[Analysis on corneal deformation and corneal biomechanical changes after small incision lenticule extraction]

Study on change in corneal biomechanics and effect of percent tissue altered in myopic laser-assisted in situ keratomileusis

Purpose

To evaluate corneal biomechanical changes and their correlation with the percentage of tissue altered (PTA) in myopic femtosecond (FS)-flap LASIK.

Methods

Prospective longitudinal observational study of 80 eyes of FS LASIK. Demographic details, LASIK parameters, preoperative and postoperative (day 1, month 1, 3, and 6), UCVA, BCVA, refraction, corneal topography, corneal hysteresis (CH), and a corneal resistance factor (CRF) were noted. Change in CH and CRF and its correlation with PTA were analyzed. Data were analyzed in three subgroups [subgroup 1: PTA 23 to <27%; subgroup 2: 27 to <33%; subgroup 3: 33 to <40%].

Results

FS LASIK for MRSE -3.5D ± 1.6D with mean PTA of 31.6 ± 4.4% (range 23.8-39.8%), showed statistically significant decrease in CH and CRF. Mean CH decreased from a preoperative value of 10.4 ± 1.9 to 8.1 ± 1.1; mean CRF from 10.5 ± 1.6 to 7.5 ± 1.3 at 6-months postoperative period, respectively. Mean preoperative CH decreased by 25%, 24%, 23%, and 21% and mean preoperative CRF decreased by 34%, 28%, 28%, and 28% at postoperative day 1, month 1, 3, and 6 follow-ups. Mean CH and CRF showed a significant negative correlation with PTA (CH: r = - 0.33 [P = <0.0001], CRF: r = -0.34 [P = <0.001]. Subgroup analysis noted greater decrease in CRF and CH in eyes with higher PTA (subgroup 3).

Conclusion

Myopic FS LASIK causes a decrease in corneal biomechanics with a significant negative correlation with PTA indicating a greater decrease in corneal biomechanics with higher PTA.

Future Developments in SMILE: Higher Degree of Myopia and Hyperopia

Small incision lenticule extraction (SMILE) is a novel 1-step refractive procedure with femtosecond laser for the correction of myopia and myopic astigmatism. Although it has shown good clinical results in efficacy, safety, predictability, and stability, there are still some concerns. In this study, we review the published clinical outcomes of high myopia correction and exploration in hyperopia correction. Results have suggested that SMILE has acceptable outcomes in correction for high myopia <10.0 diopters (D), and it is a feasible and effective procedure for the treatment of hyperopia. However, it is unsuitable for the treatment of extremely high myopia because there is undercorrection and regression as existed in laser-assisted in situ keratomileusis (LASIK), and compound hyperopic astigmatism currently could not be corrected either. More technical and clinical improvements are required to make SMILE competitive.