Scheimpflug imaging was used to measure in six meridians the shape of the anterior and posterior cornea of the right eye of 114 subjects, ranging in age from 18 to 65 years. Subsequently, a three-dimensional model of the shape of the whole cornea was reconstructed, from which the coma aberration of the anterior and whole cornea could be calculated. This made it possible to investigate the compensatory role of the posterior surface to the coma aberration of the anterior corneal surface with age. Results show that, on average, the posterior surface compensates approximately 3.5% of the coma of the anterior surface. The compensation tends to be larger for young subjects (6%) than for older subjects (0%). This small effect of the posterior cornea on the coma aberration makes it clear that for the coma aberration of the whole eye, only the anterior corneal surface and the crystalline lens play a role. Consequently, for the design of an intraocular lens that is able to correct for coma aberration, it would be sufficient to only take the anterior corneal surface into account.

*c*is the curvature (inverse radius

*r*) at the vertex (

*x*

_{0},

*y*

_{0}) and

*k*is the conic constant, which indicates the asphericity of the surface (e.g., hyperbola:

*k*< 0; parabola:

*k*= 0; circle:

*k*= 1). The

*y*-axis is the axis of revolution of both the conic axis and the optical axis of the cornea. By combining the Scheimpflug images in six meridians, it is possible to determine the astigmatism (Dubbelman et al., 2006) and spherical aberration (Sicam et al., 2006) of the anterior and posterior cornea. Nevertheless, the coma aberration cannot be obtained using Equation 1, which, therefore, has to be expanded to

*t*describes corneal tilt and

*m*describes coma. The mathematical formulation is analogous to the primary Seidel aberrations (Atchison & Smith, 2000). For each of the six meridians, Equation 2 was fitted to a 7.5-mm corneal zone as in Dubbelman et al. (2006). The 3D corneal profile is reconstructed by applying the following fit functions to the measured values of the shape parameters from all six meridians:

*α,*

*β,*and

*γ*are the angles of the meridian where

*r,*

*k,*and

*m*are maximal. For the 3D modeling of the corneal surfaces, the measured tilt

*t*of the corneal shape appeared to have no influence on the coma aberration and was, therefore, not taken into account.

*Z*

_{3}

^{1}(vertical coma) and

*Z*

_{3}

^{−1}(horizontal coma) for this wave aberration (Thibos, Applegate, Schwiegerling, & Webb, 2002). In our study, the corneal wave aberration was calculated for a 6-mm pupil size. Using similar principles, the contribution of the anterior surface to the coma wave aberration was also calculated, which allows to determine how the posterior corneal surface influences the coma of the anterior surface.

*m*of Equation 2 of the anterior and the posterior corneal surface as a function of meridian. The meridional variation of

*m*of both corneal surfaces could be well fitted using the cos function ( Equation 5). The average

*r*

^{2}was .66 and .61 for the anterior and posterior corneal surface, respectively. Because the goodness of fit varied among subjects, a weighted linear regression was performed, and the weighted mean, weighted standard deviation, and weighted standard error of the mean are presented (Bevington, 1969). Figure 2 shows the age dependence of the meridional variation of

*m*(Δ

*m*) of the anterior and posterior corneal surface. For clarity, the data are also grouped in four bins (±

*SEM*) of equal age range between 18 and 65 years, which shows the trend more clearly. The weighted linear regression was applied to all subjects. The Δ

*m*of both the anterior and the posterior corneal surface changes significantly with age (

*p*< .00001) but in the opposite direction. The Δ

*m*of the anterior surface increases, whereas that of the posterior surface decreases with age. Figure 3 shows the ratio of the Δ

*m*of the posterior corneal and that of the anterior corneal surface, which significantly changes with age. At the age of 20, the Δ

*m*of the posterior corneal surface is almost twice that of the anterior surface. With age, the difference becomes smaller.

*p*< .001) between the axes of the coma of both surfaces. For the anterior surface, the average axis

*γ*(±

*SD*) was 54° ± 21°, whereas it was 64.5° ± 21° for the posterior surface. The mean of the paired difference in axis (±

*SD*) was 13° ± 16°, which makes it clear that the coma axes of both surfaces are almost equal.

*p*< .01). Figure 4b shows the age dependence of the coma aberration of the whole cornea, that is, both the anterior and posterior corneal surfaces. The difference between the aberration of the anterior and whole cornea is hardly visible, which indicates the small effect of the posterior cornea on the coma aberration. Figure 5 shows the ratio between the coma of the whole cornea and that of the anterior surface as a function of age. A ratio smaller than 1 indicates that the posterior surface reduces the coma aberration of the anterior surface; a value of 1 indicates no change, and a value above 1 indicates that the posterior surface has an additive effect to the coma of the anterior corneal surface. It can be seen that the compensation of the posterior corneal surface is small. Average compensation (±

*SD*) is 3% ± 3.5%. Furthermore, because of the propagation of the uncertainties, the error in the ratio becomes large and no significant change with age can be determined. Nevertheless, a trend can be seen. For young subjects, the posterior surface compensates approximately 6% of the coma of the anterior surface, but this compensation disappears completely with age.

*m*) of both the anterior and posterior surfaces significantly changes with age. At the age of 20, the Δ

*m*is almost two times larger than that of the anterior surface, whereas it becomes almost equal to that of the anterior surface at the age of 65. Then, the results on the shape of the corneal surfaces were used to calculate the coma aberration, and it was found that the contribution of the posterior surface is almost negligible. At the age of 20, the posterior surface compensates approximately 6% of the coma of the anterior surface. This compensation decreases with age and disappears at the age of 60. This means that the dynamics of the refraction is different for coma aberration compared to astigmatism and spherical aberration. Calculations show that when Δ

*m*is almost the same for both the anterior and posterior surfaces, the posterior corneal surface does not contribute to the coma aberration of the whole cornea. This can be explained by the fact that after refraction of the anterior corneal surface, the wave front that approaches the posterior corneal surface has the same form as the coma shape feature of the posterior surface. As a result, there will be no change in the coma aberration at the posterior corneal surface. This is particularly true for older subjects: At the age of 60, the Δ

*m*of the posterior surface is equal to that of the anterior, which, therefore, results in minimal compensation of the corneal coma aberration by that of the posterior surface. This makes it clear that the contribution of the coma aberration of the posterior corneal surface is, thus, almost negligible and that the coma aberration that remains when the coma aberration of the anterior corneal surface has been subtracted to that of the whole eye is due to the crystalline lens. For the design of an IOL that is able to correct for coma aberration, it is sufficient to only take into account the anterior corneal surface.

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