Key Features
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Changing phacoemulsification (“phaco”) “power” or “amplitude” is achieved by changing the stroke length of needle vibration, not by changing the frequency.
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Evidence is accumulating that direct mechanical action is the most important factor in phaco.
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Power modulation significantly increases the efficiency of longitudinal phaco as well as improving the thermal safety. It is less important with torsional phaco.
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Modern pump systems are efficient and high vacuums can be achieved very quickly with modern flow-based (peristaltic) systems.
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In a flow-based machine, the aspiration flow rate can be adjusted completely independently of the preset vacuum limit.
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In a vacuum-based (Venturi) machine, the aspiration flow rate is generated by the pressure difference between the vacuum chamber and the eye. In most machines, the two cannot be completely dissociated, and a high vacuum results in higher flow rates compared with a lower vacuum.
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Modern machines feature a variety of strategies to minimize postocclusion surge. Postocclusion surge potential is directly related to the maximum set vacuum for any given needle/sleeve/tubing complex.
Introduction
As surgical techniques for the removal of cataract along with drug modulation of the consequent biological responses have become more refined, the problems of postoperative infection and inflammation are less important concerns of lens surgeons. As a consequence, it has become possible to concentrate on the further refinement of the actual process of lens removal. Phacoemulsification (“phaco”) offers the surgeon the possibility to break the nucleus into smaller pieces and even into a fine emulsion of material, all of which can be removed through the probe used to achieve the breakup. As a result, it is now possible to minimize trauma to the structures of the eye and to have minimal impact on its shape as a consequence of modern cataract surgery. Achieving this, however, requires the use of very powerful tools. Unfortunately, many surgeons fail to understand the principles that underlie the machines they use. As a consequence of this relative ignorance, surgery is sometimes performed less efficiently and possibly more dangerously than necessary.
Handpieces and Tips
The phaco handpiece houses an ultrasonic transducer—a device that converts electrical energy into mechanical vibratory energy. Standard handpieces couple the crystal to the phaco tip in such a way that the tip moves backward and forward when the crystals deform. In 2006, Alcon Surgical (Fort Worth, TX) introduced a handpiece (OZil) that can cause the tip to tort or twist when the crystals deform. It is constructed in such a way that when oscillating at 32 kHz the crystals produce torsion, and when stimulated at 44 kHz, they produce traditional linear movement. If a tip with a bend in the shaft (Kelman tip) is attached to such a handpiece, then the twisting of the shaft is converted into a sweeping side-to-side motion at the end of the tip. Following this, Abbot Medical Optics (AMO, Santa Ana, CA) introduced a third modality whereby with their new handpiece (Ellips FX), the phaco needle is made to traverse laterally while it moves forward, taking what they call an elliptical path.
The frequency at which a handpiece is set to work depends on the design and materials used. Adjustment of the power setting on the machine affects the stroke length (the distance traveled by the tip during one cycle), but not the frequency. Power is expressed as a percentage of the maximum travel the crystal-tip complex can produce. 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 be greater. It is important to recognize 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 minimum power setting) also varies among systems. In one commercially available system, 20% power produces tip travel of 50 µm, whereas this travel is reached only at 60% power in another machine. As a consequence, any comparisons between the “efficiency” of different phaco machines based on comparisons of “power used” are spurious.
The physical mechanisms that break up nuclear material when a phaco tip is used have been difficult to elucidate, and the relative importance of the various factors is still unclear. For example, a phaco tip operated at a frequency of 44 kHz has a maximum speed of 66 ft/second (20 m/second) when operated at full power, and the acceleration of the tip is >168 300 ft/second (>51 000 m/second). Under these conditions, the direct impact of the tip breaks the frictional forces within the nuclear material. This direct effect is reduced, however, 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 with the tip. However, some still postulate that the acoustic shock waves themselves tend to weaken or break some of the bonds that hold nuclear material together. The role of cavitation in breaking down lens material remains controversial, but some evidence exists that it is not required for effective phaco.
Various tip designs are available for the surgeon, but there are three key design variations, and each of the tip designs usually is available with a “cutting tip” angle of 30° or 45°. The tip may be straight with a uniform diameter along its length. The Kelman tip has a 22° angle in the shaft 3.5 mm from the tip. This design is thought to enhance the emulsification action of the tip, as well as allowing the surgeon to use it as a manipulator. Some tips have a flared termination of the tip (i.e., the outer and inner diameter at the end of the tip is greater than that 1–2 mm behind). While the larger mouth creates more holding force, the restriction of the inner lumen behind the flare helps suppress postocclusion surge. More recently, a completely new tip design has been introduced. When torsional phaco was developed, the already available Kelman design tip was used. It seemed logical that rotating the shaft along its long axis would result in a sweeping side-to-side action at the tip end. Although this proved to be the case, it was later realized that there was not just simple rotation of the shaft with this design. This realisation resulted in a completely new tip shape, specifically designed for torsional phaco (the “balanced” tip) ( Fig. 5.9.1 ). By significantly reducing the unwanted movement in the shaft, more of the energy produced by the ultrasound crystals is translated into sweeping movement at the very end of the tip, giving a stroke length of 190 µ at maximum amplitude compared to 130 µ with the Kelman tip.
Power Modulation
Although some form of simple power modulation (pulsed phaco) has been available for a long time, the introduction in 2001 of the Whitestar software for the AMO Sovereign phaco machine marked a paradigm shift in the way surgeons controlled the application of phaco power. Breakup of phaco into pulses or bursts has two advantages. First, the pauses (off-period) allow the machine fluidics to pull material back into contact with the tip following repulsion caused by the jack-hammer effect in traditional longitudinal phaco. Second, the pauses prevent significant buildup of heat as a result of frictional movement within the incision, making thermal damage to the cornea less likely. 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 (i.e., quicker surgery and reduced amount of phaco energy used). With the introduction of torsional phaco the reduced repulsion and reduced thermal effect mean that power modulation is less important, although many surgeons continue to apply modulations.
When first introduced, pulses had a fixed duty cycle of 50% (i.e., the period with power on and with power off were equal), but power was variable, whereas bursts were of fixed width, usually with fixed power also. First Bausch & Lomb (Bridgewater, NJ) and now most other manufacturers have enhanced the various possible combinations of modulation, and this distinction has become blurred and is probably no longer helpful to try to distinguish between them in advanced machines.
Pumps and Fluidics
The function of the phaco pump is twofold—to hold the nucleus onto the tip and to remove debris created by the tip. With modern techniques the pump also is used increasingly to aspirate directly the softer parts of the nucleus. There are two pump principles in general use—flow-driven and vacuum-driven. Some machines now have the ability to switch between the two modes during surgery.
Flow-Based (Peristaltic)
Roller pumps that rotate against compressible tubing or membrane generate flow; this “milks” fluid along the lumen and creates a pressure gradient between pump and anterior chamber (AC). Recent design changes in the pumps and sophisticated microprocessor controls have resulted in powerful and well-controlled pump systems. The rate at which fluid is aspirated through the unoccluded phaco tip is set at the machine console in milliliters per minute (mL/min). A low value allows events within the AC to happen slowly; a high value speeds up events and generates more “pulling power.” Fine adjustments of flow, achieved by changing the speed at which the pump turns, allow for personal surgical style or different operating conditions. Recent advanced systems sense when the tip is occluded partially and then make adjustments to the pump to compensate for reduced aspiration.
The second pump parameter that can be adjusted is the vacuum level at which, once achieved, the pump stops. When the tip becomes occluded, the pump continues to turn and move fluid into the cassette, increasing the vacuum level in the tubing between tip and cassette. Once the preset vacuum has been reached, the pump effectively stops (or turns slowly intermittently to compensate for vacuum loss) for as long as that vacuum level holds. The rate at which the maximum set vacuum level is reached is directly proportional to the flow rate ( Fig. 5.9.2 ).
Vacuum-Based
These systems generate an adjustable level of vacuum in a chamber in the machine; usually, a Venturi pump is used. It is the pressure difference between this chamber and the tip that generates flow. Once the tip is occluded, fluid continues to be removed from the tubing until the pressure within it equals that in the vacuum chamber. It is possible, however, to introduce a damping effect into the system so that the equilibration of pressures does not take place instantaneously. In a standard vacuum system, because the flow rate is generated by the pressure gradient, increasing the vacuum results in increased flow, whereas reducing vacuum lowers the flow. These two parameters cannot normally be modulated independently, although with at least one advanced machine, this is now possible.
Anterior Chamber Hydrodynamics
It is important to understand the correct meaning of various terms used to describe the fluid dynamics of phaco. “Vacuum (Limit)” is taken to mean the preset maximum vacuum level indicated on the console. In neither peristaltic nor vacuum-based systems is this vacuum present in the AC, although some degree of vacuum must be present along the aspiration line (including in the phaco handpiece) to generate outflow. Normally, “flow” is used to mean aspiration flow rate, which is evacuation flow out of the eye. Fluid also flows out of the eye at a variable rate through the incisions, often called the incision leakage flow. To avoid confusion, if flow into the eye is being described, it is necessary to use the term “inflow.” In traditional phaco systems, the rate of fluid inflow is determined by the height difference between the drip chamber of the fluid reservoir (usually a hung bottle) and the eye. Inflow has been passive in traditional phaco machines; it is reduced in varying amounts by the resistance of the tubing and by any compression of the inflow sleeve around the phaco tip. The recently introduced “Centurion” phaco machine (Alcon) has an active inflow system allowing much more precise control of the AC dynamics and intraocular pressure. The system monitors the pressure in the inflow line, the vacuum pressure being generated in the outflow line and the aspiration flow rate, in real time, and adjusts the pressure being applied to the bag containing the inflow fluid in order to generate sufficient inflow to maintain (within limits) the intraocular pressure (IOP) chosen by the surgeon.
In any system (gravity-fed or with active fluidics) it is essential that the inflow potential (the maximum possible under free-flow conditions) at least equals, and if possible exceeds, the maximum transient outflow (combined incisional flow and machine-generated flow); otherwise AC collapse occurs (see “ Postocclusion Surge ” later). In a gravity-fed system (the traditional system), as the fluid bottle or bag is generally fixed during a given step in the procedure, the height of the bottle is set so that it will generate sufficient inflow during postocclusion surge to prevent AC collapse. For most surgeons, this is set at 80–95 cm above eye level, although some surgeons set the bottle even higher. When the phaco (or I/A) handpiece is in the eye with irrigation active (foot pedal position 1) then the eye is pressured to 80–95 cm H 2 O, which equates to 59–70 mm Hg. In an ex-vivo model it has been shown that during unoccluded aspiration (foot-pedal position 2) the IOP drops significantly because of resistance to inflow in a gravity-fed system. With an aspiration flow rate of 30 cc/min for example, with a fixed bottle height of 95 cm, the IOP drops from 70 mm Hg to 50 mm Hg ( Fig. 5.9.3 ). The effect of this is that during normal surgery with a gravity-based phaco system with these typical settings, the IOP will fluctuate during cataract removal between 70 and 50 mm Hg, depending on the foot-pedal position and the degree of tip occlusion. Only very briefly during postocclusion surge will it drop significantly lower than 50 mm Hg. The same ex-vivo study showed that the monitored forced infusion system produced no significant IOP changes with aspiration flow rates up to 60 cc/min.
