Australian researchers have developed a simple tool which helps distinguish between atypical femur fractures (AFF) and typical femur fractures unrelated to severe trauma in women.

Its developers said it would help identify fractures that are a rare but serious complication of long‐term bisphosphonate use and which may otherwise may go unrecognised.

The score, published in the Journal of Bone and Mineral Research, was developed in a discovery cohort of patients from Royal North Shore Hospital. The Sydney AFF Score was then validated in a second independent cohort from Westmead Hospital.

It found age ≤ 80, femoral neck width <37mm and lateral cortical width ≥5 mm at the lesser trochanter were the independent predictors of fracture status, after adjustment for bisphosphonate use.

The researchers proposed several mechanisms by which the three factors related to the development of AFF.

“Subjects younger than 80 years may be more mobile and prone to greater micro-fracture accumulation due to increased femoral loading,” they said.

“Smaller, more compact femurs, as reflected in the Score by femoral neck width <37 mm, may accumulate greater doses of bisphosphonate per unit mineral matrix, with greater suppression of bone turnover.”

“Smaller bones may also result in targeted convergence of local stress reactions, facilitating the propagation of micro-cracks and the evolution of a macroscopic stress fracture.”

“Cortical thickening may represent an early adaptive response to stress accumulation and an attempt to resist failure of the cortical bone.”

The researchers, including senior investigator and RNSH endocrinologist Dr Christian Girgis, said the score was not able to predict AFF in the population at large or amongst patients on bisphosphonates.

“Rather, the score was derived retrospectively to distinguish AFF and TFF among a set of femoral shaft fractures.”

“Clinical decisions regarding the use of bisphosphonates and potential drug-holidays to alleviate the risk of AFF cannot be made, at this time, on the basis of the Sydney AFF Score.”

They concluded that a prospective study was necessary to confirm the predictive value of the score and its potential contribution to decision making regarding long-term bisphosphonate use.