Piggyback IOL refers to a second IOL that is placed in the eye so that there are now two lens implants. Usually the first IOL is placed in the capsular bag and then at a later date the piggyback IOL is a three-piece design that is placed in the ciliary sulcus. This is helpful in cases where there is a hyperopic refractive surprise. The calculation of the piggyback IOL power is based on the refraction with about 1.5x the power needed for the IOL as compared to the refraction spherical equivalent. For example if the desired correction is +2 diopters, then the piggyback IOL with a power of +3 diopters is placed in the sulcus. For myopic post-op refractive surprises, I generally prefer doing a keratorefective procedure like PRK or LASIK. If you perform a piggyback IOL for a myopic surprise, then cut down the factor to 1.3x for the piggyback IOL power needed to avoid post-op hyperopia. Also note that these negative power IOLs can have a thick edge which may scrape the posterior surface of the iris when placed in the capsular bag.