As light rays from an image travel through the eye to reach the retina, they undergo a certain amount of distortion due to the inherent imperfections of the eye. These imagery distortions are known as optical aberrations. Total optical aberration is contributed by all portions of the visual system, mainly y the cornea and the crystalline lens. About 95% of optical aberration in normal eye is contributed by spherical and cylindrical refractive errors. The remaining 5% of aberrations are due to field or curvature, distortion, spherical aberration, chromatic aberrations, trefoils, quatrefoils and coma. Currently, not all of these remaining aberrations can be fully corrected by using customized ablation. It can probably overcome aberrations due to distortion, spherical aberration and coma. The total optical aberration of the eye also changes as we age. The aberration in a young eye is different from the aberration in an old eye due to physiological changes mainly in the lens and cornea.

Customized ablation is a technology believed to overcome all these optical aberrations of the eye by altering the corneal shape based on individual variation of aberrations in each eye. Therefore the cornea reshaping made, is theoretically 'tailor-made' for each eye. The main advantage of customized ablation is to minimize visual symptoms resulting from optical aberrations, such as haloes, glare, loss of contrast sensitivity and of best-corrected visual acuity. The corneal tissue removed during cornea reshaping for a specific amount of correction is less as compared to the conventional method. This would allow us to treat more severe refractive errors while maintaining sufficient residual corneal tissue for cornea stability. Maximum visual potential with a stable cornea configuration is the ultimate goal.

Due to the dynamic and progressive changes of the eye as we age, new optical aberrations will arise and the maximum potential visual acuity initially achieved after treatment will deteriorate. This would require frequent re-treatments to correct these changes in optical aberration in order to maintain the vision. Furthermore, even if maximum visual potential is achievable by perfect laser ablation, corneal healing pattern after surgery varies from individual to individual. During corneal healing, new optical aberrations, the amount of which could not been measured prior to treatment, will be formed. This again hinders the achievement of ideal perfect vision. Another factor that still cannot be achieved during customized ablation is the lack of real time feedback between optical aberration measurement and the laser machine during ablation. The presence of excessive eye and head movement during laser ablation, excessively large papillary diameter and refractive changes and aberrations due to accommodation will result in less perfect visual outcome. 

The procedure of performing a customized ablation will include:

1. Measuring the optical aberration using a wave front analysis system. These measuring devices have the ability to measure refraction to submicron levels of about 0.01 dioptre.
   
   
2. Linking the measured refraction and aberration to the laser machine. This is currently accomplished by downloading the wave front data onto a floppy disk, which is then inserted into the laser computer. This step is the presently the most challenging component, as it requires an accurate orientation and registration of the wave front data with the laser optics and eye tracker to the eye undergoing laser ablation to achieve accurate ablation.

The laser machine must also have at least two essential components. They are a very fast eye tracker of at least 150 Hz to guide the laser beam to the location during normal eye movements and a small scanning spot laser, capable of delivering ablation with a spot size of 1 mm or less.

Currently customized ablation is still undergoing clinical trails in U.S.A. It is NOT an FDA approved procedure yet.

We currently feel customized ablation has a role for some patients with optical aberrations which cannot be treated by the conventional excimer laser ablation and it may extend the limit of refractive error correction due to its cornea tissue saving feature.

Most refractive errors can still be treated by the conventional method with good results. The difference in final visual acuity for most patients between using customized and conventional ablation has not been found to be very significant, due to the factors discussed above.

Information provided by: http://www.optimax.com.my