Fundamentals of Ultrasonic Phacoemulsification Power – Phacoemulsification accomplishes three simultaneous functions through a closed system: emulsification, aspiration and irrigation. It is important to understand each component to perform safe and effective cataract surgery.
The goals of phaco fluidics are to have sufficient power to remove emulsified particles, maintain the stability of the anterior chamber while removing the cataract and minimise heat production by the oscillating phaco needle.
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.
To increase the amount of ultrasound power, the machine simply increases the stroke length of the probe.
Changing the power only changes the stroke length; frequency remains the same. Power produces heat and could generate flow resistance, but too much power generates energy dispersion in the eye, releasing free radicals in the anterior chamber, which can be harmful.
Free radical production is time and power related, so we need to use as little power as we can for the shortest time possible. However, too little power stresses the zonules.
Power modulation reduces ultrasound power in the eye during surgery, reduces free radicals, and engenders less endothelial damage.
It can be done with linear control, micropulsing and microbursts, and we can change pulse modes between unoccluded and occluded. Alternative phaco tip movements such as torsional and transversal are also useful.
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 goal in cataract surgery is perfection in nuclear disassembly performed with just the right amount of phaco energy— not too much, not too little.
Excessive ultrasound can result in endothelial cell loss, corneal edema, and corneal wound burn. Too little can result in movement of the capsular bag or zonular stress, difficult nuclear disassembly, too much irrigation fluid used, and long surgical times.
Traditional longitudinal phaco is a powerful technology to break up very dense nuclei using a back-and forth jackhammer motion, but that comes with associated disadvantages.
The process, with its long stroke length and high power, can result in repulsion of nuclear fragments at the phaco tip and no cutting performed during the backward movement of the tip—making it important to limit the power settings that cause excessive heat buildup, which has the potential to damage ocular tissues.
New adjunctive developments in phaco technology can alter the direction by which the power is delivered using lateral and rotational motions, referred to as transversal and torsional phaco.
In contrast to the jackhammer effect of longitudinal phaco, a combination approach results in transversal ultrasound that combines simultaneous longitudinal back-and-forth motion with transversal movement in an elliptical fashion.
This adds to the cutting efficiency because the nuclear material is emulsified in more than one direction. In torsional phaco, the tip oscillates in a rotational manner along its primary axis, works best with an angled phaco needle.
Using this technology, the rotation of the tip provides an additional cutting movement through the nucleus. The primary advantage increases energy efficiency with minimal repulsion at the tip; the disadvantages are that a curved phaco tip can be difficult to move through a dense nucleus and there is significant movement of the tip.
The timing of power delivery can be modified to increase phaco efficiency, with the basic settings of continuous, pulse, and burst power. When power is continuous, the maximum power delivered is controlled by depressing the foot pedal; the power increases with the degree of depression of the foot pedal.
The amount of power delivered can be limited by using phaco pulse mode; after each pulse is delivered, there is a period of time in which no energy is delivered. These periods of rest in power delivery allow cooling of the phaco needle and reduce heat and energy delivered to the eye.
These on-and-off periods are referred to as the duty cycle; standard pulse has a 50% duty cycle, during which the ultrasound is on half of the time no matter how many pulses per second are delivered.
The duty cycle can be altered to change the ratio of the on and off time. For example, a 20% duty cycle results in 20 milliseconds on and 80 milliseconds off during each cycle. The long off time allows easy aspiration of the nuclear fragments.
There can be scenarios when higher or lower duty cycles serve a better purpose. During sculpting, energy is needed to create a groove in the lens, meaning that higher rates work better because the shorter time between pulses results in smoother delivery of energy; when nuclear quadrants are being removed, a lower duty cycle is better to emulsify the fragments because there is a long interval between the pulses, which facilitates aspiration.
Finally, using burst mode, each burst has the same power, but the interval between bursts decreases with foot-pedal depression. With more depression of the foot pedal, the shorter the off time is between bursts.
The identical bursts of energy are delivered more and more rapidly with foot-pedal depression; with maximal depression, the time interval between bursts is very small, making for continuous energy delivery.
This mode allows for true phaco-assisted aspiration of the nucleus; the fluidics and vacuum can be used for aspiration and give small bursts of power as needed. Lower phaco power settings should be used in burst mode compared with pulse or continuous mode because of the absence of linear control at the phaco power level.
Regarding the choice of these 3 modes, the ability to program timing and duration combined with the directional modes (ie, transverse and torsional phaco) allow the surgeon to have highly elegant control and precise ultrasound power delivery.
Femtosecond laser-assisted cataract surgery (FLACS) is another technology that has resulted in increased efficiency in nuclear disassembly. FLACS allows the surgeon to decide the pattern of lens fragmentation.
The femtosecond pretreatment significantly reduces the effective phaco time compared with conventional phaco surgery, thus reducing loss of endothelial cells and allowing faster visual rehabilitation.
FLACS works through photodisruption, which vaporizes the targeted tissues. This is similar to the mechanism of phaco, but it occurs at the near-infrared wavelength, which is not absorbed by optically clear tissues.
This laser is focused to 3 microns and is especially useful for targeting specific depths. The waves dissipate approximately 100 microns from the targeted tissue, and the endothelium is not affected.
Using FLACS to create the quadrants, there is no need to phaco to sculpt the lens; the surgeon can go directly to removal of the quadrants after nuclear disassembly along the lines created by the laser.
As a result, lower total energy is used in the eye, making nuclear removal easier. The disadvantages of FLACS for experienced highvolume surgeons are the cost, increased surgical time, and minimal long-term enhanced visual outcome.
However, FLACS may be advantageous for use in brunescent cataracts of patients with Fuchs’ dystrophy. There are many means of modulating phaco power to achieve outstanding results in cataract surgery and provide the best patient care.