NONINVASIVE MEASUREMENT OF DISTAL RADIUS INSTABILITY

Except far subjective clinical criteria, there is no formal definition of distal radius fracture instability in the literature. The purposes of this ex vivo biomechanical study were (1) to provide an objective mechanical definition of fracture instability and (2) to demonstrate a noninvasive method that allows for direct measurement of instability. The following 3 questions are addressed: (1) Can the stability of dis tal radius fractures be measured using computed tomography (CT)? (2) A re the stability measurements reproducible? (3) How does external fix ation change stability?. A CT technique is described that was used to measure displacement of fracture fragments and measure the compliance of ex vivo distal radius fractures before and after external fixation. Validation studies of the CT technique revealed a mean coefficient of variation of 0.38. There was a linear relationship between measured a nd known displacements for all 3 orthogonal planes (coefficient of det ermination 0.99; p <.01). There was significant fracture displacement with loads as small as 20 N. The slope of the load-displacement curve (structural compliance) provided a quantitative measure of fracture in stability. Fracture compliance decreased up to 69% after application o f an external fixator.