Personal eye model with more individualised LASIK approach

A more individualised approach to LASIK ablations using a personal virtual eye model for each patient could lead the way to greater predictability in the treatment of refractive errors, said Arthur Cummings MD, FRCS, Dublin, Ireland at the XXXII Congress of the ESCRS in London.

There are numerous types of laser ablation profiles available to corneal refractive surgeons today, including wavefront-optimised, wavefront-guided and topography-guided ablations. However, the ablation patterns used in each approach are based on the Gullstrand Eye model, and therefore assume a corneal power of 43D and an axial length of 24mm.

“Irrespective of how sophisticated the data is that you’re going to use to drive the treatment, when you enter it into the laser, the laser thinks you are treating Gullstrand’s Eye,” Dr Cummings said.

In addition, measurement errors can further reduce the predictability of the procedure. Research shows that subjective refraction measurements are repeatable to within 0.5D only around 90 per cent of the time. A similar proportion of eyes are within 0.5D of target refraction in most published LASIK series.

INDIVIDUAL VIRTUAL EYE MODEL 

A superior option to using the Gullstrand Eye model would be to base a laser ablation profile on a customised virtual eye model in each patient, he maintained. Such a model would be built from wavefront refraction instead of manifest refraction, the actual OCT-measured axial length, instead of the Gullstrand Eye’s arbitrary 24mm, and topography data with 24,000 data points for the corneal shape instead of the arbitrary 43D for the cornea’s power.

Additional measurements would include the refractive effects of the posterior cornea and the anterior and posterior lens surfaces. An additional enhancement would be a further modification in the pattern of the placement of the laser shots to better compensate for the reduction of their effect as they move more peripherally on the cornea’s surface and hit it more obliquely as the cornea slopes away toward the periphery.

The wavefront-optimised treatment, designed by Michael Mrochen PhD, already includes that kind of compensation but is based on the amount of compensation needed with the Gullstrand Eye model. A new treatment algorithm is available in which the mathematical compensation is based on the personal eye model of each patient.

Dr Cummings noted that results of a multicentre study in which he participated showed that an individualised ray tracing LASIK approach based on the personal virtual eye model he described can provide visual acuity that is as good if not better than that achieved with wavefront-optimised, wavefront-guided or topography-guided approaches, particularly in the higher ranges of myopia.

The study involved 127 eyes of 71 patients. Part of the study’s inclusion criteria was myopia of -4.0D or greater or myopic astigmatism between 2.0D and 6.0D. Therefore their refractive errors were higher than average for LASIK patients, he noted.

Dr Cummings and his associates performed diagnostic measurements with three devices. They used the Pentacam® (Oculus) or the Wavelight Allegro Oculyser® (Alcon) for topography and pachymetry, the Lenstar® (Haag-Streit) or Wavelight OB820 (Alcon) optical biometer for the axial length measurements, and the Wavelight Analyzer® (Alcon) for the wavefront measurements. They performed surgery using femtosecond laser or a microkeratome for flap creation. They used either the Wavelight® 400 Eye-Q or the Wavelight® 500 Concerto excimer laser to perform the ablation using a personalised compensation algorithm for each eye.

Despite the high levels of myopia treated, the new treatment’s predictability was good. At three months follow-up, around 87 per cent of patients were within half a dioptre of emmetropia, and 84 per cent had an uncorrected visual acuity of 20/20 or better. The mean cylinder decreased from -1.06D preoperatively to 0.31D postoperatively.

Postoperative uncorrected distance visual acuity was equivalent to or better than the preoperative corrected distance visual acuity in 73 per cent of eyes, and 12 per cent had a postoperative decimal uncorrected visual acuity of 1.6. In addition, corrected visual acuity improved from 0.05 logMAR preoperatively to -0.109 logMAR postoperatively.

He added that the results compared favourably with those of FDA trials with wavefront-optimised and wavefront-guided LASIK. For example, the proportion achieving 20/20 among those undergoing correction of -4.0D to -7.0D of myopia was 80 per cent in the FDA Wavelight study with wavefront-optimised ablation, 91.2 per cent in the FDA wavefront-guided study, and 93.7 per cent with the new ray tracing method.

Dr Cummings added that further improvements in predictability may in the future come from an integration of the different measurement instruments into one single instrument, itself closely integrated with the laser.

“Having one device that provides all the diagnostic information would in turn provide quicker acquisition of data, better registration. It would also be more convenient for our staff, and most importantly more convenient for the patients,” Dr Cummings added.

Arthur Cummings: abc@wellingtoneyeclinic.com