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Optimum on-time and off-time combinations for micropulse phacoemulsification in venturi vacuum mode
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LABORATORY SCIENCE
Optimum on-time and off-time combinations for micropulse phacoemulsification in venturi vacuum mode Ashlie A. Bernhisel, MD, Judd M. Cahoon, MD, Ruti Sella, MD, Brian Zaugg, MD, William R. Barlow, MD, Brian C. Stagg, MD, Natalie A. Afshari, MD, Randall J. Olson, MD, Jeff H. Pettey, MD
Purpose: To measure the time to fragment removal and number of chatter events using various combinations of micropulse on times and off times (measured in milliseconds) of longitudinal ultrasound (US) using a venturi-based phacoemulsification system. Setting: John A. Moran Eye Center, University of Utah, Salt Lake City, USA.
Results: There was a statistically significant difference between on/off duty cycle combinations. The 6 on/7 off group had higher efficiency than the 5 on/6 off and 7 on/7 off groups. Six on/5 off was more efficient than 5 on/6 off. When data were pooled and on times alone were used, 6 milliseconds on time was more efficient than 5 or 7 milliseconds. No efficiency differences in off times were found. No significant chatter differences were observed.
Design: Experimental study. Methods: Pig lenses were hardened with formalin and cut into 2.0 mm cubes. The time to fragment removal (efficiency) and frequency of fragments bouncing off the tip (chatter) were measured with the venturi-based system. Micropulse longitudinal US was tested. Parameters were combinations of 5, 6, and 7 milliseconds on, with 5, 6, and 7 milliseconds off. Twenty runs each of 9 combinations were completed.
P
hacoemulsification of lens material is the most common way in which cataract surgery is performed in the developed world. During phacoemulsification, a complex interaction occurs between ultrasound (US) energy, intraocular pressure, and fluidics. The settings that control these parameters can be altered in a variety of ways. These technologies are constantly being changed to become more surgeon-friendly and efficient as well as to ensure safer procedures for patients. One setting that can be altered is power. Ultrasound power is measured by the frequency at which the US tip moves and the distance the tip travels, known as the stroke length. Frequency is predetermined by the machine manufacturer,
Conclusions: Using micropulse longitudinal US in venturi vacuum mode, 6 milliseconds on was the most efficient on time. Five, 6, and 7 milliseconds off times had similar efficiency. These data suggest that the most efficient setting with lowest US energy use is 6 milliseconds on and 7 milliseconds off. J Cataract Refract Surg 2019; 45:1797–1800 Q 2019 Published by Elsevier Inc. on behalf of ASCRS and ESCRS
while stroke length can be varied by adjusting the percentage power for the device. The direction in which the tip moves also differs between handpieces and US platforms, which is why there are longitudinal, transverse, and torsional US variations.1 Power can be further modulated to pulsing versus continuous US energy. Pulsing allows the surgeon to automate the amount of off times and on times. Pulsing the US rather than applying constant energy reduces the total amount of phacoemulsification power used, which in turn should reduce endothelial damage and the resulting corneal edema.2 Furthermore, less heat is generated because there are brief moments of cooling, which should diminish the incidence of wound burns.3–5
Submitted: February 23, 2019 | Final revision submitted: June 28, 2019 | Accepted: July 1, 2019 From the Department of Ophthalmology and Visual Sciences (Bernhisel, Cahoon, Zaugg, Barlow, Stagg, Olson, Pettey), John A. Moran Eye Center, University of Utah, Salt Lake City, the Department of Ophthalmology (Bernhisel, Sella, Afshari), Shiley Eye Institute at University of California San Diego Health, San Diego, the Stein Eye Institute (Cahoon), University of California Los Angeles, and the Duke Eye Center and Department of Ophthalmology (Stagg), Duke University, Durham, North Carolina, USA. Supported in part by an unrestricted grant from Research to Prevent Blindness, Inc., New York, New York, to the Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City, USA. Presented at the ASCRS$ASOA Annual Meeting, San Diego, California, USA, May 2019. Corresponding author: Jeff H. Pettey, MD, John A. Moran Eye Center, University of Utah, 65 Mario Capecchi Dr, Salt Lake City, UT 84132, USA. Email: jeff.pettey@hsc. utah.edu. Q 2019 Published by Elsevier Inc. on behalf of ASCRS and ESCRS.
0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2019.07.001
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LABORATORY SCIENCE: ON/OFF TIMES FOR MICROPULSE PHACOEMULSIFICATION
Our laboratory measures the efficiency of lens particle removal in 2 ways, first in seconds to fragment removal and second in the number of chatter events, with fewer chatter events suggesting improved efficiency. Our previous work has shown that micropulsed US (4 to 10 milliseconds on/off) removes lens fragments in less time with fewer chatter events than long-pulsing US (30 to 60 milliseconds on/ off) and continuous longitudinal US.6 This difference is more notable at moderate-flow vacuum (30 mL/min and 300 mm/Hg) than at high-flow vacuum.6 This is likely because high-flow, high-vacuum systems are more efficient than low-flow systems; therefore, statistically significant differences might not be detectable.7 In further studies, we found an on time of 6 milliseconds to be the most efficient using longitudinal US and transversal US.8,9 Separately, 6 milliseconds has been found to be the most efficient off time.10 To further elucidate the most efficient micropulse parameters, our team found a linear improvement in efficiency while increasing power from 10% to 100%.11 In a further study,12 we compared peristaltic vacuum with venturi pump at vacuum levels of 300 mm Hg, 400 mm Hg, and 500 mm Hg and determined that venturi was more efficient than peristaltic and had fewer chatter events. Despite these ongoing advancements, little data are available to support claims regarding the different US platforms. The goal of this study was to gather data on various parameters using different US machines to objectively define which settings are the most proficient and safe. We determined the most efficient on and off combinations with micropulsed US using a previously validated method to test phacoemulsification efficiency. This is the first study in our laboratory that did not test on times and off times independently of each other. It is difficult to compare multiple variables in a single study because of the large number of runs it takes to perform such an experiment. In this study, we honed in on the range previously determined to be the most efficient for on times and off times and tested 9 various combinations using our most efficient parameters, including the use of the venturi vacuum mode. MATERIALS AND METHODS Pig lenses were prepared in a previously described manner.13 In brief, whole pig eyes purchased from a supplier (Visiontech, Inc.) were dissected within 48 hours of arrival at the Moran Eye Center, Salt Lake City, Utah, USA. After the lens nuclei were soaked individually in 10 mL of 10% neutral buffered formalin for 2 hours, all nuclei were placed in 10 mL of a balanced salt solution for 24 hours to increase the uniformity of the formalin’s hardening effect. A manual lens cutting apparatus was used to cut the lenses into 2.0 mm cubes.13,14 The lens cubes were stored in a balanced salt solution in a moisture chamber until the experiment started. The study procedures took place no more than 36 hours after cutting. The lenses prepared in this manner are comparable to human cataracts in terms of their density and behavior during phacoemulsification.13 All lens cubes were placed in a single container, which was shaken before random selection of cubes for each experiment. The phacoemulsification experiments were performed using the Signature machine (Johnson & Johnson Vision Care, Inc.). In this preset study design, longitudinal power was set at 100%, the bottle Volume 45 Issue 12 December 2019
Figure 1. Mean time to fragment removal based on micropulse on time off time. The asterisks indicate a statistically significant difference between groups.
height at 50 cm, and venturi vacuum at 400 mm Hg. A 0.9 mm straight tip with a 30-degree bevel (Microsurgical Technology, Inc.) was used. A previous study15 determined that these settings and the straight tip were the most efficient. Longitudinal power was chosen because the goal was to optimize longitudinal off times and on times separately from torsional US and transverse US. Twenty runs of each on/off combination were performed. Efficiency was measured as the time to complete fragment removal, not including any chatter events. Efficiency was also measured by chatter events, defined as the number of times in which a lens fragment was seen to bounce off the tip. These times and events were recorded as previously described.13,14 Statistical Analysis Measured times were averaged, and a standard deviation (SD) was calculated. Previous studies have explained that outliers greater that 2 SDs likely represent an experimental run in which microchatter has occurred.7,14–17 Microchatter is when small lens fragment repulsions occur at the phaco tip. This results in the lens fragment being seated near the tip but is neither being emulsified nor is bouncing completely away. This phenomenon cannot be measured with standard techniques and therefore contributes to noise. Thus, to optimally interpret the data, timepoints more than 2 SDs from the mean were removed from all analyses. The new mean and SD were then recalculated. We assessed efficiency data for normality with the Shapiro-Wilk test. The Kruskal-Wallis nonparametric test followed by the Dunn multiple comparison test were used to compare the efficiency of fragment removal and chatter events between 9 preset micropulse longitudinal US on/off duty cycle times. The Mann-Whitney U nonparametric test was used for post hoc analysis. A P value less than 0.05 was considered statistically significant. Statistical analysis was performed with GraphPad Prism software (version 8.0.0, GraphPad Software, Inc.).
RESULTS Direct Comparisons
Overall, the efficiency of time to fragment removal differed between separate on/off duty cycle times (P Z .0008). When comparing the on/off settings head to head, the 6 on/7 off group showed higher efficiency than the 5 on/6 off and 7 on/7 off groups (P Z .0452 and P Z .0053, respectively). The 6 on/5 off group was more efficient than the 5 on/6 off group (P Z .0206) (Figure 1).
LABORATORY SCIENCE: ON/OFF TIMES FOR MICROPULSE PHACOEMULSIFICATION
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Figure 2. On times. The asterisks indicate a statistically significant difference between groups.
Pooled Data
The 6 milliseconds on-time measurements, which were pooled regardless of “off” status, were more efficient than the 5- or 7-millisecond on-time measurements (P Z .0003 and P Z .0026, respectively) (Figures 2 and 3). No significant difference was found between the 7 -millisecond and the 5-millisecond on-time measurements, regardless of the off-time status. No differences were found when comparing the 3 off times. Chatter
Chatter events were similar between the groups without a statistically significant difference. Overall, the chatter events were very low (0.05 to 0.4 mean chatter events per run). DISCUSSION Since the advent of US modalities with horizontal motion, exclusive use of longitudinal US is becoming increasingly uncommon. This stems from the observation that longitudinal US creates repulsion at the tip as a result of its forward motion physically pushing the fragment away, which results in poor efficiency.18,19 For this reason, torsional US and transverse US have become mainstream. However, our previous data using longitudinal micropulse rather than continuous US have shown a significant decrease in chatter events and similar efficiency when operating under optimized settings.11 In particular, micropulsing using longitudinal settings seems to have a very low number of chatter events, even when high power is used; this is in contrast to torsional US and transverse US at high power settings when used in continuous mode. For example, an earlier study from our laboratory14 showed that in the time
Figure 3. On-times comparison. The asterisks indicate a statistically significant difference between groups.
required to remove the lens fragment under high power settings, there were as many as 50 chatter events in which the fragment bounced away from the tip. Once we started collecting micropulse data, our chatter events averaged fewer than 1 per experiment, which is consistent with the results we are reporting here as well as with our previous micropulse data.8,11,12,20 We hypothesize that the on-and-off nature of the power reduces the repulsion effect of the forward motion of the tip, thus allowing the vacuum to work more effectively by keeping the nuclear fragment seated. Using torsional micropulse appears to have similarly low chatter events even at high powers8; however, it remains unknown whether micropulsing overall negates the benefits of torsional US and transverse US. The goal of this study was to finalize and report our data collection regarding the most efficient micropulse on-time/ off-time combinations using longitudinal micropulse US. Our laboratory’s previous work found a linear increase in efficiency times when the on time was increased from 2 milliseconds to 6 milliseconds. After 6 milliseconds, there did not appear to be further improvement in efficiency.9 We wanted to hone in on this range and determine whether any on/off combinations were more efficient than others. We did not find a single combination that outperformed all others; however, we did find that 6 on/7 off and 6 on/5 off were more efficient than several 5 milliseconds and 7 milliseconds on combinations. Pooled on-time data also showed that 6 milliseconds on performed better than pooled 5 milliseconds and 7 milliseconds on. Similar to our on-time data, an increase in efficiency occurred as the off time was increased from 2 milliseconds off to 6 milliseconds off; efficiency began declining at approximately 8 milliseconds off continued to decrease with longer times.10 In the present study, there was no statistically significantly difference between off times of 5, 6, or 7 milliseconds. However, this nonsignificant finding had clinical implications. When attempting to apply as little US power as possible in a given time, using longer off times would give the tip more time to cool between pulses. If a fragment is removed in the same amount of time, whether one uses an off time of 5 milliseconds or 7 milliseconds, it would be prudent to use 7 milliseconds off because no US energy would be applied for a greater percentage of time. In other words, the percentage on time or duty cycle for 6 milliseconds on/7 milliseconds off is 46.15% compared with a duty cycle of 54.54% with 6 milliseconds on/5 milliseconds off. Volume 45 Issue 12 December 2019
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This study has limitations. First is the in vitro nature of the experiment. However, our in vitro system has been well validated and provides data that are objective and clinically relevant. Another weakness of this study is that we tested 2 variables only (on times and off times) while the remaining parameters were preset (% power, vacuum, aspiration, bottle height). Each of the preset variables was previously tested in isolation and shown to maximize phacoemulsification efficacy.7–12 We believe a clinical study in which all possible parameters are tested would be unfeasible to perform if powered adequately. Last, in an attempt to fully optimize on/off efficiency times, the preset parameters used might surpass what a surgeon would feel comfortable using in a surgical practice. Our aim was to determine maximal efficiency settings with the understanding that this data will be extrapolated to guide surgeons when they set phacodynamic parameters with which they are comfortable. Based on the results in this study and in our previous work, a phacoemulsification surgeon should feel confident that 6 milliseconds on and 7 milliseconds off are the most efficient longitudinal micropulsed settings and reduce the total percentage time the US power is engaged. In this experiment, venturi vacuum was used; however, our work with peristaltic vacuum found similar results.6,8,9 In the WhiteStar system (Johnson & Johnson Vision Care, Inc.), the pulse rate is set in absolute milliseconds of on time and off time, whereas the Centurion system (Alcon Surgical, Inc.) uses a pulse rate (pulses per second) and can be set as a percentage on time.
WHAT WAS KNOWN Micropulse in the longitudinal setting is more efficient than continuous pulsing and long pulsing. Micropulsing using longitudinal ultrasound (US) has an overall improved efficiency when using 6 milliseconds on and 6 milliseconds off, when tested as independent variables. Venturi vacuum is more efficient than peristaltic vacuum when using longitudinal micropulse US.
WHAT THIS PAPER ADDS When comparing combinations of on and off times, 6 milliseconds on was the most efficient setting. Off times were similar in efficiency; therefore, a longer off time of 7 milliseconds would reduce the overall US energy use with similar efficiency.
REFERENCES 1. Devgan UBasic principles of phacoemulsification and fluid dynamics. San Francisco, CA, American Academy of Ophthalmology, Focal Points; Clinical Modules for Ophthalmologists, 2010; 28; 1–14 2. Schmutz JS, Olson RJ. Thermal comparison of infiniti OZil and signature ellips phacoemulsification systems. Am J Ophthalmol 2010; 149:762–767 3. Fine IH, Packer M, Hoffman RS. Use of power modulations in phacoemulsification; choo-choo chop and flip phacoemulsification. J Cataract Refract Surg 2001; 27:188–197
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4. Fishkind W, Bakewell B, Donnenfeld ED, Rose AD, Watkins LA, Olson RJ. Comparative clinical trial of ultrasound phacoemulsification with and without the WhiteStar system. J Cataract Refract Surg 2006; 32:45–49 5. Payne M, Waite A, Olson RJ. Thermal inertia associated with ultrapulse technology in phacoemulsification. J Cataract Refract Surg 2006; 32:1032–1034 6. Gardiner GL, Garff K, Gupta I, Kramer GD, Farukhi MA, Stagg BC, Zaugg B, Olson RJ. Effect of pulsing ultrasound on phacoemulsification efficiency. J Cataract Refract Surg 2015; 41:2560–2564 7. Gupta I, Cahoon JM, Gardiner G, Garff K, Henriksen BS, Pettey JH, Barlow WR Jr, Olson RJ. Effect of increased vacuum and aspiration rates on phacoemulsification efficiency. J Cataract Refract Surg 2015; 41:836–841 8. Bohner AD, Wright AJ, Ta BT, Bernhisel AA, Zaugg B, Barlow WR, Pettey JH, Olson RJ. Optimum on-time duty cycle for a transversal ultrasound machine. J Cataract Refract Surg 2018; 44:1140–1143 9. Kirk KR, Ronquillo C Jr, Jensen JD, Zaugg B, Barlow WR Jr, Stagg BC, Pettey JH, Olson RJ. Optimum on-time duty cycle for micropulse technology. J Cataract Refract Surg 2014; 40:1545–1548 10. Jensen JD, Kirk KR, Gupta I, Ronquillo C Jr, Farukhi MA, Stagg BC, Pettey JH, Olson RJ. Determining optimal ultrasound off time with micropulse longitudinal phacoemulsification. J Cataract Refract Surg 2015; 41:433–436 11. Garff K, Jensen JD, Cahoon J, Gupta I, Stagg B, Zaugg BE, Barlow WR Jr, Olson RJ. Impact of micropulsed ultrasound power settings on the efficiency and chatter associated with lens-fragment removal. J Cataract Refract Surg 2015; 41:1264–1267 12. Cahoon JM, Gupta I, Gardiner G, Shi D, Zaugg B, Pettey JH, Barlow WR Jr, Olson RJ. Comparison of venturi and peristaltic vacuum in phacoemulsification. J Cataract Refract Surg 2015; 41:428–432 13. Oakey ZB, Jensen JD, Zaugg BE, Radmall BR, Pettey JH, Olson RJ. Porcine lens nuclei as a model for comparison of 3 ultrasound modalities regarding efficiency and chatter. J Cataract Refract Surg 2013; 39:1248– 1253 14. DeMill DL, Zaugg BE, Pettey JH, Jensen JD, Jardine GJ, Wong G, Olson RJ. Objective comparison of 4 nonlongitudinal ultrasound modalities regarding efficiency and chatter. J Cataract Refract Surg 2012; 38:1065– 1071 15. Stagg BC, Gupta I, Cahoon J, Ronquillo C Jr, Shi D, Zaugg B, Gardiner G, Barlow WR Jr, Pettey JH, Farukhi MA, Jensen J, Olson RJ. Bent versus straight tips in micropulsed longitudinal phacoemulsification. Can J Ophthalmol 2015; 50:354–359 16. Farukhi AM, Stagg BC, Ronquillo C Jr, Barlow WR Jr, Pettey JH, Olson RJ. Effect of phaco tip diameter on efficiency and chatter. J Cataract Refract Surg 2014; 40:811–817; erratum, 1576 17. Ronquillo CC Jr, Zaugg B, Stagg B, Kirk KR, Gupta I, Barlow WR Jr, Pettey JH, Olson RJ. Determining optimal torsional ultrasound power for cataract surgery with automatic longitudinal pulses at maximum vacuum ex vivo. Am J Ophthalmol 2014; 158:1262–1266 18. Wright AJ, Bohner AD, Bernhisel AA, Zaugg B, Barlow WR Jr, Pettey JH, Olson RJ. The effect of pulsing on transverse ultrasound efficiency and chatter. Am J Ophthalmol 2017; 183:107–110 19. Fernandez de Castro LE, Dimalanta RC, Solomon KD. Bead-flow pattern: quantitation of fluid movement during torsional and longitudinal phacoemulsification. J Cataract Refract Surg 2010; 36:1018–1023 20. Zacharias J, Ohl CD. Fluid dynamics, cavitation, and tip-to-tissue interaction of longitudinal and torsional ultrasound modes during phacoemulsification. J Cataract Refract Surg 2013; 39:611–616
Disclosures: Dr. Olson is on the scientific advisory board of EyeGate Pharma and Perfect Lens LLC. None of the other authors has a financial or proprietary interest in any material or methods mentioned.
First author: Ashlie A. Bernhisel, MD Department of Ophthalmology and Visual Sciences, John A. Moran Eye Center, University of Utah, Salt Lake City, USA
LABORATORY SCIENCE
Optimum on-time and off-time combinations for micropulse phacoemulsification in venturi vacuum mode Ashlie A. Bernhisel, MD, Judd M. Cahoon, MD, Ruti Sella, MD, Brian Zaugg, MD, William R. Barlow, MD, Brian C. Stagg, MD, Natalie A. Afshari, MD, Randall J. Olson, MD, Jeff H. Pettey, MD
Purpose: To measure the time to fragment removal and number of chatter events using various combinations of micropulse on times and off times (measured in milliseconds) of longitudinal ultrasound (US) using a venturi-based phacoemulsification system. Setting: John A. Moran Eye Center, University of Utah, Salt Lake City, USA.
Results: There was a statistically significant difference between on/off duty cycle combinations. The 6 on/7 off group had higher efficiency than the 5 on/6 off and 7 on/7 off groups. Six on/5 off was more efficient than 5 on/6 off. When data were pooled and on times alone were used, 6 milliseconds on time was more efficient than 5 or 7 milliseconds. No efficiency differences in off times were found. No significant chatter differences were observed.
Design: Experimental study. Methods: Pig lenses were hardened with formalin and cut into 2.0 mm cubes. The time to fragment removal (efficiency) and frequency of fragments bouncing off the tip (chatter) were measured with the venturi-based system. Micropulse longitudinal US was tested. Parameters were combinations of 5, 6, and 7 milliseconds on, with 5, 6, and 7 milliseconds off. Twenty runs each of 9 combinations were completed.
P
hacoemulsification of lens material is the most common way in which cataract surgery is performed in the developed world. During phacoemulsification, a complex interaction occurs between ultrasound (US) energy, intraocular pressure, and fluidics. The settings that control these parameters can be altered in a variety of ways. These technologies are constantly being changed to become more surgeon-friendly and efficient as well as to ensure safer procedures for patients. One setting that can be altered is power. Ultrasound power is measured by the frequency at which the US tip moves and the distance the tip travels, known as the stroke length. Frequency is predetermined by the machine manufacturer,
Conclusions: Using micropulse longitudinal US in venturi vacuum mode, 6 milliseconds on was the most efficient on time. Five, 6, and 7 milliseconds off times had similar efficiency. These data suggest that the most efficient setting with lowest US energy use is 6 milliseconds on and 7 milliseconds off. J Cataract Refract Surg 2019; 45:1797–1800 Q 2019 Published by Elsevier Inc. on behalf of ASCRS and ESCRS
while stroke length can be varied by adjusting the percentage power for the device. The direction in which the tip moves also differs between handpieces and US platforms, which is why there are longitudinal, transverse, and torsional US variations.1 Power can be further modulated to pulsing versus continuous US energy. Pulsing allows the surgeon to automate the amount of off times and on times. Pulsing the US rather than applying constant energy reduces the total amount of phacoemulsification power used, which in turn should reduce endothelial damage and the resulting corneal edema.2 Furthermore, less heat is generated because there are brief moments of cooling, which should diminish the incidence of wound burns.3–5
Submitted: February 23, 2019 | Final revision submitted: June 28, 2019 | Accepted: July 1, 2019 From the Department of Ophthalmology and Visual Sciences (Bernhisel, Cahoon, Zaugg, Barlow, Stagg, Olson, Pettey), John A. Moran Eye Center, University of Utah, Salt Lake City, the Department of Ophthalmology (Bernhisel, Sella, Afshari), Shiley Eye Institute at University of California San Diego Health, San Diego, the Stein Eye Institute (Cahoon), University of California Los Angeles, and the Duke Eye Center and Department of Ophthalmology (Stagg), Duke University, Durham, North Carolina, USA. Supported in part by an unrestricted grant from Research to Prevent Blindness, Inc., New York, New York, to the Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City, USA. Presented at the ASCRS$ASOA Annual Meeting, San Diego, California, USA, May 2019. Corresponding author: Jeff H. Pettey, MD, John A. Moran Eye Center, University of Utah, 65 Mario Capecchi Dr, Salt Lake City, UT 84132, USA. Email: jeff.pettey@hsc. utah.edu. Q 2019 Published by Elsevier Inc. on behalf of ASCRS and ESCRS.
0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2019.07.001
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LABORATORY SCIENCE: ON/OFF TIMES FOR MICROPULSE PHACOEMULSIFICATION
Our laboratory measures the efficiency of lens particle removal in 2 ways, first in seconds to fragment removal and second in the number of chatter events, with fewer chatter events suggesting improved efficiency. Our previous work has shown that micropulsed US (4 to 10 milliseconds on/off) removes lens fragments in less time with fewer chatter events than long-pulsing US (30 to 60 milliseconds on/ off) and continuous longitudinal US.6 This difference is more notable at moderate-flow vacuum (30 mL/min and 300 mm/Hg) than at high-flow vacuum.6 This is likely because high-flow, high-vacuum systems are more efficient than low-flow systems; therefore, statistically significant differences might not be detectable.7 In further studies, we found an on time of 6 milliseconds to be the most efficient using longitudinal US and transversal US.8,9 Separately, 6 milliseconds has been found to be the most efficient off time.10 To further elucidate the most efficient micropulse parameters, our team found a linear improvement in efficiency while increasing power from 10% to 100%.11 In a further study,12 we compared peristaltic vacuum with venturi pump at vacuum levels of 300 mm Hg, 400 mm Hg, and 500 mm Hg and determined that venturi was more efficient than peristaltic and had fewer chatter events. Despite these ongoing advancements, little data are available to support claims regarding the different US platforms. The goal of this study was to gather data on various parameters using different US machines to objectively define which settings are the most proficient and safe. We determined the most efficient on and off combinations with micropulsed US using a previously validated method to test phacoemulsification efficiency. This is the first study in our laboratory that did not test on times and off times independently of each other. It is difficult to compare multiple variables in a single study because of the large number of runs it takes to perform such an experiment. In this study, we honed in on the range previously determined to be the most efficient for on times and off times and tested 9 various combinations using our most efficient parameters, including the use of the venturi vacuum mode. MATERIALS AND METHODS Pig lenses were prepared in a previously described manner.13 In brief, whole pig eyes purchased from a supplier (Visiontech, Inc.) were dissected within 48 hours of arrival at the Moran Eye Center, Salt Lake City, Utah, USA. After the lens nuclei were soaked individually in 10 mL of 10% neutral buffered formalin for 2 hours, all nuclei were placed in 10 mL of a balanced salt solution for 24 hours to increase the uniformity of the formalin’s hardening effect. A manual lens cutting apparatus was used to cut the lenses into 2.0 mm cubes.13,14 The lens cubes were stored in a balanced salt solution in a moisture chamber until the experiment started. The study procedures took place no more than 36 hours after cutting. The lenses prepared in this manner are comparable to human cataracts in terms of their density and behavior during phacoemulsification.13 All lens cubes were placed in a single container, which was shaken before random selection of cubes for each experiment. The phacoemulsification experiments were performed using the Signature machine (Johnson & Johnson Vision Care, Inc.). In this preset study design, longitudinal power was set at 100%, the bottle Volume 45 Issue 12 December 2019
Figure 1. Mean time to fragment removal based on micropulse on time off time. The asterisks indicate a statistically significant difference between groups.
height at 50 cm, and venturi vacuum at 400 mm Hg. A 0.9 mm straight tip with a 30-degree bevel (Microsurgical Technology, Inc.) was used. A previous study15 determined that these settings and the straight tip were the most efficient. Longitudinal power was chosen because the goal was to optimize longitudinal off times and on times separately from torsional US and transverse US. Twenty runs of each on/off combination were performed. Efficiency was measured as the time to complete fragment removal, not including any chatter events. Efficiency was also measured by chatter events, defined as the number of times in which a lens fragment was seen to bounce off the tip. These times and events were recorded as previously described.13,14 Statistical Analysis Measured times were averaged, and a standard deviation (SD) was calculated. Previous studies have explained that outliers greater that 2 SDs likely represent an experimental run in which microchatter has occurred.7,14–17 Microchatter is when small lens fragment repulsions occur at the phaco tip. This results in the lens fragment being seated near the tip but is neither being emulsified nor is bouncing completely away. This phenomenon cannot be measured with standard techniques and therefore contributes to noise. Thus, to optimally interpret the data, timepoints more than 2 SDs from the mean were removed from all analyses. The new mean and SD were then recalculated. We assessed efficiency data for normality with the Shapiro-Wilk test. The Kruskal-Wallis nonparametric test followed by the Dunn multiple comparison test were used to compare the efficiency of fragment removal and chatter events between 9 preset micropulse longitudinal US on/off duty cycle times. The Mann-Whitney U nonparametric test was used for post hoc analysis. A P value less than 0.05 was considered statistically significant. Statistical analysis was performed with GraphPad Prism software (version 8.0.0, GraphPad Software, Inc.).
RESULTS Direct Comparisons
Overall, the efficiency of time to fragment removal differed between separate on/off duty cycle times (P Z .0008). When comparing the on/off settings head to head, the 6 on/7 off group showed higher efficiency than the 5 on/6 off and 7 on/7 off groups (P Z .0452 and P Z .0053, respectively). The 6 on/5 off group was more efficient than the 5 on/6 off group (P Z .0206) (Figure 1).
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Figure 2. On times. The asterisks indicate a statistically significant difference between groups.
Pooled Data
The 6 milliseconds on-time measurements, which were pooled regardless of “off” status, were more efficient than the 5- or 7-millisecond on-time measurements (P Z .0003 and P Z .0026, respectively) (Figures 2 and 3). No significant difference was found between the 7 -millisecond and the 5-millisecond on-time measurements, regardless of the off-time status. No differences were found when comparing the 3 off times. Chatter
Chatter events were similar between the groups without a statistically significant difference. Overall, the chatter events were very low (0.05 to 0.4 mean chatter events per run). DISCUSSION Since the advent of US modalities with horizontal motion, exclusive use of longitudinal US is becoming increasingly uncommon. This stems from the observation that longitudinal US creates repulsion at the tip as a result of its forward motion physically pushing the fragment away, which results in poor efficiency.18,19 For this reason, torsional US and transverse US have become mainstream. However, our previous data using longitudinal micropulse rather than continuous US have shown a significant decrease in chatter events and similar efficiency when operating under optimized settings.11 In particular, micropulsing using longitudinal settings seems to have a very low number of chatter events, even when high power is used; this is in contrast to torsional US and transverse US at high power settings when used in continuous mode. For example, an earlier study from our laboratory14 showed that in the time
Figure 3. On-times comparison. The asterisks indicate a statistically significant difference between groups.
required to remove the lens fragment under high power settings, there were as many as 50 chatter events in which the fragment bounced away from the tip. Once we started collecting micropulse data, our chatter events averaged fewer than 1 per experiment, which is consistent with the results we are reporting here as well as with our previous micropulse data.8,11,12,20 We hypothesize that the on-and-off nature of the power reduces the repulsion effect of the forward motion of the tip, thus allowing the vacuum to work more effectively by keeping the nuclear fragment seated. Using torsional micropulse appears to have similarly low chatter events even at high powers8; however, it remains unknown whether micropulsing overall negates the benefits of torsional US and transverse US. The goal of this study was to finalize and report our data collection regarding the most efficient micropulse on-time/ off-time combinations using longitudinal micropulse US. Our laboratory’s previous work found a linear increase in efficiency times when the on time was increased from 2 milliseconds to 6 milliseconds. After 6 milliseconds, there did not appear to be further improvement in efficiency.9 We wanted to hone in on this range and determine whether any on/off combinations were more efficient than others. We did not find a single combination that outperformed all others; however, we did find that 6 on/7 off and 6 on/5 off were more efficient than several 5 milliseconds and 7 milliseconds on combinations. Pooled on-time data also showed that 6 milliseconds on performed better than pooled 5 milliseconds and 7 milliseconds on. Similar to our on-time data, an increase in efficiency occurred as the off time was increased from 2 milliseconds off to 6 milliseconds off; efficiency began declining at approximately 8 milliseconds off continued to decrease with longer times.10 In the present study, there was no statistically significantly difference between off times of 5, 6, or 7 milliseconds. However, this nonsignificant finding had clinical implications. When attempting to apply as little US power as possible in a given time, using longer off times would give the tip more time to cool between pulses. If a fragment is removed in the same amount of time, whether one uses an off time of 5 milliseconds or 7 milliseconds, it would be prudent to use 7 milliseconds off because no US energy would be applied for a greater percentage of time. In other words, the percentage on time or duty cycle for 6 milliseconds on/7 milliseconds off is 46.15% compared with a duty cycle of 54.54% with 6 milliseconds on/5 milliseconds off. Volume 45 Issue 12 December 2019
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LABORATORY SCIENCE: ON/OFF TIMES FOR MICROPULSE PHACOEMULSIFICATION
This study has limitations. First is the in vitro nature of the experiment. However, our in vitro system has been well validated and provides data that are objective and clinically relevant. Another weakness of this study is that we tested 2 variables only (on times and off times) while the remaining parameters were preset (% power, vacuum, aspiration, bottle height). Each of the preset variables was previously tested in isolation and shown to maximize phacoemulsification efficacy.7–12 We believe a clinical study in which all possible parameters are tested would be unfeasible to perform if powered adequately. Last, in an attempt to fully optimize on/off efficiency times, the preset parameters used might surpass what a surgeon would feel comfortable using in a surgical practice. Our aim was to determine maximal efficiency settings with the understanding that this data will be extrapolated to guide surgeons when they set phacodynamic parameters with which they are comfortable. Based on the results in this study and in our previous work, a phacoemulsification surgeon should feel confident that 6 milliseconds on and 7 milliseconds off are the most efficient longitudinal micropulsed settings and reduce the total percentage time the US power is engaged. In this experiment, venturi vacuum was used; however, our work with peristaltic vacuum found similar results.6,8,9 In the WhiteStar system (Johnson & Johnson Vision Care, Inc.), the pulse rate is set in absolute milliseconds of on time and off time, whereas the Centurion system (Alcon Surgical, Inc.) uses a pulse rate (pulses per second) and can be set as a percentage on time.
WHAT WAS KNOWN Micropulse in the longitudinal setting is more efficient than continuous pulsing and long pulsing. Micropulsing using longitudinal ultrasound (US) has an overall improved efficiency when using 6 milliseconds on and 6 milliseconds off, when tested as independent variables. Venturi vacuum is more efficient than peristaltic vacuum when using longitudinal micropulse US.
WHAT THIS PAPER ADDS When comparing combinations of on and off times, 6 milliseconds on was the most efficient setting. Off times were similar in efficiency; therefore, a longer off time of 7 milliseconds would reduce the overall US energy use with similar efficiency.
REFERENCES 1. Devgan UBasic principles of phacoemulsification and fluid dynamics. San Francisco, CA, American Academy of Ophthalmology, Focal Points; Clinical Modules for Ophthalmologists, 2010; 28; 1–14 2. Schmutz JS, Olson RJ. Thermal comparison of infiniti OZil and signature ellips phacoemulsification systems. Am J Ophthalmol 2010; 149:762–767 3. Fine IH, Packer M, Hoffman RS. Use of power modulations in phacoemulsification; choo-choo chop and flip phacoemulsification. J Cataract Refract Surg 2001; 27:188–197
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4. Fishkind W, Bakewell B, Donnenfeld ED, Rose AD, Watkins LA, Olson RJ. Comparative clinical trial of ultrasound phacoemulsification with and without the WhiteStar system. J Cataract Refract Surg 2006; 32:45–49 5. Payne M, Waite A, Olson RJ. Thermal inertia associated with ultrapulse technology in phacoemulsification. J Cataract Refract Surg 2006; 32:1032–1034 6. Gardiner GL, Garff K, Gupta I, Kramer GD, Farukhi MA, Stagg BC, Zaugg B, Olson RJ. Effect of pulsing ultrasound on phacoemulsification efficiency. J Cataract Refract Surg 2015; 41:2560–2564 7. Gupta I, Cahoon JM, Gardiner G, Garff K, Henriksen BS, Pettey JH, Barlow WR Jr, Olson RJ. Effect of increased vacuum and aspiration rates on phacoemulsification efficiency. J Cataract Refract Surg 2015; 41:836–841 8. Bohner AD, Wright AJ, Ta BT, Bernhisel AA, Zaugg B, Barlow WR, Pettey JH, Olson RJ. Optimum on-time duty cycle for a transversal ultrasound machine. J Cataract Refract Surg 2018; 44:1140–1143 9. Kirk KR, Ronquillo C Jr, Jensen JD, Zaugg B, Barlow WR Jr, Stagg BC, Pettey JH, Olson RJ. Optimum on-time duty cycle for micropulse technology. J Cataract Refract Surg 2014; 40:1545–1548 10. Jensen JD, Kirk KR, Gupta I, Ronquillo C Jr, Farukhi MA, Stagg BC, Pettey JH, Olson RJ. Determining optimal ultrasound off time with micropulse longitudinal phacoemulsification. J Cataract Refract Surg 2015; 41:433–436 11. Garff K, Jensen JD, Cahoon J, Gupta I, Stagg B, Zaugg BE, Barlow WR Jr, Olson RJ. Impact of micropulsed ultrasound power settings on the efficiency and chatter associated with lens-fragment removal. J Cataract Refract Surg 2015; 41:1264–1267 12. Cahoon JM, Gupta I, Gardiner G, Shi D, Zaugg B, Pettey JH, Barlow WR Jr, Olson RJ. Comparison of venturi and peristaltic vacuum in phacoemulsification. J Cataract Refract Surg 2015; 41:428–432 13. Oakey ZB, Jensen JD, Zaugg BE, Radmall BR, Pettey JH, Olson RJ. Porcine lens nuclei as a model for comparison of 3 ultrasound modalities regarding efficiency and chatter. J Cataract Refract Surg 2013; 39:1248– 1253 14. DeMill DL, Zaugg BE, Pettey JH, Jensen JD, Jardine GJ, Wong G, Olson RJ. Objective comparison of 4 nonlongitudinal ultrasound modalities regarding efficiency and chatter. J Cataract Refract Surg 2012; 38:1065– 1071 15. Stagg BC, Gupta I, Cahoon J, Ronquillo C Jr, Shi D, Zaugg B, Gardiner G, Barlow WR Jr, Pettey JH, Farukhi MA, Jensen J, Olson RJ. Bent versus straight tips in micropulsed longitudinal phacoemulsification. Can J Ophthalmol 2015; 50:354–359 16. Farukhi AM, Stagg BC, Ronquillo C Jr, Barlow WR Jr, Pettey JH, Olson RJ. Effect of phaco tip diameter on efficiency and chatter. J Cataract Refract Surg 2014; 40:811–817; erratum, 1576 17. Ronquillo CC Jr, Zaugg B, Stagg B, Kirk KR, Gupta I, Barlow WR Jr, Pettey JH, Olson RJ. Determining optimal torsional ultrasound power for cataract surgery with automatic longitudinal pulses at maximum vacuum ex vivo. Am J Ophthalmol 2014; 158:1262–1266 18. Wright AJ, Bohner AD, Bernhisel AA, Zaugg B, Barlow WR Jr, Pettey JH, Olson RJ. The effect of pulsing on transverse ultrasound efficiency and chatter. Am J Ophthalmol 2017; 183:107–110 19. Fernandez de Castro LE, Dimalanta RC, Solomon KD. Bead-flow pattern: quantitation of fluid movement during torsional and longitudinal phacoemulsification. J Cataract Refract Surg 2010; 36:1018–1023 20. Zacharias J, Ohl CD. Fluid dynamics, cavitation, and tip-to-tissue interaction of longitudinal and torsional ultrasound modes during phacoemulsification. J Cataract Refract Surg 2013; 39:611–616
Disclosures: Dr. Olson is on the scientific advisory board of EyeGate Pharma and Perfect Lens LLC. None of the other authors has a financial or proprietary interest in any material or methods mentioned.
First author: Ashlie A. Bernhisel, MD Department of Ophthalmology and Visual Sciences, John A. Moran Eye Center, University of Utah, Salt Lake City, USA