Ultrasound velocity in heavy ocular tamponade agents and implications for biometry.
Résumé
Purpose: Heavy ocular tamponade agents are shown to be an effective tamponade in complicated retinal detachments. Combined oil removal and cataract surgery may be performed and ultrasound (US) provides a reliable means to measure the axial length of eyes. The aim of study was to evaluate the velocity of US in various tamponade agents. Methods: Five tamponade agents (SO 1000, SO 5000, Oxane HD, Densiron 68, and F-Decalin) were studied in vitro. Time of flight (T1) was measured between an ultrasound transducer and the bottom of a container of the agent, and remeasured (T2) after reducing the separation by 1 cm. The speed of sound in the particular material was calculated from the difference between T2 and T1. Measurements were repeated over a range of temperatures from 18 - 42°C. Results: The speed of sound at 37°C ranged from 645±8 m/s to 976±10 m/s depending on the tamponade agent. In Densiron 68 and F-Decalin, the speed of sound was markedly reduced to 914±10 m/s and 645±8 m/s respectively. The temperature dependence of speed of sound varied between -2.2 and -3.6 m/s/°C depending on the particular oil. With 95% confidence, the true speed is believed to lie within ± 5m/s of these values. The deviation from intended refraction was between 0.38 to 30.15 dioptres depending on the tamponade agent by using correction factor (CF) for SO 1000cs. Conclusions: Variability in the velocity of US should be taken into account when performing biometry. Biometry machines should be adjusted for various tamponade media when calculating IOL power.
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