To investigate the effect of surgically induced corneal astigmatism (SICA) and total corneal astigmatism (TCA) estimation on the anterior corneal astigmatism (ACA)-based toric intraocular lens (IOL) calculation.
Data from preoperative and postoperative corneal astigmatism, postoperative visual acuities, and refractive outcomes were collected. The incision was superior in with the rule anterior corneal astigmatism (WTRA) eyes and temporal in against the rule anterior corneal astigmatism eyes. The following five methods of calculating the toric IOL were compared: (1) ACA only and estimated SICA; (2) ACA with a fixed posterior corneal astigmatism (PCA) and estimated SICA; (3) ACA with a fixed PCA value and actual SICA; (4) and (5) TCA derived from the regression equations of ACA and actual SICA. The residual astigmatism was simulated. The Alpins method was used to analyze the astigmatism.
Sixty eyes from 46 patients were enrolled. Thirty eyes had WTRA and the other thirty had against the rule anterior corneal astigmatism. The vector and arithmetic means of the difference vector decreased when the information regarding the actual SICA and PCA was added to the calculation (from 0.59 diopters [D] @ 87.5° to 0.15 D @ 48.5°, and from 0.95 ± 0.53 to 0.71 ± 0.63 D, respectively;
Researchers may improve the accuracy of post-implantation predictions by calculating toric IOL using exact SICA and TCA, with consideration of the PCA derived from the regression equation of ACA.
The aim of modern refractive cataract surgery is not only to correct spherical errors but also to correct astigmatism. The prevalence of at least 1.0 diopter (D) pre-existing astigmatism is 36.2% to 45.0% [
The main variables in the calculation of toric power of the IOL include preexisting corneal astigmatism and surgically induced corneal astigmatism (SICA). Total corneal astigmatism (TCA) is the sum of anterior and posterior corneal astigmatism (ACA and PCA). In most cases, ACA plays a vital role, because its magnitude tends to be much larger than that of PCA [
The study protocol followed the tenets of the Declaration of Helsinki. This study was approved by our institutional review and ethical board (1603-031-747). We retrospectively reviewed the medical records of patients who met the following inclusion criteria: patients who had implanted Tecnis toric IOL (ZCT 150, ZCT 225, ZCT 300, and ZCT 400; Abbott Medical Optics, Santa Ana, CA, USA) from August, 2013, through September, 2015 at Seoul National University Hospital; 20 years or older; regular corneal astigmatism between 1.0 and 3.63 D; requirement of a spherical equivalent lens power between +6.0 and +28.0 D. Patients with any of the following characteristics were excluded: irregular astigmatism, amblyopia, keratoconus, previous refractive or keratoplasty surgery, advanced glaucoma, corneal scarring, uveitis, significant macular disease, postoperative corrected distance visual acuity <20 / 40 or zonule/pupil abnormalities. Patients were also excluded if there were intraoperative complications, including posterior capsular rupture, vitreous loss, and zonular damage. According to the keratometric data, corneal astigmatism was defined by the presence of with the rule (60°–120°) and ATR (0°–30° or 150°–180°). Therefore, we only included patients who had WTRA or ATRA in this study.
Preoperatively, patients underwent a complete ophthalmic evaluation. This included uncorrected distance visual acuity and corrected distance visual acuity measurements, slit lamp examination, tonometry, and fundoscopy. Preoperative corneal keratometry was performed using three different devices, as follows: (1) autokeratorefractometer (KR-8900; Topcon, Tokyo, Japan), (2) IOL Master (Carl Zeiss Meditec, Dublin, CA, USA), and (3) Orbscan IIz topography (Bausch & Lomb, Claremont, CA, USA). Each examination was performed in triplicate and the results were averaged.
The selection of the IOL cylinder was determined using the IOL manufacturer's web-based calculator (
We selected the toric IOL spherical power based on the IOL Master (Carl Zeiss Meditec). The target postoperative spherical equivalent was defined by the nearest negative emmetropic value or myopic value (range, −1.0 to −3.0 D) based on the patient's preoperative spherical equivalent refractive error and visual needs.
At one month postoperatively, manifest refraction was performed. We also measured corneal astigmatism at this time using an auto kerato refractometer. All refractive cylindrical errors were described as minus cylinder form. The refractive cylinder at the spectacle plane was converted into the corneal plane using a vertex distance of 12 mm for the astigmatism analysis.
The TECNIS toric IOLs are one-piece, aspheric, hydrophobic, foldable acrylic lenses. They have ultraviolet light absorbing clear IOLs [
The same experienced surgeon (MKK) performed all of the surgeries using topical and subtenon anesthesia. Preoperatively, limbal reference marks were made under the slit lamp at the 3- and 9-o'clock meridians using an insulin needle with the patient sitting upright. During surgery, the axis was marked using a Mendez ring (K3-7900; Katena, Denville, NJ, USA) and a toric axis marker (K3-7910; Katena) at the preplaced reference marks. A 2.75-mm self-sealing 3-step clear corneal incision was made using a disposable metal blade (Alcon Laboratories, Fort Worth, TX, USA) on the superior (90°) or temporal (0° or 180°) eye according to the steep corneal meridian. Phacoemulsification was performed using the Ozil Infiniti Vision System (Alcon Laboratories). After toric IOL insertion, the IOL was placed approximately 20° counterclockwise from the planned axis. After the viscoelastic material was removed, the IOL was rotated clockwise and aligned to the intended axis.
All data were compared among the following five models. The predicted residual astigmatism values in model 1, and in models 3 to 5, were re-calculated from model 2. The PCA and SICA values were adjusted to correspond to each model.
Model 1: TCA = preoperative keratometric astigmatism (PKA; no consideration of PCA) and an estimated SICA of −0.2 D
Model 2: TCA = PKA + PCA values of −0.3 D @ 90° and −0.2 D @ 90° for WTRA and ATRA, respectively; an estimated SICA of −0.2 D
Model 3: TCA = PKA + PCA values of −0.3 D @ 90° and −0.2 D @ 90° for WTRA and ATRA, respectively; the actual (not estimated) average SICA
Model 4: TCA = adjustment of PKA according to the Baylor nomogram, which is calculated using the published regression equations [
Model 5: TCA = PKA + a PCA that is estimated on the basis of the regression model reported by Eom et al. [
The results obtained 1 month after surgery were analyzed. TCA was used as the preoperative corneal astigmatism value. All calculations were performed using vector analysis [
Categorical variables were compared using the Fisher exact test or the chi-square exact test, as appropriate. For differences between continuous variables, the independent
The study enrolled a total of 60 eyes from 46 patients. Thirty eyes had WTRA, while the other 30 eyes had ATRA.
The accurate calculation of toric IOL implantation requires precise measurements. This study investigated how to improve the accuracy of toric IOL calculations, and particularly ACA-based calculations. The accuracy of the calculation improved when we considered the presumed effect of the PCA (estimated using regression models). It also improved when we used the mean of the actual SICA.
The real TCA can be calculated from 3-dimensional anterior segment optical coherence tomography [
In order to predict the PCA on the basis of the ACA, many studies have used the regression equation method [
The mean vector of the DV was lowest in model 5, although the arithmetic mean was not significantly different across models 3 through 5. In model 5, the CI was closest to 1.0 in the WTRA group. Taken together, the results indicate that model 5 may have yielded the best results, for several reasons. For instance, the regression used in model 5 is derived from a larger sample of subjects than was that in the other models. In addition, the regression equation used in model 5 was applied to the same ethnic group as in this study.
In this study, the mean age of the ATRA group was older than that of the WTRA group. This finding is in line with previous studies, which have shown that corneal astigmatism changes toward ATRA with age. PCA also changes to ATR with age, although this change is small and/or insignificant [
Tejedor and Perez-Rodriguez [
The DV and CI tended to be better when using the actual mean SICA (model 3) rather than the estimated SICA (model 2). This outcome agrees with that of a previous study, in which error was reduced when the IOL was recalculated using the actual mean SICA in subjects who had undergone toric IOL implantation [
In summary, the mean vector of the DV improved from 0.59 D @ 87.5° to 0.15 D @ 48.5° when using (1) the actual mean SICA, and (2) the TCA calculated by adding the PCA derived from the ACA regression equation. Our study confirms the results of previous studies, in which the toric IOL calculation (neglecting PCA) resulted in overcorrection and undercorrection in eyes with WTRA and ATRA, respectively. The actual mean SICA (calculated on the basis of real surgical results), and a TCA that accounts for PCA, would improve the accuracy of the toric IOL calculation. In order to predict PCA, it is advisable to use a regression equation model derived from a large number of subjects. Our results will be meaningful for surgeons who calculate the toric IOL on the basis of the anterior corneal surface only.
This paper was presented as an oral presentation at the 115th Korean Ophthalmologic Society Meeting, 2016 in Pusan, Korea.
WTRA = with the rule anterior corneal astigmatism; ATRA = against the rule anterior corneal astigmatism; D = diptor; SD = standard deviation; IOL = intraocular lens.
*Chi-square exact test; †Independent
Values are presented as means ± standard deviations or vectors.
SIA = surgically induced astigmatism; D = diopter; TIA = target induced astigmatism; DV = difference vector.
*Repeated-measure one-way analysis of variance.
Values are presented as means ± standard deviations or vectors.
WTRA = with the rule anterior corneal astigmatism; SIA = surgically induced astigmatism; D = diopter; TIA = target induced astigmatism; DV = difference vector.
*Repeated-measure one-way analysis of variance.
Values are presented as means ± standard deviations or vectors.
ATRA = against the rule anterior corneal astigmatism; SIA = Surgically induced astigmatism; D = diopter; TIA = target induced astigmatism; DV = difference vector.
*Repeated-measure one-way analysis of variance.