The role of occipital condyle and atlas anomalies on occipital cervical fusion outcomes in Chiari malformation type I with syringomyelia: a study from the Park-Reeves Syringomyelia Research Consortium
Authors: Yahanda Alexander T. , Koueik Joyce , Ackerman Laurie L. , Adelson P. David , Albert Gregory W. , Aldana Philipp R. , Alden Tord D. , Anderson Richard C. E. , Bauer David F. , Bethel-Anderson Tammy , Bierbrauer Karin , Brockmeyer Douglas L. , Chern Joshua J. , Couture Daniel E. , Daniels David J. , Dlouhy Brian J. , Durham Susan R. , Ellenbogen Richard G. , Eskandari Ramin , Fuchs Herbert E. , Grant Gerald A. , Graupman Patrick C. , Greene Stephanie , Greenfield Jeffrey P. , Gross Naina L. , Guillaume Daniel J. , Hankinson Todd C. , Heuer Gregory G. , Iantosca Mark , Iskandar Bermans J. , Jackson Eric M. , Jallo George I. , Johnston James M. , Kaufman Bruce A. , Keating Robert F. , Khan Nickalus R. , Krieger Mark D. , Leonard Jeffrey R. , Maher Cormac O. , Mangano Francesco T. , Martin Jonathan , McComb J. Gordon , McEvoy Sean D. , Meehan Thanda , Menezes Arnold H. , Muhlbauer Michael S. , O’Neill Brent R. , Olavarria Greg , Ragheb John , Selden Nathan R. , Shah Manish N. , Shannon Chevis N. , Shimony Joshua S. , Smyth Matthew D. , Stone Scellig S. D. , Strahle Jennifer M. , Tamber Mandeep S. , Torner James C. , Tuite Gerald F. , Tyler-Kabara Elizabeth C. , Wait Scott D. , Wellons John C. , Whitehead William E. , Park Tae Sung , Limbrick David D. , Ahmed Raheel 
Published: April 2024

OBJECTIVE
Congenital anomalies of the atlanto-occipital articulation may be present in patients with Chiari malformation type I (CM-I). However, it is unclear how these anomalies affect the biomechanical stability of the craniovertebral junction (CVJ) and whether they are associated with an increased incidence of occipitocervical fusion (OCF) following posterior fossa decompression (PFD). The objective of this study was to determine the prevalence of condylar hypoplasia and atlas anomalies in children with CM-I and syringomyelia. The authors also investigated the predictive contribution of these anomalies to the occurrence of OCF following PFD (PFD+OCF).

METHODS
The authors analyzed the prevalence of condylar hypoplasia and atlas arch anomalies for patients in the Park-Reeves Syringomyelia Research Consortium database who underwent PFD+OCF. Condylar hypoplasia was defined by an atlanto-occipital joint axis angle (AOJAA) ≥ 130°. Atlas assimilation and arch anomalies were identified on presurgical radiographic imaging. This PFD+OCF cohort was compared with a control cohort of patients who underwent PFD alone. The control group was matched to the PFD+OCF cohort according to age, sex, and duration of symptoms at a 2:1 ratio.

RESULTS
Clinical features and radiographic atlanto-occipital joint parameters were compared between 19 patients in the PFD+OCF cohort and 38 patients in the PFD-only cohort. Demographic data were not significantly different between cohorts (p > 0.05). The mean AOJAA was significantly higher in the PFD+OCF group than in the PFD group (144° ± 12° vs 127° ± 6°, p < 0.0001). In the PFD+OCF group, atlas assimilation and atlas arch anomalies were identified in 10 (53%) and 5 (26%) patients, respectively. These anomalies were absent (n = 0) in the PFD group (p < 0.001). Multivariate regression analysis identified the following 3 CVJ radiographic variables that were predictive of OCF occurrence after PFD: AOJAA ≥ 130° (p = 0.01), clivoaxial angle < 125° (p = 0.02), and occipital condyle–C2 sagittal vertical alignment (C–C2SVA) ≥ 5 mm (p = 0.01). A predictive model based on these 3 factors accurately predicted OCF following PFD (C-statistic 0.95).

CONCLUSIONS
The authors’ results indicate that the occipital condyle–atlas joint complex might affect the biomechanical integrity of the CVJ in children with CM-I and syringomyelia. They describe the role of the AOJAA metric as an independent predictive factor for occurrence of OCF following PFD. Preoperative identification of these skeletal abnormalities may be used to guide surgical planning and treatment of patients with complex CM-I and coexistent osseous pathology.