Optometry & Vision Science:
Corneal Shape and Optical Performance After One Night of Corneal Refractive Therapy for Hyperopia
LU, FENGHE MD, PhD; SORBARA, LUIGINA OD, MSc, FAAO; SIMPSON, TREFFORD Ph,D; FONN, DESMOND MOptom, FAAO
CIBA Vision® Corporation, Atlanta, Georgia (FL) and Centre for Contact Lens Research, School of Optometry, University of Waterloo, Waterloo, Ontario, Canada (LS, TS, DF)
Received March 10, 2006; accepted August 1, 2006.
Purpose. To investigate the corneal shape and optical performance following one night of Corneal Refractive Therapy for hyperopia (CRTH®).
Methods. Twenty subjects (spherical equivalent: −2.14 ± 2.54 D) were fit with a Paragon CRTH® lens (Dk = 100) on one eye randomly. The other eye served as the control. Aberrations, refractive error, and corneal topography at various locations along the horizontal meridian were measured at baseline prior to lens insertion, and immediately after lens removal and at 1, 3, 6, 12, and 28 hours later. Root mean square wavefront errors were measured using a 4.5 mm pupil size.
Results. After one night of CRTH® lens wear, the central cornea steepened and paracentral region flattened in the experimental eyes (p < 0.001), whereas no significant location effect was found in the control eyes (p = 0.139). Refractive error (mean ± SE) changed by 1.23 ± 0.21 D (p < 0.001). The defocus increased by 0.58 ± 0.09 μm (p < 0.001). Higher-order aberrations, coma, and spherical aberrations increased by factors of 2.69, 2.58, and 4.07, respectively (all p < 0.001). Spherical aberrations shifted from positive to negative. Astigmatism did not change over time (p = 0.771). All parameters returned to baseline by 28 hours (all p ≥ 0.808). Aberrations and refractive error did not change in the control eyes (all p ≥ 0.082).
Conclusions. The CRTH® lens steepens the central cornea and flattens the paracentral region, which alters the ametropia by inducing a myopic shift. It appears to be effective for correcting hyperopia and also is reversible.
Corneal Refractive Therapy (CRT®), also known as orthokeratology or nonsurgical corneal reshaping, is used to correct refractive error by altering the corneal shape with rigid contact lenses. Recently, rapid improvement in technology and understanding of the modality has renewed clinical interests in orthokeratology.1–8 The advent of high-Dk (oxygen permeability) materials, reverse geometry multicurve lens designs, and novel corneal topographers partly account for this renewed interest in orthokeratology.9 It has been demonstrated by a number of groups that corneal reshaping can correct myopia by flattening the central cornea and steepening the mid-periphery.1–6,8,10 Corneal reshaping lenses may also correct hyperopia by steepening the central cornea and flattening the mid-periphery. In 1962, Jessen proposed that the techniques of “orthofocus” could reduce hyperopia by attempting to mold the cornea with a contact lens, which was fit steeper.11 Other attempts12,13 have been made to correct hyperopia without clear conclusions, but recently, Swarbrick et al.14 reported that steeply fitted rigid contact lenses could induce corneal steepening and myopic shifts in refraction over a 4-hour period.
Optical quality is perhaps most sensitively measured by wavefront sensors that quantify ocular aberrations. Although the optical performance after corneal refractive surgery15–20and after myopic nonsurgical corneal reshaping21–23 has been monitored, the change in optical characteristics over time after overnight hyperopic corneal reshaping has not yet been determined. In this study, therefore, we investigated the dynamic variation of corneal shape and optical performance after one night of Corneal Refractive Therapy for hyperopia (CRTH®). The diurnal variation of the optical performance in the control eyes without lens wear was also determined.
MATERIALS AND METHODS
Twenty ametropes participated in this study after a screening appointment for eligibility (15 women and 5 men; mean age, 30.1 ± 7.5 years; range, 22 to 48). All subjects were free of any ocular and systemic diseases, with no history of contact lens wear. Spherical ametropia ranged from +1.25 to −7.00 D, and the cylinder was −0.25 to −1.50 D (the numbers of hyperopic, emmetropic, and myopic participants were 4, 2, and 14, respectively).
Informed consent was obtained from all participants before enrolment in the study. This work received approval from 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.
Lens Characteristics and Fitting
The rigid gas permeable material used for CRTH® lens was Paragon HDS 100 (fluorosilicone acrylate). A summary of the lens characteristics and parameters is given in Table 1.
CRTH® lens fit was based on that for myopia using the Paragon slide rule. The flat K reading and a refraction of –0.25 D (the minimum value on the slide rule) were used to calculate the base curve, return zone depth, and landing zone angle. The selected base curve was 0.7 mm steeper than the flat K, the depth of mid-peripheral return zone was 175 μm deeper than the calculated return zone depth (because of the small back optic zone diameter used for hyperopia, i.e., 5 mm, rather than 6 mm in the trial lens to counter the former looser lens/corneal fitting relationship), and the landing zone angle was kept the same as the myopia fit. The lenses were adjusted to achieve centration, appropriate apical clearance (2 to 4 mm wide), paracentral touch (“knee”), proper mid-peripheral pooling, peripheral alignment and edge clearance. Once an acceptable fit (Fig. 1) was obtained with the trial lenses, they were ordered for each subject.
This was a single masked randomized control study. Paragon CRTH® lenses (Paragon Vision Sciences, Mesa, AZ) were fit on one eye of 20 ametropes (eyes randomly selected). The other eye served as the control. A designated investigator (L.S.) fit the lens, and the examiner (F.L.) was masked as to which was the experimental eye. During the study visit, a technician placed the lens on the eye and removed the lens in the morning in the lab.
Corneal topography, root mean squared wavefront errors, and refractive error were measured at baseline, the night prior to lens insertion. After the lenses were correctly positioned on the eye, participants retired in the laboratory at about 10 p.m. and were awakened at 7 a.m. the next morning. These measurements were repeated immediately after lens removal, and 1, 3, 6, 12, and 28 hours later.
Corneal topography (Atlas Mastervue, Humphrey Zeiss Instruments, San Leandro, CA) was used to quantify the anterior corneal curvature. Topographical data were collected over an 8-mm chord in 1-mm steps using the tangential power map.
A Shack-Hartmann wavefront sensor (LADARWave™ CustomCornea Wavefront System, Alcon Laboratories, Inc., Orlando, FL) was used to quantify aberrations (using OSA standards). Calibration was performed daily using a test eye provided by the manufacturer. The center of the pupil served as the alignment target, i.e., 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.24 Five measurements were acquired and the three most similar wavefront shapes were used to generate a composite result. The root mean squared wavefront error (μm) was used to quantify optical quality. The measurements were taken through undilated pupils, and data were collected using 4.5-mm pupils.
Lower-order aberrations include defocus (z20) and astigmatism (z2±2), which are part of the Shack-Hartmann output that make up the clinical spherocylindrical refractive error. The total amount of the higher-order aberrations (HOAs, including the sum of the third- to sixth-order Zernike coefficients), third-order coma (z3±1) and fourth-order spherical aberration (z40, SA) were analyzed.25
Refractive error was measured with a Nikon autorefractor and autokeratometer (NRK-8000, Japan).
Two- or three-way repeated measurement analysis of variance (RE-ANOVA) was used for overall effects and Tukey Honestly Significantly Different post hoc tests was used to determine the difference over time in aberrations, refractive error, and corneal curvature. Planned paired t tests were used to determine the difference in baseline aberrations and refractive error between experimental and control eyes and to determine the difference in aberrations, refractive error, and corneal curvature after CRTH® lens wear relative to baseline. Polynomial regression was used to quantify the change of the horizontal corneal curvature from baseline immediately after the lens removal. Pearson correlation was used to determine the association between the changes of signed SA and refractive error. Significant difference was set at p < 0.05. Data analysis was conducted using STATISTICA 6.0 (StatSoft, Inc., Tulsa, OK), and bivariate regressions26 were obtained using ProFit 5.01a (QuantumSoft, Zurich, Switzerland).
The refractive error distribution and corneal curvature (autokeratometry) are listed in Tables 2 and 3. There were no significant differences between the experimental and control eyes (t test, all p ≥ 0.111).
There was no difference in corneal curvature between experimental and control eyes at baseline (RE-ANOVA, F(1,19) = 0.101, p = 0.754). After one night of CRTH® lens wear, the central cornea steepened by 0.85 ± 0.15 D (mean ± SE) and the paracentral region flattened by 1.34 ± 0.19 D from baseline (averaged temporal and nasal sides) in experimental eyes (RE-ANOVA, F(8,152) = 6.823, p < 0.001) (Fig. 2). The central corneal steepening and paracentral corneal flattening regressed over time (RE-ANOVA, F(5,95) = 16.63 to 20.24, both p ≤ 0.001) and recovered by 28 hours (post hoc test, both p ≥ 0.830). No significant location effect was found in control eyes (RE-ANOVA, F(8,152) = 1.568, p = 0.139). However, the control cornea was flatter than baseline immediately after eye opening (0.18 ± 0.05 D vs. 0.38 ± 0.10 D for the center and paracentral region) and at 1 hour (t test, all p ≤ 0.031), but this flattening disappeared by 3 hours (t test, all p ≥ 0.130).
The profile of the change of horizontal corneal curvature in experimental and control eyes was different immediately after the lens removal (Fig. 3). To characterize these differences, polynomial regression was used to quantify the change of corneal curvature from baseline versus corneal locations. Polynomial analysis showed that only a parabolic component in control eyes was different from zero (p < 0.001). However, in experimental eyes, there were linear, quadratic, cubic, quartic, and quintic components (all p ≤ 0.005). The fitting functions and correlation coefficients are YEXP = 0.651 + (−0.885)x + (−0.407)x2 + (0.202)x3 + (0.024)x4 + (−0.009)x5, r = 0.99; YCON = (−0.136) + (−0.038)x2, r = 0.98.
Refractive Error (Spherical Equivalent)
There was no significant difference in refractive error between experimental and control eyes at baseline (paired t test, t(df = 19) = 1.257, p = 0.224). After one night of CRTH® lens wear, refractive error (mean ± SE) changed by 1.23 ± 0.21 D (from −2.14 ± 0.57 D to −3.38 ± 0.60 D, t(df = 19) = 5.975, p < 0.001). It regressed over time (RE-ANOVA, F(6,114) = 26.893, p < 0.001) and returned to baseline by 28 hours (post hoc test, p = 0.458). However, it did not change in control eyes (from −2.28 ± 0.60 D to −2.26 ± 0.60 D, t(df = 19) = 0.167, p = 0.869) (Fig. 4).
There were no significant differences between experimental and control eyes in defocus, astigmatism, HOAs, coma and SA at baseline (paired t test, all p ≥ 0.323). After one night of CRTH® lens wear, defocus (mean ± SE) increased by 0.58 ± 0.09 μm (from 1.76 ± 0.37 μm to 2.34 ± 0.37 μm, t(df = 19) = −6.830, p < 0.001) (Fig. 5). No difference was found in astigmatism between eyes (RE-ANOVA, F(1,19) < 0.001, p = 0.993), and astigmatism did not change over time for either eye (F(6,114) = 0.547, p = 0.771). HOAs, coma, and SA increased by factors of 2.69 (from 0.17 ± 0.02 μm to 0.46 ± 0.04 μm) (Fig. 6), 2.58 (from 0.10 ± 0.02 μm to 0.27 ± 0.03 μm) (Fig. 7), and 4.07 (from 0.05 ± 0.01 μm to 0.20 ± 0.03 μm), respectively (t(df = 19) = −6.879 to −4.440, all p < 0.001). Signed SA shifted from positive to negative (from 0.04 ± 0.01 μm to −0.11 ± 0.05 μm, t(df = 19) = 2.837, p = 0.011) (Fig. 8). All parameters [except SA (post hoc test, p = 0.390)] (Fig. 8) had not returned to baseline by 12 hours in experimental eyes (post hoc test, all p ≤ 0.009), but did so by 28 hours (post hoc test, all p ≥ 0.808, Figs. 5 to 7). Aberrations did not change in control eyes (RE-ANOVA, F(6,114) = 0.370 to 1.927, all p ≥ 0.082) (Figs. 5 to 8).
Association of Changes of Signed Spherical Aberration and Refractive Error
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.
Centre for Contact Lens Research
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Optometry & Vision ScienceErratumOptometry & Vision Science
contact lens; hyperopia; Corneal Refractive Therapy; orthokeratology; corneal topography; aberrations
© 2007 American Academy of Optometry
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we'll ask you to re-enter your password when you access these services.
What if I'm on a computer that I share with others?
If you're using a public computer or you share this computer with others, we recommend
that you uncheck the "Remember me" box.
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