‘Remarkable’ differences occur in A constants given by companies

A "remarkable difference" can be seen between the A constant provided by the manufacturer of an IOL and the refractive result observed postoperatively, according to two physicians.

Wieke Zuidervaart, MD, and Gregorius P.M. Luyten, MD, PhD, at the University Medical Center Rotterdam in the Netherlands evaluated the refractive and visual outcomes in 288 eyes of 271 patients after implantation of five different IOLs. All eyes underwent uncomplicated cataract surgery between 1996 and 1997; surgical procedure was standard extracapsular cataract extraction with phacoemulsification through a 3.2-mm beveled two-step tunnel incision, 1 mm posterior to the superior corneoscleral limbus. Surgeon preference determined which lens was inserted: the Centra55B (Domilens), a PMMA lens, or one of four foldable lenses, Advanced Medical Optics’ AMO SI40, the STAAR AA-4203, the Silens 5 (Domilens) or the Pliolens (Iovision).

Six different IOL power calculation methods were used, calculated first with the SRK-T formula that used an A constant of 118.5 and recalculated for the remaining five formulas. Pre- and postoperative uncorrected visual acuity, best corrected visual acuity and refraction were measured via a Snellen chart.

The STAAR lens had the best UCVA compared with the Centra55B, SI40 and Pliolens. Based on the A constant provided by the company, the Pliolens revealed a significant overcorrection and the STAAR lens an undercorrection. The achieved postop refraction minus the intended postop refraction caused significant myopia with the Pliolens and hyperopia with the STAAR lens.

Additionally, the SRK-T and Holladay 1 formulas gave the best predictive lens calculations in relation to the axial length. “The AMO SI40, the STAAR AA and the Silens 5 showed the most remarkable difference between the A constant given by the company and the recalculated A constant,” the authors said.

The study is published in the November issue of Ophthalmologica.