Contribution of Posterior Corneal Astigmatism to Total Corneal Astigmatism among Saudi Population

Background: Astigmatism is an error of refraction in which incident parallel rays cannot be focused on a single point. It could be corneal astigmatism (CA), lenticular astigmatism (LA), or both. Regarding CA, it is due to the difference in the corneal curvature in the different meridians of both the anterior and posterior surfaces. The anterior and posterior corneal surfaces contribute to total corneal astigmatism (TCA). Newer technologies, such as slit-scanning technology, Scheimpflug devices, and optical coherence tomography, are now used in the clinical setting for measuring the posterior corneal surface. However, it has been shown that relying only on the anterior corneal surface measurement and neglecting the relationship between the anterior and posterior corneal surfaces can lead to unacceptable intraocular lens (IOL), power calculation results after corneal refractive surgery, and errors during the calculation of toric IOLs. Objectives: This study aimed to determine the percentage of contribution of the magnitude of posterior corneal astigmatism (PCA) to total corneal astigmatism (TCA) and to investigate the correlation between anterior corneal astigmatism (ACA), PCA, and TCA. Methods: A prospective cross-sectional study including 55 subjects aged 18 - 31 years old and divided into 3 groups based on spherical equivalent (SE), Emmetropia, Hyperopia, and Myopia were included. All subjects underwent full ophthalmological examinations and corneal topography. ACA and PCA were evaluated. TCA was computed by the addition of the ACA and PCA using vector analysis. The percentage of contribution of the PCA to the magnitude of the TCA and the correlation between ACA, PCA, and TCA were calculated. Results: All subjects had anterior corneal with the rule (WTR) astigmatism. The contribution of PCA to TCA for all subjects in 3 groups (Emmetropia, Hyperopia, and Myopia) was 35%, 37%, and 37%, respectively, with P > 0.05, which was not statistically significant. There was a correlation between ACA, PCA, and TCA in all study groups. Conclusion: TCA is composed of both corneal surfaces, and PCA impacts the outcomes of refractive surgeries for correcting astigmatism and in toric intraocular lens (IOL) calculation when just based on the data of the anterior corneal surface.