High- and low-frequency phaco energy behave differently

September 1, 2008

High- and low-frequency energy in porcine and human eyes displayed on an oscilloscope showed that the energy levels behave differently in the eye during phacoemulsification.

Key Points

"The phaco tip generates both low-frequency energy at the manufacturer-selected frequency and high-frequency cavitational energy," said Dr. Fishkind, clinical professor of ophthalmology, University of Utah, Salt Lake City, and director of the Fishkind, Bakewell, and Maltzman Eye Care and Surgery Center in Tucson, AZ.

"Each kind of energy interacts differently with cataract fragments and intraocular structures," said Dr. Fishkind, who is also a clinical instructor at the University of Arizona in Tucson.

"We found that during longitudinal phaco, the amount of energy in the eye was 0.94 mW/10 units on the axis," he said.

During back-sculpting of the nucleus, more energy was transmitted to the eye than during the forward movement of the needle, Dr. Fishkind said.

"The reason for this is the attenuation of the big lens," he said. "Sculpting is almost free-energy use as long as the needle is embedded in the nucleus. This was an interesting finding."

Energy peak

Additionally, it was found that emulsification of fragments resulted in both a peak of low-frequency energy and a peak of high-frequency energy due to cavitational energy, according to Dr. Fishkind.

"The low-frequency peak is due to coupling," he said. "When materials are on the phaco tip, low-frequency energy is emitted from the coupled material on the tip, and the blast of high-frequency energy is cavitational energy."

Dr. Fishkind and his colleagues looked at elliptical configurations and found that at 37% power, much less energy was generated due to the lower-driver frequency and the lateral movement of the phaco tip.

When data from the experiment were combined, it was found that low-frequency energy radiates from the phaco tip in all directions at 28 KHz and bounces off tissue, whereas high-frequency energy at the phaco tip with cavitation is focused in a 1- to 2-mm diameter cylinder radiating from the phaco tip. The inflow of fluid acts as fuel for cavitation.

With elliptical configurations, a prolate spheroid of energy radiated from the phaco tip at 24 kHz. Less high-frequency energy was generated.

With torsional configurations, Dr. Fishkind said he observed a bow-tie pattern of energy with an as-yet-undetermined amount of energy.

"Energy . . . disrupts the blood aqueous barrier and damages the endothelial tight junctions and cell membranes," he said. "Acoustic energy causes damage. Low-frequency energy can be minimized by using micropulse energy. Energy can be quantified in mW.

"We can guess that longitudinal configurations may have more radiating energy than elliptical-and probably torsional-configurations," Dr. Fishkind concluded. "High-frequency acoustic energy can be minimized by using an elliptical configuration or a torsional configuration."

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