NEW VARIATION ON CROSSLINKING

Ophthalmic surgeons should consider the benefits of significantly strengthening the cornea and treating keratoconus with a new variation on crosslinking, delegates attending the United Kingdom & Ireland Society of Cataract & Refractive Surgeons (UKISCRS) XXXV Annual Congress were told. Discussing the new Avedro microwave system and crosslinking, John Marshall PhD, FRCPath, FMedSci, FRCOphth(Hon), Frost Prof of Ophthalmology, and Institute of Ophthalmology in association with Moorfields eye Hospital London, voiced his concern about the negative effect of all forms of refractive surgery on the strength of the eye.

The non-invasive, incisionless, Keraflex® procedure, performed with the Vedera® machine, thermally remodels the cornea without the removal of any tissue so the procedure induces refractive change without weakening the cornea's biomechanical integrity, as happens with LASIK and other refractive correction procedures. Explaining the background to the new system, Prof Marshall said he had been concerned about the loss of tissue following PRK and especially LASIK. 'And that worried me from an engineering standpoint. What worried me more was that the wound healing never actually brings those tissues back together with the previous mechanical integrity. That is the bed is stuck to the flap with fibronectin and tenascin (an extracellular matrix glycoprotein) as if it were biological glue. 'So over the years I've looked at the maximum dioptre rate of change per year as a function of refractive surgery, and all refractive surgery results with progress of time in a greater magnitude of dioptre change,' he commented.

Working with a number of other experts, Prof Marshall looked at a means of measuring stability and confirmed that cutting down through corneal fibres is what weakens the system, while delamination doesn't interfere with overall strength. 'So wouldn't it be wonderful if you could devise a system that would change the orientation of collagen without actually cutting, and the technique you use is a thermal one using microwaves,' he said. The resulting new Avedro Vedera system accommodates flattening or steepening of the cornea, and if coupled with crosslinking the change becomes more permanent; increasing rigidity by three to four times, in a very time-efficient fashion, Prof Marshall explained. However, all thermal processes seem to have a transitory effect and therefore a second component was needed in order to ensure longevity of the induced change. Focussing on the second step of the procedure, Prof Marshall discussed the impact of beam profile and riboflavin shielding using the Avedro KXL treatment, an accelerated crosslinking system. Prof Marshall explained that photochemical reactions within similar time domains exhibit something called reciprocity which means that the relationship between power and time to deliver a given energy to tissue is constant. Classical systems use 3 mW power sources for 30 minutes, precisely the same energy will be delivered using a 30 mW system for three minutes.

For example, treating keratoconus a 10 minute presoak; three minutes at 30 mW/ cm2, Prof Marshall said the top-hat beam of the KXL provides uniform, predictable distribution of UV energy. He also said that by using a shorter soak time very little riboflavin reaches the endothelium this removes a potential harmful complication typical of other treatment methods. He explained that, the concept of riboflavin protecting tissues is incorrect as absorption by old revile it into riboflavin is the fundamental process by which crosslinking takes place. Using the Avedro system ensures that the energy is delivered in a short time at high power with uniform energy distribution across the treatment area.