Vertical atlantoaxial index: a new craniovertebral radiographic index.

STUDY DESIGN: An index for measuring the vertical relationship of atlas and axis is described.

OBJECTIVE: Deduction and application of vertical atlantoaxial index (VAAI) for quantifying the vertical atlantoaxial relationship of atlas and axis and classifying basilar invagination (BI) based on the VAAI.

SUMMARY OF BACKGROUND DATA: A number of craniospinal parameters have been described to quantify the amount of BI in relation to herniation of odontoid into the cranium. Successful treatment of BI by distraction of atlantoaxial facet joint, placement of a spacer to maintain the distraction, and lateral mass fixation has been described recently. The treatment is based on the understanding that BI is in fact a disease of the atlantoaxial facet joints and is a result of progressive vertical instability. With this new concept and technique that involves preserving all bony elements and reduction of BI in the form of reducing the vertical atlantoaxial subluxation, we thought it was prudent to formulate an index to quantify the relation of atlas and axis in the sagittal plane.

METHOD: Mid-sagittal computerized tomography (CT) scan films of 90 cases of BI treated by us between October 1999 and May 2005 with distraction and lateral mass plate and screw fixation were analyzed before and after surgery. The age of the patients ranged from 8 to 55 years and the male:female ratio was 2.5:1. Additionally, mid-sagittal CT scan films of hundred normal subjects in the same age group were analyzed as a control group. The VAAI was measured in all cases. VAAI is an index that measures the vertical relationship of the atlas and axis. The images were compiled and copies of the compilation were made. Two observers independently performed the measurements and intraobserver as well as interobserver agreement was assessed using the intraclass corelation (ICC) ) test (SigmaStat).

RESULTS: The preoperative mean and mode values of VAAI in this series of patients were 0.53 (0.20 to 0.67) and 0.61, respectively. The postoperative mean and mode values of VAAI are 0.78 (range, 0.60 to 0.89) and 0.80, respectively. The mean and mode values of VAAI in general population were 0.80 (range, 0.76 to 0.85) and 0.80, respectively. The results in preoperative patients with BI demonstrated excellent intraobserver (ICC=0.96 and 0.98) and interobserver corelation coefficient (ICC=0.96). The results in normal subjects and postoperative CT scans of operated patients with BI also showed excellent intraobserver (ICC=0.97 and 0.98) and interobserver corelation coefficient (ICC=0.97).

CONCLUSIONS: VAAI can be an excellent measurement tool for the assessment of relationship of atlas and axis. Nonrheumatoid BI can be graded and classified depending on the value of VAAI.