Phacoemulsification was first established by Kelman in 1967.1 Since that time, the technique has undergone almost 45 years of improvement to become the dominant surgical cataract treatment used in hospitals today. However, potential damage induced by thermal energy around the tip during phacoemulsification has always been a concern because the heat can burn the corneal tissue, leading to postoperative corneal damage.2–4
Ultrasound (US) phacoemulsification uses piezoelectric crystals to convert electrical energy to mechanical energy, which causes rapid vibration of the phaco tip at frequencies between 28 kHz and 40 kHz. It has been proposed that the cause of the temperature rise may be the amplitude of the US tip in conjunction with the number of US vibrations. As a result, there has been a diversification of the vibration styles in different phacoemulsification machines.5,6
Most studies that examined thermal energy production during ultrasonic phacoemulsification specifically focused on wound injury to the cornea. However, few studies have assessed the change in temperature in the anterior chamber during phacoemulsification.7,8 Several authors4–7,9 have proposed methods that use a thermal camera to measure changes at the wound. Unfortunately, it has been difficult to measure extremely fine temperature changes in the anterior chamber during phacoemulsification using a thermal camera only. Therefore, we performed a study to evaluate the change in the anterior chamber temperature during an actual surgical procedure that used US oscillation in porcine eyes.
Materials and Methods
Phacoemulsification was performed in porcine eyes obtained from a local abattoir using 2 common phacoemulsification units, the Stellaris 28.5 kHz device (Bausch & Lomb) and the Whitestar Signature 40 kHz device (Abbott Medical Optics, Inc). The peristaltic pump mode of the phacoemulsification machines was used during all procedures. The surgery was performed via a superior 2.4 mm corneal incision with aspiration.
Instead of the usual hook into the anterior chamber, the microprobe of the device was inserted from the 3 o’clock direction and then the temperature probe was secured in front of the US tip (Figure 1). Temperature changes were measured and recorded for 1 minute 30 seconds during US generation. Also measured were variations in the temperature that occurred after the settings were changed and when different methods were used to generate US. Temperature measurements were obtained using the SE-305 (Thermo Datalogger, Center Technology Corp.). Phacoemulsification was performed using a 20-gauge, 30-degree tip for the Stellaris Microflow 2.2 and the Signature Laminar 20-gauge US tip.
Experiment 1: Ultrasound Power
First, the effect of the US power setting on the changes in the temperature was evaluated. In this experiment, the 28.5 kHz phaco unit was used. Commonly used settings were determined for this machine, with the aspiration set at 18 mL/min, vacuum pressure at 50 mm Hg, and a bottle height of 50 cm. Ultrasound power was raised in a stepwise fashion, with measurements obtained at the following levels: 15%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%. All temperatures were recorded and compared during the 1 minute 30 second observation period.
Experiment 2: Ultrasound Frequency and Stroke Length
In this experiment, the effect of the 40 kHz phaco unit and the 28.5 kHz phaco unit was compared at the 100% power setting. Based on the product specifications, the stroke of the 28.5 kHz phaco unit at the 100% power setting is apparently longer than that of the 40 kHz phaco unit (stroke length 138 μm versus 89 μm). Therefore, the effect at a 66% power setting for the 28.5 kHz phaco unit was also measured because this was thought to be equivalent to the 100% power setting of the 40 kHz phaco unit. All temperatures were recorded and compared after 1 minute 30 seconds. Five porcine eyes were used for the experiments in each group.
Experiment 3: Aspiration Flow Rate
This experiment used the 28.5 kHz phaco unit to measure changes in the temperature in conjunction with variations in the aspiration flow rate. Commonly used settings were determined for this machine, with the US power set to 100% and a bottle height set to 50 cm. Measurements were obtained for 3 device settings that included vacuum pressure set at 0 mm Hg with an aspiration flow rate of 18 mL/min, vacuum pressure set at 50 mm Hg with an aspiration flow rate of 18 mL/min, and vacuum pressure set at 50 mm Hg with an aspiration flow rate of 35 mL/min. All temperatures were recorded and compared during the 1 minute 30 second observation period.
Experiment 4: Horizontal and Longitudinal Vibration
This experiment used the 40 kHz phaco unit to assess the effect of the horizontal vibration. The results were compared with the findings for elliptical vibration (Ellips FX) and normal longitudinal vibration. Commonly used settings were determined for the machine, with aspiration set at 18 mL/min, vacuum pressure set at 50 mm Hg, and bottle height set at 50 cm. Four US power levels were examined, with the levels raised in a stepwise fashion as follows: 20%, 40%, 60%, and 80%. All temperatures were measured after 1 minute 30 seconds of exposure to the power level. Five porcine eyes were used for the experiments in each group.
Experiment 5: Pulse and Clinical Setting
This experiment used the 28.5 kHz phaco unit to evaluate the effect of the pulse mode. The parameters were as follows: US power set at 100%, vacuum pressure set at 50 mm Hg, aspiration flow rate set at 18 mL/min, and bottle height set at 50 cm. Then, the continuous mode and pulse mode in which the US rate was set for an on time of 8.0 mm seconds and an off time of 5.0 mm seconds were compared. Further examined was a practical setting currently used in the surgery of grade 2 to 3 nuclei; power was set at 20%, vacuum pressure was set at 150 mm Hg, the aspiration flow rate was set at 18 mL/min, and bottle height was set at 50 cm. The pulse mode was set at an on time of 8.0 mm seconds and an off time of 5.0 mm seconds.
In experiments 2 and 4, the Student t test was used to compare results. All data were analyzed using Excel 2010 software (Microsoft Corp.) on a personal computer. A P value less than 0.05 was considered statistically significant.
Experiment 1: Ultrasound Power
Figure 2 shows the temperature changes. The US vibration not only caused an incision but also increased the temperature in the anterior chamber. A rise in the temperature occurred soon after the US was applied, with the temperature becoming constant after approximately 20 seconds. The temperature rise was in the range of 2°C to 20°C. Also, the heat rise was proportional to the length of the amplitude of the US tip.
Experiment 2: Ultrasonic Frequency and Stroke Length
Figure 3 shows the results of the 2 phaco machines. When the power was set at 100%, the 28.5 kHz phaco unit induced a significantly higher temperature than the 40 kHz phaco unit. Because the 28.5 kHz phaco unit has a longer stroke than the 40 kHz phaco unit, it was thought to be more susceptible to heat generation. When the power of the 28.5 kHz phaco unit was set to 66%, a setting at which the stroke length should be similar to that of the 40 kHz phaco unit, temperatures for both machines were not statistically significantly different.
Experiment 3: Aspiration Flow Rate
When the vacuum was set at the same value (50 mm Hg), the temperature rise was inversely proportional to the aspiration rate (Figure 4). However, when the vacuum was set at the extremely low value (0 mm Hg), the temperature rose, even with a sufficient aspiration rate (18 mL/min). This seems to be because the actual aspiration value was lower than the setting value. With the 0 mm Hg vacuum setting, the actual aspiration value detected by the device was about 6 mL/min, even when the aspiration rate was set at 18 mL/min.
Experiment 4: Horizontal and Longitudinal Vibration
When the US power settings were low, the temperatures in the anterior chamber were similar for both the longitudinal vibration and the elliptical vibration (Figure 5). When the US power settings were high, elliptical vibration led to a significantly lower temperature in the anterior chamber than the longitudinal vibration.
Experiment 5: Pulse and Clinical Setting
The commonly used pulse mode was effective in preventing the temperature rise in the anterior chamber (Figure 6). When clinical settings for the surgery of grades 1 to 3 nuclei were used, the rise in the anterior chamber temperature was very small.
In phacoemulsification, potential damage induced by the thermal energy around the tip during surgery has always been a concern because this heat can burn the wound.2–4 Before these experiments, we had strong expectations that US vibration would not only result in an incision but would also have the potential to increase the temperature in the anterior chamber. Our current findings support our theory.
In experiment 1, our data clearly showed that increases in US power caused an increase in the temperature in the anterior chamber. The temperature rise was in the range of 2°C to 20°C depending on the US power. Changes in US power settings are accomplished by changing the stroke length. Therefore, extending the stroke length will cause a rise in the anterior chamber temperature. It took only a few seconds after the US power was turned on for the temperature in the anterior chamber to rise. A US power setting of more than 50% caused a temperature rise that could exceed 40°C. This indicates that the continuous mode of the US high power setting should not be used, even when the on time is as short as a few seconds.
Experiment 2 compared the Stellaris 28.5 kHz phaco unit and the Signature 40 kHz phaco unit. The 28.5 kHz machine tended to cause a higher temperature. We further confirmed that this rise in anterior chamber temperature was the result of the extension of the stroke and not due to the frequency. At a power setting of 100%, the stroke of the 28.5 kHz phaco unit was significantly longer than that of the 40 kHz phaco unit (138 μm versus 89 μm). When 100% US power was compared between the 2 machines, there was a significantly increased temperature with the 28.5 kHz phaco unit compared with the 40 kHz phaco unit. The difference in the temperature rise was approximately 4°C. However, when the machines were set to use approximately the same stroke length, no significant difference was noted between them. A longer stroke creates more friction, resulting in a higher tip temperature. Thus, this result suggests that the anterior chamber temperature cannot be lowered by reducing the US frequency only. Therefore, surgeons should also pay attention to stroke length. Our findings are consistent with those in a study by Mackool and Sirota.5 They compared 3 phaco devices and demonstrated that the tip temperatures for the Legacy unit (Alcon) were consistently lower than those for the Sovereign Whitestar (Abbott Medical Optics) and Millennium (Bausch & Lomb) devices. They suggested that the reason for these findings was related to the longer stroke lengths of the Sovereign Whitestar and Millennium tips.
Experiment 3 underscored the importance of adequate irrigation/aspiration (I/A), which may play a significant role in cooling the tip and provide a way of preventing rises in temperature. When we used an extremely low vacuum setting, the actual aspiration rate was far lower than the setting value. Thus, a low vacuum setting can cause heating of the anterior chamber.
There are a large variety of US generation methods in use today.10 In the current study, we were particularly interested in whether the Ellips FX elliptical mode could affect the changes in anterior chamber temperature. In experiment 4, heat generation by the elliptical mode was significantly lower than that by the longitudinal mode at the high US power setting. Because the elliptical vibration is a combination of horizontal and longitudinal movements, the suppressed heat generation seems to be due to a reduction in the longitudinal vibration.11 Our findings support the notion that longitudinal vibration is the major factor in heat generation.
In experiment 5, we compared the effect of 3 settings; that is, continuous mode, pulse mode, and a practical setting that is currently used in the surgery in eyes with grade 2 to 3 nuclei. The pulse mode was apparently effective in preventing the temperature rise, which suggests that the temperature rise depends on the total quantities of the US energy. Turning the US vibration on and off is not only effective for making the incision12,13 but can also suppress the rise in anterior chamber temperature. When the practical setting was used, the temperature rise was minimal, which suggests that not only the mode of US oscillation but also the balance of I/A is important.
In summary, for phacoemulsification that is safer to the corneal tissue, the anterior chamber temperature should be carefully taken into consideration. To prevent increases in the temperature of the anterior chamber, surgeons should pay attention to the following points: (1) Reduce continuous vibration at high US power settings, even if for very short periods of time. (2) Use the pulse mode when raising the US power. (3) Maintain an appropriate infusion volume. (4) Use horizontal vibration.
What Was Known
- Potential damage induced by thermal energy around the tip during phacoemulsification has always been a concern because the heat can potentially burn the corneal tissue.
What This Paper Adds
- The aqueous humor temperature rose proportionally to the amount of US energy.
- The pulse mode or elliptical vibration can inhibit the temperature rise.
1. Kelman CD. Phaco-emulsification and aspiration; a new technique of cataract removal; a preliminary report. Am J Ophthalmol
2. Bissen-Miyajima H, Shimmura S, Tsubota K. Thermal effect on corneal incisions with different phacoemulsification ultrasonic tips. J Cataract Refract Surg
3. Ernest P, Rhem M, McDermott M, Lavery K, Sensoli A. Phacoemulsification conditions resulting in thermal wound injury. J Cataract Refract Surg
4. Osher RH, Injev VP. Thermal study of bare tips with various system parameters and incision sizes. J Cataract Refract Surg
5. Mackool RJ, Sirota MA. Thermal comparison of the AdvanTec Legacy, Sovereign WhiteStar, and Millennium phacoemulsification systems. J Cataract Refract Surg
6. Olson MD, Miller KM. In-air thermal imaging comparison of Legacy AdvanTec, Millennium, and Sovereign WhiteStar phacoemulsification systems. J Cataract Refract Surg
7. Reepolmaha S, Limtrakarn W, Uthaisang-Tanechpongtamb W, Dechaumphai P. Fluid temperature at the corneal endothelium during phacoemulsification: comparison of an ophthalmic viscosurgical device and balanced salt solution using the finite element method. Ophthalmic Res
8. Innocenti B, Diciotti S, Bocchi L, Mencucci R, Corvi A. A comparison between internal and surface temperature measurement techniques during phacoemulsification cataract surgery: thermocamera versus thermocouple. J Appl Biomater Biomech
9. Rose AD, Kanade V. Thermal imaging study comparing phacoemulsification with the Sovereign WhiteStar system to the Legacy with AdvanTec and NeoSoniX system. Am J Ophthalmol
10. Han YK, Miller KM. Heat production: longitudinal versus torsional phacoemulsification. J Cataract Refract Surg
11. Schmutz JS, Olson RJ. Thermal comparison of Infiniti OZil and Signature Ellips phacoemulsification systems. Am J Ophthalmol
12. Payne M, Waite A, Olson RJ. Thermal inertia associated with ultrapulse technology in phacoemulsification. J Cataract Refract Surg
13. Brinton JP, Adams W, Kumar R, Olson RJ. Comparison of thermal features associated with 2 phacoemulsification machines. J Cataract Refract Surg