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Wavefront-optimized myopic LASIK improves vision but induces significant higher-order aberrations
Indian J Ophthalmol. 2010;58(4):307-312.
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Wavefront-optimized LASIK yielded strong visual outcomes but induced significant higher-order aberrations, a study showed.
"Induced changes in seven of the 22 higher-order Zernike terms showed a significant linear correlation with the refractive correction attempted," the study authors said. "Larger ablation zones induced less spherical aberration."
The authors assessed changes in ocular aberrations after wavefront-optimized LASIK for correction of myopia and analyzed potential causes.
The prospective study included 117 eyes of 59 patients with a mean age of 25 years. LASIK flaps were created with the Hansatome microkeratome (Bausch + Lomb). Ablation was performed with the Allegretto excimer laser (Alcon). The Allegretto wave analyzer was used to measure ocular aberrations preoperatively and 1 month and 6 months postoperatively.
Mean preoperative spherical equivalent was –5.33. Mean postoperative spherical equivalent was –0.21.
Study data showed that 102 eyes (87%) attained uncorrected visual acuity of 20/20 or better; 104 eyes (89%) were within 0.5 D of attempted correction.
Results also showed that total root-mean square of higher-order aberrations increased by a factor of 1.96, the authors said.
This paper discusses the higher-order aberrations that occur as a result of wavefront optimized treatments using the WaveLight Allegretto laser and Hansatome microkeratome.
It was not stated in the abstract that an elliptical pattern for myopia and astigmatism was used in 84% (98/117) of the eyes treated. The major findings included demonstrate that 7 out of 22 Zernike terms showed an increase proportional to the amount of spherical myopia correction. Larger diameter ablations predictably induced less spherical aberration. Fourth- and sixth-order (spherical) aberration overall increased by a factor of 1.86 postoperatively when achieving this. The ablation depth was up to 35% greater in the periphery than conventional treatments.
The actual study "showed that 3rd order (astigmatism) aberrations continue to dominate after LASIK" in contrast to an earlier Seiler study. The study demonstrated "a five-fold increase in the amount of vertical coma and represented the highest magnitude of induced change among all the HOAs."
It is puzzling why these significant findings were omitted from the abstract. Three inconclusive explanations are offered, but the paper failed to discuss the internal optical aberrations quantified by the ocular residual astigmatism that occur with prevailing differences between corneal and refractive astigmatism parameters. Examination of this calculated vector value might have provided an explanation for the excess resultant astigmatism aberrations detailed in the published paper.
— Noel A. Alpins, MD
OSN Refractive Surgery Board Member