Extending the conventional limits for the correction of high ametropia by corneal laser refractive surgery

Abstract

Hypothesis: It is possible to safely increase the conventional limits of corneal laser refractive surgery for high ametropia and irregular astigmatism using ablation profile modifications, pre and postoperative safety screening and monitoring, surgical technique improvements such as centration, introduction of the small incision lenticule extraction (SMILE) surgical procedure as an alternative to laser in-situ keratomileusis (LASIK), and the use of therapeutic options including topography-guided custom ablation and trans-epithelial phototherapeutic keratectomy.

Results: A method for the safe treatment of high myopia up to -14.00 D by corneal laser refractive surgery was described through the use of aspherically optimized ablation profiles, residual stromal thickness mapping by VHF digital ultrasound for safe retreatment planning, a two-stage treatment protocol for high corrections and thin corneas, and flap creation by a femtosecond laser enabling the use of thinner flaps. Results were found to be comparable to those achieved by intraocular surgery, while avoiding the catastrophic risks of intraocular surgery.

A method for the safe treatment of high hyperopia up to +7.00 D by corneal laser refractive surgery was described through the use of large optical zones and transition zones to maximize refractive stability, residual stromal thickness mapping by VHF digital ultrasound for safe retreatment planning, and a two-stage treatment protocol for high corrections. The safety of retreatments was also assessed by monitoring the epithelial thickness by VHF digital ultrasound as a more reliable parameter to avoid apical syndrome than using corneal curvature. This enabled retreatment to be safely performed in eyes with relatively thick epithelium but steep corneal curvature, and no retreatment performed in eyes with thin epithelium but otherwise flat corneal curvature. Results were found to be comparable to those achieved by intraocular surgery, while avoiding the catastrophic risks of intraocular surgery.

The influence of ocular residual astigmatism on refractive predictability was evaluated and compared to prior publications to investigate potential methodological flaws. The method of grouping by a ratio was found to bias the results towards lower predictability in the high ocular residual astigmatism group due to the treated cylinder not being matched between groups. However, a small difference was still found after matching for treated cylinder with a slightly lower predictability in the group of eyes with high corneal astigmatism, high refractive cylinder, and high ocular residual astigmatism. But this result must take into account the potential for bias in this group due to potential errors in the manifest refraction, poor scan quality of the topography measurement, or the presence of non-orthogonal corneal astigmatism or other higher order aberrations. This study highlighted the importance of comparing the manifest refraction with the corneal astigmatism before surgery and to re-refract the patient if the ocular residual astigmatism is abnormally high.

The ability to repair the complications of corneal laser refractive surgery is a critical component of achieving optimal outcomes. Quality of vision can be compromised by optical zone decentration or a small topographic optical zone. Topography-guided custom ablation was shown to be effective at improving the optical zone centration relative to the corneal vertex and enlarging the topographic optical zone. These topographic changes were associated with a significant reduction in corneal higher order aberrations.
The last chapter introduced small incision lenticule extraction (SMILE) as a potential procedure that might be suited for the correction of high ametropia due to the preservation of the anterior stromal lamellae and Bowman’s layer by not requiring a LASIK flap. A number of aspects of SMILE were evaluated. Firstly, cylinder predictability was found to vary according to the cylinder axis indicating the need for an angular dependent nomogram. Secondly, spherical aberration induction was shown to decrease for larger optical zones, with SMILE achieving greater spherical aberration control compared to aspherically-optimized LASIK. Thirdly, it was demonstrated that there was clinically negligible astigmatism effect due to the small incisions.

Conclusion: Corneal laser refractive surgery can offer safe and effective treatment for a wide range of myopia, hyperopia, and astigmatism by using optimized ablation profile design, residual stromal thickness mapping, and a two-stage treatment protocol. Consideration of the epithelial thickness as the risk factor for apical syndrome in hyperopia also offers greater safety than that provided by corneal curvature. The methods described in this thesis may facilitate corneal laser refractive treatment in a cohort of patients in which intraocular surgical techniques have traditionally been preferred.

Date of Award	May 2018
Original language	English
Sponsors	London Vision Clinic
Supervisor	Andrew Nesbit (Supervisor), Tara Moore (Supervisor) & Dan Z Reinstein (Supervisor)