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Laboratory science

Optimum on-time duty cycle for micropulse technology

Kirk, Kevin R. BS; Ronquillo, Cecinio Jr. BS, BA; Jensen, Jason D. BS; Zaugg, Brian MD; Barlow, William R. Jr. MD; Stagg, Brian C. MD; Pettey, Jeff H. MD; Olson, Randall J. MD*

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Journal of Cataract & Refractive Surgery: September 2014 - Volume 40 - Issue 9 - p 1545-1548
doi: 10.1016/j.jcrs.2014.02.033
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Abstract

Phacoemulsification is the standard of care for cataract removal.1 Since its infancy in the 1960s to today, the technology, techniques, and outcomes have greatly improved. New machines, handpieces, and techniques are constantly being developed.2,3 These new advancements require careful evaluation to focus our understanding of improving efficiency and patient outcomes.

In contrast to traditional continuous or pulsed US power, micropulsed longitudinal ultrasound (US) delivers just a few microseconds of US energy. The rapid pulsing of US with completely quiet energy-free periods has been suggested as a means to reduce the total amount of energy delivered to the eye, which may lead to decreased thermal heat and iatrogenic injury as well as improved efficiency.2–8 The only study to evaluate actual thermal incision contracture9 did not show that micropulse was protective.

Little work has been done on what might represent the most efficient on time and whether there is a “sweet spot” for efficiency or the total work simply represents the total energy put into the system (ie, 3 times 2 milliseconds [ms] on equals 6 ms on once). It would seem that if the pulses were made too short, little effective work would be produced. Increasing the off time also may increase efficiency; however, there is a limit and no one has validated the most efficient duty cycle (total on time and off time of 1 cycle divided by the on time) at this time.8,10–12 Theoretically, longer on times could result in more chatter events, leading to decreased efficiency.

The purpose of this study was to use a laboratory protocol to identify the optimum on time to minimize the amount of phaco energy required to perform efficient lens fragment removal while minimizing chatter events.

Materials and methods

Porcine lenses were prepared in the manner previously described.13 In short, 20 whole porcine eyes were purchased from a supplier (Visiontech, Inc.) and dissected within 48 hours of arrival at the Moran Eye Center, Salt Lake City, Utah. Lens nuclei were soaked individually in 10 mL of 10% neutral buffered formalin for 2 hours. After soaking in formalin, 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 lens-cutting apparatus was used to cut the lenses into 2.0 mm cubes.13,14 They were placed in a pool of a balanced salt solution in a moisture chamber until the study procedures took place, no more than 36 hours after cutting. A previous study13 showed that porcine lenses prepared in this manner are comparable to hard (3 to 4+) human cataractous lenses in terms of density and behavior during phacoemulsification. All lens cubes were placed in a single container, which was shaken before random selection of cubes for each experiment.

All phaco experimentation was performed with a Signature machine (Abbott Medical Optics, Inc.). All parameters were at the settings previously determined to be most efficient.14 A 0.9 mm bent phaco tip with a 30-degree bevel (Microsurgical Technology, Inc.) was used. Power was set at 50%, bottle height at 50 cm, aspiration rate at 40 mL/min, and vacuum at 550 mm Hg. Ultrasound and vacuum were only used at their maximum setting (full pedal on). Runs of 20 were performed while varying the on time, starting at 2 ms and continuing to 10 ms in 1 ms steps. The off time was kept constant at 10 ms.

Efficiency (total time until fragment removal not including any chatter time) and chatter (when the fragment was seen to bounce off the tip equals 1 event) were compared and recorded in a previously described manner.11,13,14 In short, 1 randomly chosen lens cube was placed inside a chamber filled with a balanced salt solution. The pedal was depressed and once the lens fragment occluded the phaco tip, the pedal was fully depressed to initiate US. A stopwatch recorded the time from US initiation until fragment removal and was stopped if the particle bounced from the tip. When the particle bounced from the tip, the pedal was again depressed to vacuum until the particle was aspirated to the tip. Once the particle reoccluded the tip, the pedal was fully depressed and the timer restarted. This allowed the chatter delay time to be distinct from the total particle removal time. Efficiency was measured in terms of the amount of time in seconds that US was used to remove the lens particle.

Efficiency times were averaged, and a standard deviation (SD) was calculated. Efficiency times that were more than 2 SDs from the mean were considered outliers and removed from the data set. New means and SDs were then calculated. Linear regression with a calculated R2 was used to compare the efficiency arms between the different on times tested. Finally, a chi-square analysis was used for comparison of chatter.

For the linear regression, the power ranged from 0.11 to 0.99 using the weakest R2 of 0.02945 to the largest R2 of 0.8163 for a sample size of 20 and α equal to 0.05.

Results

Phacoemulsification efficiency improved in a nearly linear fashion from on times of 2 ms to 6 ms (Table 1). This is apparent in the left linear regression graph in Figure 1, with an R2 value of .82 and a P value of .04. After 6 ms, there was no improvement of efficiency up to 10 ms. An additional linear regression graph for on times of 6 to 10 ms is shown in the right graph of Figure 1, with an R2 value of .03 (which is essentially a flat line) and a P value of .78.

Figure 1
Figure 1:
Linear regression of on times from 2 to 6 ms (left graph) and from 6 to 10 ms (right graph).
Table 1
Table 1:
Times of removal of 2.0 mm lens cubelets (not including time during a chatter event). Each chatter event is when the nucleus fragment bounced away from the phaco tip.

There was little or no chatter from 2 to 8 ms on; however, when chatter of 9 to 10 ms was compared with 2 to 8 ms of on time using a chi-square analysis, there was a statistically significant difference (P<.0001).

Discussion

Micropulse is a software modification that allows for extremely short bursts of US energy. Studies2–4,6,8,11 suggest that this allows for a decreased amount of thermal energy buildup and greater efficiency than continuous US. This is thought to be at least partly due to decreased microchatter (chatter or bouncing of the lens fragment at the tip occurring at the microscopic level). During US, chatter can only occur after contact, and contact is important in the removal of nuclear fragments. It is surmised that microchatter is greatly increased during continuous US and traditional pulsed US because of the lack of a rest period, during which aspiration forces can reacquire the lens fragment to the tip before repulsion can occur. It is further surmised that for very brief periods of on time, inertia is such that the fragment does not get a chance to bounce off before an off cycle occurs and aspiration recaptures the lens fragment.8,15–17 Six milliseconds appears to be the optimum combination of these forces to minimize the phaco energy required and chatter while maximizing efficiency (time to removal).

It has been shown that micropulse functions as or more efficiently than continuous US.2–4,6,8 Our results show a linear relationship of phacoemulsification on time from 2 ms to 6 ms. Greater on times showed no improvement in efficiency. This is explainable if we assume that microchatter occurs at approximately the 6 ms mark and any amount of US longer than this is wasted and does not contribute to lens fragment removal. Furthermore, our findings suggest that at longer on times, there is an increased amount of gross chatter events, which may increase the risk for breaking the capsule and are clearly related to less overall efficiency.14 Although there was little chatter at times other than 10 ms, chatter was common and significantly greater at 9 ms and 10 ms. It will be interesting to study times longer than 10 ms on to see whether this finding is verified.

Another explanation for our findings is transient versus steady-state cavitational US. Cavitational energy is most robust just after tip actuation and then drops significantly at about 6 ms so the total cavitational energy at the same power level for 6 ms on and 12 ms off is the same as continuous US.11 It has also been shown that compared with continuous US, micropulse significantly decreases microstreaming (an US-induced repulsive fluid stream that drives lens fragments away from the phaco tip).4 This is another potential explanation for our results. We do not have the data to help in the controversy about how important the jackhammer effect is versus cavitational energy.

Limitations of our study include its in vitro nature, the limited number of US runs performed, and the single-size fragment that we used. In addition, we only tested the Signature phaco machine and did not compare other common phaco units. Nevertheless, we would expect analogous results for other micropulse technology phaco machines due to principles and forces relevant to all similar machines. In this study, we used previous work to start at efficient parameters; however, many variables could be tried. Nevertheless, our parameters were based on thorough testing and were the most efficient ones we had tested to date. Furthermore, a clinical study in which all possible variables are considered would be impossible to control. We believe our study procedures closely followed the clinical situation and optimally isolated the condition being tested.

In conclusion, we found a linear improvement in efficiency going from 2 ms to 6 ms on time, with a rare chatter event in quite hard nuclear fragments. There was no improvement in efficiency at on times longer than 6 ms, with significantly more chatter at 10 ms on time. Thus, 6 ms on seems to be optimum for maximizing efficiency and minimizing chatter. Future studies will help identify an ideal off time, further refine optimum needle shape and bore, and compare optimized micropulsed US with other newer US modalities. We hope this body of work will lead to improved cataract surgery efficiency and safety.

What Was Known

  • Micropulse technology provides efficient and safe phacoemulsification software.
  • Little work has been done to identify an optimum on time.

What This Paper Adds

  • The study identified 6 ms as the optimum on time for micropulse technology that provides maximum efficiency but also limits the amount of phaco energy required and chatter events that occur.

References

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