Changes in Keratocyte Density and Visual Function Five Years After Laser In Situ Keratomileusis: Femtosecond Laser Versus Mechanical Microkeratome

Abstract

Purpose: To determine the effects of keratocyte loss on optical properties and vision after laser in situ keratomileusis (LASIK) with the flap created with a femtosecond laser or a mechanical microkeratome.

Design: Randomized clinical paired-eye study.

Methods: Both eyes of 21 patients received LASIK for myopia or myopic astigmatism. One eye of each patient was randomized by ocular dominance to flap creation with a femtosecond laser and the other eye to flap creation with a mechanical microkeratome. Before LASIK and at 1, 3, and 6 months and 1, 3, and 5 years after LASIK, keratocyte density was measured using confocal microscopy, and high-contrast visual acuity and anterior corneal wavefront aberrations were measured by standard methods. At each visit, all variables were compared between methods of creating the flap and to the same variable before treatment using paired tests with Bonferroni correction for multiple comparisons.

Results: Keratocyte density in the flap decreased by 20% during the first year after LASIK and remained low through 5 years (P < .001). High-order wavefront aberrations increased and uncorrected visual acuity improved immediately after surgery, but these variables did not change further to 5 years. There were no differences in any variables between treatments.

Conclusions: A sustained reduction in keratocyte density does not affect vision or optical properties of the cornea through 5 years after LASIK. The method of creating a LASIK flap does not influence the changes in keratocyte density in the flap.

Figure 1

Mean cell density through the corneal stroma in untreated control subjects (left) and LASIK patients (right), before LASIK and at 5 years. The vertical broken line indicates the average depth of the surgical interface. Depth of each frame was scaled as a percentage of the total stromal depth, based on landmarks of the stromal boundaries in each scan. In control participants, fellow eyes were randomized for comparison to eyes in LASIK patients with flaps created by the femtosecond laser (FS Laser) or mechanical microkeratome (Mechanical). Cell density decreased in the flap after surgery and remained low at 5 years. There were no differences between treatments.

Figure 2

Change in mean cell density in untreated control participants (left) and LASIK patients (right). After LASIK cell density in the anterior flap decreased during the first year and remained low through 5 years. In the anterior retro-ablation zone (RAZ, region 100 µm deep to the surgical interfaced) and posterior third of the stroma, cell density did not change. In control participants, fellow eyes were randomized for comparison to eyes in LASIK patients with flaps created by the femtosecond laser (FS Laser) or mechanical microkeratome (Mechanical). There were no differences between treatments. Horizontal lines represent pre-LASIK densities.

Figure 3

Mean backscatter (haze) in control subjects and after LASIK at 1 year (left) and 5 years (right). Backscatter was significantly lower in the flap than in the equivalent region of un-operated control corneas at 1 year, and this reduction was consistent through 5 years (p<0.001, Bonferroni-adjusted for 20 layers).

Figure 4

Relationship between mean backscatter (confocal image brightness) and cell density in control subjects and LASIK patients. Each point represents the mean image brightness and cell density in one layer of stroma from all subjects at one visit. After LASIK, backscatter was lower in the flap (three right-most open circles) than would be expected from the decrease in cell density based on the relationship between image brightness and cell density in the control group. The frames closest to the endothelium were excluded because the image brightness was dominated by reflectance from the endothelial surface. Data were from visits at 1, 3, and 5 years.