The aim of this research was to study the anatomical aspects of abnormal foramen over the posterior arch of atlas vertebra. Posterior arch of atlas vertebrae was studied for abnormal foramen in sixty-seven adult human atlas vertebrae and findings were noted. In 2.98% of cases, unilateral complete abnormal foramen on the posterior arch of atlas vertebra was found. Clinicians should be aware about this variation on posterior arch of atlas, which may produce headache, vertigo, vertebrobasilar insufficiency and shoulder pains.
