The Appendicular Skeleton: Structure, Function, and Development

The Appendicular Skeleton

Overview of the Appendicular Skeleton

The appendicular skeleton includes the bones of the upper and lower limbs, as well as the girdles that attach them to the axial skeleton. It is essential for movement and manipulation of the environment.

• Main components: Pectoral girdle, upper limbs, pelvic girdle, lower limbs

• Function: Facilitates locomotion and interaction with surroundings

• Attachment: Girdles anchor limbs to the axial skeleton

The Pectoral Girdle

Structure and Function

The pectoral girdle consists of the clavicle and scapula, which attach the upper limb to the trunk. This structure allows for a wide range of motion at the shoulder.

• Clavicle: S-shaped bone that connects the arm to the trunk at the sternum

• Scapula: Flat, triangular bone providing attachment for muscles and articulation with the humerus

• Function: Supports arm movement and stabilizes the shoulder

Anatomy of the Scapula

The scapula has several important landmarks for muscle attachment and articulation.

• Acromion: Articulates with the clavicle

• Glenoid cavity: Articulates with the head of the humerus

• Spine, coracoid process, borders, and angles: Serve as muscle attachment sites

Bones of the Upper Limb

Divisions and Major Bones

The upper limb is divided into the arm, forearm, and hand, each with distinct bones and joints.

• Arm: Humerus (single bone)

• Forearm: Radius (lateral), Ulna (medial)

• Hand: Carpals (wrist), Metacarpals (palm), Phalanges (fingers)

Forearm Anatomy

The radius and ulna are connected by an interosseous membrane and articulate with each other at proximal and distal radioulnar joints.

• Radius: Lateral bone, important for wrist movement

• Ulna: Medial bone, forms the elbow joint with the humerus

Bones of the Hand

The hand consists of the carpal bones (wrist), metacarpals (palm), and phalanges (fingers).

• Carpals: Eight bones arranged in two rows

• Metacarpals: Five bones forming the palm

• Phalanges: Fourteen bones in the fingers (three per finger, two in the thumb)

Radiographic Anatomy of the Hand

X-rays reveal the arrangement of the hand's bones and joints, which are crucial for diagnosing fractures and other conditions.

The Carpal Tunnel

The carpal tunnel is a passageway on the anterior side of the wrist, formed by carpal bones and the flexor retinaculum. It allows passage of tendons and the median nerve into the hand.

• Clinical relevance: Compression of the median nerve leads to carpal tunnel syndrome

Hand Grip Mechanics

During a firm grip, the fourth and fifth metacarpals move anteriorly, increasing contact and grip strength.

Common Upper Limb Fractures

Fractures can occur at various sites in the upper limb, with the distal radius (Colles fracture) being the most common in the forearm.

• Surgical neck of humerus: Common fracture site in elderly

• Colles fracture: Distal radius fracture, often from falls

The Pelvic Girdle and Pelvis

Structure and Function

The pelvic girdle is formed by the hip bones, which attach the lower limbs to the axial skeleton. The pelvis includes the hip bones, sacrum, and coccyx.

• Function: Supports body weight, protects pelvic organs, and provides attachment for lower limbs

Regions of the Hip Bone

The adult hip bone is composed of three fused regions: ilium, ischium, and pubis.

• Ilium: Superior, fan-shaped portion

• Ischium: Posteroinferior portion

• Pubis: Anteromedial portion

Ligaments of the Pelvis

Strong ligaments stabilize the pelvis and form important foramina for neurovascular structures.

• Posterior sacroiliac ligament: Supports sacroiliac joint

• Sacrospinous and sacrotuberous ligaments: Form greater and lesser sciatic foramina

Sex Differences in the Pelvis

The female pelvis is adapted for childbirth, being broader and more spacious than the male pelvis.

• Female pelvis: Broader, larger subpubic angle, rounder pelvic brim, wider lesser pelvic cavity

• Male pelvis: Narrower, heavier, smaller subpubic angle

Table: Comparison of Female and Male Pelvis

	Female pelvis	Male pelvis
Pelvic weight	Bones of the pelvis are lighter and thinner	Bones of the pelvis are thicker and heavier
Pelvic inlet shape	Pelvic inlet has a round or oval shape	Pelvic inlet is heart-shaped
Lesser pelvic cavity shape	Lesser pelvic cavity is shorter and wider	Lesser pelvic cavity is longer and narrower
Subpubic angle	Subpubic angle is greater than 80 degrees	Subpubic angle is less than 70 degrees
Pelvic outlet shape	Pelvic outlet is rounded and larger	Pelvic outlet is smaller

Bones of the Lower Limb

Divisions and Major Bones

The lower limb is divided into the thigh, leg, and foot, each with specialized bones for weight-bearing and movement.

• Thigh: Femur (single bone)

• Leg: Tibia (medial, weight-bearing), Fibula (lateral, slender)

• Foot: Tarsals (ankle), Metatarsals (mid-foot), Phalanges (toes)

The Q-Angle

The Q-angle measures the lateral deviation of the femur from the vertical axis of the tibia. It is typically larger in females due to a wider pelvis.

• Clinical relevance: Increased Q-angle can predispose to knee injuries

Leg Bones: Tibia and Fibula

The tibia is the main weight-bearing bone of the leg, while the fibula provides lateral stability and muscle attachment.

• Tibia: Medial, larger, supports body weight

• Fibula: Lateral, slender, not weight-bearing

Bones of the Foot

The foot is structured for support and movement, with tarsal bones forming the ankle, metatarsals the mid-foot, and phalanges the toes.

• Tarsals: Seven bones, including the calcaneus (heel) and talus (ankle joint)

• Metatarsals: Five bones

• Phalanges: Fourteen bones (three per toe, two in the big toe)

Development of the Appendicular Skeleton

Embryonic Limb Buds

Limb buds appear in the embryo by the end of the seventh week, marking the beginning of appendicular skeleton development.

Ossification Centers and Congenital Deformities

Primary and secondary ossification centers form in the developing bones. Congenital deformities, such as clubfoot, can occur but are often treatable.

• Primary ossification: Begins in the diaphysis (shaft) of long bones

• Secondary ossification: Occurs in the epiphyses (ends) after birth

• Clubfoot: Common congenital deformity, usually correctable