Basic Sciences of the Musculoskeletal System
What this domain tests
Basic-science items on the Iranian orthopedic board are applied, not rote: you must use anatomy, biomechanics, bone-healing physiology, microbiology, and pharmacology to justify a clinical decision or explain a failure.
Core rules
- Plate bending stiffness is proportional to E·w·t³, but strength only to t². Thickness dominates stiffness; width and elastic modulus matter less.
- A tension-band construct requires an intact cortex opposite the implant, and the implant must sit on the tension side so tensile load is converted into compression.
- Primary (direct) bone healing needs anatomic compression and interfragmentary strain well under 2%; secondary healing proceeds through callus with both intramembranous and endochondral ossification.
- Implant biofilm, usually Staphylococcus aureus or S. epidermidis, can raise minimum inhibitory concentrations a thousand-fold. Rifampin is added for susceptible staphylococcal biofilm because it penetrates the glycocalyx; it must never be used alone.
- Vancomycin dosing is AUC/MIC-driven and nephrotoxic; aminoglycosides are concentration-dependent; tranexamic acid reduces transfusion requirements in major arthroplasty and trauma.
Worked example
A 3.5 mm plate is exchanged for a 4.2 mm plate. Thickness ratio = 4.2/3.5 = 1.2. Bending stiffness ratio = 1.2³ = 1.73, so the construct is roughly 73% stiffer; strength rises by 1.2² = 1.44. This explains why a modest increase in plate thickness outperforms adding screws, and why a thin plate used as a compression plate on the tension side of a femur can fatigue and break before union.
Traps that cost marks
- Confusing stiffness with strength, or forgetting stress shielding: a very rigid implant unloads cortex and produces porosis.
- Grading open fractures or judging contamination before debridement.
- Assuming negative cultures exclude infection in a patient who received antibiotics.
- Mixing up Seddon and Sunderland nerve-injury grades, or the arterial versus venous end of a microvascular repair.