What are light-adjustable intraocular lenses?
One of the most revolutionary systems on the intraocular lens market today is what is known as light-adjustable intraocular lenses (LAL) or Calhoun. It is a type of lens which can change its refractive power thanks to ultraviolet rays.
After this lens is implanted in the eye, its refractive power can be changed by up to two spherical dioptres (for myopia or hypermetropia) and up to one and a half dioptres of cylindrical power (for astigmatism). This is possible through the radiation of ultraviolet rays emitted from outside the eyeball. No additional surgical intervention is required and it allows for optimum adjustment and complete personalisation.
How does this type of intraocular lens work?
The technique used to insert this lens is the same as for traditional lenses, and is a routine cataract operation. Surgeons are very familiar with this procedure. However, the subsequent treatment is different to other crystalline lens operations.
This post-operative procedure starts two or three weeks after the surgery. Firstly, the patient’s visual acuity is assessed and if necessary, ultraviolet rays are applied. This is done using a machine designed especially for this technique which is able to provide the appropriate light energy for each type of situation. LALs are made to react to this light, meaning that they are able to change shape and optical performance. This procedure may need to be repeated several times to achieve the definitive refraction. The final step involves fully blocking the refraction of the lens (lock-in) to ensure the refractive power does not undergo further changes with the stimulus of ultraviolet rays. Until this final step is performed, the patient must wear special sunglasses for protection from solar radiation and avoid accidental changes being caused to the power of the LAL. The probability of achieving the desired power of refraction is very high thanks to this method.
Fundamental Principle of the Light-adjustable Intraocular Lens
Light-adjustable lenses are different from others because they contain special materials, known as macromers. These materials are biocompatible with the human eye and sensitive to ultraviolet light of a specific wavelength. Macromers undergo changes when they are stimulated by this light. This process is known as photo polymerisation. This involves a change in the shape of the lens and as a result, in its power.
When ultraviolet light is applied to the LAL it starts the photo polymerisation of the macromers in the irradiated area and the macromers in this area are eliminated.
This causes an imbalance in the lens, as macromers remain in the non-irradiated areas, but are no longer present in other areas. To restore the balance in the lens, the macromers in the non-irradiated area migrate to the area touched by the ultraviolet rays and cause this part to swell, thereby changing the curvature of the lens.
Once the desired refraction has been achieved, the lens is blocked so that it cannot suffer any accidental changes with sunlight. This is done by applying ultraviolet light again, but this time throughout the surface of the lens to eliminate all the remaining macromers. This ensures that no other changes can be made to the curvature.
Different treatments can be performed through the use of light-adjustable lenses. Firstly, to achieve good visual acuity for distance vision, residual myopia, hypermetropia or astigmatism is corrected.
This system also takes into account solutions for near vision. Monovision is used to customise the required level of vision. One eye (the dominant) is corrected for distance and the other eye (the non-dominant) for near vision.
As the refractive power can be changed by up to two dioptres in both eyes, the patient has the opportunity to see if the graduation needs to be improved or not in different steps until the desired result is achieved.
What are the advantages and disadvantages of this procedure?
LALs constitute a major advance in the intraocular lens market as they offer the possibility to change the refractive power without the need for additional surgery, by simply applying ultraviolet light.
It is an interesting treatment for patients who have previously had LASIK surgery and who then undergo cataract surgery. Although today the pre-operative calculations are done in a very precise way, these patients have a higher risk of suffering a post-surgical refraction, as their cornea has been modified and it is more difficult to predict the results.
Furthermore, it is a technique which achieves very good vision for distance but it is not a multifocal system, meaning that near vision may suffer in this respect or at least may not be at the same quality as with other Premium intraocular lenses. Another disadvantage is that the patient must depend on solar protection glasses for the duration of the treatment, which can last for several weeks.