Orthokeratology (ortho-k), also called corneal refractive therapy or corneal reshaping, is a nonsurgical method that temporarily reduces myopic refractive error and improves uncorrected vision by the programmed application of a specially designed rigid contact lens called reverse geometry lens.1,2 Swarbrick et al.1,3 demonstrated in their studies that refractive change occurs in a very short time and stabilizes after 7 to 10 nights of lens wear and that the change in refractive error in ortho-k mostly results from central thinning and midperipheral thickening in corneal epithelium.
Although overnight ortho-k improves daytime unaided visual acuity in a predictable manner, it inevitably increases higher-order aberration and decreases contrast sensitivity function (CSF).4–6 Changes in these indices have been suggested to be significantly affected by the amount of myopic correction and optical zone size. The most frequent visual problems associated with ortho-k resemble those of corneal refractive surgeries, which are glare and halo, both compromising visual quality and in worst cases, leading to failure of the treatment.
In corneal refractive surgeries like photorefractive keratectomy and laser-assisted in situ keratomileusis, even with the presence of pupil tracking, a slightly decentered laser ablation from the center of entrance pupil would reduce optical quality.7–9 In small incision lenticule extraction (SMILE) surgeries, where pupil tracking is absent, decentration of laser ablation is more likely and is associated with an increase in coma.10,11 In contrast to corneal refractive surgeries, the position of treatment zone (TxZ) of ortho-k is subject to the corneal shape, lid tension, lens fitting, and the like. The reported amount of decentration associated with ortho-k treatment is larger than that of corneal refractive surgeries.7,10–13 The question whether clinically acceptable lens decentration is associated with a compromise in optical and visual quality remains open.
Therefore, we performed this prospective study to investigate the correlation between the amount of lens decentration, the size of TxZ, and changes in optical and visual quality in subjects undergoing ortho-k treatment. We used CSF as the main indicator of visual performance.12 We also monitored modulation transfer function (MTF) and intraocular scattering using a relatively new device, the optical quality analysis system (OQAS) II, to better understand the effect of ortho-k treatment on the real retinal image quality.
This self-controlled study was conducted at the Fudan University Eye and ENT Hospital (Shanghai, China) between July 2015 and September 2015. This study adhered to the tenets of the Declaration of Helsinki and was approved by the Institutional Ethical Committee Review Board of the Fudan University Eye and ENT Hospital.
Twenty-seven eyes of 27 subjects were enrolled. The inclusion criteria were age between 9 and 14 years, myopic spherical refractive error between −0.75 diopters (D) and −5.50 D, refractive astigmatism up to −1.50 D, visual acuity correctable to 20/20 or better, no active inflammatory or ocular surface diseases, and no strabismus at near or distance. None of the subjects had previously undergone ortho-k treatment. Detailed explanation of all possible risks was made to subjects' parents, and they signed the written informed consents before the study.
The ortho-k contact lenses used in this study were spherical four-zone reverse geometry gas-permeable rigid contact lenses (Alpha, Nagoya, Japan) composed of fluorosilicone acrylate (Boston EM; Alpha, Japan), with an oxygen permeability (Dk) of 104 × 10−11 (cm2/s) (mL O2/mL·mm Hg). The lens has a back optical zone diameter of 6 mm, a reverse curve of 0.6 mm width, an alignment curve of 1.3 mm width, and a peripheral curve of 0.4 mm width. The total diameter of the lens is 10.6 mm, and the central thickness was 0.22 mm.
Lens fitting was performed following the manufacturer's fitting guidelines. In brief, for the first trial lens selection, noncycloplegic manifest refraction, horizontal visible iris diameter (IOL Master; Carl Zeiss, Wetzlar, Germany), corneal flat-K (keratometric value along the flattest meridian restricted to a 3-mm-diameter ring, ARK-510A; NIDEK, Japan), and corneal eccentricity (over a 10-mm chord diameter; Pentacam) were collected to determine the back optical zone radius and the alignment curve radius.
Lens fitting evaluations using fluorescein were performed 1 and 2 hours after lens insertion. To achieve a good fitting, the ortho-k lens should be well centered on the cornea (a slight decentration without lens edge going beyond the limbus is acceptable) and move approximately 1 mm on a blink. The overall fluorescein pattern revealed a classic bull's-eye, with a central touch surrounded by a narrow and deep annulus of tears trapped in the reverse curve area. Overrefraction was performed before the final lenses were ordered. After lens delivery, subjects were provided with Boston rewetting drops (Bausch & Lomb, NY) for daily lens rewetting and lubricating, Boston Simplus multiaction solution (Bausch & Lomb) for daily lens cleaning, rinsing, and disinfecting, and Menicon Progent intensive cleaner (Menicon, Kasugai, Japan) for lens cleaning every 2 weeks. Manuals on lens wear and care procedures were given to the parents. Subjects were required to wear ortho-k lenses for at least 8 hours every night during this 1-month study.
Subjects were instructed to return for study visits 1 day, 1 week, and 1 month after lens delivery. At all follow-up visits, subjects were requested to register between 9 and 11 AM, approximately 2 to 4 hours after lens removal. Parents were requested to log their children's wearing schedule and provide this log to the researchers at every face-to-face follow-up visit. Subjects were reevaluated for compliance, visual acuity, corneal topography, and ocular health during every visit.
Corneal topography, CSF, MTFcutoff, Strehl ratio (SR), and objective scattering index (OSI) were measured before and 1 month after ortho-k treatment.
Corneal topography was measured with the Pentacam (Oculus, Wetzlar, Germany) at the time of enrollment and 1 month after ortho-k treatment. During topography measurement, the Pentacam uses a blue Light Emitting Diode (LED) slit beam to illuminate a section of the cornea and anterior chamber, and the subject was instructed to fixate on a target in the center of the slit beam. The Pentacam detects the most anterior point of the cornea—the corneal apex and uses this point as the center of the Scheimpflug camera's rotation point and the center of the topography map. It is reasonable to compare curvature measurements taken before and after procedures that reshape the corneal surface as long as the subject fixated on the same target on both scans. The actual shift in corneal apex after ortho-k treatment in our previous study was less than 0.1 mm on average,14 which had a slight effect on comparisons between the pretreatment and post–ortho-k topography maps.
A previous study has shown that corneal topography usually stabilizes within 1 month after the initiation of ortho-k lens wear,3 and thus the 1-month topographic outputs were taken as representative of the post–ortho-k topography in the current study. To optimize reliability of the Pentacam readings, we evaluated the quality of topography maps with the Pentacam software and only included those with an acceptable quality score as defined by the instrument software (no significant eye movement, blinking, etc) in the final analysis. A difference map was obtained by subtracting the post–ortho-k tangential curvature map from the pre–ortho-k tangential curvature map. Eight points were plotted surrounding the central flattened area on which the power is all zero on the difference map (Fig. 1). These points were loaded into the Matlab program to simulate the best fitting circle by a circle fit function using a program code in a numerical computing environment. The detail of the function is as follows:
The center of the circle was defined as the center of the TxZ, and its distance from the corneal apex was the amount of decentration of the ortho-k lens. The diameter of the circle was defined as the diameter of TxZ.
Contrast Sensitivity Function
Contrast sensitivity function was measured with CSV-1000E (VectorVision, OH) in photopic condition and mesopic condition with and without glare, respectively. The room illuminance at testing level was about 85 lux under photopic condition and was less than 5 lux under mesopic condition. Tests were performed monocularly with best spectacle correction at the testing distance of 2.5 m. We tested each spatial frequency twice, with one descending trial and then one ascending trial, then scored contrast sensitivity (CS) as the lowest contrast correctly detected twice in a row. This testing method has been proved to have a significant improvement in the test-retest reliability in children.15 Contrast sensitivity was tested at 4 spatial frequencies (3, 6, 12, 18 cyc/deg) with 8 levels of contrast at each spatial frequency. Contrast levels in logarithmic values range from 0.70 to 2.08, 0.91 to 2.29, 0.61 to 1.99, and 0.17 to 1.55 log units for 3, 6, 12, and 18 cyc/deg, respectively. Contrast levels diminish in a uniform logarithmic fashion in steps of 0.15 log units for contrast levels 3 through 8 and 0.17 log units for steps 1 through 3.16
Using these data, the area under the log CSF (AULCSF), under three ambient lighting conditions, was calculated, respectively, according to the method of Hiraoka et al.17 In brief, the AULCSF was determined as the integration of the fitted third-order polynomials of the log CS units between the fixed limits of 0.48 (corresponding to 3 cyc/deg) and 1.26 (18 cyc/deg) on the log spatial frequency scale.
Optical Quality Analysis System
The values of MTFcutoff, SR, and OSI were measured by the double-pass optical quality analysis system (OQASII; Visiometrics, Terrassa, Spain). In the double-pass system, different routes are used for the inlet of the initial laser beam and the outlet of the detection mirror to prevent confusion between detection beams and induction. Previous studies have demonstrated that OQAS achieves outstanding repeatability and reproducibility for objective measurements of optical quality in clinical setting.18 In this study, tests were performed monocularly under mesopic condition. Before ortho-k treatment, OQAS measurements were performed with best spectacle correction to eliminate the effect of lower-order aberrations on visual performance. After 1-month of ortho-k lens wear, uncorrected visual acuity achieved 20/20 or better in all of the subject eyes, and thus, they did not need additional correction. The optical quality parameters (MTFcutoff, SR, OSI) were recorded over a 4-mm pupil size.
The MTF is a function of spatial frequency, which displays the loss of contrast between the retinal image and the original scene. To avoid the interference of high-frequency noise produced by built-in cameras, the OQAS uses an MTF threshold value of 0.01, which corresponds to 1% contrast. Thus, the cutoff MTF refers to the frequency up to which the eye can image an object in the retina with 1% contrast. Under optimum conditions (low level of optical aberration and diffraction), the maximum spatial frequency a human eye can detect is close to 60 cyc/deg.19 A higher MTFcutoff value indicates a better optical quality.
The SR is defined as the ratio of the intensity at the peak of the image formed by an aberrated optical system to the intensity of an aberration-free system. The SR provides information on the relative impact of an increased intraocular scatter on the retinal image.20 Strehl ratio measured by OQAS is the ratio of the area under the MTF curve of the measured eye and that of the ideal eye. The SR value ranges from 0 to 1, and SR of 1 is related to a perfect optical system that is only limited by diffraction.21 A higher SR value indicates a better optical quality.
The OSI is an objective parameter that describes intraocular scattered light. In OQASII, OSI value is the ratio of the amount of light within an annular area of 12 and 20 minutes arc and that recorded within 1 minute arc of the central peak.22 An OSI below 1 is normal, an index value between 1 and 3 corresponds to early cataract, an index value between 3 and 7 corresponds to developed cataracts that indicate surgery, and an index value greater than 7 corresponds to severe cataracts.23,24 A lower OSI indicates a better optical quality.
Data were analyzed using SPSS (version 19.0; IBM, NY). Refractive sphere, astigmatism, AULCSF, MTFcutoff, SR, and OSI before and after ortho-k treatment were compared using paired-samples t test. The relationship between refractive sphere and astigmatism and between diameter and decentration of TxZ were analyzed using Pearson correlation test. Refractive sphere, astigmatism, and diameter and decentration of TxZ were tested against the change in photopic AULCSF, MTFcutoff, SR, and OSI after ortho-k treatment using stepwise multiple linear regression models. The F probability test with P values of 0.05 and 0.1 was set for selecting each variable's enter and exit criteria in the model, respectively (in collinearity diagnostic tests; all Variance Inflation Factors (VIF) were <10, which indicated no multiple collinearities). Only data from the right eyes of the subjects were analyzed. P<0.05 was considered as statistically significant.
All the 27 subjects (27 eyes) completed this 1-month study. There were 14 male and 13 female subjects, and the mean age was 11.6±1.6 years (range, 9–14 years). The baseline spherical refractive error was −3.23±1.30 D (range, −0.75 to −5.50 D), and refractive astigmatism was −0.51±0.42 D (range, 0 to −1.25 D). After 1 month of ortho-k lens treatment, refractive astigmatism did not significantly change (−0.55±0.40 D; P=0.739), but the mean spherical refractive error significantly decreased to −0.17±0.30 D (t=−13.05; P<0.001). Uncorrected visual acuity achieved 20/20 or better in all the subject eyes after 1-month of ortho-k lens wear. The mean TxZ diameter was 3.61±0.26 mm; TxZ diameter was independent of baseline spherical and astigmatic refractive error (r=0.228, P=0.252 and r=0.238, P=0.231, respectively). The mean magnitude of decentration of the TxZ was 0.60±0.16 mm (range, 0.22–0.97 mm), mostly toward the temporal side (25/27) and the inferior side (19/27); TxZ decentration was independent of baseline spherical and astigmatic refractive error (r=0.009, P=0.964 and r=−0.066, P=0.742, respectively).
Contrast sensitivity function significantly decreased after 1-month ortho-k treatment under the three ambient lighting conditions being tested (photopic, mesopic, and mesopic with glare; all P<0.001). Detailed AULCSF values before and after ortho-k treatment were shown in Table 1. Stepwise multiple linear regression analysis, including baseline spherical and astigmatic refractive error, diameter and the magnitude of decentration of TxZ as potential predictive factors, showed that the decrease in AULCSF values after ortho-k significantly correlated with the amount of lens decentration (standardized β=0.425; P=0.027) but not with baseline spherical, astigmatic refractive error, or diameter of TxZ (all P>0.05).
For the data collected by OQAS, MTFcutoff values and SR significantly decreased from 45.68±5.99 to 39.09±5.75 (t=16.31; P<0.001) and from 0.27±0.05 to 0.21±0.04 (t=8.54; P<0.001), respectively, after 1 month of ortho-k lens wear. However, OSI significantly increased from 0.39±0.19 to 0.71±0.22 (t=−13.73; P<0.001). Stepwise multiple linear regression analysis showed that the decrease in MTFcutoff after ortho-k significantly correlated with the amount of lens decentration (standardized β=0.499; P=0.005) and the diameter of TxZ (standardized β=−0.494; P=0.005). The decrease in SR significantly correlated with the diameter of TxZ (standardized β=−0.434; P=0.024). The increase in OSI did not correlate with any of the potential predictive factors being tested (all P>0.05).
The influence of ortho-k on optical quality is one of the major concerns for this treatment modality. In this study, MTFcutoff, SR, and OSI were used to quantify the objective optical quality, and CSF to measure the subjective visual performance after ortho-k treatment. We found that all these parameters deteriorated after ortho-k treatment.
Contrast sensitivity function is a reliable measure of the real-world visual performance under varied circumstances and provides valuable information about visual function that may not be measurable by standard visual acuity testing. Previous studies showed controversial results as regard to the effect of ortho-k on CSF.12,25 To avoid multiple pairwise comparisons between pre–ortho-k and post–ortho-k CS measurements, we represented CS data as one number (AULCSF) to make statistical analysis more straightforward. Our study revealed a significant decrease in CS under various ambient lighting conditions after 1-month ortho-k lens wear. This finding agrees with that of the 3-month study by Hiraoka et al.12 but disagrees with that of the 1-year study by Stillitano et al.25; the discrepancies among these studies are likely to be attributable to the timing of observation because the latter study found a tendency for the higher-order aberration after ortho-k to recover over time. Longer follow-up periods are needed to validate the finding of this study.
A novel method of the current study is the use of the OQAS system to evaluate the optical quality after ortho-k treatment. This technique is noninvasive and takes into consideration the intraocular scattering in the optical quality assessment. We found that MTFcutoff values and SR significantly decreased and OSI significantly increased after ortho-k treatment. In recent years, the OQAS system has been used to assess the optical quality in several conditions, including toric intraocular lens implantation, forme fruste keratoconus eyes, dry eyes, and various corneal refractive surgeries; the parameters of MTFcutoff, SR, and OSI collected by this system were considered superior to traditional wave front aberration and CS measurements in many cases.26–32 However, a few studies have investigated the effects of ortho-k on OQAS measurements. The only relevant study available was published in a Chinese journal showing that MTFcutoff value decreased and OSI increased after 3 months of ortho-k lens wear to a greater degree than our study.33 The discrepancy between the 2 studies may be attributable to different baseline myopic refractive errors to be corrected (−3.23 D vs. −3.65 D), as higher myopic correction is usually associated with greater change in optical quality after ortho-k treatment.17 Surprisingly, the changes in visual and optical quality measures were not significantly correlated to the refractive target of ortho-k treatment, but rather to the size and dislocation of the resultant TxZ, meaning that the dimension of TxZ in ortho-k plays a more critical role than the amount of refractive correction does in visual performance.
The decentration of ortho-k lens is common because the temporal corneal region is, in most cases, steeper than the nasal one, and thus lens is not stabilized until it moves to the steeper direction.13 A tight upper lid tension, especially in Asians, may also contribute to the downward displacement of the lens during overnight treatment. A few studies have investigated the influence of ortho-k lens decentration on optical and visual quality. For example, Hiraoka et al.12 found a significant correlation between the change in wave front aberration and CSF and the amount of lens decentration by intentionally leaving the ortho-k lenses decentered to an average of 0.85 mm. Yang et al.13 showed that glare and halo was significantly more manifest in subjects with lens decentration greater than 0.50 mm. However, the MTFcutoff, SR, and OSI values after ortho-k in this study were still within the normal range as compared with the data collected from a healthy adult population.34,35 A better tear film quality in the younger subjects enrolled in our study is supposed to have minimized the deterioration of optical quality after ortho-k treatment.36 This also helps explain a paucity of visual disturbances in our study cohort.
The influence of optical zone size on optical quality has been extensively discussed in refractive surgeries.37–40 A larger ablation zone is usually associated with a better optical quality, yielding fewer visual disturbances. This relationship is more significant when the pupil size is larger than the ablation zone diameter.39 Because of the similarity in the pattern of refractive correction between refractive surgery and ortho-k, it is not surprising that the change in optical quality was affected by the TxZ size in this study because the average TxZ diameter was 3.61 mm, whereas the mesopic pupil size was larger than 4 mm among all the subjects. However, the real-time pupil size was not monitored either during CSF testing or OQAS testing. It is of knowledge that the increase in the positive spherical aberration of eye after ortho-k is a factor responsible for the decreased retinal optical quality, and the high levels of positive spherical aberration will lead to a degradation of lower spatial frequencies that is more significant under low illumination levels and it changes appreciably with the increase in pupil diameter.41 The interaction between dynamic pupil size, TxZ diameter, and visual quality warrants further investigation.
One of the limitations of this study is that we only monitored the optical quality parameters once after ortho-k lens wear. Studies should be performed to investigate the changes in these clinical parameters over a longer term. Sample size should be expanded to validate the correlation between lens decentration and the change in optical quality.
In conclusion, visual and optical quality decreases after ortho-k treatment, which can be alleviated by a larger TxZ diameter and a smaller lens decentration.
This study was supported by the National Natural Science Foundation of China (81570879), Outstanding Academic Leaders Program of Shanghai (XBR2013098), and Science Committee Nature Science Foundation of Tianjin (11JCYBJC26000).
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