Nerve Plexuses

Several nerve plexuses exist within the pelvis and send branches to somatic structures (skin and skeletal muscle) of the pelvis and lower limb. The lumbar plexus is composed of the ventral rami of spinal nerves L1-L4, which give rise to two large nerves, the femoral and obturator nerves, and several smaller branches (Fig. 6-6). The femoral nerve (L2-L4) innervates muscles of the anterior thigh, whereas the obturator nerve (L2-L4) innervates muscles of the medial thigh.

The sacral plexus is composed of the ventral rami of spinal nerves L4-S4. Its major branches are summarized in Figure 6-7 and Table 6-4. The small coccygeal plexus has contributions from S4-Co1 and gives rise to small anococcygeal branches that innervate the coccygeus muscle and skin of the anal triangle (see Chapter 5). Often the lumbar and sacral plexuses are simply referred to as the lumbosacral plexus.

Access to the Lower Limb

Structures passing out of or into the lower limb from the abdominopelvic cavity may do so through one of the following four passageways (see Figs. 5-3 and 6-10):

Neurovascular Structures

The nerves innervating the gluteal muscles arise from the sacral plexus (see Figs. 6-7 and 6-8 and Tables 6-4 and 6-5) and gain access to the gluteal region largely by passing through the greater sciatic foramen. The blood supply to this region is via the superior and inferior gluteal arteries, which are branches of the internal iliac artery in the pelvis (see also Fig. 5-13 and Table 5-6) and also gain access to the gluteal region via the greater sciatic foramen. These neurovascular elements pass in the plane deep to the gluteus medius muscle (superior gluteal neurovascular bundle) or deep to the gluteus maximus muscle (inferior gluteal neurovascular structures). Also passing through the gluteal region is the largest nerve in the body, the sciatic nerve (L4-S3), which exits the greater sciatic foramen, passes through or more often inferior to the piriformis muscle, and enters the posterior thigh passing deep to the long head of the biceps femoris muscle (see Fig. 6-8).

The internal pudendal artery and pudendal nerve (a somatic nerve, S2-S4) pass out of the greater sciatic foramen, wrap around the sacrospinous ligament, and reenter the lesser sciatic foramen to gain access to the pudendal (Alcock’s) canal (see Figs. 5-22 and 6-8). The pudendal nerve innervates the skeletal muscle and skin of the perineum (see Table 6-4). The internal pudendal artery is the major blood supply to the perineum and external genitalia.

Clinical Focus 6-5   Pressure (Decubitus) Ulcers

Pressure ulcers (bedsores) are common complications in patients confined to beds or wheelchairs. They form when soft tissue is compressed between a bony eminence (e.g., greater trochanter) and the bed or wheelchair. Comatose, paraplegic, or debilitated patients cannot sense discomfort caused by pressure from prolonged contact with hard surfaces. Common ulcer sites are shown in the figure, with more than half associated with the pelvic girdle (sacrum, iliac crest, ischium, and greater trochanter of femur). The four stages of these ulcers are as follows:

• Stage I: Changes in skin temperature, consistency, or sensation; persistent redness

• Stage II: Partial-thickness skin loss, similar to an abrasion with a shallow crater or blister

• Stage III: Full-thickness skin loss with subcutaneous tissue damage and a deep crater

• Stage IV: Full-thickness skin loss with necrosis or damage to muscle, bone, or adjacent structures

Clinical Focus 6-6   Iliotibial Tract (Band) Syndrome

Iliotibial tract syndrome is common in runners and presents as lateral knee pain, often in the midrange of flexion, between 20 and 70 degrees of knee flexion. The iliotibial tract, often referred to as “iliotibial band” by clinicians, rubs across the lateral femoral condyle, and this pain also may be associated with more proximal pain from greater trochanteric bursitis.

Bones

The femur, the longest bone in the body, is the bone of the thigh. It is slightly bowed anteriorly and runs slightly diagonally, lateral to medial, from the hip to the knee (Fig. 6-9 and Table 6-6). Proximally the femur articulates with the pelvis, and distally it articulates with the tibia and the patella (kneecap), which is the largest sesamoid bone in the body. The proximal femur is supplied with blood from the medial and lateral femoral circumflex branches of the deep femoral artery (see Fig. 6-13), an acetabular branch of the obturator artery, and by anastomotic branches of the inferior gluteal artery. The shaft and distal femur is supplied by femoral nutrient arteries and by anastomotic branches of the popliteal artery, the distal continuation of the femoral artery posterior to the knee.

Clinical Focus 6-7   Fractures of the Shaft and Distal Femur

Femoral shaft fractures occur in all age groups but are especially common in young and elderly persons. Spiral fractures usually occur from torsional forces rather than direct forces. Fractures of the distal femur are divided into two groups depending on whether the joint surface is involved. If reduction and fixation of intraarticular fractures are not satisfactory, osteoarthritis is a common posttraumatic complication.

Anterior Compartment Thigh Muscles, Vessels, and Nerves

Muscles of the anterior compartment exhibit the following characteristics (Figs. 6-10 and 6-11 and Table 6-7):

Additionally, the psoas major and iliacus muscles (which form the iliopsoas) pass from the posterior abdominal wall to the anterior thigh by passing deep to the inguinal ligament to insert on the lesser trochanter of the femur. These muscles act jointly as powerful flexors of the thigh at the hip joint (Table 6-7; see also Fig. 4-32).

Medial Compartment Thigh Muscles, Vessels, and Nerves

Muscles of the medial compartment exhibit the following characteristics (see Figs. 6-10 and 6-11 and Table 6-8):

The pectineus muscle, while residing in the medial compartment, is largely innervated by the femoral nerve, although it also may receive a branch from the obturator nerve. The adductor magnus, being an exceptionally large and powerful muscle, also receives some innervation via the tibial portion of the sciatic nerve, which runs in the posterior compartment of the thigh.

Posterior Compartment Thigh Muscles, Vessels, and Nerves

Muscles of the posterior compartment exhibit the following characteristics (Fig. 6-12 and Table 6-9; see Fig. 6-8):

Clinical Focus 6-8   Thigh Muscle Injuries

Muscle injuries are common and may include pulled muscles (muscle “strain,” actually a partial tearing of a muscle-tendon unit) from overstretching, or actual muscle tears, which can cause significant focal bleeding. Groin injuries usually involve muscles of the medial compartment, especially the adductor longus. Because the hamstring muscles cross two joints and are actively used in walking and running, they can become pulled or torn if not adequately stretched and loosened before vigorous use. Likewise, a “charley horse” is a muscle pain or stiffness often felt in the quadriceps muscles of the anterior compartment or in the hamstrings. Additionally, quadriceps muscle tears and tendon disruptions can occur, especially in athletes (see images).

Femoral Triangle

The femoral triangle is located on the anterosuperior aspect of the thigh and is bound by the following structures (see Fig. 6-10):

Inferiorly, a fascial sleeve extends from the apex of the femoral triangle and is continuous with the adductor (Hunter’s) canal; the femoral vessels course through this canal and become the popliteal vessels posterior to the knee. The femoral triangle contains the femoral nerve and vessels as they pass beneath the inguinal ligament and gain access to the anterior thigh (see Fig. 6-10). Within this triangle is a fascial sleeve called the femoral sheath, a continuation of transversalis fascia and iliac fascia of the abdomen, that contains the femoral artery and vein and medially the lymphatics. Laterally the femoral nerve lies within the femoral triangle but outside this femoral sheath. The most medial portion of the femoral sheath is called the femoral canal and contains the lymphatics that drain through the femoral ring and into the external iliac lymph nodes. The femoral canal and ring are a weak point and the site for femoral hernias. The femoral ring is narrow, and consequently, femoral hernias may be difficult to reduce and may be prone to strangulation.

Clinical Focus 6-9   Diagnosis of Hip, Buttock, and Back Pain

Athletically active individuals may report hip pain when the injury may actually be related to the lumbar spine (herniated disc), buttocks (bursitis or hamstring injury), or pelvic region (intrapelvic disorder). Careful follow-up should examine all potential causes of the pain to determine whether it is referred and thus originates from another source.

Femoral Artery

The femoral artery supplies the tissues of the thigh and then descends into the adductor canal to gain access to the popliteal fossa (Fig. 6-13 and Table 6-10). The superomedial aspect of the thigh also is supplied by the obturator artery. These vessels form anastomoses around the hip and, in the case of the femoral-popliteal artery, around the knee as well (see Fig. 6-13).

Clinical Focus 6-10   Revascularization of the Lower Limb

Peripheral vascular disease and claudication can usually be managed medically by reducing the associated risk factors. However, patients who are refractory to medical management have the following invasive options:

• Percutaneous angioplasty: balloon dilation (with or without an endovascular stent) for recanalization of a stenosed artery (percutaneous revascularization)

• Surgical bypass: bypassing a diseased segment of the artery with a graft (1-3% operative mortality)

Clinical Focus 6-11   Femoral Pulse and Vascular Access

The femoral pulse is felt at about the midpoint of the inguinal ligament. The femoral artery at this point lies directly over or just medial to the femoral head, just lateral to the femoral vein and about a finger’s breadth medial to the femoral nerve (see Figs. 6-10 and 6-13). The femoral artery and vein may be used to gain access to major vessels of the limbs, abdominopelvic cavity, and thorax (e.g., catheter threaded through femoral artery and into aorta for coronary artery angiography and angioplasty). Similarly, the larger veins of the inferior vena cava and the right side of the heart and pulmonary veins may be accessed through the femoral vein.

Thigh in Cross Section

Cross sections of the thigh show the three compartments and their respective muscles and neurovascular elements (Fig. 6-14). Lateral, medial, and posterior intermuscular septae divide the thigh into the following three sections:

Refer to the muscle tables to note several exceptions to these general divisions. However, learning the primary action and general innervation of the muscles by functional compartments will help you organize your study. Also, note that the large sciatic nerve usually begins to separate into its two component nerves—the tibial nerve and the common fibular nerve—in the thigh, although this separation may occur proximally in the gluteal region in some cases.