Atypical fractures: what they are and their incidence

incidence Th e fi rst case report describing atypical fractures of the femur was published in 2005 and reported on nine adults receiving bisphosphonate treatment [1]. Since that time, the number of case reports has increased markedly, and the existence of atypical femoral fractures (AFFs) is now widely accepted. Th e incidence remains low, but substantial mortality is associated with these fractures. Little is known regarding their etiology and mechanism. Th e measurable factors and individual characteristics contribut ing to AFF development in particular individuals require further investigation given the large number of postmenopausal women on these agents [2]. In 2009, the American Society of Bone and Mineral Research (ASBMR) appointed a task force to summarize the current state of knowledge and address key questions about AFFs. Based on a comprehensive literature review and the experiences of clinical experts on the task force, a case defi nition was generated (Table 1 of [3]). To defi ne a fracture as an AFF, the following fi ve major features must be present (Figure 1): 1) location along the femur distal to the lesser trochanter and proximal to the supercondylar fl are; 2) minimal or no trauma at fracture; 3) transverse or short oblique fracture confi guration; 4) lack of comminu tion; and 5) complete fractures extend through both cortices and may be associated with a medial ‘spike’ or incomplete fractures involve only the lateral cortex. An additional seven ‘minor features’ are often reported but are not required for classifi cation as an AFF, including a lateral periosteal reaction, cortical thickening, prodromal symptoms and bilateral incidence. Combined these criteria suggest contributing biomechanical factors include whole bone geometry and material properties [4,5], as described below. While no direct causation has been established, AFFs are often associated with long-term bisphosphonate treatment. In the 310 case reports examined by the ASBMR task force, 291 of the published reports (94%) identifi ed bisphosphonate use as a comorbidity, with a mean treatment duration of 7 years. While this association is present in case reports, a similar connection has not been established in the phase III clinical trial data ([6] and see Abstract