Effect of the posterior corneal surface on total corneal astigmatism in patients with age-related cataract

Abstract

Aim: To explore the effect of the posterior astigmatism on total corneal astigmatism and evaluate the error caused by substituting the corneal astigmatism of the simulated keratometriy (simulated K) for the total corneal astigmatism in age-related cataract patients.

Methods: A total of 211 eyes with age-related cataract from 164 patients (mean age: 66.8±9.0y, range: 45-83y) were examined using a multi-colored spot reflection topographer, and the total corneal astigmatism was measured. The power vector components J₀ and J₄₅ were analyzed. Correlations between the magnitude difference of the simulated K and total cornea astigmatism (magnitude difference_SimK-Tca), anterior J₀, and absolute meridian difference (AMD) between the anterior and posterior astigmatisms were calculated. To compare the astigmatism of the simulated K and total cornea both in magnitude and axial orientation, we drew double-angle plots and calculated the vector difference between the two measures using vector analysis. A corrective regression formula was used to adjust the magnitude of the simulated K astigmatism to approach that of the total cornea.

Results: The magnitude difference_SimK-Tca was positively correlated with the anterior corneal J₀ (Spearman's rho= 0.539; P<0.001) and negatively correlated with the AMDR (Spearman's rho=-0.875, P<0.001). When the anterior J₀ value was larger than 1.3 D or smaller than -0.8 D, the errors caused by determining the total corneal astigmatism with the karatometric calculation tended to be greater than 0.25 D. An underestimation by 16% was observed for against the rule (ATR) astigmatism and an overestimation by 9% was observed for with the rule (WTR) astigmatism when ignoring the posterior measurements.

Conclusion: Posterior corneal astigmatism should be valued for more precise corneal astigmatism management, especially for higher ATR astigmatism of the anterior corneal surface. We suggest a 9% reduction in the magnitude of the simulated K in eyes with WTR astigmatism, and a 16% addition of the magnitude of the simulated K in eyes with ATR astigmatism.

Keywords: cataract; multi-colored spot reflection topographer; posterior corneal astigmatism; total corneal astigmatism.

Figure 1. A correlation plot of the flat meridian values of posterior and anterior corneal astigmatisms.

Figure 2. Double-angle plots of astigmatism of the simulated keratometry (A) and that of the total cornea (B)

Each ring equals 0.5 D, and the mean aggregate astigmatism is signified with a triangle.

Figure 3. Double-angle plots of the vector difference between the astigmatism of simulated keratometry and that of the total cornea

Each ring equals 0.5 D, and the mean aggregate vector difference is signified with a triangle.

Figure 4. The results of the linear regression analysis of the relationship between the J₀ components of the posterior cornea and anterior cornea (A) and between those of the total cornea and anterior cornea (B)

A: y=0.09+0.11x, R²=0.194, P<0.001; B: y=-0.09+0.9x, R²=0.910, P<0.001.

Figure 5. The results of the linear regression analysis of the correlation between the magnitude difference between the simulated K and total cornea and anterior corneal J₀ (A) and that of the absolute flat meridian difference between the anterior and posterior surfaces (B)

A: y=-0.06+0.24x, R²=0.274, P<0.001; B: y=0.19-0.007x, R²=0.720, P<0.001.

Figure 6. The relationship between the magnitude of simulated K and total corneal astigmatism in eyes with an anterior J₀ greater than 0 (A) and in eyes with an anterior J₀ less than 0 (B)

A: y=0.91x, R²=0.959, P<0.001; B: y=1.16x, R²=0.977, P<0.001.