[Intraocular lens calculation and ultrasound biometry: immersion and contact procedures]

Ultrasound biometry for axial length measurement may be performed either by directly putting the probe on the cornea or by using a water immersion technique. Our goal was to examine whether there are unsystematic differences between both techniques present besides systematic differences that can be compensated by adjusting calculation formula constants.

We examined 288 patients in a prospective, randomized trial. There was no ocular pathology present beside cataract. Axial lengths < 21 mm and > 27 mm were excluded. We calculated which IOL power would have given the desired refractive result by using the postoperative refraction and data of the lens implanted.

A systematic difference between both techniques is present. With the contact technique, axial length is measured 0.15 mm shorter. This requires adjustment of formula constants. Furthermore, there is an unsystematic difference that leads to 18% greater calculation errors (difference between IOL calculated preoperatively and ideal IOL) with the contact technique. Mean absolute error was 0.43 +/- 0.38 dpt for the immersion group and 0.53 +/- 0.48 dpt for the contact group.

To minimize postoperative refractive errors, ultrasound biometry using immersion technique should be preferred.