20-Bio-A4 Anatomy and Physiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams May 2013 — 04-Bio-A4 Biomechanics, 3 hours, closed book (approved calculator allowed). Five questions constitute a complete exam paper; each question is of equal value (15 marks).
Reference texts: Winter, Biomechanics and Motor Control of Human Movement (4th ed.); Zatsiorsky, Kinematics of Human Motion; Nordin & Frankel, Basic Biomechanics of the Musculoskeletal System (5th ed.).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
a) Injury/finding. The image is not a native-bone fracture film: it shows a metallic femoral endoprosthesis (a hip hemiarthroplasty/arthroplasty stem with its spherical head seated in the acetabulum) already in place in the proximal femur. Both white arrows point to a thin radiolucent (dark) line running along the stem–bone interface — the radiographic sign of aseptic loosening of the femoral stem, presenting clinically as new hip pain and loss of weight-bearing.
b) Biomechanical mechanism. Cyclic loading during gait repeatedly transmits joint reaction force through the rigid metal stem into the surrounding femur. Because the stem is far stiffer than bone, it carries a disproportionate share of the load (stress shielding), leaving the adjacent femur under-loaded and prone to resorption, while at the same time repetitive micromotion at the stem–bone (or stem–cement) interface, aggravated by wear-particle-induced osteolysis, gradually debonds the fixation. The interface eventually fails mechanically, allowing the stem to move within the canal under load — exactly the radiolucent gap the arrows indicate.
c) Correction and healing. The indicated procedure is revision hip arthroplasty: removal of the loosened stem (and any associated cement/debris/osteolytic membrane), assessment and grafting of any bone stock defect, and insertion of a new, typically longer/extensively coated (or cemented) revision stem to re-establish stable fixation distal to the compromised bone. Return to full weight-bearing function is expected over roughly 3–6 months: an initial period of protected/partial weight-bearing (commonly 6–12 weeks) while the revision stem osseointegrates or the graft incorporates, followed by progressive loading and rehabilitation.
d) Possible complications. Two of: periprosthetic joint infection; a periprosthetic fracture around the new stem (bone weakened by the revision and by the original loosening); prosthetic dislocation; or deep-vein thrombosis/pulmonary embolism from the extended reduced-mobility recovery period.
