CHANGES IN CORNEAL BIOMECHANICS AFTER LASER IN SITU KERATOMILEUSIS AND PHOTOREFRACTIVE KERATECTOMY

To evaluate the changes in dynamic corneal response second-generation parameters and biomechanically corrected intraocular pressure (bcIOP) after femtosecond laser-assisted laser in situ keratomileusis (FemtoLASIK) and photorefractive keratectomy (PRK).

Refractive Surgery Unit of Ophthalmology Department of Centro Hospitalar Universitário do Porto, Oporto, Portugal

Prospective observational study that included 87 eyes (44 patients) submitted to keratorefractive surgery between November 2020 and April 2021 in Centro Hospitalar Universitário do Porto. FemtoLASIK was performed in 62 eyes and PRK was performed in 25 eyes. The dynamic corneal response second-generation parameters and bcIOP were measured with the OCULUS Corvis® ST, a dynamic Scheimpflyg analyzer. Deformation amplitude ratio 1.0 mm (DA-Ratio), stiffness parameter at first applanation (SP-A1), Ambrósio relational thickness through the horizontal meridian (ARTh), integrated radius (IR), corvis biomechanical index (CBI) and bcIOP were evaluated before the surgical procedure and at 1 and 6 months after surgery.

In the FemtoLASIK group, comparing to the preoperative values, there was an increase in IR and CBI, a decrease in SP-A1, ARTh and bcIOP (p<0.002) and no change in DA-ratio (p>0.183) both at month 1 and month 6. Between month 1 and month 6 there was an increase in ARTh1 and a decrease in bcIOP (p<0.017) and no changes in DA-ratio, SP-A1, CBI and IR (p>0.107). In the PRK group, comparing month 1 to preoperative values, there was an increase in DA-ratio, IR, bcIOP and CBI and a decrease in ARTh (p<0.021) and no change in SP-A1 (p=0.784). At month 6, compared to preoperative values, there was in increase in DA-ratio, IR and CBI and a decrease in SP-A1, ARTh and bcIOP (p<0.002). Between month 1 and month 6 there was an increase in IR and CBI and a decrease in bcIOP and SP-A1 (p<0.031) and no change in ARTh and DA-ratio (p>0.368). The changes in IR, CBI, and ARTh (p<0.001) were more significant in the FemtoLASIK group. Between month 6 and preoperative values, the changes in SP-A1 (p=0.032) were also superior in the FemtoLASIK group, but the changes in bcIOP were similar in both groups (p=0.871). Between month 1 and month 6, the changes in bcIOP were superior in the PRK group (p<0.001).

Both FemtoLASIK and PRK seem to induce corneal biomechanical changes, towards weakening of structures. FemtoLASIK seems to induce greater biomechanical changes than PRK. A longer follow-up would be interesting to evaluate the sustainability of these changes overtime.