Anatomical patterns and injury count associated with attrition in trainees with hip and pelvic bone stress injuries: a retrospective cohort study

BACKGROUND/PURPOSE: Hip and pelvic bone stress injuries (BSIs) cause significant medical attrition from US Army Basic Combat Training (BCT). While the morbidity of isolated femoral neck injuries is well documented, the impact of 'total bone stress burden' (concurrent multifocal injuries) on training completion remains unclear. This study evaluated the association between anatomical injury patterns, the number of affected regions and BCT graduation outcomes. METHODS: This retrospective cohort study analysed data from 123 555 trainees for fiscal years 2015 to 2017. MRI reports were manually reviewed to identify 1407 trainees with confirmed hip or pelvic BSIs. Multivariable logistic regression models, adjusted for age and sex, were used to assess the relationship between the cumulative injury count, specific locations and graduation odds. RESULTS: BSI cases had significantly lower graduation rates (63.6%) than the total cohort (91.4%). A significant inverse relationship was observed between total bone stress burden and graduation. Compared with single-site injuries, involvement of three (OR=0.67; 95% CI 0.46 to 0.98; p=0.037), four (OR=0.59; 95% CI 0.41 to 0.84; p=0.004), or ≥5 regions (OR=0.58; 95% CI 0.40 to 0.79; p=0.001) progressively reduced graduation odds. Anatomically, involvement of the femoral head (OR=0.56; 95% CI 0.42 to 0.73; p<0.0001) and femoral neck (OR=0.77; 95% CI 0.61 to 0.97; p=0.028) was independently associated with attrition. Paradoxically, while older trainees had a higher injury incidence, they demonstrated significantly higher odds of graduating post-injury than younger recruits. CONCLUSIONS: Among trainees who underwent hip/pelvic MRI, greater total bone stress burden and specific involvement of the femoral head or neck were associated with significantly lower odds of BCT graduation. These findings may inform future risk stratification and help identify high-risk multifocal injury patterns that could be evaluated in prospective studies of targeted management strategies to improve military trainee retention.