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The unique properties of a light-adjustable lens are designed to provide predictable results and customized refractive treatments.
Take-home message: The unique properties of a light-adjustable lens are designed to provide predictable results and customized refractive treatments.
By Pablo Artal, PhD, Special to Ophthalmology Times
Murcia, Spain-Despite technology advances achieved in the past decade, almost 40% of cataract surgery patients are dissatisfied with their visual outcomes, primarily due to missed refractive targets.1,2
Among the reasons refractive targets are missed is that surgeons cannot predict the effective lens position of the IOL after healing from cataract surgery and a lack of accuracy of the IOL power calculations procedures. As a consequence, it has been reported that only 55% of patients reach the target goal of emmetropia.3
The situation may be even worse in the future with a larger proportion of post-refractive surgery cataract patients.
A light-adjustable lens4 (Light-Adjustable Lens [LAL], Calhoun Vision) virtually eliminates those refractive target concerns. While not yet approved in the United States, the lens is commercially available in the Czech Republic, France, Germany, Italy, Mexico, Spain, and the United Kingdom.
The light-adjustable lens is similar to a monofocal IOLs in terms of implantation. Cataract surgeons do not have a learning curve with this lens-it is a foldable, three-piece, silicone, posterior chamber lens with an overall diameter of 13 mm (optic is 6 mm in diameter). Its posterior optic edges are squared, its anterior edges are rounded, and it boasts modified-C haptics to prevent posterior capsule opacification and rotation.
The lens itself is made of silicone material containing macromers that are photosensitive to UV light of a certain wavelength. These macromers are able to free-float within the lens material until photopolymerized. Light adjustability of the lens is based on the principles of photochemistry and diffusion, whereby the macromers incorporated in the silicone lens matrix are photopolymerized upon exposure to a spatial pattern of ultraviolet light (Figure 1).
The diffusion of the unpolymerized macromer to the area treated with UV light causes the lens to change shape, and then power, as the unpolymerized macromers reach equilibrium. As the macromers in the UV-exposed area become irradiated, it results in a diffusion gradient of unirradiated and irradiated macromers. Macromers from the non-irradiated areas diffuse into the polymerized area, producing a shape change that serves to alter the lens curvature.
For example, irradiating the central portion of the lens results in unreacted macromers diffusing into the center to produce an increase in IOL power and an increase in lens curvature. By irradiating the periphery of the lens, the macromer migration to the outer portion of the lens causes a decrease in IOL power and a flattening of lens curvature.
These spatial irradiance profiles can also be combined to produce spherocylindrical changes.
Numerous studies have shown this IOL can correct residual refractive errors of up to 2 D of hyperopia, myopia, and astigmatism with high precision.5,6
To prevent any possible retinal damage due to the exposure to UV light during the adjustments and lock-in procedures, the lens is coated with a UV-protective back layer or filter on its posterior section.7,8
Within the first postoperative month (although typically 2 to 3 weeks), patients return to the clinic for refraction and acuity measurements, and the process of correcting any residual refractive error commences. For the majority of patients, only one or two adjustment treatments are needed, each lasting less than 2 minutes.
A waiting period of about 2 days is given to allow the lens shape to change before initiating any subsequent adjustments. When the desired refractive and visual outcome is reached, a lock-in treatment is delivered to fully polymerize all remaining macromers in the lens, leaving it stable for the rest of a patient’s life.
In addition to correction of refractive errors, light-adjustable lenses also allow for different near-vision solutions. Adjustable blended vision (ABV) is essentially customizable addition of asphericity where extended depth of focus is added in one eye, while correcting for distance vision in the contralateral eye.9
Customization is possible with the light-adjustable lens to allow even better visual outcomes by considering each patient’s ocular variability, neural adaptation, and lifestyle.
We have been investigating ABV with the light-adjustable lens since 2011. Both refraction and spherical aberration need to be controlled accurately before the light treatments to produce the best-optimized outcomes.
In our study, 14 patients were bilaterally implanted with the light-adjustable lens (n = 28 eyes), customizing the SA value in one eye while setting the other to emmetropia.9
At 2 weeks postoperatively, patients undergo an adaptive optics vision analysis to determine spherical aberration values. During this visit, the primary adjustment is made to correct for defocus and astigmatism; a secondary adjustment about a week later then induces spherical aberration.
In another week, the patient returns for the final lock-in. All eyes with an aspheric profile were able to read J3 at 30 cm, and at 20/20 at far. In a smaller subset of four patients with higher-induced SA values, all were 20/20 or better for distance and J1 for 40 cm objects.
More recently, data were presented on optimized near vision with the lens (LAL Symposium, American Society of Cataract and Refractive Surgery, 2014). This controlled study used adaptive-optics technology (Voptica SL, Spain) to further improve the near and distance visual outcomes of the lens. The lens has also been shown to be comparable to a commonly used trifocal lens in contrast-sensitivity function (Artal P. Optimized near vision with the LAL. Unpublished data, 2014).
The next-generation, light-adjustable lens is in development and will have a more effective, UV-light-enhanced back layer, and the number of light treatments will be reduced. By using adaptive optics in combination with the light-adjustable lens, the hope is to provide the best customization for patients with cataracts.
1. Ford J, Werner L, Mamalis N. Adjustable intraocular lens power technology. J Cataract Refract Surg. 2014;40:1205-1223.
2. Brierley L. Refractive results after implantation of a light-adjustable intraocular lens in postrefractive surgery cataract patients. Ophthalmology. 2013;120:1968-1972.
3. Behndig A, Montan P, Stenevi U, et al. Aiming for emmetropia after cataract surgery: Swedish National Cataract Register study. J Cataract Refract Surg. 2012;38:1181-1186.
4. Schwartz DM. Light-adjustable lens. Trans Am Ophthalmol Soc. 2003;101:417–436.
5. Chayet A, Sandstedt C, Chang S, et al. Correction of myopia after cataract surgery with a light-adjustable lens. Ophthalmology. 2009;116:1432-1435.
6. Villegas EA, Alcon E, Rubio E, Marín JM, Artal P. Refractive accuracy with light-adjustable intraocular lenses. J Cataract Refract Surg. 2014;40:12.
7. Heinzelmann S, Hengerer FH, Maier P, et al. Is there an endothelial cell toxicity of light-adjustable lens UVA irradiation on the human corneal endothelium? Eur J Ophthalmol. 2012;22 Suppl 7:S57-S61.
8. Werner L, Chang W, Haymore J, et al. Retinal safety of the irradiation delivered to light-adjustable intraocular lenses evaluated in a rabbit model. J Cataract Refract Surg. 2010;36:1392-1397.
9. Villegas EA, Alcón E, Mirabet S, Marín JM, Artal P. Extended depth of focus with induced spherical aberration in light-adjustable intraocular lenses. Am J Ophthalmol. 2014;157:142–149.
Pablo Artal, PhD
P: +34 968357224
Dr. Artal is full professor of optics, Laboratorio de Optica, Departamento de Física, Universidad de Murcia, Murcia, Spain. He has received external funding from Calhoun Vision.