Basic Sciences and Orthopedic Principles
Principles of Basic Sciences
Candidates must understand bone biology (osteoblasts, osteoclasts, osteocytes), fracture healing (primary vs secondary), biomechanics (stress-strain, fatigue, viscoelasticity), implant materials (stainless steel, titanium, cobalt-chrome, polyethylene), and sterilization (autoclave, gamma irradiation). Know genetics (osteogenesis imperfecta, achondroplasia) and pharmacology (NSAIDs, corticosteroids, anticoagulants).
Key Concepts
- Bone healing: Primary (direct) healing requires rigid fixation (compression); secondary healing involves callus (endochondral ossification). Inflammatory phase, reparative phase, remodeling phase.
- Biomechanics: Wolff's law (bone adapts to load), stress shielding (implant stiffness leads to bone resorption), fatigue failure (cyclic loading).
- Implant materials: Titanium (osseointegration, modulus closer to bone), stainless steel (stiffer, cheaper), cobalt-chrome (wear resistance), ultra-high-molecular-weight polyethylene (UHMWPE) (wear debris).
- Sterilization: Autoclave (steam), gamma irradiation (single-use implants), ethylene oxide (gas).
- Genetics: Osteogenesis imperfecta (COL1A1/COL1A2 mutations, blue sclerae, fractures), achondroplasia (FGFR3 mutation, short stature).
- Pharmacology: NSAIDs (inhibit bone healing in animal studies, controversial), corticosteroids (osteoporosis, avascular necrosis), anticoagulants (DVT prophylaxis).
Traps
- Stress shielding: Causes bone loss around stiff implants; use implants with modulus closer to bone (titanium).
- Fatigue failure: Implants can fail after cyclic loading; important in weight-bearing.
- Osteogenesis imperfecta: Autosomal dominant; blue sclerae, hearing loss, fractures.
- NSAIDs: May delay fracture healing; use cautiously.
Worked Example
A 70-year-old woman with a total hip replacement complains of thigh pain. X-ray shows bone resorption around the femoral stem. Diagnosis: stress shielding. Rule of thumb: Stress shielding is due to stiffness mismatch between implant and bone; use titanium implants with lower modulus to reduce stress shielding. Treatment may require revision if severe.