Understanding and treatment of aseptic femoral shaft nonunion: Biological and mechanical perspectives.

Aseptic femoral shaft nonunion remains an uncommon but challenging complication despite union rates exceeding 90% after primary fixation. When nonunion occurs, it is rarely attributable to a single factor and instead reflects a complex interaction between mechanical stability and biological healing capacity, influenced by patient, injury, and surgery related factors. Contemporary understanding increasingly recognises that most aseptic femoral shaft nonunions are mechanically driven, with often preserved biological potential that fails to progress due to an unfavorable mechanical environment. This narrative review provides a comprehensive and contemporary overview of the evaluation and management of aseptic femoral shaft nonunion, with emphasis on the hierarchy between mechanics and biology. Definitions, classifications, and risk factors are discussed, highlighting the limitations of traditional radiographic classifications and the importance of integrated mechano-biological assessment. Current concepts in understanding femoral nonunion are reviewed, including interfragmentary strain theory and the unified concept of bone healing, underscoring the central role of construct stability in modulating biological response. A structured diagnostic work-up is outlined and evidence based surgical techniques are critically appraised.