Fig. 10 The infraorbital nerve (ION) is the major branch of the maxillary division (V2) of the trigeminal nerve. Three branches to the upper teeth originate prior to facial innervation. The posterior superior alveolar nerve (PSA) decussates from the infraorbital nerve prior to entering the infraorbital fissure or canal, while the middle superior alveolar nerve (MSA) and the anterior superior alveolar nerve (ASA) decussate within the orbital portion of the infraorbital nerve.