Development of the Atlas
- Three ossification centers: There is one ossification center in the anterior arch and two in the lateral neural arches (49).
Neural arch closes at age 3 years, anterior arch by age 6 years:The neural arches fuse together by about age 3 years, and the anterior arches fuse between ages 3 and 6 years.
- Fusion can be incomplete: The ossification centers can remain open, even into adulthood. Most commonly, this occurs in the dorsal midline.
Variant of two ossification centers:Failure in the development of an anterior arch results in the two neural arches meeting each other anteriorly and the formation of a complete or nearly complete ring with only two midline ossification centers.
Axial CT scan showing one of the patterns of normal ossification of the C1 ring. The smooth corticated margins of synchondroses make them readily distinguishable from fractures.
Development of the Axis
- Five ossification centers: There is one ossification center in the body, two in the odontoid, and two in the neural arches. The two odontoid ossification centers typically fuse in utero, leaving four ossification centers in the infant. There can occasionally be a sixth ossification center at the tip of the dens that appears during early childhood and fuses before adolescence.
- Neural arches fuse by age 3 years and anterior centers by age 7 years: The neural arches fuse together by approximately age 3 years. The anterior ossification centers fuse by approximately age 7 years.
Coronal CT image of normal C2 ossification pattern
Development of the Subaxial Spine (C3-C7)
- Three ossification centers: There is one ossification in the body and two in the lateral neural arches.
Neural arches close at age 3 years and anterior arch by age 6 years:The neural arches fuse by about age 3 years and then fuse to the body between ages 3 and 6 years.
- Variant of secondary ossification centers: Other ossification centers may occasionally be visualized at the tips of transverse processes, tips of spinous processes, and the end plates. These may occasionally persist into adulthood.