at its resonant frequency. For example, the resistance of the aqueous is less than the resistance of the cortex, which in turn, is less than the resistance of the nucleus. As the resistance to the phaco tip varies, to maintain maximum efficiency, depending on the machine, small alterations in frequency or stroke length are created by the tuning circuitry in the CPU. This is important to minimize the excessive generation of ultrasonic energy, which is harmful to the intraocular contents. The surgeon will subjectively determine good tuning circuitry by a sense of smoothness and power.

3.2.2 Phaco Energy

The actual tangible forces that emulsify the nucleus are thought to be a blend of the “Jackhammer” energy and cavitational energy. The “jackhammer” energy is the physical striking of the needle against the nucleus. Cavitational energy generation is more convoluted. Recent studies indicate that there are two kinds of cavitational energy: low frequency energy and high frequency energy.

3.2.2.1 Low Frequency Energy

Low frequency energy is the frequency of energy that is selected by the manufacturer for the individual machine. As noted above, it is generally 27–50 Hz. It is omni directional and will reflect from surfaces such as the cornea or sclera. It travels over great distances and has no cavitational impact. It is, however, the engine that generates cavitational bubbles. These are the fuel for high frequency energy.

3.2.2.2 High Frequency Energy

Low frequency energy is the dynamic force to generate high frequency energy. When the needle, moving at the lower frequency, excites implosion of cavitational bubbles, high frequency energy is created. In its powerful configuration, transient cavitation will generate a 1-mm cone of energy extending up to 10 mm. from the phaco tip. In its inactive form, sustained cavitation does nothing more than vibrating the cavitational bubbles.

3.2.3 Transient Cavitation

The phaco needle, moving through a liquid medium at ultrasonic speeds, gives rise to intense zones of high and low pressure. Low pressure, created with the backward movement of the tip, pulls dissolved gases out of the solution, thus producing micro bubbles. Forward tip movement then creates an equally intense zone of high pressure. This initiates compression of the micro bubbles until they implode. At the moment of implosion, the bubbles create a temperature of 7,204°C and a shock wave of 5,171,100 mbar. Of the micro bubbles created, 75% implode, amassing to create a powerful shock wave, radiating from the phaco tip perpendicular to the bevel of the phaco tip with annular spread.

However, 25% of the bubbles are too large to implode. These micro bubbles are swept up in the shock wave and radiated with it. Transient cavitation is a violent event. The energy created by transient cavitation exists for not more than 4 ms and extends from the phaco tip in a cone shape distribution, in the immediate vicinity of the phaco tip, and within its lumen. It is this form of cavitation that is thought to generate the energy responsible for emulsification of cataractous material (Fig. 3.1). Additionally transient cavitation is instrumental in clearing nuclear fragments within the phaco needle preventing repetitive needle clogging.

The transient cavitational energy can be directed in any desired direction. The angle of the bevel of the

Fig. 3.1 The micro bubbles are visible at the phaco tip. The micro void around the phaco tip is replenished with fresh BSS providing fuel for transient cavitation

phaco needle governs the direction of the generation of shock wave and micro bubbles.

3.2.4 Sustained Cavitation

If phaco is energized beyond 4ms, transient cavitation, with generation of micro bubbles and shock waves, ends. The bubbles then begin to vibrate, without implosion.

No shock wave is generated. Therefore, other than mechanical energy, no cavitation energy is produced. Sustained cavitation is ineffective for emulsification (Fig. 3.2a).

a

Fig. 3.2 (a) Transient cavitation energy is shown in

blue, stabilized cavitational b energy shown in red.

Continuous power: Only the initial energy is transient. The remaining is stabilized energy. In a 50-ms pulse, only the initial 4 ms are transient. In micro pulse phaco, the entire pulse is transient energy. (b) The x-axis represents time and the y-axis millivolts. The blue tracing shows the low power (mv) created during continuous phaco. This is stabilized cavitation. The red tracing represents shorts bursts of high power (mv) generated during whitestar phaco. This is transient cavitation

Water bath, hydrophonic studies indicate that transient cavitation is significantly more powerful than sustained cavitation (Fig. 3.2b). With this information in mind, it would appear that continuous phaco is best used to emulsify the intact nucleus, held in place by the capsular bag, during the sculpting phase of divide and conquer or stop and chop. In this setting, jackhammer energy is most important for emulsification.

Transient cavitation is maximized during micro pulse phaco. This is best used during phaco of the nuclear fragments in the later phase of the above two procedures, or during phaco chop procedures [1].