This was a prospective, longitudinal study conducted at The Eye and ENT Hospital of Fudan University in Shanghai, China between February 2016 and December 2016. The inclusion criteria for subjects were as follows: 1) age between 8 and 40 years, 2) myopic spherical refractive error between -0.75 to -5.00 diopters of sphere (DS) and with-the-rule (WTR) astigmatic refractive error less than 1.50 DC, 3) corneal toricity less than 1.5 DC and corneal elevation difference along the 8 mm chord of the two respective principal meridians of corneal toricity (CED) less than 40μm, 4) radius of corneal curvature between 39.75 to 46.00 D (7.34 to 8.5 mm), 5) horizontal corneal diameter greater than 11.0 mm, 6) agreeable to wear OK lenses for more than 8 hours during sleep, 7) and willingness to participate in the clinical trial and provide signed written consent. The parents of subjects younger than 18 years old signed the written informed consents prior to enrollment into this study.
The exclusion criteria included a history of RGP contact lens wear or any current ocular or systemic disease. The research described in this study adhered to the tenets of the Declaration of Helsinki and was approved by the ethics committee of the Eye and ENT Hospital of Fudan University.
The OK lenses used for this study were of a spherical four-zone reverse-geometry design (Emerald series, Euclid, USA) made in a Boston XO material (Bausch + Lomb, USA). The lenses measured 10.6 to 10.8mm in overall diameter, with a back optic zone (BOZ) of 6mm in diameter and a central thickness of 0.22mm. The reverse curve was 0.5mm wide, the alignment curve was 1.2-1.4mm wide and the peripheral curve was 0.5mm wide.
Orthokeratology lenses were dispensed to be worn overnight and removed soon after eye opening in the morning. A good lens centration, as indicated by a bull’s eye pattern on corneal topography maps, was expected. Should significant lens decentration (greater than 1.0 mm) occur or the unaided visual acuity drop below 20/25 during follow-up visits, new lenses would be ordered until a good lens fit and centration were achieved and visual acuity restored to better than or equal to 20/25.
All subjects underwent a thorough contact lens follow-up examination including uncorrected and corrected distance visual acuities (UDVA and CDVA), objective and subjective refraction, corneal topography, optical coherence tomography (OCT) and slit lamp biomicroscopy. Measurements were conducted in the morning within one hour of lens removal. The patients were followed 1 day, 1 week, 1 month, 3 months and 6 months after commencement of OK lens wear. The baseline and 6 months’ post-OK lens wear measurement results were analyzed for this study.
A Pentacam analysis system (Oculus GmbH, Wetzlar, Germany) was used for measurements of corneal curvature, elevation, corneal toricity (CT) and thickness. Pentacam imaging of the cornea was performed by the same experienced examiner and three measurements were averaged for each result. Only the scans marked “OK” by the instrument were saved and analyzed. Corneal power was measured and presented in the power distribution display and corneal powers from SimK which were derived from the axial curvature map were used in the current study. The steep (Ks) and flat (Kf) keratometry values and their axes were displayed corresponding to diameter, with diameters ranging from 1.0 to 8.0 mm centered on the corneal apex. The central CT was defined as (Ks-Kf) at 3mm (1.5mm in radius) and the peripheral CT was defined as (Ks-Kf) at 6mm (3.0mm in radius). The pre-OK corneal elevation map (front) was used to analyze the corneal elevation difference (CED) along the 8 mm chord of the two respective principal meridians of corneal toricity. CED values were determined by subtracting the average height along the steep meridian from the average height along the flat meridian
Optical Coherence Tomography
An FD-OCT tomography setup (RTVue S, Optovue, Inc., CA) with a corneal adaptor module was used to measure the central corneal epithelial thickness of chords with a diameter of 6mm. A high magnification corneal lens adapter (CAM-L) with a pachymetry scan pattern (6mm scan diameter, 8 radials, 1024 axial scans each, repeated five times) was used for imaging the cornea. Subjects were asked to focus their vision on a provided fixation target. Images were acquired when the targeting windows demonstrated a specular reflex on the corneal apex, which indicated that the incident OCT beam was perpendicular to the corneal apex. Epithelial thickness was derived using an FD-OCT system calculating the distance between the air–tear interface (first curve) and the epithelium–Bowman’s layer boundary (second curve). Processing and verification of the OCT images were conducted as previously described by Li et al. OCT calibration was performed as previously described by Kim and Ehrmann. Repeatability of the OCT measurements was generally good, matching the findings demonstrated by Li et al. All examinations were performed by the same examiner.
A 6mm diameter epithelial thickness map was generated automatically. The observer placed a transparency with multiple concentric circles on top of the epithelial thickness map and situated the cursor on the individual point of interest (Figure 1). The epithelial thickness map was divided into a central circle and 5 concentric annuli with an interval of 0.5mm in radius (Figure 2). The two principle meridians of central corneal toricity were marked on the epithelial thickness map and each meridian was divided into 12 segments with an interval step of 0.5mm. Mean values of each segment were calculated using a custom-made software. The software collected several sample points from each segment and calculated the mean of these values after removing any outliers. The epithelial thickness along the steep (ETS) and flat (ETF) meridians were recorded and compared. The lens-induced changes in epithelial thickness (ET) were calculated as the difference (△ET) between the ET at 6 months post-OK lens wear and the baseline ET (6m-pre). △ET of the steep meridian (△ETS) was compared to △ET of the flat meridian (△ETF). The means of △ET at the same radius along the same meridian (△ETSm and △ETFm, Figure 2) was calculated; for example: △ETSm of 0.5mm = (△ET of S0.5mm+△ET of I0.5mm)/2; △ETFm of 0.5mm = (△ET of N0.5mm+△ET of T0.5mm)/2 (S: superior to apex, I: inferior to apex, N: nasal to apex, T: temporal to apex).
Results for the descriptive statistics are presented as the mean ± standard deviation (SD). Simple comparisons between pre- and post-wearing were performed using Paired-t-test and comparisons between steep and flat axes using the Student’s t-test. The correlations between the different variables were studied using Pearson’s correlation coefficients. Probability values <0.05 were considered statistically significant. Data analyses were performed using statistical analysis software (PASW 18.0, SPSS Inc., Chicago, IL).