5.2 How to Use Power Modulation in MICS

Randall J. Olson

Core Messages

ßSafety and efficiency in phacoemulsification can conflict.

ßComparisons, such as percent power, are meaningless if not standardized.

ßThe incidence of wound burn can vary from 0.2 to 0.03% depending on approach and ultra-

sound power modulation.

ßMachine-indicated parameters may not be very accurate.

ßThe control of anterior chamber depth associated with post-occlusion surge can vary

dramatically depending on the parameters chosen, between machines and between patients, even when using the same machine and parameters.

5.2.1 Introduction

Phacoemulsification of the cataract is all about two core elements: (1) efficiency, and (2) safety. Unfortunately, improved efficiency can result in decreased safety; therefore, maximizing both is an important process, which will increase the speed of cataract removal without sacrificing safety. A full understanding of how energy modulation works to enhance both safety and efficiency is the subject of this chapter and has been an ongoing research effort for the last several years on behalf of my research group at the John A. Moran Eye Center, at the University of Utah, in Salt Lake City, UT.

R. J. Olson

Department of Ophthalmology and Visual Sciences, Moran Eye Center, University of Utah School of Medicine, 65 N Medical Drive, Salt Lake City, UT 84132, USA e-mail: randall.olson@hsc.utah.edu

Our original interest in this was generated due to the statements made about comparisons of machines that were not physically possible. It was unlikely that physics had been thrown out of the window and, therefore, some core assumptions could not be correct. Our early work looked at measuring actual power output and determining actual vs. machine-indicated parameters. What we learned is that flow can be different from what is indicated on the machine [1–3], in particular if flow restrictors are in place, such as Cruise control (STAAR Surgical, Monrovia, CA). We also found that vacuum can be off by as much as 23% and that features such as a bypass port in the phaco needle (ABS, Alcon Surgical, Fort Worth, TX) decreases the actual vacuum at the tip by an average of 7.5% [4]. While some of this may not seem clinically important, it certainly is important in any comparative studies between different machines. Our clinically most important early finding is that percent power had no correlation between any of the machines, including the two Alcon machines, Infiniti and Legacy. Furthermore, the power output with and without a load varied even more dramatically among different machines: with Millennium and Stellaris (Bausch & Lamb, Beverly Hills, CA) and Sovereign and Signature (Advanced Medical Optics, Santa Ana, CA), all power-based, as the foot pedal increases, the power increases; and with Legacy, being stroke length-based, the power is varied to maintain the stroke length, such that the power, even without changing the foot pedal under a load, can change dramatically.

Infiniti, interestingly, is not the same as Legacy and is somewhere between the two different extremes (Table 5.1) [1, 3].

So using percent power for any type of comparison between machines is impossible because it is different for every machine we have tested so far. For instance, a common comparison would be testing the machines in water, and based upon our work, Legacy at 100% power would be the same as Infiniti at 67%, Sovereign at 43%, and Millennium at 36%.

5.2.3The Concept of Unoccluded Flow Vacuum

We also noted that there was quite a difference in the vacuum necessary to maintain flow, particularly as

Table 5.1 Temperature Increase (C) at 1 min per 20% increment of power increase (40% divided by 2, 60% by 3) for four phacoemulsification machines with 20 ml/min of free flow (n 25 for each category)

p 0.0001 for all unweighted vs. weighted comparisons and for all vertical comparisons except: 1p 0.06, 2p 0.34, and 3p

.0016. The weight used was 200 g. Sovereign and Legacy data from [5]

flow increased above 30 ml/min of actual flow. It has generally been assumed in peristaltic vacuum systems that vacuum is zero until there is occlusion of the tip, at which point vacuum will build up. What we found is that once flow went above 20 ml/min,

active vacuum was needed to maintain that level of flow, in particular with 20-gauge tips. With a flow restrictor such as Cruise Control, the unoccluded vacuum measured at the tip was greater than what we have measured with Venturi flow systems with similar flow [3]. This unoccluded flow vacuum may be a safety concern at higher flow. At the very least, at high flow we need to be aware that there is likely to be significant active vacuum at the tip without occlusion, so one must be careful when working near the capsule or iris (Table 5.2).

5.2.4The Intricacies of Ultrasound Power Modulation

An additional area of our research deals with power modulation. Pulsing ultrasound with a variable ontime and off-time has been around for a long time; however, there was evidence accruing that very short pulses, first pioneered by Sovereign White Star, with on-times in the 4–6 ms range, seemed to provide additional efficiency and were potentially protective of wound burn as we showed in early eye bank eye studies [7, 8]. Other papers using percent power to compare machines suggested that very short pulses like signature White Star were not better temperature-wise

Table 5.2 A Comparison of actual vacuum created at the actual flow for the range of 26 through 36 ml/min of flow in 2-ml/min steps (0.9-mm tip used), for the Alcon Infiniti (INF) and Legacy (LEG), Bausch and Lomb Millennium Peristaltic (MILL-P) and Venturi (MILL-V), and AMO Sovereign (SOV), also with Staar Cruise control (SOV-CC) [6]

For all comparisons, p < 0.0001 except LEG to INF (p = 0.75). Sovereign is used as the reference with the mean set at 1.0 for each 2-ml/min step. Millennium is only compared through 34 ml/min limit of actual range tested and Sovereign with Cruise Control through 32 ml/min of flow (also limit of actual range tested). Sovereign and Legacy values from reference number 1. INF Alcon Infiniti phacoemulsification machine; LEG Alcon Legacy phacoemulsification machine; MILL-P Bausch & Lomb Millennium phacoemulsification machine with peristaltic vacuum; MILL-V Bausch & Lomb Millennium phacoemulsifi-cat- ion machine with venturi vacuum; SOV AMO Sovereign phacoemulsification machine; SOV-CC AMO Sovereign phacoemulsification machine with the Staar Cruise Control attached to the aspiration line

than long pulses in Legacy; however, we can easily explain those results based on the fact that percent power has no meaningful correlation between these two machines [1, 3]. That still left unanswered the issue of any safety or efficiency advantages with these very short on-times.

One comparative study looked at Sovereign with and without the use of White Star. This multicentered study, in which we participated, did show that these very short on-times did clinically and statistically significantly decrease the amount of energy necessary to complete a case. Furthermore, we did show significantly less corneal endothelial cell loss with White Star [9].

With regard to wound burns, we found that the overall amount of heat produced, whether with long or short ultrasound pulse, made no difference; the only issue of importance was the percent of on-time (i.e., if ultrasound is on half the time you produce half the heat) [1]. This is no surprise, as it fits the law of thermodynamics, and cannot explain any wound burn safety protection from these very short ultrasound pulses (Fig. 5.11).

Fig. 5.11 A weight is hung with 5-0 Prolene suture around the artificial test chamber at the same point throughout, producing consistent friction of the sleeve against the phaco tip [10]

5.2.5The Variable Incidence of Wound Burn Rates

Because no large study had ever been done with regard to wound burn, and we had no idea how common a problem this might be, we decided that the best thing to do was to survey the problem. Wound burn occurs when the collagen is heated by the phaco tip to 60°C, or higher, at which time a burn will occur in a matter of seconds. Many people call wound edema, which can result from localized endothelial destruction, wound burns; therefore, we defined this as increased astigmatism, and actual traction lines around the wound showing collagen were permanently shortened by the increase in temperature. By conducting a survey in the northwestern United States we were able to show that

the incidence of wound burn was about 1:1,000 and indeed did vary between machines with Sovereign and Millennium statistically less than Legacy (p = 0.014) with regard to overall wound burn. When looking at the technique, we also found that chopping, in particular vertical chopping, was safer than either a carousel or divide-and-conquer approach (p = 0.003). The most important finding, however, was that the very short power modulations (i.e., White Star, hyper pulse, or ultra pulse) decreased wound burn in comparison to continuous ultrasound by eightfold (p = 0.0001), and in a multivariate analysis showed the strongest relationship to wound burn protection (Table 5.3) [12]. So, if indeed the duty-cycle (the amount of time on) is the only thing that is important in regard to heat generation, then why would this be protective?

Cavitational energy peaks after a few seconds of ultrasound and rapidly drops to a low steady state. So very short pulses of ultrasound create more cavitational energy than continuous ultrasound at the same power setting. Furthermore, one of the great inefficiencies of

ultrasound utilization is chatter where the longitudinal stroke repels the lens particle and no emulsification occurs. These very short on-times do not allow for much chatter because during the off-phase, flow and vacuum can recapture the particle at the tip ready for the next ultrasound pulse. So these two means of using energy more efficiently, which we have documented, result in less ultrasound time necessary to remove a cataract [9]. There would also be a commensurate protection from thermal injury.

Another possible way in which thermal injury is prevented is thermal inertia. This represents the lag time before heat is transferred to the tissue. If your on-time is brief and the off-time allows cooling, then this is another possible thermal protective mechanism for these very short pulses. By looking at the differences in heat creation in vitro, vs. 100µm from the tip in eye bank eyes, we showed that there was about 11% thermal inertia in an eye bank model for protection of wound burn [13]. At a wound burn temperature of 60°C, an energy saving of 11% would equate to 6.6°C for short vs. long ultrasound pulses. Thermography work of the wound has shown that dangerous temperatures are easily reached, so a 10% temperature saving could often spell the difference between a safe case and a wound burn.

A newer power modulation is torsional ultrasound, where the phaco tip sweeps laterally, rather than longitudinally. This virtually eliminates chatter and, as a result, improves efficiency. Furthermore, with the needle rotating in the wound rather than moving longitudinally, there is less friction generated at any power setting. A concern is that rather than using 10–15% energy, many use torsional ultrasound at the 90–100% energy setting, at which point thermal advantages may not be apparent. We do not have any study looking at wound burns with torsional ultrasound, so all of this is conjecture. One clinical finding is that torsional ultrasound does increase clogging of the phaco needle, so many surgeons add short bursts of longitudinal ultrasound, especially with harder cataracts, to minimize this clogging.

5.2.6Measuring the Amplitude of Post-Occlusion Surge

An underlying reason in determining the accuracy of phacoemulsification instrumentation has been the

Fig. 5.12 Experimental setup for postocclusion surge studies: human eye-bank eye is sitting in a 35 ml syringe (big arrow), which is fixated in the center of a styrofoam box (asterisk). The phaco tips are inserted in the eye through scleral tunnel incisions placed 90° apart and the wounds are sealed with cyanoacrylate glue. The A-scan probe, which is fixated on a burette arm holder, is sitting 1 mm above the cornea (small arrow) [14]

measurement of post-occlusion surge (POS). POS is the result of the inherent elasticity of the system, such that when the occlusion is broken, the release of this elasticity will result in a sudden inflow of fluid into the phaco tip and at its most severe presentation, will completely evacuate the anterior chamber. The parameters I safely use today on the machine I first used over 30 years ago would evacuate the entire anterior chamber after each occlusion break! Obviously things have changed dramatically over the years and an example of some of the breakthroughs to control POS are smallbore, thick-walled, low-compliance tubing to minimize this elastic rebound, and constant monitoring of the vacuum level through computerized pumps, which not only stop when the required vacuum level is reached, but can also change direction at the start of POS.

We decided that the clinically most relevant way to look at POS was to measure the anterior chamber collapse in eye bank eyes (Fig. 5.12). This is the most noticeable problem, and movement of the capsule toward the phaco tip is the most dangerous scenario we encounter. By placing multiple instruments into the same eye we could have a very fair comparison of each of the instruments. Furthermore, with our work on machine accuracy, we could adjust each of the machines; so we are comparing actual parameters and not machine-indicated parameters, which could bias the results.

Our first comparison was between Infiniti, Legacy, Millennium and Sovereign. We did find that all four machines were statistically significantly different from each other, with Millennium being the best of the lot followed closely by Sovereign, then Infiniti, and the clear outlier being Legacy, which had twice as much POS as Infiniti. We then looked at different parameters and found, not surprisingly, that if you elevate the bottle, you decrease POS, and also if you elevate flow, you increase POS. In fact, we found that Legacy with the appropriate parameters was as good as any of the other machines. We were very surprised to find that changing from a 20-gauge to a 19-gauge phacoemulsification needle for Sovereign and Infiniti almost tripled the level of POS; so this is another way to tame POS by moving to a 20-gauge needle. We found that the ABS port used with Infiniti and Legacy did improve POS dramatically for Legacy, but did not have a statistically significant effect on Infiniti, showing that the machines clearly behaved differently in how they respond to POS [15].

We were interested in POS suppression with Cruise Control and found that it did not seem to have much of an effect where POS was minimal such as with Millennium; however, in machines such as Legacy, we did have a significant effect. When we went to Millennium with a Venturi pump, however, the impact was dramatic, with POS decreased by at least fivefold and probably closer to tenfold [16].

With Stellaris (Bausch & Lomb) and Signature (Advance Medical Optics) new to the market, we then compared these machines. We looked at very aggressive parameters, which would be a low bottle height of 60cm, a 19-gauge needle, and the maximum vacuum that we can obtain at our altitude of about 550 mmHg and the maximum flow attainable by all of the machines. Because the machines were new, we did look at flow and found that when trying to get 60ml/min, Stellaris had the least flow; when it said 60ml/min, it was at 53.5 + 0.0 ml, followed by Infiniti at 55.8 ± 0.4ml/min, and the closest to actual was Signature at 58.5 ± 0.0ml/min. All of them were very close with regard to vacuum, with Stellaris overestimating vacuum (526 ± 3.3mmHg actual vs. 550mmHg machine-indicated vacuum), Infiniti closest to the actual (551.6 ± 3.5mmHg actual vs. 550mmHg machine-indicated vacuum), and Signature actually underestimating the vacuum (569.4 ± 6.0mmHg actual vs. 550mmHg machine-indicated vacuum).

So in POS comparisons of these machines in two eye bank eyes, we found that Stellaris and Signature

Fig. 5.13 Bar graph demonstrating postocclusion surge comparisons (in millimeters) in a 81-year-old human cadaver eye using three phacoemulsification machines. Vacuum was at 400 mmHg (actual), flow was at 40ml/min (actual), bottle height was at 70cm, and 19-gauge tips were used (n = 20 for each machine) [17]

were quite similar to each other and both had half or less POS than Infiniti (Fig. 5.13). Signature had a small edge over Stellaris, which was statistically significantly different in one of the eye bank eyes. With regard to the amount of vacuum required to produce flow, the clear outlier was Signature, which required approximately half the vacuum to provide the same amount of high flow as Infiniti or Stellaris (Fig. 5.14).

Additional work is ongoing for the refinement of our model. Because minor variations with regard to the size of the phacoemulsification needles can have a big impact with regard to our measurements, we are now using a stopcock to run all the machines through the same hand piece and the same phaco needle. This will be used to compare the new cassettes that have come out with Infiniti to improve their overall POS, and also for comparisons of the new vacuum-based pumps in both Stellaris and Signature, which are said

P<0.0001 all comparisons

Fig. 5.14 Bar graph demonstrating, with the machines set at 550mmHg vacuum (actual), 60ml/min machine indicated flow, 60cm bottle height, and with 19-gauge tips to measure actual flow, the mean vacuum (actual) necessary to maintain that flow for three phacoemulsification machines (n = 10 for each machine)s [18]