Morphometric analysis of the C1-2 zygapophysial joint in atlantoaxial dislocation patients with sandwich fusion of the craniovertebral junction.

The combination of congenital C1 occipitalization and C2-3 non-segmentation (i.e. "sandwich fusion") results in early development of atlantoaxial dislocation (AAD). A thorough understanding of the morphometry of the C1-2 zygapophysial joints is important to ensure a safe surgery. This study was aimed to evaluate the C1-2 zygapophysial joint in AAD patients with sandwich fusion by morphological research based on CT scans. This was a retrospective case-control study including 155 AAD patients with sandwich fusion in sandwich group and 55 with os odontoideum (OO) in control group. The C1 listhetic distance, sagittal inclination angle (SIA), coronal inclination angles (CIA) were measured from the CT and compared between two groups. The listhetic grade was defined according to the ratio of the listhetic distance and length of the superior facet of C2: less than 25% as mild, 25-75% as moderate, and greater than 75% as severe. Patients in the sandwich group had higher listhetic grade than the OO group (17.4% vs. 70.6%, 50.3% vs. 29.4% and 32.3% vs. 0 were of mild, moderate, and severe, respectively). The SIA and CIA in the sandwich group were greater than those in the OO group (30.1° vs. 5.0°, 36.6° vs. 31.0°, respectively). Asymmetric listhesis was found in 0.0%, 63.3%, and 85.7% of sandwich patients with mild, moderate and severe listhetic grade, respectively. AAD with sandwich fusion was a three-dimensional deformity characterized by the high prevalence of asymmetric and moderate-to-severe listhesis.