KEY CONCEPTS
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Corneal laser surgery for keratoconus has become possible, thanks to the advent of corneal cross-linking and major improvements in customized excimer laser ablation algorithms.
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Laser surgery can be performed via various modalities, such as topography-guided and corneal wavefront–guided photorefractive keratectomy (PRK), ocular wavefront–guided PRK, and transepithelial phototherapeutic keratectomy.
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The main target is to fix relevant corneal aberrations rather than refractive errors, to keep stromal ablations to a minimum and avoid excessive biomechanical insult, haze formation, and refractive surprises.
Introduction
Corneal laser surgery for keratoconus has become possible, thanks to the advent of corneal cross-linking (CXL) and major improvements in customized excimer laser ablation algorithms. Reviled and revered by surgeons across the globe, this procedure has been the subject of controversy and fierce debates since 2009. A close, objective review of this procedure identifies its strengths, realistic goals, and limitations, and delineates surgical and clinical strategies. The target of corneal laser surgery in keratoconus is visual rehabilitation: improving corrected distance visual acuity (CDVA) and visual symptoms. It is especially useful when rigid gas permeable lenses are not an option, whether because of clinical or social intolerance. Laser treatments should not aim to eliminate or minimize spectacles dependence but should focus on improving corneal aberrations.
Procedure Planning
In planning for the procedure, the surgeon should keep in mind that the main issues in any customized excimer laser treatment are overcorrection, induction of refractive errors, epithelial and stromal healing, biomechanical adjustment, stromal haze, and, especially for keratoconus, biomechanical integrity. There are many reasons for overcorrection, including inputting a refractive correction that has higher-order aberrations (HOAs) embedded in it, such as spherical aberration and coma, among others. Hence, HOAs and corneal irregularities might be treated twice. Additionally, a larger stromal ablation to offset corneal irregularities entails greater chances to impact the refractive error and can lead to a larger biomechanical adjustment that can also affect the final refractive outcome. Additionally, the deeper the stromal ablation, the higher the chance of developing corneal haze. The latter is not uncommon in irregular eyes, especially in eyes with concomitant or previous procedures such as CXL. Finally, ablating too deeply might affect the biomechanical integrity of the cornea and reverse any benefit from a previous or concomitant cross-linking procedure. It has been shown that ablating a total of 50 µm was overall safe and did not result in ectasia progression, either simultaneously or sequentially with cross-linking. This was further revisited and confirmed 10 years after the procedure. However, this number should be regarded as a maximum to avoid rather than a safety allowance. The general rule in customized ablations for keratoconus eyes should be the less, the better. The target is to fix relevant corneal aberrations rather than refractive errors, so the less the stromal ablation, the less the chance of refractive overcorrection, induced refractive errors or aberrations, corneal haze, or ectasia progression. The eye can always be revisited when more tissue is spared, and lower-order aberrations (LOAs) can be “outsourced” to spectacles, soft contact lenses, or phakic intraocular lenses (IOLs; Fig. 29.1 ).
Treatment Modalities
Many methodologies are used to perform customized corneal ablation: ocular wavefront–guided (OWG), topography-guided (TG), and phototherapeutic keratectomy (PTK) treatments. OWG treatment aims to correct the aberrations of the whole eye and has the advantage of directly deriving and treating the refractive error from the measured wavefront. However, pupil size often limits treatment, as the ablation profile must be extrapolated outside the boundary margins, and a small optical zone treatment in keratoconus fails to address the irregular corneal changes that typically extend to the midperiphery. The latter concept of a large optical zone reflects the new trend in treating corneal irregularities in keratoconus and is a sharp departure from the small optical zone initially advocated in the Athens protocol.
Additionally, traditional aberrometers have low dynamic range and suffer from aliasing when it comes to measuring highly aberrated wavefronts, typically encountered in keratoconus eyes. The advent of pyramidal aberrometers has made it possible to measure complex ocular wavefront patterns, including those of eyes with corneal ectasia, and they can be used to formulate accurate and reliable treatment profiles.
TG ablations measure the very surface affected by the disease in a highly predictable and accurate way. Corneal wavefront–guided (CWG) treatment is a spin-off of the TG method but relies on decomposition of the topography into a wavefront map. By doing so, it provides a way to select and deselect aberrations to be treated, allowing for decreased tissue ablation volume and depth while treating the relevant aberrations in a specific eye. Both modalities suffer from the fact that refraction is estimated and never measured directly. More importantly, topography is based on the anterior curvature, which is partially offset by the posterior curvature, sitting on a negative refractive meniscus. Treating the total corneal wavefront (ray-traced sum of anterior and posterior) might provide a better functional treatment and save on tissue ablation. It has already been shown that OWG treatments ablate up to 44% less tissue than CWG ablations, planned on the same eyes using the same excimer platform (Schwind Amaris). This difference probably represents the effect of the posterior corneal curvature, which is embedded in OWG treatment profiles. Whether total CWG or OWG treatment more accurately addresses the corneal aberrations is yet to be determined, as overcorrecting total corneal aberrations by performing an anterior CWG ablation might be offset by an inherent undercorrection of the target aberrations. All the above treatments can be performed via a transepithelial approach.
Finally, PTK, which is usually reserved for irregularly irregular corneas, can be used in the keratoconus eye to ablate the epithelium and shave tissue from the cone where the epithelium is thinnest. This would result in partial treatment of coma and ensures minimal tissue ablation. This treatment modality is useful in severe keratoconus with not much stromal tissue to ablate. It is also useful in mild keratoconus with relatively good CDVA, in which slight acuity boosting with minimal tissue ablation is desired. In addition, this modality can be especially useful when there is clear discrepancy in epithelial thickness in a decentered cone, and where a transepithelial photorefractive keratectomy (PRK) approach might lead to excessive ablation over the cone, often resulting in “double treatment” of the coma and other components ( Fig. 29.2 ).
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