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Topography-guided and wavefront-guided customised laser treatments complement each other well in the customised treatment of irregular corneas, according to Michael C Knorz MD, University Hospital Mannheim, Germany.
“Topography-guided ablations perform best in the regularisation of highly irregular surfaces, and wavefront-guided ablations perform best in the correction of smaller irregularities and in the fine-tuning of the refractive result,” Dr Knorz told the XXXII Congress of the ESCRS in London. 
He noted that the customised laser treatment of irregular corneas traces its origins from the early pre-topography days of LASIK. At that time the surgeon had to painstakingly and individually plan each treatment in such cases. The advent of topography-guided treatment helped provide a greater standardisation of the technique. There followed the introduction of the more scientifically-based wavefront-guided ablations.
Topography-guided ablations have the advantage of being based on the surface irregularities that are causing the visual disturbance, he said. They are typically used for large errors and for grossly irregular corneas. Their most reliably beneficial effect is in eyes with decentred ablations and those undergoing the treatment for the enlargement of small optical zones. 
“There is typically a lot of variability in the outcome, but you do improve the topography, although you do not typically achieve a very good refractive result. And obviously a lot of re-treatments are necessary and you have a lot of regression, but again, these are patients who really need our help and we really need these techniques to improve their vision,” Dr Knorz noted.
In contrast to topography, aberrometry measures the aberrations of the eye’s total optical system. As a result, the refractive outcomes of wavefront-guided ablations are typically much better than those of topography-guided ablations. However, aberrometry provides no information on the treated surface and cannot measure irregular corneas reliably. 
“Grossly irregular corneas cannot be treated with wavefront-guided ablations because you don’t really have a measurement with which to plan the treatment. Therefore, a wavefront-guided ablation is typically more for the smaller areas or for fine-tuning the refraction. In a grossly irregular case, we typically use a topography-guided treatment first, followed about six months later with a wavefront-guided retreatment to get rid of the refractive error,” he added.
Causes of unpredictability in customised topography- or wavefront-guided surgery in irregular corneas include the cornea’s healing and biomechanical responses to the ablations. 
“These limitations could help explain why not too many people are really enthusiastic about customised ablations. However, the treatments can provide meaningful improvements in patients with decentrations and other problems,” he added. 

Michael Knorz: 

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