Influence of intraocular lenses on ultrasound axial length measurement: in vitro and in vivo studies.

Refraction affects ultrasound as it passes obliquely through tissues or materials with different propagation speeds. Therefore, axial length measurements could theoretically be affected after implantation of an intraocular lens which has a propagation speed that exceeds those of the ocular humors by a factor of 1.8 (2718 m X s-1 vs. 1532 m X s-1). The in vitro studies we report clearly demonstrate that ultrasound axial length measurements were not affected significantly by inclining the IOL up to 25 degrees or varying the transducer-lens distance from 0 to 25 mm. However, length measurements were found to be directly proportional to the lens power. A 27-diopter lens induced a 0.032-mm increase in the apparent axial length for a lens-reflector plate distance of 20 mm (P less than 0.05). In vivo the retina's curvature counterbalances this apparent increase: axial lengths measured in 26 patients were virtually the same after IOL implantation. From these data we conclude that axial lengths can be accurately measured by ultrasound after IOL implantation if the different propagation speeds are considered for calculation.