QUANTITATIVE ASSESSMENT OF CORNEAL ASTIGMATIC SURGERY - EXPANDING THEPOLAR VALUES CONCEPT

The purpose of astigmatic corneal surgery is to flatten the steeper me ridian of the preoperative cylinder, to steepen the flatter meridian, or both. Therefore, it may be useful to quantitate the surgical effect by calculating the equivalent dioptric value of the postoperative cyl inder in these principal meridians. In this study, the dioptric value projected on the preoperatively steeper meridian is termed the with-th e-power (WTP) component, the portion projected on the flatter meridian , the against-the-power (ATP) component. Consider a preoperative net a stigmatism of the power N in the meridian a. After astigmatic corneal surgery, the postoperative corneal cylinder is M in the meridian b. Fo r the postoperative cylinder, the WTP component = M x sin2([b + 90]-a) . The ATP component = M X cos2([b + 90] - a). The astigmatic polar val ue is defined as the difference between these magnitudes: AKP = M x (s in2[(b + 90)-a]-cos2[(b + 90)-a]). By calculating the astigmatic polar value, the surgeon immediately knows the outcome of the surgical proc edure (i.e., whether the preoperative astigmatism has been undercorrec ted, overcorrected, or perfectly corrected). We describe the theory be hind this new formula and discuss its applications and limitations.