Letter to the editor

Corneal flap thickness and tissue laser ablation in myopic lasik

To assess the repeatability and reproducibility of posterior corneal curvature (PCC) measurements made by combined scanning-slit/Placido-disc topography (Orbscan II) after LASIK.

Experimental instrument validation study.

We recruited 22 consecutive postmyopic LASIK patients for the repeatability study and another 50 consecutive postmyopic LASIK patients for the reproducibility study.

To analyze intrasession repeatability, 1 examiner measured 22 postmyopic LASIK eyes 10 times successively in the shortest time possible. To study intersession reproducibility, the same operator obtained measurements from another 50 eyes with stable refraction in 2 consecutive visits at the same time of the day between 6 and 9 months after myopic LASIK. We explored any association between residual stromal bed thickness and measurement variability.

Orbscan II scanning-slit PCC data, precision, within-subject coefficient of variation (CV_w), limits of agreement (LoA), and intraclass correlation coefficient (ICC).

For intrasession repeatability, precision was 0.067 mm (best-fit sphere [BFS]), 0.110 diopters (D; power within 5 mm), 0.158 D (power within 3 mm), and 0.46 (eccentricity). Repeatability was high for PCC BFS and power measurements within 3-mm and 5-mm zones (CV_w ranged from 0.5%–1.2%) but poor for eccentricity data (CV_w, 31.6%). Correspondingly, ICCs ranged from 0.89 to 0.98 for PCC BFS and power, and the ICC was 0.20 for PCC eccentricity values. For intersession reproducibility, on average, no difference in PCC measurements could be found, indicating that when there is variability, it is due to random factors. The width of the 95% LoA between sessions was clinically acceptable for BFS (0.25 mm) and power (0.4 D [within 5 mm] and 0.6 D [within 3 mm]). Similarly, ICCs indicated good intersession reliability for BFS and power (0.98, 0.96, and 0.85 for BFS, power within 5 mm, and power within 3 mm, respectively) but poor reliability for eccentricity (0.59). Repeatability and reproducibility were unrelated to stromal bed thickness.

Orbscan II provides reliable post-LASIK PCC data for symmetrical parameters (BFS and power), independent of the residual stromal bed thickness, but is unreliable for measurements that are radially asymmetrical (eccentricity). Orbscan II is useful for monitoring the PCC after LASIK once the early postoperative period is over.

To analyze LASIK retreatment-induced changes in the posterior corneal curvature (PCC) with undersurface ablation of the flap (UAF) and in-the-bed techniques.

Nonrandomized, comparative, interventional study.

Forty-six eyes with a residual spherical equivalent refraction between −0.37 and −2.75 diopters (D) and astigmatism between 0.0 and −1.25 D were included prospectively. In 23 eyes, the calculated postenhancement flap thickness exceeded 150 μm using micropachymetric optical coherence tomography, whereas with further ablation of the bed, the residual bed thickness (RBT) would have been <250 μm, or <55% of the pre-LASIK central pachymetry. In another 23 eyes, RBT allowed the planned ablation for a calculated post-retreatment RBT exceeding 250 μm, >55% of the pre-LASIK central pachymetry.

Eyes with insufficient RBT for further ablation underwent UAF retreatment, whereas those with adequate RBT received conventional in-the-bed LASIK retreatment. Examinations were performed before retreatment and 3 and 6 months postoperatively. No eye was lost to follow-up.

Micropachymetry, Orbscan II scanning-slit PCC data, and visual acuity (VA).

The groups did not differ in age, intraocular pressure, or retreatment ablation depth, but the UAF eyes had a lower mean pre-retreatment RBT (270.7±25.4 μm) than conventional enhancement eyes (353.0±41.5 μm) (P = 0.001). Eyes undergoing UAF had no significant change in PCC, whereas eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone (P = 0.008) 3 months after retreatment. No significant changes occurred thereafter. The amount of change in posterior corneal power within the 3-mm central zone from before to after retreatment differed significantly between the groups (mean difference, 0.135 D; 95% confidence interval, 0.022–0.248 D; P = 0.02). No keratectasia developed clinically, and no retreated eye lost or gained ≥2 lines of best-corrected VA. Six months after retreatment, the efficacy and safety indices for the UAF procedure were 0.96 and 1.01, respectively, and 1 and 1.06 for conventional LASIK enhancement.

Undersurface ablation of the flap retreatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia.

Eighty-five cases of post laser in situ keratomileusis ectasia were reviewed and analyzed. Cases of keratoconus or forme fruste keratoconus were eliminated; many remaining case reports lacked key information. The current literature is unable to define a specific residual corneal thickness or a range of preoperative corneal thickness that would put an eye at risk for developing ectasia. The most logical cause for eyes without preexisting pathology to develop ectasia is a postablation stromal thickness that is mechanically unstable; this “minimal” thickness is probably specific to each eye. The preoperative and postoperative corneal thickness, measured flap thickness, and microkeratome and laser parameters used in a given case are required to determine the range of residual corneal thickness that puts the eye at risk for developing ectasia. Other as yet undetermined factors may play a role in the development of this complication.

To determine the factor(s) that influences measurement of residual stromal thickness (RST) after laser in situ keratomileusis (LASIK) surgery.

Clinical office-based excimer laser refractive surgery center.

In this retrospective comparative interventional case study of 6235 eyes, ultrasonic corneal pachymetry was performed immediately before and after flap creation and immediately after laser ablation in the primary procedure and after 647 enhancements. Differences in the methods for calculating RST were compared statistically.

Using the RST measured at enhancement as the actual RST, measurements of RST immediately after laser ablation underestimated residual thickness due to laser-induced stromal dehydration and microkeratome effects (P<.001). Estimates of RST using a “standard” or estimated flap thickness were less accurate predictors of residual thickness (P<.001) than use of the theoretical laser resection with a measured flap thickness (RST–4) (P = .78) or a modified flap thickness subtracted from the postoperative corneal thickness (RST–8) (P = .98), which provided the best RST estimates.

Before LASIK, the best means of estimating RST is to subtract the theoretical laser resection obtained from the laser computer and the expected flap thickness normally obtained with a given microkeratome system from the preoperative central corneal thickness. After LASIK, the most accurate means of calculating RST is to subtract the original flap thickness from the postoperative central corneal thickness.

To develop a novel technique, undersurface ablation of the flap (UAF), for laser in situ keratomileusis (LASIK) retreatment in eyes with insufficient posterior stroma.

Noncomparative, interventional case series.

From 30 eyes examined, 25 eyes with a spherical equivalent residual refraction between −0.75 and −3.25 diopters (D) and astigmatism between 0.0 and −1.5 D were prospectively included in the study. In these eyes, calculated postenhancement flap thickness was >150 μm using micropachymetric optical coherence tomography (OCT), whereas with further ablation of the bed, posterior stromal thickness would have been <250 μm. Primary LASIK procedures had been performed with the Hansatome microkeratome.

The flap was lifted and the eye deviated downward, so that the corneal visual axis mark aligned with the laser beam. Mirror pattern ablations with an optical zone of 5 mm were performed on the flap stroma using either the Summit Apex Plus excimer laser or the Technolas Keracor 217 spot-scanning excimer laser. New axis orientation for toric ablations was calculated with the formula: β = 180° − α.

Refraction, visual acuity, OCT pachymetry, tangential videokeratography, and patient satisfaction.

The average follow-up was 6.36 ± 2.64 months (range, 3–12 months). Mean preenhancement spherical equivalent (−2.05 ± 0.75 D) was reduced to −0.19 ± 0.38 D at the last visit (P = 0.001). Mean cylinder decreased from −0.48 ± 0.53 D before retreatment to −0.23 ± 0.28 D at the last follow-up (P = 0.003). Best-corrected visual acuity worsened by 1 line in two eyes (8%), and no eye lost 2 or more lines. Satisfactory globe stabilization and stromal smoothness during ablation were more difficult to achieve than with conventional LASIK enhancements. The average central flap thickness before UAF, 187 ± 13 μm, decreased to 164 ± 12 μm after 1 month (P = 0.001). No keratectasia developed. Finally, 92% of cases were satisfied with surgery compared with 48% before UAF retreatment (P = 0.001).

UAF retreatment for low residual refractive errors after LASIK in eyes with sufficient flap stroma seems to be effective and may prevent future keratectasia.