Intraocular Lens Power Calculation

Excerpt

Sir Harold Ridley implanted the first intraocular lens (IOL) in 1949. Postoperatively the patient had a refractive error of -18 dioptre sphere and -6.00 dioptre cylinder at 120 degrees. Later Binkhorst introduced an 18.0 D pre-pupillary lens. It left the patient with the same refractive error he had before the cataract surgery. Thus it was realized that merely an implant is insufficient, and accurate IOL power calculation is necessary. It is one of the most critical factors for the refractive outcome of cataract surgery.

In most patients, emmetropia is the targeted post-operative refraction, while some patients are targeted for residual myopia. To reach the targeted refraction, the selection of the IOL formula must be performed according to the anatomical and optical parameters of the eye. Axial length is an essential parameter in IOL power calculation. 1 mm of change in axial length can alter the IOL power by 2.5-3.0D. Corneal power is another important factor. The cornea accounts for two-thirds of the total optical power. A change in corneal power by 1 Dioptre alters the IOL power by 1D.

Apart from the axial length and keratometry, other parameters may also be required, such as anterior chamber depth and the corneal white-to-white distance.