Definition and Purpose

2.2.3 Shaping Ideas

Changing the curve of the cornea in its center of about six mm, one needs to know the different factors influencing the shape and refraction of the cornea:

1.the law of thickness
1.cornea's biostatics
i ocular pressure acting on cornea
ii innercorneal tension release

the law of thickness
Changing the thickness of the cornea follows the idea that the cornea is a stable lens, removing tissue in the center or adding tissue on the periphery therefore flattens the cornea. This is called the law of thickness.

cornea's biostatics
Instead of removing or adding tissue, the shape of the cornea can be changed by weakening the strength of the cornea outside the center. Then, either the ocular pressure leads to a different curve of the cornea center, or the release of innerconreal tension changes the shape of the cornea. Most procedures, explained in brief later on, take into account either one of these two factors.

The difficulty of the first approach is that changing the thickness of the cornea can induce a change in the eye's biostatics causing undesired additional refraction changes.

Changing the biostatics of the cornea by incisions, so that the ocular pressure then reshapes the cornea often gives incalculable surprises due to unknown individual wound healing effects or due to difficulties in accuracy during the operation.

New theories try to model innercorneal tensions in order to explain observations which can not be explained by the thickness law in relating to the Bowman's corset function [Zei95p.21ff]. Some scientists suspect that the refractive change due to superficial PRK procedure is rather a result of destroying the Bowman's layer than to applying the thickness law. This would explain the observation why the results above six diopters become so unpredictable. The radius of totally destroyed Bowman's layer rises expotencionally above six diopters by now used ablation programmes.