Comparing the proven way to the latest way has always made for a tough
choice. Some would gladly stick with the tried-and-true, while early adopters
always push for more and better despite potential risks.
Ophthalmologists are plowing ahead with research about epithelium-on
cross-linking, even though epithelium-off methods are not yet approved in the
United States. Epithelium-on techniques are controversial even in the European
Union, where epithelial-off procedures have been the standard of care for years
and are approved for use.
Both modalities are used under regulatory investigational exemptions in
the United States, but removing the epithelium has been the standard of care in
Europe for 5 years, with 7 years of prior research to back up its approval
Yet, epithelium-on procedures promise efficacy with reduced pain,
quicker healing and quicker visual acuity improvement, which has led some to
engage in clinical trials to prove not only that it works, but also that it can
be done with less riboflavin preparation penetrating the stroma and less
exposure time of the ultraviolet A (UVA) light source than has been used
|With more than a decade of experience with this technique, A.
John Kanellopoulos, MD, sees cross-linking as an integral part of customizing
corneal refractive procedures.
Image: Kanellopoulos AJ
William B. Trattler, MD, is helping run the CXL-USA Study Group
multicenter clinical trial, which has a dozen investigational sites and began
looking at epithelium-on procedures starting in May 2010. He said that his
results with epithelium-on cross-linking show that visual results are as good
as epithelium-off techniques but with much faster visual recovery.
With epithelial-on cross-linking, patients are more comfortable
during the early postoperative period, with a faster visual recovery, lower
risk of infection and a faster return to contact lens wear, he said. In
contrast, with epithelium off, the recovery is similar to PRK, and patients
need to wait for 1 to 2 weeks to return to RGP contact lenses.
With epi-on cross-linking, we do not even use a bandage contact
lens, Dr. Trattler, OSN SuperSite Board Member, said.
But approval from the U.S. Food and Drug Administration for
cross-linking needs to come first. Once both procedures receive FDA approval,
then head-to-head randomized clinical trials could show which method is better.
People would love to see an epithelium-on technique that
works, said Peter S. Hersh, MD, medical monitor to Avedro Inc., which is
currently completing data analysis on multicenter phase 3 studies of
epithelium-off corneal collagen cross-linking for the treatment of progressive
keratoconus and post-LASIK ectasia.
Peter S. Hersh
To date however, there is little real evidence and virtually
nothing from the point of view of controlled clinical trials in the literature
that looks at the results of epithelial-on cross-linking, Dr. Hersh, an
OSN Refractive Surgery Board Member, said. In contrast, there is
extensive literature on the results of epi-off cross-linking. For instance, in
our study, we found stability or topography improvement in 96% of eyes, with an
average flattening of maximum keratometry of 1.7 D and average improvement in
visual acuity measures. With increased interest in epi-on cross-linking,
Im very sanguine that companies will ultimately get to the methodology
and delivery system that will allow us to do it. For instance, Avedro is
currently working on a higher-power UV light source that may have advantages in
epi-on cross-linking, and Topcon is working with a new epi-on riboflavin
Off or on?
Cross-linking was first performed in Europe in 1999, and by September
2006, it had been approved by the European Union. There, per the Dresden
protocol, the epithelium is removed, the cornea and the anterior chamber are
soaked with dextran-based 0.1% riboflavin solution, and UVA light is delivered
via a 3 mW/cm2 source for 30 minutes.
UVA light interacts with the riboflavin, producing reactive oxygen
species that create chemical bonds between and within corneal collagen fibrils,
making them stiffer and increasing corneal strength and stability by inducing
new cross-links between or within collagen fibers.
Many studies have shown the halt of keratoconus progression and even the
gains of one or more lines of best corrected visual acuity 1 year
But one issue confounds ophthalmologists, in either epithelium-off or
epithelium-on procedures: How exactly does cross-linking work?
A. John Kanellopoulos, MD, was heavily involved in the European
experience with cross-linking, with more than a decade of experience with the
procedure that has been extensively presented and published. But he noted that
while the stiffening of the cornea has been measured through sophisticated
mechanics, nobody has shown that this is what happens to collagen on the
molecular level. For example, he said, cross-linkings effect could be
related to a shrinking of individual collagen fiber rather than a cross-linking
Theres obviously some validity in higher amount of
cross-linking with epithelium-off, Dr. Kanellopoulos, an OSN Europe
Edition Editorial Board Member, said. The problem is, we are still
limited by the fact that we cannot measure how much we are cross-linking. So we
are all awaiting a measuring method and potentially measuring unit to
quantitate the cross-linking effect of each individual technique.
The modification in the epithelium-on procedure is to pass riboflavin
through about 50 µm of epithelium and into the stroma. Dextran is a large
molecule, and ophthalmologists in Europe have been dissatisfied with the
penetration available, said Parag A. Majmudar, MD, one of the surgeons working
on the CXL-USA study of epithelium-on procedures and an OSN SuperSite Board
Member. So in the U.S., ophthalmologists dissolve the riboflavin in gum
Parag A. Majmudar
Surgeons may also use benzalkonium chloride to loosen up tight junctions
in the epithelial cells that normally prevent the diffusion of molecules across
the epithelium, Dr. Majmudar said. If the cornea is less than 400 µm
thick, they will use sterile water or, more commonly, a hypotonic riboflavin
solution to swell it to that minimum thickness.
The riboflavin saturation takes a little more time to occur, up to 60
minutes instead of 30 minutes with an epithelium-off procedure, Dr. Majmudar
said. He does not use a corneal sponge to instill the riboflavin uniformly,
although other surgeons do utilize this.
The UVA exposure is still 3 mW/cm2 for 30 minutes, per the
investigational protocol, but there is already interest in intensifying the UVA
to 6 mW/cm2 for 15 minutes or even 9 mW/cm2 for 10
Dr. Majmudar is awaiting his own 1-year results but said that so far
epithelium-on procedures have offered at least as good a result as results in
the literature with epithelium-off procedures, with patients generally gaining
a line of BCVA with quicker and less painful results.
I would use a bandage contact lens after the UVA light procedure
if there were a significant irregularity to the epithelium, if it looks
ratty or beat up, or if we feel that theres any
concern of a patient having a significant amount of pain, Dr. Majmudar
said. If the patient is in any discomfort, well put a bandage
contact lens on prophylactically. But thats been less than 5% of the time
that Ive had to do that.
Eric D. Donnenfeld, MD, an OSN Cornea/External Disease Board Member, was
part of the original FDA trial on cross-linking and performed a series of
epithelium-off procedures 4 years ago. Concerned about the long healing process
that occurs after removing the epithelium, he sought a way to expedite it.
As a modification of the epithelium-off procedure, he designed a device
with 80-µm prongs that is pressed onto the cornea without penetrating
into the stroma or Bowmans membrane. (The device is a concept similar to
one developed simultaneously and independently by Sheraz Daya, MD.)
Dr. Donnenfeld then applies 2% riboflavin drops every 2 minutes for 30
minutes. If the patient has a thinner cornea, he uses hypotonic riboflavin and
then after 30 minutes checks pachymetry and fluorescence in the stroma. He
applies UVA light for 30 minutes of 4 mW/cm2. He then applies
steroids, antibiotics and a bandage contact lens.
Most of the research that Ive looked at, the work that has
come out of several labs, has shown that you dont get the good
penetration of riboflavin without removing the epithelium, Dr. Donnenfeld
said. In his youngest patients who have the most aggressive forms of
keratoconus, he still performs the traditional epithelium-off procedure. But
for the other 95% of cross-linking patients, he performs the hybrid procedure
with his device.
He said he will not switch to an epithelium-on procedure.
Until someone shows me a good epithelium-on study where you get
equal amounts of corneal cross-linking, Ill continue to completely remove
the epithelium or partially remove the epithelium, he said.
Dr. Trattler said that one of the main differences in the epithelium-on
technique being used in the CXL-USA study is that the researchers do not stick
to the 30-minute riboflavin loading time used in Europe with both epithelium on
and epithelium off.
William B. Trattler
Rather, our experience has been that 60 to 80 minutes of
riboflavin loading is required to ensure saturation of the corneal stroma to a
similar extent seen with epi-off riboflavin loading, he said. With
the longer loading time, our patients with epi-on have similar results to our
epi-off patients in our study, and the results can be seen looking at the
improvement in the corneal shape as seen on difference maps of the preoperative
and postoperative cornea.
When we look at topography/tomography, it may not be that obvious
from one map to another that theres been a change, Dr. Trattler
said. A difference map shows that the cornea got flatter in some areas
and steeper in other areas. The reshaping of the cornea seen with cross-linking
can result in improvement in UCVA and BCVA.
Haze has been noted in cross-linking. Surgeons have not been able to
pinpoint why it occurs or what its clinical effect might be.
I have seen patients develop haze with epithelium-off
procedures, Dr. Trattler said. Visually significant haze with
epithelial off is fortunately uncommon and responds to treatment with topical
steroids, similar to the way that PRK-related haze responds to topical steroid
Dr. Hersh described the haze that results in cross-linking as something
the physician sees under the slit lamp, not something the patient sees. He has
used Scheimpflug densitometry to quantify the degree of haze and has found that
it is greatest in the first month, plateaus at 3 months and then significantly
decreases over the next 3 months. It then returns to baseline at 1 year in most
In our clinical trial, haze did not correlate with changes in
visual acuity, did not affect visual recovery time and did not affect the
1-year cross-linking topography outcomes, Dr. Hersh said.
A significant proportion of patients that have the epithelium-off
process will have haze in the cornea, and that is likely a function of the
epithelium being removed, not necessarily an indication that cross-linking is
working, Dr. Majmudar said. The biggest misconception that other
surgeons have is that if you dont see haze, cross-linking did not take
effect. But I dont think thats true. Ive seen results in our
epithelium-on cases in which theres been no haze and were seeing
results that are similar to epithelial on.
Dr. Hersh said that although the source of haze is associated in some
way with the healing process after cross-linking, its ultimate effect on
positive or negative differences between epithelium-off and epithelium-on
cross-linking remains a mystery. Is the haze secondary to the loss of
keratocytes or to their repopulation? Is it related to collagen or changes in
lamellar architecture? Is it keratocytic activity or something that involves
the entire wound healing process? Or is it simply secondary to the corneal
thinning seen after the procedure?
We only know the rudiments of what collagen cross-linking is
doing, Dr. Hersh said. This is something that is early in its
development, and I think youre going to see great advances.
New uses for cross-linking
Francis W. Price Jr., MD, an OSN Cornea/External Disease Board Member,
has been using epithelium-off cross-linking under a physician-sponsored
investigational device exemption to treat infectious keratitis, primarily
bacterial but also including fungal and Acanthamoeba infections.
He limits treatments to smaller areas of obvious infection, taking care
to mask clear cornea and the limbus. Epithelium-on treatments do not deliver as
much riboflavin into the cornea and decrease the penetration of the UV light
with the intensity to do what is necessary for cross-linking, so he sticks with
Thats particularly important for fungus and
Acanthamoeba, which are full-thickness infections, he said.
Cross-linking treatments appear to get their effect by creating reactive oxygen
molecules, or singlet oxygen, which is cytotoxic.
Riboflavin is a flat molecule that slides into the RNA and DNA in cells,
disrupting them once the UVA light reaches it. This kills all the keratocytes
in the area of treatment.
The interesting thing is that UVA is used alone to sterilize
fluids, Dr. Price said, comparing it to simple pen-shaped devices that
can be dropped into canteens to sterilize water while camping, as well as
medical uses to sterilize blood products.
Dr. Kanellopoulos sees a future for cross-linking as a method of
prophylaxis for ectasia in LASIK and PRK procedures. By definition, LASIK
weakens the cornea, and by definition, cross-linking strengthens it.
Cross-linking in general was born as a remedy of corneal
ectasia, he said. In my mind, it is an integral part of customizing
corneal refractive procedures.
Dr. Kanellopoulos has also employed cross-linking for temporary relief
in patients with bullous keratopathy, and he has routinely employed
cross-linkings modulation of keratocytes in patients with severe corneal
scarring. Specifically, he is addressing several cases of severe corneal
blindness with partial topography-guided PRK and cross-linking via the Athens
Dr. Kanellopoulos has shown that placing riboflavin in a corneal pocket
created by a femtosecond laser and concentrating cross-linking in that area to
treat early stages of keratoconus appears to be equally effective as the
Dresden protocol. On the horizon, he foresees wider applications of the
techniques he has introduced and has experimented with UVA light fluences of 5
mW/cm2, 6 mW/cm2, 7 mW/cm2 and 12
mW/cm2, which have created quite a buzz in the past 6 months, he
But again, the new modalities of cross-linking are unproven, compared
with the epithelium-off Dresden protocol, the standard of care in that it
is the only protocol that carries all the basic science with it. Dr.
Dr. Hersh said, I most certainly embrace doing epithelium-on
cross-linking within the confines of a controlled clinical trial. It would make
life much easier. It clearly would be easier for the patient, because
youd be able to get the patient into a contact lens more quickly.
Theres less of a potential risk of infection because theres an
epithelial barrier thats been broken.
At the crux of the matter is whether epithelium-on techniques provide as
much efficacy as epithelium-off techniques. Epithelium-on methods allow less
riboflavin into the cornea and absorb more UVA light before it reaches the
stroma. But, Dr. Hersh said, does that mean there is less clinical effect?
I cant tell you how frustrating it was for all these years.
Patients would come in with keratoconus, and I would tell them theres
nothing we can do aside from a good contact lens fitting: When you need
to have a transplant, come see me, Dr. Majmudar said. Even if
you dont do anything else, we can stabilize and improve in the majority
of cases, and later on, if you have to add additional treatments, such as
Intacs (Addition Technology) or topo-guided PRK, you can have additive
treatment on top of the building block of cross-linking.
I have no doubt that the science will get better and better,
Dr. Majmudar said, and some of these preconceptions that people have of
epithelium-on vs. epithelium-off will diminish once people see what kinds of
results are possible. And some of the results that were seeing are just
phenomenal. by Ryan DuBosar
What should be the goal
for a standard of care in cross-linking?
Collagen cross-linking a great advancement for surgeons,
- Boxer Wachler BS, Pinelli R, Ertan A, Chan CC. Safety and efficacy
of transepithelial crosslinking (C3-R/CXL). J Cataract Refract
Surg. 2010;36(1):186-188; author reply 188-189.
- Greenstein SA, Fry KL, Bhatt J, Hersh PS. Natural history of
corneal haze after collagen crosslinking for keratoconus and corneal ectasia:
Scheimpflug and biomicroscopic analysis. J Cataract Refract Surg.
- Greenstein SA, Shah VP, Fry KL, Hersh PS. Corneal thickness changes
after corneal collagen cross-linking for keratoconus and corneal ectasia:
one-year results. J Cataract Refract Surg.
- Hersh PS, Greenstein SA, Fry KL. Corneal collagen cross-linking for
keratoconus and corneal ectasia: One-year results. J Cataract Refract
- Kanellopoulos AJ. Collagen cross-linking in early keratoconus with
riboflavin in a femtosecond laser-created pocket: initial clinical
results. J Refract Surg. 2009;25(11):1034-1037.
- 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.
- Kanellopoulos AJ. Cross-linking plus topography-guided PRK for
post-LASIK ectasia management. In: Garg A, Alió JL, Lin JT, et al, eds.
Mastering the Advanced Surface Ablation Techniques. New Delhi,
India: Jaypee Brothers Medical Publishers; 2008:204-214.
- Kanellopoulos AJ. Cross-linking plus topography guided PRK for
post-LASIK ectasia management. In: Garg A, Pinelli R, Kanellopoulos AJ,
OBrart D, Lovisolo CF, eds. Mastering Corneal Collagen Cross
Linking Techniques (C3-R/CCL/CxL). New Delhi, India: Jaypee Brothers
Medical Publishers; 2009:69-80.
- Kanellopoulos AJ. Cross-linking plus topography-guided PRK for
post-LASIK ectasia management. In: Garg A, Rosen E, Goes FJ, et al, eds.
Instant Clinical Diagnosis in Ophthalmology: Refractive Surgery.
New Delhi, India: Jaypee Brothers Medical Publishers; 2009:258-269.
- Kanellopoulos AJ. IntraLase-assisted LASIK, Video V; Comparison of
topography guided to standard LASIK for hyperopia, Video VI; Limited topoguided
PRK followed by collagen cross linking for keratoconus, Video VII. In: Garg A,
Fine IH, Pallikaris IG, et al, eds. Video Atlas of Ophthalmic
Surgery. New Delhi, India: Jaypee Brothers Medical Publishers; 2008.
- Kanellopoulos AJ. Post-LASIK ectasia. Ophthalmology.
- Kanellopoulos AJ. PRK and C3-R. In: Boxer Wachler BS, ed.
Modern Management of Keratoconus. New Delhi, India: Jaypee
Brothers Medical Publishers; 2008:219-228.
- Kanellopoulos AJ, Binder PS. Collagen cross-linking (CCL) with
sequential topography-guided PRK: A temporizing alternative for keratoconus to
penetrating keratoplasty. Cornea. 2007;26(7):891-895.
- Kanellopoulos AJ, Binder PS. Management of corneal ectasia after
LASIK with combined, same-day, topography-guided partial transepithelial PRK
and collagen cross-linking: The Athens protocol. J Refract Surg.
- Krueger RR, Kanellopoulos AJ. Stability of simultaneous
topography-guided photorefractive keratectomy and riboflavin/UVA cross-linking
for progressive keratoconus: case reports. J Refract Surg.
- Krueger RR, Ramos-Esteban JC, Kanellopoulos AJ. Staged intrastromal
delivery of riboflavin with UVA cross-linking in advanced bullous keratopathy:
laboratory investigation and first clinical case. J Refract Surg.
- Raiskup-Wolf F, Hoyer A, Spoerl E, Pillunat LE. Collagen
crosslinking with riboflavin and ultraviolet-A light in keratoconus: long-term
results. J Cataract Refract Surg. 2008;34(5):796-801.
- Vinciguerra P, Albè E, Trazza S, Seiler T, Epstein D.
Intraoperative and postoperative effects of corneal collagen crosslinking on
progressive keratoconus. Arch Ophthalmol. 2009;127(10):1258-1265.
- Wollensak G, Spoerl E, Seiler T. Riboflavin/ultraviolet-A-induced
collagen crosslinking for the treatment of keratoconus. Am J
- Eric D. Donnenfeld, MD, can be reached at OCLI, 2000 North Village
Ave., Rockville Centre, NY 11570; 516-766-2519; email:
- Peter S. Hersh, MD, FACS, can be reached at The Cornea and Laser
Eye Institute, 300 Frank W. Burr Blvd., Suite 71, Teaneck, NJ 07666;
201-883-0505; email: firstname.lastname@example.org.
- A. John Kanellopoulos, MD, can be reached at 115 East 61st St., New
York, NY 10065; 917-770-0586; email: email@example.com.
- Parag A. Majmudar, MD, can be reached at Chicago Cornea
Consultants, 1585 N. Barrington Road, Doctors Building 2, Suite 502, Hoffman
Estates, IL 60169; 847-882-5909; email
- Francis W. Price Jr., MD, can be reached at Price Vision Group,
9002 N Meridian St., Suite 100, Indianapolis, IN 46260; 317-814-2990; email:
- William B. Trattler, MD, can be reached at the Center for
Excellence in Eye Care, 8940 N. Kendall Drive, Suite 400E, Miami, FL 33176;
305-598-2020; email: firstname.lastname@example.org.
- Disclosures: Drs. Donnenfeld, Majmudar and Price have no relevant
financial disclosures. Dr. Hersh is medical monitor to Avedro Inc. Dr.
Kanellopoulos is on the speakers bureau for Priavision and Seros Medical. Dr.
Trattler is a consultant to CXL-USA.