Evaluation of corneal stromal changes in vivo after laser in situ keratomileusis with confocal microscopy

Purpose: To assess by in vivo confocal microscopy the modifications of the corneal stroma after laser in situ keratomileusis (LASIK) for myopia. Design: Nonrandomized comparative (self-control led) trial. Participants: Sixteen eyes of 13 patients were examined before surgery and at days 8, 30, and 90, and 9 eyes were examined at 6 months postoperatively using an in vivo confocal microscope. Testing/Intervention: Stromal morphologic changes, keratocyte density, flap thickness, and subclinical haze were evaluated and compared at different time points. LASIK was performed with a Flapmaker microkeratome (Solan Ophthalmic products, Jacksonville, FL) and a Lasersight LSX excimer laser (LaserSight Technologies Inc., Winter Park, FL). Main Outcome Measure: Confocal microscopy results. Results: Microfolds at the Bowman's layer were found in most eyes, as well as variable reflectivity particles (pa) located at the interface level in all eyes examined postoperatively. The density of these particles significantly decreased with time with, respectively, 504 +/- 101 pa/mm(2) at day 8 and 380 +/- 111 pa/mm(2) at day 30 (P = 0.003), 332 +/- 100 pa/mm(2) at month 3 and 312 +/- 40 pa/mm(2) at month 6. The mean flap and the activated-cells area thicknesses were, respectively, 102 +/- 26 mum and 61 +/- 19 mum and showed significant negative correlation (P < 0.0001). The intensity of the added peak (47.3 <mu>m 8.6%), corresponding to the subclinical haze, realized by Z-scan measure, was also negatively correlated with flap thickness (P = 0.01). Keratocyte (k) density quantified in the posterior stroma significantly increased from day 0 (480 +/- 67 k/mm(2)) to day 8 (701 +/- 41 k/mm(2), p < 0.0001 compared with day 0) and day 30 (917 +/- 143 k/mm(2), p = 0.0006, compared with day 0) but significantly decreased at 3 months postoperatively (597 +/- 56 k/mm(2), p < 0.0001 compared with day 30) to reach the initial level at month 6 (502 +/- 41 k/mm(2), nonsignificant compared with day 0). There was no correlation between preoperative or postoperative spherical equivalent and the density of particles, keratocytes, and the haze intensity. Conclusions: This study confirms the presence of microfolds and particles at the interface level, as well as subclinical impairment. Evaluation of keratocyte density constitutes a major contribution of confocal microscopy toward an understanding of the keratocyte response to corneal wound healing after corneal refractive surgery. Moreover, flap thickness seems to be involved in the postoperative cellular activation with a higher response when thin. Ophthalmology 2001;108:1744-1750 (C) 2001 by the American Academy of Ophthalmology.