In patients with a simple femoral shaft fracture, researchers examined the association of age and gender on femoral canal diameter and compared the outcomes and complications of interlocking nailing between wide and extremely narrow intramedullary canals. Consecutive patients with simple femoral shaft fractures were recruited between January 2009 and December 2016. This retrospective cohort study included 340 femoral shaft fractures. Femoral shaft fractures are more common in women than in men associated with extremely narrow medullary canals. In cases with extremely narrow and wider canals, a comparable union rate was observed when using interlocking nailing in a femoral shaft fracture. When using interlocking nailing in femoral shafts with extremely narrow canals, iatrogenic fracture is the only significant risk.