Corneal Keratocyte Deficits After Photorefractive Keratectomy and Laser In Situ Keratomileusis

Keratocyte density distribution throughout the normal full-thickness stroma.¹⁷ Photorefractive keratectomy removed the keratocyte-dense anterior stroma during surface ablation (light gray area). Laser in situ keratomileusis (LASIK) removed the less-keratocyte-dense middle stroma (dark gray area).

Schematic representation of the five stromal layers studied in the pre-PRK (photorefractive keratectomy) and post-PRK cornea. Stromal layers in the pre-PRK cornea are compared with corresponding layers in the post-PRK cornea.

Schematic representation of the six stromal layers studied in the pre-LASIK (laser in situ keratomileusis) and post-LASIK cornea. The thickness of stroma equivalent to the ablation depth (that is, ablation zone) in the pre-LASIK cornea was omitted from analysis to allow comparison of the same tissue layers preoperatively and postoperatively. RAZ = retroablation zone; ANT = anterior; POST = posterior.

Method to determine keratocyte density. Keratocyte nuclei (Top) were manually counted to determine keratocyte density (cells/mm³) in two images from each stromal layer, as described by Patel and associates.¹⁷ By convention, nuclei overlapping the edges of the bounding box were counted on only two sides (Bottom).

Keratocyte density before and after photorefractive keratectomy (PRK). In the anterior 10% of the stroma, keratocyte density was diminished at all post-PRK visits relative to density before PRK. Cell density in most remaining stromal layers was not decreased until five years after PRK. *P < .001 and ^†P < .05, compared with densities before PRK. ANT = anterior.

Keratocyte density before and after laser in situ keratomileusis (LASIK). In the anterior and posterior stromal flap and the anterior RAZ, keratocyte density was decreased at all post-LASIK visits from density before LASIK. Cell densities in all remaining stromal layers were first decreased at five years after LASIK. *P < .005 and ^†P < .05, when compared with densities before LASIK. RAZ = retroablation zone; ANT = anterior; POST = posterior.

Estimated cell density in anterior 10% to 33% of stroma of 212 untreated normal corneas from other studies, and in the posterior flap of corneas after LASIK throughout this study. The solid line is the least-squares regression of cell densities in corneas treated with laser in situ keratomileusis (LASIK) (average rate of cell loss = 1897 cells/mm³ per year, r = 0.51, P <. 001), and the dashed line is the regression of cell densities in untreated corneas (average rate of cell loss = 445 cells/mm³ per year, r = 0.20, P = .004). Densities in untreated corneas were estimated by three investigators, whereas all densities in corneas treated with LASIK were estimated by one investigator.

Change in cell and background brightness over time. Mean brightness of cells (open symbols) and the area immediately surrounding the cells (solid symbols) in the layers immediately anterior and posterior to the interface, and the posterior 90% to 100% of stromal thickness after laser in situ keratomileusis (LASIK). Brightness was adjusted for variations in the illumination brightness and sensitivity of the video camera by using measurements of a fluorescent glass standard. Units of brightness are arbitrary and represent the digitized output of the video camera. Brightness remained steady between one and five years when keratocyte density decreased, suggesting that potential changes in confocal image brightness did not affect our ability to identify cells.

Change in contrast. Mean contrast between cells and background was calculated in the same frames used to determine cell density. After laser in situ keratomileusis (LASIK), contrast increased to a maximum at six months and then returned to pre-LASIK contrast by one year. Contrast changed by 0.01 or less between one and five years, suggesting that potential changes in image contrast did not affect our ability to identify cells.