Human vision is a binocular process. Having two eyes gives binocular summation, with which the ability to detect faint objects is enhanced.1 It can make stereopsis possible, in which the parallax provided by the two eyes’ different positions on the head gives precise depth perception.2 Such binocular vision is usually accompanied by binocular fusion, in which a single image is seen despite each eye's having its own image of any object.2

Literature suggests that marked anisometropia is uncommon, in terms of either the magnitude of sphere or the amount of astigmatism,3 with few notable exceptions4 concluding that the axis of astigmatism does not follow any particular rule (mirror or direct symmetry) between right and left eyes.

Howland and Howland, employing the cross-cylinder aberroscope method they invented,5 found that the optical aberrations of the eye differ greatly from subject to subject and are seldom symmetrical. Liang and Williams, using a Shack-Hartmann wave-front sensor,6 found that although the pattern of aberrations varies from subject to subject, the aberrations (including irregular ones) of the left eye and those of the right eye of the same subject are correlated, indicating that they are not just random defects. Porter et al.7 confirmed this observation in a large population.

The Indiana Aberration Study by Thibos et al.8 characterized the aberration structure and the effects of these aberrations on vision, for a reasonably large population of normal, healthy eyes in young adults, and verified the hypothesis of bilateral symmetry.

Marcos and Burns9 found that not only aberrations but also cone directionality varies across subjects, and that these two functions show a left-right eye symmetry.

Wang et al.10 found that event though the anterior corneal surface's wave aberration varied greatly across subjects, a moderate-to-high degree of mirror symmetry existed between right and left eyes.

To our knowledge, very few studies in the literature have addressed the issue of symmetry of aberrations between eyes after corneal laser refractive surgery.11 Jiménez et al.11 found that binocular function deteriorates more than monocular function after LASIK, and that this deterioration increases as the interocular differences in terms of aberrations and corneal shape increase. They found that interocular differences above 0.4μm of the Root-Mean-Square (RMS) for a 5-mm analysis diameter, lead to a decrease of more than 20% in binocular summation.

If binocular symmetry is manifested on virgin human eyes and it is an important factor for binocular vision, it shall be interesting to assess whether or not the existing symmetry is maintained after treating the cornea for ametropia correction using corneal laser refractive surgery.

Taking into account that corneal ablation in standard refractive-surgery treatments induces aberrations (one of the most significant side-effects in myopic LASIK is the induction of spherical aberration,12 which causes halos and reduced contrast sensitivity13), special ablation patterns were designed to preserve the preoperative level of high-order aberrations (HOAs).14-16

In the current study we present measurements of the corneal wavefront aberration in 50 eyes (right and left eyes of 25 subjects), both preoperatively and 6 months after non-customised treatment. We analyzed the correlation of individual aberrations across the population, as well as the correlation of aberrations between the right and left eyes of the same subjects. In this study, we used non-customised “Aberrationneutral” profiles, i.e., ablations were optimized to induce no change in wavefront aberration within the Optical Zone (OZ) other than sphere and cylinder components, leaving all existing HOAs unchanged because the best-corrected visual acuity (BCVA), in this patient, has been unaffected by the pre-existing aberrations (Artal P, What aberration pattern (if any) produces the best vision?, 6th International Wavefront Congress, Athens, Greece; February 2005). Thus, to compensate for the aberrations induction observed with other types of profile definitions,17 several sources of aberration might be considered. For example, some of those sources of aberration are related to the loss of efficiency of the laser ablation for non-normal incidence.18-20

50 eyes (25 patients) that had been treated with the AMARIS “aberration neutral” (Aberration-Free™) aspheric ablation profiles were retrospectively analysed.

Inclusion criteria for review were bilateral surgery on the same day targeted for emmetropia, preoperative best spectacle corrected visual acuity (BCVA) ≥ 20/25 (logMAR ≤ 0.1) in both eyes, no signs of amblyopia, and successful completion of the 6-month follow-up.

Six-months follow-up data were available for all 50 eyes (100%), and their preoperative data were as follows: mean manifest defocus refraction: -2.47±2.51 D (range, -8.13 to +5.63 D) and mean manifest astigmatism magnitude: 2.02±0.91 D (range, 0.00 to 4.75 D). For all eyes we measured corneal topography21 and derived corneal wavefront aberrations,22,23 up to the 7th Zernike order (35 terms) (Keratron-Scout, OPTIKON2000, Rome, Italy), manifest refraction, uncorrected visual acuity24 (UCVA) and BCVA. Measurements were performed preoperatively and also 1, 3, and 6 months after surgery.

All ablations were non-wavefront-guided but were based on aspheric25 aberration-neutral (Aberration-Free™) profiles (and not on the profiles proposed by Munnerlyn26) to balance the induction of spherical aberration27,28 (prolateness optimization29,30). This approach included a multidynamic aspherical transition zone, aberration and focus shift compensation due to tissue removal, pseudo-matrix-based spot positioning, enhanced compensation for the loss of efficiency;31 all based on theoretical equations validated with ablation models and clinical evaluations.

A 6.5mm central and fully corrected ablation zone was used in all eyes, together with a variable transition size that was automatically provided by the laser depending on the planned refractive correction (6.7mm to 8.9mm). The ablation was performed using the AMARIS excimer laser (SCHWIND eye-tech-solutions, Kleinostheim, Germany), which is a flying-spot laser system that uses real ablative spot volume locally considered through a self-constructing algorithm and that controls for the local repetition rates to minimize the thermal load of the treatment.32 Therefore, the ablated surface with the aspheric aberration-neutral (Aberration-Free™) profiles should be very smooth, possibly leading also to some benefits in terms of HOAs. Finally, all these optimizations theoretically diminish the induced wavefront aberration after myopic LASIK.The AMARIS laser system works at a true repetition rate of 500Hz and produces a beam with a size of 0.54mm (Full-Width-at-Half-Maximum, FWHM) and a super-Gaussian spot profile.33,34 High-speed eye tracking (pupil and limbus tracker with cyclotorsional tracking) with a 1050Hz acquisition rate is accomplished with a 3-ms latency time.35

All flaps were created using a LDV femtosecond laser (Ziemer Group) with a 100μm nominal flap thickness.

Optical errors centred on the line-of-sight,36 representing the wavefront aberration, are described by means of Zernike polynomials37 and their corresponding coefficients using OSA standard,38 and analysed for a standardised diameter of 6mm in order to derive corneal wavefront aberrations.

Dioptrical Differences in Corneal Wavefront Aberration Between the Right and Left Eyes of the Same Subjects

For our analysis, the concept of equivalent defocus (DEQ) has been used as metric to be able to associate a dioptric power with the RMS of the Zernike coefficients.

DEQ is defined as the amount of defocus required to produce the same wavefront variance produced by one or more HOAs. A simple formula allows us to compute the DEQ in diopters if we know the total RMS wavefront error of the Zernike modes in question:

where Me is the DEQ in diopters, RMS is the RMS wavefront error of the Zernike modes in question, and PD is the pupil diameter considered for the wavefront-aberration analysis.

On virgin eyes, DEQ as proposed by Thibos et al.8 seems to be relatively insensitive to a change of analysis diameter.

The aim of this study was to evaluate the effects of laser corneal refractive surgery on the bilateral symmetry of the corneal wavefront aberration; in particular, following a treatment performed with the AMARIS system, which is based on an Aberration-Free™ ablation profile. The advantage of the Aberration-Free™ ablation profile is that it aims to be neutral for HOA, leaving the visual print of the patient as it was preoperatively with the best spectacle correction. If the aimed Aberration-Free concept would have been rigorously achieved, the bilateral symmetry between eyes would have been automatically obtained. In our group of patients, the aimed Aberration-Free concept does not hold rigorously true, but we had a very minor increase in corneal aberrations for a 6mm pupil.

Our follow-up included 100% of the cases at 6 months and, although no nomogram adjustments were applied, in none of the cases retreatment was necessary. As a proof of stability, longer follow-ups and larger number of eyes would be more convincing, even though refractive spherical and astigmatic results are stable after 3 months.

Residual defocus averaged about -0.1 D, and residual cylinder about 0.4 D, with 84% of the eyes being within 0.50 D and 98% within 1.0 D of the target correction.

In our study, at the 6-month follow-up the percentage of eyes with an UCVA of 20/32 or better was 100% and 84% had an UCVA of 20/20 or better. No single eye had a loss of more than 1 line of BCVA, and 5 eyes had gained 2 or more lines of BCVA (P<.01).

As can be seen from the data presented herein, noncustomised femtosecond LASIK performed with the combination LDV and AMARIS platforms is safe, effective, and it preserves reasonably well the bilateral symmetry of the corneal wavefront aberration between eyes. This may be related to the advantages of profiles aiming to be neutral for HOAs or to the fact that the high-speed AMARIS system reduces variability from stromal hydration effects, which increase with the duration of treatment.39,40 Recognizing the high levels of defocus and astigmatism in this study, analysis of pre- and postoperative binocular vision41 would be of interest and is a partial limitation of this study.42 Further analysis of bilateral symmetry as a function of the analysis diameter is also of interest. Long-term follow-up on these eyes will help determine the stability of these accurate results. Comparing similar outcomes from other lasers to see if any of the parameters we measured are really different for other lasers or microkeratomes and analyses to determine if these parameters are clinically relevant will help to determine the impact of this work.

Cuesta et al.43 found that even differences in corneal asphericity may affect the binocular visual function by diminishing the binocular contrast-sensitivity function. Jiménez et al.11 found that following LASIK, binocular function deteriorates more than monocular function, and that this deterioration increases as the interocular differences in terms of aberrations and corneal shape increase. They also found that interocular differences above 0.4μm of RMS for a 5 mm analysis diameter (0.4 D) lead to a drop in binocular summation of more than 20%.

In our study, only 4 out of 25 patients showed preoperative clinically relevant OS-vs.-OD differences (i.e., larger than 0.25 D), whereas 6 months postoperatively only 2 out of 25 patients showed clinically relevant OS-vs.-OD differences (i.e., larger than 0.25 D).

RMS(ΔHOAb) analysis for interocular differences accounts for the RMS of the differential corneal wavefront aberration and not for the difference of the corneal RMS(HOAb). RMS(ΔHOAb) is a rigorous analysis metric, because it accounts for any deviation (i.e. both inductions and reductions of the wavefront aberration, since both contribute positively to increase the RMS value). Furthermore, it can be mathematically demonstrated that:

Six months postoperatively 3 Zernike terms lost significant OS-vs.-OD correlation symmetry and 4 Zernike terms gained significant correlation symmetry. However, two of them showed borderline correlations. 6 months postoperatively, for 6 Zernike terms the differences in OS-vs.-OD symmetry increased significantly whereas for 4 Zernike terms the differences in symmetry decreased significantly. However, 6 of them showed borderline significances of the difference. Also 6 months postoperatively, 3 patients lost significant OS-vs.-OD correlation symmetry, whereas 1 patient gained significant correlation symmetry. However, two of them showed borderline significances of the difference. All these borderline situations, actually shall be seen as “almost preserved” bilateral symmetry.

Despite large defocus and astigmatism magnitudes, our study shows that HOAs are either minimally increased or unchanged after surgery with the LDV and AMARIS systems (Tables 1 and 2). The main post-op HOAs effects (coma and spherical aberration) are caused by decentration and “edge” effects: i.e., to the strong local curvature change between the OZ and the Transition Zone (TZ) and from the TZ to the non-treated cornea. As a result, it is necessary to emphasize the use of huge OZs, covering the scotopic pupil size plus tolerance for possible decentrations, as well as welldefined TZ. In our study this was approached by the use of a 6.5-mm-diameter fully-corrected ablation zone with a multidynamic aspherical transition zone automatically calculated by the laser depending on the planned refractive correction (6.7mm to 8.9mm diameter).

Although this small series of treated eyes does not allow for definitive conclusions or evidence-based statements, our preliminary results are promising.

Limitations of our study include the moderate number of eyes, limited follow-up and the lack of a control group. The method to determine whether or not symmetry is maintained consist of comparing individual terms in a variety of ad hoc ways both before and after refractive surgery, ignoring the fact that retinal image quality for any given individual is based on the sum of all terms. However, similar methodologies have been already used before.9 At this stage, we did not perform any specific visual tests on binocular vision; for example, stereotests. Some patients may not have good stereopsis but they may show good aberration symmetry. The analysis of bilateral symmetry should be related to the patients’ binocular vision status. Despite these limitations, we were able to demonstrate that “aberration neutral” ablation profiles reasonably preserve the bilateral symmetry between eyes in terms of corneal wavefront aberration. The presented results cannot be extrapolated to patients with symptoms of amblyopia,44 anisometropia, nystagmus, or aniseikonia45 without further studies.

This does not mean any “good or bad” point for binocular vision. Taking into account that we cannot precisely evaluate the role of aberrations monocularly (patients with a high level of aberrations can have an excellent visual acuity and vice versa), it is even more difficult to do it binocularly. The important question in binocular vision is “the role of interocular-differences”, and if they can influence significantly binocular performance. Interocular-differences can be minor but significant for visual performance. Further studies shall help to determine the impact of this on binocular visual performance.

In summary, this study demonstrated that “aberration neutral” profile definitions (as implemented at the SCHWIND AMARIS system), which are not standard in refractive surgery, yield very good visual, optical and refractive results, and reasonably preserve the bilateral symmetry between eyes (which influence binocular summation44) of the corneal wavefront aberration with no clinically relevant induction of HOAs (which influence contrast sensitivity46). In this sense, “Aberration neutral” ablation profiles, as demonstrated here, have therefore the potential to replace current standard algorithms for the non-customised correction of refractive ametropias in laser corneal refractive surgery.