(29) Lower Limb Anatomy: Thigh, Tibia, and Fibula Study Guide

THE TIBIA AND FIBULA

The tibia and fibula are the two bones that comprise the leg. These bones serve distinct mechanical and anatomical purposes within the lower limb.

The Tibia

The tibia is the larger of the two bones and is primarily responsible for weight-bearing. It possesses significant strength and establishes articulations at both its proximal and distal ends:

  • Proximal Articulation: It articulates with the femur to form the knee joint.

  • Distal Articulation: It articulates with the talus of the foot to form the ankle joint.

  • Manual Landmark: On the anterior and proximal surface, there is a prominence known as the tibial tuberosity. This is the insertion point for the quadriceps femoris, the large muscle group of the thigh responsible for extending the leg.

  • Clinical Significance: The tibia is the most frequently fractured long bone in the human body. Because its location is superficial (just beneath the skin), fractures are often compound, meaning the bone breaks through the skin.

The Fibula

The fibula is located on the lateral side of the leg. Unlike the tibia, the fibula is not weight-bearing. Its primary roles include acting as a site for muscle attachment and contributing to the stability of the ankle.

  • Articulations: The fibula does NOT articulate with the femur. However, it does participate in the formation of the ankle joint distally.

  • Attachment: It is connected to the tibia both proximally and distally by strong ligaments. Additionally, a strong interosseous membrane runs between the tibia and fibula along their lengths.

The Ankle Joint Landmarks

The tibia and fibula together form a bracket-like structure that surrounds the talus at the ankle joint. These bones create palpable landmarks just beneath the skin:

  • Medial Malleolus: The bony prominence on the medial side, formed by the tibia.

  • Lateral Malleolus: The bony prominence on the lateral side, formed by the fibula.

SUPERFICIAL ANATOMY

Deep Fascia

The muscles of the lower limb are encased in a thick layer of fibrous connective tissue known as deep fascia. This fascia has specific names based on its location:

  • Fascia Lata: The deep fascia surrounding the thigh.

  • Crural Fascia: The deep fascia surrounding the leg.

The Iliotibial Tract (ITB)

Along the lateral aspect of the thigh, the fibers of the fascia lata are oriented vertically, forming a distinct band called the iliotibial tract or iliotibial band. This tract represents part of the insertion of hip muscles onto the lateral aspect of the knee and leg.

Functions of Deep Fascia

  1. Venous Pump: The fascia lata and crural fascia assist in pumping venous blood out of the lower limb. During muscle contraction, the fascia prevents the muscles from bulging outward, forcing them instead to compress thin-walled veins.

  2. Efficiency: It helps various muscles work in unison more efficiently during activities like walking and running.

Superficial Fascia, Nerves, and Veins

The superficial fascia is the loose, fatty layer situated between the deep fascia and the skin. It contains superficial veins and cutaneous nerves.

Superficial Veins

  • Great Saphenous Vein: This is the most important superficial vein. The term "saphenous" means obvious, reflecting its large and constant nature.     * Path: It drains blood from the medial aspect of the foot, leg, and thigh.     * Termination: it returns blood to the femoral vein by passing through the saphenous opening, an aperture in the fascia lata of the proximal thigh.     * Landmark: It is almost always found running just anterior to the medial malleolus at the ankle.

Nerves of the Lower Limb

  1. Saphenous Nerve: A branch of the femoral nerve. It follows the great saphenous vein through the leg and ankle and provides sensory innervation to the skin over the medial leg, ankle, and foot.

  2. Superficial Fibular Nerve: Innervates the majority of the dorsum (top) of the foot.

  3. Deep Fibular Nerve: Innervates the specific cleft between the big toe and the second toe.

  4. Sural Nerve: Originates from both the tibial and common fibular nerves. It supplies the lateral side of the ankle and foot.     * Surgical Use: Because it is large, constant, superficial, and redundant, it is frequently used as a source of nerve sheath during reconstructive surgery.

OVERVIEW OF THIGH COMPARTMENTS

The thigh is divided into three primary compartments, each with specific actions, innervations, and blood supplies:

  • Anterior Compartment:     * Primary Actions: Extend leg and flex thigh.     * Innervation: Femoral nerve.     * Blood Supply: Femoral artery.

  • Medial Compartment:     * Primary Actions: Adduct thigh.     * Innervation: Obturator nerve.     * Blood Supply: Obturator artery.

  • Posterior Compartment:     * Primary Actions: Flex leg and extend thigh.     * Innervation: Sciatic nerve.     * Blood Supply: Perforating branches of the deep femoral artery.

ANTERIOR COMPARTMENT OF THE THIGH

This compartment contains muscles that flex the hip and extend the knee. While most follow the general rules of femoral nerve innervation and femoral artery supply, there are notable exceptions.

Quadriceps Femoris

This is the largest muscle in the body and consists of four heads of origin. All four heads insert onto the tibial tuberosity via the patella and the patellar ligament.

The Patellar Reflex

Physicians tap the patellar ligament with a hammer to test the patellar reflex. A positive response (the leg extending) indicates that the femoral nerve and its reflex arc are intact. This corresponds to spinal levels L2L2, L3L3, and L4L4. The mnemonic used is "L2, L3, L4 kick the foot off the floor."

The Four Heads

  1. Rectus Femoris: Originates from the anterior inferior iliac spine (AIIS). It crosses two joints (hip and knee), allowing it to flex the hip and extend the knee.

  2. Vastus Lateralis: Takes origin from the femur; extends the knee only.

  3. Vastus Medialis: Takes origin from the femur; extends the knee only.

  4. Vastus Intermedius: Takes origin from the femur; extends the knee only.

Sartorius

This is the longest muscle in the body. It is a strap-like muscle that runs obliquely from the anterior superior iliac spine (ASIS) to the medial part of the tibia. Due to its circuitous path, its actions are:

  • Flexes, abducts, and laterally rotates the hip.

  • Flexes the knee (Exception: most anterior muscles extend the knee).

Iliopsoas

This is the primary flexor of the hip joint and is comprised of two muscles that insert together on the lesser trochanter of the femur:

  • Iliacus: Originates from the iliac fossa in the abdomen. It is usually innervated by the femoral nerve.

  • Psoas Major: Originates from the lumbar vertebrae. It is innervated by branches of the lumbar plexus (Exception: not the femoral nerve).

Pectineus

This is a small muscle running from the superior pubic ramus to the pectineal line on the posterior femur. It flexes and adducts the thigh. It occupies the border between the anterior and medial compartments.

  • Innervation: Usually the femoral nerve, but can be the obturator nerve.

Tensor Fasciae Latae (TFL)

Located on the lateral aspect of the thigh near the iliac crest, it lies between the anterior compartment and the gluteal region.

  • Origin: Iliac crest and ASIS.

  • Insertion: Lateral side of the head of the tibia via the iliotibial tract.

  • Function: Helps steady the trunk over the thigh when standing; assists in flexing and abducting the thigh.

  • Innervation: Superior gluteal nerve (Exception: not the femoral nerve).

MEDIAL COMPARTMENT OF THE THIGH

Muscles in this compartment primarily adduct the thigh at the hip. Most are innervated by the obturator nerve.

The Obturator Nerve and Canal

The obturator nerve reaches the thigh by passing through the obturator foramen. The foramen is almost entirely covered by fascia and muscles (from the word "obturate," meaning to close over), leaving only a small gap called the obturator canal for vessels and nerves.

Adductor Group

These muscles generally take origin from the pubis and insert on the linea aspera of the femur.

  • Adductor Brevis: Short adductor.

  • Adductor Longus: Longer than the brevis.

  • Adductor Magnus: The largest of the three. It has a complex arrangement:     * Adductor Part: Inserts on the linea aspera.     * Hamstring Part: Originates from the ischial tuberosity and inserts on the adductor tubercle of the femur. It acts like a hamstring (extends the thigh) and is innervated by the tibial part of the sciatic nerve rather than the obturator nerve.

Adductor Hiatus

The lower border of the adductor magnus arches to create a gap known as the adductor hiatus. This allows femoral vessels to pass from the anterior thigh into the popliteal fossa (becoming popliteal vessels).

Gracilis

A long, strap-like muscle running from the pubis to the medial tibia. It adducts the thigh and can also flex the leg at the knee.

Obturator Externus

Located on the external surface of the obturator membrane. It inserts on the femur in the gluteal region and helps to rotate the thigh laterally.

THE FEMORAL TRIANGLE

The femoral triangle is a critical anatomical region in the proximal thigh.

Boundaries and Floor

  • Superior Boundary: Inguinal ligament (running from the pubis to the ASIS).

  • Lateral Boundary: Sartorius muscle.

  • Medial Boundary: Medial margin of the adductor longus muscle.

  • Floor: Composed of the iliopsoas, pectineus, and adductor longus muscles.

Contents

Major structures pass deep to the inguinal ligament. From lateral to medial, they are organized by the mnemonic NAVEL:

  1. Nerve (Femoral Nerve)

  2. Artery (Femoral Artery)

  3. Vein (Femoral Vein)

  4. Empty space (Femoral Canal)

  5. Lacunar Ligament

Clinical and Structural Details

  • Femoral Canal: A weak spot in the abdominal wall medial to the vein. It is the site for femoral hernias, where a loop of small intestine protrudes, appearing as a lump in the triangle.

  • Femoral Sheath: A sleeve of abdominal fascia that surrounds the femoral artery, femoral vein, and lymphatics within the canal. Crucially, the femoral nerve is NOT encased by the femoral sheath.

  • Femoral Artery: A continuation of the external iliac artery. It provides the principal blood supply to the lower limb.     * It gives rise to the deep femoral artery, which provides perforating branches to the posterior compartment and medial/lateral femoral circumflex arteries to the hip.     * It continues deep to the sartorius toward the knee, eventually becoming the popliteal artery.

  • Anatomical Rule: Major vessels and nerves cross joints along the flexor side, making them superficial and prone to damage.

POSTERIOR COMPARTMENT OF THE THIGH

This compartment contains the hamstring muscles, which extend the thigh and flex the leg. They are generally innervated by the tibial part of the sciatic nerve and supplied by the perforating branches of the deep femoral artery.

The Hamstrings

True hamstrings originate from the ischial tuberosity and insert in the proximal leg.

  • Semitendinosus: Inserts medially on the tibia. Its name comes from being half muscle belly and half cord-like tendon.

  • Semimembranosus: Inserts medially on the tibia. It has a broad, sheet-like (membranous) tendon.

  • Biceps Femoris (Long Head): Inserts laterally on the head of the fibula.

Short Head of the Biceps Femoris

This muscle is the "oddball" of the posterior compartment because it is NOT a hamstring.

  • Origin: Shaft of the femur (not the ischial tuberosity).

  • Insertion: Head of the fibula (with the long head).

  • Action: Acts ONLY on the knee joint (flexion).

  • Innervation: Common fibular nerve (not the tibial nerve).