Subbasal nerve density and corneal sensitivity after laser in situ keratomileusis: femtosecond laser vs mechanical microkeratome

Abstract

Objective: To compare changes in subbasal nerve density and corneal sensitivity after laser in situ keratomileusis (LASIK) with the flap created by a femtosecond laser (bladeless) vs a mechanical microkeratome.

Design: In a randomized paired-eye study, 21 patients received myopic LASIK with the flap created by a femtosecond laser in one eye and by a mechanical microkeratome in the fellow eye. Eyes were examined before and at 1, 3, 6, 12, and 36 months after LASIK. Central subbasal nerve density was measured by using confocal microscopy. Corneal mechanical sensitivity was measured by using a gas esthesiometer and was expressed as the ratio of mechanical threshold in eyes that received LASIK to mechanical threshold in concurrent control eyes.

Results: Subbasal nerve density and corneal sensitivity did not differ between methods of flap creation at any examination. Mean (SD) nerve density was decreased at 1 month (bladeless, 974 [2453] μm/mm(2); microkeratome, 1308 [2881] μm/mm(2)) compared with the preoperative examination (bladeless, 10,883 [5083] μm/mm(2), P < .001; microkeratome, 12,464 [6683] μm/mm(2), P < .001) and remained decreased through 12 months (P < .001). Mechanical threshold ratios did not differ from that at the preoperative examination through 36 months for either LASIK treatment; when all LASIK eyes were combined, the mechanical threshold ratio was transiently higher (decreased sensitivity) at 1 month (1.29 [0.85]) compared with the preoperative examination (0.89 [0.73], P = .05).

Conclusions: The planar configuration of the femtosecond laser flaps is not associated with faster reinnervation compared with the microkeratome flaps. The prolonged decrease in subbasal nerve density after LASIK is not accompanied by a prolonged decrease in corneal sensitivity.

Trial registration: clinicaltrials.gov Identifier NCT00350246.

Figure 1. Corneal subbasal nerve density before and after LASIK

Subbasal nerve density did not differ between femtosecond laser and mechanical microkeratome treatments at any examination before or after LASIK. For both treatments, subbasal nerve density was decreased at 1 month after LASIK (p<0.001) and remained decreased through 12 months (p<0.001). At 36 months, subbasal nerve density did not differ from preoperative for either treatment; minimum detectable differences were 7,081 μm/mm² and 7,930 μm/mm² for femtosecond laser and mechanical microkeratome, respectively (α=0.05/5, β=0.20, paired analyses).

Figure 2. Absolute mechanical threshold before and after LASIK and in concurrent controls

Central corneal mechanical thresholds (inversely related to corneal sensitivity) did not differ between femtosecond laser and mechanical microkeratome treatments before or after LASIK. Mechanical threshold appeared to increase transiently at 1 month after LASIK, but the threshold was not significantly greater than it was before LASIK within each treatment. The increase in absolute mechanical threshold from 12 to 36 months after LASIK was accompanied by a similar increase in mechanical threshold in concurrent, unoperated control corneas.

Figure 3. Mechanical threshold ratio before and after LASIK

The mechanical threshold ratio was the ratio of absolute mechanical threshold in LASIK corneas to absolute mechanical threshold in control corneas, and was calculated to account for the drift in absolute mechanical threshold noted in control corneas between 12 and 36 months (see Figure 1). The mechanical threshold ratio did not differ between femtosecond laser and mechanical microkeratome treatments at any examination before or after LASIK. The transient increase in the mechanical threshold ratio at 1 month after LASIK was not statistically significant within each treatment group (femtosecond laser, p=0.65; microkeratome, p=0.39), but when all eyes were combined, the increase from preoperative (i.e. decreased sensitivity) was significant (p=0.05), and returned to preoperative by 3 months (p=0.99) with no subsequent change.