The prediction of stress fractures using a 'stressed volume' concept

This paper addresses an anomaly which exists in the Current literature rega rding stress fractures. Analysis of the data on fatigue strength of bone sa mples in vitro would conclude that these fractures should never occur at th e strain levels known to occur in vivo. This anomaly can be resolved by inc luding in the analysis the effect of stressed volume, whereby larger volume s of material are expected to have worse fatigue properties. A Weibull anal ysis was used to predict the probability Of failure, Pt: this was an upper- bound prediction because it did not include the effects of remodelling and adaptation. Combining this analysis with a finite element model of the huma n tibia, we predicted a P-f value of 21% after five weeks of strenuous exer cise, which is comparable with reported incidences in military personnel. T he high incidence of stress fractures in the cannon bone of racehorses coul d also be predicted (P-f = 62%. compared to 70% experimentally). The approa ch can be used to investigate the effect of variables in the exercise regim e such as the distance run per day and (he use of improved footwear. It can also predict the increased risk of stress fractures in elderly people. The results Suggest certain simple rules which may be of clinical value in des igning exercise regimes and in understanding the risk factors for this type of injury. (C) 2001 Orthopaedic Research Society. Published by Elsevier Sc ience Ltd. All rights reserved.