Iatrogenic intraoperative bleeding during C1 surgeries is difficult to manage.To investigate the potential causes of iatrogenic intraoperative bleeding in atlas surgeries.This was a retrospective study, observational cohort of patients with DICOM.High-resolution head and neck computed tomography angiography (CTA) images from 551 subjects were included.Ponticulus posticus (POPO), vertebral artery (VA), venous plexus communication.Three dimension rendering was utilized in the present study. Potential arterial bleeding was evaluated based on the variation in the VA and the polymorphism of the POPO over the groove for VA (GVA). The communication of the venous plexus in the occipitoatlantal region was investigated to assess the venous hemorrhage.Among the 551 atlases examined, POPOs were identified on 155 sides, resulting in a prevalence of 14.07% (155/1102). These POPOs (n = 155) were reclassified into four types: tiny spur (54.84%), long spur (7.10%), ossified bridge (30.32%), and ossified canal (7.74%). In 42.92% (473/1102) of cases, the VA did not directly contact the sulci of the GVA, creating space for the passage of the rich venous plexus that drained intracranial venous blood outflow to various extracranial layers. Moreover, in 12.7% of the subjects, the study revealed the presence of additional foramens in the posterior lamina of C1, which served as a conduit for the communicating vein CONCLUSION: The potential underestimation of polymorphism in POPOs and VAs can lead to arterial bleeding, whereas a lack of understanding of the intricate condylar emissary venous plexus can result in venous hemorrhage. To mitigate iatrogenic hemorrhage during C1 surgeries, a preoperative HEAD AND NECK CTA is recommended, and heightened caution should be exercised during dissection in the lateral half of the C1 lamina. Furthermore, unknown causes of intraoperative bleeding may arise during the posterior C1 approach; modifications should be considered based on the specific circumstances encountered.Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.