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
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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)