Analysis of the dynamic changes of cornea after myopic excimer laser in situ keratomileusis using an Orbscan II topography system

Objective To study the changes and influence factors of an Orbscan Ⅱ topography system in myopia after excimer laser in situ keratomileusis(LASIK).Methods Two hundred and sixteen myopia patients(230 eyes)were treated by LASIK,their corneal thickness and curve of corneal posterior surface was measured with an Orbscan Ⅱ topography system before and after surgery.The results of measurement in different time points were compared.Results The thinnest point of cornea,the mean keratometry of the posterior best fit surface(BFS)and the distance from the apex to the BFS were significant difference before and after operation(P0.05).There were significant difference among those groups in different degree of myopia in the same time point(P0.05)after surgery,but there were no significant difference in the same group in difference time(P0.05).A significant difference in the area of the thinnest cornea before surgery and after surgery was detected among those groups(P0.01).The regression equation for preoperative Diff value was established as Y=0.0941-0.000 53X_1-0.000 471X_2- 0.0063X_3+0.001 22X_4(P0.01),X_1=corneal thickness of the thinnest point(μm),X_2=central corneal thickness(μm),X_3=corneal diameter(mm),X_4=length of ocular axis(mm).The regression equation for postoperative decrease Diff value between the third month and the first month was produced as Y=-0.027-0.000 78X_1+0.002 01X_2-0.0055X_3(P0.01.X_1=decrease in corneal thickness of the thinnest point(μm),X_2=intraocular pressure pre-surgery(mmHg),X_3=decrease in curvature of posterior corneal surface(D).Conclusions The application of Orbscan Ⅱ has an important clinical significance before and after IASIK.The Diff value measured before LASIK for myopia is influenced by the corneal thickness,corneal diameter and ocular axis.The postoperative decrease in Diff value between the third and first month is influenced by the postoperative decrease of corneal thinnest point and curvature of posterior corneal surface,and preoperative intraocular pressure.