[Equivalent ultrasonic velocity for intraocular lens implanted eyes in A-mode biometry]

With the standard ultrasonic velocity in commercially available instruments, there is a large difference of the ultrasonic velocity between the ocular medium and the implanted intraocular lens (IOL). If the PMMA or silicon IOL-implanted eye is assessed by using the ultrasonic velocity of the phakic or aphakic eye (1,550 m/s or 1,532 m/s), the error induced by the mismatch of the velocity is considerable. Therefore, we estimated the axial length error of pseudophakic eyes in ultrasonic biometry, and also derived theoretically the overall average sound speed (equivalent ultrasonic velocity, EUV) in IOL-implanted eyes. We found that the EUV (VE) can be estimated from the equation, VE = LV1V2/ T(V1-V2)+LV2], where L is the axial length of the eye, V1 the ultrasonic velocity of aqueous and vitreous body, V2 the ultrasonic velocity of the IOL, and T the central thickness of the IOL. The EUV for a PMMA IOL-implanted eye is approximately 1,560 m/s, and the EUV for a silicon IOL-implanted eye is approximately 1,500 m/s in an eye of standard length (24 mm) and an IOL of standard central thickness (1 mm). However, the EUV is a function of the axial length of the eye (L), the central thickness of the IOL (T), and the sound velocity in the implanted IOL (V2), and thus the EUV can not be applied for all cases. To perform more accurate ultrasonic biometry in pseudophakic eyes, we should rectify the apparent axial length assessed by the aphakic sound speed (1,532 m/s) with the central thickness of the implanted IOL.