At first glance, the article by Mackool and Sirota1 seems to report that the temperature of the Bausch & Lomb (B&L) Millennium and AMO Sovereign WhiteStar is significantly higher than that of the Alcon AdvanTec Legacy at equal power settings. The authors have designed experiments to obtain bench data across the 3 platforms. Data thermal energy of the ultrasonic tips performing under different conditions about the showed that the AdvanTec Legacy has lower operating temperatures in air and in vitro at the same on-screen power settings (publication Figures 1 through 6). However, the results of the calibrated video stroke tester showed that the stoke length of each of the 3 phacoemulsification systems are significantly different at the same on-screen power settings (publication Tables 2 and 3, Figure 7). In the results section, the authors state that the heat produced at the incision site is directly related to the amount of friction produced. It is further stated that friction is directly proportional to the velocity of the tip, which is equal to the product of stroke length and frequency. Going with the logic and data presented, we can predict the relative amount of friction/heat generated by each of the machines at the different on-screen power settings. According to the authors, the B&L Millennium operates at 27 kHz, while the other 2 systems function at 38 kHz. The 3 systems also work at different duty cycles, and we modified the method proposed by the authors for calculating velocity to account for this. This modification only substantially impacted the calculation for the AMO handpiece. Using the stroke data from publication Table 2, multiplying twice the stroke length by the frequency and the duty cycle will result in the mean tip velocity, which is proportional to the friction and heat produced, as previously defined by the authors. The curves for the AMO and B&L handpieces presented in Figures 7 and 8 do not match the data presented in Tables 1 or 2. The calculated values in (Table 1) do match those charted in Figure 8 for the Alcon handpiece and provide corrected data for the other 2 handpieces. A conversion factor of 0.0254 was used to get meters per second because the stroke is in millimeters (0.001 inches).Table 1: Comparison of stroke lengths between the 3 platforms.There is general agreement in the industry that delivered phaco power is related to stroke length, but the displayed power for any particular machine is arbitrary and not standardized. For example, at 10% console displayed power, the Alcon machine is delivering 36% of the stroke of the Millennium for the same console setting. It should, therefore, be no surprise that at 36% of the stroke, the Alcon machine produces less tip velocity because it is only producing 36% as much power by Alcon's definition. It is noteworthy that with even a significantly greater stroke length, the B&L Millennium generates less heat than the Alcon platform at the higher power settings (70% and 100%). The lower mean velocity for the Sovereign tip is due to the lower duty cycle, which may be set to any value the surgeon wishes with the B&L Custom Control Software. If we calculate tip velocity for equal stroke lengths and duty cycles, the Legacy would be predicted to produce significantly greater heat than the Millennium (Table 2).Table 2: Tip velocity of each of the different platforms in meters per second.Two other concerns are that the Millennium was apparently not tested with its Custom Control Software and that the friction tests were performed with manual decentration, which does not produce consistent force of contact between the sleeve and the needle. Greater force or less force of contact will result in higher or lower frictional heat. Finally, this article's methodology is analogous to evaluating automobile engine temperatures of 2 manufacturers using tachometers with 2 different calibrations. When console revolutions appear equal, one engine is actually running slower and cooler but also generating less power. The correct measurement of generated heat should be based on equal parameters; in this case, equal stroke lengths. Using the same data and reasoning provided, the results run contrary to the stated conclusions. The lack of standardization described in this article has long been recognized as a major obstacle to comparing various measures of phaco time between manufacturers. When studies such as this one are designed, it is essential that data interpretation take into account the discrepancies between various manufacturers' methods of producing, defining, and displaying power. Joel A.D. Javier MD Terence M. Devine MD aSingapore bSayre, Pennsylvania, USA
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Jun 1, 2008
Ke Yao + 3
Open Access