Automated evaluation of hip axis length for predicting hip fracture

The hip axis length has been shown in previous studies to be predictive of hip fracture independent of age and femoral bone density. The first studies of hip axis length were performed by manual measurement of dual x-ray absorptiometry (DXA) scan printouts. In this study, an automated analysis procedure is defined using software tools provided by the DXA manufacturer. Manual and automatic hip axis length measurements in 198 women were highly correlated (r = 0.98). Because of scaling factors of the printout, the automatic measurement was 58% longer than the manual value. Precision of the automatic measurement, based upon triplicate DXA scans of 33 women, was 0.07 cm or 0.68%. To define normative data, the hip axis length was measured from femoral DXA scans of 471 female volunteers aged 40–92 scanned on 14 different Hologic QDR-1000 systems. Mean hip axis length was 10.5 cm, with a standard deviation of 0.62 cm. No significant relationship between hip axis length and age was found (r = 0.07, P = 0.15). Based on previously reported odds ratios corrected for femoral bone density, age, height, and weight, an automatic hip axis length measurement of 11.0 cm is associated with a twofold increase in hip fracture risk compared with a woman with an average hip axis length. A hip axis length value of 11.6 cm increases hip fracture risk by a factor of 4 compared with a woman with a normal hip dimension. We conclude that the hip axis length can be easily incorporated into existing DXA hip analysis software in combination with a bone density measurement. With further research, the hip axis length may become a clinically useful tool for the evaluation of hip fracture risk.