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Refractive Surgery | May 2009

The Basics of Ultrasound

Understanding differences in tip movement, power modulation, and more.

As a complement to the basic guide to fluidics on page 30, by Richard Packard, MD, FRCS, FRCOphth, of Windsor, England, this article provides a brief guide to ultrasound, including the basics of phaco power and its potential modulations.

The phacoemulsification handpiece houses an ultrasonic device that converts electric energy into mechanical vibratory energy. Certain crystals exhibit a relationship between mechanical stress and electricity; they are piezoelectric. When an electric charge is applied to opposite faces of these crystals, a strain appears in the structure, which results in deformation. This is how ultrasonic movement and energy are created in phaco handpieces.

In a standard phaco handpiece, the crystal is coupled to the phaco tip in such a way that the tip moves backward and forward when the crystals deform. (In this article I use the word tip to mean the titanium structure fixed to the business end of the handpiece. This is often also called a needle.) This is the traditional, longitudinal form of phaco.

In September 2005, Alcon Laboratories, Inc. (Fort Worth, Texas) introduced the OZil handpiece, which can cause the tip to tort or twist when the crystals deform. The handpiece is constructed in such a way that when the crystals oscillate at 36 kHz they produce torsion, and when they are stimulated at 43 kHz they produce traditional, linear movement. If a tip with a bend in the shaft is attached to such a handpiece, the twisting of the shaft is converted into a sweeping, side-to-side motion at the end of the tip.

In 2008, Advanced Medical Optics, Inc. (now Abbott Medical Optics, Inc., Santa Ana, California) introduced the Ellips Transversal Ultrasound handpiece, which causes deviation of the phaco tip to one side as it is moving backward and forward in the traditional longitudinal manner. The company says that this produces complex 3-D movement, in which an elliptical, transversal movement occurs at the same time as the longitudinal movement we are familiar with from traditional phaco. This type of movement is independent of the tip's shape.

The frequency at which a handpiece is set to work depends on the design and materials used. For each combination of mass and material, a particular frequency exists at which the transducer works most efficiently. Adjustment of the power by depressing the footpedal in position three affects the stroke length—the distance traveled by the tip from rest to its maximum forward displacement during one cycle—but not the frequency.

The machine displays power as a percentage of its maximum power. Usually, this is the electric power delivered to the crystals. It is clear that if the frequency remains constant but the distance traveled in each stroke increases, the acceleration of the tip and the maximum speed it reaches must also increase. The tip and the handpiece, particularly the crystals it contains, tend to heat up during use, and this can have a significant impact on the functioning of the tip. The most efficient phaco machines have electronic circuits that frequently, or even constantly, monitor the performance of the tip and make small adjustments to the frequency of vibration to take account of this. This is called auto-tuning.

Some machines also adjust the voltage (ie, power) applied to the crystals to account for the loading on the tip—taking account, for example, of the difference in effective weight of a tip if a large nuclear fragment is impaled on it. It is important to recognize therefore that the power settings on the machine console are indicative only.

Some systems have a nonlinear relation between commanded power and stroke length. The smallest stroke at the minimum power setting also varies among systems. In one commercially available system, 20% power produces tip travel of 50 µm, whereas this amount of travel is reached only at 60% power in another machine. As a consequence, any comparison between the efficiency of different phaco machines based on power are spurious, even when considering only longitudinal phaco. With the torsional and transversal modes now available, the question of exactly how power should be expressed has become even more problematic.

The physical mechanisms that break up nuclear material when a phaco tip is used have been difficult to elucidate, and the relative importance of various factors remains unclear. We know that the direct mechanical hammer-like effect of the extremely hard titanium tip coming into contact with the lens material is significant. A phaco tip operated at 44 kHz has a maximum speed when operated at full power of 20 m/second, and the acceleration of the tip is greater than 51,000 m/second2.

Under these conditions, direct impact of the tip breaks frictional forces within the nuclear material. This direct effect is reduced by the forward-propagating acoustic waves or fluid and particle waves generated by the tip, which tend to push away any piece of nucleus in contact. However, it is possible that the acoustic shock waves themselves tend to weaken or break some of the bonds that hold nuclear material together.

The role of cavitation—the formation of partial vacuums in a liquid by rapidly moving pieces of metal, such as the phaco tip or a ship's propellor—in breaking down lens material is controversial. Some manufacturers claim that adjustment to the shape of the power applied to the tip to enhance the amount of transient cavitation results in more efficient emulsification. However, in one laboratory study of phacoemulsification performed in a hyperbaric chamber, the effectiveness of phaco was seen to be undiminished in conditions where cavitation was suppressed, while another study has shown the opposite.

Certainly we now know that there is little or no cavitation at the tip with torsional phaco, yet that mode is nonetheless surprisingly efficient and effective.

Although some form of simple power modulation (ie, pulsed phaco) has been available for a long time, the introduction in 2001 of the WhiteStar software for the Sovereign phaco machine (Abbott Medical Optics, Inc.) marked a paradigm shift in the way surgeons controlled the application of phaco power. Breaking up phaco energy into pulses or bursts has two advantages. First, the pauses, or off periods, allow fluidics to pull lens material back into contact with the tip after repulsion caused by the jackhammer effect in traditional longitudinal phaco. Second, the pauses reduce—but do not prevent—build-up of heat due to frictional movement within the incision, making thermal damage to the cornea less likely.

In reality, phaco is never cold. A cornea can be burned quickly with any machine, despite power modulation, if significant power is used with little or no cooling flow of fluid around or through the tip (eg, in an anterior chamber filled with a retentive ophthalmic viscosurgical device).

Several machines now allow almost infinite variation of both duty cycle (the ratio of on-time to off-time) and the length of the on period. It has been shown that such power modulation significantly improves the efficiency of phacoemulsification, resulting in faster surgery with reduced amount of phaco energy used.

The WhiteStar software has a specific occlusion mode in which, when it is enabled, the machine can make adjustments to the power modulation when occlusion is detected.

When pulsed phaco was first introduced, pulses had a fixed duty cycle of 50%—that is, the periods with power on and with power off were equal—but power was variable. On the other hand, phaco bursts were of fixed width, usually also with fixed power. Now, with the enhanced modulations possible on most new phaco platforms, the distinction between pulses and bursts has become blurred, and it is probably no longer helpful to try distinguishing between them in advanced machines. However, the terminology remains, and the following is an attempt to clarify.

At its most basic, pulsed phaco produces a pattern of on- and off-time to the phaco energy, and the lengths of on and off are fixed for any one setting. However, the surgeon can retain linear control of power with travel of the pedal through footpedal position 3.

It is common now to refer to the duty cycle of pulsed phaco. When pulsed phaco was first introduced, it broke the phaco stream down into two to approximately 10 pulses per second, but within each pulse period the power was on half the time and off half the time. Now, we would say that is a 50% duty cycle. In other words, the duty cycle is the percentage of the time from the start of one pulse to the start of the next when the phaco tip is active. Figure 1 shows standard pulsed phaco graphically.

The increasingly sophisticated software that allows us to control the ultrasound generator more precisely now allows us to also adjust the proportion of a pulse that is on- versus off-time. For instance, phaco can be on 20% of the time, with 80% of the pulse period inactive (Figure 2).

With pulsed phaco, the pulse rate and duty cycle remain constant once set, and only the power varies with footpedal travel.

Burst phaco is a mode in which initially the phaco is on for a short period—say 20 milliseconds. The power level is normally fixed. Bausch & Lomb (Rochester, New York) introduced the earliest developments of sophistication for burst phaco on its Millennium phaco machine. In burst mode, the bursts are initially about 1 or 2 seconds apart, and as the pedal is pressed down in position 3 they gradually come closer together with further pedal travel. By the time the pedal is on the floor, the bursts have run into each other, and phaco is continuous (Figure 3).

Although it is not usually described in this way, what the surgeon is doing with this traditional type of burst mode is using fixed power but simultaneously increasing both the duty cycle and the frequency by pressing down on the pedal through foot position 3.

The latest generation of software driving the ultrasound generators of some current phaco machines allows more flexibility with burst mode. With some machines, the surgeon can regain linear control of power, and therefore have linear power simultaneously with linear frequency and duty cycle. On other machines, the surgeon can set a minimum limit for how far the off-time reduces so that the bursts never become continuous.

As noted above, power modulations have the effect of significantly reducing the repulsion caused by the jackhammer effect. With torsional phaco, repulsion is not an issue, so these modulations are not necessary in that mode. Our experience with transversal phaco is at the moment too brief to know whether the same applies to this new mode as well. However, many surgeons using torsional phaco continue to use power modulations to reduce the potential heat build-up, even though frictional heating is reduced to approximately 30% that of longitudinal phaco.

It is hoped that this brief summary of the basics of ultrasound will equip the beginning surgeon with sufficient knowledge to understand the basics of phaco power and its potential modulations. This information, coupled with the explanation of basic fluidics given by Richard Packard, MD, FRCS, FRCOphth, on page 30 of this issue can serve as a primer for surgeons to better understand what is happening in the eye during phacoemulsification surgery. We hope that this knowledge will help you to make appropriate adjustments to your parameters, to take account of differences in patients and their cataracts, and to prepare for and prevent problems and complications.

David Allen, FRCOphth, is a Consultant Ophthalmologist specializing in cataract surgery at Sunderland Eye Infirmary in the United Kingdom. Mr. Allen states that he is not a paid consultant but does receive ad-hoc payments from Alcon Laboratories, Inc. He also states that he has no direct financial interest in the products or companies mentioned. He may be reached at tel: +44 191 5699071; e-mail: david.allen@chs.northy.nhs.uk.

May 2009