Concluding this group of chapters, I present my current thinking with respect to the functional biodynamics of the eyelid crease.
In 1993 when I was preparing the manuscript for the original Asian Blepharoplasty atlas, in its last chapter I made an analogy that the eyelid crease is somewhat like the Great Wall of China, where from outer space an astronaut may merely observe a line crawling across the planet’s surface while in actuality the Wall meanders along, covering vastly different terrain and with forts scattered every 80–120 meters. These outposts were built with different functions in mind, their interiors having varying sub-sections serving as sleeping quarters, grain storage, weapon depots, animal shelters, signal and command stations. Similarly, the eyelid crease changes subtly as you traverse across the width of the eyelid. It is attaching to various substrates of tissues as the curvature of the lid margin, the tarsus and its opposing canthal ligamentous attachments come into play.
As with a naturally born eyelid crease, there are different techniques that you can apply within sections of the single eyelid as you create (or add) a natural crease, coursing continuously across the upper eyelid with its outer appearance showing as a homogeneous line. (We have discussed the depth of a crease and how to titrate this, the apparent crease versus the tilted crease height, as well as attachment to different anatomic terrain across the width of the crease.) Here we are discussing the linearity of the crease with respect to the curvature of the eye.
What about the opposing vectors that act across the length of the eyelid crease? In other words, what about the forces acting along the crease which allow the anterior lamella (preseptal layer) to slide down relative to the posterior lamella of levator and tarsus when the eyelid opens?
I prefer to conceptualize the eyelid crease as the external manifestation of a stringed series of unipoints (unipoint = single point in space). In Figure 23-1 the differently sized yellow dots represent unipoints that have vector forces of different magnitudes acting on them. Each of the yellow dots ( Figure 23-2 ) represents a highly magnified unipoint (vector interface), where the red vector is in line with the pull of the levator, and the blue arrow, the gravitating weight of the anterior skin–muscle layer when the facial nerve input is relaxed. Note that the blue vector’s direction may vary between 45° relative to horizontal, to almost vertical (90°) if the facial features are comparatively flat, as is often the case with Asian eyes.
Let us conceptualize that each unipoint is a nano-scaled virtual point (like rotating nano ball-bearings, or mathematical definitions of a point in space) that is bounded by vector forces going in opposite directions (sloped-up on the backside, and sloped-down on its front side). This string of unipoints is like a string of nano-scaled rosary beads that straddles the two lamellae of the upper lid at a location approximately along the superior tarsal border. When the levator is activated (‘ON’ for third nerve) and pulls the upper lid open, there is an inhibitory nerve signal that happens at the same time towards the orbicularis muscle (supplied by seventh nerve), turning it ‘OFF’. The weight of the relaxed preseptal layer of skin and orbicularis slides down passively, and covers part of the pretarsal skin and becomes the eyelid fold. The exact interphase between the posterior layer going up and the skin–orbicularis layer coming down occurs under the skin and within the eyelid, and its external manifestation on the skin side is the eyelid crease. The phasic change is where these virtual unipoints (or virtual nano-balls) are located. The slip-slide is facilitated by healthy fat in the middle space that I have termed the glide zone (see Chapter 17 ).
A good analogy is the wise saying that ‘an avalanche can start with the added weight of a snowflake’. We can see this easily by watching layers of fine colored sand trapped in liquid between two panes of glass. As we rotate the glass panel, we can see sand dunes topple, and slip-slides as the layers on top gain just a bit more sand.
In mimicking a crease through surgery, it is important to recognize where this occurs. To create it accurately, the precise placement of the crease along where these phasic changes (series of unipoints) naturally may occur (with surgical clearance of impeding tissues) should be the most important goal. Deviation from it has significant physiological and functional side effects.
Figure 23-3 shows pre-and postoperative images of a single-lidded individual, after undergoing Asian blepharoplasty performed by the author. Yellow conceptual unipoints are pasted here to show the lower pretarsal zone vectoring against a relaxed upper preseptal zone.