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Scientific models and laboratory performance
Clinical and field-testing of new technology is critical to demonstrate improvements over the existing design, but just as important are experiments that can demonstrate improvements in closed, controlled laboratory settings. If improvements can be shown in the lab environment, they can be extrapolated to the real world. Sometimes these lab experiments can help to answer the critical “why” questions that surround a new innovation.
The case of torsional ultrasound is no different. Decreased repulsion resulting in improved emulsification with reduced balanced salt solution usage and an improved thermal safety profile are important markers of a better system. The mechanism by which torsional leads to better phacoemulsification can be measured and answered in lab studies, where testers are better able to study the parameters of phaco and perform testing that would not be possible during surgery.
Creating a fluidics model
Kerry D. Solomon, MD, a professor of ophthalmology at the Storm Eye Institute and director of the Magill Research Center at the Medical University of South Carolina, Charleston, constructed a unique model to demonstrate the lack of repulsion and improved efficiency with torsional ultrasound. His model may be the first clear visual model to demonstrate the fluidics inside the eye during phaco surgery. He and his team developed this model to evaluate intraocular fluidics and applied it to both torsional phacoemulsification and traditional longitudinal phacoemulsification.
The team used microcarrier beads (SoloHill Engineering, Ann Arbor, Mich.) with a core material made of cross-linked polysterene, and a surface coating of collagen in conjunction with 30 fresh porcine eyes and 16 fresh human globes from bilateral donors (that were not suitable for transplantation). Each bead diameter was between 125 to 212 microns with a cumulative surface area of 390 cm2 per gram. These beads had a specific gravity of 1.02, which meant they would stay suspended in balanced salt solution and not immediately collect at the bottom of the anterior chamber.
The beads, suspended in balanced salt solution, were injected at a steady rate by Luis De Castro, MD. Phaco surgeries were video recorded in the anterior view and from the side view with high-speed cameras capable of capturing 300 frames per second. Torsional phaco surgeries were performed using the Infiniti Vision system with the OZil handpiece, and traditional phaco surgeries were performed with the Legacy 20000 (Alcon Laboratories, Inc., Fort Worth, Texas).
The use of the microcarrier beads allowed Dr. Solomon to demonstrate a lack of repulsion, reduced chatter and increased followability with torsional vs. traditional phaco. Using the high-speed camera footage, Dr. Solomon and his team were able to manually trace bead movement within the anterior chamber.
In the first phase of the analysis, Dr. Solomon showed that torsional phaco was more efficient at clearing the beads from the anterior chamber of the eyes, with results showing that torsional ultrasound cut clearance times in half. “This may be why clinicians are finding that their procedures are smoother,” said Dr. Solomon, “because the whole system is more efficient.” (Figure 1)
| Comparison of clearance rates of microcarrier beads Figure 1. Torsional phaco, regardless of power settings, showed reduced clearance times vs. traditional ultrasound in Dr. Solomon's study.2 |
Fluidic model demonstrates reduced turbulence
The second phase of Dr. Solomon’s analysis focused on why torsional phaco was more efficient at removing the microcarrier beads. Showing high-speed camera footage from a traditional pulse phaco surgery, Dr. Solomon showed that although some of the beads, suspended in fluid, moved towards the tip, some beads drifted away from the vacuum due to the motion of the vibrating tip. This repulsion was further demonstrated when Dr. Solomon isolated 10 beads at random and animated the flow of motion – as some of the beads moved towards the tip, they were repelled, only to be drawn back by the fluid in the eye. (Figure 2)
| Visualizing fluidics with high speed video Figure 2. Visualizing repulsion (orange arrows) and movement in the anterior chamber (green arrows) with longitudinal phaco (A) and torsional phaco (B). Blue arrows show beads moving towards the tip.2 |
Again using high-speed camera footage, Dr. Solomon repeated the experiment using actual lens material. Analysis of bead movement in traditional phaco, with nuclear material included, demonstrated that when occlusion was broken, fluidics did not drive the beads into the tip as efficiently. Repulsion of nuclear material following occlusion was observed.
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The opposite was seen on high-speed camera footage of torsional phaco surgery. Instead of the repulsion typical of traditional ultrasound, the microcarrier beads moved towards the phaco tip.
“There are little eddies and curves of the beads, but once occlusion is broken these beads find their way to the tip, and there is really not much repulsion. The beads find their way much more efficiently to the tip in torsional phaco,” said Dr. Solomon.
“In conventional ultrasound, the lens material itself is being repelled away from the tip, the fluid itself is being repelled from the tip, so some of the efficiencies in torsional ultrasound are both fluidic in nature as well as the action of the lens material,” said Dr. Solomon. “Part of the reason that the lens may stay at the tip better is that the fluidics are driving everything right to the tip, and part of it is that the tip is not pushing the lens material away.”
Modeling emulsification force
Using simulated cataracts, Mikhail Boukhny, PhD, a research associate director with Alcon Laboratories, Inc., demonstrated that torsional movement of the cutting tip requires less force to cut through dense material with less thermal energy than linear movement of the tip.3 He also found that the handpiece was audibly quieter than the conventional ultrasound handpiece.
Human cataracts are difficult to simulate, given the variability in patient and surgical circumstances. However, the study was useful, Dr. Boukhny said, because it focused on reproducing cutting efficacy rather than simulating a human cataract. Using a hard polymer “closest in consistency to the very hardest cataracts,” Dr. Boukhny found that conventional phaco required 1.63 oz of force to cut through a simulated cataract, compared to 0.86 oz of force for torsional. Combining torsional and longitudinal ultrasound in a continuous cycle required even less force: 0.35 oz of pressure. (Figure 3)
| Force needed to break cataract in lab model
Figure 3. Dr. Boukhny found a significant reduction in the amount of force needed to break simulated cataracts in a laboratory model.3 |
In combination mode, which may be used for very dense cataracts, it is typical to use the torsional phase for 80 ms followed by 20 ms of traditional, Dr. Boukhny said, adding that longitudinal is set at half the power of torsional, so incremental increase in heat generation at the incision is small.
To better understand and study the thermal effects of torsional phaco, Dr. Boukhny opted to increase the chance of thermal injury, purposely moving the tip off center to apply more force to the incision. With conditions and settings being equal, torsional ultrasound performed “much better from the thermal standpoint than traditional ultrasound.”
“Our analysis, both theoretical and experimental, predicts about a three-fold reduction in temperature increase. This is because temperature rise is approximately proportional to the stroke of the tip at the incision and to the frequency with which the tip moves at the incision,” said Dr. Boukhny.
“We know from our measurements that stroke at the incision is reduced with the bent Kelman phaco tip by a factor of two in torsional mode compared with the same tip driven in conventional longitudinal mode. That thermal difference is also impacted in that torsional is at a lower frequency at 32 KHz, about 20% lower than the 40 KHz used during traditional, longitudinal phaco.” Dr. Boukhny said.
Another outcome of the study was that the OZil torsional handpiece was far quieter than the traditional handpiece, which may be a marker for less cavitation compared to traditional phaco.
References
- Solomon K. Performance of the Infiniti System: torsional vs conventional phacoemulsification handpieces. Presented at: Annual Meeting of the American Society of Cataract and Refractive Surgery; March 17-22, 2006; San Francisco.
- Solomon K. Longitudinal vs torsional phacoemulsification. Presented as a sponsored booth presentation at: Annual Meeting of the American Society of Cataract and Refractive Surgery; March 17-22, 2006; San Francisco.
- Boukhny M. Laboratory performance comparison of torsional and conventional longitudinal phacoemulsification. Presented at: Annual Meeting of the American Society of Cataract and Refractive Surgery; March 17-22, 2006; San Francisco.