This prospective study was conducted from April 2017 to December 2018 at the Optometry Center of Peking University People's Hospital. Informed consent was obtained from each participant or their guardians before their participation in this study.
The inclusion criteria were as follows: age between 8 and 18 years, myopic spherical refractive error between −1.0 and −4.0 D, refractive astigmatism up to −1.5 D, visual acuity correctable to 20/20 or better, and no contact lens-related contraindications. None of the participants had previously received orthokeratology treatment. All procedures performed in this study were in accordance with the ethical standards of the Institutional Research Ethics Committee of Peking University People’s Hospital and with the 1964 Helsinki declaration and its later amendments.
In all, 35 participants were included in this study. They underwent orthokeratology treatment for one year and participated in regular follow-up visits.
A corneal topographer (Sirius, CSO, Italy) was utilized to capture the corneal topography at baseline and during all measurement sessions for all participants. Three repeated measurements were carried out, and the best-focused image (with an accuracy over 95%) was used. Several pretreatment corneal parameters were directly obtained from the topography profile, including the flat and steep K readings, corneal astigmatism, corneal asphericity (Q-value) values (including the mean and four quadrant values), and horizontal visible iris diameter.
Pretreatment corneal elevation asymmetry vector——Calculated according to the anterior elevation map
The asymmetry of the cornea at the chord of 7.2 mm, which lies at the boundary between the reverse curve and the alignment curve, was considered to be important in lens centration (Figure. 1A).19 To calculate the overall sagittal height at this boundary corresponding to the cornea at all 256 semimeridians (Figure. 1B), we decomposed the sagittal height into Sagx and Sagy by a vector-based method (Sagx=Sag*cosα, Sagy=Sag*sinα) (Figure. 1C). Then, another customized MATLAB (version 7.0; MathWorks Corporation, Natick, Massachusetts, USA) program was used to calculate the sum of the sagittal height vectors in both the x and y directions. For lens centration, the sagittal height asymmetry represents the power to tilt, which is defined as the corneal elevation asymmetry vector. The two-dimensional corneal elevation asymmetry vectors were summed using the following equations:
where Sagi and αi are the sagittal height and angle at each semimeridian, respectively. Vx represents the corneal elevation asymmetry vector at the horizontal axis, and Vy represents the corneal elevation asymmetry vector at the vertical axis. The original point of this coordinate system is the corneal vertex. The calculation was conducted by two experienced, independent researchers.
Orthokeratology Lens Fitting
The orthokeratology contact lenses used in this study were four-zone reverse-geometry gas-permeable rigid contact lenses (Euclid, Herndon, VA, USA) composed of oprifocon A (Boston Equalens II; Bausch & Lomb, Rochester, NY) (DK: 85*10-11 [cm2/s] [mL O2/mL*mm Hg]). The lens was fitted using the conventional fitting method. The first trial lenses were determined by the same experienced clinician based on the horizontal visible iris diameter, the simulated corneal curvature of the flattest meridian, the corneal eccentricity and anterior corneal sagittal height difference over an 8-mm chord.
The lens fitting suitability was evaluated using static and dynamic corneal ﬂuorescein pattern and topography map collected after a 30-minute eye-closure trial. The desired lenses were ordered based on the fluorescein evaluation, the post-trial wear topography map and the over-refraction result.
After lens delivery, all participants were asked to wear their orthokeratology lenses for at least 8 hours every night during this study. The routine follow-up visits were scheduled at 1 day, 1 week, 1 month, 3 months, 6 months, 9 months and 12 months. Each visit included compliance with lens wearing and lens care procedures, unaided visual acuity measurements, corneal topography and slit-lamp examinations. Data at 1 week, 1 month, 3 months, 6 months, 9 months and 12 months was collected for analysis.
Location of Orthokeratology Lens Decentration
The results were analyzed by two experienced, independent observers. To determine the lens decentration from the corneal topography maps, the tangential curvature of the cornea of each participant before and after orthokeratology treatment at each visit was exported from the Sirius system as a 31*256 matrix, and a tangential power difference map was calculated by a customized MATLAB program. Then this program automatically selected the turning point from minus to plus on the power difference map. These 256 points composed the boundary of the treatment zone. Then, a least-squares method in the MATLAB program was used to determine the best-fit circle of these boundary points, and the center of the circle was identified as the treatment zone center (Figure. 1D).14
The vector from the center of the circle to the corneal vertex was defined as the Type I lens decentration (illustrated on both the x- and y-axes, with the original point of this coordinate system at the corneal vertex, where H represents the center of the treatment zone relative to the corneal vertex in the horizontal direction and V represents the center of the treatment zone relative to the corneal vertex in the vertical direction). The vector from the circle center to the pupil center was defined as the Type II lens decentration. The lens decentration magnitude was recorded as the absolute value, which could be calculated using the following equation.
The lens decentration vector could also represent the decentration direction, i.e., an H below 0 on the topography map depicted the lens decentered to the temporal side of the right eye, while an H above 0 indicated the lens decentered to the nasal side of the right eye, a V below 0 indicated the lens decentered to the inferior side, and a V above 0 indicated the lens decentered to the superior side.
Both the lens decentration angle and the corneal asymmetry vector angle were recorded using absolute degrees, which could be calculated using the following equation.
The 0° semimeridian starts from 3 o’clock on the topography map and moves counterclockwise to the 360° semimeridian (back to 3 o’clock).
The lens decentration data were analyzed using the SPSS software package (version 23.0; IBM Corporation, Armonk, NY). Data from the right eyes were used for analysis. The baseline corneal and lens decentration parameters at different visits were first tested for normality using the Shapiro-Wilk test. Differences in Type I and Type II lens decentration were compared using paired-sample t-tests. Lens decentration parameters over different visits were analyzed by a repeated-measures analysis of variance (RM-ANOVA), with post hoc tests. The baseline parameters, including the Vx, Vy, K readings, corneal astigmatism, Q values, horizontal visible iris diameter, and spherical equivalent refraction, were analyzed against the averaged lens decentration coordinates using linear regression analysis, respectively. The intraclass correlation coefficient (ICC), coefficient of repeatability (COR), and standard deviation (SD) were used to evaluate the repeatability and reproducibility of the experiment. 20,21 A critical p value of 0.05 was used to represent statistical significance.