Skeletal System: Functions, Bone Classifications, Anatomy, and Disorders

1) Functions of the Skeletal System

Support**: Provides framework for the body and supports soft tissues.

Protection**: Shields vital organs (e.g., skull protects brain, rib cage protects heart and lungs, vertebrae protect spinal cord).

Movement**: Osteons and joints act as levers; muscles attach to bones and allow locomotion.

Mineral Storage and Homeostasis**: Reservoir for minerals, especially calcium and phosphorus; releases them as needed.

Blood Cell Production (Hematopoiesis)**: Red bone marrow generates red blood cells, white blood cells, and platelets (primarily in long bones of children and in certain adult bones).

Triglyceride Storage**: Yellow bone marrow stores fats in adipose tissue.

Endocrine Regulation**: Osteoblasts release osteocalcin, which influences glucose metabolism and fat storage.

2) Four Classifications of Bones

Bones are classified by shape.

Long Bones**: Longer than they are wide; consist of a diaphysis (shaft) and epiphyses (ends). Examples: femur, humerus, radius, tibia.

Short Bones**: Cube-like; about as wide as they are long; mostly spongy bone with a thin outer compact layer. Examples: carpals (wrist), tarsals (ankle).

Flat Bones**: Thin, flattened bones with two parallel plates of compact bone surrounding a layer of spongy bone; often protect organs and provide broad surfaces for muscle attachment. Examples: cranial bones, sternum, ribs, scapulae.

Irregular Bones**: Complex shapes that do not fit other categories. Examples: vertebrae, some facial bones, pelvic bones.

Note: Some classifications also recognize sesamoid bones (e.g., patella) embedded within tendons, and sutural (Wormian) bones within sutures of the skull.

3) Major Anatomical Areas of a Long Bone

A typical long bone (e.g., femur) has regions including:

Epiphysis**: Expanded ends (proximal and distal) that articulate with other bones.

Articular cartilage: Hyaline cartilage covering the joint surfaces to reduce friction.

Epiphyseal plate/line: Growth plate in children (cartilaginous); becomes the epiphyseal line after growth ends.

Metaphysis*: Narrow portion between the epiphysis and diaphysis; contains the *epiphyseal plate in children; where bone growth occurs.

Diaphysis**: The shaft; a hollow, cylindrical region containing:

Medullary (marrow) cavity: Contains yellow marrow in adults (fat), and red marrow in certain bones of children.

Compact bone: Dense bone surrounding the cavity; provides strength.

Periosteum**: Dense connective tissue sheath covering the outer surface (except at articular cartilage); contains nerves, blood vessels, and osteoprogenitor cells.

Endosteum**: Thin membrane lining the medullary cavity and inner surfaces of trabeculae; contains osteoprogenitor cells.

Medullary Cavity**: Central cavity within the diaphysis; houses marrow.

Row of Vascular/Nutrient Entry Points**: Nutrient foramina allow blood vessels to enter the bone.

4) Bone Anatomy and Matrix

Bone Matrix**:

Inorganic components: Hydroxyapatite crystals (calcium phosphate) that give hardness and rigidity.

Organic components:

Collagen fibers (Type I): Provide tensile strength and flexibility.

Ground substance: Proteoglycans and glycoproteins.

The matrix is organized into two main types of bone tissue:

Compact (cortical) bone: Dense, solid exterior; organized into osteons (Haversian systems).

Spongy (trabecular or cancellous) bone: Network of trabeculae with spaces filled by red or yellow marrow; lighter but strong.

Bone Cells**:

Osteoblasts: Build bone; synthesize and secrete organic components of the matrix; become osteocytes.

Osteocytes: Mature bone cells embedded in lacunae; maintain bone matrix.

Osteoclasts: Break down bone (resorption) to release minerals and remodel bone.

Osteon Structure (Compact Bone)**:

Lamellae: Concentric rings of mineralized matrix.

Haversian (central) canal: Contains blood vessels and nerves.

Lamellar organization: Provides strength and resistance to torsion.

5) Components of the Axial and Appendicular Skeleton

Axial Skeleton**: The central core of the body, comprising:

Skull: Cranial bones and facial bones.

Hyoid bone: U-shaped bone in the neck supporting the tongue.

Auditory ossicles: Malleus, incus, stapes (in the middle ear).

Vertebral column: Cervical, thoracic, lumbar vertebrae; sacrum; coccyx.

Thoracic cage: Ribs and sternum.

Appendicular Skeleton**: The girdles and limbs:

Pectoral (Shoulder) girdles: Clavicles (collarbones) and scapulae (shoulder blades).

Upper limbs: Humerus, radius, ulna, carpals, metacarpals, phalanges.

Pelvic girdle: Hip bones (coxal bones) – ilium, ischium, pubis.

Lower limbs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

6) Fractures and Disorders of the Skeletal System

Example: Fractures

Simple (Closed) Fracture**: Bone crack or break that does not penetrate the skin.

Compound (Open) Fracture**: Break that breaks the skin; higher risk of infection.

Comminuted Fracture**: Bone broken into several small fragments.

Greenstick Fracture**: Incomplete break common in children; one side bent, the other broken.

Pathological Fracture**: Fracture due to weakened bone from disease (e.g., osteoporosis, bone cancer).

Example: Osteoporosis (Disorder)

Description**: A condition characterized by reduced bone mass and deterioration of bone tissue, leading to increased fracture risk.

Risk Factors**: Aging, postmenopausal status in women, low body weight, inactivity, calcium/vitamin D deficiency, smoking, certain medications.

Symptoms**: Fragile bones, fractures with minimal trauma, back pain from vertebral compression, loss of height over time.

Prevention/Management**: Adequate calcium and vitamin D, weight-bearing exercise, lifestyle changes, medications to strengthen bone density (e.g., bisphosphonates), fall prevention.

Example: Scoliosis (Disorder)

Description**: Abnormal lateral curvature of the spine, often with vertebral rotation.

Causes**: Idiopathic in many cases, can be congenital or due to neuromuscular conditions.

Symptoms**: Uneven shoulders/hips, rib prominence, imbalance when bending forward.

Management**: Observation for mild cases; bracing or surgery (spinal fusion) for severe cases.

Example: Osteoarthritis (Disorder)

Description**: Degenerative joint disease resulting in the breakdown of articular cartilage and changes in the underlying bone.

Symptoms**: Joint pain, stiffness, reduced range of motion, swelling.

Management**: Pain relief, physical therapy, weight management, sometimes joint replacement.