Retrotransverse foramen of the atlas: prevalence and bony variations

Purpose: To analyze the prevalence of the retrotransverse foramen (RTF) and its bony variations. Methods: One hundred ten atlases of living adult subjects, 161 twentieth century dry adult cervical atlases and four dry adult cervical atlases from medieval skeletons were studied to detect the RTF and its abnormal bony variations. The 110 living adult subjects underwent a computed tomography study to detect the RTF. Results: In the in vivo sample (n = 110; 100%), the RTF was found in four (3.6%) atlases. It was bilateral in all cases, but three (2.7%) patients showed complete RTF and the other patient presented a complete RTF in the left transverse process and an unclosed RTF in the right transverse process. In addition, the RTF was observed in combination with an unclosed transverse foramen in two cases (1.8%). In the twentieth century skeletal sample (n = 206; 100%) the RTF was found in 15 (7.3%) C1 vertebrae. It was bilateral in three (1.5%) vertebrae and unilateral in another 12 (5.8%) vertebrae. In the medieval skeletal sample (n = 4; 100%) one cadaveric atlas (25%) presented a bilateral RTF with special bony characteristics which presented an unexpected spicula in the left RTF. Conclusions: The RTF is a nonmetric variant of the atlas vertebra that can present non-degenerative and non-traumatic spiculae or it can be unclosed. In addition, it can be associated with the presence of unclosed transverse foramina.