IOL CALCULATION AND ULTRASOUND BIOMETRY - IMMERSION VS. CONTACT TECHNIQUE

Background Ultrasound biometry for axial length measurement may be per formed either by directly putting the probe on the: cornea or by using a water immersion technique. Our goal was to examine whether there ar e unsystematic differences between both techniques present besides sys tematic differences that can be compensated by adjusting calculation f ormula constants. Patients, materials and methods We examined 288 pati ents in a prospective, randomized trial. There was no ocular pathology present beside cataract. Axial lengths <21 mm and >27 mm were exclude d. We calculated which IOL power would have given the desired refracti ve result by using the postoperative refraction and data of the lens i mplanted. Results A systematic difference between both techniques is p resent. With the contact technique, axial length is measured 0,15 mm s horter. This requires adjustment of formula constants. Furthermore, th ere is an unsystematic difference that leads to 18% greater calculatio n errors (difference between IOL calculated preoperatively and ideal I OL) 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 grou p. Conclusions To minimize postoperative refractive errors, ultrasound biometry using immersion technique should be preferred.