The phaco ultrasound probe delivers energy into the eye that can be 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 three. When we titrate the amount of ultrasound energy we place into the eye, we are keeping the frequency constant but we are increasing the stroke length and therefore, the total amount of energy.
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.
The phaco pinch test is a simple way to determine if your ultrasound power settings are likely to cause an incision burn in the eye. During wet lab testing, program your selected settings into the phaco machine, remove the protective silicone sleeve from the phaco needle, grasp the needle between your fingers, and push the foot-pedal all the way down. If your settings cause excessive heat build-up, the needle will get hot and may even burn your fingers. But it’s better to singe your fingertips than fry your patient’s cornea.
It is important to give as little ultrasonic phaco energy as possible during the cataract surgery as possible. The ultrasonic energy can easily damage the corneal endothelial cells, and excessive phaco energy can cause pseudophakic bullous keratopathy and corneal decompensation. The most important way to decrease the energy is to use a mechanical method of nucleus disassembly such as phaco chop. This is far more efficient than techniques like divide-and-conquer, resulting in less energy delivery as well as a quicker procedure.
To decrease the energy maximally, we need to decrease the phaco time and we need to decrease the average phaco power. The average phaco power can be decreased by limiting the foot pedal depression in position three or by decreasing the maximum phaco power level on the machine.
The phaco time can be decreased by only applying the ultrasonic power when cataract pieces are at the phaco tip and are not aspirated by the vacuum forces alone. Additionally, phaco time can be reduced by delivering smaller pulses or bursts of phaco energy instead of continuous ultrasound. This method of breaking up the ultrasonic energy into smaller packets of pulses and bursts is called phaco power modulation.
With optimized ultrasonic phaco power parameters, it is possible to remove cataracts with a minimal amount of phaco time and energy, giving immediately clear corneas and happy patients.
The basic power settings are continuous, pulse and burst. In the continuous power setting, continuous energy is delivered with variable power depending on how long the foot pedal is depressed. The maximum power setting can be preset and then one has control on the maximum amount of phaco power delivered, ie., the longer the foot pedal is depressed, the greater the phaco power will be.
In the pulse mode, the pulses of energy delivered have variable power depending on how long the foot pedal is depressed. The more time it is depressed, the greater the power of each sequential pulse of energy will be. 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 the “on” and “off” pulse, reduces heat and delivers half the energy into the eye.
Finally, in burst mode, each burst of energy has the same power but the interval between each burst increases as the foot pedal is depressed. The longer the foot pedal is depressed, the shorter the “off” period between each burst will be. As a result, at maximum foot pedal depression, the bursts of energy will become continuous delivery of energy.
When referring to modulations of phaco power, the terms “burst” and “pulse” may seem similar, but they refer to two entirely different concepts.
Surgeons are used to the concept of “continuous” phaco energy which is delivered in a linear fashion: as the phaco foot-pedal is depressed, the energy level increases. “Pulse” mode simply gives the same linear control of phaco energy, however the energy is always delivered in pulses. “Burst” mode defines a specific and identical “burst” of phaco energy, then 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.
Burst mode allows a true phaco-assisted aspiration of the lens nucleus. We use the vacuum and fluidics of the phaco machine to aspirate the cataract and then give small bursts of phaco energy only when necessary. Because we can program these bursts of phaco power to be very short (as quick as a few milliseconds), we 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, we do 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. We can program our machine to avoid this continuous phaco energy end-point when using burst mode.
For surgeons using a divide-and-conquer technique of surgery, the foot-pedal can be maximally depressed during grooving, thereby delivering continuous phaco energy to facilitate sculpting of the nucleus. Then to remove the quadrants, the foot-pedal is only partially depressed in position 3 so that only bursts of phaco power are used for segment removal. Finally for the epi-nucleus removal, the foot position 3 is barely entered and just a few bursts of energy are delivered for removal of the softer cataract portions.
Most phaco machines have two settings for burst mode: single burst and multiple burst. Single burst delivers just one single burst of energy, for burying the phaco probe into a nucleus for chopping. I do not ever use this mode, but instead prefer multiple burst mode because I can still deliver just one single burst by barely entering foot-position 3 and I still have the ability to deliver many more burst and varying intervals with further foot-pedal depression.
click below to watch the video explaining continuous, pulse, and burst modes: