Radiology of postnatal skeletal development. XIII. C1-C2 interrelationships.

Composites of C1 and C2 were analyzed in various roentgenographic projections to elucidate osseous interrelationships and the effect of overlap of different portions of these two vertebrae in standard radiographic projections during differing stages of postnatal chondro-osseous transformation. In anteroposterior projections the dentocentral synchrondroses of C2 normally was located below the inferior rim of the C1 anterior ossification center. The upper extent of the dens ossification center was behind this anterior C1 center. The overlap made visualization of the ossiculum terminale difficult. The spinous process of C1 could be confused with the ossiculum. In transverse projections, the normal laxity characteristic of young children allowed considerable variation in rotational interrelationships. Various degrees of such instability are illustrated. In lateral views variation of the anterior contour of the dens was significant. Such variation must be considered developmental due to the location and direction of growth of the chondrum terminale and interactive modeling between C1 and C2 to allow extension at this particular joint.