A three-dimensional study of the atlantodental interval in a normal Chinese population using reformatted computed tomography

The atlantodental interval has been usually used for the evaluation of atlantoaxial instability. However, the asymmetry of the lateral atlantodental interval is occasionally found in healthy individuals. Controversy therefore exists as to the clinical significance of this asymmetry in patients after trauma. The purpose of this study was to determine the normal range of atlantodental intervals in normal individuals using reformatted computed tomography. In this study, C1–C2 vertebrae were imaged in 230 adult patients by a Lightspeed Vct CT (General Electric, CT, USA) with a slice thickness of 0.625 mm. After reformatting the original images, the anterior atlantodental interval (AADI) and lateral atlantodental interval (LADI) were measured. The AADI was found to be 1.83 ± 0.46 mm (0.9–3.4 mm) in males and 1.63 ± 0.43 mm (0.5–3.2 mm) in females. The AADI was significantly greater in males than in females (p < 0.05). The 95% confidence interval for AADI was 1.75–1.90 mm in males and 1.54–1.72 mm in females. No statistically significant differences were found between males and females in the left and right LADI, and LADI asymmetry. The left LADI was found to be 3.38 ± 0.87 mm (1.7–6.0 mm), and the right LADI was 3.42 ± 0.84 mm (1.7–5.9 mm) in males, while the left LADI was 3.30 ± 0.73 mm (1.5–5.3 mm) and the right LADI was 3.37 ± 0.92 mm (1.7–5.9 mm) in females. The 95% confidence interval for left LADI was 3.23–3.52 and 2.94–3.25 mm, and for right LADI was 3.27–3.56 and 3.18–3.56 mm in males and females, respectively. The mean asymmetry of LADI was 0.76 ± 0.66 mm (0.0–3.5 mm) in males and 0.73 ± 0.70 mm (0.0–3.7 mm) in females. The 95% confidence interval for LADI asymmetry was 0.65–0.87 mm in males and 0.59–0.88 mm in females. Most of the population was found to have an asymmetry ranging between 0.1 and 2.0 mm. The current study shows that LADI asymmetry is common in patients without any cervical spine abnormalities. LADI asymmetry may be a normal anatomic variant and there is no evidence to confirm that LADI asymmetry is a sensitive or specific indicator of traumatic atlantoaxial instability. Radiologists and clinicians should be aware of this normal range of asymmetry when interpreting CT scans of the atlantoaxial region.