During cataract surgery or refractive lens exchange, an intraocular lens is implanted to replace the natural lens. Without this the eye would be left very longsighted and the vision very blurry (termed aphakia). In NHS cataract surgery centres, the only lens option is a monofocal (single focal point) intraocular lens, although some NHS centres do also provide toric lens for select patients with high astigmatism. Privately there is no limitation to the intraocular lens available, and there are hundreds of different lens models available from different manufacturers. Many of these have advanced optical designs and so are known as “premium lens.”
In brief, the main intraocular lens types available are monofocal (so similar to the standard NHS lens), multifocal (multiple focal points) and extended depth of focus (EDoF) intraocular lens. Toric lens are those that additionally correct higher astigmatism.
Monofocal lens provide excellent vision with one focal point. In practice this means the eye is usually focussed for distance and glasses are needed for intermediate and near tasks. Intermediate tasks are typically those around ⅔ to a meter away- meaning computer work, cooking preparation and other tasks. Micro-monovision is where one eye is set for distance and for the other, the focal point pulled back to try to improve intermediate vision without spectacles but this is at the expense of the clarity of the distance vision in this eye.
Multifocal lens provide multiple focal points, which are typically blended but are usually bifocal or trifocal in design. This means there are two or three main focal points – distance, intermediate and near. With this lens design the aim is to try to maximise spectacle independence and they work well in the correct patient who is keen to be spectacle independent for most tasks and willing to accept some compromise. Typically these lens types cause mild glare or halos around light sources in low vision or at night, but that is the compromise for the multiple focal points. Most patients with multifocal lens have good distance vision for daily tasks, and can perform most intermediate and near tasks without glasses. Glasses may still be needed for intensive near tasks such as reading a novel or small print, or in poor light situations. For multifocal lens to work well, both eyes need the same lens type. Again micro-monovision can be used to slightly vary the focal points between the eyes. Overall this lens type works well in patients who are keen for as much spectacle independence as possible, but are willing to accept some compromise in image optical quality to have this.
A newer design of intraocular lens is “extended depth of focus” (abbreviated to EDoF). These lens have been available in Europe for a few years now. In summary these lens use advanced optical properties – with zones of differing surface curvature or have a continuous change in power from periphery to centre which results in the light being focused in an extended longitudinal plane, instead of discrete points. Multifocal lens are not the same as extended depth of focus lens, although some manufacturers have confusingly re-termed some multifocal lens as EDoF lens to try to move away from the stigma of high glare/ halos associated with historic multifocal lens designs.
Examples of EDoF lens include the Johnson and Johnson Technis Eyhance lens or the Alcon Vivity lens. These lens are designed to give high-quality distance and improved intermediate vision to make everyday tasks easier, with reading glasses typically needed for smaller print or fine near work activities. Unlike multifocal lens, study data shows these lens cause minimal glare or halos, with any being comparable to that with a monofocal lens. These lens are therefore an excellent choice for patients not willing to compromise on optical quality who want good distance and intermediate vision, and are happy to accept they will need reading glasses for most near tasks other than larger print.