To introduce a method of predicting effective lens position (ELP) among congenital ectopia lentis (CEL) patients undergoing transscleral fixation of intraocular lens (IOL), and evaluate its effect on improving refractive outcome by utilizing the Sanders-Retzlaff-Kraff / theoretical (SRK/T) formula.

Retrospective cross-sectional study.

A training set (93 eyes) and validation set (25 eyes) was included. Z value as the distance between the iris plane and a hypothetic postoperative IOL position was introduced in this study. The Z-modified ELP consisted of corneal height (Ch) and Z (ELP = Ch + Z), and Ch was calculated by keratometry (Km) and white-to-white (WTW). The value of Z was identified by linear regression formula with the involvement of axial length (AL), Km, WTW, age, and gender. The comparison of mean (MAE) and mediate absolute error (MedAE) among Z-modified SRK/T formula, SRK/T, Holladay I, and Hoffer Q formula was performed to evaluate the performance of Z-modified SRK/T formula.

Z value was associated with AL, K, WTW, and age (Z = offset + 15.1093 × lg (AL) + 0.0953899 × Km – 0.3910268 × WTW + 0.0164197 × Age – 19.34804). The Z-modified ELP has good accuracy with no difference to back-calculated ELP. The accuracy of Z-modified SRK/T formula was better than other formulas (P < .001) as the MAE was 0.24 ± 0.19 diopter (D) and MedAE was 0.22 D (95% CI: 0.01-0.57 D). Sixty-four percent of eyes had a refractive error smaller than ±0.25 D, and none of the subjects had a prediction error greater than ±0.75 D.

ELP of CEL can be accurately predicted by AL, Km, WTW, and age. Z-modified SRK/T formula improved on the current formula by improving predicting accuracy of ELP and may serve as a promising formula for CEL patients with transscleral fixation of IOL.