When would this need to be done or why would it be done?
Suppose a patient has a first operation, for example, a cataract procedure or refractive lens exchange procedure, and they need glasses afterwards for seeing in the distance for reading or both. In that case, we can reduce their dependence on glasses by implanting a second piggyback or add on lens just in front of the first lens.
The piggyback lens can be monofocal or multifocal, and it can correct either the distance vision, in which case a patient will still need glasses for reading, or it can be multifocal and correct the distance vision as well as the near vision.
If a patient needs glasses for reading, we can use a multifocal lens. We slide this just in front of the first lens so that the light coming in the eye goes through both the multifocal lens and the monofocal lens and focus on the retina behind. That way, we can turn a monofocal eye that only sees far into a multifocal eye that can see far and near.
The advantage of a multifocal piggyback lens is to improve the range of vision that a patient can enjoy without needing to resort to their glasses.
There are some potential side effects of a piggyback lens implant, but these are very rare. For example, it’s possible that a piggyback lens could create a little bit more inflammation in the eye than otherwise would be the case, but that usually settles down with drops. However, these lenses are very safe, and the side effect profile is incredibly good.
The final thing to mention about piggyback or add on lenses is that as well as being monofocal or multifocal, they also come in toric varieties for both monofocal and multifocal. Piggyback lens implants can correct residual astigmatism as well as near vision and distance vision.