Atlas vertebra is the first and atypical cervical vertebra. It has anterior and posterior arches and is devoid of body. The posterior arch has a neurovascular groove on the lateral part of superior surface. This groove is sometimes converted into a ring or canal. Atlas vertebra has high degree of variation in morphology. Its knowledge is essential for orthopedic surgeons, neurosurgeons, radiologists, and anthropologists. Aims and Objectives: To study the formation of abnormal foramina and canal

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... on posterior arch of atlas vertebrae. Materials and Methods: The present study was carried out in Shyam Shah Medical College and SGM Hospital, Rewa, Madhya Pradesh, India. During routine demonstration classes in the Department of Anatomy, we found atlas vertebrae with unilateral complete retroarticular ring. There are collections of 34 atlas vertebrae. These vertebrae were examined for evidence of exoostosis from the posterior margin of superior articular facet. Results: On the basis of observations, we found that the formation of retroarticular ring and canal occurs in six stages. Conclusion: Retroarticular groove may convert into ring and canal, knowledge of it is beneficial for neurosurgeons and orthopedicians in the treatment of vertebrobasilar insufficiency. It is also beneficial for an anthropologist in correlating the morphological changes from primate to human species.