Bioptics, the combination of cataract surgery and laser eye surgery, is a strategy for compensation of the inherent prediction error of intraocular lens formulae. Unlike a pair of glasses where lens choice is refined by simply presenting the patient with a number of slightly different lenses until the clearest image is obtained, intraocular lens choice must be determined by theoretical prediction, as it is not possible to swap lenses and ask the patient which is clearest. These predictive formulae take into account various measurements from the patient's eye to predict the 'ideal' lens. These formulae are based on thousands of measurements of normal eyes and the results as with any normally distributed measurement follow a bell curve. As such 95% of patients will fall within 1 dioptre of targeted post-operative outcome, whilst a small percentage of outliers will fall outside this prediction accuracy [to some degree]. Laser treatment after cataract surgery simply allows the final outcome to be refined if it is significantly outside the predicted refractive outcome.
In truth, very few patients need bioptics. In some patients who demand absolute accuracy and are adamant that they don't want to wear glasses after the surgery, the surgeon tends to aim for a initial myopic lens prediction (myopia is easier to treat with the laser) and then fine tunes the final outcome to perfection some weeks later. Other patients with extremely unusual prescriptions or very long or short eyes may also need bioptics.