Obturator Nerve

Embryology in General

As the embryo develops, neuronal axons follow the routes laid down by growth cones toward their target end points, distal to the neuronal soma. Continuous with the growth cones, these developing axons enter Schwann’s cell tubes during migration. The growth cones consist of a triad: a peripheral zone made of actin within the filopodia and internally associated with microtubules; a central zone that provides for axon elongation with bundled microtubules; and a transitional zone that allows for traction between the central and peripheral zones. ¹ Microtubules are crucial for development, providing two pathways for the growth cone to navigate forward. First, microtubules join in the peripheral zone to form an anchor that influences growth toward the adhesion. Second, microtubules can merge toward the growth of new axonal bodies, within the space provided by actin bundles. Eventually, the peripheral nerves congregate and form fascicles. Each fascicle is enveloped by a two-layer cellular mass that covers the mature nerve trunk. Its outer layer is composed of Schwann’s cells that help proliferate axons, while the inner layer is composed of connective tissue components such as mesenchymal progenitor cells. ¹

This process of peripheral nerve elongation has been described in the 12th week of gestation in humans. Between the 14th and 16th weeks of pregnancy, Schwann’s cells begin the process of myelination. Once formed, peripheral nerves consist of a triad of connective parts, namely endoneurium, perineurium, and epineurium. The endoneurium is the innermost neural layer, composed of two layers of collagen, directly supporting the individual nerve fiber. The perineurium, sandwiched between the endoneurium and epineurium, is formed by tight junctions that function to maintain ion concentrations required for neural function. As nerve fibers form clusters with surrounding connective tissue, they are labeled a “bundle.” This bundle is wrapped again by a larger perineurium and then finally by the epineurium, the outermost layer of connective tissue that provides structural support. ¹

8.2 Spinal Cord Origin and Function

The obturator nerve (▶ Fig. 8.1 and ▶ Fig. 8.2) is derived from the second to fourth ventral rami, sharing the same roots as the femoral nerve. Its largest and smallest contributors are typically from L3 and L2, respectively. ²^, ³^, ⁴^, ⁵ Its main functions are to supply motor function to the major adductor muscles of the lower limb (obturator externus, adductor brevis, adductor longus, adductor magnus, gracilis, and, at times, pectineus) and the cutaneous sensation of the medial thigh. Through communicating fibers, it can also contribute to articular innervation of the knee and cutaneous sensation of a small area of the medial leg.

Fig. 8.1 Schematic drawing of the course of the obturator nerve.

(Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012. Illustration by Karl Wesker.)

Fig. 8.2 (a, b) Internal topography of the obturator nerve. A, arterial; B, adductor brevis; G, gracilis-cutaneous; J, hip joint; L, adductor longus; M, adductor magnus; O, obturator externus.

8.3 Anatomy

Following its exit from the spinal cord, the obturator nerve runs a complex pathway deep within the pelvis. It descends medially along the posterior wall, just lateral to the lumbar vertebral column, and posterior to the psoas major muscle. It passes the sacroiliac joint, posterior to the common iliac artery and lateral to the internal iliac vessels. As it reaches the pelvic inlet, it travels anteriorly and slightly inferiorly, adjacent to the medial side of the pelvic wall, following the arcuate line, just superior to the obturator internus muscle. Throughout this course, the nerve is enveloped in the subperitoneal cellular tissue of the region, flattening and enlarging the nerve. ⁶ In this trajectory, it reaches the obturator canal, a small opening within the superior aspect of the larger, obturator foramen. The obturator foramen is mostly covered by the obturator membrane, apart from the opening of the canal, located 2.7 cm lateral and 1.7 cm inferior to the pubic tubercle. ⁷ The canal is bordered by the obturator sulcus of the pubic bone superiorly, and the internal and external obturator muscles inferiorly, with the obturator membrane forming the floor of the tunnel. ⁸ Branching off from the internal iliac vessels, the obturator artery and vein travel just inferior to the nerve along the pelvic wall and through the obturator canal to exit the pelvis. ⁶

As the nerve exits the obturator canal, it splits immediately into an anterior and posterior branch. Leaving the obturator canal, the anterior branch supplies the hip joint via the articular branch and branches a “twig” to the accessory obturator nerve, if that nerve is present. ⁴ The anterior division continues and then descends within a flat plane on the anterior surface of the obturator externus and adductor brevis muscles, deep to the pectineus and adductor longus muscles. ⁴^, ⁹ Within this pathway, it supplies the adductor longus and gracilis muscles, and at times the adductor brevis and pectineus muscles. ¹^, ⁵ As it continues to the inferior border of the adductor longus, an arterial branch is supplied to the femoral artery, and a cutaneous branch communicates with the medial cutaneous nerve of the thigh and the saphenous nerve, forming the subsartorial nerve plexus; this plexus provides cutaneous sensation to the medial thigh. ¹^, ²^, ⁵ Occasionally, this “subsartorial” cutaneous branch can continue on to provide sensation to the medial leg. ³^, ⁴

The posterior branch of the obturator nerve pierces the anterior surface of the obturator externus muscle and then descends posterior to the adductor brevis muscle and anterior to the adductor magnus muscle. Within this pathway, the nerve divides into branches to supply the obturator externus, adductor magnus, and adductor brevis muscles. Typically, articular filaments are also sent to the knee joint, either by entering the adductor magnus distally or by passing via the adductor hiatus to the posterior knee, with the femoral artery. Within the popliteal fossa, the nerve descends with the popliteal artery and then pierces the oblique posterior ligament, thus providing sensory innervation to the cruciate ligaments and synovial membrane. ²^, ⁴^, ⁵^, ¹⁰

8.4 Variations in Topography

Many variations can occur as the obturator nerve distinguishes itself from its lumbar roots. The obturator nerve can have additional roots from the first or fifth lumbar nerve. More specifically, the obturator nerve can arise in a “high” or “prefixed” form from L1–L4 or more rarely L1–L3. It can also arise in a “low” or “postfixed” form from L2–L5. In its typical form, the branch arising from the third lumbar nerve is the largest, while that arising from the second is often very small and may not contribute in some variations. ¹^, ²^, ⁵

Among the variations, changes in route are possible. The branch from the main nerve trunk to the obturator externus muscle can pass to the lateral instead of the usual medial side. ¹ The articular branch to the hip has been seen to leave the main branch and run independently to the hip joint before passing the obturator foramen; its origin has also been seen to be the posterior, rather than the anterior, branch. ⁴^, ⁵

The distribution of the obturator nerve branches can also vary. The nerve sometimes gives a branch to the pectineus or obturator internus. ⁵ The anterior cutaneous branch can be absent, in which case the cutaneous sensation of the medial thigh is supplied by the femoral nerve. ¹ Similarly, when the anterior branch is missing, the posterior branch will innervate the adductor brevis and obturator externus. ² Overall, multiple branches have been reported to the following structures: obturator internus, obturator artery, pectineus, and periosteum of pubis pelvic surface. ⁵ As well, when the communicating cutaneous nerve from the obturator is large, it can continue on from the subsartorial plexus, pierce the deep fascia lata at the knee, and communicate with the saphenous nerve to provide cutaneous sensation for the medial leg. ²^, ³^, ⁵ In this variation, the medial cutaneous nerve of the thigh is typically small and ends within the subsartorial plexus after providing a few cutaneous filaments. ²

A final variation includes the presence of an accessory obturator nerve. Its prevalence has been reported to be approximately 8 to 9% ⁵^, ⁸; however, numbers as high as 17 and 30% have also been reported. ⁵^, ¹¹ If this is present, it is usually small and arises from the ventral rami of the third and fourth lumbar nerve roots; however, combinations of single to triple nerve roots ranging between L2 and L5 have been seen. ¹^, ⁵ After leaving the nerve roots, it typically descends just anterior (often adherent) to the main obturator trunk, medial to the psoas major muscle, and then traverses the superior pubic ramus. ⁴^,