Calculations of actual corneal astigmatism using total corneal refractive power before and after myopic keratorefractive surgery

PLOS ONE, Dec 2019

Purpose To calculate actual corneal astigmatism using the total corneal refractive astigmatism for the 4-mm apex zone of the Pentacam (TCRP4astig) and keratometric astigmatism (Kastig) before and after photorefractive keratectomy or laser in situ keratomileusis Methods Uncomplicated 56 eyes after more than 6 months from the surgery were recruited by chart review. Various corneal astigmatisms were measured using the Pentacam and autokeratometer before and after surgery. Three eyes were excluded and 53 eyes of 38 subjects with with-the-rule astigmatism (WTR) were finally included. The astigmatisms were investigated using polar value analysis. When TCRP4astig was set as an actual astigmatism, the efficacy of arithmetic or coefficient adjustment of Kastig was evaluated using bivariate analysis. Results The difference between the simulated keratometer astigmatism of the Pentacam (SimKastig) and Kastig was strongly correlated with the difference between TCRP4astig and Kastig. TCRP4astig was different from Kastig in magnitude rather than meridian before and after surgery; the preoperative difference was due to the posterior cornea only; however, the postoperative difference was observed in both anterior and posterior parts. For arithmetic adjustment, 0.28 D and 0.27 D were subtracted from the preoperative and postoperative magnitudes of Kastig, respectively. For coefficient adjustment, the preoperative and postoperative magnitudes of Kastig were multiplied by 0.80 and 0.66, respectively. By arithmetic or coefficient adjustment, the difference between TCRP4astig and adjusted Kastig would be less than 0.75 D in magnitude for 95% of cases. Conclusions Kastig was successfully adjusted to TCPR4astig before and after myopic keratorefractive surgery in cases of WTR. For use of TCRP4astig directly, SimKastig and Kastig should be matched.

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Calculations of actual corneal astigmatism using total corneal refractive power before and after myopic keratorefractive surgery

April Calculations of actual corneal astigmatism using total corneal refractive power before and after myopic keratorefractive surgery Kyoung Yul Seo 0 1 Hun Yang 1 Wook Kyum Kim 1 Sang Min Nam 1 0 Department of Ophthalmology, Institute of Vision Research, Eye and Ear Hospital, Severance Hospital, Yonsei University College of Medicine , Seoul , Korea , 2 SU Yonsei Eye Clinic , Seoul , Korea , 3 B&Viit bright eye center , Seoul , Korea , 4 Department of Ophthalmology, CHA Bundang Medical Center, CHA University , Seongnam , South Korea 1 Editor: Dimitrios Karamichos, Oklahoma State University Center for Health Sciences , UNITED STATES - Data Availability Statement: All relevant data are within the paper. Funding: For this study, SMN received a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number: HI14C1607010116). URL: http://www.mohw.go.kr/ eng/. The funders had no role in study design, data Purpose Methods To calculate actual corneal astigmatism using the total corneal refractive astigmatism for the 4-mm apex zone of the Pentacam (TCRP4astig) and keratometric astigmatism (Kastig) before and after photorefractive keratectomy or laser in situ keratomileusis Uncomplicated 56 eyes after more than 6 months from the surgery were recruited by chart review. Various corneal astigmatisms were measured using the Pentacam and autokeratometer before and after surgery. Three eyes were excluded and 53 eyes of 38 subjects with with-the-rule astigmatism (WTR) were finally included. The astigmatisms were investigated using polar value analysis. When TCRP4astig was set as an actual astigmatism, the efficacy of arithmetic or coefficient adjustment of Kastig was evaluated using bivariate analysis. Results The difference between the simulated keratometer astigmatism of the Pentacam (SimKas tig) and Kastig was strongly correlated with the difference between TCRP4astig and Kastig. TCRP4astig was different from Kastig in magnitude rather than meridian before and after surgery; the preoperative difference was due to the posterior cornea only; however, the postoperative difference was observed in both anterior and posterior parts. For arithmetic adjustment, 0.28 D and 0.27 D were subtracted from the preoperative and postoperative magnitudes of Kastig, respectively. For coefficient adjustment, the preoperative and postoperative magnitudes of Kastig were multiplied by 0.80 and 0.66, respectively. By arithmetic or coefficient adjustment, the difference between TCRP4astig and adjusted Kastig would be less than 0.75 D in magnitude for 95% of cases. collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Conclusions Kastig was successfully adjusted to TCPR4astig before and after myopic keratorefractive surgery in cases of WTR. For use of TCRP4astig directly, SimKastig and Kastig should be matched. Introduction Uncorrected refractive astigmatism, even as low as 1.00 D, can affect distance and near vision as well as patients' quality of life.[ 1 ] The prevalence of refractive astigmatism increases in old age due to changes in the magnitude and axis of corneal astigmatism.[2±4] Therefore, accurate measurement of total corneal astigmatism in senile cataract patients before surgery is crucial to avoid significant astigmatism after removal of the crystalline lens. The common solution for correcting corneal astigmatism during cataract surgery is implantation of a toric intraocular lens (IOL) according to keratometric astigmatism (Kastig). Patients with regular corneal astigmatism 0.75 D may be considered for a toric IOL.[ 5 ] In traditional keratometry measuring only the anterior corneal surface, a fixed correlation between the anterior and posterior corneal surface is assumed and the standardized keratometric refractive index of 1.3375 is used.[ 3 ] However, the standardized keratometric refractive index is selected arbitrarily, and the net power of the cornea is less than the standardized keratometric power.[ 6 ] In addition, the relationship between the anterior and posterior corneal astigmatism is not fixed as a function of age:[3,7±9] the anterior corneal astigmatism is usually with-the-rule (WTR) astigmatism in younger age groups, but predominantly against-the-rule (ATR) astigmatism in older age groups. In contrast, the posterior corneal astigmatism remains relatively stable in magnitude and ATR axis, regardless of age. Consequently, two types of adjustment to inaccurate Kastig have been suggested. The first is an arithmetic method, in which some diopters would be subtracted in WTR astigmatism and added for ATR astigmatism according to a nomogram.[ 10 ] The other is a coefficient method, in which Kastig is multiplied by the coefficient for each type of astigmatism.[ 11 ] (...truncated)


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Kyoung Yul Seo, Hun Yang, Wook Kyum Kim, Sang Min Nam. Calculations of actual corneal astigmatism using total corneal refractive power before and after myopic keratorefractive surgery, PLOS ONE, 2017, Volume 12, Issue 4, DOI: 10.1371/journal.pone.0175268