The Importance of Arcuate Foramen, A Variation of the Atlas: A Microsurgical Cadaveric Study and Review of the Literature

Introduction:Arcuate foramen (AF) is an osseous variation located on the vertebral artery (VA) groove (sulcus arteriae vertebralis) at the posterior arch of the atlas. Its prevalence may vary due to regional and ethnic factors. Although it is thought that it causes compression of the structures inside AF and causes vertebrobasilar insufficiency, there are not enough cadaver studies to explain this clinical condition.Methods:To evaluate the incidence of AF variation and its relationship with VA, 20 sides of 10 randomly selected and silicone injected cadaveric heads were examined by microsurgical method at the University of Wisconsin Neuroanatomy Laboratory.Results:Complete AF variation in atlas was detected in 30% of the samples. This variation was unilateral on the right side in two cases and bilateral in one case. The mean dimensions of AF were D1=6 mm and D2=5.5 mm on the right, and D1=7.5 mm and D2=6.5 mm on the left. The mean pre-foraminal, foraminal and post-foraminal diameters of VA were 4 mm, 2.5 mm and 3 mm, respectively, on the right, and 6 mm, 4 mm and 5 mm, respectively, on the left. In all cases, the suboccipital nerve was accompanying VA in the AF.Conclusion:Being aware that this variation may have a high incidence may help to determine surgical strategy before craniocervical junction surgery and prevent life-threatening arterial injuries. In addition, although the dimensions of the AF is adequate for the passage of VA, presence of arterial compression due to other structures passing through the foramen may explain the reason for the relief of most cases with foramen decompression.