What is the fine-grained topic of the following text: The incidence of a subsequent atypical femur fracture occurring in the other thigh was 53.9 percent in patients who continued bisphosphonates for three or more years after their first fracture, compared to 19.3 percent in patients who discontinued bisphosphonate use. Only return the topic.
Risk of subsequent atypical femur fractures related to prolonged bisphosphonate use.