The Prevalence and Morphometry of the Atlas Vertebra Retrotransverse Foramen.

Objective: The current study records the prevalence of the accessory foramen, located posterior to the transverse foramen (TF), the so-called the retrotransverse foramen (RTF), its morphometry, exact location, and coexistence with ossified posterior bridges. Additionally, factors associated with the length of the RTF are investigated.

Materials: One-hundred and forty-one dried atlas vertebrae were examined.

Results: Thirty-seven out of the 141 vertebrae (26.2%) had at least one RTF. The RTF was unilateral in 67.6% and bilateral in 32.4%. The mean RTF anteroposterior diameter (length) was 4.2±1.4 mm on the right and 3.8±1.0 mm on the left side. The mean RTF laterolateral diameter (width) was 2.6±1.2 mm on the right and 2.5±0.8 mm on the left side. Both dimensions were symmetrical. The RTF was symmetrically located from the TF, at a mean distance of 4.6±1.1 mm on the right and of 4.5±0.9 mm on the left side. For the given TF-RTF distance, laterality, and presence of posterior bridges, each mm increase in the RTF width was associated with a 0.74 mm increase in the relevant length.

Conclusion: The estimated prevalence was higher than most of those reported in other studies. However, the between-studies prevalence varies to a significant degree. Hence, a systematic review and meta-analysis should be performed to identify a more precise estimate due to the clinical importance of the RTF.