The Asian periocular anatomy is very different from that of a Caucasian or African American on many levels. Firstly, the shape of the orbital bones makes a tremendous impact on the 3D shape and angle of the eyelids. In an Asian, the lateral orbital rim, or the rim of bone that can be felt at the outer corner of the eye, sits on average 3mm more anterior than in Caucasians, and 5mm more anterior than in African Americans (Migliori and Gladstone 1984, Tsai et al 2006). This difference in the skull shape significantly affects the angular corners of Asian eyelids, as explains the more rounded outer corners in Caucasians and African Americans.
Secondly, the fibrous strip of tissue that defines the upper and lower lid margins, called the tarsus, is significantly narrower in the Asian patient (6-8mm) compared to Caucasians and African Americans (10-12mm). The upper edge of tarsus is where fine filamentous attachments link the eyelid skin to the eyelid, and corresponds to the height of the lid crease. It is these attachments that can be weak in Asians who do not have a visible lid crease, often called a “single eyelid” configuration.
Another difference is the additional layer of fatty tissue in the Asian eyelid that is not found in either Caucasians or African Americans. This extra layer of fat helps to separate the eyelid skin from the muscle and tarsus, which is typically firmly attached in Non-Asians. By increasing the thickness of Asian eyelids, this layer may also prevent a lid crease from forming at the upper border of tarsus.