The Mechanism of Action
The LAL appears similar to a conventional third-generation silicone IOL, with two haptics connected to a 6-mm optic. The difference is that, at the molecular level, the lens is only partially polymerized. It's actually a silicone matrix full of unpolymerized silicone macromers that are tied to photosensitive molecules.
Using a specific wavelength of light emitted from a slit-lamp sized device from Carl Zeiss Meditec, a surgeon can titrate the lens power after the lens is already in the eye by polymerizing the unpolymerized macromers. The patient places his head on a chin rest and looks at the fixation light in what resembles a YAG laser. The user then adjusts the power by as little as 0.25 D. This adjustment usually is made one to four weeks postop.
Before the light reaches the LAL, it passes through a filter that shapes it to the precise spatial intensity distribution needed to create a specific power change. This distribution could involve placing more light in the periphery, the center or any place necessary to effect the change the surgeon needs.
The light source used with the LAL adds thickness to the lens wherever it strikes it by causing the silicone to build up in those particular regions. For example, if the LAL patient has a hyperopic refractive error after his cataract surgery, the IOL doesn't have enough power. The surgeon shines a light pattern with more intensity centrally, thickening the lens there and increasing its power to correct the hyperopia. Similarly, to correct a myopic error postoperatively, the light pattern is more intense in the periphery of the lens. Astigmatism is corrected by shining the light in a band that crosses the lens center on the astigmatic axis. The cylinder correction can be performed in plus cylinder (irradiating a band through the center) or minus cylinder (irradiating down the lens edge without touching the center), depending on the patient's refractive error.
Multiple power adjustments to the LAL are possible until the vision is at an acceptable level. For instance, if the surgeon inadvertently added too much thickness to the center of the lens, inducing myopia, he could irradiate the periphery to counteract it. When the patient's vision is optimal, the lens power is permanently locked in by irradiating the entire lens at once, usually a day or two after the adjustment is made. Once this is done, the power can no longer change. It's important that the patient wear UV-protective eyewear between the adjustment and lock-in steps, because UV light can induce a power change during this time.
Results and New Developments
Calhoun Vision recently added a UV-backing layer to the posterior lens surface in an effort to block UV rays from reaching the retina.
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"We were looking for potential issues from the administration of UV light, and some patients were reporting erythropsia," explains the LAL's current investigator, Arturo Chayet, MD, of Tijuana, Mexico. "So, just to be safe, [Calhoun Vision's founder] Dr. Dan Schwartz wanted to include this backing layer to absorb more of the light and prevent more of it from going behind the lens. However, erythropsia wasn't an issue after a week with the previous lens, and it's a fairly common occurrence—we see it in most cataract surgeries."
The addition of the UV-blocking layer has called for a change in the patterns of light used to adjust the refraction.
"The new layer causes a differential absorption of photons," explains Dr. Schwartz. "When you change the number of photons being absorbed by the UV-blocker as opposed to the photosensitive silicone, it changes the patterns and intensities of light you need to use to adjust the lens power."
So far, Dr. Chayet has implanted 15 of the lenses with the UV-blocking layer, and they've turned out to be within about 0.5 D of the intended refraction, so some refinement is still necessary to get this accuracy honed even further.
At this point, the original iteration of the LAL has been implanted in 16 patients who were deliberately left between 1.5 to 2 D over- or undercorrected before adjustment and lock-in. At one year after having their refractions locked in, all patients are within 0.25 D of emmetropia. None of these patients has lost any lines of best-corrected vision; in fact, their uncorrected acuities are equal to their preop best-corrected acuities.
The next substantial step for the lens will be to do more toric corrections, rather than just spherical ones.
"We've done three toric corrections so far, and they've been very accurate in terms of the axis, they're not even one degree off," says Dr. Chayet. "The toric corrections are undercorrected by about 30 percent at this point, so it's a matter of adjusting the nomogram. However, we're not inducing any astigmatism, or shifting the axis."
To perfect the toric adjustment, Dr. Chayet says they'll have to deliver the energy differently, possibly irradiating a little more in the periphery and allowing for a longer exposure of the lens to the light.
"Another exciting potential use for the lens is as a piggyback implant," says Dr. Chayet. "For example, say a patient undergoes cataract surgery but there's a lot of astigmatism postop. We're going to see if we can use this lens in the sulcus, and then just adjust it to correct the astigmatism. Instead of doing LASIK on the patient, you just put in the LAL and adjust it. If that approach is viable, then this technology could be used in pretty much everyone." He also says it could be a great help in cases of cataract surgery on post-refractive patients, in whom it's always been a challenge to get an accurate lens power.
Dr. Schwartz says the lens could be a boon to the cataract surgeon who's hesitant to get involved with accommodative IOLs or multifocal lenses because he doesn't have the equipment or the inclination to perform postop LASIK on these patients to get their refraction to an acceptable level.
"Since this technology is adjustable, cataract surgeons won't have to buy excimers or microkeratomes or forge relationships with refractive surgeons, but instead can perform the postop adjustment themselves in their practice."
