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
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
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, (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
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.
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
“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.
“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
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.
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.
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.
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
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
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
“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
“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
“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
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?
- Baiocchi S, Mazzotta C, Cerretani D, Caporossi T, Caporossi A.
Corneal crosslinking: riboflavin concentration in corneal stroma exposed with
and without epithelium. J Cataract Refract Surg.
- Caporossi A, Baiocchi S, Mazzotta C, Traversi C, Caporossi T.
Parasurgical therapy for keratoconus by riboflavin-ultraviolet type A rays
induced cross-linking of corneal collagen: preliminary refractive results in an
Italian Study. J Cataract Refract Surg. 2006;32(5):837-845.
- Coskunseven E, Jankov MR 2nd, Hafezi F. Controlateral eye study of
corneal collagen cross-linking with riboflavin and UVA irradiation in patients
with keratoconus. J Refract Surg. 2009;25(4):371-376.
- Ertan A, Colin J. Intracorneal rings for keratoconus and
keratectasia. J Cataract Refract Surg. 2007;33(7):1303-1314.
- Ertan A, Karacal H, Kamburoglu G. Refractive and topographic
results of transepithelial cross-linking treatment in eyes with Intacs.
Cornea. 2009 Jul 1. [Epub ahead of print].
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crosslinking with riboflavin and ultraviolet A to treat induced keratectasia
after laser in situ keratomileusis. J Cataract Refract Surg.
- Hafezi F, Mrochen M, Iseli HP, Seiler T. Collagen crosslinking with
ultraviolet-A and hypoosmolar riboflavin solution in thin corneas. J
Cataract Refract Surg. 2009;35(4):621-624.
- Kerautret J, Colin J, Touboul D, Roberts C. Biomechanical
characteristics of the ectatic cornea. J Cataract Refract Surg.
- Knox Cartwright N, Tyrer J, Marshall J. Corneal biomechanical
change with age and following cross-linking treatment. Paper presented at:
ESCRS Winter Meeting; February 2009; Rome.
- Koller T, Iseli HP, Hafezi F, Vinciguerra P, Seiler T. Scheimpflug
imaging of corneas after collagen cross-linking. Cornea.
- Koller T, Seiler T. Therapeutic cross-linking of the cornea using
riboflavin/UVA. Klin Monatsbl Augenheilkd. September
- Mazzotta C, Traversi C, Baiocchi S, et al. Corneal healing after
riboflavin ultraviolet-A collagen cross-linking determined by confocal laser
scanning microscopy in vivo: early and late modifications. Am J
- Mazzotta C, Traversi C, Baiocchi S, Sergio P, Caporossi T,
Caporossi A. Conservative treatment of keratoconus by riboflavin-uva-induced
cross-linking of corneal collagen: qualitative investigation. Eur J
- Spoerl E, Mrochen M, Sliney D, Trokel S, Seiler T. Safety of
UVA-riboflavin cross-linking of the cornea. Cornea.
- Spoerl E, Wollensak G, Seiler T. Increased resistance of
crosslinked cornea against enzymatic digestion. Curr Eye Res.
- Vinciguerra P, Albè E, Trazza S. Refractive, topographic,
tomographic and aberrometric analysis of keratoconic eyes undergoing corneal
cross-linking. Ophthalmology. 2009;116(3):369-378.
- Wollensak G, Spoerl E, Reber F, Seiler T. Keratocyte cytotoxicity
of riboflavin/UVA-treatment in vitro. Eye. 2004;18(7):718-22.
- Wollensak G, Spoerl E, Seiler T. Riboflavin/ultraviolet-a-induced
collagen crosslinking for the treatment of keratoconus. Am J
- Wollensak G, Spoerl E, Wilsch M, Seiler T. Keratocyte apoptosis
after corneal collagen cross-linking using riboflavin/UVA treatment.
- Wollensak G, Wilsch M, Spoerl E, Seiler T. Collagen fiber diameter
in rabbit cornea after collagen crosslinking by riboflavin/UVA.
- 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: firstname.lastname@example.org.
- 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;
- Aylin (Ertan) Kılıç, MD, can be reached at
Kundret Eye Hospital, Kennedy Street 71, Kavaklidere, Ankara, Turkey;
+90-312-4466464; e-mail: email@example.com.
- 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: