Abnormal circle of Willis morphology Abnormal posterior circulating artery morphology 2018-01-27T11:25:13Z Any structural anomaly of the posterior circulating artery (PCOM). The posterior circulating artery (PCOM) forms the posterior portion of the circle of Willis, connecting the interior carotid artery (proximal to its bifurcation into the anterior cerebral artery and the middle cerebral artery) with the posterior cerebral artery.