NERVES OF THIGH
The femoral nerve (L2, 3, 4) is the largest branch of the lumbar plexus (see Plate 2-5). It originates from the posterior divisions of the ventral rami of the second, third, and fourth lumbar nerves, passes inferolaterally through the psoas major muscle, and then runs in a groove between this muscle and the iliacus, which it supplies. It enters the thigh behind the inguinal ligament to lie lateral to the femoral vascular sheath in the femoral triangle. Twigs are given off to the hip and knee joints and adjacent vessels, and cutaneous branches are given off to anteromedial aspects of the lower limb.
Muscular branches supply the pectineus, sartorius, and
quadriceps femoris muscles. The nerve to the pectineus muscle arises at the
level of the inguinal ligament, whereas the branches to the sartorius muscle
enter the upper two thirds of the muscle, several arising in common with the
anterior femoral cutaneous nerves. The branches to the quadriceps femoris
muscle are arranged as illustrated, and those to the rectus femoris and vastus
lateralis muscles enter the deep surfaces of the muscles. The branch to the
vastus intermedius muscle enters its superficial surface and pierces the muscle
to supply the underlying articularis genus muscle. The branch to the vastus
medialis muscle runs in the adductor canal for a variable distance, on the
lateral side of the femoral vessels and saphenous nerve, giving off successive
branches to this muscle, some of which end in the vastus intermedius and
articularis genus muscles.
The anterior femoral cutaneous nerves arise in the femoral
triangle. All these branches pierce the fascia lata 8 to 10 cm distal to the
inguinal ligament and descend to knee level, supplying the skin and fascia over
the front and medial sides of the thigh.
The saphenous nerve is the largest and longest of the femoral
branches. It arises at the femoral triangle and descends through it on the
lateral side of the femoral vessels to enter the adductor canal. Here, it
crosses the vessels obliquely to lie on their medial side in front of the lower
end of the adductor magnus muscle. In the canal, the saphenous nerve
communicates with branches of the anterior femoral cutaneous and obturator
nerves to form the subsartorial plexus. At the lower end of the canal,
it leaves the femoral vessels and gives off its infrapatellar branch, which
curves around the posterior border of the sartorius muscle, pierces the fascia
lata, and runs onward to supply the skin over the medial side and front of the
knee and the patellar ligament. This branch assists off shoots from
the anterior and lateral femoral cutaneous nerves in forming the patellar
plexus. The saphenous nerve continues its descent on the medial side of the
knee, pierces the fascia lata between the tendons of the sartorius and gracilis
muscles, courses downward on the medial side of the leg close to the greater saphenous
vein, and gives off its medial crural cutaneous branches. In the lower leg, it subdivides terminally the smaller
branch follows the medial tibial border to the level of the ankle and the
larger passes anterior to the medial malleolus to distribute to the skin and
fascia on the medial side and dorsum of the foot.
Articular branches arising from the nerve to the rectus
femoris muscle accompany the corresponding branches of the lateral femoral
circumflex artery to the hip joint. Twigs from the branches to the vastus
muscles and from the saphenous nerve supply the knee joint.
LATERAL FEMORAL CUTANEOUS NERVE
The lateral femoral cutaneous nerve (L2, 3) emerges from the lateral
border of the psoas major muscle, passes obliquely over the iliacus muscle
behind the parietal peritoneum and iliac fascia (which it supplies) toward the
anterior superior iliac spine, and enters the thigh by passing under or through
the lateral end of the inguinal ligament (see Plate 2-5). The nerve then
passes over or through the proximal part of the sartorius muscle and descends deep to the fascia lata. It gives
off a number of small branches to the overlying skin before piercing the fascia
about 10 cm below the inguinal ligament. The terminal branches of the lateral
femoral cutaneous nerve supply the skin and fascia on the anterolateral
surfaces of the thigh between the levels of the greater trochanter of the femur
and the knee.
OBTURATOR NERVE
The obturator nerve (L2, 3, 4) supplies the obturator externus and
adductor muscles of the thigh, gives filaments to the hip and knee joints, and
has a variable cutaneous distribution to the medial sides of the thigh and leg
(see Plate 2-6).
The obturator nerve arises from the anterior divisions of the ventral
rami of the second, third, and fourth lumbar nerves. The contribution from L2
is commonly the smallest and is sometimes absent. These roots unite within the
posterior part of the psoas major muscle, forming a nerve that descends through
the muscle to emerge from its medial border opposite the upper end of the
sacroiliac joint. The obturator nerve runs outward and downward over the sacral
ala and pelvic brim into the lesser pelvis, lying lateral to the ureter and
internal iliac vessels. It then bends anteroinferiorly to follow the curvature
of the lateral pelvic wall (anterior to the obturator vessels and lying on the
obturator internus muscle) to reach the obturator groove at the upper part of
the obturator foramen. The nerve passes through this groove and foramen to
enter the thigh and divides into anterior and posterior branches shortly
thereafter.
The anterior branch runs in front of the obturator externus and
adductor brevis muscles and behind the pectineus and
adductor longus muscles. Near its origin, it gives off an articular twig that
enters the hip joint through the acetabular notch. Rarely, it supplies a branch
to the pectineus muscle and sends muscular branches to the adductor longus,
adductor gracilis, and adductor brevis muscles. The anterior branch finally
divides into cutaneous, vascular, and communicating branches.
The cutaneous branch is inconstant. When present, it unites with
branches of the saphenous and anterior femoral cutaneous nerves in the adductor
canal to form the subsartorial plexus and assists in the innervation of
the skin and fascia over the distal two thirds of the medial side of the thigh.
Infrequently, this branch is larger and passes between the adductor longus and
gracilis muscles to descend behind the sartorius to the medial side of the knee
and the adjacent part of the leg, where it assists the saphenous nerve in the
cutaneous supply of those areas.
The vascular branches end in the femoral artery. Other fine communicating
branches may link the obturator nerve with the
anterior and posterior femoral cutaneous nerves and the inconstant accessory obturator
nerve.
The posterior branch pierces the anterior part of the obturator
externus muscle and supplies it. There-after, the nerve runs downward between
the adductor brevis and adductor magnus muscles and splits into branches that
are distributed to the upper (adductor) part of the adductor magnus and
sometimes to the adductor brevis (especially if the latter does not receive a
supply from the anterior branch of the obturator nerve). A slender branch
emerges from the lower part of the adductor magnus, passes through the hiatus
of the adductor canal together with the femoral artery, and then
continues to the knee joint. The posterior branch contributes filaments to the
femoral and popliteal vessels and ends by perforating the oblique popliteal
ligament to supply the articular capsule, cruciate ligaments, and synovial
membrane of the knee joint. The fibers to the capsule and ligaments are mostly
of somatic origin, whereas those to the synovial membrane are mainly
sympathetic.
The accessory obturator nerve (L3, 4) is inconstant, small, and
derived from the anterior divisions of the ventral rami of L3 and L4. It
descends on the medial border of the psoas muscle and then crosses the superior
pubic ramus to lie behind the pectineus muscle. It ends by
helping to supply the pectineus but may also supply one twig to the hip joint
and another twig that joins the anterior branch of the obturator nerve.
SCIATIC NERVE
The roots of the sciatic nerve (L4, 5; S1, 2, 3) arise from the ventral
rami of the fourth lumbar to third sacral nerves and unite to form a single
trunk that is ovoid in cross section and 16 to 20 mm wide in adults (see Plate 2-7). In the
lesser pelvis, the nerve lies anterior to the piriformis muscle, below which it
enters the buttock through the greater sciatic foramen (in about 2% of
individuals, the nerve pierces the piriformis). Next, the nerve inclines
laterally beneath the gluteus maximus muscle, where it rests on the posterior
surface of the ischium and the nerve to the quadratus femoris muscle. On its
medial side, it is accompanied by the posterior femoral cutaneous nerve and by
the inferior gluteal artery and its special branch to the nerve.
On reaching a point about midway between the ischial tuberosity and the
greater trochanter of the femur, the nerve turns downward over the gemellus
muscles, the obturator internus tendon, and the quadratus femoris muscle (which
separate it from the hip joint) and leaves the buttock to enter the thigh
beneath the lower border of the gluteus maximus muscle.
The sciatic nerve then descends near the middle of the back of the
thigh, lying on the adductor magnus muscle and being crossed obliquely by the
long head of the biceps femoris muscle. Just above the apex of the popliteal
fossa, it is overlapped by the contiguous margins of the biceps femoris and
semimembranosus muscles. In about 90% of cases the sciatic nerve divides into
its terminal tibial and common peroneal branches near the apex of
the popliteal fossa, whereas in 10% of cases the division occurs at higher
levels. Rarely, the tibial and common peroneal nerves arise independently from
the sacral plexus but pursue closely related courses until they reach the apex
of the popliteal fossa.
In the buttock, the sciatic nerve supplies an articular branch to
the hip, which perforates the posterior part of the joint capsule (see Plate
2-4). It may also supply vascular filaments to the inferior gluteal
artery. (The entrance of the sciatic nerve and its variable relationship to the
piriformis muscle are described on page 60.)
At levels below the quadratus femoris muscle, two branches of the tibial
division of the sciatic nerve spring from its medial side to supply the
so-called hamstring muscles of the thigh. The upper branch passes to the long
head of the biceps femoris muscle and the upper portion of the semitendinosus;
the lower branch innervates the lower portion of the semitendinosus and the
semimembranosus muscles and the ischiocondylar portion of the
adductor magnus muscle. The nerve to the short head of the biceps femoris
muscle arises from the lateral side of the sciatic nerve (common peroneal
division of the sciatic nerve) in the middle third of the thigh and enters the
superficial surface of the muscle. From this nerve, an articular branch
continues to the knee, providing proximal and distal branches that accompany
the lateral superior genicular and lateral inferior genicular arteries to the
knee joint.
The tibial and common peroneal nerves arise by a division of the sciatic
nerve, usually at the upper limit of the popliteal fossa. The tibial nerve
continues the vertical course of the sciatic nerve at the back of the knee and
into the leg (see Plate 2-7). The common peroneal nerve follows the
tendon of the biceps femoris muscle along the upper lateral margin of the
popliteal space and into the leg, curving forward around the neck of the fibula
(see Plate 2-7).
