Wavefront-supported Photorefractive Keratectomy: 12-month Follow-up

Wavefront-supported Photorefractive Keratectomy With the Bausch & Lomb Zyoptix in Patients With Myopic Astigmatism and Suspected Keratoconus

PURPOSE

To evaluate the safety, efficacy, and visual outcome of wavefront-supported photorefractive keratectomy (PRK) for the correction of myopia and myopic astigmatism in patients with suspected keratoconus.

METHODS

Forty eyes of 20 patients with myopia -4.0 to -8.0 diopters (D) (mean: -6.25 +/- 1.04 D), cylinder -1.0 to -2.50 D (mean: -1.61 +/- 0.71 D), and corneal thickness 440 to 488 microm were treated with wavefront-supported PRK. Corneal topography evaluation revealed a significantly irregular cylinder (inferior-superior difference > 1.5 D) with possible mild or forme fruste keratoconus. Aberrometry was performed with the Hartmann-Shack aberrometer, and corneal data were evaluated with the Orbscan system. Eyes were treated with the Technolas 217z Bausch & Lomb excimer laser and followed for a minimum of 40 months.

RESULTS

Following surgery, mean spherical equivalent refraction was +0.33 +/- 0.8 D. It was within +/- 0.5 D of the intended refraction in 95% of eyes and within +/- 1.0 D in 100% of eyes. Mean uncorrected visual acuity improved from 20/400 preoperatively to 20/25 postoperatively. Mean best spectacle-corrected visual acuity (BSCVA) remained unchanged (20/20) or improved to 20/20 in 92.5% of eyes and to 20/25 in 7.5% of eyes. Individually, BSCVA did not change in 28 (70%) eyes and increased by > or = 2 Snellen lines in 9 (22.5%) eyes; 3 (7.5%) eyes lost 1 Snellen line because of corneal haze. Laser treatment induced a significant flattening of the preoperative inferior corneal steepness in all eyes. Wavefront analysis demonstrated a significant decrease in high order aberrations (total root-mean-square and coma).

CONCLUSIONS

Wavefront-supported PRK appears to be effective for the treatment of myopia and astigmatism in patients with suspected keratoconus and thin, irregular corneas. Longer follow-up is needed to prove the safety of the procedure in this patient population.

Spatial Distribution of Keratan Sulfate in the Rabbit Cornea Following Photorefractive Keratectomy

PURPOSE

To examine the keratan sulfate content of the stroma and to assess its correlation with the healing process (epithelialization and keratocyte density) after photorefractive keratectomy (PRK).

METHODS

Using an Aesculap Meditec MEL 70(G-scan) excimer laser, -6.0 diopters (6.0-mm diameter, 82 microm photoablation depth), PRK was carried out on the right eye of 32 New Zealand pigmented rabbits. After enucleation (at days 1, 4, 7, 14, and 28 and months 2, 3, and 7; sub-groups of 4 animals), fluorescent immunohistochemistry was performed on sections from the central comea using monoclonal mouse anti-keratan-sulfate antibody, immunohistochemistry with proliferative cell nuclear antigen antibody, and hematoxylin-eosin histology. The left, untreated eyes served as controls. Cellular morphology and spatial distribution of keratan sulfate were recorded, stromal thickness measured, and keratocyte density calculated.

RESULTS

Keratan sulfate was found on the surface of migrating epithelial cells in the early stage (from days 1 to 7). In the stroma, three phases were noted. (Phase 1) Day 1 to 14, intense granular fluorescence appeared in the anterior stroma with hypocellularity. (Phase 2) Month 1 to 2, newly synthesized lamellar keratan sulfate restored the repopulating anterior stroma. Endothelial cells became keratan sulfate positive, while in the posterior stroma, lamellar-form keratan sulfate increased from week 1 and peaked at month 1 (100% increase). (Phase 3) Month 2 to 7, remodeling and deposition of keratan sulfate was noted, which was produced in phase 2.

CONCLUSIONS

Keratan sulfate was found in the epithelium, stroma, and endothelium. By controlling the interlamellar spacing, keratan sulfate plays a role in postoperative edema, remodeling of the corneal stroma, and simultaneous regulation of inflammation after PRK.

Strategic Planning in Topography-guided Ablation of Irregular Astigmatism After Laser Refractive Surgery

PURPOSE

To identify an optimal customized ablation strategy in the treatment of eyes with secondary irregular astigmatism.

METHODS

Corneal anterior surface elevation maps of 50 eyes with secondary irregular astigmatism after decentered laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) and 50 virgin eyes were used for customized ablation simulations. Two ablation simulations with targeted postoperative surfaces perpendicular to either the visual or corneal morphological axis were made for each eye. All ablations were programmed for correction of corneal irregularities, including corneal astigmatism. The manifest refractive error was not corrected. Optical diameter was 6.5 mm and total diameter was 7.5 mm. Maximum ablation depths and maximum transition zone gradients were registered and analyzed.

RESULTS

In eyes with secondary irregular astigmatism, mean maximum ablation depth was 48.21 +/- 25.96 microm and 26.31+/- 14.08 microm, whereas mean maximum transition zone gradient was 29.07 +/- 25.15 microm and 9.88 +/- 6.41 microm in ablation simulations based on the visual and corneal morphological axes, respectively. The difference between the ablation strategies was highly statistically significant for both parameters (P < .001). In virgin eyes, only a minor difference was noted between the visual and corneal morphological axis ablation simulations (P = .15 for maximum ablation depths and P=.19 for maximum transition zone gradient).

CONCLUSIONS

In secondary irregular astigmatism, ablation based on the corneal morphological axis appears to minimize corneal tissue consumption and allows a smoother transition zone.

Involvement of Neural Adaptation in the Recovery of Vision After Laser Refractive Surgery

PURPOSE

To investigate whether the gradual improvement in unaided visual acuity commonly seen after laser refractive surgery is attributable, in part, to neural adaptation to blur.

METHODS

Unaided logMAR visual acuity was measured at presentation (blur adapted) and immediately after refraction and removal of refractive correction (correction adapted) in 26 patients with low myopic residual refractive error after laser in situ keratomileusis (LASIK). The difference in unaided visual acuity (correction adapted--blur adapted) indicates a dominance of practice effect (if negative) or blur adaptation (if positive). A combination of blur adaptation and practice effect is possible.

RESULTS

Overall, the mean unaided visual acuity at presentation (blur adapted) was 0.16+/-0.16 (mean+/-standard deviation) logMAR, and the mean unaided visual acuity immediately after refraction and removal of refractive correction (correction adapted) was 0.14+/-0.14 logMAR, giving a difference (correction adapted--blur adapted) of -0.02+/-0.06 logMAR. This difference was not significant (analysis of variance [ANOVA] F1,25=0.204, P>.05), suggesting neither blur adaptation nor practice effect. However, during the first 10 weeks after surgery, the difference in unaided visual acuity was -0.07+/-0.05 logMAR, suggesting a practice effect. After 10 weeks, the mean difference was +0.02+/-0.05 logMAR, suggesting any practice effect is offset by blur adaptation. These values were significantly different (ANOVA F1,25= 13.53, P<.01).

CONCLUSIONS

These data suggest that patients do not adapt to surgically induced blur, on average, until 10 weeks after LASIK. The reason for this delay is uncertain; perhaps instability of blur hinders adaptation during the early postoperative period. Part of the gradual visual improvement after LASIK appears to be due to neural adaptation to blur.