In Vivo Corneal Biomechanical Response to Three Different Laser Corneal Refractive Surgeries.

PURPOSE: To compare the effects of three common refractive surgeries on corneal biomechanics. METHODS: Two hundred seven patients who had refractive surgery were included in this study, of whom 65 received transepithelial photorefractive keratectomy (tPRK), 73 received femtosecond laser-assisted laser in situ keratomileusis (FSLASIK), and 69 received small incision lenticule extraction (SMILE). Each patient had biomechanical measurements using the Corvis ST (Oculus Optikgeräte GmbH) preoperatively and at 3 and 6 months postoperatively. The measurements included five parameters expected to be associated with corneal biomechanics: deformation amplitude ratio at 2 mm (DAR2), integrated inverse radius (IIR), stiffness parameter at first applanation (SP-A1), highest concavity time (HCT), and the updated stress-strain index (SSIv2). The variations in these parameters postoperatively among the three surgeries, and their relationship with corneal thickness (CCT) and intraocular pressure measured by the Dynamic Contour Tonometer (DCT-IOP) were analyzed. RESULTS: SP-A1 decreased significantly from preoperatively to 3 months postoperatively in all three groups, whereas DAR2 and IIR increased significantly, all indicating stiffness losses. Between 3 and 6 months postoperatively, the results were inconsistent, with DAR2 decreasing (indicating stiffness increases) and IIR increasing (denoting stiffness decreases) in the FS-LASIK and SMILE groups. The decrease in SSIv2 (the only measure of corneal material stiffness) postoperatively was comparatively less pronounced at both 3 and 6 months postoperatively. On the other hand, HCT remained generally stable after all three surgeries. Unlike DAR2, IIR, and SP-A1, the changes postoperatively in stiffness parameters HCT and SSIv2 were independent of the corresponding changes in both DCT-IOP and CCT. CONCLUSIONS: Among the stiffness parameters considered, SSIv2 was not correlated with CCT or DCT-IOP, and holds promise for representing the corneal material stiffness and how it remains largely unaffected by refractive surgeries. Overall, FS-LASIK had the most significant impact on corneal stiffness, followed by SMILE, and finally tPRK. [J Refract Surg. 2024;40(5):e344-e352.].

Performance of Corvis ST Parameters Including Updated Stress-Strain Index in Differentiating Between Normal, Forme-Fruste, Subclinical, and Clinical Keratoconic Eyes.

PURPOSE: This study seeks to evaluate the ability of the updated stress strain index (SSIv2) and other Corvis ST biomechanical parameters in distinguishing between keratoconus at different disease stages and normal eyes. DESIGN: Diagnostic accuracy analysis to distinguish disease stages. METHODS: 1084 eyes were included and divided into groups of normal (199 eyes), forme fruste keratoconus (FFKC, 194 eyes), subclinical keratoconus (SKC, 113 eyes), mild clinical keratoconus (CKC-Ⅰ, 175 eyes), moderate clinical keratoconus (CKC-Ⅱ, 204 eyes), and severe clinical keratoconus (CKC-Ⅲ, 199 eyes). Each eye was subjected to a Corvis ST examination to determine the central corneal thickness (CCT), biomechanically corrected intraocular pressure (bIOP), SSIv2 (updated stress-strain index), and other 8 Corvis parameters including the stress-strain index (SSIv1), stiffness parameter at first applanation (SP-A1), first applanation time (A1T), Ambrósio relational thickness to the horizontal profile (ARTh), integrated inverse radius (IIR), maximum deformation amplitude (DAM), ratio between deformation amplitude at the apex and at 2 mm nasal and temporal (DARatio2), and Corvis biomechanical index (CBI). The sensitivity and specificity of these parameters in diagnosing keratoconus were analyzed through receiver operating characteristic curves. RESULTS: Before and after correction for CCT and bIOP, SSIv2 and ARTh were significantly higher and IIR and CBI were significantly lower in the normal group than in the FFKC group, SKC group and the 3 CKC groups (all P < .05). There were also significant correlations between the values of SSIv2, ARTh, IIR, CBI, and the CKC severity (all P < .05). AUC of SSIv2 was significantly higher than all other Corvis parameters in distinguishing normal eyes from FFKC, followed by IIR, ARTh and CBI. CONCLUSION: Corvis ST's updated stress-strain index, SSIv2, demonstrated superior performance in differentiating between normal and keratoconic corneas, and between corneas with different keratoconus stages. Similar, but less pronounced, performance was demonstrated by the IIR, ARTh and CBI.

Corneal Epithelial Remodeling in a 6-Month Follow-up Period in Myopic Corneal Refractive Surgeries.

PURPOSE: To investigate corneal epithelial thickness changes during a 6-month follow-up period after transepithelial photorefractive keratectomy (tPRK), femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK), and small incision lenticule extraction (SMILE). METHODS: This prospective study included 76 eyes of 76 participants who underwent myopic refractive surgery (23 FS-LASIK, 22 SMILE, and 31 tPRK). Epithelial thickness and anterior curvature were averaged over 4 regions (subdivided into 25 areas) and measured by spectral-domain optical coherence tomography and Scheimpflug tomography before the operation (pre) and at 1 or 3 days (pos1-3d), 1 week (pos1w), and 1 month (pos1m), 3 months (pos3m), and 6 months (pos6m) postoperatively. RESULTS: The epithelial thickness of the three groups was similar in both the pre and pos6m (all P > .05), but the tPRK group fluctuated the most during the follow-up period. The largest increase was in the inferior-temporal paracentral area (7.25 ± 2.58 µm for FS-LASIK; 5.79 ± 2.41 µm for SMILE; 4.88 ± 5.84 µm for tPRK; all P < .001). Only the epithelial thickness of tPRK increased from pos3m to pos6m (P < .05), whereas all changes for FS-LASIK and SMILE were not significant (P > .05). A positive correlation of thickness changes with curvature gradient in the paracentral region of tPRK was found (r = 0.549, P = .018), but not in other regions in all groups. CONCLUSIONS: Epithelial remodeling followed different trends after different surgeries from the early postoperative stage onward, but exhibited similar values at pos6m. Although remodeling after FS-LASIK and SMILE stabilized by pos3m, it remained unstable at pos6m after tPRK. These changes may affect corneal profile and lead to deviation from the intended surgical outcome. [J Refract Surg. 2023;39(3):187-196.].