Watch the lasers

Laser use in ophthalmology has seen many great advances over the years

Aidan Hanratty

Posted: Friday, March 1, 2019

Paul Rosen and John Marshall in the Eye Contact studio at the 36th Congress of the ESCRS in Vienna, Austria

John Marshall PhD has seen many advances in laser surgery over the years. While he has been wary of some changes, he can admit when things don’t turn out as he expected. Many years ago, he warned about the potential risk posed by LASIK surgery to the biomechanics of the cornea.

“Any surgical intervention in the cornea does change the biomechanics, and these weaken the system,” he said in a EuroTimes Eye Contact interview with Paul Rosen MD, PhD. “Do they weaken it sufficiently to cause problems? I don’t think so. Are we going to see continuing problems in later years? Again, I think the evidence now after 20 years of surface surgery is suggesting that we aren’t going to see significant problems generating.”

In relation to LASIK itself, Dr Marshall, Frost Professor of Ophthalmology at the Institute of Ophthalmology in association with Moorfield’s Eye Hospital, UCL, London, points out an inbuilt redundancy in the structure of the eye.

“If you cut through the fibres all the way around the cornea, as you do in a LASIK flap, you’re cutting around three-and-a-half million collagen fibrils. But I think we’ve been very lucky, there is a huge redundancy in the way in which the cornea is designed.”

He finds small-incision lenticule extraction (SMILE®) quite interesting because it is based on a theoretical concept: “That is, if you cut out a lenticule but preserve continuity of the fibres in the strong part of the cornea, the superficial part of the cornea, it should actually be biomechanically less weakening than a LASIK incision. And although we don’t have a lot of detailed biomechanical evidence, the laboratory evidence would suggest that SMILE is going to be less biomechanically invasive.”

When he was first introduced to the idea of corneal cross-linking (CXL), Dr Marshall delivered a paper entitled “How would you like to age your cornea 600 years in 600 seconds?”

“I thought to prematurely age a system wasn’t really a good idea. However, I was wrong, and I think the whole of cross-linking is really interesting, especially in pathological corneas, like keratoconus,” The procedure can only get better, he believes.

“At the moment, we’re probably only getting a dioptre or so of correction, but that’s not the end of the story. I think there are now beginning to be changes in technology which should give us more, so for very low order corrections, I think cross-linking does have a future in refractive surgery.”

Some patients are left dissatisfied following surgery because their brain has become used to the inadequacies of their optical system. The benefit of cross-linking, according to Dr Marshall, lies in its ability to alter specific aspects of the aberrated cornea.

While laser surgery will weaken the cornea and cross-linking strengthens it, combining the two procedures works on a complementary basis. Asked about the order in which the procedures should be performed – laser then cross-linking, vice versa or combined, as in LASIK Xtra – he sees only one potential approach.

“If you cross-link a cornea, you slightly change the ablation rate, so you would have to modify the algorithms. I think the easiest way to do this and the quickest way to do this is to do laser and then cross-link before you put the flap back.”

Dr Marshall believes in the potential of using femtosecond laser to modify intraocular lenses. “Using two-photon phenomena, you can actually change the refractive index. And certainly, in materials, that is viable.”

He doesn’t see the value of attempting to change the refractive index in biological tissue, however. “Who knows what wound healing is going to do? Those changes are so small and so subtle, that I’m pretty sure that wound healing over a period will wipe them out.”

With lasers a permanent feature of the ophthalmological landscape, Prof Marshall sees the future going in a different direction. “Personalised medicine and genetics are going to play an incredibly important role. The more we understand of the gene control mechanisms, the more we will have the potential to manipulate them.” He cites technology such as the CRISPR genome-editing system as having a big future.

“As a scientist, I love the idea of changing the genes to making the system as it should be.”

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