Purpose: In the present study, two different types of astigmatism (i. e., K-value astigmatism were compared. The comparisons were made using keratometric reading [K value], which was based on the measurement of the anterior corneal curvature, and total corneal astigmatism, which was calculated from total refractive power [real power] based on the measurement of the anterior/posterior corneal curvature. The patients were categorized into the eyes with-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism groups.
Subjects and methods: A total of 505 eyes of 505 subjects with a K-value astigmatism of ≥0.75 D were classified into the following three groups: WTR, ATR, and oblique astigmatism groups. K-value astigmatism was obtained from the anterior corneal curvature measurement of anterior segment optical coherence tomography (OCT). Total corneal astigmatism was calculated from the real power using both anterior and posterior corneal curvature measurements.
Results: There were 296 (age, 44.9±21.4 years), 160 (age, 73.1±9.0 years), and 49 eyes (age, 65.9±16.5 years) in the WTR, ATR and oblique astigmatism groups, respectively. When K-value and total corneal astigmatism was compared, K-value astigmatism was significantly greater in the WTR group (p< 0.0001), total corneal astigmatism was significantly greater in the ATR group (p< 0.0001), and no significant difference was found in the oblique astigmatism group (p=0.5731). The discrepancy in cylinder axes between K-value and total corneal astigmatism was 6.6±2.8° in the oblique astigmatism group, which was significantly greater than those in the WTR (2.2°±2.6°) and ATR groups (2.3°±2.0°) (p< 0.0001).
Conclusions: Relationship between K-value and total astigmatism varied according to the type of corneal anterior astigmatism. When K-value and total corneal astigmatism was compared, K-value astigmatism was greater in the WTR group, whereas total corneal astigmatism was greater in the ATR group. In the oblique astigmatism group, no difference was observed in cylinder power; however, differences in cylinder axis were greater.
Nippon Ganka Gakkai Zasshi (J Jpn Ophthalmol Soc) 126: 567-573,2022.