[Influence factors and differences of posterior corneal elevation measured by Pentacam system combined with Corvis...

Objective: To investigate the influence factors and differences of abnormal posterior corneal elevation by Pentacam system and Corvis ST. Methods: This retrospective case series study included 227 eyes of 144 patients (90 males, 139 eyes; 54 females, 88 eyes) from December 2017 to October 2018 who were going to receive corneal refractive surgery at the Corneal Refraction Department of Qingdao Eye Hospital. The general data of the patients including gender, age, refractive parameters, optimal correction of spherical and cylindrical diopters were collected. All patients underwent Pentacam system and Corvis ST measurement. According to the back difference (BD) of Pentacam parameters, BD<12 μm was set as the control group (59 patients, 118 eyes) and BD≥12 μm as the high BD group (85 patients, 109 eyes). In the high BD group, BD≤16 μm was set as the suspicious group (44 patients, 53 eyes), while BD>16 μm was set as the abnormal group (41 patients, 56 eyes). Seven parameters of Pentacam and 15 parameters of Corvis ST were selected. The Pentacam parameters included BD, anterior surface keratometry (ASK), posterior surface keratometry (PSK), anterior surface astigmatism (AAstig), posterior surface astigmatism (PAstig), central corneal thickness (CCT), and corneal diameter (W-W). The parameters of Corvis ST included the first applanation time (AT(1)), the first applanation length (AL(1)), the first applanation velocity (AV(1)), the second applanation time (AT(2)), the second applanation length (AL(2)), the second applanation velocity (AV(2)), highest concavity time (HCT), highest concavity peak distance (HC-PD), highest concavity deformation amplitude (HC-DA), highest concavity radius (HC-R), the ratio of deformation amplitude (DA ratio), Integr. Radius, corneal thickness thinnest/pachymetric progression (ARTh), SPA1 (resultant pressure divided by deflection amplitude at the first applanation), and the Corvis Biomechanical Index (CBI). The comparison between the groups was analyzed with Independent sample t test, Kruskal-Wallis H test, and Bonferroni test. Spearman rank correlation analysis was used to explore the correlation factors of BD, and the main factors affecting BD were found through multiple linear regression. Results: There were no statistically significant differences between the control group and the high BD group in age, spherical diopters, and cylindrical diopters (t=-3.311, -1.808, -2.359; P=0.071, 0.072, 0.121, respectively). In Pentacam parameters, ASK, PSK, PAstig, and W-W showed significant differences among groups (Z=18.492, 31.547, 10.773, 70.167; P<0.05). AAstig and CCT showed no statistical difference between groups (P>0.05). Compared with the control group [42.80 (41.98, 44.00)], ASK increased in the abnormal group [43.40 (42.20, 44.40)] significantly (t=-4.292; P<0.05). PSK of the suspicious group [-6.50 (-6.60, -6.35)] and the abnormal group [-6.50 (-6.70, -6.33)] increased significantly compared with the control group [-6.30 (-6.50, -6.20)] (t=4.492, 4.618; P<0.05). Compared with the control group [0.40 (0.30, 0.50)], PAstig of the suspicious group [0.40 (0.30, 0.40)] and the abnormal group [0.40 (0.30, 0.40)] increased significantly (t=2.796, 2.515; P=0.016, 0.036). Compared with the control group [11.50 (11.40, 11.80)], W-W of the suspicious group [11.40 (11.00, 11.60)] and the abnormal group [11.10 (10.90, 11.30)] decreased, and W-W of the abnormal group also decreased significantly compared with the suspicious group (t=3.235, 8.353, 4.282; P<0.05). The correlation analysis between BD and Pentacam parameters of patients in each group showed that BD was negatively correlated with W-W (r=-0.614, -0.304, -0.396, -0.661, P<0.05) in the control group, the suspicious group, the abnormal group, and all patients, while BD had a low correlation with other parameters or no significant correlation. The correlation analysis of BD and Corvis ST parameters in patients showed that only in the suspicious group, BD was positively correlated with AV(1), HCT, and HC-DA (r=0.332, 0.361, 0.382, P<0.05), while no significant correlation was found between BD and other Corvis ST parameters in each group. In order to further explore the main factors affecting BD, Pentacam parameters and Corvis ST parameters were selected as independent variables with BD as the dependent variable to establish a multivariate linear regression analysis model. There was no collinearity between variables W-W, ASK, PSK, HC-PD, SPA1, and CCT (tolerance<0.100). The equation test result was F=37.221, P<0.001, adjusted r(2)=0.504, and the fitting was good. Conclusions: Among the Pentacam parameters, W-W, ASK, and PSK are the main factors affecting the change of BD. HC-PD and SPA1 in the Corvis ST parameters may also have some influence on BD. The Pentacam system combined with Corvis ST is a very useful differential diagnosis system for patients with abnormal BD. (Chin J Ophthalmol, 2020, 56:110-117).