Page 266 - Clinical Anatomy

ECA4 7/18/06 6:47 PM Page 251

Course and distribution of nerves 251

ligament and may then be pressed upon by it with resultant pain and
anaesthesia over the upper outer thigh (meralgia paraesthetica). This is
relieved by dividing the deeper fasciculus of the inguinal ligament where
the nerve passes over it.
The obturator nerve (L2–4) emerges from the medial aspect of the psoas
and runs downwards and forwards, deep to the internal iliac vessels, to
reach the superior part of the obturator foramen. This the nerve traverses,
in company with the obturator vessels, to enter the thigh.
Its branches are:
•◊◊muscular—to obturator externus, the adductor muscles and gracilis;
•◊◊cutaneous—to an area of skin over the medial aspect of the thigh;
•◊◊articular—to the hip and knee joints.

Clinical features

1◊◊Spasm of the adductor muscles of the thigh in spastic paraplegia can be
relieved by division of the obturator nerve (obturator neurectomy). This can
be performed through a midline lower abdominal incision exposing the
nerve trunk extraperitoneally on each side as it passes towards the obtura-
tor foramen.
2◊◊Rarely, an obturator hernia develops through the canal where the obtura-
tor nerve and vessels traverse the membrane covering the obturator
foramen. Pressure of a strangulated obturator hernia upon the nerve causes
referred pain in its area of cutaneous distribution, so that intestinal obstruc-
tion associated with pain along the medial side of the thigh should suggest
this diagnosis.
3◊◊The femoral and obturator nerves, as well as the sciatic nerve and its
branches, supply sensory fibres to both the hip and the knee; it is not
uncommon for hip disease to present disguised as pain in the knee.

The sacral plexus (Fig. 182)
This plexus originates from the anterior primary rami of L4–5, S1–4.
Note that L4 is shared by both plexuses, a branch from it joining L5 to
form the lumbosacral trunkwhich carries its contribution to the sacral plexus.
The sacral nerves emerge from the anterior sacral foramina and unite in
front of piriformis where they are joined by the lumbosacral trunk.
Branches from the plexus supply:
•◊◊the pelvic muscles;
•◊◊the muscles of the hip;
•◊◊the skin of the buttock and the back of the thigh.
The plexus itself terminates as the pudendal nerve and the sciatic nerve.
The pudendal nerve (S2–4) provides the principal innervation of the per-
ineum. It has a complex course, passing from the pelvis, briefly through the
gluteal region, along the side-wall of the ischiorectal fossa and through the
deep perineal pouch to end by supplying the skin of the external genitalia
(Fig. 183).