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Advanced Custom Ablation: New Technology for Better Quality of Vision

Dr Keiki R. Mehta
Dr Cyres K. Mehta

Mehta International Eye Institute,Mumbai India

About the author: Dr Keiki Mehta is counted amongst worlds finest cataract surgeons. Apart from being a brilliant clinician and a superlative surgeon he is a visionary. He is widely credited for the IOL revolution in India. He has many firsts in his name like his association with worlds first soft foldable intraocular lens.  He is always in demand for his live surgical demonstrations and has performed through out the world. He is an exceptional teacher. His lectures and surgical videos have taught and inspired an entire generation of surgeons in India and abroad. He is a pioneer in refractive surgery in this part of the world. But above all he is relentless sight giver for rich and poor alike and has recently been awarded with Padmashree by the honorable President of India for his unparalleled contribution towards global ophthalmology.

 

LASIK is the most widely used technology for the correction of most spherocylindrical ametropias because of its safety and its predictability that leads to excellent outcomes for both the patient and the surgeon.  Over the years there has been a progressive refinement of the technique.  Customised correction of refractive errors using wavefront technology has revolutionised the way we plan and execute the treatment of refractive errors there are two specific reasons why customised treatment is transforming the entire LASIK arena.  The first is that wavefront technology allows us to treat or not the only second order aberrations such as sphere and cylinder but also a higher order aberrations such as coma  and spherical aberrations which exist in normal individuals as well as post  refractive surgery patients.  A major shift in the thought process of refractive surgery is that the LASIK surgeon now seeks to enhance vision beyond what nature itself and designed.  With the advent of adaptive optics it would seem possible to enhance and correct not only the eye is aberrations but also improve contrast significantly.  For some tasks this improvement can be significant.

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Let us first of all define what is customisation  Chambers 21st Century dictionary defines Customize as (verb) adapt, convert, modify, tailor, alter, adjust, suit, fit, transform colloq. fine-tune . Thus customised correction seeks to  optimises the clarity of the eyes optical system and attempts to correct not only the sphere ,the cylinder and astigmatism ,but also the aspherical and asymmetrical anomalies which may exist.  This is an ideal technique for not only correcting decentred ablations,  corneal scarring , following injury,  penetrating keratoplasty and  even following inappropriate catarct surgery. .

There are various factors which one needs to considered before applying the proceedure

Age as a criteria

Experience has shown that the agent is an important factor to be considered when applying customisation procedures.  Many studies have shown that my occupations over the age of 40 to 50 are  much more susceptible to hyperopic overcorrection.  In addition reports have shown that treating younger patients with myopia more aggressively and reciprocally hyperopia leads to higher patient acceptance.  The reason why that is so is that the young eyes  generally have a large range of accommodation so a slight hyperopic overcorrection does not really affect the ability to see clearly.  On the other hand it is always preferable to over correct  older persons more aggressively for hyperopia and less for myopia so as to leave them   slightly myopic since the obviously have limited accommodative amplitudes.  Over correcting an older myopic  can blur both distance and near while a slight  under correction would provide a functional near vision for many tasks .

Size of the LASIK flap

Flaps are usually designed on to match the intended the size of the ablation.  Thus one would prefer a larger flap  for hyperopic ablations as compared to myopia.  Very often when treatment is expected to be difficult and the surgeon can assume that he is going to retreat this patient having a slightly larger flap is far better as it allows more room in case a hyperopic pretreatment needs to be superimposed on  and over a myopic primary treatment.  It is important for the surgeon to know as to what is flap thickness is, vis a vis, the microkeratome he utilises.  But in our and definitive study utilising the ACS microkeratome  it was found that the mean corneal thickness measured 124.8+/- 18.5 um , indicating that the flaps are thinner than predicted by the manufacturers plate depth measurement.  While another study showed progress of thinning/thickening of the flap in the direction towards the hinge.

Larger estimating errors also require a larger transition zone to ensure smooth transition and avoid relegation.  It also makes sense to utilise a cross cylinder ablation in eye is with a large amounts of myopic or mixed and astigmatism as it significantly reduces the quantum of cylinder to be corrected in each Meridian by almost 50% and thus is very tissue saving.

Factors to be considered for customised correction

Customisation based on corneal topography

Corneal surface aberrations and irregularities can be easily calculated from corneal abberation data derived from corneal topographic measurements using  a computerised topographer .  Using this technique is very useful when the aberrations is as essentially corneal in origin typically corneal scars or  excess/irregular astigmatism induced following penetrating keratoplasty  It has an excellent potential for enhancing visual ability as it seeks to reduce the induced aberrations which impinge on the ability to see clearly. It is  also ideal for decentred ablations with the irregular induced astigmatism.  Interestingly it is the penetrating keratoplasty patient can see 6/6 and read N5 , in theory, with excellent vision albeit with glasses, but who in reality is acutely unhappy with the distortion of the induced residual  astigmatism and higher order abberations. The rationale  of corneal based customisation is that it  significantly improves the quality of life and brings great benefit to these patients . Often an examination of the Ocular wavefront abberometry  maps of   these eyes will show a fair quantum of higher order aberrations which are responsible for inducing the blurred vision  and which if eliminated , significantly improves contrast. 

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Click the pictures to enlarge them

There are often questions asked as to the role of optimised LASIK as compared to wavefront guided LASIK.  Optimised LASIK essentially utilises a software which gives the best possible result by optimising the placement of the excitement of device LASIK gives good results when the astigmatism is regular and there is a reasonable quantum of consistency between refraction and astigmatism and that noted on corneal topography.  For eyes with low powers and with low quantum of astigmatism perhaps optimisation gives excellent results.  Usage of a optimisation technique diminishes the quantum of lower order aberrations of the eye and results in good vision.     To summarise if aberrations on the corneal surface irregular, investing when his is irregular, if examination of corneal topography maps is mediation in quantum of astigmatism in the upper and lower quadrant or indifferent accordance then corneal wavefront guided LASIK perhaps gives the best results

In many ways Corneally wavefront guided Lasik has a fair number of advantages. 

There are however some disadvantages attached to doing only a corneally wavefront guided Lasik.

 
There is a word of caution  . It must be clearly understood that :-.