The case definition, community incidence, and characteristics of atypical femoral shaft fractures (FSFs) are poorly understood. This retrospective study utilized electronic medical records and radiograph review among women !50 years of age and men !65 years of age from January 1996 to June 2009 at Kaiser Permanente Northwest to describe the incidence rates and characteristics of subgroups of femur fractures. Fractures were categorized based on the American Society for Bone and Mineral Research (ASBMR) as atypical fracture major features (AFMs) (low force, shaft location, transverse or short oblique, noncomminuted) and AFMs with additional minor radiograph features (AFMms) (beaking, cortical thickening, or stress fracture). There were 5034 fractures in the study. The incidence rates of FSFs (without atypical features) and AFMs appeared flat (cumulative incidence: 18.2 per 100,000 person-years, 95% CI ¼ 16.0-20.7; 5.9 per 100,000 person-years, 95% CI ¼ 4.6-7.4; respectively) with 1,271,575 person-years observed. The proportion of AFMs that were AFMms increased over time. Thirty percent of AFMs had any dispensing of a bisphosphonate prior to the fracture, compared to 15.8% of the non-atypical FSFs. Years of oral glucocorticosteroid dispensing appeared highest in AFM and AFMm fractures. Those with AFMs only were older and had a lower frequency of bisphosphonate dispensing compared to those with AFMms. We conclude that rates of FSFs, with and without atypia, were low and stable over 13.5 years. Patients with only AFMs appear to be different from those with AFMms; it may be that only the latter group is atypical. There appear to be multiple associated risk factors for AFMm fractures. ß

Both radiographic techniques used in the study recorded high sensitivity and negative predictive value. However, positive predictive value was poor, especially with fine detail radiographs. Nevertheless, CT with reformatted images did appear to be superior to fine detail radiographs in accurately identifying nonunions in this animal model.

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.