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Customised ablation research produces some answers but raises even more questions

WHAT is the effect of flap creation on higher order aberrations following custom LASIK procedures and how do these results compared with custom LASEK? Scott M MacRae MD reviewed the latest findings at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery.

He presented data from two studies designed to define the role of flap creation as a contributing factor in surgery-induced higher order aberrations (HOA). One randomised trial investigated the wavefront effects of flap creation alone and a second randomised study compared customised LASEK and conventional LASIK.

In addition, Dr MacRae reviewed results from the FDA Zyoptix trial of customised LASIK as a historical control for assessing the outcomes of customised LASEK and to provide some perspective on the functional improvements achievable with wavefront-guided surgery.

"The investigations conducted so far demonstrate that developing surgical procedures to reduce higher order aberrations is a complex challenge because there are multiple patient and procedure-related factors which can affect outcomes.
"So far, our experience is encouraging in showing that the customised procedures appear to offer some improvement over conventional surgery and certainly are not making vision worse. However, much further study is needed to determine the optimal way to optimise vision with excimer laser ablation surgery," Dr MacRae said.

He cited previous studies from other researchers indicating that customised PRK or LASEK might lower HOA. Other studies provide some evidence that the creation of the LASIK flap increases HOA by approximately 20% to 30%. Accordingly, he performed a biomechanics study to further investigate the contribution of the flap to LASIK-associated changes in HOA.

His study enrolled patients undergoing bilateral LASIK and randomised the eyes to flap creation, followed immediately by ablation or a two-stage procedure with a two-month delay between flap construction and ablation. Surgeons created a superior hinge flap using the Hansatome microkeratome and performed the ablation with the Technolas 217Z excimer laser.

Serial wavefront evaluations in the eye which had the delayed ablation showed horizontal trefoil was increased by creation of the flap and remained unchanged after the ablation, while spherical aberration increased only slightly after flap creation, but changed more significantly after the ablation. Both horizontal trefoil and spherical aberration increased similarly after one-stage LASIK in the fellow eye.

"We believe the effect on horizontal trefoil is induced by the arc pattern of this microkeratome’s head as it passes over the eye. We were somewhat surprised to find that the ablation itself, rather than the flap, is the predominant contributor to LASIK-related increases in spherical aberration," Dr MacRae commented.

To investigate further the effect of the flap on HOA, he conducted a LASEK study in which the eyes of 19 patients undergoing bilateral surgery were randomised to receive customised LASEK in one eye and conventional LASIK contralaterally using identical ablation zones. Mean sphere and cylinder for the group were -3.28 D and -0.79 D respectively.

At three months after surgery, mean cylinder was similar in both groups (mean -0.3 D). While there was a slight overcorrection of sphere in the customised LASEK eyes (mean +0.41 D), compared with the conventional LASIK (mean +0.06 D) group, logMar visual acuity results were similar for the two procedures. Mean logMAR UCVA was -0.07 for customised LASEK and -0.05 for conventional LASIK. Corresponding values for logMAR BCVA were -0.11 and -0.12.

Wavefront studies performed through a 6.0mm pupil showed both procedures induced only low amounts of HOA. At three months after surgery, mean HOA RMS was 0.51 microns (+13%) after customised LASEK and 0.49 microns (+12%) after conventional LASIK respectively.

The most remarkable wavefront finding, however, was that while both procedures were associated with increases in spherical aberration, customised LASEK had a lesser effect.
Mean spherical aberration RMS increased by 58% after customised LASEK, from 0.17 microns preoperatively to 0.27 microns. In the eyes treated with conventional LASIK, mean spherical aberration RMS increased by 94% from 0.17 microns to 0.33 microns.

The difference between the two groups, however, was not statistically significant and more patients are being treated to see if this trend remains the same with a larger group.

"An increase in spherical aberration is not uncommon with the B&L laser. However, secondary to our findings in the biomechanics study we conducted, the software is being modified to incorporate a correction factor and we believe better results with less induction of spherical aberration will be achieved in the future," Dr MacRae said.
Based on protocol design consultations with the FDA, Dr MacRae was unable to perform a study directly comparing customised LASEK and customised LASIK.

Therefore, he reviewed data from the FDA Zyoptix study to gain some perspective of how the results of customised LASEK compare with customised LASIK. The baseline characteristics of the 340 eyes in the FDA cohort were comparable to those of his customised LASEK population.

In the FDA study, total higher order RMS at three months was 0.53 microns and comparable to the value for eyes treated with customised LASEK. Further analysis showed the eyes with low levels of HOA experienced slight increases after surgery, while those with larger amounts (RMS Ž0.5 microns) tended to benefit with a decrease IN RMS VALUE.

Visual acuity outcomes from the Zyoptix trial showed 91% of eyes achieved 20/20 or better UCVA, while 70% were seeing 20/16 or better at six months after surgery. About 60% achieved a one line or greater gain in BCVA. Contrast sensitivity testing at three, six, 12, 18 and 24 cycles/degree showed the surgery was associated with a one patch improvement at all spatial frequencies.

"This data shows that customised ablation is yielding vision improvements which are more subtle than we initially expected and measurable mostly in changes in contrast rather than visual acuity.

For the average population, the benefit of wavefront treatment might be equivalent to the effect of a 0.3 D improvement in sphere. That is not overwhelming overall, although select patients with more significant wavefront errors will likely benefit with a more significant effect.

"In the future, with correction for spherical aberration correction, most patients with three or more dioptres of myopia will benefit from customised treatments," Dr MacRae said.