The Appendicular Skeleton: Structure, Function, and Clinical Correlates

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The Appendicular Skeleton

Overview of the Appendicular Skeleton

The appendicular skeleton consists of the bones of the limbs and the girdles that attach them to the axial skeleton. It is essential for movement and manipulation of the environment. The appendicular skeleton is divided into the pectoral (shoulder) girdle, upper limbs, pelvic (hip) girdle, and lower limbs.

Pectoral girdle: Attaches the upper limbs to the trunk.
Pelvic girdle: Attaches the lower limbs to the trunk.
Upper and lower limbs differ in function but share a similar structural plan.

The Pectoral Girdle

Structure and Function

The pectoral girdle consists of the clavicle and scapula, providing attachment points for muscles and allowing a wide range of motion for the upper limb. Only the clavicle articulates directly with the axial skeleton, while the scapula is highly mobile.

Clavicle: Extends horizontally across the superior thorax; sternal end articulates with the manubrium, acromial end with the scapula.
Scapula: Lies on the dorsal surface of the rib cage, between ribs 2–7; has three borders (superior, medial, lateral) and three angles (lateral, superior, inferior).
Glenoid cavity: Shallow socket for the humerus, allowing flexibility but less stability.

Articulated pectoral girdle showing clavicle and scapula Right scapula, posterior aspect

The Upper Limb

Bone Groups and Key Features

Each upper limb contains 30 bones, grouped into the arm, forearm, and hand. These bones provide structure, leverage, and articulation for complex movements.

Arm: Humerus is the sole bone, articulating with the scapula and forearm bones.
Forearm: Consists of the radius and ulna, which articulate with each other at proximal and distal radioulnar joints.
Hand: Includes the carpal bones (wrist), metacarpals (palm), and phalanges (fingers).

Table 8.1 Bones of the Upper Limb (1 of 2)

Articulations of the Upper Limb

The elbow joint is a complex hinge joint involving the humerus, radius, and ulna. The radius and ulna are connected by an interosseous membrane, allowing pronation and supination of the forearm.

Humerus: Features include the head, anatomical and surgical necks, greater and lesser tubercles, and condyles for articulation with the forearm.
Radius and Ulna: The ulna forms the elbow's hinge with the humerus; the radius allows rotation of the hand.

Anterior view at the elbow region Posterior view of extended elbow

The Pelvic Girdle

Structure and Function

The pelvic girdle consists of two hip bones (os coxae), each formed by the fusion of the ilium, ischium, and pubis. It attaches the lower limbs to the axial skeleton and supports the weight of the upper body.

Ilium: Superior, flaring portion of the hip bone.
Ischium: Posteroinferior part of the hip bone.
Pubis: Anterior portion of the hip bone.
Acetabulum: Deep socket for the femoral head, providing stability for the hip joint.

Bones of the pelvic girdle Lateral view, right hip bone Medial view, right hip bone

Comparison of Male and Female Pelves

The male and female pelves differ in structure due to functional requirements, such as childbirth in females. These differences are evident in the shape, size, and angle of the pelvic bones.

Characteristic	Female	Male
General structure	Adapted for childbirth; lighter, thinner, broader	Heavier, thicker, more prominent markings
Pelvic inlet	Broader, more oval	Narrower, more heart-shaped
Pubic arch angle	Broader (80–90°)	More acute (50–60°)

Comparison of the male and female pelves

The Lower Limb

Bone Groups and Key Features

The lower limb is specialized for weight-bearing and locomotion. It consists of the thigh, leg, and foot, each with distinct bones and articulations.

Thigh: Femur is the largest and strongest bone in the body.
Leg: Tibia (medial, weight-bearing) and fibula (lateral, stabilizing).
Foot: Tarsals (ankle), metatarsals (sole), and phalanges (toes).

Table 8.3 Bones of the Lower Limbs (1 of 3)

Disorders of the Appendicular Skeleton

Common Disorders

Several congenital and acquired disorders can affect the appendicular skeleton, impacting mobility and quality of life.

Hip dysplasia: The head of the femur slips out of the acetabulum, often present at birth.
Clubfoot: Soles of the feet turn medially, a congenital condition.
Bone fractures: Various types, including comminuted, compression, spiral, epiphyseal, depressed, and greenstick fractures.

Congenital disorders of the lower limb: hip dysplasia and clubfoot

Common Types of Fractures

Fracture Type	Description and Comments
Comminuted	Bone fragments into three or more pieces; common in aged, brittle bones.
Compression	Bone is crushed; common in porous bones subjected to trauma.
Spiral	Ragged break from excessive twisting; common in sports injuries.
Epiphyseal	Epiphysis separates from diaphysis along growth plate; occurs where cartilage cells are dying.
Depressed	Broken bone portion pressed inward; typical of skull fractures.
Greenstick	Bone breaks incompletely, like a green twig; more common in children.

$Common types of fractures (1 of 3)$ $Common types of fractures (2 of 3)$ $Common types of fractures (3 of 3)$

Additional info: The appendicular skeleton is crucial for movement, support, and interaction with the environment. Disorders and injuries can significantly impact function, highlighting the importance of understanding its anatomy and clinical relevance.