Ultrasound velocity in heavy ocular tamponade agents and implications for biometry

Aim: Heavy ocular tamponade agents have been 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 this 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 a US 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°C to 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 per °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 D depending on the tamponade agent by using correction factor (CF) for SO 1000 cs. 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 intraocular lens power.