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Study finds that cavitation plays no role in cutting cataract tissue
Findings prove that the jackhammer effect dominates the phaco process.
The “jackhammer effect” is the mechanism of action responsible for phacoemulsification’s cataract lens cutting power, according to a study that demonstrates that equivalent phacoemulsification occurs even when cavitation is not present. The study, which I performed at the Clinica Oftalmologica Pasteur in Santiago, Chile, was documented in “Jackhammer or Cavitation: The Final Answer.” The film, which was shown at the annual meeting of the American Society of Cataract and Refractive Surgery in San Francisco, was awarded the 2006 ASCRS Film Festival Grand Prize.
Our findings support the belief that has long been held by some — that the forward displacement of the phaco tip directly disrupts the lens tissue in an action known as the jackhammer effect. The outcome casts doubt to the equally long-held proposal of others — that the lens-disrupting power of phaco lies in energy released by violently collapsing bubbles created by acoustic cavitation occurring at areas of low pressure during tip oscillation.
Practical implications
The ongoing debate regarding whether the jackhammer effect or cavitation is responsible for phacoemulsification inspired our study, but our desire to provide answers sprung from an impetus far greater than simple intellectual curiosity. There are practical implications involved in determining the true mechanism underlying phaco. For example, if cavitational energy is irrelevant to lens cutting, it makes sense to design and use phaco tips that enhance the jackhammer effect and to avoid those that rely on cavitation, mainly to prevent the known harmful effects of cavitation, such as free radical formation and increased turbulence.
Study methods
In our effort to determine the mechanism responsible for the actual destruction of cataractous lens tissue, we first had to document the occurrence of cavitation (Figure 1). We did this by using a combination of complex light sources, recording methods and custom-made, state-of-the-art electronics. Our efforts revealed that cavitation mainly occurs in close proximity to the tip of the phaco needle and only appears at high ultrasonic powers of 50% or more. We also found that cavitation not only occurred at the tip of the phaco needle, but also along the shaft and in proximity to the hub in the proximal portion of the phaco needle.
We used a slow-motion technique to document that all cavitation activity at the tip of the phaco needle is actually transient cavitation (Figure 2a). This means that bubbles are born and fully collapse during a single cycle of tip oscillation. During forward displacement of the tip, all cavitation bubbles collapsed; therefore, we observed that cavitation only took place on the backstroke or as the tip moved away from the lens material.
Once we documented the acoustic cavitation activity of phaco probes, we set out to determine the relationship between cavitation and phacoemulsification of the crystalline lens. We needed a model that would eliminate ultrasonic cavitation so that we could determine the efficacy of phaco under suppressed cavitation conditions. We started out with the knowledge that the fast backward displacement of the phaco tip during oscillation creates areas of low local pressure — below water vapor pressure — that lead to the formation of cavities filled with water vapor, or cavitation bubbles. Additional research led us to the understanding that cavitation suppression could be re-created using a system that mimicked the pressure conditions present in the deep sea.
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Images: Zacharias J |
Under pressure
Acting on this new-found knowledge, we built a hyperbaric system (Figure 2b) that contained the complete fluidics system of a phaco console, including the peristaltic pump, the handpiece and the phaco tip. Although this was complicated, enclosing the complete fluidics system was necessary to maintain standard operating conditions, such as irrigation, aspiration and vacuum, during deep-sea pressure experiments.
We used computer controls to increase and decrease the pressure of the system through actuators that allowed pressurized air into the hyperbaric chambers based on absolute pressure sensor readings, and we monitored cavitation during cycles of rising and decreasing pressure of the probe chamber filled with balanced salt solution. We found that when we increased pressure, cavitation was reduced, and when we further increased the pressure, cavitation at the tip of the handpiece was completely eliminated.
Having first documented cavitation and then totally suppressed it under hyperbaric conditions, our next and final step was to test the efficacy of phacoemulsification with cavitation fully suppressed.
The final answer
For this purpose, we performed a series of experiments with real cataract fragments (Figure 3). We developed a technique to feed lens fragments to the phaco probe at ambient and hyperbaric conditions in a controlled manner. Interestingly, we observed that phacoemulsification was performed with equal efficiency under normal conditions and under suppressed cavitation conditions. Essentially, we documented that when cavitation is completely inhibited, the phacoemulsification process is unaltered.
The fact that phacoemulsification occurs when no cavitation is present is conclusive evidence that cavitation plays no role in phacoemulsification, leaving the jackhammer effect as the only important mechanism responsible for the lens disrupting power of phaco.
For more information:
- Jaime Zacharias, MD, can be reached at Clinica Oftalmologica Pasteur, Av. Luis Pasteur 5917, Vitacura, Santiago, Chile; 56-2-520-59-00; e-mail: jamie.zacharias@pasteur.cl.
- The study documented in Jackhammer or Cavitation: The Final Answer” was supported by a research grant by Alcon Laboratories Inc. To see this film, go to ascrs2006.conferencefilms.com.