A retrospective study of 42 patients was performed in order to compare keratometric values and corneal structure preoperatively and 6 weeks after CPS. The data of 46 eyes were included in the study between December 2016 and February 2017. Inclusion criteria were an age of at least 18 years with visual impairment caused by existence of senile cataract. Only patients with senile cataract and no other ocular pathology who underwent uneventful routine phacoemulsification surgery with implantation of a posterior chamber IOL were included. Patients were randomly assigned to either of three experienced surgeons involved in the audit who used the same surgical technique in every case. After inclusion, corneal topography was performed using a Scheimpflug tomography system (OCULUS Pentacam®, Wetzlar, Germany) pre- and 6 weeks postoperatively.
Exclusion criteria were a history of previous ocular surgery, any documented complications during cataract surgery, irregular astigmatism, previous corneal scarring, corneal dystrophy, suture placement, and postoperative endothelial dysfunction as evidenced by clinically visible corneal stromal edema at the follow-up examination.
This research was conducted in accordance with the regulations of the ethics committee of the Medical University of Innsbruck and the Declaration of Helsinki.
Conventional phacoemulsification cataract surgery was undertaken using a bevel up 2.4-mm blade for a two-step main CCI that was always performed at 100° and two paracenteses at 140° and 60°. For phacoemulsification, the Alcon Infiniti Vision System (Alcon, Fort Worth, TX, USA) was used in conjunction with a 45-degree Kelmann tip. An aspheric acrylic posterior chamber IOL Acrysof natural SN60WF was implanted using a preloaded system (UltraSert®, Alcon) and wound-assisted IOL implantation technique in order to ensure stability of CCI size in all cases. The corneal incisions were hydrated and no sutures used for any included patient. All patients received routine postoperative topical dexamethasone and gentamicin eye drops (Dexagenta-POS, Ursapharm, Arzneimittel GmbH, Saarbrücken, Germany) for 4 weeks and topical ketorolac eye drops (Acular, Axicorp Pharma GmbH, Friedrichsdorf, Germany) for 2 weeks.
On the day of surgery and 6 weeks postoperatively, complete eye examinations were performed. Collected data included best-corrected visual acuity (BCVA) measured on a Snellen chart and converted to logarithm of the minimum angle of resolution (logMAR), refractive astigmatism measured by ARK-30 autorefractor (Nidek Co. Ltd, Aichi, Japan), corneal tomography using the Pentacam, slit-lamp biomicroscopy, and indirect fundoscopy. The timing of the follow-up examination was chosen with respect to the findings of Hayashi et al. [14
], who did not observe significant keratometric changes later than 4 weeks after surgery with a 2.4-mm CCI.
Keratometry and calculation of IOL was conducted with the IOLMaster (Carl Zeiss Meditec, Dublin, CA, USA) and refraction was calculated at a spheric equivalent of −0.50 D. The expected postoperative refraction, as targeted by the chosen IOL at a range between 0.00 D and −0.50 D, was recorded for every patient and compared with actual postoperative refraction measurements on follow-up.
For comparison of pre- and postoperative corneal power and astigmatism, Pentacam readings were assessed using k values of the central 3‑mm zone, namely, steep axis, flat axis, refractive power in both axes, anterior corneal astigmatism (ACA), and mean corneal power (k-mean). Surgically induced astigmatism was defined as the change in ACA. Additionally, manifest refraction was measured and compared with objective autorefractor refraction.
Coupling ratio (CR) and a coupling constant (CC) were calculated using k readings in the incisional meridian, 90 degrees, and mean corneal power. The CC has recently been defined as the change in mean corneal power in relation to the difference between the change in corneal power at the treatment meridian and the change in corneal power at the opposite meridian: CC = ∆ KMean
/ (∆ KT
− ∆ KO
]. A CC of 0.33 therefore equals a 0.33 diopter change of SE for each diopter change in astigmatism. The CR is defined as the flattening of the incised meridian in relation to the opposite meridian: CR = –∆ KT
/ ∆ KO
]. A CR of 0.33 therefore indicates that the change in corneal power at the opposite meridian was one third of the change in the incisional meridian.
Astigmatic change, calculated as the difference between preoperative and postoperative ACA, was defined as SIA.
For the analysis, patients were categorized into three groups according to the individual relation of the steep axis of corneal astigmatism to the incisional axis at 100°. Group 1: Patients with steep axis within 20 degrees to the incision (steep axis at 80–120 degrees). Group 2: Patients with steep axis at 70–110 degrees to the incision (flat axis at 80–120 degrees). Group 3 consisted of all other cases (steep axis from 120–170 and 30–80 degrees).
All statistical analyses were performed using SPSS version 14.0 (IBM SPSS Statistics, Armonk, NY, USA). The Shapiro–Wilk test of normality was used to test the distribution of continuous data. Hence, the nonparametric Wilcoxon signed rank test was used within groups and the Mann–Whitney U test or Kruskal–Wallis was used to compare differences between groups. Means are given with standard deviation. All p values <0.05 were considered significant.