Patients
Institutional review board approval was obtained for the project and this study followed the tenets of the Declaration of Helsinki. After a detailed explanation, informed consent was obtained from each patient prior to enrollment. Prospectively, 85 consecutive eyes of 63 patients having 2.2-mm coaxial microincision phacoemulsification with monofocal toric IOL (AcrySof Toric) implanted were enrolled between May 2018 and February 2019 at the Shanxi Eye Hospital (Taiyuan, Shanxi, China).
Inclusion criteria were a 3-month postoperative follow-up and no intraoperative or postoperative complications. All patients had cataracts with preexisting regular corneal astigmatism and wanted a toric IOL implantation; their cylindric values were between 0.75 D and 5.0 D. Exclusion criteria were pregnancy, lactation, irregular corneal astigmatism, diabetic retinopathy, iris neovascularization, congenital eye abnormalities, severe unstable tear film, retinal detachment, glaucoma, pseudoexfoliation syndrome, uveitis, long-term anti-inflammatory treatment, amblyopia, advanced age-related macular degeneration, previous ocular surgery, severe corneal and retinal disease, and history of eye trauma.
Preoperative Assessment
All patients had a full ophthalmologic examination including subjective refraction, uncorrected distance and best-corrected visual acuity measurements, a slit-lamp examination, Goldmann applanation tonometry, and fundoscopy in mydriasis. Ocular biometry was performed using a partial coherence interferometry device (IOL Master 500, Carl Zeiss Meditec AG). Corneal topography was measured using the Oculus Pentacam (Optikgeräte GmbH, Wetzlar, Germany) and iTrace Surgical Workstation. All measurements were acquired in automatic release mode for each eye before using any eye drops or performing other contact-based examinations. Eye alignment evaluations and measurements with good quality (graded as “ok”) obtained via Pentacam, were used in the final analysis. The participant was placed in front of the iTrace and his or her head was carefully aligned with the chin and forehead fixed with the help of an assistant. All measurements were performed in a semidark room with undilated pupils. A single experienced operator (JZ) performed all examinations.
The aberrometer iTrace was used for the wavefront analysis. It uses the ray-tracing principle, sequentially projecting 256 near-infrared laser beams into the eye in a specific scanning pattern; parameter detection takes less than 200 ms. Topographies were captured using the Placido based corneal topographer mounted on the same device. Corneal aberrations were calculated using anterior topography data; internal aberrations were calculated by subtracting the corneal wavefront aberrations from those of the entire eye measured by the ray-tracing aberrometer using the built-in program[7].
Intraocular Lenses
The Acrysof IQ toric SN6AT (Alcon Laboratories, Inc., Geneva, Switzerland) has an acrylic biconvex toric optic (overall diameter: 13 mm, optic diameter: 6 mm, refractive index: 1.55). The toric IOL has 2 marks at the edge of the optic. The anterior surface of the lens is aspheric and the toric component is on the posterior surface. The lens also filters ultraviolet and blue light and is available in 0.5 D increments from +6.0 D to +30.0 D and in 1.0 D increments from +31.0 D to +34.0 D. Lenses are available with a cylinder power of 1.0 D to 6.0 D at the IOL plane. The models SN6A-T3 (toricity 1.50 D) to SN6A-T9 (6.0 D) were implanted based on the WFK readings from the built-in iTrace toric calculator. The steep power and axis of the WFK astigmatism were calculated based on the best Zernike mathematical fit from all topological data within a 4 mm circle instead of simulating the keratometer using the iTrace topographer SimK, which uses only 4 points based on topography data on a 3 mm ring.
A clear corneal incision was made at 120° with an estimated surgically induced astigmatism (SIA) of 0.25 D created with a 2.2 mm keratome on all patients. The spherical power was calculated using biometry measurements obtained with the IOL Master 500 and calculated using the SRK/T formula. The goal in all patients was emmetropia.
Slit-lamp Marking
Before surgery, with the patient seated upright, the same experienced surgeon (SZ) marked the anterior stromal puncture inside the limbus of the operative eye with reference markings (e.g., 0°, 90°, and 180°) using a 26-gauge needle. Intraoperatively, a Mendez ring was used to localize the incision site and IOL placement axis. An anterior stromal puncture was made by a 26‑gauge needle with sterile blue ink on the tip to mark the actual IOL placement axis.
Surgical Technique
Preoperatively, patients were prescribed 0.1% pranoprofen (0.1% Niflan) and 0.5% levofloxacin eyedrops for the operative eye 4 times daily for 48 hours. The same experienced surgeon (SZ) performed all surgeries. A 2.2 mm primary 2-plane cataract incision and a 1.0 mm single-plane paracentesis were created. Phacoemulsification was performed using the Infiniti Vision System (Alcon Laboratories, Inc.). The folded IOLs were implanted into the capsular bag then aligned with the pre-marked axis.
Postoperative Assessment
Postoperative examinations were performed at 1 week, 1 month, and 3 months and included uncorrected and corrected distance visual acuity, intraocular pressure, subjective and objective (autorefractometry) refractions, slit-lamp evaluation, and corneal topography (Pentacam HR and iTrace). The IOL axis was assessed with toriCAM (Graham Barrett, AppStore, USA) at the slit-lamp following mydriasis. The toriCAM is a newly developed smartphone application, invented by Professor Graham Barrett, based on the accelerometer and gyroscope built into modern smartphones that can provide a photographic analysis to identify the actual IOL axis postoperatively.
Astigmatism Vector Analysis
Postoperative refractive cylinder (adjusted to the corneal plane) and preoperative corneal WFK astigmatism were assessed by vector analysis using the Alpins method (Assort software, Assort Pty Ltd.)[1,2]. This method allows for the evaluation of the effective astigmatism changes while considering astigmatic axis alterations. Therefore, 3 fundamental vectors and the relationship between them were examined as follows: the target-induced astigmatism vector (TIA), defined as the astigmatic change in magnitude and the axis the surgery was intended to induce; the surgically induced astigmatism vector (SIA), defined as the amount and axis of the astigmatism the surgery actually induced; and the difference vector (DV), defined as the vectorial “difference” between the TIA and the SIA vectors. This absolute measure of success has a preferable value of 0. Furthermore, relations between these 3 fundamental vectors were calculated. The correction index (CI), the ratio of SIA to TIA, is preferably 1.0. If the CI is greater than 1.0, overcorrection has occurred; if it is less than 1.0, undercorrection has occurred. The magnitude of error is the arithmetic difference between the SIA and TIA magnitudes. The magnitude of error is a positive value in overcorrection and a negative value in undercorrection. The angle of error is that between the SIA and TIA vectors; it is positive or negative depending on whether the achieved correction is counterclockwise or clockwise to the intended axis, respectively. The amount of corneal incision SIA was calculated using vector analysis based on the preoperative and postoperative iTrace topography simulated keratometry data.
Statistical Analysis
All data were collected in an Excel database (version 2019, Microsoft, Redmond, WA); statistical analyses were performed with SPSS for Windows (version 23, IBM, Armonk, NY, USA). Data normality was assessed via the Kolmogorov–Smirnov test. Descriptive statistics are presented as the mean ± standard deviation or as the median (range). The iTrace WFK astigmatism results were compared with other data measured by various devices using the Student’s paired t-test. IOL rotation results were analyzed by the multiple comparison test, that is, one-way analysis of variance (ANOVA). The Bonferroni correction was applied for multiple comparisons. Differences were considered statistically significant according to the Bonferroni-corrected significance level for each comparison. A p-value < 0.05 was considered significant; all statistical tests were 2-sided.