Does fracture-extension into the pars interarticularis alter outcomes in odontoid failure? a technical note on pars interarticularis osteotomy and atlantoaxial distraction arthrodesis.

Purpose: Odontoid process fractures can extend rostral into the C2 arch. We investigated the clinical impact of a concurrent fracture of the pars interarticularis on odontoid failure. To overcome the surgical challenges related to the morphology of these fractures, we describe a novel surgical strategy using atlantoaxial joint distraction arthrodesis.

Methods: We conducted a single centre cohort study of 13 consecutive patients with odontoid fractures extending into the pars treated between June 2016 and June 2018. Criteria for a stable fibrous non-union were: Atlanto-Dens Interval (ADI) < 3 mm, Posterior Atlanto-Dens Interval (PADI) > 14 mm and lack of symptomatic motion at the fracture site. Atlantoaxial instability was defined as greater than 50% subluxation across the C1-C2 joint. Return to pre-injury performance status was considered a satisfactory clinical outcome.

Results: The mean age of the patient population was 77.2 years (SD 11.9). The mean follow-up time was 15 months (SD 5.2). 69% had an associated atlantoaxial instability (P-value 0.0005). Cervical orthosis treatment was associated with a high non-union rate (70%) (P-value 0.04) although it did not affect the overall clinical outcome. 2 cases presented with cord compression were treated surgically with pars interarticularis osteotomy and atlantoaxial distraction arthrodesis.

Conclusions: Odontoid fracture with extension into the pars interarticularis often present with atlantoaxial instability and may result in stable fibrous non-union if treated non-operatively. The C1-C2 segment can be stabilised with atlantoaxial distraction arthrodesis achieved through an osteotomy of the pars interarticularis.