[Nonunion: cause before treatment : From time-based definition to cause-directed treatment].

Fracture nonunion is a multifactorial complication of fracture healing. A solely time-based definition does not adequately reflect the underlying disturbances in healing and provides an insufficient basis for treatment decisions. Instead, management should be guided by a systematic assessment of the factors preventing osseous union. These include fracture-related infection, mechanical deficits, impaired biological healing potential, and patient-related risk factors. Occult infection should also be considered in clinically presumed aseptic nonunion. Mechanical instability requires restoration of adequate stability, whereas biological deficits may necessitate targeted augmentation. Modifiable patient-related risk factors should be optimized in parallel. As several contributing factors may coexist, treatment should not be based solely on temporal or morphological criteria but should address the individual causes of impaired fracture healing. A systematic, cause-directed approach therefore enables targeted treatment and, in multifactorial cases, the appropriate combination of therapeutic strategies.