Safety of non-osseous union of type II odontoid fractures-a multi-institutional cohort study.

Background Context: The management of type II odontoid fractures in elderly patients presents significant clinical challenges. Surgical treatment may lead to operative complications, while conservative management may increase the risk of non-osseous union, potentially compromising fracture stability.

Purpose: This study aims to evaluate the safety of non-osseous union subtypes in type II odontoid fractures following conservative treatment and to identify risk factors for unstable fractures.

Study Design: A multi-institutional retrospective cohort study.

Patient Sample: A total of 307 patients with acute type II odontoid fractures treated conservatively between 2005 and 2022 were included. The mean age was 76±17 years, with a median follow-up of 24 months (IQR 9-55 months).

Outcome Measures: Fracture healing and stability were assessed. Safety of each healing subtype was determined by the incidence of new neurological deficits post collar removal or the need for surgical fixation. Risk factors for unstable fractures were also determined.

Methods: Fracture healing was classified as osseous union, fibrous nonunion, or unstable nonunion based on CT and dynamic X-rays at collar removal. Fracture stability was assessed using only dynamic X-rays, with unstable fractures demonstrating active displacement. Neurological outcomes and the necessity for surgical fixation in each group were compared. Multivariable logistic regression was used to analyze risk factors for fracture instability.

Results: Unstable nonunion occurred in 25% of patients, while fibrous nonunion occurred in 48% after a median collar wear of 3.7 months (IQR 2.9-6.2 months). New neurological deficits after collar removal were seen in 6% of patients with unstable nonunions during follow-up, but in none of those with fibrous nonunions or osseous unions, even after subsequent trauma. Risk factors for unstable nonunion included male sex (OR 2.14; 95% CI: 1.02-4.49), osteoporosis/osteopenia (OR 2.50; 95% CI: 1.17-5.37), and baseline fracture displacement (OR 4.81; 95% CI: 2.35-9.86).

Conclusions: Fibrous nonunion is a viable outcome in conservatively managed type II odontoid fractures, reducing the need for surgery or prolonged collar wear. Risk factors for unstable nonunion included male sex, osteoporosis/osteopenia, and baseline fracture displacement. Unstable nonunions may lead to new neurological deficits occurring after collar removal in a small percentage of cases.