A method of refractive error management that involves using special rigid gas permeable lenses to modify and shape the corneal epithelium, therefore changing the way light is refracted into the eye. Patients will typically wear the rigid lenses to sleep, during which the corneal epithelium becomes remodeled overnight. Full correction in patients with myopia can be achieved within a week but lenses must be continued to be worn nightly to retain the correction. The side effects that accompany this method of correction include glare, especially during nights and decreased contrast sensitivity. The effect of orthokeratology is completely reversible by discontinuing lens wear
The oxygen demands of the cornea are very high and vary from individual to individual. Closed eye demands range from as low as 1µl/cm²/h to as high as 10µl/cm²/h. Demand increases with temperature. The epithelium is thought to consume up to 95% of the oxygen available to the cornea.
In order to reduce the level of overnight corneal oedema to a physiological level of 3.2% (the mean level of no lens overnight oedema), the critical lens Dk/t would need to be around 125 x 10-9 (cm x ml O2)/(s x ml x mmHg).