Ultrasound velocity in heavy ocular tamponade agents and implications for biometry

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.

Five tamponade agents (SO 1000, SO 5000, Oxane HD, Densiron 68 and F-Decalin) were studied in vitro. Time of flight (T(1)) was measured between a US transducer and the bottom of a container of the agent, and remeasured (T(2)) after reducing the separation by 1 cm. The speed of sound in the particular material was calculated from the difference between T(2) and T(1). Measurements were repeated over a range of temperatures from 18°C to 42°C.

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 ± 5 m/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.

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.