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Amira M Eltony, Peng Shao, Theo G Seiler, Amy Scally, Roberto Pineda II, Theo Seiler, Seok-Hyun (Andy) Yun; Longitudinal Brillouin measurements of the human cornea before and after LASIK. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1400.
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© ARVO (1962-2015); The Authors (2016-present)
Changes in the cornea’s mechanical response after LASIK have been observed using instruments based on air-puff deformation, but it has not been possible to distinguish between the effects of larger structural changes and possible underlying material changes in the tissue. Brillouin light-scattering microscopy measures local longitudinal modulus in corneal tissues. This study investigates biomechanical changes in corneal tissue before, one day, and one month after LASIK treatment in vivo using Brillouin micro-spectroscopy.
Brillouin frequency shift was measured using a 780-nm laser in the central corneal zone (<1 mm from the pupil center, averaged over the stromal depth) of 26 eyes from 20 subjects (mean age: 49.3 years old, 60% female) receiving LASIK surgery. All surgeries comprised standard femtosecond-laser flap creation followed by wavefront-guided excimer laser ablation of depths ranging from 7 to 114 µm (mean: 55.2 µm).
Before surgery, the mean Brillouin frequency shift measured was 5.722 ± 0.027 GHz (mean ± std. dev.). One day after LASIK, the mean value was significantly lower (5.687 ± 0.027 GHz, p < 0.001, unpaired t-test), but by one month after surgery the average Brillouin value recovered to the pre-surgery level (5.721 ± 0.020 GHz, n.s.). The mean individual change (paired difference) in Brillouin value after one day was -31.4 ± 21.4 MHz (p < 0.001, paired t-test), and after one month was 0.4 ± 36.7 MHz (n.s.).
We have measured Brillouin values of patients undergoing LASIK for the first time to our knowledge. We attribute the transient reduction in Brillouin frequency shift one day after LASIK to an increase in hydration (swelling) immediately after surgery. The Brillouin value one month after LASIK is the same as its pre-surgical value within the instrumental resolution (10 MHz), which is consistent with previous findings that the collagen fiber structure in the corneal flap and underlying stromal bed are unchanged following LASIK. Unlike air-puff measurements, Brillouin value is independent of structural factors such as corneal thickness. Therefore, the recovery of Brillouin value suggests that the material properties of the cornea are not significantly altered by LASIK, and that the global change in biomechanical parameters measured with air-puff-based instruments is likely due to the reduced corneal thickness and incomplete flap adhesion.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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