Asian Single Eyelid(monolid)-Reasons why Asian have single eyelids
4.Reasons why Asian have single eyelids
The single eyelid and epicanthus is a outward phenotype which is the result of evolution for adaptation to relavant environments. Genetically, All modern humans are included into the subspecies Homo sapiens sapiens, i.e. the subspecies of Homo sapiens. It has been reasoned that the racial differences of modern humans are just phenotypic variations. The epicanthus exists as a normal charicteristics in Asian ethnicity, and it is peculiar to East Asians. The true incidence of the Asian epicanthus is uncertain. But at least most people have the epicanthi of varying degree with absence or incomplete presence of double eyelid.
An eyelid is anatomically composed of various different tissues (skin, Levator muscle, orbicularis muscle, fat, tarsal plates, conjunctiva and so on). The tissues that help the opening of the eyes, called the levator muscle, is attached to the upper tarsal margin, and some parts of the muscle tissues are also attached to the back side of the eyelid skin through penetration into orbicularis muscles. Based on this mechanism, a double eyelid is formed by the pulling up of the eyelid skin when the eyes open. Dr. Kwon think that the single eyelid and epicanthus is a remnant manifestations of eyelid evolution which resulted from hypertrophy of upper orbicularis muscle and related aponeurotic attenuation/loss.
Previously, Sayoc’s theory has been accepted to explain absence of supratarsal crease in Asian since 1956. Sayoc suggested that in Caucasian, parts of the levator muscle of the upper eyelid are attached to the skin, While in Asians, it is congenitally absent or sparse. So, the natural double eyelid is absent or incomplete in Asian. It had been main medical theory for double eyelid up to early 2000s.
However, the electron microscopic studies confirmed that levator aponeurotic fibers penetrate the orbicularis muscle in both single and double eyelids of Asian, of course Caucasian. This finding suggested that Sayoc’s theory of the “levator expansion” had been incorrect.
Three parts of the eyelid which are affected in evolutional process. In eyes with a double eyelid, parts of levator muscle are attached to the back of the skin. On the other hand, an eye without a double eyelid has a structure called a epiblepharon which result from orbicularis oculi muscle displacement . Also, the preaponeurotic fat tissue is drooped down like pseudoherniation.
If so, why does the supratarsal crease not exist although aponeurotic fibers do penetrate the orbicularis muscle in Asian single eyelid? Dr. Kwon reconsidered basically without stereotype for previous theory about Asian double eyelid and the epicanthus from 2005, because Sayoc’s theory could not explain absence of Asian double eyelid and presence of epicanthus clearly. Dr. Kwon’s research tried to address the formative causes of epicanthal deformity and loss of double eyelid at the same time in evolutional aspect. Dr. Kwon’s research concluded that the answer for Asian double eyelid is not “not previously existed” but “lost” during evolution. Environmental adaptation is the basic cause for loss of supratarsal crease and formation of the epicanthus. Genetically, all modern humans are included into the subspeciesHomo sapiens sapiens,i.e. the subspecies of Homo sapiens. It has been reasoned that the racial differences of modern humans are just phenotypic variations. The stereotype of Sayoc’s theory may mislead plastic surgeons to misunderstand Asian double eyelid and epicanthus incorrectly. So, the evolutionary approach to Asian epicanthus or absence of supratarsal crease has not been researched and not suggested ever. Dr. Kwon suggested Kwon'stheory for the evolution of Asian eyelid.
The stage of the eyelid evolution according to the main action of orbicularis oculi muscle in Kwon’s theory.
Loss of medial crease, formation of the epicanthus, loss of mid-lateral crease, formation of the epiblepharon occur in sequence.
I. Stage of hypertrophy: The hypertrophy of orbicularis oculi muscle and depressor supercilli muscle develop by repeated frowning. The hypertrophy of orbicularis oculi muscle cause attenuation of the aponeurotic expansion which penetrate through orbicularis muscle. The aponeurotic penetrations
loosen and become sparse.
II. Stage of sheer stress: Complete detachment of aponeurotic expansions with loss of supratarsal crease takes place on point S by sheer stress. The detachment of aponeurotic fibers can occur beneath the orbicularis muscle or on the orbicularis muscle. Orbicularis oculi muscle contraction acts as sheer tensional stress with depressor supercilli muscle. The depressor supercilli muscle plays an ancillary role.
III. Stage of tensional stress: The successive loss of medial crease below point S results from displacement of preseptal orbicularis oculi muscle toward eyelash and the vicious cycle between malpositioned muscle hypertrophy and crease loss. The orbicularis oculi muscle contraction mainly act as tensional stress on remaining aponeurotic fibers and medial crease.
IV. Stage of compressional stress: Excessive contraction of orbicularis oculi muscle acts as compressional stress on overlying skin of upper medial eyelid with resultant vertical skin shortage and tension. The tight tension band develops due to vertical skin shortening of upper medial eyelid.
V. Stage of traction and tensional stress: Contraction of orbicularis oculi muscle acts as tensional stress on remaining supratarsal crease resulting in successive loss of supratarsal crease on the upper eyelid superiorly and/or act as traction force causing skin tent toward point P on the lower eyelid inferiorly. During and after formation of a tight tension band by vertical skin shortage, the orbicularis oculi muscle contraction causes a prominent skin fold on the lower eyelid by pulling-up the muscle point of origin like setting up a tent pole during contraction. The preseptal orbicularis muscle pulled up its origin superolaterally with skin tent on lower eyelid during contraction.
VI. Stage of degeneration: The atrophy of orbicularis oculi muscle and accompanying degenerative fibrosis develop, leaving the epicanthus with incomplete crease or epicanthus without crease.
The hypertrophied orbicularis oculi muscle becomes atrophied and replaced by fibrosis, but there remains vertical skin shortage and tension left on upper eyelid and skin fold and wrinkles on lower eyelid. Also there has been left centripetal fibrosis toward point p under the skin of lower eyelid.
The formation of epicanthus and loss of supratarsal crease are remnant manifestations of eyelid evolution which resulted from hypertrophy of upper orbicularis muscle and related aponeurotic attenuation.Increased blood supply induced by hypertrophied orbicularis muscle and climate factors would cause hypertrophy and pseudoherniation of preaponeurotic fat. Displacement of orbicularis muscle and fat pad would affect the levator muscle and Muller’s muscle. A relatively longer medial canthal ligament and underdeveloped nasal bone would be additional evolutional manifestations by hypertrophy of the orbicularis oculi and excessive tension.
The Classification of the epicanthus in relation to supratarsal crease in Kwon’s theory
Type I : Attenuated original crease without epicanthus (exopthalmic Asian eyelid)
Type II : Minimal epicanthus without crease. (without epibepharon)
Type III : Epicanthus with attenuated original crease
Type IV : Epicanthus with lowered infold crease
Type V : Epicanthus without crease (epicanthus and epiblepharon).
Examples) Only a double eyelid surgery performed with severe epicanthus. They look fierce .
Examples) Structural double eyelid surgery combined with magic epicanthoplasty.
Examples) Structural double eyelid surgery combined with magic epicanthoplasty for bigger and parallel type double eyelid.
Dr. Kwon emphasize that epicanthoplasty needs to be considered as a core surgery for Asian double eyelidplasty, especially when forming the parallel-type of double eyelid. Epicanthoplasty is not an ancillary surgery in Asian double eyelidplasty, but combined epicanthoplasty and double eyelidplasty deserve one complete "structural eyelidplasty" for Asians.
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