The human superior tarsal muscle (Müller's muscle): a morphological classification with surgical correlations.

The superior tarsal muscle (STM) is a smooth muscle that originates from the undersurface of the levator palpebrae superioris muscle (LPSM) and inserts onto the superior tarsal plate (STP) of the upper eyelid. We have performed a morphometrical investigation of the STM in 49 adult human cadavers (34 males, 15 females). Histological analysis has shown a transitional area between the skeletal striated muscle (LPSM) and the adjacent smooth muscle (STM). We propose an original morphological classification based upon the attachment of STM to the upper border of the STP. Accordingly, we describe four patterns of STM. Pattern 1 (P1) consists of STM attachment to the central portion of the STP. Pattern 2 consists of both medial (P2M) STM attachment to both the central and medial regions of the STP and lateral (P2L) STM attachment to both the central and lateral regions of the STP. Pattern 3 (P3) consists of STM attachment along the whole extent of the STP. Pattern 3 was the most frequently observed pattern (63.27%) followed by patterns P2M (24.49%), P2L (8.16%) and P1 (4.08%). P3 was the predominant pattern in males (73.52%), while in females, both patterns P2M (46.66%) and P3 (40.00%) were equally prevalent. The analysis of paired specimens revealed a symmetrical arrangement in 72.20% of all cases, with the remaining cases (27.80%) displaying left-right STM asymmetries. To the best of our knowledge, this is the first description of the STM asymmetries in the medical literature. This innovative classification provides anatomical parameters for interpreting morphological variations of the STM with relevant applications in both plastic surgery and ophthalmology.