The relationship between dens height and alar ligament orientation: a radiologic study

Objective: This study examined the anatomical assumptions underlying multiplanar alar ligament stress testing. The alar ligament has been described as occurring in 1 of 3 planes: caudocranial, horizontal, and craniocaudal. This has been stated to result from variation in dens height. Stress testing in all 3 planes is suggested, with increased translation present in all positions to infer instability. Methods: Computed tomography scans with no diagnosed bony or ligamentous abnormally were prospectively collected over a 3-month period from a teaching hospital in Newcastle, Australia, and sequentially analyzed. The height of the dens relative to the occipital condyles was measured using McRae's line and the bimastoid line. The orientation of the alar ligament was measured relative to the vertical axis of the dens as well as a vertical line defined by specified occipital and spinal bony landmarks. These results were correlated with dens height. Results: After exclusions, 42 individual computed tomography studies were analyzed yielding 64 clearly discernible ligaments. A vertical line derived from the digastric line provided the smallest variation in results. The mean ligament orientation given by this measure was 110.06° (85°-127°). There was no correlation between measured dens height relative to the occiput and ligament orientation. Conclusion: Our findings reinforce the existence of normal anatomical variation in dens height and alar ligament orientation. However, variation in dens height as a cause of variation in ligament orientation was not supported by this study.