Change of cornea posterior elevation after laser in situ keratomileusis

Background Keratectasia after laser in situ keratomileusis (LASIK) is one of the serious complications which affect the vision.The measurement of cornea posterior elevation is an important way helping to find this change.Objective The aim of this study was to investigate the change of cornea posterior elevation after LASIK in myopic eyes with astigmatism and affecting factors.Methods A series case-observational study was adopted.One hundred and twenty-seven myopic eyes with astigmatism of 66 patients who received LASIK in 2008 May through 2010 January in Beijing Tongren Hospital were retrospectively analyzed to evaluate the change of cornea posterior elevation following the LASIK.The parameters related to cornea posterior elevation were measured and compared before and 3 months,6 months and 1 year after surgery,respectively,with Oculyzer anterial segment analysis system.Results The height values from posterior cornea highest point,the lowest point and central vertex central elevation zone were (12.20±3.39),(-19.02±7.38) and (1.05 ±3.25) μm respectively before LASIK and were (14.38±3.80),(-18.55±7.11),(2.83±4.81)μm in3 months and (13.99±3.38),(-17.57±6.54),(2.45±4.61) μm in 6 months after LASIK.They were (14.40±3.85),(-17.76±6.00),(2.16±5.00) μm in 1 year after surgery.Significant increases were found in the vertex height and central elevation after LASIK compared with before surgery(highest pointq=6.813,5.594,6.875,all at P＜0.001.central vertex:q=4.488,P=0.002;q =3.530,P=0.013 ;q =2.799,P =0.047).However,no significant difference was seen in various time points after LASIK (P＞0.05).A positive correlation was obtained between the height value of the posterior cornea central vertex with the spherical equivalent (SE),maximum cutting depth or cutting proportion 1 year after LASIK (r =0.295,0.297,0.295,all at P=0.001),and there was a negative correlation between it with residual stroma (r=-0.208,P=0.019).The intraocular pressure (IOP) measured with noncontact tonometry (NCT) was (14.24±3.33) mmHg before LASIK,and those of 3 months,6 months and 1 year were (8.42± 1.90),(8.61 ± 1.64) and (8.76± 1.64)mmHg after LASIK,showing a significant lowing in IOP after LASIK in comparison with before LASIK (q =29.851,28.317,26.337,all at P＜0.001).But no significant change was found in the different time points after LASIK (P＞0.05).There was significant difference in the IOP corrected by Ehlers after LASIK (P＞0.05).The IOP before LASIK had positive correlation with the central elevation of posterior cornea surface 1 year after LASIK(r=0.258,P =0.003).Conclusions The cornea posterior elevation increases slightly early stage after LASIK but retains stable level with lapse of time.The refraction diopter before LASIK is the main factor affecting the posterior corneal shape.To reduce the risk of corneal ectasia,more attention should be paid to the reserving of appropriate corneal residual stroma and smallest degree of eccentricity during the ablation procedure. Key words: Laser in situ keratomileusis;  Cornea posterior elevation;  Cornea thickness;  Keratectasia