Factors Influencing Manifest Refractive Cylinder and Ocular Residual Astigmatism in Keratoconus

PURPOSE: To investigate the factors influencing manifest refractive cylinder (MRC) and ocular residual astigmatism (ORA) in keratoconus. METHODS: This was a retrospective study including data of 145 eyes with keratoconus. MRC power was correlated to corneal tomographic variables including higher order aberrations measured with Scheimpflug corneal topography, using univariate and multivariate regression analysis. The vector difference between keratometric astigmatism (KA) and MRC, termed ORA, was also explored to detect tomographic variables associated with this disparity. Axis of MRC was compared and correlated to the axis of corneal coma. RESULTS: KA showed the strongest association with MRC. ORA was associated with KA, posterior keratometric astigmatism, and posterior corneal power. MRC axis was more closely aligned with coma axis than with keratometric axis. However, coma magnitude was not significantly associated with the difference between MRC axis and coma axis, and sensitivity analysis showed that the association between coma and ORA weakened after exclusion of extreme coma values. CONCLUSIONS: MRC and ORA were associated with several tomographic and aberration parameters in keratoconus, with KA showing the strongest and most consistent relationship with MRC. Coma axis was more closely aligned with manifest refractive axis than keratometric axis. However, the observed relationships were modest and should be interpreted as associative rather than strongly predictive at the individual-eye level.