Among women with atraumatic femur fractures, the Sydney AFF Score showed efficacy in distinguishing atypical femur fractures (AFF) from typical femur fractures (TFF), according to the results of a study published in the Journal of Bone and Mineral Research. This score is based on age and 2 femur measurements.
Atypical femur fractures are a rare complication of long-term bisphosphonate or denosumab use in patients with osteoporosis. Prompt identification of AFF is essential to ensuring appropriate surgical management and reducing the risk of fracture in the contralateral femur by allowing for screening for cortical stress lesions and stopping bisphosphonate use, according to the study authors.
Although the American Society for Bone and Mineral Research (ASBMR) diagnostic criteria for atypical subtrochanteric and diaphyseal femoral fractures are the gold standard for AFF diagnosis, these criteria require a time-consuming qualitative assessment of each fracture. In some cases, AFFs may go unrecognized or may be confused with TFF. These drawbacks warrant the development of a quantitative measure that rapidly and accurately identifies AFF.
The researchers examined retrospective data from 110 women presenting to a single center with AFF (n=65) or TFF (n=45). They used multiple logistic regression to identify independent predictors of fracture status and a decision-tree analysis to develop the Sydney AFF score.
This analysis showed that patients with AFFs were significantly younger (76.4 vs 81.9 years, P <.001), were 4 times more likely to be taking bisphosphonates (P <.001), and 2.5 times more likely to be using calcium supplements (P =.002) than patients who experienced TFFs.
Patients with AFFs had smaller femur neck width (P <.001), femoral head diameter (P =.001), femoral width at inferior trochanter (P =.014), hip axis length (P =.037), and medulla width 20 mm above lesser trochanter (P =.024) compared with those with TFFs. Patients with AFFs also had a wider lateral cortical width at the lesser trochanter than those with TFFs (P =.004).
The Sydney AFF score was then developed based on the following 3 dichotomized independent predictors: age, femoral neck width, and lateral cortical width at the lesser trochanter. The score adds 1 point for each predictor: [age ≤80 years] + [femoral neck width <37 mm] + [lateral cortical width at lesser trochanter ≥5 mm] = (score 0-3).
Validation testing was conducted in a separate cohort of 53 women who presented to a different center. This testing showed that a Sydney AFF score of 2 or more had a 73.3% sensitivity and 69.6% specificity for AFF and remained an independent predictor of AFF after adjustment for bisphosphonate use.
Limitations to this study include its retrospective design and its use of fracture radiographs, which may make measuring femoral geometry difficult due to rotational deformity and altered posture.
The study authors said this research represents “the first steps” in the development of an AFF prediction score and hope that the findings “will inspire groups to develop a large-scale prospective study which would lead to a clinical algorithm to predict the risk of AFF.”
Crouch G, Dhanekula ND, Byth K, et al. The Sydney AFF Score: a simple tool to distinguish females presenting with atypical femur fractures versus typical femur fractures. J Bone Miner Res. Published online February 2, 2021. doi:10.1002/jbmr.4255