Fundamentals of Ultrasonic Phacoemulsification Power
The phacoemulsification ultrasound probe delivers energy into the eye that is used to break up the cataract to facilitate emulsification and aspiration. It accomplishes this by vibrating at a fixed frequency when the foot pedal is depressed to position 3. To increase the amount of ultrasound power, the machine simply increases the stroke length of the probe.
Traditionally the probe delivers power only in a longitudinal manner, with the phaco needle moving forward and back. Recent innovations in phaco technology also allow for the delivery of power through a lateral motion. Delivering ultrasound power through lateral motion can increase cutting efficiency by reducing repulsion of lens material.
The 2 types of lateral motion in phacoemulsification are torsional, in which the phaco tip oscillates in a rotational manner along its primary axis, and transversal, where the phaco tip moves in an elliptical path. Because of their types of motion, torsional works best with an angled phaco needle while transversal works equally well with a straight or angled needle. Combining lateral motion phaco with traditional longitudinal phacoemulsification can aid cutting efficiency, since the cataract material is emulsified in more than one direction (Figure 6).
The stroke of the phaco needle creates a mechanical impact as the metal phaco needle hits the cataract material. It also creates cavitation and implosion as a microvoid is created just in front of the phaco needle. A fluid and particle wave is propagated into the cataract material and finally, heat is created as a by-product. It is important to avoid choosing phaco power settings that cause excessive heat build-up as this can burn the cornea and damage the delicate ocular structures. Unrestricted flow through the surrounding irrigating sleeve is also very important, as the constant cooling effect of balanced salt solution moving around the phaco probe helps to prevent heat build-up.
During surgery, the phaco machine keeps track of the average phaco power, given as a percentage of maximum, as well as the total time during which phaco ultrasonic power was delivered. The machine displays these settings as “U/S AVE,” which stands for “ultrasound average,” and “EPT,” which is “elapsed phaco time.” The total energy delivered into the eye is the product of the phaco power multiplied by the time the power is on, known as the absolute phaco time (APT). The phaco machine will automatically calculate the APT by multiplying the “U/S AVE” by the “EPT,” so that the surgeon can compare the total ultrasonic energy delivered in different cases.
Delivering 15 seconds of 100% power is the same energy as delivering 30 seconds of 50% power, or 60 seconds of 25% power. For each of the 3 examples in Figure 7, the APT is 15 seconds.
It is important to use as little ultrasonic phaco energy as possible during the cataract surgery. The ultrasonic energy can damage the corneal endothelial cells, with excessive damage leading to corneal decompensation.
To decrease the APT maximally, the surgeon needs to decrease the phaco time and/or the average phaco power. The average phaco power can be decreased by limiting the foot pedal depression in position 3 when using a linear controlled power mode or by decreasing the maximum phaco power level on the machine. The phaco time can be decreased by applying the ultrasonic power only when cataract pieces are at the phaco tip and vacuum alone is insufficient to aspirate the piece. Additionally, phaco time can be reduced by delivering shorter pulses or bursts of phaco power instead of continuous ultrasound power or by decreasing the duty cycle (the ratio of the on:off pulses). This method of breaking up the ultrasonic power into smaller packets of pulses and bursts is called phaco power modulation.
Continuous, Pulse, and Burst Phacoemulsification Modes
The basic power settings are continuous, pulse, and burst. In the continuous power setting, energy delivery is continuous with variations in power, controlled by the amount of foot pedal depression.
In the pulse mode, the pulse power increases linearly by how far down the foot pedal is depressed. The farther it is depressed, the greater the power will be of each sequential pulse of energy. The defining feature of pulse mode is that after each pulse of energy delivered, there is a period of time in which no energy is delivered between increasing pulses of energy, the “off” period. Alternating between equal “on” and “off” pulse times reduces heat and delivers half the energy into the eye.
In burst mode, each burst has the same power but the interval between each burst decreases as the foot pedal is depressed. The farther the foot pedal is depressed, the shorter the “off” period will be between each burst. As a result, at maximum foot pedal depression, the bursts of energy will become continuous delivery of energy.
In referring to modulations of phacoemulsification power, the terms burst and pulse may seem similar, but they refer to entirely different concepts. Surgeons are used to the concept of “continuous” phacoemulsification power that is delivered in a linear fashion: as the phacoemulsification foot pedal is depressed, the power level increases. “Pulse” mode simply gives the same linear control of phacoemulsification power; however, the energy is always delivered in pulses. “Burst” mode defines a specific and identical “burst” of phacoemulsification energy. As the foot pedal is depressed, these identical bursts of energy are delivered more rapidly, until the interval of time between bursts is infinitely small. A key advantage of burst mode is that it allows the surgeon to titrate the rate of delivery of these tiny bursts, which can be as short as a few milliseconds.
Burst mode allows a true phaco-assisted aspiration of the lens nucleus. The surgeon uses the vacuum and fluidics of the phaco machine to aspirate the cataract and then gives small bursts of phaco power only when necessary. Because one can program these bursts of phaco power to be very short (as quick as a few milliseconds), one can effectively give hundreds of tiny bursts and still total less than 1 second of total phaco time. Because the phaco foot pedal now controls the rest interval between identical bursts, one does not have linear control of the phaco power level. For this reason, it is important to use a lower phaco power setting when using burst mode as compared to pulse or continuous modes. When the foot pedal is maximally depressed, the rest interval between bursts is zero and the phaco probe essentially delivers continuous energy (Figure 8).
Hyper Settings for Power Modulation
The range of programmability of the pulse and burst phacoemulsification settings has expanded considerably. While previous generations of phaco platforms had pulse rates of up to 20 pulses per second, the newer-generation machines have the ability to deliver up to 120 pulses per second. Similarly, the older machines had burst widths as narrow as 30 milliseconds, while the new platforms are able to deliver burst widths as fine as just 4 milliseconds.
The advantage of this upgraded range of programmability is the smoothness and precision of power delivery. With the standard settings in pulse mode, where each pulse is as long as each rest period, the pulse mode can deliver good cutting power with half the energy of continuous phaco power. The more pulses per second given, the smoother the power delivery will be—very similar to serrations on a knife.
Power modulation using hyper settings also allows the surgeon to reduce the total amount of energy released in the eye. For example, simply changing from continuous phaco power to a hyper pulse rate of 100 pulses per second allows the surgeon to cut total energy delivery in half. This halving of the ultrasound energy will result in less endothelial cell damage, less heat production, clearer corneas, and sharper vision immediately post‑op. Surgeons who perform the divide-and-conquer method of nucleus phaco can switch to a hyper pulse mode and immediately perform better surgery without a change in technique.
Note, however, that simply changing the number of pulses per second alone does not change the amount of energy delivered into the eye. Whether the surgeon gives 2 pulses per second or 8 pulses per second, the total energy, as represented by the green blocks (Figure 8) is the same. The same applies when comparing 10 pulses per second to 100 pulses per second. Reduction in the energy delivered is accomplished by decreasing the time that the power is on by altering the ratio of the on:off pulses (Figure 9). The lower pulse rates tend to be better for emulsifying nuclear fragments since there is a significant time interval between pulses for the fluidics to keep the cataract piece attracted to the phaco needle. The higher pulse rates tend to work better for sculpting or grooving the nucleus since the narrow time interval between pulses produces a smoother delivery of ultrasonic energy.
Variable Duty Cycle
Ultrasound energy creates helpful cavitation and mechanical forces that are used to break up the cataract nucleus; however, this energy also can create significant heat. The jackhammer effect of ultrasound energy can cause repulsion of the nuclear fragments from the phaco tip. It is helpful to alternate periods of phacoemulsification energy with rest periods that serve to achieve cooling of the phaco needle and aspiration of the nuclear fragments. If the surgeon changes the ratio of the on period, when ultrasound energy is delivered, to a shorter duration, then the surgeon can favor the aspiration and cooling of the phaco needle over the heat generation and jackhammer repulsion effects of the ultrasound.
With the choice of a mode such as pulse mode, which alternates phaco power pulses with periods of rest, the default ratio is 50:50. This is called a 50% duty cycle, as each complete cycle is composed of power on for 50% of the time, then power off for 50% of the time. This default ratio can be changed to alter the ratio of ultrasound energy to the rest interval. For example, 40% results in 40 msec on, 60 msec off giving a ratio of 40:60. The surgeon can then harness the benefits of a lower duty cycle, which results in longer cooling time for the phaco needle, thus decreasing the amount of phaco energy delivered to the eye. In addition, during the extended “off” time, no energy is delivered and nuclear fragments can be easily aspirated (Figure 10).
When are higher or lower duty cycles preferred? The answer depends on the phase of surgery. For sculpting the nucleus, such as with the technique of divide-and-conquer, the surgeon needs to deliver sufficient energy to be able to cut the grooves. This requires a duty cycle of about 40% to 60%. Once the surgeon has placed grooves in the nucleus and has achieved cracking that results in quadrants, a lower duty cycle can be used during the phaco-assisted aspiration of the quadrants. For this quadrant removal, a lower duty cycle of 20% to 40% can be used since the principal force for aspiration is the fluidics and not the ultrasound.
Using the variable duty cycle programming allows the surgeon to deliver just the right amount of ultrasound energy during each phase of surgery. The concept to remember is that a higher duty cycle results in better cutting power but increased heat generation and more energy-related damage to the corneal endothelium. Using the lower duty cycle allows more fluidic aspiration of nuclear fragments while minimizing heat and phaco power, resulting in clearer corneas immediately after surgery. Clear corneas on postoperative day 1 make for good visual acuity and very satisfied patients.