3.3 Modification of Phaco Power

Application of the minimal amount of phaco power/ energy necessary for adequate emulsification of the nucleus is desirable. Unnecessary power/energy is a cause of heat with subsequent wound trauma, endothelial cell trauma, trabecular meshwork trauma, and iris trauma with an alteration of the blood-aqueous barrier. Phaco power/energy can be modified by the alteration

1.In stroke length

2.Of duration

3.Of emission

3.3.1 Alteration of Stroke Length

Stroke length is determined by foot pedal adjustment. When set for linear phaco, depression of the foot pedal will increase stroke length and therefore power.

3.3.2 Alteration of Duration

The duration of the application of phaco power has a dramatic effect on overall power delivered. Usage of burst or pulse mode phaco will considerably decrease the overall power delivery. New machines allow for a period of power alternating with a period of aspiration only.

Burst mode and Pulse mode are opposite sides of the same coin.

3.3.2.1 Burst Mode

There is a surgeon set panel level of energy, which is fixed, with linear intervals between bursts. The interval shortens as the foot-pedal is depressed.

3.3.2.2 Pulse Mode

There is a fixed interval between pulses, but linear power, so that as the foot-pedal is depressed, the number (singular) of bursts remain the same but the power is increased.

Micro Pulse (Hyper-Pulse)

Through the development of highly responsive and low mass piezo crystals, combined with software modifications, the manufacturers of phaco machines have shortened the cycle of on and off time. This process, patented by AMO (Advanced Medical Optics) is called “micro pulse” or “Whitestar®.” This technology is now available in most phaco machines.

A duty cycle is defined as the length of the time of power on combined with power off. (Fig. 3.3) Within the duty cycle, the duration of on and off time is variable depending on the phase of the procedure.

The short bursts of phaco energy followed by a short period without phaco energy allows two important events to occur. First, the period without phaco energy permits the nuclear material to be drawn toward the phaco tip to increase efficiency. Second, the absence of power allows inflow of the irrigating fluid into the micro cavity between the phaco tip and the nuclear fragment. This renewal of fluid is important to provide new fuel for transient cavitation as well as for the cooling of the phaco tip (Fig. 3.1).

The cool phaco tip has been termed “cold phaco.” This is a misnomer as the phaco tip is not cold, but warm. However, studies indicate that it will not develop a temperature greater than 55°C which is the temperature required to create an incision burn [2, 3]. Phaco techniques such as phaco chop utilize minimal periods of power in pulse or burst mode, or micro pulse mode, to reduce superfluous power delivery to the anterior chamber. In addition, the use of pulse mode, or micro pulse mode, to remove the epinucleus provides for an

Fig. 3.3 A duty

cycle consists of a period of phaco power on (blue) followed by a period of power off (white)

Pulse

Photo Courtesy of AMO