Odontoid fracture type and angulation affect nonunion risk, but comminution and displacement do not: A propensity score matched analysis of fracture morphology

Objective: Odontoid fractures disproportionately affect older patients who have high surgical risk, but also high rates of fracture nonunion. To guide surgical decision-making, we quantified the effect of fracture morphology on nonunion among nonoperatively managed, traumatic, isolated odontoid fractures. Methods: We examined all patients with isolated odontoid fractures treated nonoperatively at our institution between 2010 and 2019. Multivariable regression and propensity score matching were used to quantify the effect of fracture type, angulation, comminution, and displacement on bony healing by 26 weeks from injury. Results: 303 consecutive traumatic odontoid fracture patients were identified, of whom 163 (53.8 %) had isolated fractures that were managed nonoperatively. Selection for nonoperative management was more likely with older age (OR=1.31 [1.09, 1.58], p = 0.004), and less likely with higher fracture angle (OR=0.70 [0.55, 0.89], p = 0.004), or higher presenting Nurick scores (OR=0.77 [0.62, 0.94], p = 0.011). Factors associated with nonunion at 26 weeks were fracture angle (OR=5.11 [1.43, 18.26], p = 0.012) and Anderson-D'Alonzo Type II morphology (OR=5.79 [1.88, 17.83], p = 0.002). Propensity score matching to assess the effect of type II fracture, fracture angulation> 10^o, displacement≥ 3 mm, and comminution all yielded balanced models (Rubin's B<25.0, 0.5 <Rubin's R<2.0). By 26 weeks, controlling for confounders, 77.3 % of type I or III fractures healed, compared to 38.3 % of type II fractures (p = 0.001). 56.3 % of non-angulated fractures healed compared to 12.5 % of fractures angled> 10^o (p = 0.015), and there was an 18.2 % lower rate of bony healing for each 10^o increase in fracture angle. Fracture displacement≥ 3 mm and comminution had no significant effect. Conclusion: Type II fracture morphology and fracture angle > 10^o significantly increase nonunion among nonoperatively managed isolated traumatic odontoid fractures, but fracture comminution and displacement ≥ 3 mm do not.