Skeletal System: Structure, Function, and Disorders

Skeletal System

Overview

The skeletal system is composed of bone, cartilage, tendons, and ligaments. It provides protection, support, and movement for the body. The system is also responsible for blood cell production and mineral storage. There are approximately 206 bones in the adult human skeleton.

• Axial skeleton: Bones of the head, neck, and trunk (skull, vertebral column, rib cage).

• Appendicular skeleton: Bones of the limbs and girdles (arms, legs, pelvis, shoulder girdle).

Functions of the Skeletal System

• Support: Provides a framework for the body.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to act upon.

• Mineral Storage: Stores calcium and phosphate ions.

• Blood Cell Production: Houses red bone marrow for hematopoiesis.

• Triglyceride Storage: Yellow bone marrow stores fat.

Bone Structure and Classification

Major Classification of Bones

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, and usually curved (e.g., skull, sternum, ribs).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

• Sesamoid Bones: Develop within tendons (e.g., patella).

• Sutural (Wormian) Bones: Small bones found between skull sutures.

Gross Anatomy of a Typical Long Bone

• Epiphysis: Ends of the bone, usually spongy bone covered by compact bone.

• Diaphysis: Shaft, composed of compact bone surrounding the medullary cavity.

• Metaphysis: Region between diaphysis and epiphysis, includes the epiphyseal (growth) plate.

Bone Tissues

• Compact Bone: Dense outer layer; provides strength and protection.

• Spongy Bone (Cancellous Bone): Honeycomb of trabeculae; contains red bone marrow.

Bone Membranes

• Periosteum: Dense fibrous membrane covering the external surface of bones.

• Endosteum: Thin membrane lining the medullary cavity.

Bone Cells

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; synthesize bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue, found in lacunae.

• Osteoclasts: Large, multinucleated cells that resorb bone matrix.

Microscopic Structure of Bone

Compact Bone

• Composed of structural units called osteons (Haversian systems).

• Each osteon contains a central (Haversian) canal for blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae for nutrient/waste exchange.

Spongy Bone

• Composed of trabeculae; spaces filled with red bone marrow.

• No osteons; nutrients reach osteocytes by diffusion through canaliculi.

Bone Growth and Remodeling

Growth in Length

• Occurs at the epiphyseal (growth) plate via endochondral ossification.

• Epiphyseal plates close after puberty, forming the epiphyseal line.

Growth in Thickness

• Appositional growth adds new bone to the periosteal surface.

Bone Remodeling

• Continuous process of bone resorption (by osteoclasts) and deposition (by osteoblasts).

• Regulated by hormones (parathyroid hormone, calcitonin) and mechanical stress.

Bone Markings

Bone markings are features on bones that serve as attachment sites for muscles, ligaments, and tendons, or as passageways for nerves and blood vessels.

Marking	Description
Tuberosity	Large, rounded projection; may be roughened
Crest	Narrow ridge of bone; usually prominent
Trochanter	Very large, blunt, irregularly shaped process (only on femur)
Line	Narrow ridge of bone; less prominent than a crest
Tubercle	Small, rounded projection or process
Epicondyle	Raised area on or above a condyle
Spine	Sharp, slender, often pointed projection
Process	Any bony prominence
Head	Bony expansion carried on a narrow neck
Facet	Smooth, nearly flat articular surface
Condyle	Rounded articular projection
Ramus	Armlike bar of bone
Groove	Furrow
Fissure	Narrow, slitlike opening
Foramen	Round or oval opening through a bone
Notch	Indentation at the edge of a structure
Meatus	Canal-like passageway
Sinus	Cavity within a bone, filled with air and lined with mucous membrane
Fossa	Shallow, basinlike depression in a bone

Fractures and Bone Repair

Types of Fractures

Type	Description
Closed (simple)	Bone does not penetrate skin
Open (compound)	Bone penetrates skin
Comminuted	Bone fragments into three or more pieces
Compression	Bone is crushed
Depressed	Broken bone portion is pressed inward
Impacted	Broken bone ends are forced into each other
Spiral	Ragged break due to twisting forces
Greenstick	Bone breaks incompletely, like a green twig

Bone Repair Process

1. Hematoma forms at the fracture site.

2. Fibrocartilaginous callus forms.

3. Bony callus forms.

4. Bone remodeling occurs.

Joints (Articulations)

Classification of Joints

• Fibrous Joints: Bones joined by dense connective tissue; immovable (e.g., sutures of the skull).

• Cartilaginous Joints: Bones joined by cartilage; slightly movable (e.g., intervertebral discs).

• Synovial Joints: Bones separated by a fluid-filled cavity; freely movable (e.g., shoulder, hip).

Types of Synovial Joints

• Plane – intercarpal and intertarsal joints

• Hinge – elbow and interphalangeal joints

• Pivot – proximal joint between radius and ulna

• Condylar – knuckles

• Saddle – carpometacarpal joint of thumb

• Ball and Socket – shoulder and hip

Sinuses

• Frontal

• Ethmoid

• Sphenoidal

• Maxillary

Fetal Skull

• Fontanels are soft spots on the fetal skull that allow for growth and compression during birth.

• Main fontanel to know: anterior fontanel.

Homeostatic Imbalances and Disorders

• Rickets: Vitamin D or calcium deficiency in children; causes bone deformities.

• Fracture: Break in bone.

• Sinusitis: Inflammation of sinuses.

• Herniated disc: Protrusion of intervertebral disc.

• Scoliosis: Abnormal lateral curvature of the spine.

• Kyphosis: Hunchback; exaggerated thoracic curvature.

• Lordosis: Swayback; exaggerated lumbar curvature.

• Dislocation: Bone forced out of normal position in joint cavity.

• Bursitis: Inflammation of a bursa.

• Sprain: Ligament stretched or torn.

• Strain: Injury to muscle or tendon.

• Arthritis: Inflammation of joints.

• Osteoarthritis: Degenerative joint disease affecting articular cartilage.

• Crepitus: Crunching noise in joints.

• Gouty arthritis: Uric acid crystal deposition in joints.

• Osteoporosis: Bone-thinning disease, common in postmenopausal women and elderly men.

Additional Information

• Bone turnover is a dynamic process; about 10% of the skeleton is replaced annually.

• Bone health is influenced by diet, hormones, and physical activity.