Impact of Fracture Deficit Volume on Fusion Success in Anterior Odontoid Screw Fixation.

OBJECTIVE: Anterior odontoid screw fixation (AOSF) has several advantages over posterior C1-2 fusion for Grauer type II and shallow type III odontoid fractures. However, the risk factors for fusion failure, particularly in terms of 3-dimensional (3D) measurements, remain unclear. This study investigated the impact of fracture deficit volume (FDV), a novel 3D measurement, on fusion outcomes in patients undergoing AOSF. METHODS: We enrolled 44 patients with Grauer type II or shallow type III odontoid fractures treated with AOSF at a single institution. Radiological assessments included preoperative and postoperative measurements of the fracture gap and fracture displacement on computed tomography (CT) scans. FDV was calculated through 3D CT reconstruction of preoperative and immediate postoperative CT to quantify the spatial gap between the edges of the fractures. Fusion outcomes were defined as solid union, fibrous union, or nonunion. Logistic regression and a generalized additive model (GAM) were used to identify risk factors for fusion failure after AOSF. RESULTS: Solid fusion was achieved in 77.3% of patients. A reduction in the FDV with respect to the preoperative value was significantly associated with successful fusion (p=0.028), whereas patients presenting an increased FDV postoperatively were more likely to exhibit fusion failure (p=0.006). Age≥65 years, a fracture gap≥2 mm, and an increased FDV postoperatively were significant risk factors for fusion failure. GAM analysis revealed a linear relationship between a reduced FDV and improved fusion rates (adjusted R2=0.186, p=0.018). CONCLUSION: The risk of fusion failure is greater in elderly patients, those with a fracture gap greater than 2 mm, and those with an increased FDV postoperatively. Among the modifiable risk factors, FDV had the greatest impact on fusion outcomes after AOSF.