PHACO FOCUS

Ever since Charles Kelman and Anton Banko introduced phacoemulsification in 1967, phaco machines have continued to evolve, giving greater precision, effectiveness, safety and speed. With continuing advances and newer power modulations, surgeons need to keep abreast of technology.

This article discusses terminologies that need to be understood with regard to phaco power.

Nucleus emulsification is achieved using ultrasound energy by applying an electrical field to a piezoelectric crystal causing it to oscillate.

These oscillatory movements are transmitted to the phaco tip causing it to vibrate back and forth. Back and forth movement of the phaco tip creates jackhammer and cavitation effects as well as acoustic and fluid waves. The transient cavitation effect creates small air bubbles that implode releasing large amounts of energy. This together with the jackhammer effect causes breakdown of nuclear fragments.

Phaco power is used to emulsify the nucleus. Frequency and stroke length contribute to phaco power. Frequency is fixed and ranges from 27 to 50kHz for most machines. Stroke length is the length of movement of the needle and ranges between 2-4 mil.

Though the back and forth movement emulsifies nucleus, the forward movement of the tip also causes lens material to be pushed away, thereby decreasing followability. Phaco power modulation is important to increase followability and to decrease chatter and heat production.

Excess phaco power can cause wound burn and endothelial loss. Tuning the phaco machine prior to using it is important to make sure that the electronic circuits are working well. This is of importance to increase efficiency while decreasing thermal effects.

PHACO POWER DELIVERY

Continuous Mode phaco: Continuous delivery of ultrasound occurs with no time in-between for cooling the tip or for nuclear fragments to move back towards the tip. This results in decreased followability and increased heat generation, thereby decreasing efficiency and safety.

Non-continuous modes of phaco have on and off periods of ultrasound energy. During the off period, the pieces get attracted to the tip again, thereby increasing followability and decreasing energy delivered. Pulse mode phaco delivers phaco in pulses with on and off times.

Duty cycle refers to percentage of phaco-on to total phaco time and default value set is 50 per cent.

Cycle time refers to time taken for one phaco pulse and its subsequent off period. The cycle time determines pulses per second and can generally go up to 20pps. The total energy delivered can be changed by changing duty cycle but not by changing cycle time.

Hyperpulse mode can go up to 120pps. Using very high pulse rates gives the same efficiency as continuous phaco but with less energy delivered.

Burst mode phaco gives bursts of phaco energy at fixed power but variable control of burst period to as low as 80 milliseconds. On depressing the foot pedal, the bursts come closer together at the fixed phaco power setting till at the bottom of the foot pedal excursion, it is continuous phaco.

A minimum interval or endpoint duty cycle may be programmed to prevent continuous phaco and overheating at the lowest foot pedal depression. Single and multiple bursts are also possible.

Hyperburst mode can go as low as four milliseconds. These modulations also result in less total energy used and less
heat build up.

Maximum phaco power is preset by the surgeon. In linear phaco delivery, with depression of the foot pedal, increasing levels of energy are delivered up to the preset level at the bottom of the foot pedal. This can be set in both continuous and pulse phaco.

Panel controlled phaco delivery delivers the full preset phaco power as soon as the pedal is depressed into foot position 3. Actual phaco power in linear mode denotes the actual power being delivered at that instant.

Phaco time refers to the time that phaco is used. EPT refers to effective phaco time at 100 per cent power and equals the product of phaco time and average phaco power.

Normally, power level delivered in each pulse or burst starts directly at the full level proportionate to the foot pedal position. A gradual rising up of the power level in individual pulse or burst allows a variable rise time. This decreases the power used for phaco, decreases chatter and also increases efficiency. A variable duty cycle similarly prevents continuous phaco energy even with maximum foot pedal depression.

LONGITUDINAL AND NON-LONGITUDINAL PHACOEMULSIFICATION

Phacoemulsification can be done using traditional longitudinal ultrasound or the newer torsional and transversal ultrasound modes. In longitudinal phaco, the needle tip moves back and forth in a longitudinal direction coring the nucleus and decreasing followability.

Non-longitudinal modes have better followability, generate less heat, are safer and more effective and act by shaving nuclear material. However, when wanting to impale the nucleus as for phaco chop, longitudinal phaco offers a better grab of the nucleus than non-longitudinal modes.

In torsional phaco, the phaco tip oscillates around its axis. With an angled Kelman phaco tip, significant movement of the end of the tip is attained. It shaves nuclear material effectively and decreases repulsion of nuclear material from the tip that occurs with longitudinal phaco.

By virtue of the bent tip, it also has the advantage of achieving a longer stroke path at its tip while the movement within the corneal tunnel is much less, thereby decreasing chances of wound burn.

In transversal phaco, an elliptical lateral movement is used. Straight and curved tips may be used. The Stellaris (Bausch & Lomb) has longitudinal power mode, Infiniti (Alcon) has both longitudinal and torsional modes, whereas Whitestar Signature (AMO ) has both longitudinal and transversal modes.

* Dr Soosan Jacob is a Senior Consultant Ophthalmologist at
Dr Agarwal's Eye Hospital, Chennai, India, and can be reached at dr_soosanj@hotmail.com