How comparable are ultrasound and optical biometry measurements? How do different methods of measurement contribute to measurement differences?
Ultrasound instruments measure the distance between the anterior surface of the cornea and the limiting membrane, whereas the IOLMaster measures the distance between the anterior cornea (strictly speaking, the anterior surface of the tear film) and the pigmented epithelium. A great number of comparative measurements have been performed regarding these methodological differences. The “correction factors” resulting from these measurements were integrated in the device- internal software so that the well-known biometrical formulas with customized IOL constants for immersion technique can be used without any problems. In measurements using the ultrasound contact technique, the amount of applanation (shortening of anterior chamber due to the pressure exerted on the cornea by the ultrasound probe by approximately 100-300 m) also must be considered. Measurement deviations vary from patient to patient due to differences in anatomy and, in the case of ultrasound measurements, they also depend o
