Appendicular skeleton

Appendicular Skeleton Overview

Main Takeaway: The appendicular skeleton comprises 126 bones organized into the pectoral girdle, upper limbs, pelvic girdle, and lower limbs, specialized for movement and support.

1. Pectoral Girdle

Consists of the clavicle and scapula.

Clavicle

• Shape: S-shaped.

• Articulations: Medial (sternum/manubrium) and lateral (scapula/acromion).

Scapula

• Structure: Flat triangle with three borders and angles.

• Key Features:

• Glenoid cavity: Articulation for the humerus.

• Spine: Posterior bony ridge.

• Acromion: Lateral projection.

• Coracoid process: Anterior projection.

2. Upper Limb (30 bones per arm)

Humerus

• Proximal: Head (articulates with scapula), greater/lesser tubercles, and anatomical/surgical necks.

• Shaft: Deltoid tuberosity.

• Distal (Fossae):

• Anterior: Radial fossa (lateral) and Coronoid fossa (medial).

• Posterior: Olecranon fossa (largest).

• Distal (Articulations): Capitulum (lateral, for radius) and Trochlea (medial, for ulna).

Radius and Ulna

• Radius (Lateral): Head, neck, radial tuberosity, and styloid process.

• Ulna (Medial): Olecranon (elbow), trochlear notch, coronoid process, and styloid process.

Hand

• Carpus: 8 bones in two rows of four.

• Distal: Trapezium, Trapezoid, Capitate, Hamate.

• Proximal: Scaphoid, Lunate, Triquetrum, Pisiform.

• Metacarpals: 5 bones (I–V, thumb to pinkie).

• Phalanges: 14 total (3 per digit II–V; 2 in the pollex).

3. Pelvic Girdle

• Composition: Right and left ossa coxae, sacrum, and coccyx.

• Os Coxae: Formed by the fusion of ilium, ischium, and pubis (fused by ages 13–15).

• Acetabulum: Lateral socket where all three hip bones meet to articulate with the femur.

Ilium

• Largest bone; includes the ala, iliac crest, iliac fossa, and greater sciatic notch.

Ischium

• Features the ischial spine, ischial tuberosity (weight-bearing), and ischial ramus.

Pubis

• Features superior/inferior rami, pubic crest, pubic tubercle, and obturator foramen.

Sex Differences

| Feature | Female Pelvis | Male Pelvis | | :--- | :--- | :--- | | Greater sciatic notch | Wide | Narrow | | Pubic body | Rectangular | Triangular | | Obturator foramen | Large, oval | Triangular | | Subpubic angle | $> 100^\circ$ | $< 90^\circ$ |

4. Lower Limb (30 bones per leg)

Femur

• Heaviest and strongest bone.

• Proximal: Head, fovea, neck, greater/lesser trochanters.

• Shaft: Linea aspera, gluteal tuberosity.

• Distal: Medial/lateral condyles, intercondylar fossa, patellar surface.

Patella

• Triangular "kneecap" articulating with the femur's patellar surface.

Tibia and Fibula

• Tibia (Medial): Main weight-bearing bone. Includes tibial tuberosity, anterior border ("shin"), and medial malleolus (ankle).

• Fibula (Lateral): Not weight-bearing. Includes head and lateral malleolus.

Foot

• Tarsals (7): Calcaneus (heel), talus (articulates with tibia), navicular, cuboid, and three cuneiforms.

• Metatarsals: 5 bones (I–V, medial to lateral).

• Phalanges: 14 total (3 per toe II–V; 2 in the hallux).