Tehran University of Medical Sciences Journal of Orthopedic and Spine Trauma 2538-2330 8 2 2022 05 28 44 7 Hamidreza Zafari Department of Orthopedic Surgery, Joint Reconstruction Research Center, Tehran University of Medical Sciences, Tehran, Iran Seyed Arman Moein Department of Orthopedic Surgery, Bone and Joint Diseases Research Center, Chamran Hospital, Shiraz University of Medical Sciences, Shiraz, Iran Mohamadreza Razzaghof Department of Orthopedic Surgery, Joint Reconstruction Research Center, Tehran University of Medical Sciences, Tehran, Iran SM Javad Mortazavi Department of Orthopedics, Joint Reconstruction Research Center, Imam Khomeini Hospital Complex, Tehran University of Medical Sciences, Tehran, Iran 2022 03 04 2022 05 25 The skeletal system has a high healing capacity. A nonunion fracture occurs when the natural course of bone healing is impaired. Numerous local and systemic factors participate in the development of a nonunion fracture. Patients with diabetes mellitus (DM), smoking history, obesity, and malnutrition are at risk for nonunion. Moreover, the major local risk factors for impaired bone healing are malalignment, infection, mechanical stability, and tissue loss. In this brief review, we discuss the definition, epidemiology, and diagnosis of nonunion. We further explain the major contributing factors which must be considered in patient selection for nonunion revision surgeries. https://jost.tums.ac.ir/index.php/jost/article/view/370 https://jost.tums.ac.ir/index.php/jost/article/download/370/265