Purpose: To assess for alterations in the microscopic anatomy that occur as a result of the Müller muscle-conjunctival resection (MMCR) ptosis procedure and to better understand the mechanisms by which MMCR elevates the eyelid. Methods: Sixteen orbits from 8 fresh frozen Caucasian cadaver heads, ranging from 38 to 100 years of age were used. For each head, MMCR was performed on one side. The contralateral, unoperated orbit served as an anatomic control. Each exenterated orbital contents and excised MMCR specimen was evaluated. The histopathology of the eyelids and MMCR specimens were studied microscopically by staining with hematoxylin-eosin, elastic, and Verhoeff-Masson trichrome. Results: Müller muscle and conjunctiva were present in all 8 of the excised MMCR specimens. Elastic fibers consistent with Müller muscle tendon or among the smooth muscle fibers were seen within all excised MMCR specimens. The levator aponeurosis was intact in 8 of 8 operated eyelids; however, the aponeurosis was plicated in all. The accessory lacrimal gland tissues were intact in all of the operated and unoperated eyelids. Conclusions: MMCR works by shortening the posterior lamella, which results in advancement of the levator palpebrae superioris muscle and plication of the levator aponeurosis. Plication of the levator aponeurosis likely contributes to the increased volumetric effect seen clinically after MMCR. Phenylephrine testing can help in fine-tuning the amount of resection, but given the mechanism of action of MMCR, adequate levator muscle function remains a critical factor in the success of the surgery. Moreover, MMCR preserves accessory lacrimal gland tissues.
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