Skeletal System: Structure, Function, Diseases, and Medical Terminology

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Orthopedics and Medical Terminology

Definition and Etymology

Orthopedics is the medical specialty focused on the anatomy and physiology of the skeletal and muscular systems. The term is derived from Greek roots:

orth/o-: means straight
ped/o-: means child
-ics: means knowledge; practice

Breakdown of the word orthopedics into its roots and meanings

Orthopedists use diagnostic tests, medical and surgical procedures, and drugs to treat skeletal and muscular diseases.

Skeletal System Overview

Structure and Organization

The skeletal system is a widespread, connected body system consisting of 206 bones and other structures. It extends from the top of the head to the tips of the fingers and toes.

Axial skeleton: Central structure including the head, neck, chest, and spinal bones.
Appendicular skeleton: Limbs, including the shoulder, arm, hip, and leg bones.

Full body skeleton showing axial and appendicular skeleton

Bones of the Skull

Lateral View

The skull consists of cranial and facial bones. The lateral view shows:

Frontal bone: Forms the forehead
Parietal bones: Form the upper sides and posterior
Occipital bone: Forms the base
Temporal bones: Form the sides
Sphenoid bone: Forms the central base
Ethmoid bone: Forms the nasal septum

Lateral view of the skull with labeled bones

Frontal View

The facial bones support the nose, cheeks, and lips. The mandible is the only moveable bone of the skull. The palatine bones fuse to form the hard palate (not shown).

Frontal view of the skull with labeled facial bones

Fontanels

Fontanels are soft spots on an infant's skull where the bones have not yet fused. The anterior fontanel is between the two frontal bones, and the posterior fontanel is between the two parietal bones. These areas are covered by fibrous connective tissue.

Infant skull showing anterior and posterior fontanels

Bones of the Chest and Shoulder

Thorax and Shoulder Girdle

The sternum and ribs form the thorax, a bony cage that protects the heart and lungs. The clavicle and scapula are bones of the shoulder.

Bones of the chest and shoulder, including sternum, ribs, clavicle, and scapula

Bones of the Spine

Vertebral Column

The spine supports the head, neck, and trunk and protects the spinal cord. It consists of five regions:

Cervical vertebrae (C1–C7)
Thoracic vertebrae (T1–T12)
Lumbar vertebrae (L1–L5)
Sacrum
Coccyx

Lateral view of the vertebral column with labeled regions

Bones of the Upper Extremity

Shoulder and Arm

The scapula joins the humerus at the glenoid fossa. The acromion of the scapula connects to the clavicle, allowing free movement of the shoulder.

Posterior view of the shoulder showing scapula, clavicle, and humerus

Forearm, Wrist, and Hand

The humerus joins with the radius and ulna. The radius and ulna rotate around each other, allowing the hand to turn. The wrist contains eight carpal bones, the hand has five metacarpal bones, and each finger has three phalanges (except the thumb, which has two).

Bones of the upper extremity including humerus, radius, ulna, carpals, metacarpals, and phalanges

Bones of the Hip and Lower Extremity

Hip Bones

The ilium and ischium on each side of the hip flow into each other, with the pubis containing a symphysis joint.

Bones of the hip including ilium, ischium, pubis, and sacrum

Leg, Ankle, and Foot

The femur joins the tibia to support body weight. The fibula is the smaller bone of the lower leg. The patella protects the knee joint. The tarsal bones in the ankle connect to the metatarsal bones in the foot, and each toe contains three phalanges (except the great toe, which has two).

Bones of the lower extremity including femur, tibia, fibula, patella, and foot bones Bones of the foot including tarsals, metatarsals, and phalanges

Joints, Cartilage, and Ligaments

Types of Joints

Joints (articulations) are formed where two bones meet. Types include:

Suture joints: Immovable, found between cranial and facial bones.
Symphysis joints: Slightly moveable, found at the symphysis pubis and vertebrae.
Synovial joints: Fully moveable, including hinge and ball-and-socket joints.

Sutures of the skull

Structure of a Bone

Osseous tissue is a type of connective tissue. The surface of a bone is covered with periosteum. A long bone has a diaphysis (shaft) and epiphyses (ends). The internal structure includes dense cortical bone, a medullary cavity with yellow marrow, and cancellous bone filled with red marrow.

Structure of a long bone showing periosteum, cortical bone, medullary cavity, and marrow

Bone and Cartilage Diseases

Types of Fractures

Fractures are classified by the pattern and cause of the break:

Colles fracture: Distal radius broken by falling on an outstretched hand.
Comminuted fracture: Bone is crushed into several pieces.
Compression fracture: Vertebrae are compressed together.
Depressed fracture: Cranium is fractured inward.
Greenstick fracture: Bone is broken on one side, common in children.
Hairline fracture: Very thin fracture line, bone pieces still together.
Oblique fracture: Bone is broken at an oblique angle.
Transverse fracture: Bone is broken perpendicular to its long axis.

$X-ray of a forearm fracture involving radius and ulna$ $Colles fracture illustration$ $Comminuted fracture illustration$ $Compression fracture illustration$ $Depressed fracture illustration$ $Greenstick fracture illustration$ $Hairline fracture illustration$ $Oblique fracture illustration$ $Transverse fracture illustration$

Osteoporosis

Osteoporosis is abnormal thinning of bone structure due to bone breakdown exceeding new bone formation, resulting in demineralization and increased fracture risk.

Normal bone versus bone with osteoporosis, showing progressive demineralization

Spinal and Lower Extremity Deformities

Scoliosis

Scoliosis is an abnormal, excessive, C-shaped or S-shaped lateral curvature of the spine. It can impair movement, posture, and breathing.

Comparison of normal spine and spine with scoliosis X-ray and model used to measure scoliosis Scoliosis screening in a child

Hallux Valgus and Clubfoot

Hallux valgus is a deformity where the great toe is angled laterally, often resulting in a bunion. Clubfoot (talipes equinovarus) is a congenital deformity where the foot is pulled downward and inward.

Hallux valgus and bunion X-ray of clubfoot

Joint Diseases

Osteoarthritis and Rheumatoid Arthritis

Osteoarthritis is a chronic inflammatory disease of joints, especially weight-bearing joints, characterized by cartilage degeneration and bone spur formation. Rheumatoid arthritis is an autoimmune disorder causing inflammation and deformity of joints, most often in the hands and feet.

Normal knee joint x-ray Knee joint with osteoarthritis, showing narrowed joint space and malalignment Hands with severe joint deformities from rheumatoid arthritis

Diagnostic and Radiologic Procedures

Bone Densitometry

Bone density tests measure bone mineral density to assess for osteoporosis. The hip and spine are optimal sites for testing.

Patient undergoing bone mineral density test

Key Abbreviations

Abbreviation	Definition
AKA	Above-the-knee amputation
BKA	Below-the-knee amputation
BMD	Bone mineral density
DJD	Degenerative joint disease
NSAID	Nonsteroidal anti-inflammatory drug
ORIF	Open reduction and internal fixation
RA	Rheumatoid arthritis
ROM	Range of motion
THR	Total hip replacement