There was a significant association between the changes in signed SA and refractive error from baseline immediately after the lens removal (r = 0.603, p = 0.005, the bivariate regression is shown in Fig. 9). The larger the myopic shift (hyperopic correction), the greater the amount of negative SA induced by the CRTH® lenses.
Rigid contact lenses can be used to correct hyperopia by steepening the central cornea.11,14 However, previous studies12,13 have not unequivocally shown that steep lenses steepened the corneal curvature, perhaps because of lens fitting and/or lens design differences. In addition, because of changes in corneal curvature, an initially steep lens might not be steep after it is worn for some time. Recently, Swarbrick et al.14 reported that lenses with base curves about 0.3 mm steeper than the flattest K could successfully correct hyperopia or induce a myopic shift, about 0.4 D, after 4 hours of PMMA lens wear, but not in the Boston XO lens. In the current study, the selected base curve was 0.7 mm steeper than the flat K, and the depth of the mid-peripheral return zone was 175 μm deeper than the calculated return zone depth, which determined the final sagittal depth of the initial lens.27 The lenses were worn for a single night for about 8 to 9 hours. As a result, the cornea steepened centrally and flattened paracentrally (Figs. 2 and 3). There was a predictable increase in the defocus component, in myopic direction (Fig. 5), and refractive error became more myopic or less hyperopic (1.23 D on average) (Fig. 4), suggesting that CRTH® could be used to treat hyperopia. Furthermore, after one night of CRTH® lens wear, the optical effects did not return to baseline by 12 hours (except SA which lasted 6 h), indicating that this overnight lens wear modality may be feasible for retaining reasonable daytime vision.
Change in corneal shape alters the ocular aberration structure. After one night of hyperopic corneal reshaping, the defocus component increased because of the mostly myopic subjects enrolled (Fig. 5). As might be expected, the HOAs, including coma and SA, increased (Figs. 6 to 8) but astigmatism did not change.
SA was the major component of HOAs induced after CRTH®, increasing by a factor of 4.07. Similar to hyperopic corneal refractive surgery,15,28 signed SA shifted from positive to negative after one night of CRTH® lens wear. SA is relatively low in the population,29–31 and this low SA is believed to be largely a result of the balance of the corneal shape and the crystalline lens.32,33 It has been hypothesized that the cornea reduces overall ocular SA by its aspheric prolate shape, which is steeper centrally and flatter in the periphery,34,35 and perhaps by variation of the refractive index across the cornea.36 CRTH® corrects hyperopia by steepening the central cornea and flattening the paracentral region. This shape change alters the positive30,31,37 or negative32,38 corneal SA to be less positive or more negative after hyperopic corneal reshaping. The balance of SA between the cornea and crystalline lens was disturbed, shifting ocular SA from positive to negative. Furthermore, uneven epithelial distribution (discussed later) may also alter the refractive index across the cornea, contributing to the imbalance of the SA.
The amount of the signed SA change was associated with the refractive error change. Similar to hyperopic corneal refractive surgery15,39 and myopic nonsurgical corneal reshaping,22 the increment of SA was significantly related to the change of the refractive error immediately after the lens removal. The greater the amount of the refractive error corrected, the more the negative SA induced (Fig. 9).
Increased coma may have been caused by slight lens decentration. Topography data in this study showed the center of the treatment zone displaced 0.47 ± 0.30 mm temporally and 0.09 ± 0.27 mm inferiorly on average. This decentration outcome is comparable to the myopic corneal reshaping.40 Lid-lens interaction has been hypothesized to affect the lens centration during blinking,41 especially the forced and squeezed blinks from lens discomfort immediately after rigid lens insertion. Coma caused by lens decentration after myopic corneal reshaping has been reported,21,22,40 and it also happened after hyperopic corneal reshaping in this study. Therefore, decentration-induced coma may be difficult to completely avoid in corneal reshaping.
After one night of CRTH® lens wear, the central cornea steepened and paracentral cornea flattened (Figs. 2 and 3). We hypothesize that the compression from the junction (“knee”) between the return zone and the optic zone (Fig. 1) presses the epithelium and forces it to migrate centrally (toward the optic zone) and also squeezes the epithelium to the mid-periphery (toward the return zone). Central and mid-peripheral negative forces (the space between the lens and cornea generating capillary suction) are also hypothesized to drive the tissue toward the center and mid-periphery. The effect of lid tension42,43 through the contact lens may induce squeeze pressure on paracentral epithelial cells through the “knee” although the eyelid tension may be low during sleep. In addition, Allaire and Flack44 demonstrated that different thickness tear film profiles between a contact lens and the cornea induced different hydraulic forces or pressures on the cornea. Similarly, Mountford45 simulated a tear film profile to explain the hydraulic force underneath the myopic corneal reshaping lens. The different hydraulic forces resulting from the uneven tear film profile underneath the lens can also be assumed in hyperopic corneal reshaping. These hypotheses have been supported by a corneal morphological study46 suggesting that corneal reshaping was caused by epithelium accumulation centrally, and by histological data in cats47,48 suggesting that the epithelium was thicker centrally47,48 and thinner paracentrally.47
Full recovery is a critical clinical issue in corneal reshaping. In this study, the corneal shape and all optical parameters had returned to baseline by 28 hours, indicating that CRT® for hyperopia was reversible. This temporary effect is a drawback in nonsurgical corneal reshaping, but it may also be attractive for candidates who are concerned about the safety of corneal refractive surgery.
Diurnal variation of ocular aberrations and corneal shape might potentially have affected the outcome in this study. The contralateral eyes without lenses served as controls and no diurnal variation of aberrations was found in these eyes during this study, suggesting that the robust treatment effect was valid in experimental eyes. Although the control cornea was slightly flatter (0.18 ± 0.05 D) than baseline after one night of sleep (consistent with previous experiments49,50), this corneal flattening disappeared by 3 hours of eyes being open. In addition, the profile of the change of corneal curvature in experimental and control eyes was significantly different, i.e., the central corneal steepening and paracentral flattening in experimental eyes, and no significant location effect in control eyes (Figs. 2 and 3), also indicating an effective corneal reshaping in experimental eyes.
Our result of no diurnal variation of HOAs in control eyes, especially coma and SA, is in accord with the report of Mierdel et al.,51 who demonstrated no change of Zernike coefficients during the day in 22 eyes, except for the coefficient z4 ±2 (quantifying secondary astigmatism 90/180). This lack of diurnal variation in aberrations reflected relatively stable corneal shape at different corneal locations in the control eyes over time (Fig. 2).
There are a number of issues that might warrant further investigation. An example of this is the large 95% confidence intervals in the experimental eyes (Figs. 4 to 8) suggesting high treatment variability. The cause of this needs to be determined to make the clinical outcome more predictable. For instance, lens decentration might lead the “knee” to touch the central cornea, resulting in a myopic-like correction, an opposite effect. In addition, only one night of lens wear might be a source of transient variability. For example, as in myopic corneal reshaping,52 central topographic irregularities that appear as “central islands” and that occur in the earlier stage of treatment might resolve over time. A second issue is that the majority of subjects in this study were myopes, whose corneal shapes perhaps are not exactly the same as those of hyperopes,53,54 and who, theoretically, may have subtly different ocular structure. Therefore, further work is needed to clarify the treatment effect on hyperopes. A third issue relates to long-term effects beyond a single night that need to be investigated. Finally, the safety of these lenses should be evaluated in a much lager clinical trial.
After one night of CRTH® lens wear, CRT® steepens the central cornea and flattens the paracentral region, altering the ametropia by inducing a myopic shift. It therefore appears to be effective for correcting hyperopia. HOAs increased in predictable ways and the optical effects did not return to baseline by 12 hours, but did so by 28 hours. No significant diurnal variation in optical performance was found in control eyes.
This work was supported by Paragon Vision Sciences and Canadian Optometric Education Trust Fund. Lu is a recipient of the Ontario Graduate Scholarship, Ontario, Canada.
This study was presented in part at the annual meeting of American Academy of Optometry, December 2004, Tampa, FL.
None of the authors of this study have any financial or other interests/arrangements with the products/companies mentioned in the manuscript.
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