Orthokeratology (Ortho-K) is the process by which the corneal epithelium is reshaped overnight by a gas permeable lens to provide corrected vision during the following day after the lens is removed.
The process is temporary and completely reversible. Normally it is used for the correction of myopia and minor astigmatisms, but it has also been used for the correction of small amounts of hyperopia and even presbyopia.
Ortho-k is best fitted using a topographer. It produces a height map that allows a laboratory to calculate the lens that will sit just above the apex of the cornea, but without causing any surface erosions. Lenses are normally calculated to provide 8-12 microns clearance over the apex of the cornea so that, while being worn overnight, gentle pressure from the lids allows the epithelium to be reshaped by the lens. Normally the power is altered at a rate of one dioptre per night. This means that an average -3.00 dioptre myope will achieve fully corrected vision after three night’s wear. In fact, the vision normally regresses at a rate of about 0.75 dioptres across a full day, so target correction is normally set to produce a slight overcorrection to offset this effect. Overall, the patient should expect the treatment to take around a week to be fully effective.
The area of treatment is very small, only around 2.5mm, and therefore it is critical that lenses remain centred over the apex of the cornea during sleep. Ortho-k lenses are normally slightly larger than regular GP lenses to minimise movement. Some designs are even larger than the visible cornea.
The topographer provides essential data during the aftercare period. Initially, it may reassure the practitioner that the treatment is being delivered in the correct area and not displaced during sleep, but as time progresses it will also demonstrate the effectiveness of the treatment by comparing the image with the pre-treatment map. This is called a difference map.
Once the province of just a few specialist practitioners, orthokeratology is rapidly gaining ground as a mainstream practice and is being recognised as the most effective method of controlling the progression of childhood myopia.