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 cataract patients 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, history of eye trauma and serious intraoperative complications.
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 .
AcrySof Toric IOL (Alcon Labs, Fort Worth, TX) is a one-piece hydrophobic acrylic lens. The optic is measured to be 6.0 mm and can be inserted through incision sizes of 2.2 mm. The lens 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. 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.
Before surgery, with the patient seated upright, the same experienced surgeon (SZ) marked the corneal epithelium 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. A long fine scratch was left on the corneal epithelium by a 26-gauge needle with sterile blue ink on the tip to mark the actual IOL placement axis.
Preoperatively, patients were prescribed 0.1% pranoprofen (0.1% Niflan) and 0.5% levofloxacin eyedrops for the operative eye 4 times daily for 48 h. 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. A continuous curvilinear capsulorhexis measuring approximately 5.5 mm in diameter was 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 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.
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]. The four main outcomes for Alpins analyses were target-induced astigmatism (TIA), surgically induced astigmatism (SIA), difference vector (DV), and correction indices (CI).
TIA was the intended magnitude and axis of astigmatic correction, where the magnitude was equivalent to preoperative corneal WFK astigmatism. SIA was defined as the actual magnitude and axis of astigmatism created during surgery. DV was the postoperative refractive cylinder (adjusted to the corneal plane). CI was defined as SIA/TIA, where values > 1 or < 1 represented overcorrection or undercorrection, respectively. The magnitude of error was the arithmetic difference between the SIA and TIA magnitudes. The magnitude of error was a positive value in overcorrection and a negative value in undercorrection. The angle of error was the axis angle difference between the SIA and TIA; it was positive or negative depending on whether the achieved correction was 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.
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.