Conversion of keratometer readings to polar values.
Corneal astigmatism is a complex entity that has direction and magnitude. This study reports a new method to describe corneal astigmatism within the with- and against-the-rule concept. Each net astigmatism of the maximal power M in the meridian alpha may be divided into two dioptric components: a with-the-rule astigmatism projected on the 90-degree meridian and an against-the-rule component projected on the 180-degree meridian. The former figure has the dioptric value M X sin2 alpha, the latter M X cos2 alpha. The polar value is defined as the difference between these magnitudes: M X (sin2 alpha - cos2 alpha). The polar value calculates the balance between the with- and against-the-rule components for any given net astigmatism. The entire model allows an exact description of surgically induced with- or against-the-rule astigmatism following cataract extraction. The advantage of the model is that a corneal astigmatism may be expressed by a single figure. The system enables each surgeon to evaluate the contribution of the preoperative astigmatism, incision type, suture technique, and postoperative treatment on the final astigmatism. This in turn allows the surgeon to estimate a number of different surgical techniques empirically. By disposing of and choosing between several known techniques the surgeon may be able to minimize final astigmatism even in cases of significant preoperative astigmatism.