: Postoperative axial deviations and delayed unions are possible complications after flexible intramedullary nailing (FIN). The goals of this study were to determine if a correlation exists between occurrence of the above complications and the ratio of the diameter between nail and medullary canal [nail diameter (ND)/MCD ratio], to study the interobserver variability in the measurement of MCD, and to define a threshold to be respected to optimize the results.