Comparision of ultrasound A- scan with optical A-scan Dr.Somshekar.N Consultant Refractive Services Narayana Nethralaya, Bangalore No Finanacial Interest

Purpose To compare the two biometry machines to check for their accuracy in getting near emmetropia post operatively.

Materials and Methods 66 eyes of 64 patients were scanned. With the ultrasound A-scan (Echorule2, Biomedix) and with optical biometer (Lenstar, Haag Streit). The difference between achieved and predicted refraction, the axial lengths, and IOL powers was noted.

Results The mean difference between the achieved refraction and predicted refraction with the ultra sound system (US) was-0.33±0.79D. The mean difference between the achieved refraction and predicted refraction with the optical system (OS) was 0.49±0.81D.

Results The mean difference in axial length between US and OS was 0.5±0.25mm The difference in the mean IOL power between the two systems was 1.90D±1.0.

Discussion The IOL power measured with the optical system was more tightly clustered than the ultrasound system. This may be explained by the way the measurements are taken. The OS measures from the anterior surface of cornea to the Retinal Pigment Epithelium and the US measures from anterior surface of cornea to the Internal limiting membrane of the retina.

Discussion This translates into a 0.5mm difference in the axial length or 1.5D of IOL power. Hence the OS gives lower IOL power compared to the US The additional 0.4D difference can be explained by taking into account the indentation of cornea while measuring with US.

Conclusion The IOL power predicted by the OS was closer to emmetropia than the US.