Effect Of Higher-order Aberrations Research Articles

The effect of different angle kappa on higher-order aberrations after small incision lenticule extraction

This study aimed to compare higher-order aberrations (HOAs) after small incision lenticule extraction (SMILE) in patients with different angle kappa. This is a retrospective report in which 341 right eyes of 341 patients who were subjected to SMILE, which used coaxially sighted corneal light reflex (CSCLR) as the treatment zone centered, treated by the same experienced surgeon (LHB) for correction of myopia and myopic astigmatism, preoperative and postoperative spherical equivalent (SE), angle kappa, total higher-order aberrations (total HOA), spherical aberration (SA), vertical coma (VC), horizontal coma (HC), oblique trefoil (OT), and horizontal trefoil (HT), were compared. SMILE showed outstanding performance in terms of safety, efficacy, and predictability. In addition, a comparison of preoperative and postoperative HOAs exhibited the difference of total HOA (P < 0.01), SA (P < 0.01), VC (P < 0.01), and HC (P < 0.01), which was statistically significant; however, for OT and HT with the longer follow-up time, the statistical difference gradually decreased. For stratification of angle kappa into groups based on decantation, angle kappa was divided into three major groups: r < 0.1 mm, 0.1 ≤ r < 0.2 mm, and r ≥ 0.2 mm; the changes of SA (F = 4.127, P = 0.021) and OT (F = 3.687, P = 0.031) exhibited significant difference after 1 year of SMILE. We performed a correlation analysis of all preoperative and postoperative parameters, and the results indicated that the preoperative total HOA was negatively correlated with preoperative cylindrical diopter (DC), and postoperative total HOA, SA, and coma were affected by spherical diopter (DS) and SE. Moreover, we also found a significant difference of SA and VC in the early postoperative with preoperative. SA was positively correlated with Y values and r of 1 year after SMILE. All of the analyzed parameters in the three groups, except for the trefoil, gradually increased over time; however, the trefoil could gradually stabilize over time. We also divided angle kappa into four groups by quadrants; the result showed that the effects of higher-order aberrations were markedly different from the various quadrants. Patients with large angle kappa were able to increase VC and SA postoperatively, and higher HOAs were more significant in patients with high myopia. The differences in quadrants exhibited a diversity of HOAs; this could be attributed to the corneal surface reestablishment and the alteration of angle kappa, but the trend was not apparent. Although all patients displayed increased HOAs after SMILE, the potential application of CSCLR as the treatment zone centered still showed excellent safety, efficacy, and predictability.

Effect of Higher Order Aberrations and Intraocular Scatter on Optical Quality Based on an Optical Eye Model

The impact of intraocular scatter and higher order aberrations (HOAs) on ocular optical quality was investigated. An optical eye model was constructed using the measured ocular aberrations, corneal surfaces, axial length, and scatter fraction, and the impact of HOAs and scatter on modulation transfer functions (MTFs) was studied based on the newly established optical eye model. For uniform intraocular scatter, the monochromatic and polychromatic MTF decreased as the HOAs or scatter fractions increased independently at each spatial frequency, which implied that both were essential for visual quality. In addition, the scatter effect on MTF was more significant for the eye with less HOA. The combined deterioration effect of these two factors on the MTF was less than their summation, suggesting a potential compensatory mechanism between HOAs and scatter.

Impact of higher-order aberrations on depth-of-field.

It is well known that depth-of-focus (DOF) is influenced by optical factors (such as pupil size and monochromatic aberrations). However, neural factors such as blur sensitivity and defocus adaptation may play an important role on the extent of DOF. A series of experiments were conducted to study if optical or neural factors are most pertinent in explaining the variability of DOF across subjects. An adaptive optics system with a black and white target, a 3.8-mm artificial pupil, and a subjective criterion (based on objectionable blur) were used to measure depth of field ([DOFi]; DOF computed in the object space) in 11 participants, after at least 6 min of adaptation. This was done under three conditions: (a) with their own higher order aberrations (HOA); (b) after correction of their monochromatic HOA; and (c) after altering the HOA pattern for some participants to reflect the HOA pattern measured for a different participant. Natural DOFi and DOFi after HOA correction were positively correlated (R2 = 0.461), but a significant decrease in DOFi (21% on average) was found after HOA correction (p = 0.042). Effect of HOA on the intersubject variability of DOFi was 3.9 times smaller than the effect of the image neural processing. This study shows that DOFi depends on both optical and neural factors, but the latter seems to play a more important role than the former.

Effects of higher-order aberrations on contrast sensitivity in normal eyes of a large myopic population.

To study the relation between higher-order aberrations (HOAs) and contrast sensitivity (CS) in normal eyes among a population of laser in situ keratomileusis (LASIK) candidates. In 6629 eyes of 3315 LASIK candidates, CS were measured under dark environment at the spatial frequencies of 1.5, 3, 6, 12 and 18 cycles per degree (c/d), respectively, using an Optec 6500 visual function tester. Meanwhile, ocular HOAs were measured for a 6.0 mm pupil with a Hartmann-Shack wavefront analyzer. In the study, the subjects with an average spherical equivalent of -4.86±2.07 D were included. HOAs decreased from the third to the sixth order aberrations with predominant aberrations of third-order coma, trefoil and fourth-order spherical aberration. At low and moderate spatial frequencies, CS was negatively correlated with the third-order coma and trefoil aberrations, and decreased with increasing Z31, but increased with increasing Z3-3 and Z5-1. At high spatial frequencies, CS decreased with increasing Z3-3 and increased with increasing Z5-1. At a large pupil size of 6.0 mm, the third-order aberrations, but not the total aberrations, are the main factors affecting CS. Vertical coma is negatively correlated with CS.

Effect of higher-order aberrations and intraocular scatter on contrast sensitivity measured with a single instrument

Higher-order aberrations (HOAs) and intraocular scatter lead to the degradation of image quality on the retina, and consequently deteriorate subjective visual performance. In this article, we modified an adaptive optics double-pass system to combine objective and subjective visual testing capabilities. Employing the modified DP system, we investigated the effects of HOAs and intraocular scatter on contrast sensitivity. Contrast sensitivity measurements were performed with HOAs either retained or corrected by adaptive optics, and with scatter either remaining at the natural eye-induced level or further enhanced by a set of three different scatter filters. Contrast sensitivity was found to be worse when HOAs were uncorrected or scatter increased. Quantitative analysis indicated that the joint effect of HOAs and scatter on contrast sensitivity was not a simple summation of each contributing factor, suggesting a potential compensatory mechanism between HOAs and intraocular scatter on contrast sensitivity.

The importance of grating orientation in contrast sensitivity following refractive surgery

Higher-order aberrations degrade visual performance when the pupil is large, particularly in eyes that have abnormally high amounts of aberrations, such as those that have undergone refractive surgery. Geometrical optics predicts that asymmetrical aberrations such as coma can produce orientation selective effects, much like uncorrected astigmatism. Coma is also one of the main aberrations to increase following refractive surgery. Orientation may therefore be an important parameter when testing grating based contrast sensitivity, particularly in refractive surgery patients. Contrast sensitivity to four orientations of a 12 cycles deg(-1) sine wave grating was measured in normals (n = 34) and refractive surgery patients (n = 12). In over a third of normal subjects the higher-order aberrations produced a significant orientation-specific change in contrast sensitivity (13 out of 34 eyes, p < 0.05). No significant differences existed in the aberrations between those that displayed orientation-selective changes and those that did not. In subjects who underwent refractive surgery those that displayed orientation selective changes had significantly higher amounts of overall aberrations and also higher amounts of primary coma than those that did not. These results indicate that grating orientation is an important factor when assessing the effects of higher-order aberrations on contrast sensitivity.

Effect of higher order aberrations on contrast sensitivity function in myopic eyes

To evaluate the relation between higher order aberrations (HOAs) and contrast sensitivity function (CSF) in a population with myopia. In this case series, HOAs were measured in 70 myopic eyes over a 6-mm pupil. Contrast sensitivity was also measured. From the contrast sensitivity data, the area under the log contrast sensitivity function (AULCSF) was calculated. Pupil diameter was measured under photopic conditions. Seventy right eyes of 70 myopic subjects with a mean age of 26.6 +/- 5.7 years were studied. The mean spherical equivalent and refractive astigmatism were -4.97 +/- 1.6 D and 0.93 +/- 0.5 D, respectively. AULCSF was significantly but negatively correlated with the cycloplegic spherical equivalent (r (2) = 0.57, P = 0.02), the root mean square (RMS) of total HOAs (r (2) = 0.065, P = 0.03), the RMS of fourth-order aberrations (r (2) = 0.089, P = 0.015), and the RMS of the spherical aberration (r (2) = 0.037, P = 0.004). AULCSF did not have any significant association with age, photopic pupil diameter, refractive astigmatism, or the RMS of the coma aberration. Spherical and fourth-order aberrations significantly affect CSF in myopic eyes. However, the effect of myopia on CSF cannot be attributed only to HOAs. Other factors such as neural elements in the visual pathway should be taken into account.

Optimal spherical focus in the peripheral retina

It is well known that, in most eyes, astigmatism increases with the field angle. A simple reduced-eye model is used to demonstrate that, for point imagery in the peripheral retina, the combination of oblique astigmatism with elliptical entrance and exit pupils means that the retinal image with the optimal rotational symmetry is not necessarily at a focus corresponding to a 'best-sphere' correction. Equations are derived for the position of focus at which a circularly symmetric blur patch is obtained and for the dimensions of the patch in this and other image planes. Ray tracing through a wide-angle schematic eye is used to explore the validity of the simple model. It is shown that although the latter gives good predictions of retinal imagery for very small entrance pupils, it becomes less valid for larger, more realistic pupil diameters, due to the increasing importance of the effects of higher-order aberrations. Nevertheless, the simple model can still yield useful insights into through-focus, peripheral optical imagery.

Effects of higher-order aberrations on contrast acuity as a function of light level

The effects of higher-order ocular aberrations on functional visual performance were assessed for seven young healthy subjects, as a function of light level. Contrast acuity thresholds were measured for a functional vision test with and without correction of higher-order aberrations and at a number of different pupil sizes in three of the subjects investigated. A visual simulator incorporating an adaptive optics (AO) system was used for this purpose. The results show that when light level is decreased, the drop in neural contrast sensitivity limits the impact that increased optical degradations have on vision. The expected AO benefit for functional vision is given for scotopic, mesopic and photopic regimes.

Aberration Analyses Needed for FDA Evaluation of Safety and Effectiveness of Wavefront-guided Refractive Surgical Devices

Manufacturers of refractive surgical lasers are seeking Food and Drug Administration (FDA) approval for refractive surgery based on wavefront measurements rather than manifest refractions. This article draws attention to the need for better understanding of the relationship between aberrations and visual function and better testing methods for evaluating safety and effectiveness of wavefront-guided refractive surgery. Before U.S. market approval for a new medical device, the FDA must determine whether the manufacturer has provided reasonable assurance that the device is safe and effective for its intended use, based on valid scientific evidence. The issues discussed here are based on FDA reviewers' experiences in evaluating Investigational Device Exemption (IDE) and Premarket Approval (PMA) applications for wavefront-guided refractive lasers. Even an optimal wavefront correction can provide only a marginally greater visual function benefit than a conventional spherocylindrical correction in a normal eye. Actual benefits are even smaller or nonexistent because variability from uncontrollable sources (eg, healing, aging, accommodation, pupil size, or ocular biomechanical effects) are typically larger than the errors treated. The effects of higher order aberrations and aberration changes on visual function are complex and poorly understood. The FDA's need for better tools to assess safety and effectiveness of wavefront-guided refractive surgery has stimulated major research efforts within the vision, ophthalmic, and optometric research communities. More research is needed to better understand the relationship between aberrations and visual function, and to be able to predict the visual effects of specific aberration changes in individual eyes. This research would have been more appropriately done before rather than after clinical trials for wavefront-guided refractive lasers.

Holographic optical elements (HOE) for semiconductor lasers

A two step method is described to produce off-axis holographic lenses with high diffraction efficiency and without astigmatism for semiconductor lasers. The hologram is recorded in the visible (514 nm) and reconstructed in the infrared (800 nm). The principal parameters (recording and reconstruction angles, astigmatic focal lengths) for each hologram and wavelength are calculated analytically using second order approximation. Numerical methods, based on classical ray-tracing applied to holographic diffraction, have been developed to calculate spot diagrams and to investigate the effects of higher order aberrations.