Orthopaedic anatomy, biomechanics, and injury mechanisms
What this domain tests
This domain tests whether you can connect anatomy, tissue mechanics, and mechanism of injury to a credible diagnosis. The exam expects region-specific patterns, associated injuries, and the difference between a plausible mechanism and proof of a specific lesion.
Core rules and concepts
- Tissue roles: bone resists compression and bending; cartilage distributes load; ligaments restrain translation and rotation; tendons transmit muscle force; muscle absorbs energy and controls motion; nerve injury may be compressive, tractional, or ischemic.
- Load types: tension, compression, shear, torsion, and bending often combine. Bone and tendon fail differently depending on load rate, age, and pre-existing disease. Viscoelastic tissue behaves differently under fast versus slow loading.
- Regional mechanisms: cervical strain from rapid flexion-extension; shoulder dislocation with abduction and external rotation; rotator cuff injury from traction, impingement, or fall; scaphoid fracture from a fall on an outstretched hand; posterior hip dislocation from dashboard impact; ACL injury from valgus with external rotation; meniscal tear from twisting on a loaded knee; ankle fracture from inversion or eversion with rotation; thoracolumbar fracture from compression, flexion-distraction, or shear.
- Associated injuries matter: posterior hip dislocation can injure the sciatic nerve or femoral head; knee trauma can involve ligaments, meniscus, cartilage, and bone contusion; wrist trauma can include scaphoid, distal radius, and ligament injury.
Worked example
A worker falls onto an outstretched hand and has radial wrist pain. The mechanism loads the scaphoid in compression and bending. Examination shows snuffbox tenderness and pain with axial thumb loading. Initial radiographs may be normal. The correct reasoning is not that no fracture exists because the x-ray is negative; it is that scaphoid fracture remains suspected and requires immobilization and repeat or advanced imaging. The mechanism, anatomy, and exam fit the suspected injury, while the normal early film does not exclude it.
Traps that cost marks
Equating a possible mechanism with a confirmed diagnosis; ignoring associated injuries; attributing degenerative findings to a single event without baseline evidence; missing pediatric growth-plate patterns; and relying on one examination finding without imaging or follow-up when the clinical picture warrants it.