Bilateral atypical femoral fractures: how much symmetry is there on imaging?

To evaluate bilateral atypical femoral fractures (AFFs) and to compare imaging features of paired fractures. Bilateral femoral imaging studies of 124 patients on bisphosphonate therapy with at least one AFF were retrospectively reviewed. Time between AFF diagnoses was determined. The following imaging features were evaluated for each AFF: fracture location, femoral angle, length of cortical thickening, medial spike location, fracture orientation, and comminution. Associations between imaging findings on pairs of bilateral AFFs were assessed with Spearman's correlation (r(s)) and the Kappa statistic (kappa). Bilateral AFFs were present in 78/124 (62.9 %) cases (3 men, 75 women; mean age 67.3 years). Average time between contralateral AFF diagnoses was 10.3 months. Contralateral AFFs were diagnosed within 12 months of the index fracture in 60/78 (76.9 %) cases and within 3 years in 69/78 (88.5 %) cases. There was a strong correlation between bilateral AFF locations (r(s) = 0.65), with 58/76 (76.3 %) occurring within a distance of < 5 cm and 41/76 (53.9 %) within a distance of a parts per thousand currency sign2.5 cm. Bilateral AFF pairs had moderately correlated femoral angles (r(s) = 0.42), and weakly correlated lengths of cortical thickening (r(s) = 0.28). There was substantial agreement for medial spike location (kappa = 0.68) and fracture orientation (kappa = 0.64), and moderate agreement for lack of comminution (kappa = 0.42). All findings were independent of time between AFF diagnoses. Patients with unilateral atypical femoral fractures are likely to be diagnosed with a contralateral AFF within the first year of presentation. Bilateral AFFs commonly have similar imaging features, including location along the femur.