OPTOMETRY RESEARCH PAPER. Optical quality comparison among different Boston contact lens materials

Transcription

1 C L I N I C A L A N D E X P E R I M E N T A L OPTOMETRY RESEARCH PAPER Optical quality comparison among different Boston contact lens materials Clin Exp Optom 2016; 99: Alberto Domínguez-Vicent MSc Jose Juan Esteve-Taboada PhD Teresa Ferrer-Blasco PhD Santiago García-Lázaro PhD Robert Montés-Micó PhD Optometry Research Group (GIO), Department of Optics, University of Valencia, Spain alberto.vicent@uv.es Submitted: 11 August 2014 Revised: 15 April 2015 Accepted for publication: 19 April 2015 DOI: /cxo Background: The aim was to assess the optical quality of four Boston contact lens materials with an optical device based on Schlieren interferometry. Methods: The NIMO TR1504 (Lambda-X, Nivelles, Belgium) was used to measure higherorder aberrations and their corresponding root mean square values of four different rigid gas permeable contact lenses made from four different Boston materials: EO, ES, XO and XO 2. For each lens, 30 measurements were performed with two optical apertures: 3.0 mm and 6.0 mm. The modulation transfer function, point spread function, Strehl ratio and a simulation of the image provided by the lens were computed from the Zernike coefficients measured up to the fourth order. Results: The root mean square error of higher-order aberrations varied significantly with material type for both optical apertures (p < 0.01). The largest difference was obtained between the Boston EO and the Boston ES materials (for a 6.0 mm aperture), the mean difference being(8.3±0.2) 10-2 μm. The modulation transfer functions, point spread functions and Strehl ratio values were similar among all Boston materials at the smaller optical aperture; however, differences between each material were more apparent for the 6.0 mm aperture, with the Boston ES material exhibiting the best optical quality. Conclusions: In terms of all metrics analysed, all Boston materials examined showed comparable optical quality for a 3.0 mm aperture but the Boston ES material displayed the best optical quality for a 6.0 mm optical aperture. Key words: Boston materials, Boston optical quality, contact lens aberrations, rigid contact lenses In vivo measurements have demonstrated the optical benefits of wearing rigid gas-permeable (RGP) contact lenses 1 4 in comparison to soft contact lenses and spectacles. 2,4 Roberts and colleagues 5 reported that soft contact lenses induced a significant increase in higher-order aberrations, when compared with the naked eye and several studies have reported that RGP contact lenses improve the optical quality of the eye, especially those dominated by corneal aberrations (for example, keratoconus); 1 however, several studies have reported that the effect of RGP contact lenses on the eye s wavefrontaberrationsdepends upon the habitual ocular aberrations. 2,3,6 In this sense, if the habitual aberrations of the eye were high, RGP contact lens correction would reduce them, whereas the opposite trend would be expected, if the habitual aberrations were low. Boston materials are used to manufacture RGP contact lenses. The Boston EO and Boston ES materials are manufactured with a fluoro silicone acrylate technology, combined with the AERCOR chemical architecture. This permits the maintenance and increase of oxygen delivery while reducing silicone. 7 Boston XO is a second generation fluoro silicone acrylate, which offers superpermeability 8 and is as dimensionally stable as gas permeable lenses of much lower Dk. The newest Boston material, Boston XO 2,providesexcellent oxygen permeability without compromising wettability, stability or comfort. Table 1 summarises the main physical properties of these materials; however, it has not yet been studied whether differences in the physical characteristics, chemical structure and composition of these Boston RGP materials influence the optical quality of the lens. The NIMO TR1405 (Lambda-X, Nivelles, Belgium) is an optical device based on the Schlieren interferometric principle, 9 which can be used to perform in vitro measurements to obtain the refractive power and optical aberrations from spherical, toric and refractive multifocal contact or intraocular lenses. This device is more precise than any current International Organization for Standardization (ISO) method for the measurement of monofocal contact lenses. 10 The NIMO TR1405 has been used to measure the distribution of refractive power within the optical zone of different soft contact lenses 11 and to assess the power profile of multifocal contact lenses. 12 No previous studies have used this device to assess the optical quality of RGP monofocal contact lenses. The aim of this study was to assess the optical quality of four Boston contact lens materials with the NIMO TR1405 to determine whether differences in physical properties, polymer composition and chemical structure affect optical quality in vitro. METHOD Contact lenses used The contact lenses studied were RGP monofocal BIAS-S (Conoptica, Barcelona, Spain). Four lenses were included with the 2016 Optometry Australia Clinical and Experimental Optometry 99.1 January

2 same optical design made from four different materials: Boston ES, Boston EO, Boston XO and Boston XO 2 (Table 1). According to the data provided by the manufacturer, all lenses had the same nominal characteristics: a back vertex power of D, a base curve of 7.90 mm, a total diameter of 9.60 mm and a nominal centre thickness of 0.19 mm. The optical quality of each material was assessed comparing the optical outcomes of lenses fabricated with the same design and nominal characteristics. Only one lens made from each material was examined to minimise the introduction of possible bias due to the fabrication process and this was not the purpose of the current study. The fabrication process was assumed to be within tolerance limits stipulated in the ISO : BOSTON MATERIAL ES EO XO XO 2 Refractive index Oxygen permeability (Dk) Oxygen transmissibility (Dk/t) Silicone content 5-7 % 5-6 % 8-9 % % Wetting angle 52 o 49 o 49 o 38 o Dynamic contact angle (advanced / receding) 52 o /50 o 62 o /60 o 59 o /58 o 50 o /40 o Table 1. Material characteristics of each Boston material used in the study directions are used to characterize the contact lens power within the optic zone With a single measurement, the NIMO is able to obtain the spherical and cylinder powers with their axes, Zernike coefficients (higher-order aberrations) and the contact lens refractive power profile. The measuring protocol was as follows. 1. The contact lens was removed from its blister and cleaned carefully with distilled water. 2. A small fan was used to remove all traces of distilled water without touching the lens. 3. The contact lens was transferred to the NIMO s dry cell cuvette with its back surface oriented downward (Figure 1). This step was performed delicately using a pair of tweezers and special attention was taken not to touch the optical zone of the contact lens with the tweezers. 4. The contact lens was aligned with the NIMO s optical axis, and then one measurement was taken. 5. Both the dry cell cuvette and the contact lens were removed from the plate of the Measurements The NIMO TR1504 (Figure 1) uses a quantitative deflectometric technique and combines the interferometric Schlieren principle with a phase-shifting method 9 to measure the optical properties of contact and intraocular lenses. A measuring light with a maximum radiance peak of 546 nm illuminates a liquid crystal display (LCD). In addition, the image of the LCD, which passes through the lens being measured, is formed on a charged-coupled device (CCD) camera, with a resolution of pixels. The contact lens is placed on the dry cuvette and the coupling lenscuvette is placed in the object plane of the instrument (Figure 1). A sinusoidal pattern is projected on the LCD, which is illuminated uniformly, when no lens is placed in the object plane. Schlieren fringes are projected on the CCD due to light deviations or deflections once the lens is placed in the object plane of the instrument. The light beam deviation along both x and y Figure 1. Schematic layout of the NIMO TR1504 Clinical and Experimental Optometry 99.1 January Optometry Australia

3 measurement, and the contact lens was also removed from the dry cell cuvette. 6. The contact lens was placed again in the dry cell cuvette and the coupling lenscuvette was replaced on NIMO s measuring plate. 7. Steps four, five and six were repeated 30 times to obtain a total of 30 measurements for each lens material. Prior to each measurement, the contact lens was realigned with the NIMO s optical axis. This protocol was completed twice, for 3.0 mm and 6.0 mm optical apertures. The root mean square (RMS), modulation transfer function (MTF), point spread function (PSF), and Strehl ratio were computed to assess the optical quality of each lens material with a custom-made MATLAB program (Mathworks, Natick, Massachusetts, USA). In addition, the convolution between an optotype eye chart and the point spread function was also computed. The average RMS value of each Zernike coefficient was also calculated for the 30 measurements taken. In addition, the higher-order root mean square error (HORMS) from third to fourth order, which includes trefoil (Z 3-3 ;Z 3 3 ), coma (Z 3-1 ;Z 3 1 ), tetrafoil (Z 4-4 ; Z 4 4 ), secondary astigmatism (Z 4-2 ;Z 4 2 )and spherical (Z 4 0 ) aberration, was also calculated. The MTFs were computed up to 60 cycles per degree and the retinal contrast threshold values, which were measured at a retinal illuminance of 500 td (troland), 14 were also included in the modulation transfer function curves. The intersection between each Boston MTF and the retinal curve gives information about the neural cut-off frequency given by an eye wearing a contact lens made of each material. The point spread functions were presented using a non-linear scale to enhance the side lobes of the PSF functions. Finally, the Strehl ratio was calculated in the frequency domain, as the ratio between the area under the lens MTF curve divided the area under the diffraction-limited MTF curve. Statistical analysis All data were analysed using SPSS v.17.0 (IBM, New York, New York, USA). The normality of the data was verified using the Shapiro-Wilk test and the variance equality with the Levene s tests. A twoway analysis of variance (ANOVA) of four (materials) times two (optical apertures) factorial analysis was performed. The dependent variables were the RMS of each Zernike coefficient, the HORMS error and the Strehl Ratio. The Holm-Sidak multiple comparison test was performed when the ANOVA revealed statistically significant differences. Differences were considered statistically significant when the p value was smaller than RESULTS Table 2 shows the HORMS errors for all Boston contact lens materials for the 3.0 mm and 6.0 mm optical apertures. Similar HORMS values among each Boston material were obtained for the 3.0 mm optical aperture with the standard deviation of each value smaller than μm. The multiple comparison test revealed statistically significant differences among all Boston materials (p < 0.01): Boston ES displayed the smallest HORMS (1.46 ± 0.03) 10-2 μm, whereas, Boston EO displayed the largest value (2.38 ± 0.06) 10-2 μm. Table 2 also includes the mean HORMS differences for all between-material comparisons and their significance levels. The greatest difference was observed between the Boston EO and Boston ES ( μm, p < 0.01) and the smallest difference was obtained between Boston EO and Boston XO 2 ( μm, p < 0.01). Differences between Boston materials were more apparent when the optical aperture was set to 6.0 mm (Table 2). Statistically significant differences (p < 0.01) were obtained for all between-material comparisons. The Boston ES displayed the smallest HORMS value (10.14 ± 0.05) 10-2 μm and the Boston EO material displayed the largest value (18.40 ± 0.10) 10-2 μm. Table 2 also includes the mean HORMS differences for all between-material comparisons and their significance levels. The maximum difference was obtained between the Boston EO and Boston ES materials ( μm, p < 0.01), whereas, the minimum difference was obtained between the Boston XO and Boston XO 2 ( μm, p < 0.01). Tables 3 and 4 show the RMS of each thirdand fourth-order Zernike coefficient for each material over the 3.0 mm and 6.0 mm optical apertures. When the aperture was set to 3.0 mm (Table 3), each Zernike coefficient term was similar among all four materials, as the mean difference for any given coefficient was less than μm, except for the spherical aberration term, where the mean difference was less than 0.03 μm. This table also includes all between-materials comparisons for each Zernike coefficient with statistically significant differences (p < 0.01). In terms Material 3.0 mm optical aperture 6.0 mm optical aperture Mean ± SD Mean difference (significance) Mean ± SD Mean difference (significance) Boston EO (2.38 ± 0.06) 10-2 Boston EO-Boston ES: (18.40 ± 0.10) 10-2 Boston EO-Boston ES: Boston ES (1.46 ± 0.03) 10-2 Boston EO-Boston XO: (10.14 ± 0.05) 10-2 Boston EO-Boston XO: Boston EO-Boston XO 2 : Boston EO-Boston XO 2 : Boston XO (1.82 ± 0.04) 10-2 Boston ES-Boston XO: (12.92 ± 0.08) 10-2 Boston ES-Boston XO: Boston ES-Boston XO 2 : Boston ES-Boston XO 2 : Boston XO 2 (2.22 ± 0.16) 10-2 Boston XO-Boston XO 2 : (12.29 ± 0.07) 10-2 Boston XO-Boston XO 2 : Table 2. Mean high-order root mean square value from third- and fourth-order for each Boston material at the 3.0 mm and 6.0 mm optical apertures. The mean difference with its significance level was also included for all material pairs comparisons with statistical significant differences. Significant differences were considered when p < SD: Standard deviation. Both mean and mean differences were expressed in μm Optometry Australia Clinical and Experimental Optometry 99.1 January

4 Zernike coefficient Zernike value, μm (Mean ± SD) Mean difference (significant level) Z (3, -3) Boston EO: (3.00 ± 2.00) 10-4 Boston EO Boston ES: Boston ES: (1.08 ± 0.20) 10-3 Boston EO Boston XO: Boston XO: (5.06 ± 2.00) 10-4 Boston EO Boston XO 2 : Boston XO 2 : (7.70 ± 3.00) 10-4 Boston ES Boston XO: Boston ES Boston XO 2 : Z (3, -1) Boston EO: (2.03 ± 1.60) 10-3 None Boston ES: (2.70 ± 0.60) 10-3 Boston XO: (2.38 ± 1.20) 10-3 Boston XO 2 : (2.41 ± 0.80) 10-3 Z (3, 1) Boston EO: (2.21 ± 1.50) 10-3 Boston EO Boston ES: Boston ES: (7.51 ± 7.00) 10-4 Boston EO Boston XO: Boston XO: (1.31 ± 0.50) 10-3 Boston ES Boston XO 2 : Boston XO 2 : (2.98 ± 0.80) 10-3 Boston XO Boston XO 2 : Z (3, 3) Boston EO: (2.55 ± 1.80) 10-4 Boston EO Boston ES: Boston ES: (1.07 ± 0.20) 10-3 Boston EO Boston XO 2 : Boston XO: (2.09 ± 1.03) 10-4 Boston ES Boston XO: Boston XO 2 : (1.16 ± 0.40) 10-3 Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (4, -4) Boston EO: (6.85 ± 2.00) 10-4 Boston EO Boston XO 2 : Boston ES: (3.71 ± 2.00) 10-4 Boston ES Boston XO 2 : Boston XO: (4.79± 3.00) 10-4 Boston XO Boston XO 2 : Boston XO 2 : (3.87 ± 2.00) 10-4 Z (4, -2) Boston EO: (9.03 ± 2.15) 10-4 Boston EO Boston ES: Boston ES: (1.18 ± 0.20) 10-3 Boston EO Boston XO: Boston XO: (1.59 ± 0.60) 10-3 Boston EO Boston XO 2 : Boston XO 2 : (4.89 ± 1.60) 10-4 Boston ES Boston XO: Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (4, 0) Boston EO: (2.36 ± 0.06) 10-2 Boston EO Boston ES: Boston ES: (1.41 ± 0.02) 10-2 Boston EO Boston XO: Boston XO: (1.79 ± 0.06) 10-2 Boston EO Boston XO 2 : Boston XO 2 : (2.18 ± 0.16) 10-2 Boston ES Boston XO: Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (4, 2) Boston EO: (8.60 ± 3.0) 10-4 Boston EO Boston ES: Boston ES: (1.80 ± 0.40) 10-3 Boston EO Boston XO: Boston XO: (4.17 ± 3.00) 10-4 Boston ES Boston XO: Boston XO 2 : (8.06 ± 3.00) 10-4 Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (4, 4) Boston EO: (9.32 ± 7.00) 10-4 Boston EO Boston ES: Boston ES: (7.95 ± 2.00) 10-4 Boston EO Boston XO: Boston XO: (6.72 ± 4.00) 10-4 Boston ES Boston XO 2 : Boston XO 2 : (2.47 ± 1.07) 10-4 Table 3. Mean and standard deviation (SD) of each Zernike coefficient term from third- to fourth-order for all Boston materials at the 3.0 mm optical aperture. The third column represents the mean difference and the level of significance for all between-material comparisons with statistically significant differences, which were considered when p < of spherical aberration, all between-material comparisons showed statistically significant differences: the Boston EO material demonstrated the largest spherical aberration value (2.36 ± 0.06) 10-2 μm, whereas the Boston ES showed the smallest value (1.41 ± 0.02) 10-2 μm. No statistically significant differences (p > 0.01) were obtained in terms of vertical coma among all Boston materials. The Boston EO showed the smallest value (2.0 ± 1.6) 10-3 μm; the Boston ES material demonstrated the highest coma value (2.70 ± 0.60) 10-3 μm. For horizontal coma, statistically significant differences were obtained for all between-material comparisons (p < 0.01), except for the Boston EO Boston XO 2 and BostonES BostonXOcomparisons(p> 0.05). On the other hand, when the optical aperture was set to 6.0 mm (Table 4), the RMS errors corresponding to vertical and horizontal coma were smaller than μm; however, the spherical aberration was larger than 0.1 μm and the other coefficients were smaller than μm. Table 4 also displays all between-boston material comparisons with statistically significant differences for each Zernike coefficient. The magnitude of spherical aberration varied significantly between all Boston materials (p < 0.01): the Boston ES displayed the smallest value (10.08 ± 0.03) 10-2 μm, whereas the Boston EO showed the largest value (18.25 ± 0.05) 10-2 μm. On the other hand, vertical coma was statistically significant for all between-material comparisons, exceptfortheboston ES Boston XOcomparison and comparable horizontal coma values (p > 0.01) were obtained between Boston ES Boston XO 2 and Boston EO Boston XO. Figures2and3showtheMTFsforeach material at the 3.0 mm and 6.0 mm optical apertures. Comparable curves were obtained for the smallest optical aperture (Figure2),wheretheMTFsofallfourBoston lens materials were close to the diffraction-limited curve. Additionally, the Boston ES material provided the closest match to the diffraction-limited MTF. For the 6.0 mm optical aperture (Figure 3), all curves moved away from the diffraction-limited MTF, with the MTF of Boston ES closest to the diffraction-limited MTF. The cut-off frequencies were similar among all Boston materials for the smallest optical aperture, being about 50 cycles per degree. For the 6.0 mm optical aperture, the Boston ES material displayed the largest cut-off frequency (about 40 cycles per degree) and the Boston EO demonstrated the smallest frequency (about 25 cycles per degree). Clinical and Experimental Optometry 99.1 January Optometry Australia

5 Zernike coefficient Mean ± SD (μm) Mean difference (significant level) Z (3, -3) Boston EO: (3.68 ± 2.00) 10-4 Boston EO Boston ES: Boston ES: (3.49 ± 1.60) 10-4 Boston EO Boston XO: Boston XO: (1.96 ± 0.70) 10-3 Boston EO Boston XO 2 : Boston XO 2 : (2.43 ± 0.70) 10-3 Boston ES Boston XO: Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (3, -1) Boston EO: (1.60 ± 0.20) 10-2 Boston EO Boston ES: Boston ES: (9.00 ± 2.00) 10-3 Boston EO Boston XO: Boston XO: (9.26 ± 4.00) 10-3 Boston EO Boston XO 2 : Boston XO 2 : (1.83 ± 0.20) 10-2 Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (3, 1) Boston EO: (2.01 ± 1.08) 10-2 None Boston ES: (7.85 ± 5.00) 10-3 Boston XO: (1.95 ± 0.70) 10-2 Boston XO 2 : (1.19 ± 0.30) 10-2 Z (3, 3) Boston EO: (1.82 ± 1.60) 10-3 Boston EO Boston ES: Boston ES: (3.57 ± 1.50) 10-4 Boston EO Boston XO: Boston XO: (1.99 ± 0.70) 10-3 Boston EO Boston XO 2 : Boston XO 2 : (2.07 ± 0.80) 10-3 Boston ES Boston XO: Boston ES Boston XO 2 : Z (4, -4) Boston EO: (1.97 ± 0.50) 10-3 Boston EO Boston XO: Boston ES: (2.08 ± 0.30) 10-3 Boston ES Boston XO: Boston XO: (5.31 ± 1.50) 10-4 Boston XO Boston XO 2 : Boston XO 2 : (1.71 ± 0.20) 10-3 Z (4, -2) Boston EO: (2.17 ± 0.80) 10-3 Boston EO Boston XO: Boston ES: (1.78 ± 0.40) 10-3 Boston EO Boston XO 2 : Boston XO: (1.06 ± 0.70) 10-3 Boston ES Boston XO: Boston XO 2 : (6.85 ± 3.00) 10-4 Boston ES Boston XO 2 : Z (4, 0) Boston EO: (18.25 ± 0.05) 10-2 Boston EO Boston ES: Boston ES: (10.08 ± 0.03) 10-2 Boston EO Boston XO: Boston XO: (12.71 ± 0.02) 10-2 Boston EO Boston XO 2 : Boston XO 2 : (12.09 ± 0.05) 10-2 Boston ES Boston XO: Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (4, 2) Boston EO: (1.14 ± 0.62) 10-3 Boston EO Boston XO: Boston ES: (1.48 ± 0.18) 10-3 Boston EO Boston XO 2 : Boston XO: (2.25 ± 0.50) 10-3 Boston ES Boston XO: Boston XO 2 : (7.81 ± 2.00) 10-4 Boston ES Boston XO 2 : Boston XO Boston XO 2 : Z (4, 4) Boston EO: (1.89 ± 1.09) 10-3 Boston EO Boston ES: BostonES: (1.74 ± 0.40) 10-3 Boston EO Boston XO 2 : Boston XO: (1.08 ± 0.30) 10-3 Boston ES Boston XO: Boston XO 2 : (3.67 ± 3.00) 10-4 Boston ES Boston XO 2 : Table 4. Mean and standard deviation (SD) of each Zernike coefficient term from third- to fourth-order for all Boston materials at the 6.0 mm optical aperture. The third column represents the mean difference and the level of significance for all between-material comparisons with statistically significant differences, which were considered when p < Similar Strehl ratio values were obtained among Boston materials for the 3.0 mm optical aperture; however, statistically significant differences were obtained for all between-materials comparisons (p < 0.01). In general, the Strehl ratio values were close to one at the smallest aperture: the Boston ES lens demonstrated the largest value (0.96), whereas the Boston EO showed the smallest value (0.89). When the optical aperture was set to 6.0 mm, the Strehl ratio values also revealed statistically significant differences for all between-material comparisons (p < 0.01): the Boston ES material displayed the largest value (0.25) and Boston EO the smallest value (0.19). Figure 4 shows the point spread function images of each Boston material for both optical apertures. Compact (that is, similar to a single point source), with high peak intensity PSFs were obtained for all Boston materials for the 3.0 mm optical aperture. Generally speaking, this metric was similar among all Boston materials. Broader point spread functions with lower peak intensity were obtained when the optical aperture was set to 6.0 mm. The Boston ES material displayed the most compact PSF with the highest peak intensity, while the Boston EO showed the broadest PSF with the lowest peak intensity. The Boston XO and XO 2 materials displayed similar point spread functions. Finally, Figure 5 shows the convolution of each PSF with an optotype chart. As expected, similar images were observed among all Boston materials for the 3.0 mm optical aperture. When the optical aperture was set to 6.0 mm, the Boston ES material displayed the best optotype chart and the Boston EO the worst. In addition, both Boston XO and XO 2 materials displayed similar images. DISCUSSION The Zernike coefficients (Table 3), HORMS errors (Table 2) and Strehl ratios were small in absolute value but varied significantly between all Boston materials for the 3.0 mm aperture. Nevertheless, these differences were not noticeable in the modulation transfer functions (Figure 2), point spread functions (Figure 4) or the simulations of the images provided by the different materials (Figure 5). These statistically significant differences were most likely a result of the extremely low standard deviations obtained due to NIMO s high level of repeatability. 10 Therefore, optical differences among these materials are most 2016 Optometry Australia Clinical and Experimental Optometry 99.1 January

6 Figure 2. Modulation transfer function versus spatial frequency expressed in cycles per degree for a 3.0 mm optical aperture for Boston EO, ES, XO and XO 2 materials. The diffraction-limited curve and retinal contrast threshold curve obtained by Sekiguchi, Williams and Brainward 14 at a retinal illuminance of 500 td are included. Figure 3. Modulation transfer function versus spatial frequency expressed in cycles per degree for a 6.0 mm optical aperture for Boston EO, ES, XO and XO 2 materials. The diffraction-limited curve and retinal contrast threshold curve obtained by Sekiguchi, Williams and Brainward 14 at a retinal illuminance of 500 td are included. likely not clinically significant over a 3.0 mm optical aperture. For the 6.0 mm aperture, the Boston ES displayed the best optical quality among the other Boston materials in terms of Zernike coefficients, HORMS errors and Strehl ratio values. The simulated images in Figure 5 (bottom row) suggest that these differences may be clinically significant. This figure also highlights that differences between the Boston XO and XO 2 materials are most likely not clinically significant. Since all lenses examined had the same design and nominal parameters, any observed optical differences between these lens materials might be related to the polymer structure of each material; however, small manufacturing differences may account for some of the observed optical quality differences between materials, as the ISO : allows a variance of 0.05 mm in the back optic zone radius. Finally, it should be taken into consideration that these results were obtained under in vitro conditions and neither the optical quality of the eye nor the performance of the lens on eye were taken into account during computations. For this reason, further in vivo studies should be aimed to assess whether these observed optical differences are clinically significant to RGP wearers. Applegate and colleagues 15 studied the effect of the RMS wavefront error on the visual performance using a Hartman-Shack wavefront sensor. They reported the number of letters lost as a function of the RMS of the whole eye. According to their results, RMS wavefront errors of 0.10 μm, 0.15 μm and 0.20 μm correspond to mean letter losses of two, three and four letters, respectively. Based on Applegate and colleagues results, differences in visual function between Boston materials would not be expected for the 3.0 mm optical aperture as the differences in the RMS among all materials tested were lower than 0.01 μm. In addition, differences in the wearer s visual function might not be expected due to the HORMS difference between the lens materials, which was less than 0.10 μm, for the 6.0 mm aperture. As was stated previously, a further in vivo study is needed to assess whether subtle optical differences between materials affect the subject s visualperformance. Spherical aberration was the predominant higher-order aberration for both optical apertures and was negative in sign for all materials (Tables 3 and 4). As the spherical aberration ofthewholeeyeisinmostcasespositive, 16 these particular lenses would minimise the Clinical and Experimental Optometry 99.1 January Optometry Australia

7 total spherical aberration of the combined eye-contact lens system, as was reported by Hong, Himebaugh and Thibos; 4 however, the spherical aberration of contact lenses increases with increasing (absolute) contact lens power. 17 There are other technologies to measure in vitro optical properties of RGP contact lenses. Kollbaum and colleagues 18 evaluated the accuracy and repeatability of a commercial Hartman Shack aberrometer to measure the aberrations of dry and wet contact lenses. The accuracy of third-order aberrations was verified by comparing the magnitude of induced coma after decentring a lens with a known amount of spherical aberration, while the accuracy of fourth order was evaluated by comparing the measured longitudinal spherical aberration to that expected based on raytracing through the lens design. This approach yielded accurate results with an error lower than one per cent. The authors used two different procedures to assess repeatability. First, repeated measurements were taken with the wet cell remaining in place, and second, repeated measurements were taken after removing and reinserting the lens into the wet cell. The variance using the first procedure was μm 2, when measuring RGP contact lenses and μm 2 for soft contact lenses. With the second procedure, the variance was μm 2 for RGP lenses and μm 2 for soft contact lenses. Previous studies have assessed the NIMO s repeatability for the measurement of monofocal contact lenses 10 and the power profiles of multifocal soft contact lenses; 19 however, no previous studies have assessed the repeatability and accuracy of this device for the measurement of Zernike coefficient terms. Thus, further studies should assess both repeatability and reproducibility of the NIMO for measure aberrations of both RGP and soft contact lenses. In vitro measurements were conducted in this study; however, differences in on-eye lens performance related to surface wettability, oxygen permeability and the wearing modality may impact on the quality of vision. Further studies should be aimed to assess whether differences among materials affect visual performance and to assess the visual quality of these materials over time. In summary, it can be concluded that no visual differences might be expected among Boston materials for a 3.0 mm aperture due to similar results in the optical metrics assessed. Conversely, visual differences might be expected at the 6.0 mm Figure 4. Point spread functions computed from the wavefront aberrations for each Boston contact lens material examined for both 3.0 mm and 6.0 mm optical apertures Figure 5. Simulation of the images provided by the different lens materials for 3.0 mm and 6.0 mm optical apertures optical aperture; however, in vivo studies are required to clarify these conclusions drawn from in vitro testing. ACKNOWLEDGEMENTS This research was supported in part by the Starting Grant funded by the European Research Council (ERC-2012-StG SACCO) to Professor Robert Montés-Micó and by an Atracció de talent research scholarship (Universidad de Valencia) awarded to Alberto Domínguez Vicent. REFERENCES 1. Dorronsoro C, Barbero S, Llorente L, Marcos M. Oneye measurement of optical performance of rigid gas permable contact lenses based on ocular and corneal aberrometry. Optom Vis Sci 2003; 80: Lu F, Mao X, Qu J, Xu D, He J. Monochromatic wavefront aberrations in the human eye with contact lenses. Optom Vis Sci 2003; 80: Choi J, Wee WR, Lee JH, Kim MKK. Changes of ocular higher order aberration in on- and off-eye of 2016 Optometry Australia Clinical and Experimental Optometry 99.1 January

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