Intramedullary nailing (IMN) is considered a gold-standard treatment for femoral shaft fractures because of its effectiveness in maintaining the various mechanical and physiological characteristics of the fractured bone [[1], [2], [3]]. The IMN acts as a rigid internal fixator in the medullary cavity of a bone to provide sufficient support against the axial, bending, and torsional load. In addition, it offers more remarkable soft-tissue preservation around the fracture site fixed through closed reduction [2,3]. Despite these advantages and well-established treatment modalities, femoral shaft fractures have more re-admissions due to nonunions or delayed unions than other long bone fractures [4]. Also, the costs associated with these re-admissions of femoral shaft fractures are higher than others [5]. Some observational studies have also shown that even after performing IMN of femoral shaft fractures, the prevalence of a nonunion remained as high as 6–12.5%, and most require additional operation [6,7]. Therefore, it is clinically relevant for orthopedic surgeons to find any modifiable surgical factors associated with a nonunion to resolve this serious problem.

Several risk factors of a nonunion peculiar to femoral shaft fractures after the IMN have been reported, such as postoperative fracture gaps due to poor reduction, distal fracture, narrow nail diameter, and delayed weight-bearing [[8], [9], [10]]. Among these factors, orthopedic surgeons can modify the problem of postoperative fracture gap. Although open reduction can minimize the fracture gap size, it can damage the soft tissues surrounding the fracture site, which could harm bone union [2,11]. In contrast, closed reduction does not invade the soft tissue; however, it is difficult to complete a closed reduction without a gap, even with the advent of various reduction tools [12].

Therefore, it would be helpful if some indicators could be identified to determine how much of a fracture gap needs to be corrected. In tibial shaft fractures, a fracture gap of ≥3 mm would prolong the bone union; hence, further reduction maneuvers should be performed in such cases [13]. However, the cut-off value of the fracture gap size for a nonunion in femoral shaft fractures is not known. Furthermore, the measurement method and the evaluation standard of the fracture gap have not been clearly described in previous research. Although there is no consensus regarding the measurement method for the fracture gap size, it is commonly agreed that fracture gaps are inimical to bone unions [8,13]. Development of a correct measurement method for fracture gap sizes and the knowledge of how large gaps increase the risk of delayed unions/nonunions could help in postoperative prognosis and in deciding whether open reduction should be performed, which is very useful clinically.

This study aims to clarify how we should evaluate fracture gaps when assessing simple femoral shaft fractures with radiographs and to determine the acceptable cut-off value of the fracture gap size in simple femoral shaft fractures.