Lasers have become an indispensible part of ophthalmology—but there's always room for improvement. For ophthalmic surgeons, wavelength is key; it determines penetration depth and absorption patterns. Until recently some wavelengths could only be generated in a basic way, using costly, difficult-to-maintain instruments. Now a new laser in development by Iridex Corp. (
Greg Halstead, global marketing manager at Iridex, explains how Iridex came to develop the new laser. "The first lasers were argon and krypton gas lasers," he says. "Argon lasers were a deep bluish-green, with a wavelength around 514 nm; they worked fine for coagulation. Eventually, tunable dye lasers came out, making it possible to select a wavelength, including yellows, oranges and reds.
"Yellow was a common selection at 577 nm, the pure yellow band, for a couple of reasons," he continues. "When you're targeting with the laser inside the eye, a chromophore such as melanin, xanthophyll or hemoglobin absorbs the laser light, producing heat and triggering a reaction, and different wavelengths are more or less efficient at this. As it turns out, pure yellow is very efficient. You can use less energy to achieve the same results, and there's less scatter in the eye." (A previously published study by Martin A. Mainster, PhD, MD, listed the advantages of 577-nm light as including peak absorption for oxyhemoglobin; negligible absorption by macular xanthophyll; low light scattering in intraocular transit; negligible retinal phototoxicity; good lesion visibility; limited patient pain; and a high ratio of oxyhemoglobin to melanin absorption for treatment of vascular structures with a minimum of damage to adjacent pigmented tissues.1)
"Unfortunately," Mr. Halstead continues, "maintaining those tunable lasers could be expensive and difficult, and some of the dyes in them were carcinogenic. But companies eventually created solid-state lasers in the infrared and the green ranges; we now have 532-nm and 810-nm solid-state lasers. These were quickly adopted because of their lower cost and reduced need for maintenance."
As it turned out, however, the existing solid state technology couldn't produce the 577-nm wavelength. "Several lasers on the market today can produce a 561-nm beam that's a greenish-yellow," he notes. "But once you get to 560 nm, you're off the peak absorption for hemoglobin, and you're a little less efficient than the 532s."
Getting to Yellow
Mr. Halstead explains that Iridex recently found a different technology that makes it possible to produce a solid-state 577 nm laser—with other features that make it especially useful. "We wanted these lasers to have two key attributes," he says. "The first was a true-yellow, 577-nm laser light. Second, we wanted the laser to have more power and micropulsing capabilities. Micropulsing gives the surgeon much finer control over laser delivery." The new laser, now being tested and refined, has all of these characteristics.
Mr. Halstead notes that micropulsing opens the door to new ways of treating with the laser. "Without micropulsing, when the light is absorbed by the tissue, it creates visible scarring that tells you the treatment was effective—a little white blanching. But with micropulsing it's possible to stay below the level of visible scarring and still deliver a clinically significant dose of laser energy that evokes a healing response, much like selective laser trabeculoplasty. You don't see a visible response, but you do get a biological effect." (A study presented at the Association for Research and Vision in Ophthalmology meeting in 2005 [Ingvoldstad et al, IOVS 2005;46:ARVO E-Abstract 123] compared micropulse laser trabeculoplasty to argon laser trabeculoplasty; they produced equally effective, statistically significant lowering of intraocular pressure.)
Mr. Halstead says the new technology also allows the laser to produce a more powerful beam. "The solid state lasers currently available that produce the off-peak 560 nm-range light can produce about 600 to 800 mW of power," he says. "The yellow laser we demonstrated at the
Mr. Halstead says the laser is still in development, and the company is filing for 510K clearance. "This is our top project at Iridex right now," he notes. "It has the potential to be the all-purpose laser; with its micropulsing capability you can use it in your office for general ophthalmic applications such as photocoagulation and use it in the OR for retina work as well. We hope to have full clearance by early summer.
1. Mainster MA. Wavelength selection in macular photocoagulation. Tissue optics, thermal effects, and laser systems. Ophthalmology 1986;93:952-8.