Understanding the Skeletal System: Structure, Function, and Adaptations
A comprehensive exploration of the human skeletal framework. We’ll examine bone composition, joint classifications, and clinical implications for medical practitioners.
Bone Tissue Composition
Cortical Bone
Dense outer layer. Comprises 80% of skeleton. Contains haversian systems.
Trabecular Bone
Spongy inner structure. Highly vascular. Contains red marrow.
Cellular Components
Osteoblasts build. Osteoclasts resorb. Osteocytes maintain.
Matrix Composition
35% organic (collagen). 65% inorganic (hydroxyapatite). Ground substance present.
Functional Classification of Joints
Synovial Joints
Freely movable. Contains synovial fluid. Examples: knee, hip, shoulder.
- Ball-and-socket
- Hinge
- Pivot
Cartilaginous Joints
Slightly movable. Connected by cartilage. Found between vertebrae.
- Symphyses
- Synchondroses
Fibrous Joints
Immovable. Connected by fibrous tissue. Examples: skull sutures.
- Sutures
- Syndesmoses
Axial vs Appendicular Skeleton
Axial Skeleton
Forms central axis of body. Contains 80 bones.
- Skull
- Vertebral column
- Ribs and sternum
- Hyoid bone
Primary function: protection of vital organs.
Appendicular Skeleton
Comprises limbs and girdles. Contains 126 bones.
- Upper limbs
- Lower limbs
- Pectoral girdle
- Pelvic girdle
Primary function: locomotion and manipulation.
Bone Development and Growth
Intramembranous Ossification
Forms flat bones. Occurs within mesenchymal membranes. Cranial bones develop this way.
Endochondral Ossification
Forms long bones. Replaces cartilage model. Requires growth plates.
Childhood Growth
Longitudinal growth at epiphyseal plates. Diaphyseal widening through appositional growth.
Adult Remodelling
Continuous turnover. Responsive to mechanical stress. Balances formation and resorption.
Skeletal Adaptations
Mechanical Loading
Weight-bearing exercise increases bone density. Wolff’s Law describes adaptive remodelling. Athletes show 10-15% greater BMD.
Hormonal Influence
Oestrogen preserves bone mass. Testosterone promotes bone growth. PTH regulates calcium homeostasis.
Nutritional Factors
Calcium and vitamin D are essential. Protein supports matrix formation. Magnesium affects crystal structure.
Age-Related Changes
Peak bone mass at 30. Gradual loss thereafter. Women lose 30-40% lifetime bone mass.
Common Pathologies and Clinical Relevance
Osteoporosis
Decreased bone density. Affects 1 in 3 women. Characteristic fractures include vertebral compression and femoral neck.
- DEXA scanning for diagnosis
- T-score below -2.5 diagnostic
Osteoarthritis
Joint degeneration. Cartilage breakdown. Osteophyte formation. Most common in weight-bearing joints.
- Kellgren-Lawrence grading system
- Joint space narrowing on X-ray
Fractures
Bone discontinuity. Classifications include simple, comminuted, and pathological. Healing occurs in 4 stages.
- Haematoma formation
- Fibrocartilaginous callus
- Bony callus
- Remodelling
Clinical Assessment Techniques
Physical Examination
Observe gait and posture. Palpate bony landmarks. Assess joint range of motion with goniometer.
Imaging Studies
Plain radiographs for initial assessment. CT scans for complex fractures. MRI visualises soft tissues.
Laboratory Testing
Calcium and phosphate levels. Vitamin D status. Bone turnover markers like P1NP and CTX.
Specialist Investigations
DEXA scanning quantifies BMD. Bone biopsy for definitive diagnosis. Scintigraphy detects metabolic activity.