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Biophysics

In addition to utilizing minimally invasive techniques, the new generation of double eyelid surgery incorporates elements of biophysics. Biophysics requires each surgery to be tailored for each patient and their unique eyelid characteristics. Factors that must be taken into account are the patient’s skin thickness, elasticity of skin, soft tissue mass volume, differences in eye aperture, muscle function, and brow laxity. These variables must be taken into consideration to have the most optimal results for each individual.

Previously, double eyelid surgery was performed primarily in the same fashion regardless of the differences in the patients’ eyes. This resulted in unnatural and unbalanced results. Today, each surgery is individually calibrated and calculated with all the different factors accounted for, resulting in a very natural looking fold.

The formulation is based on the spring mechanical model to understand the relationship between the height of the fold and the appearance of the eye with the constraint of the levator muscle strength.

As shown in the diagram below, the levator muscle is responsible for generating the pull to bring up the fold. It works against two forces. One is the weight of the skin and muscle, the other is the tension within the muscle (or the compression). The model treats the skin and muscle as elastic, or in other words, a spring. The spring constant k represents the upper eyelid skin stiffness.

Neglecting the damping constant of the spring, the force equations of each system for the high fold and the low fold can be expressed as (X1) and (X2) respectively,

where mH and mL are the mass of the eyelid in case of high fold and low fold respectively, g means gravitational acceleration and F is the force to lift the eyelid by the levator muscle. [18] When the velocity becomes zero so that the eye opens most widely, the second order time derivatives of the displacements in (X1) and (X2) becomes zero. Equation (X3) and (X4) show the relations between the maximum displacement of the eyelid and the force. Therefore, the maximum displacements of the eyelid for the case of high fold and low fold can be derived as shown in (X5) and (X6).

Equations (X3) and (X4) tell us that regardless of how high the fold is, levator muscle is supporting two forces. One is the weight of the muscle (mg), and the other is the tension (kx).

The equation (X5) and (X6) indicated that the maximum displacement should be inverse correlated to the mass of the eyelid.

Therefore, the maximum displacement in case of the high fold always should be less than that in case of the low fold as shown in the equation. Consequentially, the spring modeling can explain why eyelid ptosis can occur just from creation of a high double eyelid fold without any direct effect from the levator muscle. Conversely, eyelid elevation increases (resolution of eyelid ptosis) with correction of high fold to lower double eyelid fold.

The underlying factor that most affects eyelid physiology is whether the eye is protruding or recessed. This is actually based on whether the patient’s facial bone that surrounds the eyeball is protruding or recessed. When the eye is protruding, there is less eyelid skin hooding and when the eye is recessed, the eyelid skin tends to hood and sag more. When the eye is protruding, the fold height has to be smaller whereas when the eye is more recessed, the fold should be larger to prevent the eyelid skin from hooding. In addition, when the eye is recessed and more sunken in, the thicker brow skin tends to drop and push downward on the eyelid skin which then hoods over the fold.

In correcting eyelid ptosis, the protruding eye has the mechanical advantage of eyelid elevation, so even small amount of muscle tightening will give greater eyelid elevation. For recessed eyes, the same amount of muscle tightening will yield less eyelid elevation.

Graphical analysis of mechanical advantage in eyelid elevation with respect to the degree of
eye protrusion. (F2: force exerted by muscle. F1: weight of eyelid.)

Illustration to show the mechanical advantage in exophthalmic eyes in lifting eyelid.

Mechanical modeling according to the spring dynamics in order to analyze the eyelid elevation
in relations to the double eyelid fold height.

It is important to understand that each half of a person is asymmetric. For example, no two feet are the same size, no two breasts are the same size, and no two sides of the face (left and right) are the exact same for a person. That said, one side of the eye is more protruding and the other more recessed relative to each other. Therefore, when performing the double eyelid surgery, the surgeon has to take into account the biophysics to design and calibrate the fold and eyelid muscle function based on the physical entity of the preexisting asymmetrical tissue structure.