8.4 Bones of the Lower Limb

Bones of the Lower Limb

The lower limb bones form the structural framework for standing, walking, and running. From the femur in the thigh to the small bones of the toes, each bone plays a specific role in bearing body weight and enabling movement. This section covers the individual bones, how the tibia and fibula compare, and how the foot's arches work together for balance and shock absorption.

Major Bones of the Lower Limb

Femur

The femur is the longest and strongest bone in the body, located in the thigh. Its size reflects the massive forces it handles during walking, running, and jumping.

• Proximal features: The head articulates with the acetabulum of the pelvis to form the hip joint. The neck connects the head to the shaft and is a common fracture site, especially in older adults. The greater trochanter serves as an attachment for the gluteus medius and minimus, while the lesser trochanter is the attachment point for the iliopsoas.
• Distal features: The medial and lateral condyles articulate with the tibia at the knee joint. Between them, the intercondylar fossa provides attachment for the anterior and posterior cruciate ligaments (ACL and PCL).

Patella

The patella is a triangular sesamoid bone (a bone embedded within a tendon) located within the quadriceps tendon, sitting anterior to the knee joint. It articulates with the femur at the patellofemoral joint. Its main job is to increase the mechanical leverage of the quadriceps muscle, making knee extension more efficient.

Tibia

The tibia is the medial and larger bone of the leg. It bears the majority of the body's weight (roughly 85%).

• Proximal features: The medial and lateral condyles articulate with the femoral condyles. The intercondylar eminence between them provides attachment for the menisci and cruciate ligaments. The tibial tuberosity, a bony bump on the anterior surface, is where the patellar ligament attaches.
• Distal features: The medial malleolus forms the bony prominence on the medial side of the ankle.

Fibula

The fibula is the lateral and smaller bone of the leg. It bears little to no body weight but provides important muscle attachment sites and helps stabilize the ankle.

• The head of the fibula articulates with the lateral condyle of the tibia proximally.
• The lateral malleolus forms the bony prominence on the lateral side of the ankle. Together with the medial malleolus of the tibia, it creates the bony "mortise" that grips the talus at the ankle joint.

Tarsal Bones

Seven tarsal bones form the posterior half of the foot:

• Calcaneus: The heel bone and largest tarsal bone
• Talus: Sits on top of the calcaneus and articulates with the tibia and fibula to form the ankle (talocrural) joint
• Navicular: Located on the medial side, anterior to the talus
• Cuboid: Located on the lateral side, anterior to the calcaneus
• Cuneiforms (medial, intermediate, lateral): Three small bones anterior to the navicular

These bones articulate with each other, with the leg bones above, and with the metatarsals below.

Metatarsal Bones

Five long bones form the anterior half of the foot, numbered 1 through 5 from medial to lateral. They articulate with the tarsal bones proximally and the phalanges distally, forming the metatarsophalangeal (MTP) joints at the ball of the foot.

Phalanges

Fourteen phalanges form the toes. Each toe has three phalanges (proximal, middle, and distal), except the big toe (hallux), which has only two (proximal and distal). They articulate with the metatarsals proximally and with each other at the interphalangeal (IP) joints.

Arches of the Foot

The foot has three arches that distribute body weight, absorb shock during movement, and provide flexibility. Without these arches, every step would send jarring forces straight up through the skeleton.

Medial Longitudinal Arch

This is the tallest and most clinically significant arch. It runs along the medial side of the foot and is formed by the calcaneus, talus, navicular, three cuneiforms, and the first three metatarsals. Because of its height, it's the primary structure for shock absorption and weight distribution. This is the arch that "falls" in flat feet (pes planus).

Lateral Longitudinal Arch

Lower and flatter than the medial arch, it runs along the lateral side of the foot. It's formed by the calcaneus, cuboid, and the fourth and fifth metatarsals. It provides additional stability during weight-bearing, especially when standing on uneven surfaces.

Transverse Arch

This arch runs side to side (perpendicular to the longitudinal arches), formed by the cuboid, cuneiforms, and the bases of the metatarsals. It helps distribute weight across the width of the foot and maintains flexibility for adapting to different terrain.

What maintains the arches?

Three factors work together:

• Bone shape: The interlocking shapes of the tarsal and metatarsal bones create the arch framework.
• Ligaments: The plantar ligaments and plantar aponeurosis (a thick band of connective tissue on the sole) provide passive support from below.
• Tendons: Muscles like the tibialis posterior and fibularis (peroneus) longus provide dynamic support, actively adjusting arch tension during movement.

When these support structures fail or weaken, conditions like plantar fasciitis (inflammation of the plantar aponeurosis) and flat feet can develop.

Tibia vs. Fibula: Structure and Function

These two bones sit side by side in the leg but have very different roles. Understanding their differences is a common exam topic.

Similarities

• Both are long bones of the leg
• Both serve as attachment points for leg muscles and ligaments
• Both contribute to the ankle joint distally (tibia via the medial malleolus, fibula via the lateral malleolus)

Key Differences

One detail worth noting: the fibula does not articulate directly with the femur. It connects to the tibia at the proximal and distal tibiofibular joints, but it's excluded from the knee joint itself.

Functional summary: The tibia is the primary weight-bearing and stabilizing bone of the leg. The fibula's main contributions are providing muscle attachment sites and forming the lateral wall of the ankle joint for stability.

Bone Structure and Function

• The long bones of the lower limb (femur, tibia, fibula, metatarsals, phalanges) share a common structure adapted for weight-bearing: a dense cortical bone shaft (diaphysis) surrounding a medullary cavity, with spongy bone at the ends (epiphyses) to absorb compressive forces.
• The major joints of the lower limb (hip, knee, ankle) are synovial joints, meaning they have a joint capsule lined with synovial membrane that produces fluid for smooth, low-friction movement.
• Bone remodeling is an ongoing process in which osteoclasts break down old bone and osteoblasts build new bone. This allows the skeleton to repair microdamage from daily stress and adapt bone density to the mechanical loads placed on it.