To define the lateral extension of the Müller muscle and to elucidate its involvement in the development and surgical treatment of Graves (thyroid) eye disease.
Twelve lateral halves of orbits exenterated from patients with medial or posterior orbital neoplasms were fixed, embedded, and step sectioned at 250-μm intervals to produce histological sections. Gross anatomical dissections of human cadaver heads were also used to corroborate the histological findings.
Histological analysis revealed that the Müller muscle extended laterally between the orbital and palpebral lobes of the lacrimal gland in all specimens. The smooth muscle fibers were found to interdigitate with lacrimal ducts passing from the orbital to palpebral lobe and to extend close to the ductal orifices at the conjunctival surface. Gross dissections confirmed that the Müller muscle accompanied the levator aponeurosis lateral extension, which is known to pass between the orbital and palpebral lobes of the lacrimal gland.
Our anatomical findings suggest that the Müller muscle may contribute to the temporal flare frequently seen in eyelid retraction associated with thyroid eye disease. They may also explain the difficulty of treating lateral eyelid retraction in thyroid eye disease and indicate the need for new surgical approaches for severe lateral eyelid retraction.