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
200 University Avenue West
Waterloo, Ontario N2L 3G1
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Keywords:© 2007 American Academy of Optometry
contact lens; corneal refractive therapy; corneal topography; aberrations; oxygen transmissibility