Definition and Purpose

2.2.2 The Cornea

Adjusting the eye length to the given refraction seems practically impossible, whereas changing the refraction power seems to be the only solution. As already mentioned the cornea takes the highest portion of refraction, moreover its refraction is stable. To change the refraction without additional lenses like glasses or contact lenses, the most likely answer would be to change the shape of the cornea.

The corneal tissue is highly watered and it consists of various cell layers. The two interface cell layers, the epithelium and the endothelium, separate the cornea from the outside and the inside of the eye respectively. The Bowman's layer, the second layer from the anterior of the eye following the epithelium, needs a special mention as it covers different functions which must be taken into account when applying Refractive Surgery.

Bowman's layer
Firstly, as the layer is more dense and less watered , it has somewhat of a "corset-function" and gives stability to the eyeball. Destroying this layer can change the stability of the eye which can cause incalculable refraction changes due to the ocular pressure "taking the opportunity once the corset is not there anymore". Until now no reliable theory has been found to calculate this undesired effect. Some ophthalmologists even want to use this corset-function in the future to cure astigmatism and even myopia by measuring and changing the partial tension on the Bowman's membrane [Zei95].

Secondly, like the bark of a tree, the Bowman's layer is highly sensitive and partial destruction causes woundhealing effects which are followed by pain and haze. The pain disappears a week after surgery and the haze usually last for three months. However, the most annoying circumstance is that the effects due to reoperation in superficial PRK are highly incalculable, with the result that the haze often does not disappear at all. The effects of the "bark-function" of Bowman's membrane in Refractive Corneal Surgery are always undesired and in reoperation they can be even fatal.