Evaluation Of The Change In The Deformation Amplitude Ratio, Integrated Radius And Stress Strain Index As Parameters Of Corneal Biomechanics Post Different Laser Vision Correction Procedures

Recently the Corvis ST (CST; corneal visualization Scheimpflug technology) has been introduced as a clinical tool for evaluating corneal biomechanical properties in vivo.

Our aim was detecting the SD of some parameters that changed from measurement A (pre-op) to B (post-op) and whether this change was indicating a stiffening, a softening or no significant change in the cornea after PRK, LASIK and Femto LASIK. This outcome was carried out through evaluation of the changes in Deformation Amplitude ratio (DA ratio), integrated radius (IR) and Stress Strain Index (SSI) before and after LVC using CORVIS ST and comparison between delta changes in the 3 different LVC procedures regarding the same previous indices.

Candidates were divided into 3 groups, each of them underwent Corvis ST scan before and after different LVC procedure, PRK, LASIK and Femto-LASIK. The study was conducted between March 2020 and Jan 2022 in DEO eye hospitals and Research Institute of Ophthalmology, Cairo, Egypt.

It was a prospective, randomized, comparative interventional study. Patients with maximum (MRSE) of -7.00 diopters and with maximum astigmatism of -3.00 diopters had their corneas captured using the Corvis ST before and after three different laser vision correction (LVC) procedures including PRK, LASIK and Femto-LASIK to detect the deformation amplitude ratio (DA ratio), Integrated radius (IR) and Stress strain index (SSI).

Group A underwent PRK their epithelium was removed manually, Group B underwent LASIK with Moria microkeratome to create their flaps and Group C of FemtoLASIK used the platform Alcon Wavelight FS-200 femtosecond to create their flaps. The EX500 excimer laser is applied to perform corneal ablation in all patients.

Demographic data of the patients showed 33 patients with total 66 eyes with a mean age of 29.12 ± 7.13, percentage of women was 72.7% (24 patients) and men was 27.3% (9 patients). In Group A (PRK) the Mean SE was -3.35 ± 1.23 SD, Group B (LASIK) the Mean SE was -3.06 ± 1.23 SD and Group C (Femto-LASIK) the Mean SE was -3.21 ± 2.11 SD. Comparison between pre and post treatment showed significant increase in the DA ratio and IR with no significant change in SSI in Group A. While in Group B and C showed significant increase in DA ratio, IR and significant decrease in SSI. The SP A1 change was significantly decreased in all 3 groups.

The corneal biomechanical response to the three surgical procedures varied differently and the signifcant change was in the IR, where the SSI and Da ratio was non significant. The reductions in overall corneal stiffness were the highest in LASIK and lowest in PRK, whereas the Femto LASIK remained in between.