Biometric aspects of diffractive multifocal intraocular lenses.

A retrospective biometric study was done on 132 patients with diffractive multifocal intraocular lenses. The two principal focal points induced by the intraocular lens were refracted for distance in each patient. The difference in diopters between the refractions was obtained, and the A constant (SRK formula series) and anterior chamber depth (ACD, Binkhorst formula) were calculated. Our results showed the A constant was 116.46. This value was consistent and not affected by the axial length of the eye in the 22mm to 24.5mm range. In patients who had emmetropia with the intraocular lens power implanted alone, the constant was equal to 116.3. The ACD was 3.94mm. This value was lower in shorter eyes and higher than 3.94mm in longer eyes. On the basis of the data obtained, when the SRK-II formula is used, an A constant of 116.4 should be used in preoperative biometric calculations.