Introduction: Controversy exists regarding the use of reamed interlocking nailing in femoral shafts with extremely narrow medullary canals (diameter ≤ 9 mm). The aims of this study were to (1) investigate the association of age and sex on femoral canal diameter in patients with a simple femoral shaft fracture and (2) compare the outcomes and complications of interlocking nailing between wide and extremely narrow intramedullary canals. Patients and methods: For the purposes of this retrospective cohort study, consecutive patients with simple femoral shaft fractures were recruited between January 2009 and December 2016. The patient demographic data were analyzed. Then, fractures treated with interlocking nailing were divided into the wide group (canal diameter > 9 mm) and narrow group. The primary outcome was union rate, and the secondary outcomes were complications such as thermal necrosis, fat embolism syndrome, iatrogenic fracture, and implant failure. Results: This study included 340 femoral shaft fractures. The average canal diameter was 9.97 ± 1.79 mm, with significantly wider canals in men than in women. Overall, 289 of the patients had undergone interlocking nail fixation, and a similar union rate and complications were noted between the wide canal and narrow canal groups, with the exception of the incidence of iatrogenic fracture. Conclusions: Femoral shaft fractures associated with extremely narrow medullary canals are more common in women than in men. There was a similar union rate found when using interlocking nailing in a femoral shaft fracture in cases with extremely narrow and wider canals. Iatrogenic fracture is the only significant risk when using interlocking nailing in femoral shafts with extremely narrow canals.
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