This article is more than 5 years old. Information may no longer be current.
Improving the thermal safety profile of ultrasound
In any consideration of phacoemulsification surgery, safety is of paramount importance. With phaco, a long-standing concern is limiting thermal energy to the wound. In traditional longitudinal phaco, heat is generated at the incision as a byproduct of frictional energy of the back and forth movement of the phaco needle.
To produce less heat in the longitudinal system, one can reduce the coefficient of friction by providing a protective phaco sleeve, such as the Mackool tip (Alcon Laboratories, Inc., Fort Worth, Texas); reduce the dimensions of the contact surface (reduce the diameter of the shaft of the tip); or reduce power use and/or duty cycle. With traditional, longitudinal phaco, the latter can decrease the efficiency of phaco surgery.
|
Torsional phaco, on the other hand, allows surgeons to decrease thermal risk without resulting loss in phaco efficiency. With the tip operating at 32 KHz, and shaft movement reduced to only about 45 µm, friction is reduced between the tip and sleeve within the wound when compared with traditional longitudinal phaco. That means less heat transfer, effectively reducing the risk for thermal injury or damage to corneal tissue.
In fact, according to Khiun Tjia, MD, an anterior segment specialist in the department of ophthalmology of the Isala Clinics of Zwolle, The Netherlands, with 100% continuous torsional phaco, there is almost no risk for wound burn.1
The need for varying duty cycles, common with longitudinal phaco, is all but eliminated in torsional phaco, according to Dr. Tjia. The reduction in heat transfer and more efficient motion lessen the surgical dependence on less effective duty cycles to avoid thermal injury. Pauses between pulses or bursts are not necessary in torsional phaco, according to Dr. Tjia.
According to Richard J. Mackool, MD, of the Mackool Eye Institute in New York, significant reduction in temperature elevation occurs with torsional phaco (as opposed to longitudinal) as a result of the lack of frictional movement in the shaft of the handpiece.2
As part of his study, Dr. Mackool created “the nightmare situation for cataract surgery” to stress the improved thermal profile of torsional phaco. He used a cadaver eye model, entered through a 2.75-mm incision with no leakage and used 100% torsional or longitudinal ultrasound without aspiration flow. Using thermal imaging, he showed that the temperature in the wound where he used traditional phaco rose three times faster than in eyes in which torsional ultrasound was used.
| Thermal comparison of torsional vs. traditional ultrasound Figure 1. Laboratory model showing a three fold reduction in temperature with torsional ultrasound in burst mode (A). Further testing with intended occlusion in a leak-tight incision (B) demonstrated a temperature increase in 80% ultrasound power application approximately two times a fast as the temperature increase due to 100% amplitude. |
“With traditional ultrasound, the backstroke is useless,” said Dr. Mackool. “In fact, it is counterproductive. It creates heat [in the shaft] and does nothing for the procedure. Torsional cuts in each direction of the movement of the tip and there is no repulsion, making it much more efficient.”
As a result, he said, there is less heat and surgeons can use tighter incisions, so there is the added benefit of less fluid leakage.
“When [there is no] chattering, the surgeon removes nucleus, not balanced salt solution, so less fluid flows through the eye. The less fluid through the eye, the happier the cornea and iris are post-operatively,” said Dr. Mackool. “Torsional phaco surgery is not only more efficient, it is truly less traumatic.”
Many thermal injuries occur as a result of improper control of the handpiece while in the incision, according to Dr. Tjia. Thermal wounds tend to occur, he said, when the tip is pressed against the side of the incision. This action leads to close contact between the vibrating phaco tip and the silicone sleeve. The high friction and heat generated by this maneuver cannot be sufficiently cooled by the irrigation or aspiration flows in and around the tip. Excessive heating of the adjacent corneal stroma and shrinkage of collagen fibers result in a significantly deformed wound, which is difficult to suture and often leads to high induced astigmatism.
Torsional ultrasound, with its dramatically reduced friction within the incision and resulting heat build up, should greatly reduce the incidence of thermal injury or trauma to the incision. “I would strongly recommend the introduction and use of torsional ultrasound in cataract training centers,” Dr. Tjia said.
With respect to thermal wound, torsional phaco might be most beneficial when sculpting and during quadrant removal of the hardest cataracts, when higher levels of ultrasound power are needed in traditional phaco. This is a primary risk factor for thermal injury.
“Personally, I feel absolutely secure concerning wound integrity after starting to use torsional ultrasound in 2005,” Dr. Tjia said. “Thermal injury and trauma has become a nonissue for me in my cataract surgery practice.”
A laboratory model created by Mikhail Boukhny, PhD, a research associate director with Alcon Laboratories, Inc., shows a three fold reduction in temperature with torsional phaco due to the reduced tip stroke at the incision, as well as a lower operating frequency. The model used a miniature thermocouple attached to the outside of the infusion sleeve inserted directly in the incision, and with 50-ms pulses of both torsional and traditional ultrasound applied periodically. Temperatures spiked as a result of propagation of heat from the source of generation and dipped as heat dissipated due to conductivity to surrounding fluidics and eye tissues. (Figure 1)
To test this model further, Dr. Boukhny and colleagues purposely occluded the aspiration line and created a leak-tight incision to eliminate cooling. The research showed that the temperature increase as a result of 80% ultrasound power application was approximately two times a fast as the temperature increase due to 100% amplitude.
References
- Tjia K. Microcoaxial torsional phacoemulsification for 2.0 to 2.2 mm incision cataract surgery. Presented at: Annual Meeting of the American Society of Cataract and Refractive Surgery; March 17-22, 2006; San Francisco.
- Mackool R. Lens removal/torsional phacoemulsification: advantages of nonlinear ultrasound. Presented at: Annual Meeting of the American Society of Cataract and Refractive Surgery; March 17-22, 2006; San Francisco.