20-Bio-A4 Anatomy and Physiology · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2013 — 04-Bio-A4 Biomechanics, 3 hours, closed book. Four 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) Bone originally injured. The image shows a single long bone of the forearm carrying a metal fixation plate and screws along its shaft; the white arrow marks the original fracture/fixation site on the radius — the bone most commonly plated after a forearm shaft fracture (and the more frequently symptomatic of the two forearm bones for hand-function loss, since forearm rotation/pronation-supination and much of grip mechanics depend on it).
b) What has happened now. The x-ray shows the fixation plate itself has fractured (a clean break through the metal, visible at one of the screw holes), and the bone has re-fractured/displaced at that same level — i.e. this is a hardware (implant) failure with a peri-implant re-fracture, not a fresh, unrelated injury.
c) Biomechanical mechanism. A metal plate is far stiffer than bone, so once rigidly screwed in place it carries a disproportionate share of every bending/torsional load the forearm experiences during normal use (stress shielding of the underlying bone, which can also delay or prevent full union beneath the plate). Screw holes act as stress concentrators in the plate. Under the thousands of sub-maximal bending/torsion cycles of daily forearm use (grip, pronation-supination, axial loads), a fatigue crack nucleates at one such stress concentration and slowly propagates through the plate's cross-section — a classic fatigue (cyclic-loading) failure rather than a single overload event. Once the plate parts, the bone — especially if union beneath the plate was still incomplete — loses its rigid splint and fails under a subsequent normal-use load, producing the displaced re-fracture seen here.
d) Surgical procedure. Revision open reduction and internal fixation (ORIF): remove the failed plate and any loose screws, debride the fracture site of fibrous non-union tissue, and re-fix with a new, longer and/or thicker plate that spans well past the original failure zone (bridging fresh, unweakened bone on each side), typically combined with autologous bone grafting (e.g. iliac crest) to stimulate union where the fatigue failure and stress shielding have compromised local bone stock.
e) Possible complications. Two of: infection (deep or superficial, particularly with retained/removed hardware and a second surgery); non-union or delayed union (recurrence, given the first fixation had already failed to achieve solid healing); a further peri-implant fracture at the new plate's end; or iatrogenic nerve injury (e.g. superficial radial nerve) during hardware removal/revision.
