Role of Hemidivisional Corneal Topographic Astigmatisms (CorTs) in the Regularization and Reduction of Irregular Astigmatism

Purpose: To demonstrate how the concept of hemidivisional corneal topographic astigmatism (hemiCorT) enables the planning of hemidivisional corneal treatments to reduce irregularity and overall astigmatism. Methods: Whole-of-cornea corneal topographic astigmatism (CorT) is calculated from topography data derived from a corneal topographer or tomographer. The cornea is conceptually divided into 2 hemidivisions along the flat meridian of the CorT. For each hemidivision, hemiCorTs are calculated. The regularization treatment for each hemidivision is the treatment required to target the whole-of-cornea CorT, which is a symmetrical orthogonal corneal astigmatism. The regularization is then combined with astigmatism reduction treatment, which could be a conventional refractive treatment or a vector-planned treatment. For each hemidivision, the combined astigmatic effect of the regularization treatment and reduction treatment can be determined through double-angle vector summation. The 2 hemidivisional treatments together regularize and reduce corneal astigmatism. Results: A theoretical pair of hemidivisional treatments is derived from an actual example of a cornea displaying idiopathic asymmetric nonorthogonal astigmatism. Conclusions: HemiCorTs allow for the design of hemidivisional corneal treatments of asymmetric nonorthogonal astigmatism. Such treatments should be suitable in the routine treatment of commonly occurring irregular astigmatism, while also allowing the spherical refractive error to be treated concurrently.