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THE Light Adjustable Lens (LAL, Calhoun Vision), which started human trials recently, already offers the prospect of post-operative adjustment of refractive power to get better outcomes for lower order aberrations. A new light delivery system now in animal studies may also allow the correction of higher-order aberrations. Daniel Schwartz, MD of the University of California , San Francisco presented the latest information on the lens at the
XXI Congress of the ESCRS The light adjustable lens is a posterior chamber IOL designed for use in cataract patients. Its refractive power is adjusted postoperatively due to the special material of the optic. The silicone matrix of the lens is composed of free-moving photosensitive monomers which can be fixed in place through polymerisation by shining ultra-violet light in the near visual range (365 nm). When some of the monomers are polymerised, the remaining monomers redistribute through the lens, changing its shape and refractive power. By precisely targeting light on the lens the power can be adjusted over a range of four dioptres. "Standard deviation of adjustment is approximately 0.1D. In our recent animal studies 100% of outcomes fall within 0.25 D of the intended refractive target," said Dr Schwartz, who is also the Chairman of Calhoun Vision. With the development of a new Digital Light Delivery Device (DLDD), developed jointly with Zeiss, the light adjustable lens could be now used to correct higher order aberrations as well, according to Dr Schwartz. "An initial test using the DLDD was pretty exciting, because it showed that we can correct astigmatism and higher-order aberrations in vivo with the lens," he told Eurotimes. For the test, Nick Mamalis MD at Moran Eye Center in the University of Utah carried out cataract surgery on four rabbits. Two days after surgery, the rabbits were placed in a special holder and the lens was treated with ultraviolet light from the DLDD. Two eyes were adjusted for astigmatism and two for higher order aberrations using a tetrafoil pattern. One day later the rabbits were sacrificed and the lenses explanted for optical analysis at Calhoun Vision. "The optical analysis showed a nice, smooth, wavefront corresponding to the astigmatic change and tetrafoil pattern on the lenses," said Dr Schwartz. Used in conjunction with the DLDD, the lens offers the prospect of using conjugate images of diagnostic wavefront analysis patterns to correct higher-order aberrations like coma and spherical aberration, he noted. "It enables you to project any pattern onto the light adjustable lens. For example, we can use this device to project aspheric patterns, a multifocal or astigmatic pattern, and other higher order aberrations onto the IOL. Using the
Digital Light Delivery Device, the surgeon can customise the digital pattern and that pattern is faithfully captured by the IOL. It's a veryexciting development for us. Spherical aberration is the major aberration of the corneal surface and currently the potential for IOL correction rests with lenses like the Tecnis silicone lens (Pfizer Ophthalmics) which comes with a negative spherical aberration to correct the positive spherical aberration in the cornea. Eventually surgeons could perhaps pick and choose among a range of IOLs suited to the most common wavefront profiles. In contrast, the light adjustable lens could potentially correct higher-order aberrations postoperatively, in vivo. The lens can also be adjusted repeatedly, until the desired outcome is achieved. Once the desired refractive profile is achieved the lens is fixed permanently by exposing all remaining liquid monomers to UV light. The light adjustable lens has undergone pre-clinical trials in animals and a human trial has begun in Mexico . That trial stalled, however, when it was tests revealed that post-operative adjustments were not achieving the anticipated results. "It taught us the importance of keeping the cornea moist during the entire adjustment," said Dr. Schwartz. By moistening the cornea using a contact lens and methylcellulose interface, reproducible adjustments are achieved in animal trials. Human trials will re-start this fall. The company hopes to launch the lens in Europe by the end of 2004, and to be well into clinical trials in the US by then.
Further advances in the light adjustable lens are also underway. Calhoun Vision is developing an injectable, accommodative version of the lens."Ideally, to restore accommodation one wants to inject a lens material that has sufficient flexibility to allow people to see at distance or near, as the ciliary body contracts. Currently, we can make a lens material flexible enough but in terms of injecting the lens material, no one knows exactly how much to inject in the capsular bag. If you inject too much or too little the power will be incorrect. With our light-adjustable material we can inject an approximate amount and then adjust the shape of the lens after the eye has closed and healed." The company has also done preliminary work developing an acrylic light adjustable lens. Many ophthalmologists prefer acrylic lenses, so the company wants to develop one, he commented. Dr Schwartz believes Calhoun's design provides superior function to other planned adjustable IOLs
The Eagle Adjustable Lens works by using internal rotation to adjust forward and back focusing to correct refractive errors. "I have seen this design and it has the disadvantage of requiring an invasive procedure to move the parts of the lens. The light adjustable lens is adjusted with light so it's not invasive." Dr Schwartz remained discreet about plans to develop a lens that could be adjusted for the lifetime of the patient. He said that the company is developing a means to fabricate a non-locking formulation that can be adjusted multiple times without the need for a lock-in step. Even without a lifetime-adjustability, however, the light adjustable lens will have a huge impact on cataract surgery and even refractive lens exchange if it passes the human trials, he said, adding:"I think it will be like "Intel Inside". It will be a platform that will be a part of any IOL technology because even with our best measurements we can't get the power right, every time, in the eye."
Prof. Daniel Schwartz
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