One of the limitations of modern cataract surgery is the relatively low predictability of the patient's final refraction. Although, on average, the calculated power of intraocular lenses (IOLs) produces a spherical equivalent close to zero, the individual variability can be quite large, and refractive errors over 1 D are not unusual. It is well known that this poor predictability of final IOL power is even worse in certain cases, such as in patients who have undergone previous refractive surgery. In addition, it is typical that neither natural nor induced corneal astigmatism is accurately corrected, even when state-of-the-art toric IOLs are implanted.

It is the combination of residual defocus and astigmatism that accounts for retinal image degradation and reduced quality of vision following IOL implantation in the majority of patients. Naturally, residual refractive errors can be corrected with spectacles, contact lenses or laser refractive surgery; it would, however, be preferable and more elegant to offer an advanced approach to cataract surgery that produces near perfect refraction in a single procedure.

Light adjustable IOLs: a quick introduction

Light adjustable lenses (LALs) were invented and developed by Calhoun Vision in order to address this problem.¹ These novel IOLs are similar to standard 3-piece lenses, but contain photosensitive silicone molecules that enable postoperative adjustment of the final refractive power using ultraviolet (UV) light.

The lenses have four main components: a silicone matrix polymer, a photoreactive macromer, a photoinitiator, and a UV absorbent layer at the back of the lens. Irradiation of the lens through a defined spatial profile polymerizes the macromer in the exposed region: the subsequent diffusion of un-reacted macromer to re-establish equilibrium throughout the lens produces a change in the shape of the lens and therefore a modification of the IOL power. For example, if the lens power needs to be increased, the central part is irradiated to cause the macromer in that area to polymerize. Then time is allowed for free macromer to diffuse down the resulting concentration gradient. The central zone bulges, the radius of curvature decreases, and the focal length decreases. Irradiating on the periphery causes macromer to leave the central zone, producing a flattening of the lens and increasing its focal length. Once the desired refraction (shape) is reached, the entire lens may be irradiated to polymerize all the macromer, making the state of the lens permanent. After this, the LAL may be exposed to UV light (for example, sunlight) and will remain unaltered.

Figure 1

By applying different irradiation profiles, the shape of the IOL can be controlled to produce the desired final refractive outcome. This ability to adjust the shape of the IOL allows the targeted correction change of both defocus and astigmatism to be achieved (Figure 1).

Figure 2

The LAL is implanted using standard cataract surgery techniques. After implantation, patients must wear protective spectacles for a period of 10 days to two weeks: this is to prevent ambient UV light from affecting the shape of the LAL. During this time, the cornea will settle into almost the final stable shape, so the applied correction produces a definitive optimum refraction. Typically, up to two adjustment procedures — to correct any residual refractive error within a range of 2 D — are applied (Figure 2).

Once the desired refraction is achieved, two additional photo-locking treatments need to be performed, to insure that the LAL is fixed and remains stable. From this point, the LAL behaves entirely as a conventional IOL. The irradiation of the LAL is performed using a custom digital light delivery device that is easy for trained ophthalmologists to operate, and the procedure is quick and comfortable for patients.