Keratoconus: Trends in Cross-linking

A look at the treatment protocols and new developments

The corneal collagen cross-linking (CXL) procedure is a minimally invasive, conservative, early intervention for keratoconus. It is indicated for patients with progressive keratoconus or ectasia because it offers the opportunity to preserve visual function by slowing or halting progression of the condition. In 2015, a global Delphi panel published a consensus report recognizing cross-linking as the standard of care for progressive keratoconus.1 (see “Keratoconus: An Overview”).

As we go to press, only one cross-linking platform is FDA-approved: the iLink corneal remodeling platform (Glaukos), consisting of Photrexa Viscous (riboflavin 5’-phosphate in 20% dextran ophthalmic solution), Photrexa (riboflavin 5’-phosphate ophthalmic solution), and the KXL system, approved in 2016.2,3

This article aims to provide a look at the standard epithelium-off CXL and several new developments for the treatment of keratoconus (see “Communication Essential in Implementing Cross-linking”).

Standard Cross-linking

Also known as the Dresden protocol, this standard approach occurs as follows. The central corneal epithelium is debrided, and the corneal stroma is loaded with photosensitizer through the application of drops of riboflavin 5’-phosphate ophthalmic solution at 2-minute intervals for 30 minutes. The epithelial debridement step enables permeation of the riboflavin to the corneal stroma, where photoactivation of the riboflavin with ultraviolet A light (UVA) generates oxygen radicals in the corneal stroma. This effect, in turn, induces the formation of permanent collagen cross-links via photopolymerization.

The anterior chamber is then examined for the presence of a yellow flare, indicating that the photosensitizing solution has saturated the corneal stroma, with additional drops applied at 2-minute intervals as needed until flare is observed. Once flare occurs, the corneal thickness is measured with ultrasound pachymetry. If it is less than 400 µm, a hypo-osmolar riboflavin 5’-phosphate ophthalmic solution is applied at 10- to 15-second intervals to swell the cornea. Corneal pachymetry is rechecked periodically until it reaches 400 µm. The corneal surface is then irradiated with UVA at 3 mW/cm2 for 30 minutes, for a total treatment dose of 5.4 J/cm2.

Two U.S. phase 3, multicenter, prospective, randomized, sham-controlled clinical trials showed that maximum keratometry (Kmax) decreased from baseline to 1 year, whereas keratoconus continued to progress in the control group (Figure 1). A difference between the treatment and control groups of at least 1.0 D in the mean change in Kmax was chosen as a clinically meaningful endpoint.2,3 The trials demonstrated that the procedure also has an excellent safety profile, with most adverse events resolving within the first month following treatment. In the keratoconus clinical trial, there was one serious adverse event, a case of microbial keratitis, for an incidence of 0.3%. Transient corneal haze was common in the initial postoperative months, but it resolved by 1 year in all but three eyes. There was no statistically significant loss of endothelial cells.

FIGURE 1. In U.S. phase 3 controlled clinical trials, treated patients experienced Kmax improvement, while control arm patients continued to exhibit disease progression.

Additionally, cross-linking with the Dresden protocol can slow or halt the progression of keratoconus, with persistence of the treatment effect through 10 years of follow-up.7-10 Further, it has been associated with a 25% reduction in corneal transplantation over 3 years (P=.005).11

Alternative UVA Parameters

Researchers are looking to adjust the UVA delivery parameters to improve upon clinical workflow and patient experience. In accelerated CXLs, the UV irradiance is increased from 3 mW/cm2 to as high as 30 mW/cm2. This increase enables delivery of an equivalent or increased total treatment dose (J/cm2), while reducing the duration of the treatment. International clinical studies suggest that accelerated cross-linking may be successful in stabilizing keratoconus.12,13

Investigators have noted differences in the appearance and depth of the corneal stromal demarcation line that occurs when cross-linking is performed at different irradiances.12 This observation is important to mention because the depth of the demarcation line, commonly observed on optical coherence tomography (OCT) in the initial months after cross-linking, is correlated with the depth of keratocyte apoptosis in the cornea.14,15 It is often used as a convenient clinical proxy for relative (not absolute) treatment depth. Investigators suggest that adjusting the UVA delivery and demarcation line depth offers the potential to tailor the treatment to suit corneas of varying thickness. Two European groups proposed treatment algorithms to achieve this customized effect.16,17 Further study is needed to evaluate the impacts of these parameters on the long-term safety and efficacy of the procedure.

This difference in demarcation line depth between conventional and accelerated protocols is likely modulated by oxygen bioavailability within the stroma. Accelerated delivery of UVA light also increases the cornea’s oxygen consumption, reducing the efficiency of the cross-linking reaction.18 As discussed below, innovative new methods of increasing oxygen availability to the cornea are under review.


Keratoconus is a bilateral, asymmetric ectasia resulting in progressive stromal thinning, corneal steepening, and irregular astigmatism. It can lead to loss of best-corrected visual acuity (BCVA), scarring, and contact lens intolerance. In the absence of therapeutic intervention, one in five patients with keratoconus eventually undergoes corneal transplantation.1

Historically, the prevalence of keratoconus in the United States has been estimated at 1 in 2,000.2 However, research conducted more recently, after the introduction of advanced technology for the detection of anterior and posterior corneal irregularity, suggests that the prevalence may be five to 10x higher.3 With this realization, screening and early diagnosis become even more important to ensuring that patients seek cross-linking early in the course of the progressive disease, to preserve vision and avoid the most serious consequences of later-stage keratoconus, such as the inability to achieve good corrected vision or the need for a corneal transplant.

Because corneal collagen cross-linking is the only conservative intervention available to slow disease progression, It is indicated in all stages of progressive keratoconus and ectasia. Ideally, patients are diagnosed and treated in the disease’s early stage to slow progression before loss of BCVA. However, patients who present later in the disease course should still undergo cross-linking to preserve the remaining function.


  1. Pramanik S, Musch DC, Sutphin JE, Farjo AA. Extended long-term outcomes of penetrating keratoplasty for keratoconus. Ophthalmology. 2006;113(9):1633-1638.
  2. Hersh PS, Stulting RD, Muller D, Durrie DS, Rajpal RK; U.S. Crosslinking Study Group. U.S. multicenter clinical trial of corneal collagen crosslinking for treatment of corneal ectasia after refractive surgery. Ophthalmology. 2017;124(9):1475-1484.
  3. Godefrooij DA, de Wit GA, Uiterwaal CS, et al. Age-specific incidence and prevalence of keratoconus: A nationwide registration study. Am J Ophthalmol. 2017;175:169-272.

Epithelial-on Protocol

Oxygen dynamics have also likely been a limiting factor in the development of epi-on or transepithelial techniques, in which the epithelium is not debrided at the start of the procedure. Elimination of the debridement step is attractive to clinicians and researchers, due to the potential to increase patient comfort, speed visual recovery, and further reduce the already low incidence of complications, such as haze and infection, which have been associated with epithelial-off cross-linking. Because the epithelium acts as a natural barrier to all three of the key components of cross-linking photochemistry (riboflavin, UV light, and oxygen), published studies of this protocol have mostly shown reduced corneal flattening and a high rate of keratoconus progression compared with the conventional Dresden protocol.[19,20]

Proposed modifications to this protocol include the development of transepithelial riboflavin formulations to enhance delivery to the corneal stroma; pulsed, accelerated UVA irradiation protocols to reduce the rate of oxygen consumption; and, most recently, the addition of supplemental oxygen at the corneal surface to increase the rate of oxygen diffusion.

Oxygen is an important mediator in cross-linking photochemistry, which can follow either an aerobic (type I) or an anaerobic (type II) pathway. While cross-linking formation is possible under both conditions, the aerobic pathway leads to more efficient generation of oxygen radicals.18,21

The addition of supplemental oxygen may improve the efficiency of epi-on cross-linking by increasing the rate of diffusion to the corneal stroma such that oxygen is replenished faster than it is consumed. Preclinical studies show that, by providing supplemental oxygen to the anterior surface during UV illumination in high-irradiance, epi-on cross-linking could balance midstromal oxygen supply and demand (Figure 2) and produce significantly more corneal stiffening than epi-on protocols without supplemental oxygen.17

FIGURE 2. Stromal oxygen concentrations before, during, and after UV light application in two different cross-linking protocols demonstrate that the supply of and demand for oxygen are balanced when supplemental oxygen is provided.

A U.S. phase 3 clinical trial to evaluate a new epi-on protocol with supplemental oxygen is under way ( Identifier: NCT03442751).


Successful implementation of cross-linking within a cornea practice requires good communication with patients and comanaging optometrists.

Insurance coverage. The cross-linking practice should work with referring ODs to establish criteria for the documentation of progression, which will facilitate reimbursement for the procedure. Historically, insurance coverage was a significant barrier to access with cross-linking. Today, the majority of commercial health plans in the US, representing >95% of commercially covered lives, recognize cross-linking with FDA-approved drugs and devices as a covered service.1,2

In addition to more consistent payment policies, there are also now appropriate drug (J2787) and office procedure (0402T) codes for cross-linking. There is no global period, so follow-up appointments can be billed by the ophthalmologist or optometrist according to exam complexity.

Setting patient expectations. In talking with patients, it is important to appropriately set expectations that cross-linking will stabilize a degenerative condition, rather than correct their vision. Patients must be made aware that, in most cases, they will still need vision correction after the procedure, but the treatment can help them to remain in standard methods of vision correction, such as glasses or soft contact lenses, and avoid the need for specialty contact lenses or invasive transplantation. Patients who already wear specialty scleral lenses may need to continue wearing them. Patients can generally expect to return to contact lens wear within 1 month, as long as the epithelium is fully healed. However, they may ultimately need to be refit as the cornea changes postoperatively over the course of a year or so after cross-linking. Communication about the time course for stabilization of the treatment effect is essential. Additionally, patient coordinators or counselors within the cross-linking practice can be very helpful in obtaining the needed insurance preauthorization or precertification and educating patients about coverage amounts and copayments.


  1. Insurance coverage for FDA-approved cross-linking. Avedro website. Available at: . Accessed February 23, 2020.
  2. Is cross-linking covered by insurance? Avedro website. Available at: . Accessed February 23, 2020.

Customized Cross-linking

While the primary goal of conventional cross-linking is disease stabilization, multiple studies have demonstrated a degree of disease reversal in the form of post-treatment corneal flattening. Customized cross-linking treatment protocols have been proposed to harness this flattening effect to reduce corneal irregularities and improve visual function.22

In the customized procedure, tomography is used to design a personalized treatment pattern that targets the formation of more cross-link bonds in the weakened area of the cone.23,24 The treatment is applied across the cornea, using a programmable UVA delivery device with integrated eye tracking to selectively activate the riboflavin photosensitizer in the targeted areas.25 Several controlled clinical studies have demonstrated greater corneal regularization in eyes treated with epithelium-off customized cross-linking compared with conventional cross-linking, with greater flattening in the areas of steepest corneal curvature, compensatory steepening in the untreated surrounding cornea, and resultant gains in visual function.26-28 Custom cross-linking could potentially be performed with epi-off or epi-on protocols.

Keeping Tabs

Conventional epi-off cross-linking is currently the standard-of-care treatment in the management of keratoconus patients, increasing our responsibility as corneal physicians to identify these patients early and intervene with conservative treatment to preserve vision. Promising new developments with the potential to reduce treatment times, eliminate the need for epithelial removal, and improve refractive outcomes are currently under clinical investigation. CP


  1. Gomes JAP, Tan D, Rapuano CJ, et al; Group of Panelists for the Global Delphi Panel of Keratoconus and Ectatic Diseases. Global consensus on keratoconus and ectactic diseases. Cornea. 2015;34(4):359-369.
  2. Hersh PS, Stulting RD, Muller D, Durrie DS, Rajpal RK; U.S. Crosslinking Study Group. U.S. multicenter clinical trial of corneal collagen crosslinking for treatment of corneal ectasia after refractive surgery. Ophthalmology. 2017;124(9):1475-1484.
  3. Hersh PS, Stulting RD, Muller D, Durrie DS, Rajpal RK; U.S. Crosslinking Study Group. United States multicenter clinical trial of corneal collagen crosslinking for keratoconus treatment. Ophthalmology. 2017;124(9):1259-1270.
  4. Pramanik S, Musch DC, Sutphin JE, Farjo AA. Extended long-term outcomes of penetrating keratoplasty for keratoconus. Ophthalmology. 2006;113(9):1633-1638.
  5. Kennedy RH, Bourne WM, Dyer JA. A 48-year clinical and epidemiologic study of keratoconus. Am J Ophthalmol. 1986;101(3):267-273.
  6. Godefrooij DA, de Wit GA, Uiterwaal CS, et al. Age-specific incidence and prevalence of keratoconus: A nationwide registration study. Am J Ophthalmol. 2017;175:169-272.
  7. Shetty R, Pahuja NK, Nuijts RM, et al. Current protocols of corneal collagen cross-linking: Visual, refractive, and tomographic outcomes. Am J Ophthalmol. 2015;160(2)243-249.
  8. O’Brart DP, Patel P, Lascaratos G, et al. Corneal cross-linking to halt the progression of keratoconus and corneal ectasia: Seven-year follow-up. Am J Ophthalmol. 2015;160(6):1154-1163.
  9. Poli M, Lefevre A, Auxenfans C, Burillon C. Corneal collagen cross-linking for the treatment of progressive corneal ectasia: 6-year prospective outcome in a French population. Am J Ophthalmol. 2015;160(4):654-662.
  10. Raiskup F, Theuring A, Pillunat LE, Spoerl E. Corneal collagen crosslinking with riboflavin and ultraviolet-A light in progressive keratoconus: ten-year results. J Cataract Refract Surg. 2015;41(1): 41-46.
  11. Godefrooij DA, Gans R, Imhof SM, Wisse RP. Nationwide reduction in the number of corneal transplantations for keratoconus following the implementation of cross-linking. Acta Ophthalmol. 2016;94(7):675-678.
  12. Shajari M, Kolb CM, Agha B, et al. Comparison of standard and accelerated corneal cross-linking for the treatment of keratoconus: A meta-analysis. Acta Ophthalmol. 2019;97(1):e22-e35.
  13. Lang PZ, Hafezi NL, Khandelwal SS, Torres-Netto EA, Hafezi F, Randleman JB. Comparative functional outcomes after corneal crosslinking using standard, accelerated, and accelerated with higher total fluence protocols. Cornea. 2019;38(4):433-441.
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