Corneal endothelial changes 5 years after LASIK with the flap created with a femtosecond laser were similar to those after LASIK with the flap created with a mechanical microkeratome; differences in the mean density, if they exist, are likely less than 120 cells/mm2, or approximately 4% of the mean cell density. This indicates that the additional energy imparted to the cornea during LASIK with a femtosecond laser is not detrimental to the health of the endothelium.
Endothelial cell loss over the 5-year follow-up in this study was close to zero for both treatments, similar to that of normal corneas,13
showing that LASIK surgery did not have an effect. Similarly, with 9 years of follow-up, we previously found no effect of LASIK with a microkeratome on endothelial cell loss above that of age-related physiologic cell loss.2
Kato et al.14
retrospectively analyzed 779 eyes after myopic LASIK with a microkeratome and found that endothelial loss at 5 years was 1.2%, which is within the range of physiologic cell loss.
In recent years, femtosecond lasers have been increasingly used to create LASIK flaps15
; however, there are few reports of the long-term effect, if any, on the corneal endothelium. Smith et al.16
found no difference in ECD between fellow eyes receiving LASIK with a femtosecond laser versus PRK; however, their end point was 3 months after surgery, which might not have been long enough to detect small changes. Muñoz et al.17
created flaps with a 15 kHz femtosecond laser; 1 year after LASIK, they found that the ECD increased in previous contact lens wearers and remained stable in noncontact lens wearers. The results in our study with 5 years of follow-up provide further evidence that LASIK with a femtosecond laser is not detrimental to the corneal endothelium. The clinical results are supported by laboratory studies of endothelial keratoplasty tissue preparation with femtosecond lasers,18,19
in which lamellar cuts deeper than those used in LASIK were not associated with endothelial cell damage. Similarly, the depth of ablation in LASIK in this study was not correlated with endothelial cell loss, confirming previous results.2
Contact lens wear is known to induce morphologic changes in the corneal endothelium, although ECD is typically unaffected.20,21
found an improvement in ECD and morphology after photoablative refractive surgery, and this has often been attributed to cessation of contact lens wear. In this study, there was a trend toward increased cell density 5 years after LASIK in contact lens wearers. However, the trend was not statistically significant, although our statistical power prevented us from detecting an increase smaller than 148 cells/mm2
(or 5%). Although we had sufficient statistical power to detect an increase in cell density as low as 2% in noncontact lens wearers, determining the effect of cessation of contact lens wear after refractive surgery would require a larger controlled study.23
The strengths of this study were its randomized paired-eye design and 5-year prospective follow-up with careful calibration and standardization of the endothelial cell analysis. The small sample size was a limitation and was determined based on the primary outcome of the trial, which was visual acuity. Nevertheless, post hoc analyses indicated sufficient power to detect a difference in cell loss as small as 4% at 5 years, if indeed a difference existed between treatments. All femtosecond flaps were created with a 15 kHz femtosecond laser, which was standard at the time the patients were enrolled in the study. Because femtosecond lasers have since improved and now deliver much less energy to the cornea during flap creation, we expect that similar results would be found with the newer lasers. Although we used 2 different confocal microscopes (Confoscan 3 and Confoscan 4) for endothelial imaging in the initial phases of the study, the microscopes were spatially calibrated with the same scale etched into a glass slide and the ECD measured using both microscopes has been validated with a specular microscope.24,25
In summary, the energy delivered to the cornea during femtosecond laser flap creation did not affect the corneal endothelium 5 years after LASIK when compared with flap creation using a mechanical microkeratome. The results support the use of corneas that have had LASIK as acceptable donor tissue for endothelial keratoplasty with respect to endothelial cell health, enabling these corneas to expand the donor pool.2
The latter is contingent on successful preparation of endothelial grafts from donor corneas that have had LASIK and is aided by a complete donor history and careful screening of the tissue.26