TREATING ABBERATION

In theory, wavefront-guided LAsiK can correct not only spherico-cylindrical refractive errors, but also pre-existing higher-order wavefront errors. in practice, however, higher order aberrations (hOAs) cannot be completely eliminated due to limits on the precision of measuring and treating them. so the question arises: for myopes with less than 0.3 microns RMs total hOAs preoperatively, does wavefrontguided ablation offer any benefit over wavefront-optimised treatment, which may be simpler and preserve tissue? Evidence suggests benefits for both approaches, though the advantages gained with wavefront-guided ablation may be difficult to measure for patients with low pre-existing hOAs, according to presenters at Refractive surgery Day at the annual meeting of the American Academy of Ophthalmology. 'Wavefront-guided is important, but it has no measurable advantage in improving visual acuity for eyes that have preexisting higher order aberrations of low value,' said Ronald R Krueger MD, Cleveland Clinic, Ohio, Us. Wavefront guided is also of no benefit over wavefront optimised in higher myopes, with more than -4.0 D correction, and total hOAs of up to 0.4 microns RMs, he added. While wavefront-guided ablation is clearly better for patients with more hOAs, there are several reasons why it does not benefit patients with low hOAs, Dr Krueger said. For one, successfully treating low-value hOAs requires extreme precision in controlling laser beam shape, size, and power; a high repetition rate; and perfect centration and eye tracking to stay precisely on target.

Level of precision The literature suggests this level of precision is difficult to achieve, Dr Krueger said. Decentration of more than 0.8mm measurably degrades retinal image quality (Bueeler M et al. J Cataract Refract Surg 2003; 29:257- 263) and cyclorotation of more than 2.0 degrees induces aberrations – an event one study found occurred in 68 per cent of cases (Ciccio et al. J Refract Surg. 2005; 21:S772-S774). A 2003 study of 1,039 eyes found mean cyclorotation of 4.03 degrees. Plus, the track record of iris-recognition rotational eye trackers isn't the best. in a study of 275 eyes, iris recognition failed after three attempts in 10.1 per cent of cases, and iris tracking failed in 31.3 per cent (Prakash G et al. Am J Ophthalmol 2010; 149: 229-237). studies have shown that both wavefront-guided and wavefront-optimised LAsiK induce hOAs, and there is no difference in postoperative uncorrected visual acuity or spherical equivalent for patients with less than 0.3 microns pre-existing hOAs, Dr Krueger said (Perez-Straziota CE et al. J cataract Refract Surg. 2010 Mar; 36(3) 437-41. Yu J et al. J Refract Surg. 2008 May; 24(5) 477-86. ).

[caption id='attachment_2215' align='alignleft' width='284' caption='Figure 1: Comparison of mean pre-op to post-op change of HOA after LASIK in a matched group of conventional and wavefront-guided treatments where the pre-op HOA was <0.3 microns and every eye had a 6mm wavefront capture'][/caption] [caption id='attachment_2216' align='alignright' width='284' caption='Figure 2: Comparison of mean pre-op to post-op change of HOA after LASIK in a matched group of conventional and wavefront-guided treatments where every eye had a 6mm wavefront capture'][/caption]

Another study showed no difference in post-op wavefront aberrations in patients with less than 0.3 microns hOA preop, but increased induced aberrations in wavefront-optimised patients with low myopia who had more than 0.3 microns pre-op, and patients with myopia higher than -4.00 D who had more than 0.4 microns pre-op (Stonecipher et al. J Refract Surg. 2008; 24: S424-S430). A meta-analysis of seven studies with 930 eyes found no difference in uncorrected distance or best corrected visual acuity, or mean residual spherical equivalent and no difference in post-op induction of hOAs for those with preoperative hOAs of less than 0.3 microns, but found wavefront-guided eyes had significantly less post-op induced hOAs than wavefront-optimised eyes in patients with more than 0.3 microns hOA pre-op (Feng J et al. Optom Vis Sci. Sept. 2011).

Low-value hOAs may not affect vision, and may even be beneficial, Dr Krueger said. A study of the effects of Zernicke wavefront aberrations using adaptive optics, with an electromagnetically deformable mirror, found that measureable visual acuity decreases started at 0.3 microns RMs with major decreases at 0.9 microns spherical aberration (Rocah CM et al. J Refract Surg. 2007;23: 953-959). he pointed out that one goal of wavefront-optimised ablation is maintaining the ratio of central and peripheral corneal power to each other before and after surgery, preserving the prolate profile of the cornea, which is generally more beneficial than removing all aberrations. 'We need to preserve the asphericity of the cornea, and i especially believe wavefront-optimised should be used with higher myopes.' steve schallhorn MD, AMO consultant and global medical director for Optical Express, believes there are advantages to wavefront-guided ablation for eyes with low-value hOAs. he cited published studies that have found higher levels of induced hOAs in wavefront-optimised eyes than wavefrontguided eyes (Padmanabhan et al. J Cataract Refract Surg 2008; 34: 389-397. Yu J et al. J Refract Surg. 2008 May; 24(5) 477-86.)

His own research also shows advantages for a wavefrontguided ablation profile. Dr schallhorn presented a study comparing matched eyes, 194 with conventional LAsiK and 185 wavefront-guided where all eyes had a pre-op hOA 0.3 microns. Even when considering a higher hOA cutoff, fully 26 per cent of eyes had a pre-op hOA >0.4 microns.