A great variety of systems and devices have been developed as clinical tools for the assessment of visual performance, such as the ocular wavefront analyzers or aberrometers.1, 2 These devices allow the clinician to characterize one of the limiting factors of optical quality of the retinal image, wavefront aberrations.1, 2 Different optical principles have been developed for ocular aberrometry, such as the Hartmann-Shack method,3 the Tscherning principle,4 or ray tracing.5 Specifically, ocular aberrometers based on ray tracing have the advantage of avoiding the overlapping of image points that would hinder the wavefront reconstruction because laser beams are projected sequentially onto the eye.5 This facilitates the measurement of wavefront aberrations in highly aberrated eyes. To date, Tracey Technologies is the only manufacturer of sensors using this principle for clinical purposes. Several versions of its aberrometer have been marketed and all of them have been shown to provide accurate and highly reproducible measurements of spherocylindrical error and of some aberrometric coefficients determined for distance viewing conditions.6–8
Most ocular wavefront aberrometers are aimed at characterizing the wavefront aberrations simulating far distance conditions. This is especially useful for evaluating excimer laser refractive surgery outcomes9 or for planning customized procedures.10 However, the analysis of wavefront aberrations under near viewing conditions has gained importance since the new developments in cataract surgery. New optimized models of intraocular lenses (IOL) aimed at restoring visual function at distance, intermediate and near, such as the multifocal11 or accommodative IOLs,12 have been introduced in clinical practice. The evaluation of the visual function with this type of IOL requires the analysis of the wavefront aberrations not only in distance vision but also in near conditions. There are some currently available devices with the capability of providing such analysis as the Dynamic Stimulation Aberrometry of the COAS wavefront sensor or the latest version of the aberrometer developed by Tracey Technologies and previously mentioned. This ray tracing aberrometer allows the evaluation of the wavefront aberration at different distances13 and the measurement of the objective accommodative response.14, 15
The aim of the current study was to evaluate the repeatability of ocular aberrometry at distance and near vision conditions provided by the latest version of this ray tracing aberrometer to assess the consistency of the aberrometric measurements provided by this instrument and therefore its potential usefulness for clinical purposes.
PATIENTS AND METHODS
A total of 22 eyes (18 right eyes and 4 left eyes) of 22 patients with ages ranging from 21 to 65 years were included in this study. Patients were recruited by the personnel of our institute (Vissum Corporation, Alicante, Spain), where this investigation was conducted. Only one eye from each subject was chosen for the study, randomly, according to a random number sequence (dichotomic sequence, 0 and 1). All eyes achieved a best spectacle corrected visual acuity of 20/20 or better. Eyes with active ocular pathologies or previous ocular surgeries were excluded from the study. All patients were informed about the study and signed an informed consent document in accordance with the Helsinki Declaration.
A comprehensive ophthalmologic examination was performed in all cases, which included the analysis of the wavefront aberration at distance (5 m) and at near (40 cm) by means of the iTrace ray tracing aberrometer (Tracey Technologies Corp., Houston, TX). Three repeated consecutive measurements were taken by the same experienced examiner (P.J.S.P.) to assess the repeatability of the aberrometric measurements. No pupil dilation was used for measurements to avoid the interference of the cycloplegic agent.16 By means of the pupillometry mode included in this instrument, it was ensured that at least a pupil diameter of 3 mm was available in all cases. No refractive correction was in place when the aberrometric measurements were taken.
The following aberrometric parameters at distance and near vision conditions were recorded and analyzed in each examination with the iTrace system for a 3-mm pupil: sphere (SPH), cylinder (CYL), primary spherical aberration with its sign (Z40), horizontal coma (Z31), and vertical coma (Z3−1). In addition, the repeatability of the modulation transfer function (MTF) estimated for six spatial frequencies (5, 10, 15, 20, 25, and 30 cycles/deg) from the wavefront aberrations and the Strehl ratio were evaluated. This ratio was obtained from the simulated point spread function by performing the quotient between the maximum intensities of light energy of retinal image in the analyzed eye and in a diffraction-limited eye. This parameter should be as close as possible to the value of 1 (perfect optical system) and is related to the ability of the eye to produce a point-image on the retina when a point-object is observed. For MTF and point spread function calculations with the iTrace system, the complete aberrometric profile was considered, including low and higher order aberrations. As near aberrometric measurements were performed without correction to avoid the interference of the aberration of the ophthalmic lenses, the level of accommodative response was expected to be different in each eye depending on the refractive error despite presenting a test at 40 cm. For example, a myopic patient of −1 D will present a lower accommodative response (about 1.5 D) when viewing a test at 40 cm (a theoretical accommodative response required of 2.5 D). This fact should be considered because it was an important source of variability for the evaluation of the near optical performance with this instrument. Furthermore, the MTF analysis was based on a single orientation, the horizontal meridian (0 to 180°).
The iTrace uses the principle of ray tracing5 for obtaining the wavefront aberrations of the eye. It uses 250 parallel thin beams in separate and concentric arrays that are projected sequentially onto the eye through the entrance pupil within ⅛th of a second (Fig. 1). This rapid sequence of beams avoids any data confusion enabling measurement of highly aberrated eyes on a point-by-point basis and providing robust refractive information over a range of ±15 D. The location of each spot on the retina where each entering light beam reflects is imaged by the detectors of the iTrace. This information is used to reconstruct the wavefront of the eye by using the standardized Zernike polynomials. The instrument allows the change of the fixation target and then the measurement of the wavefront aberration in different viewing conditions and the objective determination of the accommodative response. Besides ocular aberrometry and accommodation measurements, this equipment has a Placido-based topographic system incorporated providing corneal topographic maps and corneal aberration data.
In the current study, all measurements were performed following a standardized procedure to minimize as much as possible the external sources of variability. As the iTrace aberrometer is a monocular system, the contralateral eye was covered with an eye patch in all cases during the measurement procedure. First, the patient's head was well positioned on the chin and forehead rests and the patient was asked to fixate on a far or near target presented through the 2.5 cm diameter wide, 20 cm long open-field instrument housing. It should be noted that the iTrace monocular open field of view permits the far and near targets to be viewed monocularly through a beam splitter. An image of the eye appeared then on the computer screen, allowing the examiner to change and optimize focus and centration. During the measurement procedure, the patient was asked to fixate on a specific optotype of a visual acuity chart presented at two distances, far (5 m) and near (40 cm). Specifically, all patients had to look at a specific letter from the 20/40 visual acuity line. In all patients, the repeated measurements for far distance conditions were performed first. The chart for far testing was a conventional Snellen chart located at 5 m from the instrument, whereas the chart for near testing was the high-contrast miniaturized Early Treatment Diabetic Retinopathy Study letter chart printed in black on white paper. A near-point rod holder on top of the instrument allowed a standard clinic near-point rod dioptrically calibrated to be attached for viewing near targets. All far and near measurements were performed immediately after a blink. If centration was lost after the blink, the measurement was not accepted if a period of time of more than 2 s was needed for recentration. Furthermore, the software of the iTrace system was configured for including all the parallel thin beams used for measurement within the central 3-mm pupil area (Fig. 1).
The statistical analysis was performed using the software SPSS version 15.0 for Windows (SPSS, Chicago, IL). Normality of all aberrometric, accommodation, and MTF data distributions was confirmed by means of the Kolmogorov-Smirnov test. Then, parametric statistics was always applied. The paired Student's t-test was used for analyzing the comparison of near and distance aberrometric and MTF measurements. All tests were two-tailed, and p values <0.05 were considered statistically significant. Correlation coefficients (Pearson or Spearman depending if normality condition could be assumed) were used to assess the correlation between different variables.
Repeatability for each analyzed parameter was assessed by means of the within-subject standard deviation (Sw) and the intraobserver precision. The within-subject standard deviation (Sw) is a simple way of estimating the size of the measurement error. The intraobserver precision was defined as (±1.96 × Sw),17 and this parameter indicates the range of error of the repeated measurements for 95% of observations. Spherocylindrical refractive errors were also expressed and analyzed as power vectors. It should be considered that power vectors are more helpful for detecting complex changes in refraction, because with this, methodology trajectories are traced in a uniform dioptric space.18 Therefore, the vector components (J0, J45) and the spherical equivalent (M) were calculated for each spherocylindrical measurement using the standard procedure defined for such purpose.18
Refractive and Aberrometric Analysis
Table 1 summarizes the repeatability outcomes for the refractive and aberrometric parameters in distance and near vision conditions that have been analyzed in the current study. As shown in Table 1, low values of Sw were found for the magnitude of sphere and cylinder at distance and near and for the power vector components of refraction. However, a more pronounced variability was observed in the repeated measurements of the coefficients associated to higher order aberrations. The values of Sw for primary spherical aberration, horizontal coma, and vertical coma were significant and clinically relevant. Specifically, extremely poor repeatability was found for the vertical coma at near, with a Sw of 0.011 μm for a mean value of 0.018 μm.
Regarding difference among distance and near conditions, a significantly more negative sphere was found at near (p < 0.01). This is a result of the eye accommodating in response to the near fixation target. However, no significant changes in cylinder (p = 0.48), primary spherical aberration (p = 0.88), horizontal coma (p = 0.15), and vertical coma (p = 0.49) were found among distance and near viewing conditions. Further, variability of measurement (i.e., Sw) for each of these variables was not significantly different at distance and near. When the power vector components of refraction were analyzed, a statistically significant difference among distance and near values was found for M (p = 0.04). No significant differences between distance and near conditions were detected in J0 (p = 0.59) and J45 (p = 0.94) components. In addition, Sw for sphere was found to be significantly larger when measured in distance vision conditions (p = 0.03) and Sw for vertical coma was significantly larger at near (p = 0.01). For cylinder (p = 0.42), primary spherical aberration (p = 0.91) and horizontal coma (p = 0.15), no significant differences among near and distance were found. No significant differences between the distance and near values of Sw for the power vector components of refraction were found (p ≥ 0.08).
Optical Quality Analysis
Fig. 2 shows the mean MTF curves obtained for distance and near for the sample of eyes evaluated. Table 2 summarizes the repeatability outcomes for the MTF parameters that have been analyzed in distance and near vision conditions in the current study. As shown in Table 2, all Sw values were low. The modulation transfer for the six spatial frequencies evaluated was significantly lower in near conditions when compared with distance (all p < 0.01). In addition, the Sw was significantly lower in near conditions when compared with distance for 10 (p = 0.01), 15 (p = 0.02), 20 (p = 0.02), 25 (p = 0.01), and 30 (p = 0.02) cycles/deg.
Regarding the Strehl ratio, it was 0.1108 (SD, 0.1145; range, 0.0003 to 0.3847) at distance and 0.0436 (SD, 0.0987; range, 0.0003 to 0.4554) at near. The difference in this parameter among conditions was statistically significant (p < 0.01). The Sw for the Strehl ratio was 0.0349 (SD, 0.0540; range, 0.0001 to 0.2115) at distance and 0.0104 (SD, 0.0184; range, 0.0001 to 0.0830) at near. This difference in Sw was also statistically significant (p = 0.03). Furthermore, no statistically significant differences were found in the MTF and Strehl ratio between eyes with a positive (10 eyes) and negative (12 eyes) M value (p ≥ 0.10).
The evaluation of the visual quality in near vision has gained importance because the introduction of new surgical options aimed at restoring the near visual function in presbyopic and pseudophakic eyes. Several studies attempting to measure and characterize the near visual quality by means of aberrometry in eyes implanted with different IOL models have been conducted.14, 19, 20 In addition, the measurement of the level of pseudoaccommodation achieved with different models of accommodating IOLs has become a challenge to assess their real clinical effect.20–22 As previously commented, one device providing simultaneously a measurement of the near wavefront aberrometry and the determination of the objective accommodative response is the wavefront sensor iTrace from Tracey Technologies. Although previous studies have evaluated the reliability of far wavefront aberration measurements and especially of the spherocylindrical error with previous versions of the iTrace system, there are no studies reporting information about the consistency of measurements of the new version of this instrument and providing a general analysis of the consistency of all aberrometric measurements (near and distance). The aim of the current study was to evaluate the repeatability of ocular aberrometry at distance and near vision conditions provided by the latest version of this ray tracing aberrometer to confirm the consistency of these clinical data.
Repeatable measurements of the subjective refraction were obtained for the 3-mm pupil, which is consistent with previous studies also assessing the consistency of the spherocylindrical measures obtained with different versions of the iTrace system.6–8 In addition, we evaluated the consistency of the power vector components of the spherocylindrical refraction to consider also the vectorial character of the astigmatism, and we found the same trends. Therefore, this device is able to provide consistent estimations of the spherocylindrical refraction, as with other types of aberrometers using other optical approaches.1–6 This excellent repeatability at distance was as good as at near, within-subject standard deviations equal to 0.20 μm or below. As expected, a statistically significant difference in sphere and M was found among distance and near conditions. This was logical because an increase in the optical power of the eye occurs when the crystalline lens accommodates for focusing a near target.23, 24 No significant change with accommodation in cylinder and in the power vector components J0 and J45 was found.
Regarding higher order aberrations, the repeatability was found to be more limited, especially for the vertical coma measured at near. The horizontal coma showed the best intraobserver repeatability at distance and at near. However, the within-subject standard deviation and the intraobserver precision for the repeated measurements of the wavefront higher order aberrations at distance was in the range of previous studies or better using the same device or other aberrometers.25–27 It should be considered that higher order aberrations for a 3-mm pupil in a normal individual are really low, being <0.1 μm in magnitude. This makes the evaluation of the repeatability of such values more difficult, because small changes in low decimal numbers imply great changes in repeatability coefficients. Future studies aimed at testing the repeatability of higher order aberrations measured with the iTrace system in highly aberrated eyes, such as in keratoconus, will be of great interest. In addition, the potential effect of the intrinsic variability of aberrations28 and the variability associated to blinks29 on the consistency of repeated measurements should be considered. Indeed, Ginis et al.30 stated that wavefront aberration data used in clinical care should not be extracted from a single measurement, which represents only a static snapshot of a dynamically changing aberration pattern. Another finding of our study that should be analyzed with care is the positive value found in all cases for the coma Zernike terms. In a normal population, negative values for the horizontal and vertical coma terms are expected. However, it should be considered that we have analyzed aberrometric data for a 3-mm pupil in the current series and for this small pupil size all higher order Zernike terms are expected to be very close to zero. Indeed, we have obtained only positive values for horizontal and vertical coma but most of them practically equal to zero. In future studies, values for vertical and horizontal coma for larger pupil sizes (5 or 6 mm pupil) must be reported in normal population to confirm the presence of a variability in the sign of the horizontal and vertical coma Zernike terms when the iTrace is used for ocular aberrometric analysis.
One curious finding of the current study was the absence of a statistically significant difference between distance and near higher order aberrations. It has been shown that significant changes occur in primary spherical aberration with accommodation, showing a trend toward negative sign.31, 32 A possible reason for our discrepant finding could be the pupil size used in the current study, which was significantly smaller than in other studies attempting to characterize the change in higher order aberrations at near. We found in the current study a slight trend to a decrease in primary spherical aberration, horizontal and vertical coma at near, but it did not reach statistical significance. However, Iida et al.13 found a significant change in primary spherical aberration also using the iTrace system in emmetropic patients but for a 4 mm pupil.
Repeatability of the MTF parameters was quite good at distance and near, but the values obtained for near vision seemed contradictory. The MTF for near was significantly worse than the MTF for distance. This was not consistent with previous experimental and clinical data.31, 32, 33 The quality of near vision in a non-presbyope emmetropic eye or in a low myopic eye is quite good, and even the accommodative lags seen in myopes are able to provide optimized retinal image characteristics.33 It seems that the software calculates the retinal image quality considering the theoretical negative sphere obtained at near. However, it should be considered that this negative sphere is estimated without considering the change in the vergence of light rays coming from a near object instead from a distant object. This is an issue that should be revised and changed by the manufacturer to avoid erroneous clinical decisions considering these data when the iTrace system is used for clinical purposes. An accurate calculation of dioptric blur or optical quality would need to account for whatever is the accommodative stimulus.
One potential limitation of the current study is the effect of accommodation microfluctuations, microsaccades, and tear film dynamics on our evaluation of intraobserver repeatability. Several recent investigations have shown the role of these factors on optical quality of the eye.34–40 After a blink, a gradual increase in optical aberration associated with the increasingly irregular tear film used to occur.34–38 This effect causes a progressive reduction in the optical quality of the eye. In any case, it should be considered that this influence of tear film is expected to be minimal considering that measurements with these instruments are taken within ⅛th of a second. Furthermore, no cases of dry-eye syndrome were included. Regarding the effect of accommodation microfluctuations and microsaccades on the consistency of aberrometric measurements, it was assumed to be minimal considering the fast procedure of measurement. A complete and perfect control of such factors is very difficult to obtain in a clinical measurement. Future studies on the effect of accommodation microfluctuations and microsaccades on the consistency of aberrometric measurements should be performed.
In conclusion, the iTrace is a device with applicability to clinical practice, although some measurements provided by the instrument should be considered with caution. It provides repeatable measurements of the spherocylindrical refraction at near and distance. The measurement of higher order aberrations at near and distance with this instrument should be repeated several times because the consistency of such data is more limited. Repeatable estimation of the MTF is also provided for distance by the iTrace, but the estimation of the MTF for near seems to be biased. Future studies are necessary to confirm these preliminary results in different groups of population according to age range and other different criteria.
The authors have no proprietary or commercial interest in the medical devices that are involved in this manuscript.
David P. Piñero
University of Alicante
Crta San Vicente del Raspeig s/n
03690 Alicante, Spain
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Keywords:© 2011 American Academy of Optometry
aberrometer; ray tracing; iTrace; accommodation