Corneal crosslinking (CCL) has been gaining popularity internationally as a first-line treatment for keratoconus and other ectatic disorders of the cornea (see Chapter 7). However, the primary objective of the procedure is to stabilize a progressive ectatic condition; the patient will likely be relegated to using optical correction (glasses or contact lenses) to optimize visual acuity.
Visual rehabilitation of ectasia must achieve stabilization (using CCL) while reducing corneal irregularity and minimizing the residual refractive error. The term corneal crosslinking plus refers to CCL plus additional procedures such as PRK, phototherapeutic keratectomy (PTK), intracorneal ring segment (ICRS) implantation, conductive keratoplasty (CK), and phakic intraocular lens (PIOL) implantation.
Photorefractive or Phototherapeutic Keratectomy and Corneal Crosslinking
Topography-guided PRK (T-PRK) may be performed either after CCL or simultaneously. One study showed that same-day simultaneous T-PRK and CCL is superior to sequential CCL and T-PRK beyond 6 months. It is believed that CCL followed by T-PRK 6 months later would remove the stiffened crosslinked cornea, thereby reducing the benefits of CCL; thus, simultaneous T-PRK and CXL is preferred. Similarly, another study demonstrated significant improvement in mean spherical equivalent refraction, defocus aberration, UCVA and BCVA, and keratometric parameters in patients undergoing simultaneous T-PRK and CCL.
The primary variables in combined T-PRK and CXL are the maximal ablation depth and the postoperative corneal thickness. Most surgeons choose a maximum ablation depth of 50 μm and a minimal postoperative corneal thickness of 350–400 μm. Although some surgeons advocate the use of mitomycin C 0.02% to prevent haze, others believe that it is not necessary.
In patients with keratoconus, the epithelium is not uniform in thickness; rather, it is thinner directly above the cone. Therefore, manual removal of the epithelium over the central 6–8 mm will “unmask” the corneal stromal irregularity. In contrast, transepithelial PTK has the advantage of removing the thinned epithelium, Bowman layer, and stroma over the cone apex. Thus, the procedure may be able to regularize the anterior corneal surface while allowing the patient’s epithelium to act as a masking agent.
Kanellopoulos AJ. Comparison of sequential vs same-day simultaneous collagen cross-linking and topography-guided PRK for treatment of keratoconus. J Refract Surg. 2009; 25(9):S812–S818.
Kymionis GD, Kontadakis GA, Kounis GA, et al. Simultaneous topography-guided PRK followed by corneal collagen cross-linking for keratoconus. J Refract Surg. 2009;25(9): S807–S811.
Stojanovic A, Zhang J, Chen X, Nitter TA, Chen S, Wang Q. Topography-guided transepithelial surface ablation followed by corneal collagen cross-linking performed in a single combined procedure for the treatment of keratoconus and pellucid marginal degeneration. J Refract Surg. 2010;26(2):145–152.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.