Subjects
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 astigmatic refractive error less than 1.50 DC, 3) with-the-rule (WTR) corneal toricity less than 1.5 DC, 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.
Study Protocol
The OK lenses used for this study were of a spherical four-zonereverse-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 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 paper.
Corneal Topography
A Pentacam analysis system (OCULUS Optikgeräte GmbH, Germany) was used for measurements of the anterior radii of the corneal curvature, corneal elevation and corneal thickness. The steep (Ks) and flat (Kf) keratometry and their axis were recorded.
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 topography of chords with a diameter of 6mm(Φ6mm). 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[5]. 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.[13] OCT calibration was performed as previously described by Kim and Ehrmann.[14] Repeatability of the OCT measurements was generally good, matching the findings demonstrated by Li et al.[13] 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 steep (Ks) and flat (Kf) meridians of the cornea were determined according to the corneal topography maps. The epithelia along the steep and flat meridians were divided into 12 zones with an interval step of 0.5mm. Mean values of each zone calculated using custom-made software at the steep (ETS) and flat (ETF) meridians were recorded and compared. Each measurement was repeated 3 times and the mean was calculated. All of the data analysis was performed by one person who didn’t know the time point of the image. 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 of the cornea pre-OK lens wear (6m-pre). △ET of the steep meridian (△ETS) was compared to △ET of the flat meridian (△ETF). The means of △ET of chords of the same radius along individual meridians (△ETSm and △ETFm, Figure 2) were calculated and compared; for example: △ETSm of 1mm = (△ET of S0.5mm+△ET of I0.5mm)/2; △ETFm of 1mm = (△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).
Statistical analysis
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 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).