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

Abstract

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.