Patients inclusion and exclusion criteria
A total of 216 patients who received Orthokeratology treatment at Joint ShanTou International Eye Center of ShanTou University and Chinese University of HongKong (JSIEC) from July 2020 to March 2021 were retrospectively studied. The study design and flow chart were shown in Fig. 1. Eyes being recruited were 36 (22 patients) and 41 (24 patients) in poor fitting and good fitting group. All research was conducted by the Helsinki Declaration, and were authorized by the ethical and academic council of JSIEC.
Orthokeratology lens
Orthokeratology lens (Eyebright Medical Technology Co., Ltd, Beijing, China) is made of acrylate polymer. It has an oxygen permeability coefficient of 125×10− 11(cm2/s) (mlO2/mmHg). The lens has a total diameter of 10.2-11mm (0.2mm interval) and a central thickness of 0.15-0.3mm. It is divided into four zones. The base curve is spherical or aspherical, with a curvature radius of 7.22-9.12mm (0.05mm interval) and an optical zone diameter of 5.8-6.6mm. The reverse curve is non-concentric, with a width of 0.4-1.0mm. The alignment curve (AC) is aspherical or toric, its width is 0.4-1.0mm. The width of the peripheral curve is 0.5mm. The replacement period of lens is 1.5-2 years.
Lens fitting, monitoring and re-fitting
Patients were chosen based on indications and contraindications of Orthokeratology. A routine ophthalmic examination was carried out. Subjective refraction was tested by a refraction device (VT-10; Topcon, Tokyo, Japan). Corneal parameters were assessed by a Corneal topography (TMS-4; Tomey, Aichi, Japan). Ocular biological parameters were recorded by an optical coherence instrument (OA-2000; Tomey, Aichi, Japan). Corneal endothelium was evaluated by a specular microscopy (SW-7000; Tianjin Suowei Electronic Technology Co., Ltd, Tianjin, China).
Flat K, e value and height difference of 8mm chord were inputted into the manufacturer's software, the recommended AC was generated. If the height difference was greater than 30um, a Toric lens was recommended. T3, T5, T7, T9, and T11 corresponded to height differences of 30–45 um, 45–60 um, 60–75 um, 75–90 um, and greater than 90 um, respectively. Static and dynamic fluorescent imaging were assessed. Parameters were adjusted until a satisfactory fitting was achieved. Patients were instructed to close their eyes and lie down. After 30 minutes, refraction with lenses on the eyes was performed. The lenses were then removed, and the corneal topography was examined. A treatment area centered on the tangent map and a defocus ring resembling "a bull's eye's sign" were indicators of good fitting. Adjustment was suggested if there was a smiling face, crying face, or central island sign. The target reduction plus 0.75 to 1.25D was chosen as the base curve. The relative distance between the lens and the corneal limbus was used to estimate lens diameter. Finally, after signing an informed consent form, lenses were ordered.
Patients were re-examined one day, one week, one month, three months, and every three months after wearing the lens. Extra follow-up was necessary if the effect was not satisfactory or complications existed. To minimize daytime regression, all patients were visited within 2 hours of having their lenses removed. The following evaluations were given: (1) Symptoms of patients: blurred vision, ghosting or glare, secretions, redness, burning, foreign body sensation, and eye pain; (2) Uncorrected and corrected visual acuity; (3) Ocular surface health: tear break-up time, corneal staining (grades 0, 1, 2, 3, and 4) [8], conjunctival or corneal inflammation; (4) Lens fitting: width and boundary of each curve, lens centration, binding, corneal indentation, lens diameter, and fluorescence dynamics; (5) Corneal topography: whether the central flattening area was enough and the defocus ring was centered.
When a lens has been worn for more than one month, poor fitting should be considered if any of the following conditions existing: (1) Uncorrected vision ≤ 20/30 or residual myopia≤-0.50D; (2) Lens decentration greater than 1mm, or lens decentration of 0.5-1mm, accompanied by unsatisfactory uncorrected vision or corneal staining of grade 2 and above; (3) Central corneal staining of grade 2 and above, or peripheral corneal staining of grade 3 and above; (4) Lens binding of grade 2 and above, or lens indentation. If patient had any of the problems, we first re-examined patients' operation rules of the lens, and suggested them to instill artificial tears. If there was no improvement, the lens parameters were adjusted after a two-week washing period.
Data collection
Original records of patients were gathered in an EXCEL file: (1) Patients' name, gender, age, date of visit, and chosen eye (both eyes were chosen if they met the inclusion criteria); (2) Refractive results; (3) Topographic information: flat K, steep K, astigmatism, steep and flat e-value (Es, Em), height difference of 8mm chord length; (4) Lens parameters: type (spherical or toric, toric value), target reduction, AC, lens diameter; (5) Re-fitting data: time, reason, and lens parameter changes in poor fitting group; (6) Follow-up information: uncorrected and corrected vision, ocular surface health, lens evaluation, and corneal topography.
Statistical analysis
SPSS 21.0 was used for data analysis. The numerical data was recorded as mean ± stand deviation, and the variance homogeneity test was performed. The paired sample t-test was used to compare lens parameters before and after re-fitting in the poor fitting group. The differences between good and poor fitting groups were compared by independent sample t test and chi-square test. Logistic regression analysis was utilized to reveal risk factors of poor fitting. The diagnostic value of each risk factor was evaluated by receiver operating characteristic curve. When P < 0.05, the difference was considered as significant.