Ectropion or scleral show resulting from weakness of the lower eyelids is not uncommon after lower blepharoplasty or midface lift via blepharoplasty incision. Denervation of the pretarsal orbicularis oculi muscle (OOM) attributes to such complications. The authors analyzed 102 patients who underwent midface lift via lower blepharoplasty incision for the past 3 years and investigated the motor nerve innervation of the lower OOM in 20 cadavers. They encountered two cases of ectropion attributed to the denervation of the pretarsal OOM: one with dry-eye syndrome and scleral show, and the other with a "polar bear-like appearance" (i.e., outer eversion of the lower eyelid). All pretarsal and preseptal OOMs were innervated by five to seven terminal twigs of the zygomatic branches of the facial nerve that approached the muscle at a right angle. The medial portion of the lower OOM was innervated by one to two terminal twigs of the buccal branch, and the middle portion was innervated with two to three twigs of the zygomatic branch. The lateral portion was supplied by the uppermost zygomatic branch, which split into two to four twigs. The mean horizontal distance between the lateral canthus and the zygomatic branch was 2.31 +/- 0.29 cm (range, 1.7-2.7 cm) and the vertical distance was 1.20 +/- 0.20 cm (range, 0.8-1.5 cm). The critical zone was a circle with 0.5-cm radius, and its center was located 2.5 cm inferolaterally (30 deg) from the lateral canthus. It is very important to understand the motor nerve innervation of the lower eyelid and the "critical zone" to avoid postoperative ectropion or weakness of the lower eyelid resulting from paralysis of the pretarsal or preseptal OOM.