Left-right asymmetries and other common anatomical variants of the first cervical vertebra.

SUMMARY. Left-right asymmetries were systematically and independently observed twice in 82 macerated atlases by three investigators using a check-list and the instructions of an examination protocol. A grid behind the vertebra served as a visual aid and was inserted in an inclinometer box used for measuring zygapophyseal joint facet inclination. Only those asymmetries observed once by each of the investigators were listed. The posterior arch showed the highest number of asymmetries and many unequal grooves for the vertebral artery were observed. The lateral compartment frequently revealed asymmetries of transverse processes and transverse foramina. These results are indicative of variants of the bony canal for passage of the vertebral artery and asymmetric lever arms for muscles, inserted at C1. A third zone could be defined at the level of the joint facets. Tropism of the superior joint facets was caused by differences in shape, size, surface area, curvature, inclination relative to the sagittal plane, implantation on the lateral mass, unilateral or asymmetric subdivision. Left-right differences of inferior joint facets were often related to degenerative enlargement. Osteophytes also caused asymmetry of the facet for the odontoid process. An important number of combined asymmetries reveals questions about their functional consequences. Copyright 1997 Harcourt Publishers Ltd.