We conducted this retrospective study with the approval of the Institutional Review Board of the Asan Medical Center and the University of Ulsan College of Medicine, Seoul, South Korea (2019 − 1244). The study adhered to the tenets of the Declaration of Helsinki and followed good clinical practice. All patients were informed of risks and benefits of the surgery and provided written consent for surgery.
This retrospective study included 48 patients who underwent cataract surgery by one surgeon (HT) at the cataract and refractive clinic of Asan Medical Center from November 2016 to July 2018. Patients who met the following inclusion criteria were included: older than 18 years, with preexisting corneal astigmatism between + 1.00 and + 2.00 D by simulated keratometry (Sim K) of topography, and agreement of femtosecond laser-assisted intrastromal AK with FLACS or toric IOL implantation during cataract surgery. Patients were excluded from the analyses if they had any optical opacities or pathologies on slit-lamp examination, previous corneal surgeries, ocular trauma, intraocular surgery, severe dry eye, corneal disease, ocular infection, or collagen vascular/autoimmune diseases. Among 48 patients, 27 patients had FLACS and femtosecond laser-assisted intrastromal AK (AK group) and 21 patients underwent conventional cataract surgery with toric IOL implantation (toric IOL group).
All surgeries were performed by one surgeon using topical anesthesia (0.5% proparacaine hydrochloride). Cataract surgeries were performed with phacoemulsification and implantation of a foldable IOL with 2.2 mm limbal incision. The incisions were located at the steep axis of cornea measured by automated keratometry. All non-toric or toric IOLs were injected into the capsular bag. Main incisions were sealed with anterior stromal hydration without corneal sutures. Topical antibiotics and steroid eyedrops were used for 1-month after surgery.
In the AK group, all 27 patients underwent cataract surgery using the Catalys femtosecond laser platform (Abbott Medical Optics, Inc, Santa Ana CA, USA). Horizontal limbal marker was done in sitting position prior to the laser to avoid cyclotorsion. After the patient lies supine, a suction ring with vacuum was aligned with corneal marks, and a safe distance for the docking interface was controlled with a joystick by the surgeon. After surgical site and depth was confirmed using the incorporated optical coherence tomography cross section, anterior capsulotomy and lens fragmentation were performed, followed by the intrastromal AK. The programmed anterior capsulotomy size was 4.8 mm in diameter. Crystalline lens fragmentation was done using a standard template with a pattern described as “lens softening: quadrants” in the system.
Intrastromal AK nomogram of paired symmetric (same length) incisions centered on the steep corneal axis was done, with 8.0 mm in diameter and central 60% of total corneal thickness in depth remaining upper 20% and lower 20% without penetration. The arc length was determined by intrastromal AK nomogram calculator v3, which is provided on the website. The length was proportional to preoperative corneal cylindrical power and was adjusted by the age and type of astigmatism, i.e., against-the-rule, with-the-rule, or oblique astigmatism. Foldable aspheric IOL (TECNIS 1-piece ZCB00 or PCB00 IOL; Abbot Medical Optics, Inc.) was implanted in patients of the AK group. The target of reduction in femtosecond laser-assisted intrastromal AK is corneal astigmatism.
In the toric IOL group, horizontal corneal-limbal marks using a sterile pen was made in each patient, while each was in a sitting position to avoid cyclotorsion. After a patient lied in a supine position in the operating room with aseptic drape, intraoperative markings were made under a surgical microscope with guidance from preoperative horizontal markings. After successful phacoemulsification, toric IOL (TECNIS Toric IOL; Abbott Medical Optics, Inc.) was implanted. The goal of toric IOL implantation is to reduce the refractive astigmatism. The appropriate cylindrical power and axis placement of the toric IOL to be implanted were determined with a program available from the IOL manufacturer and the Barrett toric calculator. After the implantation of IOL, IOL alignment and axis were re-checked.
The preoperative and postoperative (1-day, 1-month, 3-month, and 6-month) ophthalmic examinations included uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), autorefraction, manifest refraction, IOP measurements (non-contact tonometer; NT-530, NCT Nidek Co. Ltd., Aichi, Japan), dilated fundus examinations, and slit-lamp examinations (Haag-Streit, Gartenstadtstrasse, Köniz, Switzerland). Corneal astigmatism was measured by corneal topographies with the Orbscan (Bausch & Lomb, Rochester, NY, USA) and automated keratometer (KR-8800, Topcon Europe Medical BV). Additionally, for measuring ocular biometry such as axial length and anterior chamber depth, IOL Master 500 (Carl Zeiss Meditec, Oberkochen, Germany) was used.
Vector analysis of refractive astigmatism is based on the definitions and formulas given by Eydelman MB.[23, 24] We analyzed the intended refractive correction, surgically induced refractive correction, error ratio, correction ratio, error of magnitude, and error of angle. The intended refractive correction is the vector difference between the preoperative and the target postoperative cylinder vector. Assuming that target refractive state is emmetropia, it is equal to the preoperative astigmatism. The surgically induced refractive correction is defined as an amount and axis of astigmatism change achieved by surgery. The error ratio is the proportion of the intended correction that was not successfully corrected. The correction ratio is the ratio of the achieved correction magnitude to the targeted correction magnitude; whereas the error of magnitude is the difference in magnitudes between the surgically induced refractive correction and intended refractive correction. Finally, the error of angle is the angular difference between the achieved correction and the intended correction. Double-angle plots of preoperative corneal astigmatism and postoperative refractive astigmatism (3-month and 6-month) are drawn. The double-angle plot tool used in this paper is provided in the American Society of Cataract and Refractive Surgery website.
The Mann–Whitney U test, Wilcoxon signed-rank test, and Chi-square test were used for statistical analyses using the SPSS software version 21.0 (SPSS, Inc., Chicago, IL, USA). Differences were considered to be statistically significant for P values less than 0.05. Data from automated keratometer and topography were analyzed using vector analysis.