Future Challenges to Aberration-free Ablative Procedures

Relative Contribution of Central and Peripheral Aberrations to Overall High Order Corneal Wavefront Aberration

PURPOSE

To analyze the influence of specific combinations of corneal high order aberrations on the optical image quality of the cornea before and after photorefractive keratectomy (PRK) for low to high myopia and myopic astigmatism.

METHODS

Corneal topography was obtained for 80 eyes that underwent PRK using a scanning-spot excimer laser. The eyes were subdivided into three groups according to the preoperative refraction. The topographical data were imported into a custom software program that combined the Zernike high order terms having the same sign and angular frequency up to seventh order for 4-mm and 6-mm pupils, ie, coma and spherical aberrations, and midperipheral and peripheral high order aberrations.

RESULTS

Photorefractive keratectomy induced a significant amount of the root-mean-square (RMS) values of the combinations of midperipheral and peripheral high order aberrations over the smaller pupil size for deeper myopic ablations (P<.05). Over the larger pupil, spherical myopic ablations showed a significant increase (P<.05) of the RMS values of coma and spherical aberrations. In the astigmatism group, the combination of terms having higher angular frequency increased significantly (P<.05) after surgery both over 4-mm and 6-mm pupils.

CONCLUSIONS

After surface ablation, ablation depth and profile significantly influence the distribution and contribution of determined combinations of high order aberrations to the overall high order corneal wavefront aberration. Terms having high angular frequency were increased following large myopic correction and wide treatment zone. Quality of the whole corneal optics will be enhanced by designing future ablation profiles to compensate for peripheral high order optical aberrations.

Computer simulation of visual outcomes of wavefront-only corneal ablation

PURPOSE

To evaluate the effectiveness, predicted visual outcome, and limitations of a corneal ablation algorithm that uses wavefront aberration measurement alone without the need for corneal shape information.

SETTING

Contact Lens and Visual Optical Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Australia.

METHODS

Corneal topography and wavefront error data from 22 eyes of 11 potential refractive surgery candidates were used. A computer simulation of the corneal ablation was performed, and the predicted postoperative visual outcome was assessed by calculating the resulting wavefront root-mean-square (RMS) values and visual Strehl ratios. Additionally, the effect of ablation alignment error was examined. Finally, the visual outcomes of the wavefront-only corneal ablations were compared to those in an age-matched group of 20 emmetropic patients.

RESULTS

Significant improvement in total and higher-order wavefront RMS was achieved postoperatively in both an ideal setting and in the case of ablation alignment errors. The predicted improvement in visual Strehl ratio in the potential refractive surgery candidates was significantly better than that in the untreated emmetropes. After additional simulated decentration of the pupil center by 150 microm, the result was slightly worse, but the change was found to not be significant when compared to the retinal image quality of emmetropes.

CONCLUSIONS

Wavefront-only corneal ablation algorithms could potentially lead to significantly better visual outcomes than those normally encountered in untreated emmetropes, provided that the alignment error is not large. The presented methodology may be used as a screening tool to predict patients' visual outcomes before the surgery.

Differences in nasal and temporal responses of the cornea after photorefractive keratectomy

PURPOSE

To examine the differences in the biomechanical response of the peripheral regions of the cornea after photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS

Preoperative and 1-, 3-, 6-, and 12-month postoperative corneal topographies of 70 eyes that had PRK with the Technolas 217C excimer laser (Bausch & Lomb) were obtained. The eyes were divided into 4 groups according to the preoperative spherical equivalent refraction. Preoperative and follow-up topographic data were imported into custom software that computed the average composite corneal map and difference maps in each group to scientifically evaluate the corneal response to the surgery. The software was also used to analyze regional corneal changes after the laser ablation. Corneal peripheries up to 9.0 mm were evaluated.

RESULTS

The preoperative corneas had a flatter nasal periphery than temporal periphery. The corneal surfaces in the right eyes and left eyes showed a mirror symmetry. Significant differences in the regional response of the cornea were observed (P<.05), with a greater increase in the curvature of the nasal periphery than in the temporal periphery.

CONCLUSIONS

To refine modeling of the cornea, the different regional anatomic features and biomechanical responses must be considered. Modifying existing ablation algorithms to compensate for the differences between nasal and temporal corneal flattening of the preoperative corneal surface and between the nasal and temporal responses may improve the postoperative corneal shape and quality of peripheral optics.

Wavefront-guided Ablation With Bausch and Lomb Zyoptix for Retreatments After Laser in situ Keratomileusis for Myopia

PURPOSE

To evaluate the efficacy of wavefront-guided ablation for treatment of residual refractive error and higher order aberrations after laser in situ keratomileusis (LASIK) for myopia.

METHODS

Twenty-one eyes of 13 patients with residual refractive error after myopic LASIK were treated with a wavefront-guided ablation with the Zyoptix technique. In 13 eyes of 8 patients (Group A) we lifted the flap with an epitheliorhexis technique; in 8 eyes of 5 patients (Group B) we cut a new flap with a Hansatome microkeratome.

RESULTS

Preoperative root mean square (RMS) values were 1.370 for second order aberrations, 0.382 for total higher order aberrations, 0.273 for third order, 0.243 for fourth order, and 0.052 for fifth order aberrations. Three months postoperatively, RMS values decreased significantly to 0.278 (second order), 0.189 (total higher order), 0.138 (third order), 0.107 (fourth order), and 0.038 (fifth order). Ablation depth was two times greater than needed with a standard Planoscan treatment. Patients in the microkeratome recut group showed a trend toward overcorrection, and residual astigmatism was higher (-0.75 +/- 0.58 D) than in the flap lift group (-0.25 +/- 0.29 D).

CONCLUSIONS

Wavefront-guided ablation was an effective method to correct residual refractive error and higher order aberrations after myopic LASIK. Functional and refractive outcomes were better when we lifted the flap than when we recut the flap.

Comparison of Conventional Versus Wavefront-guided Laser in situ Keratomileusis in the Same Patient

PURPOSE

To verify whether there is any difference in visual outcome, including higher order aberrations, after performing laser in situ keratomileusis (LASIK) with conventional ablation and wavefront-guided ablation in myopic eyes of the same patient.

METHODS

This was a prospective randomized study of 20 myopic eyes (10 patients) who had LASIK using the Nidek NAVEX excimer laser system. Wavefront-guided customized ablation was used in the first eye of the patient (study group) and the other eye of the same patient was operated with conventional ablation (control group). Mean refractive error was similar between left and right eyes of the same patient. Preoperative examination included higher order aberration by Nidek OPD-Scan. Uncorrected and best spectacle-corrected visual acuity and higher order aberrations were recorded postoperatively.

RESULTS

Preoperative and postoperative best spectacle-corrected visual acuity was better than 20/40 (100%) after LASIK in both the conventional ablation and wavefront-guided customized ablation groups. Postoperative refraction was within +/- 0.50 D of emmetropia: 90% in the conventional group and 100% in the wavefront-guided group. No statistically significant difference in postoperative higher order aberrations was found between groups.

CONCLUSION

Postoperative visual outcome with both conventional LASIK and wavefront-guided customized ablation was not significantly different. Higher order aberrations did not significantly increase postoperatively in either group.

Comparison of Wavefront-guided Customized Ablation vs. Conventional Ablation in Laser in situ Keratomileusis

PURPOSE

To compare visual outcome after applying wavefront-guided customized ablation with conventional ablation in laser in situ keratomileusis (LASIK) patients.

METHODS

We performed a prospective, randomized controlled study that included 22 eyes of 11 patients who were treated with LASIK using the Nidek EC-5000 excimer laser system. We applied wavefront-guided customized ablation (study group) in the first eye of each patient. The conventional ablation (control group) was performed in the other eye of the same patient. Best spectacle-corrected visual acuity (BSCVA), spherical equivalent refraction, and high-order aberrations were evaluated preoperatively and at 1 month. High-order aberrations were measured using the Nidek OPD-Scan.

RESULTS

Preoperative and 1-month postoperative BSCVA were better than 20/40 (100%) after LASIK with both conventional ablation (control) and wavefront-guided customized ablation. Preoperatively, mean high-order aberrations in the control group were 0.572 and in the study group, 0.428 (Zernike coefficient unit). One-month after LASIK, mean high-order aberrations in the control group were 0.496 and in the study group, 0.476 (Zernike coefficient unit). We found no statistically significant difference between preoperative and 1-month postoperative high-order aberrations in either group.

CONCLUSION

LASIK with both conventional ablation and wavefront-guided customized ablation resulted in the same BSCVA 1 month after LASIK. Preoperative and 1-month postoperative high-order aberrations were not statistically different following LASIK between ablation types.

Effect of Acute Biomechanical Changes on Corneal Curvature After Photokeratectomy

PURPOSE

Unintended hyperopic shift is a common yet poorly understood complication of phototherapeutic keratectomy (PTK) that raises fundamental questions about the etiology of corneal curvature change in PRK and LASIK. We investigated the relative contributions of ablation profile and peripheral stromal thickening to intraoperative PTK-induced central flattening, and propose a biomechanical model of the acute corneal response to central ablation.

METHODS

Fourteen de-epithelialized eye bank globes from seven donors underwent either broadbeam ablation (approximately 100-microm depth, no programmed dioptric change) or sham photoablation in paired-control fashion. Peripheral stromal thickness changes and the pattern of thickness loss across each ablation zone were evaluated by optical section image analysis as predictors of acute corneal flattening.

RESULTS

Relative to sham ablation, keratectomy caused significant anterior corneal flattening (-6.3+/-3.2 D, P = .002). Concomitant peripheral stromal thickening (+57+/-43 microm, P = .01) was a significant predictor of acute hyperopic shift (r = 0.68, P = .047). Ablation pattern bias did not consistently favor hyperopia and was a poor lone predictor of hyperopic shift.

CONCLUSIONS

Unintended keratectomy-induced hyperopic shift is replicable in a human donor model and is associated with significant thickening of the unablated peripheral stroma. This biomechanical response may have a considerable impact on early refractive outcomes in PTK, PRK, and LASIK.

Highlights of the 1st International Congress of Wavefront Sensing and Aberration-free Refractive Correction

As we review the many new and evolving techniques for treating patients with customized ablation, it is obvious that there is a rapid evolution of technology and thought. Newly refined diagnostic technology, such as wavefront sensing, and more sophisticated spot laser delivery systems with eye tracking gives the refractive surgical team greater flexibility in tackling challenging optical abnormalities. These highlights of the 2000 Congress now set the stage for further development, outlined in the following selected papers from the 2001 Congress.