Effects of lighting conditions and accommodation on the three-dimensional position of Visian implantable collamer lens

Patients

The participants were fully informed of the details and potential risks of the procedure and provided written informed consent. This study adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee of the Eye & ENT Hospital, Fudan University (No. 2016038).

In this observational study, we recruited patients who underwent ICL V4c implantation at the Refractive Surgery Centre of Eye & ENT Hospital. The inclusion criteria were as follows: patients aged 20–42 years, those with a stable refractive error (≤ 0.50 D change per year in refractive error for the past two years), minimum anterior chamber depth (ACD) of 2.8 mm, minimum endothelial cell density (ECD) of 2000 cells/mm2, and no contact lens use for at least two weeks. The exclusion criteria were as follows: patients with comorbid eye disorders, suspicion of keratoconus and presence of comorbid systemic diseases.

Visian implantable collamer lens

The power calculation for the ICL V4c (STAAR Surgical, Nidau, Switzerland) was performed using a modified vertex formula based on the preoperative refractive parameters, according to the manufacturer’s instructions. The size of the implanted ICL V4c was determined from the white-to-white and ACD both obtained using the Pentacam (Pentacam HR, Oculus Optikgeräte GmbH, Wetzlar, Germany) and the recommended size was automatically calculated using the formula provided in the producer’s website (STAAR Surgical, https://ocos.staarag.ch/).

Surgical procedure

An experienced surgeon (XZ) performed all ICL V4c implantations, as described previously [14]. Briefly, the pupils were dilated preoperatively. A mark was made to indicate the horizontal axis on the limbus to allow toric ICL (TICL) implantation. The ICL was implanted through a 3.0 mm temporal corneal incision with an injector cartridge. A moderate viscoelastic surgical agent (1% sodium hyaluronate) was injected into the anterior chamber, and the ICL V4c was positioned in the posterior chamber. The viscoelastic surgical agent was completely washed out with a balanced salt solution, and a miotic agent was instilled. Postoperative medications included antibiotic, non-steroidal anti-inflammatory, steroidal, and artificial eye drops. Sixty-two eyes were implanted with ICL, including 34 eyes implanted with TICL, which were placed horizontally with a rotation no more than 10°.

Follow-up examination

All patients underwent preoperative and postoperative ocular examinations. The following main parameters were evaluated: uncorrected and corrected distance visual acuity (UDVA and CDVA, respectively), subjective manifest refraction, intraocular pressure (IOP; Canon, Kanagawa, Japan), corneal topography (Pentacam HR, Oculus Optikgeräte GmbH, Wetzlar, Germany), vault (distance between the posterior ICL surface and anterior crystalline lens surface; AS-OCT, CASIA2) [15], endothelial cell density (ECD; SP-2000P, Tokyo, Topcon Corporation, Japan), and UBM (Quantel Medical, Clermont-Ferrand, France; to assess and exclude abnormal structures of the anterior segment of the eye, such as crystalline lens subluxation, and etc.). Patients were followed up one year postoperatively.

Lighting conditions

Scotopic, mesopic, and photopic conditions measurements were obtained in a 3-lx (CL-200, KONICA MINOLTA, Tokyo, Japan) dark room (scotopic), with a background light environment of 80 lx (mesopic), and with a penlight, pointed at the temple level of the contralateral eye (photopic).

Accommodation

Before measurements, we provided + 2.0 D lens to relax the accommodation, gradually increased this to 0.0 D at intervals of − 1.0 D (we defined this non-accommodative stimulus condition as non-accommodation, which may have induced relative accommodation to a certain extent for near accommodation), and then gradually increased this to − 4.0 D (we defined this − 4.0 D accommodative stimulus condition as the accommodation without measuring the actual accommodation) [16, 17]. The patients were asked to specifically focus on the internal fixation target and keep clear of the visual target to achieve an accommodative state. The eye was defined as fully accommodated when the target could no longer be focused on clearly. If the patient was able to keep a clear focus on the visual target continuously under the − 4.0 D accommodative stimulus, images were collected when the pupil was constricted to its minimum. To avoid the induction of accommodation by objects around the target, we performed the measurements in a 3-lx dark room (scotopic) without occluding the fellow eye. Accordingly, we defined the scotopic condition, which was also the non-accommodative stimulus condition, as the baseline. Patients were instructed to avoided reading a book or use their cell phone for 3 h before the measurements [18].

AS-OCT measurements

The ocular anterior segment parameters were measured by a single ophthalmologist using AS-OCT. Furthermore, the ACD, ACD-ICL (distance from the corneal endothelium to anterior surface of the ICL), vault, pupil diameter, and the crystalline lens tilt value relative to the corneal topographic axis were measured automatically. During photopic, mesopic or accommodation conditions the images with a minimum pupil diameter were selected and during scotopic conditions (baseline) the images with a maximum pupil diameter were selected. Each patient was evaluated three times to exclude interference with image quality, such as eyelash occlusion and inadequate corneal limbus exposure, to achieve optimal image quality for accurate analysis. A total of 128 anterior segment tomographic images (from 0° to 179° with an average interval) of the entire circumference were obtained in 2.4 s. We chose 16 images that display patterns with different directions consisting of 0°, 11°, 23°, 34°, 45°, 56°, 68°, 79°, 90°, 101°, 113°, 124°, 135°, 146°, 158°, and 169° for analysis.

Tilt values

Images for tilt measurement of the crystalline lens were automatically captured by CASIA 2 (see Additional file 1: Supplementary Method). The measurements of tilt of the ICL V4c has been previously described by Niu et al. [13]. Briefly, the raw measurement images of AS-OCT were exported to MATLAB (R2018a, The MathWorks, Inc., Natick, MA, USA) with a purpose-designed program by Dr He. Four registration lines were manually adjusted to align the anterior and posterior corneal surfaces and ICL surfaces. The location of the marked points on the image were expressed in pixels (X and Y, per pixel equaled to 7.749 μm) relevant to the coordinate XY axis. The corneal topographic axis was defined as the connecting line of the fixation point of the machine and the corneal vertex, which was vertical to the X axis in each image. The tilt value of ICL was determined by averaging the degrees of rotation of the registration lines fitted to the anterior and posterior surfaces of the ICL in each image. The highest value of tilt within the 16 images represented the total tilt value. The horizontal and vertical tilt were the values on the 0- and the 90-degree images. Only absolute values of tilt were used for analysis.

Statistical analysis

Statistical analysis was performed using R 3.4.3 (http://www.R-project.org). Continuous and categorical variables are expressed as mean ± standard deviation (SD) and frequencies (percentages), respectively. Differences between categorical variables were assessed using the Chi-squared test. The generalized estimating equation model was used to analyze the change in lighting conditions and accommodation, adjusted for both eyes. Different measurement points in both eyes were used as repeated measures variables. Statistical significance was set at P < 0.05.

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