Normal ossification patterns of atlas and axis: a CT study

Abstract

Background and purpose: Development of the CVJ is a complex process rarely analyzed by CT. Cartilaginous remnants within the atlas and axis have been shown to variably persist throughout childhood and may be mistaken for fractures. The purpose of this study was to better estimate the fusion timeline of the synchondroses at the CVJ.

Materials and methods: We retrospectively reviewed singular CT scans in 550 children without known skeletal dysplasia or maturation delay (from neonate to 17 years) and analyzed the ossification of atlas and axis. Normal closing age for synchondroses was defined as soon as a complete ossification rate of 80% or more per age category was achieved.

Results: No separate OC was observed in 38 of 230 incompletely ossified AAAs, whereas single, bipartite, or multiple OCs were observed in, respectively, 196, 86, and 16 of 298 AAAs in which OCs could be identified. Synchondroses at the AAA closed after 12 years (range 4.5-17 years). Posterior midline synchondrosis of the atlas closed after 4 years (range 2-13 years). The axis ossified from 6 OCs and 4 synchondroses. Subdental and neurocentral synchondroses closed simultaneously after 9 years (range 7-9.5 years). Apicodental synchondrosis and chondrum terminale were completely ossified after 10.5 years (range 5.5-13.5 years).

Conclusions: Analysis of the CVJ using triplanar CT reconstructions allows secure identification of characteristic developmental features of the atlas and axis, and helps to separate normal variants from true osseous lesions.

Fig 1.

Two examples of absence of ventral ossification center of the AAA. A, Large nonossified AAA in a 2-year-old child. B, Midline cartilaginous gap at the AAA in a 12-year-old child.

Fig 2.

Singular midline OC pattern at the AAA. A, 5-month-old child with already large midline OC and large ventrolateral synchondroses. B, A 2.5-year-old child with progressive ossification of the chondral AAA anlage from both the central OC and the lateral masses of the atlas. C, 7-year-old child with complete ossification of the AAA but still-visible ventrolateral synchondroses.

Fig 3.

Multiple OC patterns at the AAA. A, 3-year-old child with 2 OCs paramedially located. B, A 2.5-year-old child with 3 irregular OCs within the AAA.

Fig 4.

Ossification timetable of the anterior atlas arch.

Fig 5.

Different aspects of the PAA. A, Normal midline synchondrosis. B, Rare paramedial posterior synchondrosis. C, Rare midline OC within the PAA.

Fig 6.

Ossification timetable of the posterior atlas arch.

Fig 7.

Coronal (A) and sagittal (B) view of the neonatal axis with 5 OCs. A, While both club-shaped centers of the dens are already fused at birth, a midline vertical fusion line can still be perceived. B, Smooth cartilaginous rounded CHT at the top of the dens is best appreciated on the sagittal view.

Fig 8.

Ossification timetable of the subdental synchondrosis.

Fig 9.

Coronal view of ossification development of the axis. Children are of the following ages (A) 30 days, (B) 5 months, (C) 1.5 years, (D) 2 years, (E) 3 years, (F) 5 years, (G) 8.5 years, and (H) 14 years.

Fig 10.

Ossification timetable of the apicodental synchondrosis.

Fig 11.

Sagittal view of ossification development of the axis. Children are of the following ages: (A) 30 days, (B) 3 months, (C) 3 years, (D) 5 years, (E) 6.5 years, (F) 8 years, (G) 13 years, and (H) 16 years.

Fig 12.

Rare posterior ossification of the CHT resembled a dislocated fragment.

Fig 13.

Large ossiculum terminale in an 8-year-old child and incomplete ossification of the ADS.