September 01, 2009
11 min read

Corneal cross-linking shows increasingly good results, gains popularity, stimulates research

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Until recently, there was no one successful way of treating keratoconus. In some cases, contact lenses could help prevent the progressive deterioration of vision; nevertheless, about 25% of patients eventually required corneal transplantation.

About 10 years ago, a new technique that was able to change the intrinsic biomechanical properties of the cornea was introduced. There is now hope, validated by several studies, that keratoconus may have a specific treatment, which can be extended to pellucid marginal degeneration and post-LASIK ectasia.

The beginnings

The idea to cross-link the collagen in the cornea was first developed and put into practice by OSN Europe Edition Editorial Board Member Theo Seiler, MD, PhD, who at the time was a professor of ophthalmology at Dresden University, Germany. The suggestion came from previous applications of this chemical-physical process in different fields.

“For over 6,000 years, cross-linking has been used to tan leather and for at least 25 years by dentists to stiffen plastic materials,” Prof. Seiler said. “More recently, it was used by dermatologists to tighten collagen fibers in sagging skin. I thought that the same tightening and stiffening effect might have been a potentially effective way of counteracting the progressive weakening of collagen fibers in keratoconus.”

Efekan Coskunseven, MD
Efekan Coskunseven, MD, (right) said he thinks ring segment implantation and corneal cross-linking should not be performed simultaneously.
Image: Coskunseven E

Preclinical studies on corneal cross-linking began in Dresden in 1995. Several chemical cross-linking substances and light irradiation with different photosensitizing agents were investigated, and in 1998, the first patient was treated with riboflavin and UV light. Because no side effects were reported, a pilot study was started 1 year later.

Prospective trials began in Europe in 2003 and 2004. In the meantime, the technique has been adopted by an increasing number of surgeons, the technology has evolved, the protocols for treatment have come to a better definition, and indications are slowly becoming clearer.

Conclusions so far

A few main points have emerged from this 10-year experience with the procedure.

In Dresden, where now more than 400 eyes have been treated, it was demonstrated that cross-linking has an impact on corneal biomechanics, as it strengthens and stiffens the cornea. It was also shown that keratoconus progression was halted in most of the cases, maximum keratometry readings decreased, and all Pentacam-measured keratoconus indexes were reduced.

These results were confirmed by controlled prospective trials.

From 1999 to 2004, side effects were investigated by the Dresden group. It was shown that the procedure induces keratocyte depopulation about 300 µm deep within the stroma and that repopulation requires up to 6 months. Therefore, a minimum corneal thickness of 400 µm, as previously recommended, was confirmed as the safety threshold for the treatment to avoid damage to the endothelium and deeper structures such as the lens and retina.

In a recent publication, however, Prof. Seiler and co-authors showed that preoperative swelling of the cornea using hypo-osmolar riboflavin solution safely broadens the spectrum of corneal cross-linking indications to thin corneas that would otherwise not be eligible for treatment.

Theo Seiler, MD, PhD
Theo Seiler

The almost complete absence of adverse reactions to the treatment has been confirmed by several studies. Transient haze occurred frequently in connection with epithelial healing, “the vulnerable phase of the procedure,” according to Prof. Seiler. Stromal scars were reported in 3% of the cases.

The success rate and complication rate were determined by Koller and Seiler in a prospective study in which complications were also established.

According to Prof. Seiler, a risk factor for failure was keratometry readings more than 58 D, and the risk factors for visual loss were best corrected visual acuity worse than 20/25 and age older than 35 years. If respecting these contraindications, the failure rate is less than 3% and the complication rate is less than 1%.

At the University of Siena, Italy, where the technique has been used for more than 5 years in about 380 patients, a series of studies were published, including the first confocal microscopy study on the micromorphological modifications induced by the technique.

Another area of this group’s research is indications in relation to age.

Aldo Caporossi, MD
Aldo Caporossi

“Following approval from the ethic committee, we have started applying the technique to very young 10- to 16-year-old patients, and now more than 60% of our treatments are performed in this age group,” Aldo Caporossi, MD, said.

When keratoconus develops at such a young age, it is particularly severe, evolves quickly and inevitably leads to early corneal transplantation.

“We are now retrospectively evaluating our results in terms of efficacy and stability and comparing them to those of different age groups,” Prof. Caporossi said.

It is likely that this study will confirm his previous conclusion: In all cases of progressive keratoconus, cross-linking should be performed as early as possible.

“On average, the time of maximum evolution of the disease is between 12 and 26 years, and it is at this stage that we should cross-link the cornea,” he said.

Protocol in progress

The cross-linking procedure entails impregnation of the cornea with a diffusible photosensitizer. Instillation is repeated every few minutes for a total of 30 minutes, then the cornea is exposed to intense 365-nm UV light irradiation for an additional 30 minutes. The induced photochemical reaction, called oxidative deamination, creates new, stable bridges between collagen molecules that reinforce the corneal structure and increase corneal elasticity by a factor of 3 to 5.

In the nearly 450 centers where cross-linking is currently performed worldwide, the procedure follows this fairly standardized protocol, but a few variations have been proposed by individual specialists to improve safety, efficacy and patient comfort and to widen indications.

OSN Europe Edition Editorial Board Member Joseph Colin, MD, recently presented a new method of corneal impregnation. As a photosensitizer, he uses Ricrolin (Sooft Italia), a standard solution of riboflavin 0.1% in 20% dextran, which makes the solution isosmotic to the corneal stroma.

Joseph Colin, MD
Joseph Colin

“Initially, we used to instill Ricrolin every 3 minutes as a preparation for surgery and continued to do so during UV irradiation,” Prof. Colin said. “Now, to facilitate this step of the procedure, we place a special suction lens on the cornea and fill it with the riboflavin solution so that there is a progressive, sustained absorption by the cornea of the photosensitizing agent before surgery. We still use the drops during irradiation, after the lens has been removed.”

One step of the procedure that is still controversial is the initial removal of the epithelium. Most surgeons agree that removing the epithelium is a necessary step for riboflavin absorption.

“The corneal epithelium represents a barrier for the molecules with molecular weight greater than 100, and riboflavin’s [molecular weight] is more than three times that,” Prof. Seiler said. “The question is, at this point, whether abrasion is the only way to make riboflavin go through this barrier or whether there are better alternatives.”

“We always remove the epithelium for better efficacy,” Prof. Colin said. “However, Sooft is now producing a new riboflavin preparation with modified physiochemical properties that will be able to penetrate the cornea through the epithelium. This will result in better comfort for the patient.”

A study on transepithelial cross-linking with the use of this new Ricrolin TE (transepithelial) is beginning in Italy at the University of Siena.

“The procedure will be performed in 10 keratoconus patients already selected for corneal transplantation. After surgery, the removed tissue will undergo immunology testing and confocal microscopy observation to assess the safety, depth and efficacy of the cross-linking treatment,” Prof. Caporossi said.

According to Aylin (Ertan) Kılıç, MD, an effective way of doing epithelium-on procedures is by impregnating the corneal surface with a 20% alcohol solution for 25 seconds.

“Alcohol breaks the tight junctions of epithelial cells, so the epithelium is no longer a barrier to riboflavin penetration,” Dr. Kılıç said.

There are several advantages in leaving the epithelium on, she said. Healing and visual rehabilitation are faster, and the patient experiences no pain. In addition, the risk of complications is significantly lowered.

“Most of the complications of cross-linking are due to removal of the epithelium,” Dr. Kılıç said. “In addition, the studies on riboflavin penetrations were conducted on porcine eyes, which have a thicker epithelium than the human keratoconic eye.”

OSN Europe Edition Editorial Board Member A. John Kanellopoulos, MD, recently experimented on a variation in UV delivery that shortens the total irradiation time to 15 minutes.

“This new approach raises the irradiancy level of the UVA laser from 3 mW/m² to 7 mW/m² and fractions the doses of the energy delivery with a pulse irradiation using cycles of 30 seconds on and 30 seconds off. The method should provide more oxygen into the collagen matrix than the standard procedure, leading to optimization of the collagen oxidative deamination reaction,” Dr. Kanellopoulos said.

Clinical results are comparable to those of the standard procedure, and the increased irradiancy levels are not phototoxic for the cornea, he said. The shorter procedure may, in fact, result in less keratocyte loss because fibroblasts are more resistant to higher energy exposure and shorter intervals rather than lower energy at higher intervals.

Combined procedures

Prof. Seiler and colleagues reported an improvement in visual acuity after cross-linking in a significant percentage of eyes treated for keratoconus or post-LASIK ectasia.

“A fact that we cannot fully explain but that is probably due to the regularization of the corneal surface induced by the procedure,” he said.

However, cross-linking primarily aims at biochemical stability rather than visual improvement.

“In most cases, there is no visual improvement with cross-linking. The procedure halts disease progression, but patients cannot see better, and this is frustrating for both them and the doctor,” Prof. Colin said.

In patients with progressive keratoconus and intolerance to contact lenses, the combination of cross-linking and intracorneal ring segments seems to be an effective way of obtaining more rewarding visual results.

“On one hand, we halt disease progression. On the other hand, we improve visual acuity by reshaping the cornea with rings,” he said.

In Bordeaux, the two procedures are carried out in one operating session. Intacs (Addition Technology) implantation is performed through femtosecond laser-created channels, and cross-linking is done immediately after in the standard way, Prof. Colin said.

Efekan Coskunseven, MD, also has a wide experience with combined corneal cross-linking and implantation of ring segments. In his initial experience, he performed corneal cross-linking first, followed by Keraring implantation (Mediphacos) at a 6-month interval.

“We were afraid that UVA could potentially damage the ring segments,” Dr. Coskunseven said.

Later he tried to reverse the sequence and found that no damage to the ring segments was caused by UV irradiation. He compared the results of two groups, in which corneal cross-linking followed by ring segments implantation vs. ring segments implantation followed by corneal cross-linking were performed, and found that the second sequence produced a greater improvement in uncorrected visual acuity, best corrected visual acuity, cylinder and spherical equivalent.

However, Dr. Coskunseven said he thinks that ring segment implantation and corneal cross-linking should not be performed simultaneously.

“Healing at the incision site is extremely important, and both manually and femtosecond laser created tunnels need some time for epithelialization and perfect healing. We prefer to wait, monitor the patient and choose the right time for the second procedure, which may vary between 1 and 6 months later,” he said.

Dr. Kılıç uses the combination of cross-linking and Intacs in almost all her patients, with the exception of early stage keratoconus with good visual acuity and young age.

“Cross-linking only halts disease progression, but flattening and regularization of the cornea, as well as keratometry and refractive changes, are due to [Intacs] implantation,” she said.

In addition, she is proposing a new way of using riboflavin in the combined procedure. She injects the substance into the channels just before Intacs implantation and proceeds with the normal cross-linking procedure, instilling further riboflavin drops in the same session.

“I found that in this way I can also treat corneas thinner than 400 µm. The combined effect of riboflavin injection and Intacs implantation creates a slightly edematous, thicker cornea that can be safely irradiated. In this way, I can now broaden my spectrum of indication to 350-µm corneas,” Dr. Kılıç said.

Another way of enhancing visual results after a cross-linking procedure is limited topography-guided PRK, which has been extensively performed for several years by Dr. Kanellopoulos.

UVA corneal cross-linking can be used as a pretreatment to strengthen and stabilize the cornea, allowing a refractive treatment that may be otherwise contraindicated.

“Our therapeutic goal is not emmetropia but improved BCVA. Therefore, we only ablate about 50 µm. The residual refractive error is compensated by spectacles or soft contact lenses,” Dr. Kanellopoulos said.

In a study of 325 patients, he found that the two procedures are better performed sequentially, with PRK preceding corneal cross-linking. The sequential procedure was also found to be easier and less stressful for the patient.

“I do epithelial removal, partial PRK with [mitomycin C] and directly go on to cross-linking,” he said.

In these cases, the aim of PRK is regularizing the cornea rather than correcting refraction.

Rudy Nuijts, MD
Rudy Nuijts

“It is more like a [phototherapeutic keratectomy] procedure,” Dr. Kanellopoulos said. “A more regular cornea will then allow a better distribution of forces by the cross-linking procedure.”

Finally, the combination of corneal cross-linking and phakic Artisan toric IOLs (Ophtec) was tested in a group of 19 patients with high refractive errors by Camille Budo, MD, and Rudy Nuijts, MD.

“Results were rewarding in terms of reduced astigmatism and BCVA lines gain. We prefer this procedure because it is reversible,” Dr. Nuijts said.

In a few cases, sequential treatment with corneal cross-linking, femtosecond-assisted Intacs and toric Artisan implantation was carried out by the same surgeons. – by Michela Cimberle

Could long-term effects of cross-linkage be a concern and a potential source of problems?


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  • Aldo Caporossi, MD, can be reached at Policlinico Le Scotte, V.le Bracci, 53100 Siena, Italy; +39-0577-233356; fax: +39-0577-233358; e-mail:
  • Joseph Colin, MD, can be reached at Hôpital Pellegrin, Place Amélie Raba-Lèon, 33076 Bordeaux, France; +33-5-56795608; fax: +33-5-56795909; e-mail:
  • Efekan Coskunseven, MD, can be reached at the Dunya Eye Hospital, Istanbul, Turkey; +90-212-3623232; fax +90-212-2750580; e-mail:
  • A. John Kanellopoulos, MD, can be reached at Laservision Institute, Tsocha Street 17, Athens 11521, Greece; +30-2107472777; fax: +30-210-7472789; e-mail:
  • Aylin (Ertan) Kılıç, MD, can be reached at Kundret Eye Hospital, Kennedy Street 71, Kavaklidere, Ankara, Turkey; +90-312-4466464; e-mail:
  • Rudy Nuijts, MD, can be reached at University Hospital of Maastricht, Department of Ophthalmology, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands; +31-43-3877344; fax +31-43-3875343; e-mail:
  • Theo Seiler, MD, can be reached at IROC AG, Institut für Refraktive und Ophthalmo-Chirurgie, Stockerstrasse 37, CH-8002 Zürich, Switzerland; +41-43-488-38-00; fax: +41-43-488-38-09; e-mail: (personal secretary).