Computed Tomographic Morphometric Analysis of Pediatric C1 Posterior Arch Crossing Screw Fixation for Atlantoaxial Instability.

Study Design.A CT-based study of pediatric atlas.Objective.The aim of this study was to identify morphometric patterns and dimensions of the pediatric C1 posterior arch to establish guidelines for the posterior arch crossing screw (PACS) fixation.Summary of Background Data.The PACS fixation is an alternative or supplement to the C1 lateral mass screw or pedicle screw constructs. However, the anatomic feasibility of this technique has not been well documented in the pediatric population.Methods.A total of 123 pediatric patients were assigned into 6 groups based on age at an interval of 3 years. The C1 posterior arch unsuited to PACS fixation was classified as type 1 with synchondrosis, and type 2 without synchondrosis. Three subtypes of type 2 were included: the underdeveloped tubercle, the flat arch, and the undersized tubercle (height < 7mm or width < 3.5mm), respectively. The C1 posterior tubercle height and arch width, the projected PACS length and angle were measured, respectively.Results.There were 19 patients of type 1 and 68 patients of type 2. Type 1 was mostly found in patients under age 3 years, whereas type 2 was largely observed in patients older than 4 years. The tubercle size was the most limiting variable for successful PACS placement. The PACS fixation was feasible for 42% patients over age 7 years and only 5% patients under age 6 years. The posterior tubercle height increased with age, whereas the tubercle width, the PACS length, and axial-plane angle increased with age up to 12 years. The PACS length was between 16 and 18mm with the angle between 18 degrees and 23 degrees for patients over age 7 years.Conclusion.The PACS fixation was anatomically feasible in patients over age 7 years, but contraindicated in patients under age 6 years. Pediatric patients for the PACS fixation should be assessed before surgery.Level of Evidence: 4