Ultrasound phacoemulsification: Physical mechanisms, cavitation, and thermal effects

PURPOSE: To perform a comprehensive technical and biomedical analysis of ultrasound-induced effects during phacoemulsification cataract surgery (PCS) and assess the potential risks associated with ultrasound emulsification. SETTING: Lucerne University of Applied Sciences and Arts, Horw, Switzerland. DESIGN: Computational modeling and laboratory bench testing. METHODS: Ultrasound wave propagation and intensity were simulated using the finite element method (FEM). The onset of cavitation was experimentally validated in a pressure chamber and observed using a high-speed camera. The thermal effects of ultrasound energy were estimated using established physical and thermal models and confirmed in a cuvette setup using thermistor probes. RESULTS: At the distal end of the PCS tip, the ultrasound wave intensity reached 0.2 W/mm2, resulting in ocular tissue displacements in the anterior chamber below 2 µm. The temperature rise in the anterior chamber was less than 0.2 °C/s. Both observations indicate negligible mechanical and thermal risks to ocular tissues. High-speed imaging confirmed that cavitation was confined to the PCS tip region. The efficiency of emulsification was maintained even under conditions that suppressed cavitation, including elevated ambient pressures and cavitation-inhibiting fluids. The results indicate that mechanical fragmentation by tip oscillation ("jackhammer effect") is the only relevant emulsification mechanism, whereas cavitation plays a negligible role. Acoustic streaming was observed during the ultrasound excitation without a phaco sleeve. During phacoemulsification, the sleeve suppresses acoustic streaming into the anterior chamber. CONCLUSIONS: These findings validate the safety of ultrasound-based PCS and confirm that lens fragmentation is primarily driven by the direct action of the oscillating tip.