Corneal collagen cross-linking (CXL) is a procedure to halt the progression of corneal ectasia. It combines riboflavin, a photosensitizer, with ultraviolet A (UV-A) light to induce new covalent bonds between collagen fibers through a photochemical reaction1). This increases the mechanical strength of the cornea and suppresses the progression of ectasia.
The mechanism of action of CXL is based on a photochemical reaction1).
The crosslinking effect is greatest in the anterior corneal layers and diminishes in deeper layers as riboflavin concentration decreases. CXL has been shown to affect corneal structure and cell density for at least 36 months. It does not significantly affect intraocular pressure measurements. Corneal resistance factor and corneal hysteresis change only slightly after CXL, but custom variables derived from the same device show changes suggesting stiffer behavior after CXL2).
The basic technology of CXL was developed at the University of Dresden in the late 1990s. Ultraviolet irradiation of riboflavin-impregnated porcine and rabbit corneas successfully induced collagen crosslinking. Human studies began in 2003, and progression was halted in all 16 patients with progressive keratoconus. Flattening of the anterior corneal curvature was seen in 70%, and visual acuity improved in 65%.
In 2011, the FDA granted orphan drug designation, and in April 2016, corneal CXL using riboflavin and UV-A was officially approved2).
CXL is performed under local anesthesia, so there is usually no pain during the procedure. However, with the epithelial-off (epi-off) method, pain, foreign body sensation, and tearing may occur for several days after surgery. Postoperative pain is managed with analgesics and a bandage contact lens. The transepithelial (epi-on) method does not remove the epithelium, thus reducing postoperative pain.
The main purpose of CXL is to halt the progression of corneal ectasia. Optimal candidates are patients with progressive corneal ectatic diseases2).
Keratoconus is a contraindication for excimer laser surgery (LASIK, PRK, etc.), and progressive keratoconus is also a contraindication for phakic intraocular lenses (phakic IOL) 12). After CXL halts progression, refractive correction with glasses, contact lenses, or phakic IOL can be considered.
The Dresden protocol is the standard CXL technique, with the most accumulated evidence 1)2).
After preoperative riboflavin soaking, confirm that the corneal stromal thickness is at least 400 µm 1). Dextran-free riboflavin formulations may reduce intraoperative stromal dehydration 1).
Epithelium-Off (Epi-off)
Standard method: Considered most effective1)
The corneal epithelium acts as a barrier to riboflavin diffusion into the stroma, so it is removed to enhance penetration.
Disadvantages: Postoperative pain, delayed epithelial healing, risk of infection.
Trans-epithelial (Epi-on)
Preserves the epithelium method.
Techniques such as using agents to loosen intercellular junctions and iontophoresis are being attempted.
Advantages: Reduced postoperative pain and corneal haze.
Disadvantages: May be less effective than epithelium-off method2)
Based on the Bunsen-Roscoe law, protocols that increase UV-A irradiance to shorten treatment time have been developed. 10 mW/cm² for 9 minutes (total energy 5.4 J/cm²) shows a good balance between standard and acceleration1). However, exceeding 45 mW/cm² results in loss of CXL effect. Accelerated protocols significantly reduce cross-linking strengthening effect, but long-term clinical stability has been confirmed6).
Examples of main accelerated protocol parameters:
A technique that uses a weak electric current to actively transport riboflavin into the corneal stroma in transepithelial CXL. It aims to enhance drug penetration into the stroma while preserving the epithelium.
Leveraging the corneal strengthening effect of CXL and the bactericidal activity of ultraviolet irradiation, this is applied in the management of keratitis with stromal melting. It is called photoactivated chromophore for keratitis-corneal cross-linking (PACK-CXL). Meta-analyses have shown that adjuvant CXL promotes healing of infectious keratitis compared to standard antimicrobial therapy alone. However, consistency of results has not yet been established, and it is currently considered only in cases resistant to standard antimicrobial therapy.
Approaches combining CXL with refractive surgery have been reported.
The following parameters are considered for assessing progression 1):
| Contraindication | Reason |
|---|---|
| Corneal thickness <400 µm | Risk of endothelial toxicity1) |
| History of herpes infection | Possible viral reactivation1)5) |
| Active infection | Risk of worsening infection |
| Severe corneal scarring or opacity | Limited efficacy of CXL |
| History of epithelial wound healing failure | Increased risk of postoperative complications |
| Severe ocular surface disease (e.g., dry eye) | Risk of delayed epithelial healing |
| Autoimmune disease | Risk of corneal melting due to immune reaction |
Risk factors for CXL failure (progression after treatment) include preoperative age ≥35 years, preoperative corrected visual acuity worse than 20/25, and preoperative maximum steepness >58 D 2). However, some reports indicate that CXL can safely stabilize vision and corneal shape even in progressive keratoconus with >58 D 2).
Contact lens users can also undergo CXL. However, to ensure the accuracy of preoperative examinations (corneal topography), a period of contact lens discontinuation is necessary. Please consult your doctor for the specific discontinuation period.
The following examinations are important for determining CXL eligibility.
To determine progression, documentation of changes over time during a follow-up period of at least 12 to 18 months is necessary1).
The procedure based on the Dresden protocol proceeds as follows.
For cases with corneal thickness 320–400 µm, use hypo-osmolar riboflavin to swell the cornea to ≥400 µm before irradiation1). Hafezi et al. introduced the Sub400 protocol (immediate hypo-osmolar riboflavin soak for 20 minutes + individualized UV-A energy irradiation) for thin corneas6).
Corneal epithelium regenerates in 3–4 days, and corneal haze often appears at 1–2 months and resolves over 6–12 months 1). Refractive and topographic stabilization after CXL takes several months to a year.
The KERALINK trial (UK) was an RCT involving 60 patients aged 10–16 years with progressive keratoconus. At 18 months, the mean K2 in the CXL group was 49.7 D, while in the standard treatment group it was 53.4 D, with an adjusted mean difference of –3.0 D (95% CI: –4.9 to –1.1 D, P = 0.002), showing significant superiority of CXL. The progression rate was 7% (2/30) in the CXL group versus 43% (12/28) in the standard treatment group (OR 0.1, P = 0.004) 3).
The authors concluded that “CXL halts keratoconus progression in the majority of young patients” and recommended it “should be considered as a first-line treatment for progressive disease” 3).
A US multicenter trial (205 patients with progressive keratoconus, Dresden protocol vs. sham control) provided the basis for FDA approval 14).
| Parameter | CXL group | Control group |
|---|---|---|
| Kmax change (1 year) | -1.6±4.2 D | Continued progression |
| Kmax change in post-surgical ectasia study (179 cases) | -0.7±2.1 D | +0.6±2.1 D |
A meta-analysis of 75 studies including follow-up of 36 months or longer reported the following results2):
In a 10-year follow-up study by Raiskup et al., corneal shape was stable in over 95% of progressive keratoconus cases treated with Dresden CXL. The mean Kmax gradually decreased from 1 to 10 years postoperatively, confirming long-term corneal flattening13). Two European studies reported a significant reduction in the number of full-thickness corneal transplants performed for keratoconus since the introduction of CXL2).
The effect of CXL is mainly exerted in the anterior corneal layers because the riboflavin concentration decreases in the deeper layers.
The effect of CXL often persists for a long time, and some studies have reported stability for more than 10 years. However, progression after CXL has been reported in about 8% of cases. Especially in young patients, there is a risk of re-progression, so regular follow-up after surgery is important. If re-progression is observed, repeat CXL may be considered.
CXL is a safe procedure, but complications mainly related to epithelial removal have been reported1)4).
| Complication | Frequency/Characteristics |
|---|---|
| Corneal haze | Frequent. Appears at 1–2 months, resolves at 6–12 months1) |
| Permanent scarring | Up to 8.6%1) |
| Aseptic infiltration | Early postoperative period. Resolves with steroid eye drops1) |
| Infectious keratitis | 0.0017%1) |
| Delayed epithelial healing | May occur with epithelial debridement |
| Excessive flattening | Accompanied by hyperopic shift |
| Endothelial damage | 1.4% (reported even with safety standards)1) |
CXL can induce reactivation of herpes simplex virus (HSV) 1)5)11). CXL is contraindicated in patients with a history of herpetic eye disease 1). The main factors for reactivation are cell damage from UV irradiation, temporary loss of the corneal nerve plexus, and postoperative use of steroid eye drops.
In a report by Bagatin et al., the incidence of HSV keratitis after CXL was 4 out of 52 cases (7.69%). All 4 patients had a history of herpes labialis. Even with prophylactic acyclovir administration starting 5 days before surgery, 2 out of 16 cases (12.5%) developed HSV keratitis 5).
Wang et al. reported 4 new cases of HSV keratitis out of 300 cases (1.33%). None of the 4 patients had a history of ocular herpes, but onset occurred between 3 days and 1 month after surgery. Some cases had few subjective symptoms, and regular follow-up was considered essential for early diagnosis 11).
CXL can induce reactivation of herpes simplex virus (HSV). Factors for reactivation include ultraviolet irradiation, damage to the corneal nerve plexus, and postoperative steroid use. Patients with a history of herpetic keratitis are not eligible for CXL. If you have a history of herpes labialis, please inform your doctor before surgery. Prophylactic antiviral medication may be considered.
Moramarco et al. reported a case of severe corneal melting after CXL in a 12-year-old boy. Microbiological tests were negative. After avoiding perforation with a conjunctival flap surgery, DALK was performed 3 months later, and visual acuity recovered to 20/25 4).
Tillmann et al. reported two cases of corneal melting and perforation after CXL. One case perforated on postoperative day 7 without signs of infection, and the other had Staphylococcus aureus detected within 24 hours. Both required emergency full-thickness corneal transplantation. An association with ZNF469 gene mutation has also been suggested 10).
Soleimani et al. reported a case of corneal edema after CXL in a patient with preoperative corneal thickness of 461 µm. AS-OCT showed a very deep CXL line, and endothelial cell density decreased to 60% of the fellow eye. Complete recovery occurred after 2 months with steroid eye drops, and final visual acuity was 20/30 9).
The KERALINK trial, as the first RCT of CXL in young patients (10–16 years), showed that CXL halts keratoconus progression in the majority3). Initial cost-effectiveness analyses report that CXL has high cost-effectiveness3).
New protocols and technologies under investigation include:
Application to bullous keratopathy is also being considered, but the effect may last only about 6 months and may remain palliative.
