Obturator Nerve

Introduction

Introduction to the Obturator Nerve

The obturator nerve is the principal motor and sensory supply to the medial compartment of the thigh. It arises from the lumbar plexus, formed by the ventral rami of the L2, L3, and L4 spinal nerves. The nerve descends through the psoas major, runs along the lateral wall of the pelvis, and passes through the obturator canal to enter the thigh. Within the thigh, it divides into anterior and posterior divisions, which supply most of the adductor muscles, provide sensory innervation to the medial thigh, and give articular branches to the hip and knee joints. The obturator nerve is clinically significant in pelvic surgery, pelvic fractures, adductor spasticity, and as a source of referred pain from hip joint pathology.

Study

1. Origin and Formation

The obturator nerve originates from the anterior primary rami of L2, L3, and L4 within the substance of the psoas major muscle. The nerve emerges from the medial border of the psoas major at the level of the pelvic brim, posterior to the common iliac vessels. It then runs inferolaterally on the lateral wall of the pelvis, lying medial to the internal iliac vessels and lateral to the ureter, and passes into the obturator canal.

2. Course and Relations in the Pelvis

Within the pelvis, the obturator nerve lies on the obturator internus muscle and is crossed anteriorly by the vas deferens in the male or the round ligament of the uterus in the female. It is accompanied by the obturator artery and vein, which lie inferior and lateral to the nerve. The nerve enters the obturator canal, a short fibro‑osseous tunnel at the upper part of the obturator foramen, bordered by the obturator membrane and the superior pubic ramus. The canal is a potential site of nerve entrapment.

3. Divisions in the Thigh

Upon exiting the obturator canal, the nerve divides into anterior and posterior divisions, separated by the adductor brevis muscle.

Anterior division: This lies on the anterior surface of the adductor brevis, deep to the adductor longus and pectineus. It gives muscular branches to the gracilis, adductor longus, adductor brevis (usually), and occasionally the pectineus (when the femoral nerve does not supply it). It also gives a sensory articular branch to the hip joint and terminates as the cutaneous branch that supplies the skin of the lower medial thigh (medial cutaneous nerve of the thigh).

Posterior division: This pierces the adductor brevis and lies on the anterior surface of the adductor magnus (adductor part). It gives muscular branches to the adductor brevis (in some cases), the adductor magnus (adductor part), and often the obturator externus muscle. It also gives articular branches to the knee joint.

4. Branches and Distribution

Motor: All muscles of the medial thigh are supplied by the obturator nerve, with the exception of the ischiocondylar (hamstring) part of adductor magnus, which is supplied by the tibial nerve, and the pectineus, which usually receives dual supply from the femoral nerve. Sensory: The nerve provides cutaneous innervation to the middle third of the medial thigh via the medial cutaneous nerve of the thigh. Articular: Branches supply the hip joint (from the anterior division) and the knee joint (from the posterior division). The sympathetic fibres within the nerve supply vascular smooth muscle.

5. Vascular Supply

The obturator nerve is accompanied by the obturator artery and vein, branches of the internal iliac vessels. The vasa nervorum of the obturator nerve are derived from the obturator artery and the medial circumflex femoral artery.

6. Clinical Relevance

Obturator nerve injury: This can occur during pelvic surgery (e.g., radical prostatectomy, pelvic lymph node dissection), pelvic fractures, or due to obturator canal entrapment. It results in weakness of hip adduction and sensory loss over the lower medial thigh. The patient may complain of difficulty crossing the legs.

Obturator nerve entrapment: Athletes may develop entrapment neuropathy due to fascial bands at the obturator canal, presenting with exercise‑induced medial groin and thigh pain, without significant motor loss.

Hip joint referred pain: The articular branch of the obturator nerve innervates the anteromedial capsule of the hip joint. Hip joint pathology (e.g., osteoarthritis) often presents with pain referred to the medial thigh and knee, mediated by the obturator nerve.

Obturator nerve block: A regional anaesthetic technique used for adductor spasticity, for hip surgery when combined with other blocks, and for analgesia following knee surgery (though it primarily blocks sensory fibres). An obturator nerve block can be used to diagnose and treat chronic hip adductor spasm.

Adductor spasticity: In conditions such as cerebral palsy, spasticity of the adductor muscles leads to a scissoring gait. The obturator nerve is often targeted with botulinum toxin injections or surgical neurectomy to reduce adductor tone.

Summary

Summary of the Obturator Nerve

The obturator nerve (L2–L4) is the primary motor and sensory nerve of the medial thigh. It descends through the psoas major, passes along the pelvic wall, and enters the thigh via the obturator canal. Its anterior division supplies the adductor longus, gracilis, adductor brevis, and skin of the medial thigh; the posterior division supplies the adductor magnus (adductor part) and gives articular branches to the knee. Injury causes adduction weakness and medial thigh sensory loss. The nerve is a conduit for referred pain from the hip joint and a target for regional blocks.