Customized Corneal Collagen Cross Linking Using The Topography Guided Photorefractive Keratectomy As Topography Guided Phototherapeutic Keratectomy (T-Cxl): A Case Report

To describe how to customize the corneal collagen cross-linking (CXL) in a patient with progressive keratoconus, turning CXL into topography guided, and adding more refractive therapeutic effect. Usually, CXL is used to halt the progression of the disease. However, this technique (T-CXL) adds a topography guided therapeutic refractive effect to regularize the cornea. The idea is to create multiple ablation zones at the corneal surface using the topography guided photorefractive keratectomy (PRK) treatment profile as topography guided phototherapeutic keratectomy (PTK) to ablate the epithelium reaching bowman’s on top of the cone area, and around the flat area next to the cone (which are automatically detected by the software).

The procedure was performed on a patient with progressive keratoconus at Ebsaar Eye Surgery Centre, Dubai, United Arab Emirates. The epithelial map (Zeiss OCT Cirrus 5000), 8 Scheimpflug images (Wavelight ocullyzer II), and the T-Cat profile (EX 500 Wavelight excimer laser platform from Alcon) were used to plan the customized CXL, with an optical zone of 6.50 mm. As the epithelial layer was used as shield to customize the CXL, the ablation planning was done including the epithelium.

A 22-year-old male with progressive keratoconus was submitted to T-CXL. Pre-surgery the patient had uncorrected distance visual acuity (UDVA) of 0.6 LogMar, best corrected distance visual acuity (BDVA) of 0.5 LogMar with 0.00/-3.00@10 and visual acuity (VA) with scleral contact lenses was 0.1 LogMar. The customized effect was achieved by modifying the sphere and cylinder magnitude of the T-CAT profile aiming to ablate only the epithelium (epithelium thickness obtained using the epithelium map by the OCT). The astigmatism and sphere magnitude were adjusted to achieve the following: 1. the area over the cone (myopic part of the treatment) achieving an ablation depth enough to ablate the epithelium on top of the cone, 2. the hyperopic area of the treatment achieving an ablation depth enough to ablate the epithelium only and 3. the area between the myopic and hyperopic zones was ablated partially such that it remained covered by a residual sheet of epithelial layer. The axis of astigmatism correction was calculated by the software using topo/tomography data. The expected residual corneal thickness was more than 350-microns in the thinnest part. Four months post-surgery the patient had UDVA of 0.5 LogMar and BDVA of 0.3 LogMar with -0.75/-3.00@15. VA with scleral contact lenses improved to 0.0 LogMar. The tissue consumption was 0-microns in the centre of the pupil (as expected) and the cornea got thicker by +11 microns at the thinnest point due to the epithelium remodelling.

We aimed to create areas where the bowman’s is exposed (will receive the full dose of the CXL) and areas still covered by epithelium partially or fully (will receive less dose of the CXL), leading to customization of the CXL (T-CXL). The T-CXL effect is created by using the topo-guided PRK as a topo-guided PTK, giving higher regularization effect compared to conventional CXL. By using the epithelium as a shield, the T-CXL allows the weak area on the cone area to receive the full dose of CXL. The same dose of CXL will be delivered to the stroma in the hyperopic ablation zone inducing more coupling effect with higher therapeutic refractive gains, and not only a stabilization effect.