Surgeons at the Eye Institute of Utah, Salt Lake City, compared the efficacy, safety, predictability and merits of LASIK and lens-based (intraocular lens exchange or piggyback IOL) approaches for correcting different types of residual refractive error after cataract surgery.

The retrospective study included 57 eyes of 48 patients who had LASIK (28 eyes) or lens-based correction (29 eyes) for residual refractive error after cataract surgery. The visual and refractive outcomes were evaluated at a mean follow-up of 20 to 24 months.

In the LASIK group, the mean spherical equivalent was reduced from -1.62 ±0.80 D preoperatively to +0.05 ±0.38 D postoperatively in myopic eyes and from +0.51 ±1.25 D to +0.19 ±0.35 D in hyperopic eyes. Ninety-two percent of eyes were within ±0.50 D of intended correction. In the lens group, the mean SE was reduced from -3.55 ±2.69 D preoperatively to -0.20 ±0.50 D postoperatively in myopic eyes and from +2.07 ±2.38 D to +0.07 ±0.85 D in hyperopic eyes. Eighty-one percent of eyes had postoperative SE within ±0.50 D of the intended correction. The UCVA improved significantly in both groups. No eye lost more than one line of BSCVA. With a similar length of follow-up, no significant difference in postoperative SE was found between the two groups (p=.453).

(J Cataract Refract Surg 2008;34:562-9.)

Jin GJ, Merkley KH, Crandall AS, Jones YJ.

 

Sources of Error in IOL Power Calculation

A researcher from AMO Groningen BV, in the Netherlands conducted a study to identify and quantify sources of error in the refractive outcome of cataract surgery. From the published literature, he took or derived mean and standard deviations of parameters that influence refractive outcomes, to the extent available. To evaluate their influence on refraction, thick-lens ray tracing that allowed for asphericity was used. The numerical partial derivative of each parameter with respect to spectacle refraction was calculated. The product of the partial derivative and the SD for a parameter equates to its SD, expressed as spectacle diopters, which squared is the variance. The error contribution of a parameter was defined as its variance relative to the sum of the variances of all parameters.

The largest contributors of error were preoperative estimation of postoperative IOL position (35 percent), postoperative refraction determination (27 percent) and preoperative axial length measurement (17 percent), with a mean absolute error (MAE) of 0.6 D for an eye of average dimensions. Pupil size variation in the population accounted for 8 percent of the error, and variability in IOL power, 1 percent.

He concluded that improvement in refractive outcome requires better methods for predicting the postoperative IOL position. Measuring AL by partial coherence interferometry may be of benefit. Autorefraction increases precision in outcome measurement. Reducing these three major error sources with means available today reduces the MAE to 0.4 D. Using IOLs that compensate for the spherical aberration of the cornea would eliminate the influence of pupil size. Further improvement would require measuring the asphericity of the anterior surface and radius of the posterior surface of the cornea. 

(J Cataract Refract Surg 2008;34:368-76.)

Norrby S.