The atlantoaxial distance ratio (AADR): a novel radiographic parameter for the diagnosis and postoperative assessment of vertical atlantoaxial dislocation

Objectives Vertical atlantoaxial dislocation (VAAD) is a clinically significant but often overlooked component of atlantoaxial instability, yet a reliable, universally applicable radiographic parameter for its quantification is lacking. This study introduces and validates a novel metric, the atlantoaxial distance ratio (AADR), for the diagnosis and postoperative assessment of VAAD. Materials and methods The AADR is calculated on lateral cervical radiographs as the ratio of the mean vertical distance from the C1 superior tubercles to the C2 inferior endplate, divided by the anteroposterior diameter of the C2 inferior endplate. A normal reference range for AADR was established in 46 asymptomatic volunteers. Subsequently, a retrospective analysis was conducted on 13 patients with VAAD who underwent single-stage posterior reduction and fusion using C2 lateral mass-isthmus screws between January 2022 and April 2023. Pre- and postoperative AADR, posterior atlanto-dental interval (PADI), spinal canal diameter on MRI (d-MRI), visual analogue scale (VAS) for pain, and modified Japanese Orthopedic Association (JOA) scores were evaluated. Results The mean AADR in the normal cohort was 1.87 ± 0.08 (range: 1.80–2.06). In patients with VAAD, the mean preoperative AADR was significantly lower at 1.78 ± 0.10 (p = 0.0015 vs. normal), and it significantly increased to 2.07 ± 0.12 postoperatively (p < 0.0001), a value not significantly different from the normal range (p > 0.05). Surgical reduction also resulted in significant improvements in PADI (from 7.31 ± 1.32 mm to 15.77 ± 1.24 mm, p < 0.0001), d-MRI (from 6.62 ± 1.26 mm to 15.00 ± 1.22 mm, p < 0.0001), VAS scores (from 4.77 ± 1.01 to 0.77 ± 0.60, p < 0.0001), and JOA scores (from 9.08 ± 3.14 to 12.46 ± 2.47, p < 0.0001). Conclusion The AADR is a simple, reliable, and reproducible radiographic parameter for quantifying VAAD. An AADR value below 1.80 can be considered indicative of VAAD, while restoring it to the normal range (1.80–2.06) signifies successful reduction. Correcting VAAD is crucial for achieving effective neural decompression and creating optimal conditions for fusion. The use of C2 lateral mass-isthmus screws is an effective technique to facilitate this reduction.