LU, FENGHE MD, PhD; SIMPSON, TREFFORD PhD; SORBARA, LUIGINA OD, MSc, FAAO; FONN, DESMOND MOptom, FAAO
High oxygen permeability (Dk) material is one of three important advancements1 accounting for the renewed interest in corneal refractive therapy (CRT®)/orthokeratology or non-surgical corneal reshaping.2–13 New higher oxygen transmissible (Dk/t) materials14 are desirable to produce the least corneal swelling beyond physiological edema15–17 when using these lenses at night. Overnight lens wear induces corneal swelling over and above eye closure. Therefore, using the highest Dk/t lens materials to minimize any swelling increment would be desirable.
In conventional gas permeable (GP) polymers, a silicone- containing methacrylate has been used to provide high oxygen permeability. The greater the proportion of silicone used in the material, the higher the Dk/t, but the less the hardness and mechanical strength of the material.18 GP lenses for myopic corneal reshaping have been made from relatively high Dk/t materials14 (Dk/t: approximately 35 to 70).2,7,8,10,13,19,20 A higher Dk/t GP material, Menicon Z (Tisilfocon A, Dk/t: 90.6, MZ) (Menicon Co. Ltd. Nagoya, Japan), has been developed and was approved by the FDA (US) for continuous wear up to 30 days in 200221; the company has claimed that MZ is the first material to possess both high oxygen permeability and high mechanical strength (www.menicon.com). Thus, it may be a promising material for corneal reshaping.
Optical quality is perhaps most sensitively measured by wavefront sensors that quantify ocular aberrations. In addition to wavefront sensors, corneal topographers can detect subtle changes of the corneal shape, which also contribute to the vision quality, improving our understanding of the ocular response after corneal reshaping.
Corneal reshaping lenses primarily alter the corneal anterior surface to correct the refractive error.3–5,7,8 The effect of higher Dk/t on corneal reshaping after overnight lens wear is largely unknown, although the effect of lower Dk/t on corneal reshaping has been studied.22,23 In this study, two lenses with identical physical lens design (Paragon CRT®), but with different Dk/t [one the Equalens II (EII) with Dk/t 47.2 units (×10−9 barrers/cm), the other with Dk/t 90.6 units, MZ] were worn for a single night to examine their effects on corneal shape and optical performance. We could expect that the higher Dk/t lens would provide more oxygen to the cornea and induce less corneal swelling compared to the lower Dk/t lens. Consequently, the corneal shape and optical performance may be different in two lens groups.
Twenty healthy myopic subjects participated in this study. Their ages ranged from 19 to 35 years (mean ± SD: 24.2 ± 3.6) and most of them were women (13 women and 7 men). All of the subjects were free of ocular and systemic diseases, and had no history of eye surgery. The subjects were instructed not to wear soft contact lenses for the week before the start of the study. Rigid contact lens wearers were excluded. Spherical ametropia ranged from –0.25 to –5.25 D, and corneal cylinder was <1.50 D. Informed consent was obtained from all participants before enrolment in the study. This work received ethics approval from the Office of Research Ethics at the University of Waterloo (Waterloo, Ontario, Canada). All subjects were treated in accordance with the tenets of the Declaration of Helsinki.
The refractive error and central/mid-peripheral corneal curvatures are listed in Tables 1 and 2. There were no significant differences between the MZ and EII lens-wearing eyes (all p ≥ 0.138).
Lens Characteristics and Fitting
MZ and EII lenses were manufactured in the same Paragon CRT® design by the same laboratory. Each lens was verified using a radioscope to measure the base curve. A summary of the MZ and EII lens characteristics and parameters that were used is found in Table 3. There were no significant differences in back optical zone radius, return zone depth and landing zone angle between the MZ and EII lenses (all p ≥ 0.163).
CRT® lens fitting has been described in detail elsewhere.13 Briefly, lenses were adjusted to achieve centration, approximately 4 mm of central touch, proper (1 to 1.5 mm width) mid-peripheral pooling, and proper (1 to 1.5 mm width) peripheral alignment. Once an acceptable fit was obtained with the trial lenses, the lenses were ordered for each eye.
This was a double masked study. Paragon CRT® lenses (MZ) were fit on one eye of 20 myopes with the same CRT® design EII lenses on the contralateral eye (eye randomized). The investigators and participants were masked which eye (right vs. left) wore which lens (MZ vs. EII). Sample size was calculated to provide at least 90% power to determine differences from baseline in CRT® treatment using corneal curvature and spherical aberration outcome variables.
Corneal topography, wavefront errors, refractive error, and visual acuity were measured at baseline on the night before lens insertion. After the lenses had settled appropriately, participants retired in our laboratory at approximately 10 p.m. and were awakened at 7 a.m. the next morning. The measurements were repeated immediately after lens removal and 1, 3, 6, 12 h later.
Anterior corneal curvature was quantified using Atlas™ Corneal topographer (Atlas Mastervue, Humphrey Ziess Instruments, San Leandro, CA). The corneal topographer was calibrated routinely using a series of spherical and aspheric surfaces.
Topographical data along the horizontal meridian were collected over an 8-mm chord in 1 mm steps using the tangential map from the computer display. To compare the corneal curvature change, we aligned the corneal curvature according to its directional position. The temporal corneal curvature in the MZ lens-wearing eyes was compared with the temporal corneal curvature in the EII lens-wearing eyes. The nasal corneal curvature in the MZ lens-wearing eyes was compared with the nasal corneal curvature in the EII eyes.
The corneal curvature measurements are repeatable. The intraclass coefficients (ICC) were 0.99 centrally and 0.95 mid-peripherally. The standard deviation of mean difference of the corneal curvature was 0.24 D centrally and 0.38 to 0.41 D mid-peripherally, respectively, which is in accord with that reported in previous studies.24,25
Root mean square wavefront errors using LADARWave™ (LADARWave™ CustomCornea Wavefront System, Alcon Laboratories, Inc. Orlando, FL) were collected using 4.5 mm pupils. Calibration of the LadarWave was performed each day using a specifically aberrated surface provided by the manufacturer. The center of the subject's pupil served as the alignment target. That is, the instrument's optical axis passed through the center of the subject's pupil, and the line of sight was coaxial with the Shack- Hartmann optical axis and fixation target.27 Five measurements were acquired and the three most similar wavefronts were used to generate a composite result. The RMS wavefront error (in microns) was used to quantify optical quality. The measurements were taken through undilated pupils and the aberration data were calculated using 4.5 mm diameter pupils based on the smallest pupil in this group of participants.
Lower order aberrations included defocus (Z20) and astigmatism (Z2±2), which were components of the Shack-Hartmann output that make up the clinical spherocylindrical refractive error. Composite higher order aberrations (HOA, including the sum of the third to sixth order Zernike coefficients), third order coma (Z3±1), horizontal coma (Z3+1), vertical coma (Z3−1), and fourth order spherical aberration (Z40, SA) were analyzed separately.28
Ladar Wave aberration measurements were repeatable, especially for defocus and astigmatism. The standard deviation of the mean difference of the defocus and astigmatism was 0.077 and 0.028 μm, respectively. ICCs were 0.99 for defocus and astigmatism. The defocus term was comparable to a previous report.29 The repeatability of the HOA, coma, and SA were slightly less than the lower order aberrations. They were 0.023 μm, 0.019 μm and 0.013 μm for the HOA, coma, and SA, respectively. ICCs were 0.86, 0.75 and 0.93, respectively. This is slightly better than that reported in previous studies.30,31
Refractive error was measured with a Nikon autorefractor and autokeratometer (NRK-8000, Japan). Spherical equivalent was used and reported.
Uncorrected visual acuity was measured with computerized log MAR charts (logarithm of the minimum angle of recognition) at 90% and 10% Weber contrast (HCVA and LCVA, respectively) with high illumination (approximately 500 lux) at 6 meters. The chart luminance was 98.2 cd m−2.
Repeated measures analysis of variance (RM-ANOVA) was used to examine the main effects of the lens type, time and location of corneal curvature, the aberrations, refractive error and visual acuity if applicable. Tukey honestly significantly different (HSD) post hoc tests were used to determine whether there were differences in corneal curvature, aberrations, refractive error, and visual acuity over time and at different locations. A planned paired t-test was used to test the difference between the two lens wearing eyes at baseline and to determine the treatment effect after lens wear relative to the baseline. Polynomial non-linear regression analysis was used to characterize the change of horizontal corneal curvature at different corneal positions after one night of lens wear. Differences were considered statistically significant when the likelihood of a type I error was ≤0.05. Data analysis was conducted using STATISTICA 7.0 (StatSoft Inc., Tulsa, OK). The data were presented as mean ± standard deviation in the text unless specified.
Averaged over location and time, MZ and EII lens-wearing eyes' horizontal corneal curvature changes were statistically different [RM-ANOVA, F(1, 19) = 8.022, p = 0.011]. The changes of horizontal corneal curvature between the MZ and EII lens-wearing eyes are illustrated in Fig. 1. The central cornea flattened similarly from baseline in both the MZ (1.22 ± 0.65 D) and EII (1.16 ± 0.58 D) lens-wearing eyes [RM-ANOVA, F(1, 19) = 0.021, p = 0.886, Figs. 1–3], and the mid-periphery (3 mm from the center) steepened in both the MZ (0.92 ± 1.76 D) and EII (1.35 ± 1.52 D) lens-wearing eyes [RM-ANOVA, F(1, 19) = 3.982, p = 0.061, Figs. 1, 3, and 4]. In addition, EII lens-wearing eyes were steeper in the mid-periphery than the MZ eyes immediately after lenses removal and at 1-h visit (both p ≤ 0.032, Figs. 3 and 4). These differences resolved from 3 h onwards (all p ≥ 0.081) (Fig. 4). The central corneal flattening and the mid-peripheral corneal steepening regressed over time [RM-ANOVA, F(4, 76) = 7.705–8.763, both p < 0.001] and did not return to baseline after 12 h without lens wear (40.3 vs. 40.7% recovery centrally and 53.8 vs. 54.4% recovery mid-peripherally for the MZ and EII lens-wearing eyes, respectively) (post hoc tests, all p ≤ 0.004, Figs. 2 and 4).
Polynomial regression analysis showed that linear, quadratic, cubic and quartic components were each significant in the fit of the horizontal corneal curvature change of both the MZ and EII lens-wearing eyes (all p ≤ 0.040). The details of the regression are shown in Fig. 3.
Refractive Error (Spherical Equivalent)
There was no significant difference in refractive error between the MZ and EII lens-wearing eyes at baseline [paired t test, t(1, 19) = 0.609, p = 0.550], nor after one night of lens wear [RM-ANOVA, F(1, 19) = 0.378, p = 0.546]. The refractive error was reduced equally by 0.84 ± 0.83 D and 0.84 ± 0.87 D for the MZ and EII lens-wearing eyes, respectively [paired t tests, t(1, 19) = 0.039, p = 0.969]. The myopic correction regressed over time [RM-ANOVA, F(5, 95) = 5.598, p < 0.001]. The refractive error did not return to baseline after 12 h without lens wear (post hoc tests, all p ≤ 0.006, Fig. 5).
There were no significant differences in defocus, astigmatism, overall HOA, coma, horizontal coma, vertical coma and SA between the MZ and EII lens-wearing eyes at baseline (paired t tests, all p ≥ 0.219), nor after one night of lens wear (RM-ANOVA, all p ≥ 0.308). Defocus decreased from 2.522 ± 0.974 μm to 1.896 ± 0.908 μm for MZ lens-wearing eyes and from 2.442 ± 0.967 μm to 1.844 ± 0.911 μm for the EII eyes (paired t tests, both p < 0.001, Fig. 6). Astigmatism did not change significantly in the MZ and EII lens-wearing eyes over time [ANOVA, F(5, 95) = 0.614 ∼ 2.290, both p ≥ 0.052]. Overall HOA increased by 1.78 × in the MZ lens-wearing eyes (from 0.139 ± 0.044 μm to 0.248 ± 0.094 μm) and 1.65 × (from 0.138 ± 0.049 μm to 0.228 ± 0.105 μm) in the EII eyes (paired t tests, both p < 0.001). Coma increased by 1.85 × in the MZ lens-wearing eyes (from 0.066 ± 0.041μm to 0.121 ± 0.059 μm) and 1.72 × (from 0.067 ± 0.036 μm to 0.115 ± 0.077 μm) in the EII eyes (paired t tests, both p < 0.001, Fig. 7). In addition, horizontal coma did not change significantly in the MZ and EII lens-wearing eyes over time [ANOVA, F(5, 95) = 0.236 ∼ 2.145, both p ≥ 0.067]. Vertical coma increased in EII lens-wearing eyes immediately after the lens removal [ANOVA, F(5, 95) = 3.679, p = 0.004; post hoc test, p = 0.002] and returned to baseline by 1 h (post hoc test, p = 0.202), and it did not change in the MZ lens-wearing eyes [ANOVA, F(5, 95) = 1.289, p = 0.275, Fig. 8]. SA increased by 4.55 × in the MZ lens-wearing eyes (from 0.03 ± 0.022 μm to 0.137 ± 0.086 μm) and 4.31 × (from 0.026 ± 0.025 μm to 0.11 ± 0.074 μm) in the EII eyes from baseline (paired t tests, both p < 0.001, Fig. 9). All aberrations, except for astigmatism and horizontal/vertical coma, lasted at least 12 h and did not return to baseline after 12 h without lens wear (post hoc tests, all p ≤ 0.007).
There were no significant differences in HCVA or LCVA between the MZ and EII lens-wearing eyes at baseline [paired t-test, t(1, 19) = −0.301, p = 0.767; t(1, 19) = −0.135, p = 0.894, respectively], nor after one night of lens wear [RM-ANOVA, F(1, 19) = 1.056, p = 0.317; F(1, 19) = 0.529, p = 0.476, respectively]. The HCVA (and LCVA) were improved similarly by 0.38 ± 0.34 D (0.27 ± 0.30 D) and 0.36 ± 0.33 D (0.23 ± 0.30 D) for the MZ and EII lens-wearing eyes, respectively [paired t tests, t(1, 19) = 0.300, p = 0.767; t(1, 19) = −0.791, p = 0.439]. The HCVA and LCVA regressed over time [RM-ANOVA, F(5, 95) = 27.971, p < 0.001; F(5, 95) = 10.068, p < 0.001]. The HCVA and LCVA did not return to baseline after 12 h without lens wear for both MZ and EII lens-wearing eyes (post hoc tests, all p < 0.001, Fig. 10).
Corneal hypoxia and corneal health are of concern in overnight corneal reshaping. Higher Dk/t material can provide more oxygen to the cornea and may minimize the corneal swelling. However, from a clinical point of view, does a higher Dk/t lens material (MZ) have the same therapeutic effect as the lower Dk/t lens material (EII) in corneal reshaping for myopia in terms of corneal shape and optical performance?
In this study, myopia was reduced by flattening the central cornea and steepening the mid-periphery, which is consistent with previous corneal reshaping studies.3–5,7–9 In addition, the flattening of the central cornea was similar after one night of MZ and EII lenses wear (Figs. 1 and 2). This similarity of central corneal shape change may be due to the similar central compression induced underneath these two lenses, resulting in similar central epithelial thinning in the lens wearing eyes, as demonstrated using the optical coherence tomography (OCT) by Haque et al. in part 1 (an accompanying publication in this issue).32 The similar central compression is presumably because of the identical physical characteristics of two lenses (Table 3) and similar baseline corneal shape (central and mid-peripheral corneal curvature, Tables 1 and 2) in the lens-wearing eyes.
MZ and EII lenses had different Dk/t (90.6 and 47.2 units) centrally, resulting in slightly but significantly different central corneal swelling,32 but the change of the central anterior corneal curvature was similar in the lens-wearing eyes. This similar anterior surface change may be due to corneal swelling occurring in the posterior direction.33–35
Our findings differ from Swarbrick et al., who compared Boston ES (Dk/t = 8 units)/Boston XO (Dk/t = 45 units),22 and EO (Dk/t = 26 units)/Boston XO,23 and found different apical corneal radius change after overnight lens wear. The difference may be due to more corneal edema induced in Boston ES and EO lens-wearing eyes compared to the Boston XO lens-wearing eyes.
EII lens-wearing corneas were steeper in the mid-periphery than those wearing MZ lenses immediately after the lens removal and at the 1-h visit (Figs. 3 and 4). This difference in steepening in the mid-periphery may be due to the different corneal swelling correspondingly in the MZ and EII lens-wearing eyes, as demonstrated using the OCT in part 1.32 The difference resolved after 3 h without lens wear (Fig. 4), which was consistent with corneal deswelling time course.10,20,39 In addition, the difference in steepening may also be due to the different amount of corneal epithelial thickness in the mid-periphery between MZ and EII lens-wearing eyes, as demonstrated using the OCT in part 1.32 Finally, it is possible that on-eye lens flexure differences may occur in the mid-periphery due to the different materials used. The higher Dk/t (MZ) lens with more silicone would be expected to have less mechanical strength,18 perhaps resulting in a narrower mid-peripheral postlens space during eye closure compared to the lower Dk/t (EII) lens. We did not have the lens flexure data in the mid-periphery for the two lenses in this study.
After one night of corneal reshaping for myopia, defocus and refractive error decreased, and HOA (particularly SA) increased. This optical alteration was in agreement with previous reports.40–42 In addition, the optical performance using 4.5 mm pupils was similar after one night of MZ and EII lens wear (Figs. 5–9). This similarity of the optical performance in the lens wearing eyes was primarily attributed to the similar anterior corneal shape change centrally (Figs. 1–3). Anterior corneal surface contributes greatly to the ocular aberrations, which is due to the greater refractive indices difference between the anterior corneal surface and air relative to that in the posterior corneal surface and aqueous. In addition, posterior corneal surface only contributes a small amount (2% at most) to ocular aberrations theoretically and in practice.43 As expected, after one night of CRT® lens wear, the equal myopic reduction and similar optical performance (Figs. 5–9) resulted in similar visual outcomes in MZ and EII lens-wearing eyes (Fig. 10).
Coma increased in amounts comparable to previous overnight studies.40–42 Increased coma may be due to the slight lens decentration. Topography data in the current study showed the center of the central treatment zone displaced temporally and inferiorly as illustrated in Fig. 11 (mean distance ± SD) by 0.55 ± 0.25 mm for MZ lens-wearing eyes, and 0.54 ± 0.29 mm for EII lens-wearing eyes (paired t test, p = 0.935). These decentration outcomes were also comparable to previous reports.44,45 The lack of the difference in lens decentration in two lens groups again suggested the centration of two lens groups was similar (Fig. 11).
Difference in corneal shape change occurred at diameters of approximately 6 mm (Figs. 3 and 4): Steeper mid-peripheral corneal curvature in lower Dk/t (EII) lens-wearing eyes was found compared to the higher Dk/t (MZ) lens-wearing eyes. Theoretically, SA with 6 mm pupils in the lower Dk/t lens-wearing eyes might be greater than the higher Dk/t lens-wearing eyes. In this sample, only 14 subjects had pupils equal to or >6 mm under the non dilated condition. The SA data in these 14 subjects were re-analyzed using the 6 mm pupil size. There was a trend that the SA in lower Dk/t (EII) lens-wearing eyes was slightly greater than that of the higher Dk/t (MZ) lens wearing eyes after overnight lens wear but it was not statistically significant [RM-ANOVA, F (1, 13) = 0.2766, p = 0.608]. This small sample size (14 subjects) may be the source of the lack of the power to detect the difference statistically.
After 12 h without lens wear (the time corneal edema would be expected to have resolved10,20,32,39), the corneal shape had not returned to baseline (Figs. 1, 2, and 4). This suggests that the corneal structural change was not solely because of the alteration in hydration. The central epithelial thinning9,20,32 and the mid-peripheral epithelial thickening20,32 did not completely return to baseline in the late afternoon, suggesting that the epithelial profile alteration is particularly important in corneal reshaping.4,9,10,20,32 This corneal shape change resulting from epithelial change contributes to the maintenance of the myopic correction and optical performance in the late afternoon.
In summary, after one night of CRT® lens wear, the central cornea flattened and mid-periphery steepened. The defocus and myopia decreased, whereas the overall HOA, coma and SA increased. In addition, optical performance and central corneal shape change was similar in the higher and lower Dk/t (MZ vs. EII) lens-wearing eyes. The mid-periphery of cornea in the lower Dk/t (EII) material lens-wearing eyes was steeper than the higher Dk/t (MZ) eyes, but these differences resolved after 3 h without lens wear.
This work was supported by Menicon Co., Paragon Vision Sciences and Canadian Optometric Education Trust Fund (COETF). FL is a recipient of the Ontario Graduate Scholarship, Ontario, Canada.
Part of this work (program number 050042) was presented in the annual meeting of the American Academy of Optometry, December 09, 2005, San Diego, CA.
None of the authors of this study have any financial or other interests/arrangements with the products/companies mentioned in the manuscript.
Centre for Contact Lens Research
School of Optometry
University of Waterloo
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Waterloo, Ontario N2L 3G1
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This article has been cited 3 time(s).
Ophthalmic and Physiological OpticsThe relationship between the treatment zone diameter and visual, optical and subjective performance in cornea refractive therapy (TM) dens wearers'Ophthalmic and Physiological Optics
Optometry & Vision ScienceErratumOptometry & Vision Science
contact lens; corneal refractive therapy; corneal topography; aberrations; oxygen transmissibility
© 2007 American Academy of Optometry