The Appendicular Skeleton: Structure, Function, and Variation

The Appendicular Skeleton

Overview of the Appendicular Skeleton

The appendicular skeleton consists of the bones of the limbs and the supporting girdles that connect them to the trunk. It plays a crucial role in movement and locomotion, providing attachment points for muscles and supporting the weight of the body, especially in the lower limbs.

• Major Divisions: Pectoral girdles, upper limbs, pelvic girdle, lower limbs

• Function: Facilitates movement, supports body weight, and provides leverage for muscles

Pectoral Girdles

Structure and Function

The pectoral (shoulder) girdle connects each arm to the trunk and provides a foundation for arm movement. Each girdle consists of one clavicle and one scapula.

• Clavicle: S-shaped bone, connects with the axial skeleton at the manubrium (sternal end) and articulates with the acromion of the scapula (acromial end).

• Scapula: Flat, triangular bone located on the posterior side of the rib cage; articulates with the clavicle and humerus.

• Mobility: The shoulder is highly mobile but less stable due to the lack of direct bony attachment of the scapula to the axial skeleton.

Clavicle: Surface Features

• Conoid tubercle and costal tuberosity: Attachment points for muscles and ligaments.

• Sternoclavicular joint: Articulation between clavicle and manubrium of sternum.

Scapula: Surface Features

• Borders: Superior, medial (vertebral), and lateral (axillary)

• Angles: Superior, inferior, lateral

• Subscapular fossa: Anterior surface depression

• Glenoid cavity: Articulates with the humerus to form the shoulder joint

• Processes: Coracoid process (anterior, superior), acromion (posterior, superior), and spine (posterior ridge)

• Fossae: Supraspinous (above spine), infraspinous (below spine)

Upper Limbs

Structure and Function

The upper limb consists of the arm (humerus), forearm (ulna and radius), wrist (carpals), and hand (metacarpals and phalanges). These bones facilitate a wide range of movements and dexterity.

• Arm (Brachium): Humerus

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

• Wrist: Eight carpal bones

• Hand: Five metacarpals, 14 phalanges

Humerus: Structure and Surface Features

• Head: Proximal, articulates with scapula

• Greater and lesser tubercles: Muscle attachment sites

• Intertubercular sulcus: Groove between tubercles

• Anatomical and surgical necks: Anatomical neck is between tubercles and head; surgical neck is a common fracture site

• Deltoid tuberosity: Attachment for deltoid muscle

• Radial groove: Path for radial nerve

• Epicondyles (medial and lateral): Muscle attachment; medial is the "funny bone"

• Condyle: Articulates with ulna and radius; includes trochlea (medial) and capitulum (lateral)

Forearm: Ulna and Radius

• Ulna: Medial bone; olecranon forms elbow point; trochlear notch articulates with humerus; coronoid process fits into humerus during flexion; radial notch for proximal radio-ulnar joint; ulnar styloid process stabilizes wrist.

• Radius: Lateral bone; head articulates with capitulum of humerus; radial tuberosity for biceps attachment; ulnar notch for distal radio-ulnar joint; radial styloid process stabilizes wrist.

• Interosseous membrane: Connects ulna and radius.

Wrist and Hand

• Carpal bones: Eight per wrist (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate)

• Metacarpals: Five per hand, numbered I–V from lateral (thumb) to medial

• Phalanges: 14 per hand; thumb (pollex) has two, other fingers have three each

• Carpal tunnel syndrome: Compression of the median nerve due to inflammation in the carpal tunnel

Pelvic Girdle

Structure and Function

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

• Ilium: Superior portion

• Ischium: Posterior and inferior portion

• Pubis: Anterior and inferior portion

Surface Features of the Hip Bone

• Acetabulum: Socket for femur head; formed by all three hip bone components

• Obturator foramen: Large opening for nerves and blood vessels

• Pubic symphysis: Joint between left and right pubic bones

• Iliac crest, iliac fossa, arcuate line: Support and muscle attachment

• Ischial spine, ischial tuberosity: Muscle and ligament attachment; supports body weight when sitting

The Pelvis

• Composed of: Two hip bones, sacrum, coccyx

• Sacroiliac joints: Articulation between ilia and sacrum

• Pelvic cavity: Enclosed by pelvic bones

• True (lesser) pelvis: Inferior to pelvic brim

• False (greater) pelvis: Superior to pelvic brim

• Pelvic inlet and outlet: Boundaries important for childbirth

• Perineum: Floor of the pelvic cavity, supports pelvic organs

Sex Differences in the Pelvis

• Female pelvis: Broader, smoother, lighter, wider pelvic outlet, triangular obturator foramen, pubic angle ≥ 100°

• Male pelvis: Narrower, rougher, more robust, oval obturator foramen, pubic angle ≤ 90°

Lower Limbs

Structure and Function

The lower limb bones are specialized for weight bearing and movement. They include the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.

• Femur: Thigh bone, longest and heaviest in the body

• Patella: Kneecap, a sesamoid bone within the quadriceps tendon

• Tibia: Medial, weight-bearing bone of the leg

• Fibula: Lateral, slender bone of the leg

• Tarsals: Seven ankle bones

• Metatarsals: Five foot bones

• Phalanges: 14 toe bones

Femur: Surface Features

• Head: Articulates with acetabulum of hip bone

• Fovea capitis: Ligament attachment site

• Neck: Connects head to shaft

• Greater and lesser trochanters: Muscle attachment

• Linea aspera: Posterior ridge for muscle attachment

• Epicondyles and condyles: Form part of the knee joint

Patella

• Sesamoid bone: Develops within the quadriceps tendon

• Base: Attachment for quadriceps tendon

• Apex: Attachment for patellar ligament

Leg: Tibia and Fibula

• Tibia: Medial, weight-bearing; medial and lateral condyles articulate with femur; tibial tuberosity for patellar ligament; medial malleolus at ankle

• Fibula: Lateral, non-weight-bearing; head articulates with tibia; lateral malleolus at ankle

• Interosseous membrane: Connects tibia and fibula

• Pott's fracture: Fracture involving both malleoli

Ankle and Foot

• Tarsal bones: Talus (articulates with tibia), calcaneus (heel), navicular, cuboid, three cuneiforms

• Metatarsals: Five per foot, numbered I–V from medial to lateral

• Phalanges: 14 per foot; hallux (great toe) has two, others have three each

• Arches: Longitudinal and transverse arches distribute weight and provide support; flatfeet result from loss of these arches

Individual Skeletal Variation

Sex and Age-Related Changes

Skeletal features can reveal information about muscle strength, medical history, sex, age, and body size. The female pelvis is modified for childbearing, and age-related changes include fusion of bones and reduction in bone mass.

• Relaxin: Hormone that loosens pelvic ligaments during pregnancy

• Age-related changes: Fusion of fontanelles, sutures, and epiphyses; reduction in bone mass and density with age

Region/Feature	Event	Age (years)
Bony matrix	Reduction in mineral content; increased osteoporosis risk	Begins 30–45; accelerates after 65
Fontanelles (skull)	Closure	By age 2
Frontal suture (skull)	Fusion	2–8
Epiphyseal cartilages (long bones)	Fusion	Begins ~3; varies by bone
Epiphyses (girdles)	Fusion	14–25 (varies by site)

Additional info: Bone markings become less prominent with age due to decreased muscle mass and strength. The timing of bone fusion can be used to estimate age in forensic and anthropological contexts.