LASERS IN SURGERY

Though first perceived as potentially blinding anti-personnel weapons, lasers have proved to be valuable instruments that can preserve, enhance and restore vision, said John Marshall PhD, UK, as he looked back on nearly 50 years of involvement with lasers in ophthalmology in his UKISCRS Lifetime Achievement Award lecture, which he delivered at the 2014 UKISCRS Congress in London.

He noted that his involvement with lasers in ophthalmology goes back to the early days of lasers themselves. Albert Einstein first postulated the possibility of lasers back in 1917. In 1953 Charles Townes and his team exploited Einstein’s theoretical work to create the first maser, in 1960 Theodore Maiman invented the first working laser, and the first eye accident was reported in 1963.

“It was all new, so I was really lucky to be headhunted by the Royal Air Force in 1965 and given a grant to do a PhD. This PhD was divided between the Institute of Aviation Medicine and the Institute of Ophthalmology. The requirement was to investigate laser damage to the retina in order to try to protect aircrew against weapons which at that time did not exist,” said Prof Marshall.

His research elucidated the different mechanisms by which lasers can damage tissue. It showed a time dependency in determining the mechanisms of damage. If a laser caused detectable tissue damage using pulse durations of a nanosecond or less then the damage results from electron-stripping and ionisation.

However, if it took a nanosecond to a millisecond the damage results from explosive heating and shockwaves. By contrast, when pulse durations range from a millisecond to a second the damage was a thermal effect, and finally, when the pulse duration was longer than a second the damage results from photochemical effects with short wavelengths being the most hazardous.

Prof Marshall’s research led to many innovations in the use of lasers for retinal pathologies and also the use of the lasers for corneal refractive surgery. His work led to the initial patents in refractive surgery and the development of photorefractive keratectomy (PRK). This resulted in the establishment of the first company to manufacture lasers for commercial use in refractive surgery, and the first to receive FDA approval in 1995.

He noted that PRK’s predictability was poor in the early days. However, refinements in laser technology, particularly increasing spot sizes, greatly reduced the variability in outcomes. The problems that remained included postoperative pain and a haze. The solutions for these problems came in the form of the use of pharmacological inhibitors of keratinocyte activity and the use of topical anaesthesia postoperatively.

Meanwhile, LASIK was under development theoretically as a means of increasing the amount of refractive correction compared with surface ablations. Although it did not achieve that primary aim, it did reduce pain and eliminated haze, and as a result has become the predominant form of corneal refractive surgery performed in some countries today.

More recently, Prof Marshall and his associates have developed a new approach, designated LASIK Xtra, which involves locking in the post-ablation stromal shape with a two-minute collagen crosslinking (CXL) procedure. The new technique has been adopted at many centres around the world and the results so far suggest that CXL improves the predictability of the LASIK procedure and extends the amount of refractive error it can correct, up to -19 dioptres.

Finally, he showed a new technique of refractive surgery (PiXL) whereby corrections are achieved by topographically applied ultraviolet radiation such that the resultant tensional changes in collagen fibres address refractive errors in myopia, hyperopia and astigmatism.

John Marshall:
eye.marshall@googlemail.com