Atypical femoral fractures are a separate entity, characterized by highly specific radiographic features. A comparison of 59 cases and 218 controls.

BACKGROUND Estimations of the risk of bisphosphonate associated atypical femoral fractures vary between different population-based studies, from considerable to neglectable. A possible explanation for these discrepancies could be different definitions of atypical fractures. We aimed to identify specific radiographic fracture characteristics associated with bisphosphonate use. METHODS In a previous nationwide study, 59 atypical and 218 ordinary fractures were diagnosed. The atypical fractures were defined by their stress-type fracture pattern. All fractures were now re-assessed by a physician in training, without information about bisphosphonate use. The fracture angle (0-180°) was measured. Presence of local lateral cortical thickening (a callus reaction), more than 2 fragments, or a medial spike was noted. The reader then made a judgment whether the fracture appeared as an atypical fracture based on the ASBMR criteria. RESULTS Frequency distribution analysis of the fracture angle showed a distinct subgroup, comprising 25% of all 277 fractures, with a mean of 89 and SD of 10°. Forty-two of 57 patients in this subgroup used bisphosphonates, whereas only 27 of 213 others did (specificity 0.93; 95% CI 0.88-0.96). Presence of a callus reaction had also a high specificity for bisphosphonate use (0.96; 95% CI 0.92-0.98). The ASBMR criteria had a lower specificity, increasing the number of atypical fractures without bisphosphonate use from 13 to 31. This led to a decrease in age-adjusted relative risk associated with bisphosphonate use from 47 (95% CI 26-87) to 19 (95% CI 12-29). INTERPRETATION Stress fractures of the femoral shaft are a specific entity, which is easily diagnosed on radiographs and strongly related to bisphosphonate use. Differences in diagnostic criteria may partially explain the large differences in relative risk between different population-based studies.