Background:

Atypical femoral fractures (AFF) are rare fractures associated with bisphosphonate use. Although excessive suppression of bone turn-over and secondary mineralization may contribute to the pathogenesis, risk factors other than bisphosphonate exposure have not been reported in prospective studies. Published case series also have not identified risk factors which predict AFF (1). We aimed to characterise AFF patients by comparing bone mineral density (BMD) and biochemical parameters to those with typical hip and femoral fractures seen in Austin Health.

Methods:

We prospectively collected data for consecutive femoral fractures seen at Austin Health over a 2 year period. AFF was defined as per ASBMR guidelines and adjudicated by 2 independent radiologists. BMD, Body Mass Index (BMI), total ALP, 25 OH Vit D and eGFR were measured. High trauma fractures and strontium ranelate treated patients were excluded. Typical femoral and hip fractures were used as comparative groups. ANOVA regression was used to assess differences between groups.

Results:

Of the 1516 consecutive low trauma fractures identified, there were 14 AFF, 13 typical femoral fractures (TFF) and 92 hip fractures (HF). All AFF subjects had bisphosphonate exposure (4 – 10 years). Age, lumbar spine (LS) T-score and biochemical parameters were similar between groups. Femoral neck (FN) T-score of AFF patients (-1.4 ± 1.1) was 39% (p<0.02) and 44% (p<0.001) higher than TFF (-2.3 ± 0.9) and HF (-2.5 ± 0.8) patients. BMI was 20% higher in AFF and TFF compared to HF patients (p<0.003). After adjusting for BMI, FN T-score remained elevated in the AFF patients.

Conclusion:

FN but not LS T-score was higher in AFF compared to control groups with proximal femoral fractures. In addition to reduced micro-fracture removal due to bisphosphonate use, higher BMI in AFF patients might alter weight-bearing biomechanics and contribute to the risk of stress fractures in the femoral shaft.