Correlation among refractive, keratometric and topographic astigmatism after myopic photorefractive keratectomy

Background: Photorefrative keratectomy can be used to flatten the curvature of the anterior cornea and reduce the myopic refraction of the eve. This l eads to unphysiological topographical changes of the cornea and may alter t he conditions for examinations of corneal surface topography. The purpose o f this study was to check for mutual agreement of three different methods o f assessment of astigmatism before and after myopic photorefractive keratec tomy (PRK). Patients ann methods: Forty-seven eyes of 28 patients (age 32.7 +/-6.6 years) following PRK using an 193-nm excimer laser were included in this study. 37 eyes were treated for pure myopia (-4.9+/-2.4 D) and 10 eyes for myopic astigmatism (sphere -2.0 to -7.0 D, cylinder -1.0 to -3.0 D). P reoperatively and at 18 months postoperatively, subjective refractometry, k eratometry and topography analysis were performed. The axes of topographic and keratometric cylinder were standardized periodically (180 degrees) with respect to the refractive cylinder axis. Results: Pre- and postoperatively , the absolute astigmatism values correlated highly significantly between a ll three methods (P less than or equal to 0.001). The mean refractive cylin der was 0.65+/-0.61 D preoperatively and 0.46+/-0.41 D postoperatively (P=0 .2). The mean keratometric astigmatism was 1.14+/-0.64 D before and 0.94+/- 0.50 D after PRK treatment (P=0.2). Among the three methods, the mean topog raphic astigmatism was the highest (P<0.001) preoperatively (1.31+/-0.56 D) and postoperatively (1.21+/-0.52 D) (P=0.3). In eyes treated for pure myop ia, no difference between pre- and postoperative refractive, keratometric a nd topographic astigmatism was detected (P>0.5). The axes of both topograph ic and keratometric astigmatism correlated highly significantly with the re fractive cylinder axis (R greater than or equal to 30.9, P<0.0001). Conclus ion: Up to 2 years after myopic PRK, the difference between refractive and keratometric astigmatism does not differ from the preoperative value, indic ating an even corneal surface. The absolute astigmatism values and the cyli nder axis correlated well between subjective and objective methods of astig matism assessment. Thus, objective measurements may be helpful in determini ng the cylinder component of best spectacle correction after PRK. However, topographic analysis overestimates astigmatism values systematically before and after PRK.