[Trial of new intraocular lens power calculation following phototherapeutic keratectomy].

Abstract

We reviewed the SRK-II method and introduce a new equation to calculate the intraocular lens (IOL) power for eyes which underwent laser phototherapeutic keratectomy (PTK). The Gullstrand series was used to determine the power and the radius of curvature of planoconvex IOLs which alter the focal point from the cornea to reach the conjugate point on the retina. The radius of anterior corneal curvature (R), axial length (AXL), predicted postoperative anterior chamber depth (ACD), and lens thickness (LT) were employed in the following formula to calculate the IOL refractive power: K = R/7.7, DC = 337. 5/R, VC = 1,000/DC*1.336 where VC is the posterior vertex focal length. A1 = -(VC-ACD), B1 = AXL-0.5* K-ACD-0. 103 LT, S = 1/A1 + 1/B1; this determined the diopter (D) of IOL in liquid to be (D) = 1,000/(1/S)* 1.336. In eyes which underwent PTK, the keratometric value prior to cataract surgery was not applied. Instead, R' defined as R-dT, where R is the radius of corneal curvature prior to PTK and dT the amount of corneal tissue removed, was introduced. Further, the corneal thickness before cataract surgery (CT') was defined as CT-dT where CT is the corneal thickness prior to PTK. Although it is important to select a lens that has an acurate predicted anterior chamber depth, the new equation appears to be more useful than the SRK-II formula.