Comparison of Residual Stromal Bed Thickness and Flap Thickness at LASIK and Post-LASIK Enhancement in Femtosecond Laser-Created Flaps
Narrative Responses:
Purpose
To compare the changes in calculated flap thickness and calculated and measured residual stromal bed thickness between initial LASIK and post-LASIK enhancement in myopic patients with LASIK flaps created by femtosecond laser.
Methods
This retrospective, non-randomized, comparative case series comprised 37 eyes of 35 patients with femtosecond laser-assisted laser in situ keratomileusis (LASIK) and post-LASIK enhancement. The main outcomes measures were: 1) comparison of residual stromal bed thickness (RSB) between initial LASIK and at time of enhancement 2) comparison of calculated flap thickness (FT) between initial LASIK and at time of enhancement. We calculated RSB (RSB-1, RSB-2, RSB-3) using three different formulas, which were compared against the measured RSB at time of enhancement.
Results
Our data show a significant difference (p <0.0001) when comparing RSB-1, RSB-2, RSB-3, and measured RSB at time of enhancement. Tukey HSD test revealed that each calculated RSB was erroneously thicker than actual measured RSB at time of enhancement. On average, the actual measured RSB was 17um thinner than the calculated RSB as estimated with a formula using the planned ablation depth. Calculated flap thickness at time of enhancement surgery was significantly thicker compared to calculated flap thickness at time of initial LASIK for both the planned 110um flap (p < 0.001) and planned 90um flap (p < 0.01).
Conclusion
In femtosecond laser-assisted LASIK, RSB measured at post-LASIK enhancement is thinner compared to calculations using ablation depths or pre-operative pachymetry. FT calculation at post-LASIK enhancement is thicker than estimated. Though femtosecond laser-assisted LASIK causes less intraoperative IOP fluctuations as compared to microkeratome-based LASIK, RSB is still thinner than estimated.