By Graham W. Lyles, BA, and Kenneth L. Cohen, MD
Edited by Sharon Fekrat, MD, Ingrid U. Scott, MD, MPH, and Steven I. Rosenfeld, MD
This article is from April 2007 and may contain outdated material.
Part of what makes modern cataract surgery so exciting is how rapidly the procedure has been refined. Recently, there has been a trend toward the use of smaller incisions and less phaco energy. Bimanual microincisional cataract surgery (MICS) has received much attention from surgeons interested in transitioning from coaxial phacoemulsification. Proponents of MICS have cited smaller, safer incisions (allowing for a more stable anterior chamber), more flexibility for placement of the hand pieces and better followability.
MICS has paved the way for advances in digital power modulation, which, due to its efficiency, allows the surgeon to use less total phaco energy over less time. This minimizes heat production and the risk of thermal injury1 and contributes to less corneal swelling and endothelial cell loss.2
Power modulation also allows improved followability with less chatter. Each time the phaco power is on, the needle moves forward, microscopically pushing the nuclear material away. Using any form of pulse mode allows for regrasping or better followability during the phaco-off periods. Before incorporating these advances into your practice it is best to become familiar with the technology that is being used. (Siebel has written a good book on this topic.3) The following terms and ideas provide a foundation for understanding the intricacies of power modulation, but be aware that different manufacturers may use their own terminology.
Terminology
Continuous phacoemulsification is exactly what the name implies - continuous ultrasound energy that is delivered with no "off" period. Continuous mode is a basic form of power modulation. It allows linear control of phaco power using the foot pedal. The maximum power is preset by the surgeon. As the foot pedal is depressed, the power rises, from 0 percent to the preset maximum power (up to 100 percent).
All other modes of power modulation are unlike continuous mode because they are various combinations of phaco-on periods followed by phaco-off periods. As an exercise, consider the analogy of incandescent vs. fluorescent light. An incandescent bulb generates light continuously - the light is adequate but the bulb is hot and energy is lost as heat. Fluorescent bulbs are not always "on"; rather they generate light by pulsing (from 60 to 100 Hz). The fluorescent bulb produces adequate light and remains cool by consuming 75 percent less energy than the incandescent bulb.
Pulse mode is a basic type of power modulation that relies on alternating phaco-on time with phaco-off time. Each shot of ultrasound energy is referred to as a pulse. Again, similar to continuous mode, the surgeon is able to linearly control the power delivered via foot pedal positioning. However, with phaco-on time automatically followed by an equal amount of phaco-off time, one effectively reduces the total amount of power delivered by one-half as compared with continuous ultrasound.4
Cycle time is the total time of one phaco pulse plus its following pulse-off period. A 500 ms pulse followed by a 500 ms pulse-off time gives a cycle time of one second. Some manufacturers use the term "pulse interval."
Duty cycle is the ratio of phaco-on time to total (or cycle) time, expressed as a percentage. In other words, phaco-on time / (phaco-on time + phaco-off time). An important concept to grasp is that duty cycle is independent of the actual number of ultrasound pulses per second. For example, setting power modulation at one pulse per second with a 50 percent duty cycle would give 500 ms phaco-on + 500 ms phaco-off. Setting power modulation at four pulses per second with a 50 percent duty cycle would give 125 ms phaco-on + 125 ms phaco-off. The cycle time in this latter example is 250 ms, while the first example has a cycle time of one second.
Traditional pulse mode has a fixed duty cycle of 50 percent, which translates to 500 ms of ultrasound energy delivered per second.
Burst mode, when set with a fixed power, is termed multiple-burst mode. Multiple-burst mode allows independent control of the length of time of the ultrasound shot (now referred to as the burst duration) and the duty cycle. After setting the burst duration (from 80 to 600 ms, which is machine-dependent), you can then set a minimum and maximum duty cycle. Depression of the foot pedal in position 3 allows linear control of the duty cycle between this minimum and maximum. An example will help clarify how the duty cycle is controlled. You can select a burst duration of 200 ms, and preset, say, a minimum and maximum duty cycle of 20 percent and 80 percent. You then control the duty cycle from 20 percent (200 ms ultrasound-on with 800 ms ultrasound-off) at the top of foot position 3 to 80 percent (200 ms ultrasound-on with 50 ms ultrasound-off) with the foot pedal maximally depressed. Essentially, the further the pedal is depressed, the shorter the phaco-off time. If the maximum duty cycle is set at 100 percent, then with maximum foot depression the phaco-off time can become infinitely small and approach continuous ultrasound delivery.4
Hyperpulse settings expand on traditional pulse technology. Hyperpulse allows the surgeon to choose from new, higher-range pulse settings that can exceed 100 pulses per second (compared with traditional pulse, which had a maximum of 20 pulses per second). The surgeon does not change the amount of phaco energy used by using more pulses per second because each short phaco pulse is followed by a short phaco-off time. The increased frequency of phaco-off periods does, however, reduce the amount of heat generated.1
Hyperburst technology allows you to use burst durations as short as 4 ms (compared with traditional burst, which had a minimum of 80 ms). These "microbursts" are very small amounts of phaco energy that reduce the total amount of energy used, and the "microrests" allow for dissipation of thermal energy.2
Variable rise time is a form of power modulation that modifies the shape of individual pulse's waveform. Traditionally, pulses were delivered in square waves. Variable rise time creates individual pulses that gradually ramp up in power over a preset duration to create waveformed pulses. This reduces the repulsive effect of the pulses and uses less total energy.4
Typical Power Modulation Settings
The following are a few typical settings on the Alcon Infiniti Vision System: for sculpting, a maximum preset power of 60 percent with 60 pulses per second and a duty cycle of 50 percent; for quadrant removal, a maximum preset power of 40 percent with 10 pulses per second and a duty cycle of 35 percent; and for an epinucleus, a maximum preset power of 20 percent with 10 pulses per second and a duty cycle of 25 percent.
Conclusion
By effective use of power modulation, the surgeon can decrease the energy used, shorten phaco time and improve efficiency, all of which may produce less endothelial cell loss, thus clearer corneas with improved outcomes.1,2
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1 Hoffman, R. S. et al. Curr Opin Ophthalmol 2005;16:38-43.
2 Fishkind, W. et al. J Cataract Refract Surg 2006;32:45-49.
3 Seibel, B. S. Phacodynamics: Mastering the Tools and Techniques of Phacoemulsification Surgery, 4th Edition (Thorofare, N.J.: Slack Inc., 2005), 114-125.
4 Devgan, U. Ophthalmol Clin North Am 2006;19(4):457-468.
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Mr. Lyles is a fourth-year medical student and will soon be a resident at the University of North Carolina, and Dr. Cohen is professor of ophthalmology there.