Methods: Ten natural atlas bone specimens' images were recorded using a digital radiographic system, and evaluations were made by 2 independent examiners using the system's software. Mean differences, standard deviations, and agreements were evaluated in regard to bilateral differences.
Results: The mean bilateral difference for both examiners was 0.96 mm, with a standard deviation of ±0.67 mm. There were no statistically significant differences between the mean values for left and right measurements.
Discussion: The mean of 0.96 ± 0.67 mm indicates that measurements up to 1.63 mm (1 SD) or 2.30 mm (2 SDs) are clearly within a reference range of variation for this sample. This information could be used to assist the clinician measuring lateral misalignment of the atlas in determining the amount of expected normal asymmetry for the individual patient before concluding that lateral misalignment of the atlas is present.
Conclusions: These 10 specimens showed an average difference of 0.95 mm ± 1 SD (0.67 mm) or 2 SDs (1.34 mm) between the left and right sides of the atlas vertebrae. Differences found on radiographs may be due to asymmetry and not actual misalignment. On the average, for these 10 vertebra specimens, a lateral disposition of 1.62 mm on either side should be allowed when arriving at a conclusion for lateral displacement of the atlas.
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