THE BASICS OF POWER MODULATION
Describing how the Millennium with CCS works involves learning some new terms, which were not necessary when power-modulation options were more limited. If the surgeon wants to modulate phaco power based on the number of distinct shots of ultrasound energy per second, each shot is usually called a pulse. The length of the off period between pulses, or the interval, varies depending on the duty cycle, which in pulse mode is the percentage of each second that the surgeon wants the power to be delivered.
Alternatively, the surgeon can modulate power by varying the relative lengths of the on and off periods. In this case, the shots of ultrasound energy are called bursts or microbursts. The duty cycle is the ratio between the burst's length and the cycle time (burst's duration + rest interval). If the surgeon chooses a mode that varies the duty cycle based on foot pedal position, cycle time also will vary as the pedal moves.
FIVE ULTRASOUND MODES WITH CCS
There is no pulse or burst modulation, and linear power control is provided between the surgeon's programmed minimum and maximum power levels.
The surgeon can choose 0 to 120 pulses per second and a 10% to 90% fixed duty cycle. Previously, only a single duty cycle (50%) and up to 20 pulses per second were available in this mode. There are two options for rise time, a term referring to the pattern in which ultrasound power is delivered. Rise time 1 gives the traditional solid spike of power with each pulse. Rise time 2 yields a linear increase in power of more than 250 milliseconds. Then, the cycle repeats (Figure 1).
Single Burst Mode
The surgeon chooses microbursts of ultrasound, ranging from 4 to 600 milliseconds. Power is fixed, and each activation of the foot pedal to the end of position three provides a single burst of ultrasound energy.
Fixed Burst Mode
The surgeon chooses microbursts of ultrasound on and off times, ranging from 4 to 600 milliseconds. Power is linear between the surgeon's programmed minimum and maximum levels.
Multiple Burst Mode
The surgeon chooses a fixed pulse duration (from
4 to 600 milliseconds), a fixed power, and a maximum duty cycle of 50% to 100%. The foot pedal controls the length of the intervals between ultrasound bursts. The cycle time (burst's duration + rest interval) starts at 1,200 milliseconds and decreases as the foot pedal is pressed.
TWO RISE-TIME SETTINGS
The CCS offers a choice of two ultrasound rise times. Rise time 1 is the conventional square wave pulse available on all phaco systems (Figure 2A). Rise time 2 gradually ramps up the power to achieve a “pulsed pulses” effect. It reaches the same peak power as in rise time 1 but with less total energy expended (Figure 2B).1
Rise time 2 is a power modulation that creates a “packet” or “envelope” of multiple micropulses in which the total on time equals 250 milliseconds. The individual micropulses within the envelope start at zero power and gradually increase in amplitude to the user's selected maximum power level. The power then immediately returns to zero and begins rising again. As a result, each envelope has a saw-toothed or waveform pattern when viewed on an oscilloscope.
When the power resets to zero, the surgeon will notice a discernible pause between each waveform. During that time, followability improves, and there is more opportunity to occlude the phaco tip to achieve a vacuum holding force. This force maximizes the efficiency of delivered power to the nucleus as it is held against the needle. Thus, less total energy is needed to emulsify the cataract, and less energy is dispersed within the eye to impact the cornea, iris, and ciliary body.
LABORATORY AND CLINICAL STUDIES WITH CCS
In a recent wound temperature study on the new power modulations,2 investigators tested five power-modulation combinations. They included three combinations of fixed microburst mode settings, with durations of 4 milliseconds on, 4 milliseconds off; 6 milliseconds on, 12 milliseconds off; and 6 milliseconds on, 24 milliseconds off. Power was linear, varying between 20% and
80% in 10% intervals. For each percentage power, the foot pedal was completely depressed to ensure maximum power.
Two hyperpulse mode settings were used with variable duty cycles (eight or 75 pulses per second with a 30% duty cycle). During each pulse, power ramped up automatically from zero to a predetermined maximum, which ranged from 20% to 80%.
For each modality, investigators measured temperatures with and without occlusion of the aspiration line. In the unoccluded state, the maximum temperature rise was 29.0ºC. In the occluded state, the maximum temperature rise was 38.6ºC, with the phaco pedal depressed for 3 minutes. This study demonstrated temperature rises that were well below the clinical thresholds for wound burn or endothelial damage during phacoemulsification.
Reports at the 2005 meeting of the ASCRS confirmed that clinicians using the CCS are experiencing clinical benefits. Uday Devgan, MD, of Sun Valley, California, compared the results of 100 phaco procedures done with the Millennium at conventional settings against 100 cases performed with the CCS.3 The CCS group required half (5%) the average phaco power and had 15% more eyes with clear corneas and 20/20 UCVA. The CCS also reduced the mean absolute phaco time to
1.5 seconds from 2.8 seconds. (Absolute phaco time, also called effective phaco time, expresses how long phaco energy would have been delivered to the eye if 100% power had been used continuously. It is calculated by multiplying the total phaco time by the average percentage of power used.4)
In a similar study, Cipres and Milla compared the results of the fixed burst versus multiple burst mode on the Millennium. They found that the total energy delivered to the eyes decreased from 12.71mJ for fixed burst eyes to 7mJ with multiple burst mode (P<.005).5 There was no trace of corneal edema in 95% of the multiple burst eyes, and the BCVA was 20/25 or better in 70% of the cases.
ADVANCED FLUIDICS CONTROL
The Millennium allows the surgeon to choose from two different fluidic strategies, the traditional venturi fluidic modality and a peristaltic modality. The new Advanced Flow System on the Millennium employs a closed fluid design for maximum patient protection against bacterial infection, minimized transducer volume, and rigid pump head tubing for low compliance (see sidebar, Safety and Benefits of the Advanced Flow System Technology, on page 78). The tubing's large inner diameter increases flow as well as the wall's thickness for improved resistance against kinking and less compliance of the aspiration tube.
PERSONAL EXPERIENCE AND SUMMARY
Individually, or in various combinations, the surgeon can use the Millennium's new software to choose linear or fixed power, a duty cycle from 10% to 90%, and pulse and interval times of 4 to 600 milliseconds—the widest range of pulsing durations available. The surgeon may preset these modulations at a single level or vary them between a personal minimum and maximum with Bausch & Lomb's proprietary Dual Linear foot pedal. The CCS allows physicians to use their own presettings to control the amount of ultrasound energy delivered to the eye.
For the quick-chop technique (all modes), the recommended parameters are a vacuum range of 140 to 325mmHg and an infusion-bottle height of 115cm. When using pulse mode, the ideal selections are eight pulses per second and a variable duty cycle with a maximum of 30%. In fixed-burst mode, one should employ 4-millisecond pulses and intervals or pulses of 6 milliseconds followed by 12-millisecond intervals and linear power varying from 0% to 30%.
When sculpting, surgeons should use a vacuum of 40mmHg and a variable power from 0% to 30%. In fixed-burst mode, 4-millisecond pulses and intervals or pulses of 6 milliseconds followed by 12-millisecond intervals are recommended.
Microbursts of ultrasound power as short as 4 milliseconds not only reduce the total amount of phaco energy in the eye, but they also virtually eliminate the cavitational effects that cause chatter. The followability and hold one can achieve is tremendous. The lenticular fragment does not move from the tip.
The Millennium allows the surgeon to access and control flow, vacuum, and ultrasound power simultaneously to the extent necessary.
Rosa Braga-Mele, MD, MEd, FRCS(C), is Assistant Professor of Ophthalmology at the University of Toronto, and Director of Cataract Unit and Surgical Teaching at Mount Sinai Hospital in Toronto. She is a consultant for Bausch & Lomb but states that she does not hold a financial interest in any product mentioned herein. Dr. Braga-Mele may be reached at (416) 462-0393; email@example.com.
1. Braga-Mele R, Devine T, Packer M. The Millennium. In: Fine HI, Packer M, Hoffman RS, eds. Refractive Lens Surgery. Berlin, Germany: Springer; 2005: 213-220.
2. Braga-Mele R. Thermal effect of microburst and hyperpulse settings during sleeveless bimanual phacoemulsification with advanced power modulations. J Cataract Refract Surg. In press.
3. Devgan U. Effect of advanced power modulation on total phaco energy and patient outcomes in 200 eyes. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; April 2005; Washington, DC.
4. Fine IH, Packer M, Hoffman R. Use of power modulations in phacoemulsification—choo-choo chop and flip phacoemulsification. J Cataract Refract Surg. 2001;27:188-197.
5. Cipres M and Milla E. Custom control software in microincision cataract surgery. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; April 2005; Washington, DC.