Risk score created to identify atypical vs. typical femur fractures in women
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A model factoring in age, femoral neck width and lateral cortical width at lesser trochanter may be able to differentiate atypical vs. typical femur fractures in women, according to study data.
Christian M. Girgis, BSc(Med), MBBS(hons), PhD, FRACP, staff specialist at the Westmead Institute for Medical Research in Australia, and colleagues created the Sydney atypical femur fractures (AFF) score, a formula that examines predictors for atypical femur fractures and assigns points to women who sustained a fracture.
“In this study, we developed a simple tool to differentiate female patients suffering a femoral shaft or subtrochanteric fracture who had experienced an atypical femur fracture as opposed to an osteoporotic fracture in the absence of severe trauma,” the researchers wrote in a study published in the Journal of Bone and Mineral Research. “Unlike risk scores confirmed by prospective validation, this score is not equipped to predict atypical femur fractures in the population at large or among patients on bisphosphonates. Rather, the score was derived retrospectively to distinguish atypical femur fractures and typical femur fractures among a set of femoral shaft fractures.”
Predictors for atypical femur fractures
Researchers identified all intertrochanteric, subtrochanteric and diaphyseal fractures at Royal North Shore Hospital in Sydney from 2008 to May 2017 from the institution’s radiology database. Fractures that fit four of the American Society for Bone and Mineral Research’s major criteria were labeled as probable atypical femur fractures, whereas those satisfying one to three criteria were described as possible atypical femur fractures. Of the remaining fractures, a group of typical femur fractures was randomly selected as a comparator group. Since atypical femur fractures were rarely seen among men, only women were included in the study.
The final analysis included 65 women with atypical femur fractures and 45 with typical femur fractures. Women with atypical femur fractures were younger than those with typical femur fractures (mean age, 76.4 years vs. 81.9 years; P < .001). No other demographics or comorbidities were associated with fracture type.
The atypical femur fracture group had more people using bisphosphonates (77% vs. 18%; P < .001) at the time of the fracture compared with the typical femur fracture cohort. Those with atypical femur fractures had a smaller femur neck width compared with the typical femur fracture group (35.3 mm vs. 38.4 mm; P < .001) as well as a larger lateral cortical width at the lesser trochanter (5.19 mm vs. 4.45 mm; P = .004).
Sydney AFF score
After excluding bisphosphonate use — because it is an established risk factor for atypical femur fractures — age, femoral neck width and lateral cortical width at the lesser trochanter were identified as independent predictors for atypical femur fractures. Researchers created the Sydney AFF score, in which women would receive a point if they were aged 80 years or younger, had a femoral neck width of less than 37 mm, and had a lateral cortical width at the lesser trochanter of 5 mm or greater. Women with a score of 2 or higher were more likely to have an atypical femur fracture than a typical femur fracture.
When the Sydney AFF score was applied to the Royal North Shore Hospital study cohort, the area under the curve was 0.81, and a score of 2 or more had a sensitivity of 74.5% and specificity of 75%.
The Sydney AFF score was tested in a second, independent cohort of 30 women with atypical femur fractures and 23 with typical femur fractures at Westmead Hospital in Sydney. In this cohort, the AUC was 0.78 and a score of 2 or more had a sensitivity of 73.3% and specificity of 69.6%, similar figures to the North Shore Hospital group. The Sydney AFF score also remained associated with identifying atypical femur fractures after adjustments were made for bisphosphonate use.
“This study represents the first step toward the development of an atypical femur fracture prediction score, and we hope that the description of the Sydney AFF score, the factors we have identified and the methodology we have employed will inspire groups to develop a large-scale prospective study, which would lead to a clinical algorithm to predict the risk of atypical femur fractures,” the researchers wrote.
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