Intraocular lens power calculation--the selection of formula

The postoperative refraction of the eye after intraocular lens (IOL) implantation is an important aspect of the quality of aphakia correction. The postoperative refraction cannot be guessed with consistent accuracy as the refractive power of an eye is multifactorial. Calculation of IOL power is based upon measurements of corneal curvature and axial length and an estimation of postoperative anterior chamber depth. There are a number of formulae available for the calculation of IOL power. The optical formulae, of which those of R. D. Binkhorst are the most popular, give results which are very similar and which all differ from the mathematical regression SRK formula by the indication of a stronger IOL power for short eyes. A statistical analysis of postoperative results confirms that the R. D. Binkhorst formula gives the author an over-powered IOL for short eyes when calculated for emmetropia but not when calculated for planned ametropia. Results with the R. D. Binkhorst formula will be improved by the modification of the anterior chamber depth according to the axial length. Both formulae give satisfactory results for the range of axial lengths commonly encountered in clinical practice. Whichever formula is used, it is important for each surgeon to analyse postoperative results and to modify the selection of IOL power according to this feedback to correct for consistent errors of instrumentation or technique.