Comparative analysis of corneal topography characteristics in primary keratoconus and secondary keratoconus after LASIK

Abstract

Objective To analyze the difference in corneal topography between primary keratoconus and secondary keratoconus after LASIK;to provide a reference for the clinical diagnosis and analysis of risk factors in secondary keratoconus. Methods In this retrospective case analysis, 13 secondary keratoconus patients (20 eyes, secondary group) were diagnosed and accepted for treatment in Shandong Eye Institute between January 1997 and December 2012. Correspondingly, 18 primary keratoconus patients (25 eyes, primary group) were randomly selected during the same period. The mean ages of the primary group and secondary group were 18.9±2.8 years (15-28 years) and 24.3±9.1 years (19-48 years), respectively. The mean axial lengths were 25.39±1.33 mm (23.3-27.8 mm) and 27.09±2.43 mm (25.3-30.1 mm), and the differences were statistically significant (t=3.942,P<0.01). The morphological features of the topography were observed with an Orbscan Ⅱ system, which primarily included the position of the thinnest point of the cornea, the maximum simulation curvature of the anterior corneal surface (Kmax), astigmatism, the average power and astigmatism in the central 3 mm region of the cornea, the Diff-values of the anterior and posterior corneal surfaces, and the morphology of the elevation map pattern. The collected data were analyzed with a t-test, one-way ANOVA and Pearson correlation coefficient. Results The distribution of the thinnest points in the corneal quadrants for the primary group were 18 eyes (72%) infratemporal, 4 eyes (16%) supertemporal, 1 eyes (4%) intranasal, 2 eyes (8%) supernasal, and the distributions for the secondary group were 8 eyes (40%), 4 eyes (20%), 4 eyes (20%) and 4 eyes (20%), respectively. Corneal astigmatisms in the primary and secondary groups were 5.85±3.26 D and 3.95±2.51 D, and the astigmatism values in the 3 mm region were 4.12±2.11 D and 2.82±1.50 D. The differences were statistically significant (t=-2.142, -2.187,P 0.05). Conclusion Compared to primary keratoconus, the morphological features of the corneal topography follow the same rules, and the astigmatism value is smaller. In addition, the thinnest point of the cornea is uniformly distributed near the central corneal area, which points out that excessive cutting may be a risk factor for secondary keratoconus. Key words: Keratoconus; Secondary keratoconus; Keratomileusis, laser in situ; Corneal topography; Astigmatism