[Evaluation of two biomechanical stiffness indexes in the diagnosis of eratoconus and their changes after corneal collagen cross-linking surgery].

Objective: To evaluate the diagnostic efficacy of stress-strain index (SSI) for different stages or degrees of keratoconus and changes of SSI and stiffness parameter A1 (SPA1) after corneal collagen cross-linking (CXL) surgery. Methods: Cross-sectional study and retrospective case series study. Ninety-four patients (113 eyes) diagnosed as clinical keratoconus (CKC) in Qingdao Eye Hospital from July 2019 to August 2021 were enrolled in the CKC group, including 69 males and 25 females, aged (20.82±4.53) years, and further divided into subgroups of mild (35 patients, 36 eyes), moderate (36 patients, 40 eyes) and severe (33 patients, 37 eyes) CKC. Fifty-six unaffected eyes of monocular keratoconus patients were enrolled in the subclinical keratoconus (SKC) group. Ninety-one healthy subjects (91 eyes) were recruited as the control group. All subjects were examined by Pentacam topography and Corvis ST measurements to obtain mean keratometry, maximal keratometry, deformation amplitude (DA) ratio at 2 mm, integrated radius (IR), Ambrósio's relational thickness to the horizontal profile, corneal central thickness, SPA1 and SSI for comparison. Forty-eight CKC patients (65 eyes) underwent CXL surgery, and the above parameters were recorded before and 3, 6 and 12 months after operation. Data were analyzed by the ANOVA test, Kruskal-Wallis H test, paired sample test, receiver operating characteristic curves and Pearson correlation. Results: The value of SPA1 in the SKC group accounted for 85.53% (87.92±12.38 vs. 102.79±11.74; t=-6.614, P<0.001) compared with the control group, but the value of SSI had no difference in the two groups (t=0.105, P=0.916). The value of SPA1 in the CKC group accounted for 52.87% (54.35±14.70 vs. 102.79±11.74; t=25.985, P<0.001) compared with the control group. The value of SSI in the CKC group accounted for 67.96% (0.70±0.14 vs. 1.03±0.14; t=-15.305, P<0.001) compared with the control group. The more severe the disease was, the smaller the SPA1 and SSI values were 64.27±12.12, 55.22±12.23, 43.75±12.33; 0.78±0.14, 0.71±0.11, 0.61±0.09, and there were significant statistical differences among groups (mild vs. moderate, mild vs. severe, moderate vs. severe; SPA1: t=3.257, -7.249, -4.159; all P<0.001. SSI: t=2.383, 5.065, 2.798; P=0.018,<0.001,=0.006). Receiver operating characteristic analysis showed that SPA1 had good diagnostic efficiency for subclinical patients [area under curve (AUC)=0.802], while the SSI had no diagnostic value (P=0.802). SPA1 had better diagnostic efficiency than the SSI for keratoconus in different stages, especially in the mild CKC and SKC groups (AUC: 0.914 vs. 0.847). The SSI had a significant positive correlation with SPA1 and a significant negative correlation with DA ratio and IR in the control, SKC and CKC groups (r=0.278, 0.368, 0.550; r=-0.346, -0.462, -0.547; r=-0.612, -0.591, -0.718; P<0.01). For patients who received CXL, maximal keratometry decreased significantly at 6 and 12 months postoperatively (t=4.029, 3.633; all P<0.001), whereas SPA1 increased significantly (t=-3.960, -4.500; all P<0.001). However, the SSI only increased significantly at 3 months (t=-2.577, P=0.012) and returned to the preoperative level at 6 and 12 months postoperatively, with no statistical difference compared with the preoperative level (t=-0.544, -0.257; P=0.589, 0.798). Conclusions: While there was no significant change in the SSI of SKC, the SSI of CKC decreased, and the more severe the disease was, the smaller the value was. The SSI was significantly and consistently correlated with DA ratio, IR and SPA1. The SSI compared with SPA1 had a lower degree of identification in different stages and degrees of keratoconus. The consistency of SPA1 with clinical effects after CXL surgery was higher than that of the SSI parameter.目的： 探讨角膜应力-应变指数（SSI）对于不同分期或程度的圆锥角膜的诊断效力，以及其与第1次压平硬度（SPA1）在角膜胶原交联术（CXL）后的变化。 方法： 横断面研究及回顾性病例系列研究。纳入2019年7月至2021年8月青岛眼科医院确诊的临床期圆锥角膜（CKC）患者94例（113只眼）作为CKC组，其中男性69例，女性25例，年龄（20.82±4.53）岁；并进一步按疾病程度分为轻（35例，36只眼）、中（36例，40只眼）、重度（33例，37只眼）3个亚组；纳入CKC组56例单眼发病患者的未发病眼作为亚临床期圆锥角膜（SKC）组；纳入健康者91例（91只眼）作为对照组。所有受试眼均行Pentacam角膜地形图和可视化角膜生物力学分析仪检查获取角膜平均曲率（Km）、角膜前表面最大曲率（Kmax）、角膜顶点和距顶点2 mm处的形变幅度比值（DAR）、综合半径（IR）、水平方向Ambrósio相关厚度（ARTH）、角膜中央厚度（CCT）、SPA1及SSI等参数进行比较。另收集CKC组中行CXL术的48例（65只眼）患者资料于术前及术后3、6、12个月分别检查上述参数。采用方差分析、Kruskal-Wallis H检验、配对t检验、受试者操作特征（ROC）曲线、Pearson相关分析等对数据进行分析。 结果： SKC组的SPA1约为对照组的85.53%（87.92±12.38和102.79±11.74；t=-6.614，P<0.001），而SSI与对照组差异无统计学意义（t=0.105，P=0.916）。CKC组的SPA1约为对照组的52.87%（54.35±14.70和102.79±11.74；t=25.985，P<0.001），SSI约为对照组的67.96%（0.70±0.14和1.03±0.14；t=-15.305，P<0.001）。轻、中、重度亚组的SPA1分别为64.27±12.12、55.22±12.23、43.75±12.33，SSI分别为0.78±0.14、0.71±0.11、0.61±0.09，组间差异均有统计学意义（均P<0.05）。SPA1对SKC具有良好的鉴别力（曲线下面积为0.802），而SSI则无诊断价值（P=0.869）；相比SSI，SPA1对不同程度圆锥角膜的鉴别力更高，其中鉴别轻度CKC与SKC的ROC曲线下面积为0.914。对照组、SKC组及CKC组SSI均与SPA1呈正相关（r=0.278，0.368，0.550）、与DAR（r=-0.346，-0.462，-0.547）、IR（r=-0.612，-0.591，-0.718）呈负相关（均P<0.001）。圆锥角膜患者CXL术后6、12个月，Kmax相比术前明显降低（t=4.029，3.633；均P<0.001）；SPA1则在术后6、12个月明显上升（t=-3.960，-4.500；均P<0.001）；但SSI仅于术后3个月有所增加（t=-2.577，P=0.012），术后6、12个月回降至术前。 结论： SKC眼SSI无明显变化，CKC眼的SSI降低，且程度越重降低越明显；SSI与DAR、IR及SPA1具有相关性；SSI对不同分期及程度的圆锥角膜的诊断效力不如SPA1。圆锥角膜眼行CXL术后中期SPA1与临床效果的一致性较SSI更高。.