Topoguided PRK for optical zone enlargement
Topoguided PRK for corneal trauma
11.2 Topography-Guided (TG) vs. Wavefront-Guided (WG) Ablation: Indications
Wavefront-guided (WG) treatments aim to correct total aberrations primarily generated by the cornea and lens. Topography-guided (TG) treatment aims to regularize any corneal irregularities based on the acquisition of topographic data from the corneal surface.
Wavefront-guided maps are obtained from models based on adaptive optics using wavefront sensors and may be affected by factors such as pupil size, accommodation, or lens opacities. In highly aberrated corneas, it is often difficult to obtain reproducible wavefront imaging on which to base a wavefront-guided treatment. In these situations, topographically based image capture is more likely to be possible.
With wavefront measurements, the Zernike expansion is based on measuring the distance from pupil entrance center. In TG treatment the measurements are centered on the corneal vertex, which more closely approximates the visual axis . Corneas considered normal are more likely to have the line of sight and the visual axis to coincide. However where there is a large topographic decentration, a different reference axis may result in dissimilar ablation profiles and less successful refractive outcomes between WG and TG treatments. This also may occur in otherwise normal eyes with a large angle kappa, as we often see in hyperopes [3, 4].
11.3 Methods: Topographical Mapping and TG Lasers
There are an increasing number of topography-guided laser manufacturers internationally, with recent limited FDA approval in the USA. We have experience with two, the topography-guided customized ablation treatment (T-CAT) planning software with the Allegretto Wave (AW) Eye-Q Excimer laser platform (Alcon Laboratories Inc. in Fort Worth, Texas, USA) approved in Europe and Canada since 2003. We have performed over 850 TG treatments with the Allegretto which we replaced in 2014 with the Schwind Amaris (SA) 1050 SmartPulse/SmartSurf. The Amaris uses Sirius wave image capture and we have treated over 450 cases. Most TG lasers interface directly with one imaging system, although it is possible to use further information from other topography/tomography instruments. Further systems include CRS-Master planning software used with the Meditec Mel 80 (Carl Zeiss Meditec, Jena, Germany) and OPD scan (Nidek) using a customized aspheric transition zone (CATZ-Nidek) with final fit software, in the corneal interactive program topographic ablation Cipta Max. Topography-guided planning systems use information from Placido disk topographers, Scheimpflug rotating cameras, or a combination of both to capture satisfactory images to be analyzed by proprietary algorithms to determine the ablation profile. For example, Allegretto Wave Eye-Q uses in its T-CAT planning software a compilation of both systems, the ALLEGRO Oculyzer.
11.4 Design and Planning Ablation Patterns
Planning requires interpretation of the topographic imaging (such as shape and elevation) to develop a customized treatment profile. The complexity of finding appropriate individual profiles has likely delayed the wider application of TG laser treatments. Each system has unique aspects that need to be understood and modified by the surgeon. An option is to use manufacturer-provided software to perform a plano topography-guided ablation and then to perform a second ablation once the topography is stable to correct for the shape and therefore refractive changes induced by the first treatment. Our clinic developed a customized topographic neutralization technique (TNT) to compensate for the changes induced by regularizing the cornea, so the procedure could be done in one step . Planning is a four-step procedure neutralizing the astigmatism, under spherical component, and then adding the spherocylinder result to the refraction which can be used for keratoconus and ectasia . The degree of refractive treatment is usually limited by the pachymetry and targeted for low myopia in anticipation of cross-linking induced hyperopic shift.
Contemporary topography-guided lasers either combine or use compatible imaging systems to provide the clinician with multiple treatment options. Fortunately, compensation for induced refractive change is provided by proprietary software. The refraction is then added to give the final treatment. The current software we use aims at creating an aspheric cornea with the advantage of providing the clinician with multiple options including depth limitation, optical zone and transition size, and centration control. The SA1050 laser (our current system) has additional options of a seven-dimensional tracker, iris-based tracker with cyclotorsion control—features we find beneficial for controlling centration and a reproducible ablation.
11.5 Topography-Guided Laser for Aberrated Corneas
Topography-guided treatments are of most value in the management of highly aberrated corneas. Frequently, it is not possible to obtain wavefront measurements other than with topography-guided techniques. Indications for TG treatment include complications of previous refractive surgery such as decentered ablations, small optical zones, eyes, and irregular astigmatism. TG PRK is increasingly used for the management of ectatic conditions, keratoconus, pellucid marginal degeneration, and post-LASIK ectasia as examples, often in combination with collagen cross-linking (CXL). Additionally highly irregular astigmatism found in corneas having had a penetrating keratoplasty may be optimized with TG PRK.
11.6 Methods: Procedure—Transepithelial PRK
Several methods of using TG laser have been described, but all in essence use topographers and tomographers for treatment planning ideally with compensation for the refractive effect induced by the shape change.
Successful results have been obtained with both LASIK and PRK using topography-guided techniques. Our preferred procedure is transepithelial PRK using the Schwind Amaris SmartSurf. Many of our referred cases are complications of refractive surgery often having had several procedures or are complex cases with irregular astigmatism. Lifting flaps may lead to further complications, and low pachymetry is also a limiting factor in how much further treatment is possible on the residual stromal bed. We use mitomycin C (MMC) 0.02% at completion to minimize stromal haze. Collagen cross-linking is used in all keratoconus cases and in postoperative ectasia cases if indicated.
Early epithelialization is critical for success, and delays in healing may lead to loss of vision due to haze and scarring. Preoperatively advisable to treat any ocular surface disorder. Bandage contact lens with close follow-up postoperatively.
The corneal epithelium is able to compensate to a certain extent over an irregular stroma [6–8]. The epithelium is thinner over elevated ectatic areas, and this finding is potentially useful in diagnosing subclinical ectasia. This can also be used clinically in advanced cases of ectasia by doing PTK for epithelial removal when insufficient thickness for a TG PRK. Transepithelial (TE)-guided PRK may correct part of stromal irregularity being masked by the epithelium, thereby enhancing the effect of the treatment. Epithelial imaging can be done with high-frequency ultrasound or spectral domain optical tomography and the information used for treatment planning. However, the data acquisition is difficult, due to the degree of variability; as such it is not yet widely adopted. Our experience is to use transepithelial TG PRK without epithelial imaging, and we postulate that it is likely that postoperative epithelial changes can also compensate for irregularities masked preoperatively.
11.7 Post-refractive Surgery Complications (Non-ectatic)
Irregular astigmatism is a common feature in many patients with post-refractive surgery complications that can be managed successfully with TG ablations . A cautious approach and a topographic neutralization technique (TNT) are advised to compensate for the refractive error induced by the treatment. Thus, TG ablation may be best reserved for those cases with decreased BCDVA. Rigid contact lens refraction is a helpful predictor of efficacy of a TG treatment.
We have found TG PRK highly effective in improving decentration and enlarging optical zones. Our results from treating decentration and small optical zones suggest that greater than 92% of eyes should be expected to achieve within 1D of target refraction with TG PRK  (Fig. 11.1).
Other indications for treating post-refractive corneas are post-LASIK flap trauma, central islands, inactive diffuse lamellar keratitis, and asymmetric astigmatism. Refractive error after cataract surgery with irregular astigmatism can be effectively treated but is very challenging if a toric lens implant has been used, and patients must be warned that they may require a two-stage treatment. Irregular astigmatism secondary to corneal scars such as central rust rings is also difficult to manage, and phototherapeutic keratectomy (PTK) may be a safer initial option to be followed by TG PRK if needed.
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11.8 Post-radial Keratotomy (RK)
TG ablation has been used successfully for post-RK irregular astigmatism . We have performed TG PRK using both the Allegretto Wave (AW) (n = 25) and Schwind Amaris (SA) (n = 12) TG lasers with follow-up at 6 months showing gain ≥2 lines of BCDVA (4% AW and 25% SA), with none losing ≥2 lines. More than half in both groups had 20/40 or better UDVA. The value of cross-linking is not yet determined with post-RK. Treatment planning, if using the AW laser, needs to compensate for the induced myopia and demonstrates the value of a topographic neutralization technique (TNT) (Fig. 11.3).
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