Redefining Optimal Nail to Medullary Canal Diameter Ratio in Stainless Steel Flexible Intramedullary Nailing of Pediatric Femur Fractures

Purpose: The ideal canal fill for flexible intramedullary fixation of pediatric femoral shaft fractures is considered to be 80% based upon relatively few clinical studies. The purpose of this study is to assess the relationship between the summed nail to intramedullary canal diameter (ND/MCD) ratio and alignment at radiographic union following flexible intramedullary nailing (FIMN) of pediatric femoral shaft fractures. Methods: An Internal Review Board approved, retrospective review of a consecutive series of patients who sustained a femoral shaft fracture treated by retrograde, stainless steel FIMN was performed at a single level 1 pediatric trauma center from 2005 to 2012. Preoperative radiographs were analyzed to determine fracture pattern, location, and isthmic canal diameter. ND/MCD ratio was calculated using the sum of the known nail diameters and the measured isthmic diameter. Radiographs at bony union were reviewed to measure shortening, coronal angulation, and sagittal angulation. ND/MCD ratio was analyzed to determine correlative factors with final radiographic outcomes. Results: In total, 261 children underwent retrograde FIMN at an average age of 8.2 years (range, 2.2 to 17.0 y). ND/MCD ratio of >= 80% was seen in 108 (41.4%) patients. When compared with those with < 80% ND/MCD ratio, there were no significant differences in age (8.8 vs. 8.0 y), sex (76.9% vs. 71.0% males), or body mass index (18.5 vs. 17.2 kg/m(2)). There were significantly more length unstable fractures in the < 80% ND/MCD ratio group (49.4% vs. 29.7%; P < 0.01). Radiographic outcome was no different with respect to coronal angulation (2.7 vs. 3.0 degrees), sagittal angulation (3.0 vs. 3.2 degrees), or shortening (2.5 vs. 4.1 mm). ND/MCD ratio of >= 70% was seen in 176 (67.4%) patients and, when compared with the < 70% ND/MCD ratio group, there were no differences in shortening (3.3 vs. 3.9 mm), coronal angulation (2.8 vs. 3.0 degrees), or sagittal angulation (3.0 vs. 3.4 degrees). Finally, 6.9% of the population (18 patients) had ND/MCD ratios < 60% and did not demonstrate a significant increase in shortening, coronal, or sagittal angulation compared with groups with higher ND/MCD ratios. No group had an increased rate of infection, implant removal, nonunion, or need for reoperation. Conclusions: In a large series of consecutive patients treated with retrograde stainless steel FIMN there does not appear to be any correlation between the ND/MCD ratio and radiographic outcome. Stainless steel flexible IM nails seem to maintain fracture alignment without an increase in complications at lower ND/MCD ratios than previously reported as "optimal."