超音波A-モードによる前房深度測定と等価音速

Abstract

Anterior chamber depth (ACD) is an important parameter for accurate determination of the intraocular lens (IOL) power as well as in the recent refractive phakic IOL or implantable contact lens (ICL). We analyzed ACD error of phakic and pseudopha- kic eyes in ultrasonic biometry, and also derived theoretically the overall average sound velocity (equivalent ultrasonic velocity, EUV). We found that the EUV (Ve) for ACD measurement can be estimated from the equation, Ve=ACD/{(ACD/V₂)+(T/V₁)-(T/V₂)}, where T is the corneal thickness, ACD is the anterior chamber depth, V₁ the ultrasonic velocity of the cornea, and V₂ the ultrasonic velocity of the aqueous. The EUV for the ACD is a function of the corneal thickness and the ACD, even if we assume the ultrasonic velocity of the cornea and aqueous. For the standard eye dimension, the EUV for the ACD measurement is approximately 1545 m/s in phakic eyes. For the IOL implanted eye (pseudophakic eye), postoperative axial length can be measured with the different velocity, 1560 m/s for PMMA IOL-implant- ed eye, 1500 m/s for silicone IOL-implanted eye and 1547 m/s for acrylic IOL-implanted eye. When we use these equivalent velocities for accurate determination of the axial length, resultant ACD from A-mode biometry shows the apparent depth with considerable errors. To perform more accurate assessment of the ACD in IOL-implanted eyes, we must use the proper velocity for the ACD ignoring the error in the axial length measurement.