To evaluate the tomography biomechanical index (TBI) value in early corneal dilatation by observing the morphological and biomechanical changes of laser in situ keratomileusis (LASIK) and femtosecond laser-assisted excimer laser in situ keratomileusis (FS-LASIK).

72 patients (117 eyes) who received LASIK or FS-LASIK in the Excimer Laser of Shanxi Eye Hospital from January to December 2018 and had complete clinical data and routine follow-up after surgery for 3 years were collected. According to the TBI, they were divided into two groups normal group (TBI<0.29) and suspicious group (TBI≥0.29). The patient′s age, the corneal diameter, the vision and refraction before and after surgery were collected. Preoperative and postoperative parameters, including corneal central thickness (CCT), thickness of thinnest point of cornea, surface height behind the thinnest point of cornea, maximum keratometry (Kmax), inferior-superior difference value (I-S value), deviation of front elevation difference map (Df), deviation of back elevation difference map (Db), deviation of average pachymetric progression (Dp), deviation of minimum thickness (Dt), deviation of ARTmax (Da), total deviation value (BAD-D), were measured using Pentacam anterior segment analyzer; Pentacam random forest index (PRFI), stiffness parameter (SPA1), Ambrósio′s relational thickness to the horizontal profile (ARTh), integrated radius (IR), deformation amplitude ratio (DA ratio), Corvis biomechanical index (CBI), were measured using the corneal visualization scheimpflug technology. Parameters conform to normal distribution and were expressed by ±s, and compared by ANOVA and t test; the age was compared by chi-square. The preoperative parameters with statistically significant differences were selected and the D-value of preoperative and 3rd postoperative years compared (Δ). The Logistics analysis was performed if variables with statistical significance differences.

The height of posterior corneal surface at the thinnest point in suspicious group was (7.74±1.65) μm, (5.19±1.68) μm and (4.67±1.73) μm before surgery, after surgery for 1 month and 3 years, respectively. There was no significant difference between them (F=0.120, 0.055, 0.407; P>0.05). The height of the corneal posterior surface of the thinnest point before surgery were higher than after surgery with statistical significance differences (t=3.731, 6.631, 3.269, 6.166; P<0.05). There was no significant difference in the surface height after the thinnest point between the two groups before surgery, after surgery for 1 month and 3 years (F=0.120, 0.055, 0.407; P>0.05). Preoperative CBI, PRFI, I-S value, Kmax, Dp, Da and BAD-D in the suspicious group were higher than those of the normal group, and the difference was statistically significant (t=-2.564, -5.629, -2.352, -2.551, -3.268, -2.596, -2.124; P<0.05). ARTh was lower than normal group, and the difference was statistically significant (t=3.239, P<0.05). There was no significant difference in ΔPRFI, ΔI-S value, ΔKmax, ΔDp, ΔDa and ΔARTh between the two groups (t=-1.778, 1.054, 1.407, 1.599, -0.628, 1.151; P>0.05). TBI was taken as the dependent variable, and PRFI, I-S value, Dp, Da, ARTh were used as concomitant variables for multiple Logistic regression analysis. The results showed that PRFI was associated with TBI value changes (β=2.717, 95%CI: 3.104 to73.784, P<0.05).

When the height pattern of the posterior corneal surface was incomplete or decreased, TBI≥0.29 was mainly associated with PRFI. The predictive role of preoperative screening for keratoconus remains to be discussed if TBI≥0.29 was caused by PRFI; the risk of postoperative corneal dilation should be vigilant if TBI≥0.29 was caused by others.