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Roibeard O’hÉineacháin
in Rome

CUSTOMISED PRK using the Zyoptix (Bausch and Lomb) system appears to increase higher order aberrations to a lesser degree than conventional PRK in patients with myopic astigmatism, according to the results of an Italian study presented at the 7th ESCRS Winter Refractive Surgery Meeting.

The study compared 30 patients who underwent PRK with the customised ablation system, with 30 patients undergoing conventional Planoscan PRK. It showed that eyes which underwent Zyoptix ablation had a significantly smaller increase in the root mean square (RMS) of their higher order aberrations at six months follow-up, said Mario Nubile MD.

“With standard Planoscan PRK there was an increase in all aberrations. Coma, trefoil and spherical aberration increased and the total RMS was nearly doubled. With Zyoptix PRK, the coma decreased and there was less increase in trefoil and spherical aberration, which were the main reasons there was less increase in the overall RMS compared to Planoscan,” Dr Nubile said.

In the prospective study the two treatment groups both had myopic astigmatism and a spherical equivalent ranging from -2.5 D to -6.0 D (mean: – 4.6 D). Both groups underwent PRK with the Technolas 217 excimer laser, one group with the wavefront-guided Zyoptix ablation profile and the other with conventional Planoscan ablation.

“Six months postoperatively all eyes in both groups were within 1.0 D of emmetropia and 75% were within 0.5 D. There was no significant difference between the two groups in terms of UCVA and BCVA and both achieved excellent visual acuity. There was a non-significant trend towards better visual acuity in the Zyoptix group,” Dr Nubile said.

He added that several studies have shown that refractive photoablations such as PRK and Lasik increases the overall corneal aberrations and that the increase is dependent on pupil size and ablation depth.

The aberrations which increase most are spherical aberration and coma. Proponents of PRK and Lasek note that these procedures do not induce the flap-induced aberrations seen with Lasik.
The main cause for the reduction in optical quality in laser-based refractive surgery is the modification of corneal asphericity. Other factors include the bio-mechanical response, corneal mechanics and the healing effect, he said.

“An ideal ablation should have different features. It should improve optical quality, induce fewer aberrations and reduce existing aberrations. It should also be aspheric and incorporate mechanical and biological response. However, we are still far from this point,” Dr Nubile said.

The superior results of wavefront-guided ablations may be partly due to the lower amount of tissue that has to be removed - almost 13% less - compared to Planoscan ablations. By removing less tissue, wavefront-guided ablation induces less of a healing response and fewer bio-mechanical changes, he explained.

In support of this hypothesis, Dr Nubile pointed out that among those in the Zyoptix group, the higher the preoperative higher order RMS value, the lower was the percentage increase of aberrations.
Conversely, among those with low levels of preoperative higher order aberration the percentage change was similar, whether they had undergone Zyoptix or Planoscan ablation.

Dr Nubile suggested that in such eyes the healing response even to customised ablation is still enough to negate any potential reduction of higher order aberrations.
Thomas Kohnen MD, who chaired the session at the Rome meeting, noted that he had observed the same trend in patients treated with wavefront-guided Lasik.

“When we treated with customised Lasik we found that we sometimes even increased the higher order aberrations in patients with low amounts of higher order aberrations preoperatively, while we maintained it among those we treated with high amounts of higher order aberrations,” Dr Kohnen said.

Dr Kohnen said that this raised the question of whether patients with only small amounts of higher order aberrations should undergo customised ablations.
In response Dr Nubile pointed out that in his study patients with low amounts of higher order aberrations undergoing Zyoptix ablations did as well as similar patients undergoing Planoscan ablations.

Therefore, his results did not indicate that customised ablations posed any hazard to such eyes. Furthermore, the majority of patients have enough aberrations to potentially benefit from the wavefront-guided approach, he said.

Leonardo Mastropasqua MD
University G d'Annunzio, Chieti, Italy
Email: mastropa@unich.it

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