"When I began doing wavefront research in the late 1980s, I realised that refractive surgery was increasing the aberrations of the eye and was causing a loss of best-corrected acuity... We used to not pay much attention to higher order aberrations because we couldn’t fix them." - Raymond Applegate, OD, PhD. Source: EuroTimes June 2003
The term "visual aberrations" describes all imperfections of the eye that cause light rays to be improperly focused.
Lower order aberrations are simple defocus such as as myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (irregularly shaped cornea), which are correctable with glasses. Higher order aberrations (HOA) are more complex imperfections of the eye's optical system. Higher order aberrations are not correctable with glasses. Irregular corneal astigmatism creates higher order aberrations.
Higher order aberrations are present in all human eyes, but in normal, unoperated eyes HOAs are so minor that they aren't noticeable. The cutting of a LASIK flap and ablation of corneal tissue creates an unnatural, irregular shape of the cornea which increased the higher order aberrations of the eye. The severity of induced HOAs depends on the profile of the flap, ablation profile, and the size of the patient's pupils. You can read more about the role of pupil size at: Link
Higher order aberrations are measured with wavefront aberrometers and expressed in terms that describe the shape and severity of the deviated light rays as they pass through the eye's optical system and strike the retina. Coma, spherical aberration, and trefoil are the most common aberrations induced by LASIK. Coma causes light to be smeared like the tail of a comet in the night sky. Double vision is a common symptoms of coma. Spherical aberrations results when the pupil dilates larger than the area of the cornea that was fully corrected by the laser treatment. Spherical aberrations are characterized by halos, starbursts, ghost images, and loss of contrast sensitivity (inability to see fine detail) in low light. Trefoil causes a point of light to smear in three directions, like a Mercedes-Benz symbol. It's quite common for a patient to have an increase in all of these aberrations, resulting in distorted night vision when the pupil opens and allows light to enter through a larger area of the irregular corneal surface.
Custom LASIK, also known as wavefront-guided LASIK, was introduced by the LASIK industry in recent years. Unlike conventional LASIK which is based solely on refraction (measurement of myopia, hyperopia and astigmatism), 'custom' or 'wavefront-guided' LASIK is based on lower and higher order (total) aberrations of the eye. A wavefront map is obtained prior to surgery which is used to program a custom treatment for each individual eye. Though the goal of wavefront-LASIK is to correct the 'small optical imperfections' (HOAs), the procedure actually makes them worse than before surgery. The increase in HOAs is greater with conventional LASIK. Read more: Link
Medical studies about higher order aberrations after LASIK
Higher order aberrations after LASIK for myopia with alcon and wavelight lasers: a prospective randomized trial.
J Refract Surg. 2005 Nov-Dec;21(6):S799-803.
Brint SF.
Brint Vision, Metairie, LA, USA. Brintmd@aol.com
PURPOSE: To evaluate the differences in higher order aberration outcomes between bilateral wavefront-guided and wavefront-optimized LASIK treatments on two closely matched patient groups 1 and 3 months following surgery.
METHODS: Thirty patients were enrolled in the study and randomly assigned to undergo bilateral LASIK with either the Alcon CustomCornea laser system or the WaveLight Allegretto Wave laser system. Standard clinical outcomes such as visual acuity and manifest refraction were evaluated as well as quality of vision measures such as subjective questionnaires and higher order aberrations. All patients completed follow-up examinations for 3 months following surgery.
RESULTS: At 1 and 3 months after surgery, the uncorrected visual acuity and the refractive outcome were similar for the two laser platforms, with 90% to 93% eyes being within +/- 0.5 D of intended outcome. At 1 and 3 months the Alcon CustomCornea group had a statistically significant lower amount of induced total higher order aberrations compared to the Allegretto Wave group (P < .05).
CONCLUSIONS: LASIK for myopia with the Alcon CustomCornea and WaveLight Allegretto Wave systems is highly effective in terms of refractive outcome and uncorrected visual acuity. However, aberrations were statistically significantly greater after treatment with the WaveLight Allegretto Wave system than with the Alcon CustomCornea.
Wavefront-guided LASIK with the Zyoptix 3.1 system for the correction of myopia and compound myopic astigmatism with 1-year follow-up: clinical outcome and change in higher order aberrations.
Ophthalmology. 2004 Dec;111(12):2175-85. Kohnen T, Bühren J, Kühne C, Mirshahi A.
Department of Ophthalmology, Johann Wolfgang Goethe-University, Frankfurt am Main, Germany. kohnen@em.uni-frankfurt.de
OBJECTIVE: To assess safety, efficacy, predictability, stability, and change in aberrations after wavefront-guided LASIK for myopia and myopic astigmatism.
DESIGN: Prospective, nonrandomized, self-controlled trial. PARTICIPANTS: Wavefront-guided LASIK was performed in 97 eyes in a 1-year trial. Treated eyes had a mean subjective manifest spherical equivalent (SE) of -5.22+/-2.07 diopters (D), with a range of -0.25 to -9.00 D of myopia and 0 to -3.25 D of astigmatism. I
NTERVENTION: After a microkeratome cut, a wavefront-based excimer ablation (Zyoptix 3.1) was performed. The full treatment to achieve emmetropia of an early nomogram provided by the system manufacturer was used in all procedures.
MAIN OUTCOME MEASURES: Safety, efficacy, predictability, and stability were evaluated at 1, 3, and 12 months postoperatively. Wavefront changes of higher order aberrations (HOAs) at 1 year were determined for pupil sizes of 3.5 and 6 mm. RESULTS: At 1 year postoperatively, uncorrected visual acuity (VA) was 20/20 or better in 83% of the eyes, and 20/40 or better in 98%. The mean subjective manifest SE at 1 year was -0.25+/-0.43 D; it was within 0.50 D in 77% and within 1.0 D in 95%. No eye lost > or =2 lines of best spectacle-corrected VA (BSCVA) at 1 year postoperatively; 40 eyes gained 1 line of BSCVA, and 5 eyes gained 2 lines. The total HOA root mean square (RMS) increased on average by a factor of 1.23+/-0.57 with a 3.5-mm pupil; for the 6 mm pupil, the increase factor was 1.52+/-0.36. No change or reduction in the total HOA RMS was observed in 45.5% of the eyes for a 3.5-mm pupil and in 20.6% for a 6-mm pupil. There was a significant increase of primary spherical aberration (Z 4,0) by a factor of 4.11+/-10.17 for 3.5-mm pupils and 4.31+/-6.76 for 6-mm pupils.
CONCLUSIONS: Wavefront-guided LASIK using Zyoptix 3.1 is an effective and safe procedure for the treatment of myopia and myopic astigmatism. Although in close to half of the eyes HOAs could be reduced, there was still undercorrection and induction of HOAs with the algorithm employed.
Refractive surgery, optical aberrations, and visual performance.
J Refract Surg. 1997 May-Jun;13(3):295-9. Applegate RA, Howland HC.
Department of Ophthalmology, School of Medicine, University of Texas Health Science Center at San Antonio, USA.
Visual optics is taking on new clinical significance. Given that current refractive procedures can and do induce large amounts of higher order ocular aberration that often affects the patient's daily visual function and quality of life, we can no longer relegate the considerations of ocular aberrations to academic discussions. Instead, we need to move toward minimizing (not increasing) the eye's aberrations at the same time we are correcting the eye's spherical and cylindrical refractive error. These are exciting times in refractive surgery, which need to be tempered by the fact that after all the research, clinical, and marketing dust settles, the level to which we improve the quality of the retinal image will be guided by the trade-off between cost and the improvement in the quality of life that refractive surgery offers.
Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis.
American Journal of Ophthalmology Volume 127, Issue 1 , January 1999, Pages 1-7
Oshika T, Klyce SD, Applegate RA, Howland HC, El Danasoury MA. Department of Ophthalmology, University of Tokyo School of Medicine, Japan. oshika-tky@umin.ac.jp
PURPOSE: To compare changes in the corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis.
METHODS: In a prospective randomized study, 22 patients with bilateral myopia received photorefractive keratectomy on one eye and laser in situ keratomileusis on the other eye. The procedure assigned to each eye and the sequence of surgery for each patient were randomized. Corneal topography measurements were performed preoperatively, 2 and 6 weeks, 3, 6, and 12 months after surgery. The data were used to calculate the wavefront aberrations of the cornea for both small (3-mm) and large (7-mm) pupils.
RESULTS: Both photorefractive keratectomy and laser in situ keratomileusis significantly increased the total wavefront aberrations for 3- and 7-mm pupils, and values did not return to the preoperative level throughout the 12-month follow-up period. For a 3-mm pupil, there was no statistically significant difference between photorefractive keratectomy and laser in situ keratomileusis at any postoperative point. For a 7-mm pupil, the post-laser in situ keratomileusis eyes exhibited significantly larger total aberrations than the post-photorefractive keratectomy eyes, where a significant intergroup difference was observed for spherical-like aberration, but not for coma-like aberration. This discrepancy seemed to be attributable to the smaller transition zone of the laser ablation in the laser in situ keratomileusis procedure. Before surgery, simulated pupillary dilation from 3 to 7 mm caused a five- to six-fold increase in the total aberrations. After surgery, the same dilation resulted in a 25- to 32-fold increase in the photorefractive keratectomy group and a 28- to 46-fold increase in the laser in situ keratomileusis group. For a 3-mm pupil, the proportion of coma-like aberration increased after both photorefractive keratectomy and laser in situ keratomileusis. For a 7-mm pupil, coma-like aberration was dominant before surgery, but spherical-like aberration became dominant postoperatively.
CONCLUSIONS: Both photorefractive keratectomy and laser in situ keratomileusis increase the wavefront aberrations of the cornea and change the relative contribution of coma- and spherical-like aberrations. For a large pupil, laser in situ keratomileusis induces more spherical aberrations than photorefractive keratectomy. This finding could be attributable to the smaller transition zone of the laser ablation in the laser in situ keratomileusis procedure.
Disclaimer: The information contained on this web site is presented for the purpose of warning people about LASIK complications prior to surgery. LASIK patients experiencing problems should seek the advice of a physician.