Technological innovations in corneal collagen cross-linking
Corneal collagen cross-linking represents a new therapeutic option for delaying or halting keratoectasia in progressive keratoconus and post-LASIK ectasia. The basic technique was developed in Dresden in 1998 by Wollensak, Spoerl and Seiler, however, it wasn't until 2003 that the first clinical trial data in 22 patients was published by the same authors.1
We introduced corneal collagen cross-linking in Italy for the first time in 2004 at Siena University in the form of the Siena Eye Cross Project, which was awarded "Best Ophthalmological Research of the Year in Italy" by the Italian Society of Ophthalmology. We, along with some other colleagues published the preliminary report of the first 10 cases treated in Italy in 2006,2 and the final report of 44 patients was presented in June 2007 at the local Ethical Committee of Siena University.
The Italian Eye Cross Study, which was conducted by Caporossi and co-workers, represents the first international in vivo analysis of the human cornea by scanning laser in vivo confocal microscopy, performed by Mazzotta,3,4 and strongly supports the safety and efficacy of the Riboflavin UV-A collagen cross-linking technique. The first international confocal results in humans were published in the European Journal of Ophthalmology in 20063 and, more recently, in the journal Cornea in May 2007.4
The basics of collagen cross-linking
The technique of Riboflavin UV-A collagen cross-linking1,2 involves the photo-polymerization of corneal collagen by increasing chemical inter- and intra-helical bond formation. This mechanism of molecular cross-linking allows the cornea to build strength and a resistance to ectasia. The mechanism of hardening and thickening the cornea is mediated by a photodynamic reaction between the photosensitizer Riboflavin 0.1 % / Dextrane 20% solution (Ricrolin; Sooft, Italy) and low-dose UV-A irradiation (3 mW/cm2 ) with a total exposure time of 30 minutes. The release of reactive oxygen species (ROS) during the reaction stimulates covalent bond formation between collagen fibres.
At our clinic, the UV-A irradiation is delivered by a solid state UV-A illuminator named C.B.M. (Caporossi-Baiocchi-Mazzotta) X-Linker VEGA, which is CE-marked and was developed by us in collaboration with the Italian firm C.S.O. (Costruzione Strumenti Oftalmici, Badia a Settimo, Florence, Italy).3,5 The clinical experience that we have now achieved at Siena University has enabled us to develop a UV-A source that optimizes the surgical technique and postsurgical outcomes of corneal cross-linking.2,3,4,5
What is different about our technology?
The first Italian prototype was developed in 2004 at Siena University by Caporossi and Mazzotta2,6 in collaboration with the National Research Council of Florence. It consisted of a dual LED UV-A illuminator similar to the first instrument used in Dresden Technical University for the cross-linking pilot study.1 The only difference between the two illuminators was in the energy stabilizing system (CM controller). The new prototype was designed to obtain a timely, homogeneous irradiation, thus avoiding the emission peak and energy decrease related to battery pack systems.2 The first spot diameter was 6 mm and the UV-A source was focused at a distance of 1 cm from the corneal apex.
In 2005 we, in collaboration with C.S.O., went on to create the "CBM X linker" (Caporossi-Baiocchi-Mazzotta cross-linker).3,5 Learning from our experience with the previous system,2 the aim of this new technology was to obtain the most circular illumination spot and the biggest and most constant in its diameter. In order to do this, we increased the cross-linkable area to 9 mm, by using an illuminator made of a five UV-A LED array (370-10), mounted on a heat sink system in the source head of the equipment and powered by a stabilized circuit to provide an energy equal to a safe and efficient value (5.4 J/cm2 , thus 3.0 mW/cm2 as power density).
A second goal of the project was to increase the working distance so that we could operate more efficiently on the cornea; the focusing distance was increased from 1 cm to 1.5-1.8 cm. A further goal was to achieve a sharper focus and improved adjustment control of the UV-A spot. This particular feature was obtained by inserting two low-power red laser sources into the optical head, thus not interfering with the "therapeutic" wavelength, as aiming-beam function.5
This particular visual-control feature plays a very important role in maintaining constant UV-A irradiation. This is particularly important because a ±1 mm defocus can diminish the energy provided to the corneal tissue by 8-10%. A 0.2 mm decentration can cause the same decrease in energy, whilst a 0.5 mm increase in decentration is associated with a 20% decrease in the energy that reaches the corneal apex. All of this does not take into account the influence of impact angle (tilting) on energy delivery. This CBM X-linker was the first of its kind in the world to receive CE mark approval in 2006.
The innovating continued...
CBM-VEGA has a new auto-balanced arm for more comfortable surgical control and an additional iris-pinhole, which can change the irradiated area from a 4.5 to 11 mm diameter, with a constant 3 mW/cm2 power density. Just as with the original CBM, the CBM-VEGA also has a stabilized power supply and uses two low-power red laser sources (blinking aiming beam) and the colour micro-camera for the real-time irradiated area display. The footswitch command provides sterility and independence to the surgeon, by reducing the need for additional members of staff.
The monitor displays important information for the surgeon during the procedure, such as the current stage of the operation, device calibration, staining phase with related timer from 10 to 30 minutes, and elapsed treatment time for each five-minute step (steps one to six). Finally, a new feature is the camera-coaxial fixation point, which makes ocular alignment fixation easier for the patient.
The results so far
Since 2004 we have treated 44 eyes (26 OD, 18 OS; 24 male, 20 female), each affected by growing keratoconus, clinically and instrumentally reported. The mean age of patients was 23.2 years of age (between 14 and 42 years). The level of keratoconus clinical and instrumental progression in each eye, as defined by Krumech and Sitrac 04 classification, was as follows: 10 stage I, 33 stage II, one stage III. Four of our patients were treated in both eyes; the second eye was operated on 12 months after the first, because of keratoconus worsening.
All patients in this Eye Cross Study were treated in accordance with the Siena Eye Cross protocol (Dresden modified) with cross-linking Riboflavin UV-A.2,3,4
Patients were followed up at 15 days, one, three, six, 12, 18 and 24 months postoperatively. The average preoperative pachymetry value was 441.5 μm (range 406 μm to 488 μm), whilst intraocular pressure was recorded at 13.9 mmHg (range 11 mmHg to 18 mmHg) and average endothelial cell density was 2,216 cells/mm2.
Up until the time of writing this report, the Eye Cross Study2 had yet to yield any complications relating to therapeutic riboflavin UV-A cross-linking application.
Temporary haze was observed in a few of our cases (15%; four within the first three months, one case after six months), however, this disappeared after one month of therapy with preservative-free topical steroids. The most common side effect reported during the first postoperative month was temporary corneal oedema (15 days/three months), which disappeared progressively after steroid and topical NSAID administration. No delayed re-epithelialization or endothelial damage was apparent during the 24-month follow-up period and there were no incidences of keratoconus worsening in any of the treated eyes during follow-up. An average decrease in K-reading of around 1.5 D was, however, reported in the fellow eye of patients (the control group). Overall, however, no adverse events were registered
Setting guidelines for the future
Keratoconus is the single largest contributor to the number of corneal transplants required in Italy and across Europe today.
Treatment of the disease is based on its clinical staging, and riboflavin UV-A collagen cross-linking is the only method currently available to treat corneal ectasia in an etio-pathogenetic manner.
Based on our experience, we have been able to develop some real staging-based guidelines for the therapeutic treatment of keratoconus and, in our opinion, corneal collagen cross-linking is mainly effective in the first and second stage of the disease. We believe that an early keratoconus diagnosis is necessary in order to optimize cross-linking efficacy, delay keratoconus progression and reduce the need for corneal transplants. We truly believe that this new equipment represents one of the most advanced technologies available for the cross-linking technique and, as we assimilate further patient data, no doubt more widespread use will follow.
Our thanks go to Luciano Sassano (C.S.O., Italy) and Riccardo Nicoletti (C.S.O., Italy) for all of their technical support.
1. G. Wollensak, E. Spoerl, T. Seiler. Am. J. Ophthalmol., 2003;135:620-627.
2. A. Caporossi, et al. J. Cataract Refract. Surg. 2006;32:837-845.
3. C. Mazzotta, et al. Eur. J. Ophthalmol. 2006;16(4):530-535.
4. C. Mazzotta, et al. Cornea 2007;26(4):390-397.
5. R. Mencucci, C. Mazzotta et al. J. Cataract Refract. Surg. 2007;33(6):1005-8.
6. A. Caporossi, S. Baiocchi, C. Mazzotta. La Voce A.I.C.C.E.R., 2006;4.