Chapter 34. Overview of the Lower Limb

Big Picture

The bones of the skeleton provide a framework that serves as an attachment for soft tissues (e.g., muscles). The bony structure of the gluteal region and thigh, from proximal to distal, consists of the pelvis, femur, patella, tibia, and fibula (Figure 34-1A). Synovial joints and fibrous ligaments serve to connect bones together.

Pelvis

The pelvis is an irregularly shaped bone consisting of right and left pelvic bones. The pelvic bones articulate posteriorly with the sacrum, via the sacroiliac joints, and anteriorly with each other at the pubic symphysis (Figure 34-1A and B). Each pelvic bone has three components: ilium, ischium, and pubis. The acetabulum is a large cup-shaped structure at the junction where the ilium, ischium, and pubis fuse. The acetabulum protrudes laterally for articulation with the head of the femur bone. The three bony components of the pelvis form an opening, called the obturator foramen.

Ilium

The ilium is the most superior and the largest bone of the three components of the pelvis.

• Iliac crest. The entire superior margin of the ilium is thick and forms a prominent crest, which is palpable. The iliac crest is the site for muscle attachment and fascia of the abdomen, back, and lower limb. The iliac crest terminates anteriorly at the anterior superior iliac spine and posteriorly at the posterior superior iliac spine, both providing a site for muscle attachments.
• Anterior inferior iliac spine. A rounded protuberance located just inferior to the anterior superior iliac spine on the anterior surface of the ilium. The anterior inferior iliac spine serves as a site for the attachment of muscles and ligaments.
• Posterior inferior iliac spine. A less prominent spine along the posterior border of the sacral surface of the ilium.
• Iliac fossa. Has an anteromedial surface of the wing, which is concave and forms a large fossa. The iliac fossa serves as a site for muscle attachment.

Ischium

The ischium is the posterior and inferior component of the pelvic bone.

• Ischial tuberosity. The most prominent feature of the ischium, a large tuberosity on the posteroinferior aspect of the bone. The ischial tuberosity is an important site for the attachment of muscles of the lower limb, primarily the hamstrings, and for supporting the body in a seated position.
• Ischial ramus. Projects anteriorly to join with the inferior ramus of the pubis.
• Ischial spine. A prominent spine that separates the lesser sciatic notch from the greater sciatic notch.

Pubis

The pubis is the anterior and inferior part of the pelvic bone. The pubis has a body and two arms called rami.

• Pubic tubercle. A rounded crest on the superior surface of the pubis.
• Superior pubic ramus. Projects posterolaterally from the body and joins with the ilium and ischium at its base, positioned toward the acetabulum.
• Inferior pubic ramus. Projects laterally and inferiorly to join with the ramus of the ischium.

Femur

The femur is located in the thigh and is the longest bone of the body. The following landmarks are located on the femur (Figure 34-1A and C):

• Head. A spherically shaped knob on the proximal end of the femur that articulates with the acetabulum of the pelvic bone. The head is characterized by a nonarticular fovea on its medial surface, which serves as an attachment for the foveolar ligament (ligament of the head of the femur).
• Neck. A cylindrical part of the bone that connects the head to the shaft of the femur. The neck has a unique superomedial projection from the shaft at an angle of about 125 degrees and a slight forward projection.
• Greater trochanter. Extends superiorly from the shaft of the femur, just lateral to the site where the neck joins the shaft. The greater trochanter is a major attachment site for muscles.
• Lesser trochanter. A smaller but prominent conically shaped protuberance. The lesser trochanter projects posteromedially from the shaft, just inferior to the junction with the neck. The lesser trochanter serves as an attachment for muscles.
• Linea aspera. A distinct vertical ridge on the posterior aspect of the femoral shaft that serves as an attachment for muscles.
• Pectineal line. Curves medially under the lesser trochanter and around the shaft of the femur to merge with the linea aspera.
• Medial and lateral condyles. Both condyles lie at the distal aspect of the femur and articulate with the tibia to form the knee joint. The medial supracondylar lines terminate at a prominent adductor tubercle, which lies just superior to the medial condyle.
• Medial and lateral epicondyles. Rounded eminences on the medial and lateral surfaces of either condyle, which serve as the attachment for the collateral ligaments of the knee joint.
• Popliteal fossa. The posterior surface of the distal shaft of the femur forms the floor of the popliteal fossa and its margins.

Patella

The patella (knee cap) is the largest sesamoid bone in the body. It is formed in the tendon of the quadriceps femoris muscle as it crosses the anterior surface of the knee joint (Figure 34-1A). The patella has a unique triangular shape.

Big Picture

The bony structure of the leg and foot, from proximal to distal, consists of the tibia, fibula, 7 tarsals, 5 metatarsals, and 14 phalanges (Figure 34-2A). The tibia and fibula are bound together by a tough, fibrous sheath known as the interosseous membrane.

Tibia

The tibia is the medial and larger of the two bones of the leg (the fibula is the other bone). It is also the only bone that articulates with the femur at the knee joint (Figure 34-2A and B). The following landmarks are found on the tibia:

• Medial and lateral condyles. Lie on the proximal aspect of the tibia. The medial and lateral condyles are flattened in the horizontal plane and overhang the shaft of the tibia.
• Intercondylar eminence. The intercondylar region of the tibial plateau lies between the articular surfaces of the medial and lateral condyles and narrows centrally at a raised site to form the eminence. The intercondylar eminence contains sites for attachments of the anterior and posterior cruciate ligaments.
• Tibial tuberosity. A palpable inverted triangular area that lies on the proximal and anterior part of the shaft. The tibial tuberosity is a large tuberosity and is the site of attachment for the patellar ligament.
• Soleal line. A roughened and oblique line that lies posteriorly on the tibia, serving as an attachment for the soleus muscle.
• Anterior border. A sharp and palpable ridge on the anterior surface of the tibia that descends from the tibial tuberosity, down the tibial shaft.
• Interosseous border. A vertical ridge that descends along the lateral surface of the tibia. The interosseous border is the attachment site for the interosseous membrane, which is located between the tibia and the fibula.
• Medial malleolus. The most distal aspect of the tibia. The medial malleolus is shaped like a rectangular box with a bony protuberance on the medial side. The medial malleolus articulates with the talus, a tarsal bone, to form a large part of the ankle joint.
• Fibular notch. The lateral surface of the distal end of the tibia at the site where the fibula articulates.

Fibula

The fibula is lateral to the tibia and has limited involvement in weight-bearing activity. The fibular shaft is much narrower than the shaft of the tibia and is mainly enclosed by muscles (Figure 34-2A and B). The following landmarks are found on the fibula:

• Head. The most proximal aspect of the fibula. The head is a globe-shaped expansion that articulates with the tibia.
• Neck. The neck of the fibula separates the head from the fibular shaft.
• Interosseous border. A subtle vertical ridge that descends along the medial surface of the fibula. The interosseous border is the attachment site for the interosseous membrane, which is located between the fibula and the tibia.
• Lateral malleolus. The most distal end of the fibula, which expands to form a spade-shaped lateral malleolus. The lateral malleolus articulates with the lateral surface of the talus, forming the lateral part of the ankle joint.

Foot

The foot is the region of the lower limb, distal to the ankle joint, and is subdivided into three sections: ankle, metatarsus, and digits. There are five digits in the foot, beginning medially with the great toe and four lateral digits (Figure 34-2A and C). The foot has a superior surface, the dorsum, and an inferior surface, the plantar surface. The three groups of bones that comprise the foot are the tarsals, metatarsals, and phalanges.

• Tarsal bones. There are seven tarsal bones, arranged in a proximal and distal group with an intermediate bone on the medial side of the foot. The proximal group consists of the calcaneus bone and the talus bone; the intermediate bone is the navicular bone; and the distal group consists of the cuboid and the three cuneiform bones.
  • Talus bone. The most superior bone of the foot; sits on top of the calcaneus bone. The talus bone articulates with the tibia and fibula to form the ankle joint, as well as articulating with the intermediate navicular bone on the medial side of the foot.
  • Calcaneus bone. The largest of the tarsal bones. The calcaneus bone forms the heel posteriorly; anteriorly, it protrudes forward to articulate with the cuboid bone on the lateral side of the foot.
  • Navicular bone. Articulates with the talus bone posteriorly and with the cuneiform bones anteriorly.
  • Medial, intermediate, and lateral cuneiform bones. Are part of the distal group of tarsal bones. The cuneiform bones articulate with the navicular bone and the three medial metatarsal bones.
  • Cuboid bone. Articulates with the calcaneus bone and with the two most lateral metatarsal bones.
• Metatarsal bones. There are five metatarsal bones in the foot, numbered one through five, from medial to lateral. The first metatarsal is associated with the great toe and is the shortest and the thickest bone; the second metatarsal is the longest bone. Each metatarsal has a head at the distal end, a base at the proximal end, and a shaft between the head and the base.
• Phalanges. The phalanges are the very short bones of the digits. Each digit, except the great toe, has three phalanges (the great toe has only two phalanges). The phalanges are named by location as proximal, middle, and distal for the lateral four digits, and only proximal and distal for the great toe. Each phalange has a base (proximal), a shaft, and a head (distal).

Big Picture

Two facial layers, known as the superficial fascia and the deep fascia, are located between the skin and bone of the lower limb. The deep fascia divides the lower limb into anterior, medial, and posterior compartments of the thigh (fascia lata) and anterior, lateral, and posterior compartments of the leg (crural fascia). Muscles are organized into these compartments and possess common attachments, innervation, and actions.

Fascia of the Lower Limb

The lower limb consists of superficial and deep fascia (Figure 34-3A and B).

• Superficial fascia. Referred to as the subcutaneous or hypodermis layer and located deep to the skin. The superficial fascia primarily contains fat and superficial veins (i.e., greater and small saphenous veins), lymphatics, and cutaneous nerves.
• Deep fascia. Lies deep to the superficial fascia and primarily contains muscles, nerves, vessels, and lymphatics. The deep fascia of the lower limb connects or is continuous with the inguinal ligament, pubic bone, and Scarpa's fascia of the inferior abdominal wall. The deep fascia develops intermuscular septae that extend to the bones, dividing the thigh and leg into three compartments. Each compartment contains muscles that perform similar movements and have a common innervation.

Muscles of the Lower Limb

The muscles of the lower limb are organized into the following groups:

• Gluteal muscles. The muscles of the gluteal region primarily act on the hip joint, producing extension, medial rotation, lateral rotation, and abduction. In addition to producing motion, the muscles of the gluteal region are important for stability of the trunk and hip joint and for locomotion. These muscles consist of the gluteus maximus, gluteus medius, gluteus minimus, piriformis, superior gemellus, inferior gemellus, obturator internus, quadratus femoris, and tensor fascia lata muscles.
• Thigh muscles. The deep fascia divides the thigh into anterior, medial, and posterior compartments, with common actions and innervation (Figure 34-3A).
  • Muscles of the anterior compartment of the thigh. Consist of the psoas major, psoas minor, iliacus, sartorius, and quadriceps muscles. The quadriceps muscle group consists of the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius muscles. Most of the muscles share common actions (extension of the knee and flexion of the hip) and innervation (femoral nerve).
  • Muscles of the medial compartment of the thigh. Consist of the pectineus, adductor longus, adductor magnus, adductor brevis, gracilis, and obturator externus muscles. Most of the muscles share common actions (adduction of the hip) and innervation (obturator nerve).
  • Muscles of the posterior compartment of the thigh. Consist of the hamstring group of muscles, which consists of the semitendinosus, semimembranosus, and biceps femoris muscles. Most of the muscles share common actions (hip extension and knee flexion), a common proximal attachment (ischial tuberosity), and innervation (tibial nerve).
• Leg muscles. The deep fascia divides the leg into anterior, lateral, and posterior compartments with common attachments, actions, and innervation (Figure 34-3B).
  • Muscles of the anterior compartment of the leg. Consist of the tibialis anterior, extensor hallucis longus, and fibularis tertius muscles. Many of these muscles share common actions (dorsiflexion and inversion at the ankle and extension of the digits) and innervation (deep fibular nerve).
  • Muscles of the posterior compartment of the leg. Consist of the gastrocnemius, plantaris, soleus, popliteus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior muscles. Many of these muscles share common actions (plantarflexion at the ankle and flexion of the digits) and innervation (tibial nerve).
  • Muscles of the lateral compartment of the leg. Consist of the fibularis (peroneus) longus and fibularis (peroneus) brevis muscles. Many of these muscles share common actions (plantarflexion and eversion at the ankle) and innervation (superficial fibular nerve).
• Foot muscles. There are four layers of intrinsic muscles of the foot. Layer 1 consists of the abductor digiti minimi, flexor digitorum brevis, and abductor hallucis muscles. Layer 2 consists of the lumbricals and the quadratus plantae muscles. Layer 3 consists of the flexor digiti minimi, flexor hallucis brevis, and adductor hallucis muscles. Layer 4 consists of the plantar and dorsal interossei muscles. The extensor digitorum brevis and extensor hallucis brevis muscles are the only two intrinsic muscles on the dorsum of the foot.

Big Picture

The lower limb receives sensory and motor innervation from anterior rami that originate from spinal nerve levels L1–S4. The anterior rami form two networks of nerves, referred to as the lumbar plexus (L1–L4) and the sacral plexus (L4–S4), which are connected via the lumbosacral trunk (L4–L5) (Figure 34-4).

Lumbar Plexus

The lumbar plexus originates from ventral rami of L1–L4. It provides motor and sensory contributions to the anterior and medial compartment of the leg as well as to the abdominal wall and pelvic areas.

• Iliohypogastric nerve (L1). Provides a portion of the motor innervation to the abdominal wall muscles and sensory innervation to the pubic region.
• Ilioinguinal nerve (L1). Provides a portion of the motor innervation to the abdominal wall muscles and sensory innervation to the superior medial thigh and pubic area.
• Genitofemoral nerve (L1–L2). Possesses two branches, the genital branch and the femoral branch. The genital branch provides motor innervation to the cremasteric muscle (male only) and sensory innervation to the skin of the anterior scrotum or labia majora. The femoral branch provides sensory innervation to the skin over the anterior region of the thigh.
• Posterior division of the lumbar plexus
  • Lateral cutaneous nerve of the thigh (L2–L3). Provides sensory innervation to the skin over the lateral region of the thigh.
  • Femoral nerve (L2–L4). Provides motor innervation to the anterior compartment of the thigh, including the quadriceps femoris muscle group. The femoral nerve provides sensory innervation via sensory branches, which include the following nerves:
    • Anterior and medial cutaneous nerves of the thigh. Provide innervation to the skin of the anterior and medial thigh.
    • Saphenous nerve. Provides innervation to the skin on the medial leg.
• Anterior division of the lumbar plexus
  • Obturator nerve (L2–L4). Provides motor innervation to the medial compartment of the thigh (excluding the pectineus and hamstring portion of the adductor magnus muscle). In addition, the obturator nerve provides sensory innervation via the cutaneous branch of the obturator nerve to the skin over the medial region of the thigh.

Sacral Plexus

Most of the sacral plexus (L4–S4) is divided into anterior and posterior divisions. The anterior division provides the primary motor innervation to the posterior compartment of the thigh, leg, and foot. The posterior division provides the primary motor innervation to the anterior and lateral compartments of the leg.

• Anterior division of the sacral plexus
  • Pudendal nerve (S2–S4). Provides motor innervation to the muscles of the pelvic floor and sensory innervation to the skin of the perineum, penis, and clitoris.
  • Posterior femoral cutaneous nerve (S1–S3). Provides sensory innervation to the posterior region of the thigh.
  • Nerve to the superior gemellus and obturator internus muscles (L5–S2). Provides motor innervation to the superior gemellus muscle and obturator internus muscles.
  • Nerve to the inferior gemellus and quadratus femoris muscles (L4–S1). Provides innervation to the inferior gemellus and quadratus femoris muscles.
  • Tibial nerve (L4–S3). Provides motor innervation to the hamstring muscles (excluding the short head of the biceps femoris muscle), hamstring head of the adductor magnus muscle, posterior compartment of the leg, and plantar muscles of the foot. The tibial nerve provides sensory innervation to the posterolateral region of the leg and the plantar surface of the foot.
• Posterior division
  • Perforating cutaneous nerve (S2–S3). Provides sensory innervation to the skin over the inferior aspect of the gluteus maximus muscle (inferior gluteal fold).
  • Nerve to the piriformis muscle (S1–S2). Provides motor innervation to the piriformis muscle.
  • Posterior femoral cutaneous nerve (S1–S3). Provides sensory innervation to the skin over the posterior region of the thigh.
  • Superior gluteal nerve (L4–S1). Provides motor innervation to the gluteus medius, gluteus minimus, and tensor fascia latae muscles.
  • Inferior gluteal nerve (L5–S2). Provides motor innervation to the gluteus maximus muscle.
  • Common fibular (peroneal) nerve (L4–S3). Provides motor innervation to the short head of the biceps femoris muscle, muscles in the anterior and lateral compartments of the leg, and the intrinsic muscles on the dorsum of the foot. The common fibular nerve provides sensory innervation to the lateral compartment of the leg, the dorsum of the foot, and a small area between the first and second digits.

Big Picture

Branches of the lumbar and sacral plexuses supply the sensory innervation to the lower limb, providing cutaneous and dermatomal distributions. The cutaneous distribution consists of multiple nerve root levels carried by a single nerve, whereas the dermatomal distribution consists of innervation from a single nerve root carried by multiple nerves.

Dermatomal Distribution

The dermatomal innervation comes from multiple peripheral nerves that carry the same spinal levels. For example, the spinal root of L2 and L3 provides cutaneous innervation to the anterior, medial, and lateral parts of the thigh. In other words, all sensory neurons originating in this region will terminate at either the L2 or the L3 spinal nerve level.

Some principal dermatomes to remember are as follows (Figure 34-5A):

• L1. Skin overlying the inguinal region.
• L4. Skin overlying the anteromedial leg.
• L5. Skin overlying the anterolateral leg.
• S1. Skin overlying fifth digit and the calcaneus.
• S2. Skin overlying the posterior thigh.

Cutaneous Nerves

Peripheral nerves supply cutaneous innervation to an area of skin on the surface of the body. For example, the lateral region of the thigh receives its cutaneous innervation from the L2 and L3 nerve roots. However, the sensory neurons from L2 and L3 are distributed to the lateral part of the thigh via the lateral cutaneous nerve of the thigh. The principal cutaneous nerves supplying the skin of the lower limb are illustrated in Figure 34-5B.

Big Picture

The common iliac arteries provide blood supply to both lower limbs. Each iliac artery bifurcates into an external and an internal iliac artery. The internal iliac artery primarily supplies blood to the pelvic and gluteal regions, whereas the external iliac supplies blood to the remainder of the lower limbs. Blood is returned to the heart via a superficial and a deep venous system. The deep venous system follows the arteries and shares the same name as the adjacent artery (i.e., popliteal artery and vein).

Arteries

The common iliac artery bifurcates into the internal and external iliac arteries (Figure 34-6A). The internal iliac artery gives rise to the obturator artery and to the superior and inferior gluteal arteries. The external iliac artery becomes the femoral artery as it passes the inguinal ligament and enters the thigh. The femoral artery gives rise to the deep artery of the thigh and continues distally to become the popliteal artery behind the knee joint. The popliteal artery bifurcates into the anterior and posterior tibial arteries, which travel distally into the leg. These arteries continue into the dorsal and plantar surfaces of the foot. Smaller vessels throughout the lower limb branch from the larger vessels to supply muscle, bone, and joints.

Veins

Generally, the superficial and deep venous system of the lower limb drains into the internal and external iliac veins before reaching the inferior vena cava. The deep veins follow the arteries and usually consist of two or more veins that wrap around the accompanying artery (vena comitantes). The superficial veins originate in the foot and primarily consist of the great and small saphenous veins.

• Great saphenous vein. Originates along the great toe (digit 1) from the dorsal venous arch. The great saphenous vein courses anterior to the medial malleolus and travels along the medial side of the lower limb, medial to the medial epicondyle of the femur. It traverses an opening, called the saphenous opening, in the fascia lata and drains into the femoral vein (Figure 34-6B).
• Small saphenous vein. Travels along the lateral foot and ascends the posterior region of the leg to drain into the popliteal vein.