Wavefront-guided transPRK for improving visual quality
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Unlike ELZA-PACE, wavefront-guided transPRK involves the removal of small amounts of stromal tissue. Since the stroma forms the cornea’s structural foundation and is inherently thinner and more fragile in corneas with keratoconus or other corneal ectasias than in healthy corneas, it is crucial to ensure this procedure is performed only on corneas that are biomechanically sufficiently robust.
Accordingly, corneas must be cross-linked before considering this procedure (either with a regular CXL procedure or ELZA-PACE customized cross-linking). After the corneal shape has stabilized (which takes up to 12 months after the procedure), we can then assess whether the cornea is suitable for lasering. We will comprehensively assess the cornea’s shape and biomechanical strength to ensure it is safe to proceed with the laser treatment.
In some cases, it is possible to combine CXL and wavefront-guided transPRK.
Wavefront-guided PRK for keratoconus can significantly improve patients’ visual quality. The excimer laser platform ELZA uses, the SCHWIND AMARIS, is unique in its ability to selectively target higher-order wavefront errors (HOAs) for correction. These HOAs arise because the cone makes the surfaces of the cornea increasingly irregular. HOAs, which cannot be corrected by spectacles, cause the hallmark keratoconus visual disturbances of ghosting, blurring, double or even triple vision, and issues with glare and starbursts at night. In keratoconus, wavefront-guided transPRK is used solely to reduce these aberrations. This minimizes the tissue removal by the laser and can result in a dramatic improvement in visual quality.
We opt for laser treatment only after cross-linking. The rationale behind this approach is that, despite the high success rate of cross-linking, it does not guarantee 100% efficacy: in 3 to 7% of cases, keratoconus may still progress even after comprehensive cross-linking. Additionally, though exceedingly rare (0.5%), some corneas may undergo significant flattening post-cross-linking. Therefore, the safest approach is to delay laser procedures until the corneas have stabilized.
Until recently, a critical piece of information was missing for accurately lasering a cornea post-cross-linking: the exact amount of corneal tissue removed by an excimer laser pulse following CXL. Is it the same amount as in a non-cross-linked cornea? Or 20% less? Perhaps 40% more?
Lacking knowledge of the precise ablation rate leads to inaccuracy in results.
Our group conducted an extensive study to determine the ablation rate in corneas post-CXL, with findings published in the American “Journal of Refractive Surgery”: https://pubmed.ncbi.nlm.nih.gov/25250420/
We discovered that in a cornea treated with CXL, each excimer laser pulse removes 12% less tissue compared to a cornea without CXL. This insight significantly enhances the precision of our treatments.
Here’s a comparison of visual acuity and simulated vision (based on corneal wavefront analysis) before and after the wavefront-guided transPRK treatment. After the treatment, issues like double vision and halos improve, and the sharpness and contrast of what you see gets better. If the patient wears glasses, their prescription might change or even become stronger . The primary goal is to improve vision and contrast.
Beyond the improvement in visual acuity, it is primarily the enhancement in image quality that benefits our patients. Unlike visual acuity, “image quality” as a concept lacks a simple metric in ophthalmology.
Thus, improving visual acuity is just one aspect of the overall enhancement. Equally crucial is that an increase in visual acuity by over 50% can augment three-dimensional vision (depth perception), offering a comprehensive improvement in visual experience.
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