Objective: To evaluate the changes of corneal transparency over the 1-year period after small incision lenticule extraction (SMILE) according to corneal Scheimpflug densitometry. Methods: Prospective cohort study. Fifty-five patients (100 eyes) with myopia (19 male and 36 female, aged 18 to 39 years with average of 24) who underwent SMILE and regular follow-up for at least 1 year at Tianjin Eye Hospital were enrolled. Examinations took place before SMILE and postoperatively at 1 day, 1 week, and 1, 3, 6, and 12 months. The preoperative spherical equivalent of surgical refractive correction was (-5.75±1.64) D. Corneal densitometry (CD) data were calculated automatically by the Pentacam Scheimpflug system and expressed in gray scale units (GSU), from 0 (completely transparent) to 100 (completely opaque). During the analysis of corneal densitometry, the average corneal optical density of three concentric radial zones centered on the apex of the cornea ≤2 mm, >2 mm and ≤6 mm, >6 mm and ≤10 mm diameter) were applied, and three layers were defined according to different corneal depths (the anterior 120 μm as 1st layer, and the posterior 60 μm as 3rd layer, and the center part between them of the cornea as 2nd layer). The automatic release mode was applied in order to avoid operator-induced errors. The preoperative and postoperative values in different corneal layers and different concentric radial zones were compared using Kruskal-Wallis. Results: Ninty-three eyes(93%) of all eyes achieved uncorrected distance visual acuity (UCVA) of 0.8 or better on the first day postoperatively, and 96% (96 of 100 eyes) achieved UCVA of 0.8 or better in the first week postoperatevely, and all eyes 100% (100 of 100 eyes) achieved UCVA 0.8 or better until the end of our follow-up (in 1 year postoperatively). In general, the difference preoperative and postoperative corneal average optical density is of no statistical significance (χ(2)=8.446, P=0.207), specifically, the overall postoperative CD increased in the first week after surgery (1 day to 15.90, 1 week to 15.65), and then decreased to the equivalent of baseline levels and maintained such level until 6 months after surgery (1 month to 15.60; 3 month to 15.50, 6 month to 15.55), though at the 12-month follow-up, densitometry values had spiked significantly again (12 month, 15.90). The difference between the preoperative and postoperative CD values in the central two zones (≤2 mm, >2 mm and ≤6 mm), but not the outer (>6 mm and ≤10 mm), were of statistical significance (χ(2)=61.961, 52.397, P<0.000). When subdivided by corneal layer, significant changes in CD were detected in the anterior 120 μm layer (χ(2)=27.774, P<0.000). The CD increased significantly (i.e., corneal transparency decreased) as compared to the preoperative baseline on the first postoperative day, and lasted through the first week. There were no significant changes in the CD of the central layer (χ(2)=0.918, P=0.986). Significant increases in the posterior 60 μm of the cornea after surgery were found only at the 6-month and 1-year follow-up examinations (χ(2)=17.469, P=0.008). No correlation was found in the preoperative refractive error, ablation depth, postoperative UCVA and the CD. The CD at 6 months was found negatively correlated with SE at 6 months (r=-0.219, P=0.028). Conclusions: SMILE reduced corneal transparency temporarily and primarily within the surgically altered area, however such change has no significant correlation with the ablation depth, SE, and the postoperative UCVA. Transparency recovered within a month, but patients should be monitored for at least 1 year. (Chin J Ophthalmol, 2018, 54: 27-32).
目的： 探讨飞秒激光小切口角膜基质透镜取出术（SMILE）术后角膜透明度在不同时间、不同部位的变化规律及其影响因素。 方法： 前瞻性队列研究。入选在天津市眼科医院接受SMILE的近视及近视散光患者55例（100只眼），其中男性19例（36只眼），女性36例（64只眼），年龄（24±5）岁。术前等效球镜度数为（-5.75±1.64）D。应用Pentacam三维眼前节分析诊断系统测量角膜光密度值，利用其分析软件，检测以角膜顶点为中心≤2 mm、>2 mm且≤6 mm、>6 mm且≤10 mm直径范围角膜光密度的平均值。并根据角膜厚度将角膜分3层，以靠近角膜上皮前表面120 μm厚度为第1层，靠近角膜内皮面的60 μm厚度为第3层，中间部分为第2层，分别进行角膜光密度值测定及分析。于术前、术后1天、1周、1个月、3个月、6个月、12个月进行常规随访与测量。不同部位、不同时间之间光密度值的差异采用Kruskal-Wallis检验，角膜中央区切除厚度、术前等效球镜度数、术后等效球镜度数、术后裸眼视力（UCVA）等因素与角膜光密度值的相关性分析采用Spearman相关分析。 结果： 术后第1天88只眼（88%）UCVA已达到术前最佳矫正视力（BCVA）水平，93只眼（93%）UCVA≥0.8，于术后1周96只眼（96%）UCVA≥0.8，术后1年所有术眼UCVA≥0.8。整体角膜平均光密度值在SMILE手术前后变化无统计学意义（χ(2)=8.446，P=0.207），由术前的15.40增加到术后1 d的15.90，术后1周为15.65，术后1个月及3个月下降并接近术前水平到15.60和15.50，术后6个月又再度增高，至术后1年维持在15.90。≤2 mm与>2 mm且≤6 mm直径区域角膜光密度值手术前后变化有统计学意义（χ(2)=61.961，52.397；P<0.000），术后1 d及1周角膜光密度值分别为17.10、16.70，>2 mm且≤6 mm区域术前、术后1 d及术后1周为14.10、15.20及14.90。>6 mm且≤10 m区域手术前后光密度值变化无统计学意义（χ(2)=4.315，P=0.634）。前120 μm与后60 μm角膜厚度层平均光密度值在SMILE手术前后变化有统计学意义（χ(2)=27.774，17.469；P<0.000，0.008），但中部角膜组织光密度值无明显变化（χ(2)=0.918，P=0.986）。角膜中央区切除厚度、术前等效球镜度数、术后UCVA与术后同期整体角膜光密度值均无明显相关关系，仅在术后6个月的等效球镜度数与光密度值间存在微弱的相关性（r=-0.219，P=0.028）。 结论： 角膜透明度在SMILE术后早期出现一过性下降，后随时间延长逐渐恢复术前水平；透明度的改变主要局限在手术操作区域。透明度改变与角膜中央区切除厚度、术前等效球镜度数、术后UCVA等无明显相关关系。（中华眼科杂志，2018，54：27-32）.
Keywords: Cornea; Corneal stroma; Corneal surgery, laser; Densitometry; Device removal; Laser, femtosecond; Lenses.