Cataract surgery may occasionally create an unexpected or unwanted refractive (focus) error, causing blurred vision. This can happen if the lens implant sits in front of or behind the position predicted from the pre-operative eye measurements, often in eyes that are unusual in some way.
Correcting the focus and vision with either a change of glasses or a contact lens may be inconvenient, not tolerated, or unsatisfactory for optical or cosmetic reasons.
The problem can sometimes be rectified at an early stage by replacing the lens implant. However, if there is a risk or there has been a delay, it may be safer to place a supplementary “piggy-back” lens in front of the first one – two lens implants in the same eye.