excessive use of the gluteal muscles, or after trauma to the buttocks associated with hypertrophy.
• The sciatic nerve could be injured in intragluteal intramuscular injections.
• The safe area for injections to avoid injury to the sciatic nerve is in the superolateral quadrant of the buttocks or superior to a line extending from the posterior superior iliac spine to the superior border of the greater trochanter
• Injection can be safely given in the triangular area between a finger on the tubercle of the iliac crest and another on the anterior superior iliac spine.