PNNL

Abstract

Mechanisms occurring in phacoemulsification using high power ultrasound remain the subject of ongoing debate. Tissue disruption is attributed to a variety of phenomena including cavitation and mechanical forces. This paper reports an investigation of the physics of phacoemulsification. Acoustic measurements were made for a commercial phacoemulsification unit operating at 40 kHz with various directional tips, operating at a range of powers, in both continuous and “pulse” (tone-burst) modes. The wavelength in water is ~ 37 mm, and the tip acts as a small source generating a non-uniform radiation pattern. Data were acquired in water, soft tissue phantoms, hard “casting resin” and freshly enucleated porcine eyes. The interaction is considered as three zones: near-tip: (~1mm) material directly interacting with the vibrating tip; near-field: (to ~2 cm) an intense acoustic field and far-field: a zone which extends out many cm. The radiation patterns were imaged in a water bath using a scanning wide-band hydrophone and the far-field was measured using wide-band receivers attached to tissue phantoms, casting resin and porcine eyes. Examples of time and frequency domain data will be shown. In the normal clinical range there was a minimal level of random cavitation occurring in the near-tip-tissue interface. This did increase, but remained at low levels, even at powers above those normally used in surgery. The number of cavitation events detected reduced with the increase in tissue hardness. The characteristic cavitation “hiss” was audible at full power. The source of this sound/cavitation was found to be at the horn shoulder, not at the end of the tip. For the small number of cavitation events recorded it was unclear how many occurred at the tip-tissue interface and how many were associated with micro-bubble excitation due to bubbles traveling down the horn in the tip in irrigation fluid. In pulse mode no evidence was detected to indicate occurrence of “transient cavitation” at the start of each tone-burst excitation. On the basis of this study, it is concluded that the primary mode for tissue disruption using a phacoemulsification unit employing a straight beveled ended tip is due to a “jack-hammer” effect.