This observational study was conducted on patients who underwent EVO-ICL implantation from January 2016 to October 2016 in the Department of Eye & ENT Hospital of Fudan University. In accordance with the Declaration of Helsinki, all patients provided written informed consent after receiving detailed explanation of the risks and benefits of the study. The study was approved by the Ethics Committee of the Eye & ENT Hospital (EENT) Fudan University.
The study was conducted on 89 eyes from 47 patients (19 males and 28 females, mean age 29.6 ± 7.42 years) who were recruited. The inclusion criteria for patients who underwent ICL implantation were as follows: age between 20 and 45 years, stable refractive error (refractive error change of ≤0.50 D in the past 2 years), spherical refraction of over −3.00 D, astigmatism of up to −6.00 D, and no contact lens use for 2 weeks. Exclusion criteria were anterior chamber depth (ACD) < 2.80 mm, endothelial cell density (ECD) <2000 cells/mm2, a history of ocular surgery, any chronic systemic disease, inflammation or trauma, and a history of ocular conditions other than myopia with or without astigmatism (suspicion of cornea, cataract, keratectasia, glaucoma, macular degeneration, retinal detachment, or neuro-ophthalmic disease).
EVO-ICL corrects −0.50 D to −18.00D myopic spherical refraction and up to −6.00 D cylindrical refraction. The size of the EVO ICL was individually selected based on the horizontal white-to-white (WTW) distance by IOLmaster and the ACD by Pentacam camera system (Oculus, Germany) following the manufacturer's recommendations. We calculated the size on the website at http://ocos.staarag.ch. The software is version 4.08 for ICL V4c. Power calculation for the EVO-ICL was performed by the software provided by the manufacturer (STAAR Surgical) using a modified vertex formula.
Surgical procedure and follow-ups
Two experienced surgeons (XZ and XW) performed all implantations. Chen X et al. have previously described the surgical procedure. Briefly, before surgery, the pupils were dilated. The anterior chamber was filled with sodium hyaluronate (ProVisc, Alcon), and an EVO-ICL was inserted through a 3.0 mm incision in the temporal corneal limbus using an injector cartridge (STAAR Surgical Co), then positioned in the posterior chamber. The remaining viscoelastic surgical agent was removed using balanced salt solution, before a miotic agent was instilled into the anterior chamber. Postoperative medications included 0.3% tobramycin, 0.1% fluorometholone, non-steroidal anti-inflammatory (NSAID) and artificial tears eye drops.
We determined manifest refraction (spherical equivalent, SE), uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), IOP (Canon, Japan), endothelial cell density (ECD, by SP-2000P, Topcon Corporation, Japan), standard slit-lamp biomicroscopic and funduscopic examinations preoperatively and all follow-ups. Anterior chamber depth (ACD) was measured from the corneal endothelium to the anterior lens using a Pentacam camera system and horizontal corneal diameter (white-to-white, WTW) using IOLMaster before surgery. Central corneal thickness (Pentacam), axial length (IOLMaster Carl Zeiss, Germany) and ultrasound biomicroscopy (UBM; Quantel Medical, France) were performed preoperatively.
Under the same illumination conditions of 330 lux, which was monitored using a photometer (Digitales Luxmeter, Sensorshop24@, Germany), all images were collected in the same room at a 10x magnification using a slit lamp anterior segment photographic system (YZ5FI Slit Lamp + SLICPS 2). Patients sat comfortably with their eyes facing straight ahead. The diffused light from the slit lamp was illuminated 60° from the temporal side. Using Visio image analysis software (version 2013), the size of the center hole was fixed to 360 μm (7.5 pixels) in both the horizontal (X-axis) and vertical (Y-axis) axis, and the image size was then adjusted accordingly. The average horizontal and vertical relative corneal diameters were 214.23 ± 14.66 pixels and 201.78 ± 12.39 pixels, respectively, and the average horizontal and vertical pupil diameters were 52.87 ± 11.95 pixels and 50.58 ± 11.65 pixels, respectively. Images confirmed that both the cornea and the pupil were elliptical (Fig. 1). The actual magnification of the image was calculated according to the following equation: WTW (mm)/relative horizontal diameter of the cornea (mm). Similar to the method reported by Hoang et al., the central hole of EVO-ICL (CH), the corneal center (CC), and the pupil center (CP) were defined as the intersection between the longest horizontal and vertical diameters of the respective ellipse. Each measurement was performed three times by the same physician, and the results were averaged.
In this study, we set the CC of the fitted corneal image as the reference point (0,0) and the position of CH and CP as (X, Y) using the following formula: D = (X2 + Y2)1/2 and calculated the relative distances from the center hole to the corneal center, D(H-C); the pupil center to the corneal center, D(P-C); and the center hole to the pupil center D(H-P) (Fig. 2). According to the equation: actual distance Ð = relative distance D * WTW (mm)/relative horizontal diameter of the cornea (mm), the actual distances of Ð(H-C), Ð(P-C) and Ð(H-P) were calculated respectively. In all images, the X-axis value of the left eye was converted to a negative value, which was converted to the same nasal and temporal direction as the right eye. The X-axis (ÐHx and ÐPx) and the Y-axis (ÐHy and ÐPy) on Ð(H-C) and Ð(P-C) are shown in Fig. 2. The X-axis (ÐHPx) and the Y-axis (ÐHPy) on Ð(H-P) were defined as the X-axis and the Y-axis of the vector of D(H-P).
Pupil offset was used as the estimate of the Kappa angle and defined as the vector between the corneal vertex and the pupil center in the corneal plane. Pupil offset was measured using Pentacam. The x-offset (K-x) and y-offset (K-y) of the vector were recorded respectively. A positive value is defined as the pupil center nasal to the pachy apex (x-offset) and as the pupil center upper to the pachy apex (y-offset).
All statistical analyses were performed using Stata statistical software, version 14.1 (Stata Corporation, College Station, TX, USA). The Wilcoxon Signed-rank test was used for statistical analysis to compare Ð(H-C) and Ð(P-C), ÐHx and ÐPx, ÐHy, and ÐPy. Spearman’s correlation analysis was used to assess the correlation between Ð(H-C) and Ð(P-C), ÐHx and ÐPx, and ÐHy and ÐPy, K-x (K-y) and ÐHx (ÐHy), K-x (K-y) and ÐHPx (ÐHPy). Results are expressed as mean ± standard deviation (SD), and statistical significance was set at p < 0.05.