4　激光引起角膜内皮细胞损伤的影响因素

　　4.1　切削深度　Marshall等［27～29］通过动物实验发现193nm准分子激光切削深达角膜厚度80%～90％或切削到与角膜内皮面相距大约40μm时，角膜内皮细胞的组织学或超微结构发生急性损伤。Lambert等［10］通过离体实验发现人角膜PRK切削深达150μm时，角膜内皮细胞密度降低到对照眼的60％，且细胞形态亦发生了改变。Carones等［3］发现PRK切削深达24～113μm时，角膜内皮细胞缺失与切削深度无相关性。Kang等［11,30～32］报道LASIK切削角膜深达250μm时，角膜内皮细胞就有损伤的危险。这些证据表明激光切削达到一定深度时才能导致角膜内皮细胞的损伤。

　　4.2　激光频率　Stulting等［6］报道在激光能量较低，脉冲重复率10～50次·min-1时，即使切削深达100％亦未发现角膜内皮细胞急性损伤；Ozler等［15］发现在激光能量恒定时，高激光频率能导致较严重的角膜内皮细胞损伤。

　　4.3　消融直径　在矫正屈光度恒定时，切削直径与切削深度呈正相关，若减少切削深度，则应使用较小光学切除区，但小光学切除区可增加切削偏中心、夜间近视和术后眩目的可能性，因此理想的消融应该是直径最大而又对角膜内皮细胞无损伤［17］。

　　4.4　激光切削次数　重复激光切除术，内皮细胞具有损伤的危险，Seiler等［33］发现再次激光手术可增加内皮细胞缺失的可能性。

　　4.5　术前配戴接触眼镜　Stulting等［6］报道术前未戴过接触镜者，PRK后追踪未发现细胞变异系数改变，而术前戴过接触镜者，术后发现中央和周边角膜内皮细胞变异系数降低，PRK后2a，中央内皮细胞和周边部细胞变异系数分别下降10.4%和12.4%；他们认为PRK引起角膜内皮细胞大小改变的作用是小的和暂时的，PRK后变异系数下降可能是由于术前配戴接触镜导致角膜内皮细胞发生变异，而PRK后停戴接触镜后细胞形态逐渐恢复之故。Carones等［3］则认为PRK术后由于角膜Bowman氏膜缺乏和基质变薄，使外界氧气易于渗入内皮细胞层，因而术后角膜内皮细胞的形态变异系数和面积变异系数得以改善。

　　4.6　术后眼压升高　术后局部长期应用皮质类固醇类药物可导致眼压升高，Korey等［34］报告眼压升高可导致中央角膜内皮细胞缺损。Carones等［3］发现PRK，后13.2%的患眼眼压超过了3.2kPa，但未发现内皮细胞明显缺失。相反，内皮细胞缺失较严重的患眼而无眼压升高。

　　4.7　其他眼病　激光角膜光学切除术能影响其他眼病对角膜内皮细胞损伤的敏感性，Lambert等［10］报道以往施行过PRK的患眼，当发生虹膜炎、青光眼、眼外伤及进行白内障手术时较易损伤角膜内皮细胞。

　　5　展望

　　PRK和LASIK已成为角膜屈光性手术的研究热点，由于LASIK适用于轻、中和高度近视眼的矫治，且术后视力恢复快、无不适感、并发症少等优点，已越来越被广大近视患者所接受，具有取代PRK的趋势，因此确定LASIK安全切削范围势在必行，LASIK最大安全切削深度、最高脉冲总数、脉宽及最大激光能量范围等有待于进一步研究。

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收稿　1998-06-08　修回　1998-08-10

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