LASIK后角膜后表面曲率变化的研究

Changes in Posterior Surface Elevation of Cornea after LASIK

目的研究准分子激光原位角膜磨镶术（LASIK）手术前后角膜后表面前凸的变化,探讨手术后应保留角膜的安全厚度。方法将135例269只眼近视患者依据近视程度分为3组,使用Orbscan裂隙扫描角膜地形图/角膜测厚系统,观察3组LASIK手术前后角膜后表面差值（Diff）的变化。结果术前3组Diff值1组为（0.023±0.008）mm;2组为（0.024±0.010）mm;3组为（0.024±0.010）mm,平均值为（0.024±0.009）mm,各组间比较,差异无显著性意义（P〉0.05）。厚度〉500μm的角膜后表面Diff值为（0.023±0.008）mm,厚度〈500μm的角膜后表面Diff值为（0.035±0.014）mm,两组相差非常显著（P〈0.001）.术后3组Diff值均较术前增大,1组为（0.052±0.014）mm;2组为（0.052±0.018）mm;3组为（0.053±0.018）mm,平均值为（0.052±0.017）mm,术前术后两组比较相差非常显著（P〈0.001）。术后根据保留角膜厚度不同分3组,1组保留角膜厚度〉450μm,Diff值为（0.041±0.008）mm;2组保留角膜厚度为410-450μm,Diff值为（0.057±0.013）mm;3组保留角膜厚度为〈410μm,Diff值为（0.080±0.014）mm。3组间比较,差异均非常显著。术后保留角膜床厚度〉280μm者,角膜后表面前凸值为（0.016±0.005）mm;术后保留角膜床厚度〈280μm,角膜后表面前凸值为（0.034±0.009）mm;角膜后表面前凸值平均为（0.017±0.006）mm。结论术前角膜后表面Diff值与近视程度无关,而与角膜厚度有关,较薄的角膜后表面前凸程度较厚的角膜明显。LASIK术后角膜后表面中央部均有不同程度的前凸。保留角膜床厚度和角膜后表面前凸值呈负相关。LASIK后角膜厚度至少应在410μm,残留角膜床厚度安全值应为280μm。

Objective To investigate the changes in corneal posterior surface elevation before and after laser in situ keratomileusis （LASIK） for myopia, and identify the safe residual corneal thickness. Methods Totally 269 eyes of 135 patients with myopia were divided into three groups according to diopter of myopia. The parameters of posterior surface elevation difference of these groups were measured with scanning slit corneal topography （ Orbsean, Orbtek, Inc. ） preoperatively and postoperatively. Results Before surgery, the parameters of posterior surface elevation difference of three groups were （0. 023 ± 0. 008 ） mm, （0. 024 ± 0. 010） mm, and （0. 024 ± 0. 010） mm,the average was （0. 024 ± 0. 009） mm,and there was no significant difference among three groups （P 〉 0. 05 ）. The posterior surface elevation of cornea with thickness of more than 500 μm was （0. 023 ± 0. 008 ） mm, the less than 500 μm was （0. 035 ±0. 014） mm, and there was significant difference between two groups（P 〈0. 001 ）. After surgery, the parameters of posterior surface elevation difference of three groups were more larger than before： （0. 052 ± 0. 014 ） mm, （0. 052 ± 0. 018 ） mm, （0. 053 ±0. 018） mm,the average was （0. 052 ±0. 017） mm,and the difference was significant（P 〈0. 001 ）. The posterior surface elevation of corneawith residual corneal thickness of more than 450 μm was（0. 041 ± 0. 008 ） mm, with that of 410- 450 μm was （0. 057 ± 0.013 ） mm, and with that of less than 410 μm was （0. 080 ± 0. 014） mm. The difference among three groups was statistically significant. According to the thickness of the residual corneal bed, the anterior protrusion of the posterior surface of cornea was （0. 016 ± 0. 005 ） mm when the thickness was more than 280 μm and （0. 034 ± 0. 009 ） mm when that was less than 280 μm, the average was （0.017 ±0.006） mm. Conclusions The changes in cornea posterior surface shape are related to cornea thickness but not to diopter of myopia. The anterior protrusion is obvious in thinner cornea. The anterior protrusion of the posterior surface of cornea is negatively related to the thickness of the residual cornea bed. After LASIK, the cornea has more or less anterior protrusion, the thickness of cornea should be no less than 410 μm, the safe threshold value of residual corneal thickness is 280 μm.