Objectives: Upper eyelid retraction was believed to be maintained solely by voluntary contraction of the levator palpebrae superioris (LPS) and frontalis muscles, together with involuntary contraction of the sympathetically innervated Müller's muscle (MM). However, several studies have suggested that the LPS also undergoes involuntary contractions, and that a visual stimulus may not be the only trigger for frontalis muscle contractions. Recent studies hypothesized that the MM contains proprioceptive neuronal structures, which elicit involuntary LPS muscle contraction by the mesencephalic trigeminal nucleus via a continuous stretch reflex. We aim to identify proprioceptive structures in MM by means of histological examinations.

Methodology: Prospective study. Collaboration of oculoplastics, ophthalmologists and a neuroanatomy specialist in Sheba Medical Center. 50 fresh MM specimens from patients undergoing Müllerectomies will undergo histologic examinations, including immunohistochemical staining and light and electron microscopy.

Significance: Identification of proprioceptive structures in MM will be the first evidence-based proof of a proprioceptive mechanism in the eyelid. This might have a significant impact on future surgical management of eyelid procedures. Comprehensive understanding of the mechanisms underlying eyelid function is especially important in the management of children with ptosis who may have coexisting amblyopia.

The investigators' goal is to find proprioceptive neuronal structures in Müller's muscle (MM). The investigatorswill recruit 50 MM specimens of patients diagnosed with either unilateral or bilateral ptosis who were referred to undergo a posterior approach surgical correction (MMCR). In this surgical approach, the proximal part of MM is removed between the levator palpebrae superioris muscle and the tarsus of the upper eyelids. The excised tissue is generally not used but simply discarded. For study purposed, the investigators will obtain the patients' consent to save these tissues and carry out histologic examinations of the samples. No change in the surgical procedure or the following management will be needed.

Inclusion Criteria:

• specimens from patients undergoing Müllerectomies

Exclusion Criteria:

• small specimens (<1 cm length and width)

patients over 18 years old undergoing Müllerectomies