To understand cavitation, we must first ask the fundamental question: What is ultrasonic power? When a surgeon steps down on the foot pedal to increase power, what he is actually doing is increasing the stroke, or back-and-forth excursion of the phaco needle. According to Anton Banko, founder of the Surgical Design Corporation (Armonk, New York), and the engineer who co-invented phacoemulsification, stroke creates four components of power: (1) acoustical wave, (2) mechanical impact of the tip, (3) fluid wave, and (4) cavitation.
THE FOUR COMPONENTS
Acoustical wave. Seldom discussed in the context of phacoemulsification, the acoustical wave has been photographed with shadow field photography. This wave travels at speeds up to 5,400 km/h. According to Mr. Banko, the acoustical wave imparts an oscillatory motion to the nucleus, which begins to break down intermolecular bonds and initiate the emulsification process.
Mechanical impact. The so-called jackhammer effect occurs as the needle accelerates forward and impacts the nucleus. The tip can reach velocities up to 72 km/h. The nucleus is essentially jackhammered between 28,500 to 50,000 times per second, depending on the phaco machine.
Fluid wave. Resulting from the same forward acceleration of the needle, the fluid wave pushes fluid and lens particles ahead from the surface of the needle at velocities up to 72 km/h. This wave tends to oppose followability and outflow.
Modern phaco technology must balance fluidics and power for efficiency. There must be adequate evacuation flow to overcome the fluid wave and draw the nucleus to the tip. Once occluded, the tip must produce adequate vacuum holding force for the jackhammer effect, and the chamber must remain stable. There is more than one way to accomplish this.
Torsional phaco (Ozil; Alcon Laboratories, Inc., Fort Worth, Texas), derives its efficiency by redirecting the fluid wave and jackhammer forces perpendicular to the longitudinal axis of the needle. This is said to reduce microchatter and improve followability; however, it may create other unwanted effects.
Teruki Miyoshi, MD, and Hironori Yoshida, MD, described another alternative. In their recent publication,1 they used high-speed video techniques to evaluate lens chatter with micropulses of 8 milliseconds of on time and 4 milliseconds off time. The investigators noted a visual demonstration of hyperfollowability, a term they coined and described as "once captured by an ultrasonic tip, nuclear fragments are emulsified and aspirated more efficiently (with micropulse) as if they were automatically rotating without drifting away from the tip."
Cavitation. To understand hyperfollowability more fully, we must first define the fourth component of ultrasonic power, transient cavitation. Cavitation bubbles are formed immediately in front of the vibrating phaco needle. As the needle accelerates forward, fluid is pushed ahead during the compression cycle. As the needle reverses, the fluid cannot follow, creating a low-pressure area. Cavitation bubbles begin to form during the expansion cycle. The needle then accelerates forward, again compressing (but not collapsing completely) the new cavitation bubble. The amount of gas diffused into or out of the bubble depends on its surface area. During the expansion cycle, more gas diffuses into the larger bubble than can diffuse out during the compression cycle. Over a series of cycles, the cavitation bubble grows until it reaches its resonant size—when it can no longer absorb energy—and implodes, releasing its energy.
The amount of energy is astonishing—heat up to 1,000° C, hotter than the surface of the sun (5,506° C) and pressures up to 1,000 atmospheres, equivalent to being at the bottom of the ocean (1,033 kilograms/cm2). Cavitation also releases energy in the form of a unique phenomenon called microjets.2 As the cavitation bubble implodes, a small microjet of fluid is formed and passes through the bubble, striking the nearest hard surface at velocities up to 400 km/h.
With such extreme energies, why doesn't cavitation destroy the eye? The scientist Kenneth S. Suslick, PhD, explains: "the heat and pressure from imploding cavitation bubbles occur in microscopic spaces in less than a microsecond. At any given time, therefore, the bulk of the liquid remains at ambient temperature."3
Mark E. Schafer, PhD, used acoustical measurements to show that a narrow beam of cavitation, less than 1 mm wide, is formed in front of the tip.4 In a separate experiment, he also demonstrated the following: More cavitation (ie, cutting) energy is delivered with pulses than with continuous ultrasound at the same power (ie, stroke) because during the off portion of the pulse cycle, more time is available for cavitation bubbles to grow and reach their resonant size (Figure 1).
The amount of energy released from a cavitation bubble is proportional to its volume. We know that at 28.5 KHz (as with Bausch & Lomb's [Rochester, New York] system), the average diameter of the cavitation bubble is 115 µm; at 40 KHz (as with the systems from Alcon Laboratories, Inc. and Advanced Medical Optics, Inc., Santa Ana, California]), the bubbles are 82 µm. When the larger bubble implodes, it releases 2.68 times the energy of the smaller bubbles. Only cavitation at the tip or inside the needle is useful to emulsify the lens and lower frequency pulsed power will deliver more energy.
MAY LEAD TO FREE RADICAL FORMATION
But what about torsional cavitation? At the European Society of Cataract and Refractive Surgeons (ESCRS) annual meeting in 2006, Jamie Zacharias, MD, of Chile reported that torsional phaco significantly reduced the "undesirable aspects of traditional phaco, such as cavitation."5 He also reported that cavitation may lead to free radical formation, loss of efficiency, induction of turbulence, and direct or indirect endothelial damage.
Subsequent to these statements, Dr. Schafer studied cavitation during torsional phaco and did not detect any cavitation when the acoustical probe was placed directly in front of the phaco tip; however, substantial cavitation formed along the sides of the needle.6 He concluded that this cavitation forms beams of energy away from the cataract. The surgical implication is that although torsional shaft cavitation does not emulsify the lens, it may lead to free radical formation along the sides of needle.
Free radicals are formed by sonolysis, a unique chemical phenomenon associated with cavitation. With the extreme temperatures and pressures created by cavitation, water molecules can be split, undergo secondary reactions with oxygen, and form superoxide radicals and hydrogen peroxide.7
Hydroxyl radicals, which are nonselective, react with any organic substrates present in the sonicated medium.8 Additionally, cavitation may not only generate cellular and intracellular damage but it may also affect tissues by chemical electromagnetic radiation or thermal or mechanical mechanisms.9 Cellular damage would affect endothelial cell membranes and would be expected to occur immediately. Intracelluar damage, on the other hand, may manifest itself over a longer time and may not be evident in the immediate postoperative period.
Conceptually, with longitudinal phaco, any free radicals formed are immediately in front of the vibrating tip and in position to react immediately with lens material or be quickly aspirated. Free radicals formed along the shaft cannot react with lens material at the tip and may persist longer in the eye. The intraocular effects of free radicals deserve further study.
With this as background information, we can examine the controversy surrounding cavitation. Dr. Zacharias won the 2006 American Society of Cataract and Refractive Surgery (ASCRS) film festival with his hyperbaric chamber study on cavitation.10 Performing phaco on a divided human lens in a hyperbaric chamber, Dr. Zacharias effectively turned cavitation off and on by increasing or decreasing the chamber's pressure.
In his study, Dr. Zacharias observed the cutting effect under both conditions and concluded that cavitation did not play a role in cutting cataract tissue, which contradicts other research.
In 1894, two British nautical engineers observed that cavitation was responsible for damaging ship propellers. In the following century, numerous investigations and publications showed that cavitation is useful for cutting metal and other substances.11,12 It is also used to cut bone in orthopedics13 and to cut other tissues in neurosurgery, urology, and plastic surgery.
Dr. Schafer studied the value of cavitation in ophthalmology. In 2007, he performed his own hyperbaric chamber experiments, suspending a phaco handpiece and needle by a solenoid above an artificial lens material equivalent to a grade 3+ cataract. He lowered the phaco tip onto the material and measured its rate of cutting to 5 mm with cavitation turned on and off (personal communication, 2008; Figure 2). As the phaco needle is lowered onto the lens material, there is an initial indentation of approximately 2 mm. At 20% power with cavitation active, the needle penetrated the artificial lens in approximately 4.7 seconds (average of 11 experiments); however, with cavitation suppressed, 20% power failed to perform any additional cutting.
When Dr. Schafer increased the power to 60%, the needle could penetrate the lens without cavitation; however, it required approximately 10 seconds. Although the jackhammering produced a coring effect, it required more energy without cavitation and was also prone to clogging, which seems to be clinically supported by various surgeons who consult for Alcon Laboratories, Inc.
Dr. Zacharias, for example, recommended using axial power with torsional power for grade 4+ cataracts, which would prevent occlusion of the phaco tip.2 Similar observations have been made by Takayuki Akahosi, MD, of Japan; Stephen S. Lane, MD, of Minnesota; and David Allen, BSc, FRCOphth, of the United Kingdom.
IS THERE AN EXPLANATION?
How do we explain these two hyperbaric experiments that appear to have reached opposite conclusions? One possible explanation is that if phacoemulsification relies on the combined energies of both the jackhammer effect and cavitation, then it stands to reason that if we maximize one we can minimize the other without necessarily observing a difference in cutting. During Dr. Zacharias' experiment, the ultrasonic energy on the display showed 100%. He used a vacuum of 454 mm Hg. In contrast, Dr. Schafer eliminated the vacuum variable and conducted his studies using the constant force of gravity. He also studied the effect at more clinically relevant powers of 20% and 60% and repeated his measurements 11 times to assure reproducibility.
Therefore, it appears that the most likely mechanism of action during phacoemulsification is: first, the jackhammer cutting burrows into the nucleus, and second, the cavitational energy emulsifying the lens into smaller particles that can be easily aspirated.
Failure to recognize and control the powerful effects of cavitation may lead to undesirable clinical outcomes. For example, Alcon calculates the cumulative dissipated energy as: torsional amplitude (torsional stroke) X torsional time X 0.4, stating that "the factor 0.4 represents approximate reduction of heat dissipated at the incision as compared to conventional phaco."14
One potential problem with this method is that it considers only one source of energy (ie, mechanical or frictional energy) at only one location (ie, the incision). Furthermore, reducing the calculation by 60% (multiplying by 40%) gives the impression that any case performed with torsional automatically uses less energy than an identical case performed with longitudinal. In fact, torsional cavitation energy in the incision is actually increased because of (1) the lower frequency (32 KHz) and (2) because it is produced on both sides of the needle as opposed to just in front of the tip on the back stroke.
One possible consequence of this was presented at the 2008 ASCRS Film Festival by Ismail Hamza, MD, of Egypt. His submission reported on 400 cases during a single year using sleeveless bimanual phaco. He encountered a 3.5% burn rate (14 cases).15 This occurred while using pulse power modulation, which would predictably amplify the cavitation energy occurring along the shaft of this 32 KHz technology.
As a point of reference, I recently reviewed the outcomes from 19,300 of my cases, extending 10 years and ending in May 2008, with the Millenium and Stellaris (both manufactured by Bausch & Lomb) power modulation. I encountered no cases of wound burn.
In my opinion, it is critical that we objectively evaluate the true effects of ultrasonic power so that we may better harness this energy for useful work. Manufacturers need to develop accurate methods to calculate all the clinically relevant energies delivered to all parts of the eye from the phaco needle. Such a calculation must ultimately account for all four components of ultrasonic power as well as their side effects, such as free radicals. The true test of their success will be the ability to show a direct correlation between the calculated energy and clinically measured outcomes, such as endothelial counts, postop inflammation, and energy-related complications, including incision burns.
Terence M. Devine, MD, is Section Chief of Ophthalmology at Guthrie Health, Sayre, Pennsylvania. Dr. Devine states that he is a paid consultant to Bausch & Lomb but has no financial interest in the company or its products. He may be reached at e-mail: firstname.lastname@example.org.
Cavitation does not play a role in phacoemulsification.
By Jaime Zacharias, MD
Cavitation—a physical phenomenon that occurs in liquids in zones of low pressure—has been associated with phacoemulsification for a long time. Recently, however, the role that cavitation may have in the phacoemulsification process has turned controversial. Some surgeons advocate its role as beneficial, claiming that cavitation enhances lens tissue disruption. Others negate its effect, even suggesting deleterious outcomes, such as free radical production, increased turbulence, and reduction in cutting efficiency.1,2
During cavitation, small bubbles form in the areas of liquid that experience a drop in pressure lower than that of the liquid's vapor. The pressure drop can either be permanent or short-lived (when the pressure in the liquid alternates between periods of low and of high pressure), leading to steady or transient cavitation, respectively (Figure 1). The violent collapse of cavitation bubbles release shockwaves that produce most of the energetic effects associated with cavitation. In fact, cavitation can be a nightmare for hydraulic engineers because it reduces efficiency and destroys parts of hydraulic pumps and improperly designed ship propellers.
Ultrasonic energy is recognized as an effective pathway to induce cavitation in the liquid state. The ultrasonic waves create zones of intermittent low pressure, thus inducing cavitation bubble formation. Its benefits have been exploited in ultrasonic baths and sonochemistry, where implosions of the small transient bubbles remove contaminant particles and speed up chemical reactions, respectively.
It appeared natural to associate cavitation with ultrasonic lens removal, and initially, cavitation was mentioned as a remote force capable of disrupting the lens tissue without direct contact between the lensectomy probe and the cataract. In fact, investigators first thought that cavitation produced its disruptive effect as a concentrated wave at a focal point located millimeters away from the phaco tip.3,4 At that time, the only way to detect cavitation was by indirect measurement, oftentimes using hydrophones. The presumed noncontact effect between the phaco probe and the cataract was not observed during real surgery, where direct contact between the metal phaco tip and the lens tissue was required for lens disruption.5 This discovery led us to further study the phenomenon some years ago.
We used a stroboscopic method (based on Nyquist theorems) to observe the ultrasonic motion patterns of phaco probes in slow motion. The slow motion video images captured footage of the phaco probes oscillating at 40 kHz with an equivalent frame rate of 10 million frames per second. This was the first time that cavitation was directly observed in association with phacoemulsification. In contrast to earlier suggestions that cavitation occurs in a remote location, our videos clearly showed it occurring in precise areas in close proximity to the tip of the phaco probes. The cavitation only occurred at phaco powers above a threshold of approximately 50%. Additionally, the cavitation was transient, meaning that the bubbles were created and destroyed during single cycles of ultrasonic tip motion (Figure 2A).
The location and distribution pattern of the cavitation bubbles, induced by ultrasonic activation of the lensectomy probes, further suggested that this form of cavitation was not acoustically induced by sonic waves but rather was fluid mechanical. In this mechanism, the fast backward displacement of the phaco probe rim reduces local pressure in the surrounding liquid. Our findings were presented at the 2002 annual meeting of the American Society of Cataract and Refractive Surgery (ASCRS).6
Our research allowed direct visualization and definition of relevant aspects of cavitation in phacoemulsification; however, it did not address its role in the phacoemulsification process. There was significant controversy surrounding the effects of cavitation. Were they beneficial, neutral, or undesirable in phacoemulsification? The controversy took hold in the industry, and some manufacturers started to design equipment that enhanced cavitation to improve phaco efficiency.
We conducted a three-stage study to determine the role of cavitation in phacoemulsification efficiency.7,8 First, the synchronous, video-photographic technique was updated, thus allowing clearer images of the individual cavitation bubbles, including the presence and location of the microbubbles (Figure 2B). Cavitation was then controlled without altering other parameters of the phacoemulsification environment. In this stage, the environmental pressure was manipulated and the threshold of ultrasonic power required for cavitation bubbles to appear was modified. A hyperbaric chamber was assembled with the complete phacoemulsification fluidics elements (ie, irrigation, aspiration, phaco handpiece; Figure 3), and the probe was inserted into the observation chamber (Figure 4). Cavitation bubbles naturally appear in zones where local pressure drops below the liquid's vapor pressure (Figure 5A); however, by increasing the overall pressure inside the hyperbaric chamber, the threshold power level at which bubbles appear shifts upward. When the environmental pressure was raised to 2 bars (approximately 2 ATM or 29 PSI) above atmospheric pressure, we were capable of shifting the power threshold beyond 100% phaco power. At this pressure level, cavitation bubbles were no longer observed throughout the entire range of available phaco powers (Figure 5B).
With stages 1 and 2 completed, we were now able to (1) clearly visualize cavitation and (2) suppress it at will by increasing environmental pressure. Our final goal was to determine if cavitation played a role in phaco. Stage three consisted of feeding the phaco probe with real cataract fragments and determining the efficiency of lens emulsification with and without cavitation; all other conditions were identical (Figure 6). Because cataracts are nonhomogeneous and can differ significantly within samples, we used the same cataract segments for testing and alternated between cavitation-enabled and cavitation-suppressed by quickly changing the environmental pressure between atmospheric and +2 bar, respectively.
The depth of the cut made into the cataract sample after single 100% power, 100 millisecond bursts was compared with and without cavitation. On average, single bursts of phaco energy at 100% power produced an advance of 380 µm of tissue with cavitation present (normal, ambient conditions) and 490 µm with cavitation suppressed at the elevated ambient pressure. These results demonstrated that suppression of cavitation did not reduce the efficacy of emulsification, given a 1.33 ±0.37 cut-rate ratio between phaco without cavitation and phaco with cavitation. Concluding that absence of cavitation definitively enhances the maximum efficiency of phaco by 30% could be considered speculative at this time; however, some theoretical considerations may suggest that the presence of cavitation bubbles creates a gaseous barrier between the lens material and cutting edge of the emulsifying tip. This phenomenon may actually inhibit material from reaching the metal edge—where it is most effectively being cut.
We have comprehensively documented cavitation, developed a reproducible technique to controllably suppress it, and demonstrated that cavitation does not enhance—and could potentially even degrade—the efficiency of phaco. These observations, paired with the known downsides of cavitation, including undesirable free radical formation and increased turbulence, allow us to conclude that cavitation should be avoided during phacoemulsification. As ophthalmologists, we should promote the use of efer phaco systems that operate with reduced or minimal cavitation.
Jaime Zacharias, MD, is an Assistant Professor of Ophthalmology at the Pasteur Ophthalmic Clinic, Santiago, Chile. Dr. Zacharias states that he is a paid consultant to Alcon Laboratories, Inc. He may be reached at e-mail: email@example.com.