Reprinted with permission from Fishkind WJ. Phacoemulsification Technology: Improved Power and Fluidics, Chapter 9. In: Wallace RB, ed. Refractive Cataract Surgery and Multifocal IOLs. 2000:87.
Introduction
All phacoemulsification machines consist of a computer to generate ultrasonic impulses, a transducer, and piezo electric crystals that turn these electronic signals into mechanical energy. The energy that is created is then harnessed within the eye to overcome the inertia of the lens and emulsify it. Once turned into emulsate, the fluidics system removes the emulsate, replacing it with balanced salt solution.
Power Generation
Power is created by the interaction of frequency and stroke length. Frequency is defined as the speed of the needle movement and is determined by the manufacturer of the machine. Presently, most machines operate at a frequency of between 35,000 to 45,000 cycles per second (Hz) (Table 1). This frequency range is the most efficient for nuclear emulsification. Lower frequencies are less efficient and higher frequencies create excess heat.
Frequency is maintained by tuning circuitry that is designed into the machine computer. Tuning is vital because the phaco tip is required to operate in varied media. 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, small alterations in frequency are created by the tuning circuitry in the computer. The surgeon will subjectively appreciate good tuning circuitry by a sense of smoothness and power.
Stroke length is defined as the length of the needle movement. This length is generally 2 mils (thousandths of an inch) to 6 mils. Most machines operate in the 2-mil to 4-mil range. Longer stroke lengths are prone to generate excess heat. The longer the stroke length, the greater the physical impact on the nucleus, and the greater the generation of cavitation forces. Stroke length is determined by foot pedal excursion in position 3 during linear control of phaco.
Energy at the Phaco Tip
The actual tangible forces that emulsify the nucleus are a blend of the "jackhammer" effect and cavitation.1 The jackhammer effect is merely the physical striking of the needle against the nucleus.
The cavitation effect is more convoluted. The phaco needle, moving through the liquid medium of the aqueous at ultrasonic speeds, creates intense zones of high and low pressure. Low pressure, created with the backward movement of the tip, literally pulls dissolved gases out of the solution, thus giving rise to micro bubbles. Forward tip movement then creates an equally intense zone of high pressure. This produces compression of the micro bubbles until they implode. At the moment of implosion, the bubbles create a temperature of 13000° F and a shock wave of 75,000 pounds per square inch (PSI). Of the micro bubbles created, 75% implode, amassing to create a powerful shock wave radiating from the phaco tip in the direction of the bevel with annular spread. However, 25% of the bubbles are too large to implode. These micro bubbles are swept up in the shock wave and radiate with it.
The cavitation energy that is created can be directed in any desired direction; the angle of the bevel of the phaco needle governs the direction of the generation of the shock wave and micro bubbles.
A method of visualization of these forces, called enhanced cavitation, has been developed. Using this process with a 45° tip, the cavitation wave can be visualized and seen to be generated at 45° from the tip and comes to a focus 1 mm from it. Similarly, a 30° tip generates cavitation at a 30° angle from the bevel, and a 15° tip, 15° from the bevel (Slide 1). A 0° tip creates the cavitation wave directly in front of the tip and the focal point is 0.5 mm from the tip (Slide 2). The Kelman tip has a broad band of powerful cavitation that radiates from the area of the angle in the shaft. A weak area of cavitation is developed from the bevel but is inconsequential (Slide 3).
Considering analysis of enhanced cavitation, one can conclude that emulsification is most efficient when both the jackhammer effect and cavitation energy are integrated. To accomplish this, a 0° tip, or the bevel of the needle, should be turned toward the nucleus or nuclear fragment. This simple maneuver will cause the broad bevel of the needle to strike the nucleus, which will enhance the physical force of the needle striking the nucleus. In addition, the cavitation force is then concentrated into the nucleus rather than away from it (Slide 4A and Slide 4B). This causes the energy to emulsify the nucleus and be absorbed by it. When the bevel is turned away from the nucleus, the cavitational energy is directed up and away from the nucleus toward the iris and endothelium (Slide 5). Finally, in this configuration, the vacuum force (discussed below) can be maximally exploited as occlusion is encouraged.
Modification of Phaco Power Intensity
Application of the minimal amount of phaco power intensity necessary for emulsification of the nucleus is desirable. Unnecessary power intensity is a cause of heat with subsequent wound burn, endothelial cell damage, and iris damage with alteration of the blood-aqueous barrier. Phaco power intensity can be modified by alteration in stroke length, alteration of duration, and alteration of emission.
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. New foot pedals, such as those found in the Sovereign (Allergan, Irvine, Calif.)
and the Legacy (Alcon, Ft. Worth, Texas), permit surgeon adjustment of the throw length of the pedal in position 3.
This can refine power application. The Millennium (Bausch & Lomb Surgical, Santa Clara, Calif.) dual linear foot pedal
permits the separation of the fluidic aspects of the foot pedal from the power elements.
Alteration of Duration
The duration of application of phaco power has a dramatic effect on overall power delivered. Usage
of pulse or burst mode phaco will considerably decrease overall power delivery. New machines allow for
a power pulse of a selected duration alternating with a period of aspiration only. In the Sovereign,
burst mode (the parameter is machine dependent) is characterized by 80-millisecond (msec) or 120-msec
periods of power, combined with fixed short periods of aspiration only. Pulse mode utilizes fixed pulses
of power of 50 msec or 150 msec with variable short periods of aspiration only (Slide 6A and Slide 6B).
Phaco techniques, such as the choo-choo chop and phaco chop, utilize minimal periods of power in pulse
mode to reduce power delivery to the anterior chamber. In addition, the use of pulse mode to remove the
epinucleus provides for an added margin of safety. When the epinucleus is emulsified, the posterior
capsule is exposed to the phaco tip and may move forward toward it due to surge. Activation of pulse
phaco will create a deeper anterior chamber to work within. This occurs because each period of phaco
energy is followed by an interval of no energy. In pulse mode during the interval of absence of energy,
the epinucleus is drawn toward the phaco tip, producing occlusion and interrupting outflow. This allows
inflow to deepen the anterior chamber immediately prior to the onset of another pulse of phaco energy.
The surgeon will recognize the outcome as operating in a deeper, more stable anterior chamber.
Alteration of Emission
The emission of phaco energy is modified by tip selection. Phaco tips can be modified to accentuate
power, flow, or a combination of both.
Power intensity is modified by altering the bevel tip angle. Noted previously, the bevel of the phaco tip will focus power in the direction of the bevel. The Kelman tip will produce broad powerful cavitation directed away from the angle in the shaft. This tip is excellent for the hardest of nuclei. New flare and cobra tips direct cavitation into the opening of the bevel of the tip. Thus, random emission of phaco energy is minimized. Designer tips, such as the "flathead" designed by Barry Seibel, MD, and power wedges designed by Douglas Mastel, modify the direction and focus delivery of phaco energy intensity.
Power intensity and flow are modified by utilizing a 0° tip. This tip will focus power directly ahead of the tip and enhance occlusion due to the smaller surface area of its orifice.
Small diameter tips, such as 21-gauge tips, change fluid flow rates. Although this tip does not actually change power intensity, it appears to have this effect, as the nucleus must be emulsified into smaller pieces for removal through the smaller diameter tip.
The Alcon ABS (aspiration bypass system) tip modification is now available with a 0° tip, a Kelman tip, or a flare tip. The flare is a modification of power intensity and the ABS a modification of flow. In the ABS system, a 0.175-mm hole in the shaft permits a variable flow of fluid into the needle, even during occlusion (Slide 7). This flow adjustment serves to minimize surge.
Finally, flow can be modified by utilizing one of the microseal tips. These tips have a flexible outer sleeve to seal the phaco incision. They also have a rigid inner sleeve or a ribbed shaft configuration to protect cooling irrigant inflow. Thus, a tight seal allows low-flow phaco without the danger of wound burns.
Phaco power intensity is the energy that emulsifies the lens nucleus. The phaco tip must operate in a cool environment and with adequate space to isolate its actions from delicate intraocular structures. This portion of the action of the machine is dependent upon its fluidics.
Fluidics
The fluidics of all phaco machines are fundamentally a balance of fluid inflow and outflow. Inflow is determined by the bottle height above the eye of the patient. It is important to recognize that with recent acceptance of temporal surgical approaches, the eye of the patient may be physically higher than in the past. This requires that the irrigation bottle be adequately elevated. A shallow, unstable anterior chamber will otherwise result.
Outflow is determined by the sleeve-incision relationship as well as the aspiration rate and vacuum level commanded. The incision length selected should create a snug fit with the phaco tip selected. This will result in minimal uncontrolled wound outflow with resultant increased anterior chamber stability.
Aspiration rate, or flow, is defined as the flow of fluid through the tubing in cubic centimeters per minute (cc/min). With a peristaltic pump, flow is determined by the speed of the pump. Flow determines how well particulate matter is attracted to the phaco tip.
Aspiration level, or vacuum, is a parameter measured in millimeters of Mercury (mm Hg) that is defined as the magnitude of negative pressure created in the tubing. Vacuum is the determinant of how well, once occluded on the phaco tip, particulate material will be held to the tip.
Vacuum Sources
There are three categories of vacuum sources, or pumps: flow pumps, vacuum pumps, and hybrid pumps.
Flow Pump
The primary example of the flow pump type is the peristaltic pump. These pumps allow for independent control
of both aspiration rate and aspiration level.
Vacuum Pump
The primary example of the vacuum pump is the venturi pump. This pump type allows direct control
of only vacuum level. Flow is dependent upon the vacuum level setting. Additional examples are the rotary
vane and diaphragmatic pumps.
Hybrid Pump The primary example of the hybrid pump is the Sovereign peristaltic pump (Slide 8) or the Concentrix pump (Bausch & Lomb Surgical) (Slide 9). These pumps are interesting in that they are able to act as either a vacuum or flow pump dependent upon programming. They are the most recent supplement to pump types and are generally controlled by digital inputs, creating incredible flexibility and responsiveness.
The challenge to the surgeon is to balance the effect of phaco intensity, which tends to push nuclear fragments off the phaco tip with the effect of flow, which attracts fragments toward the phaco tip and vacuum, holding the fragments on the phaco tip. Generally, low flow slows down intraocular events, while high flow speeds them up. Low or zero vacuum is helpful during sculpting of a hard or large nucleus, in which the high power intensity of the tip may be applied near the iris or anterior capsule. Zero vacuum will avoid inadvertent aspiration of the iris or capsule, preventing significant morbidity.
Surge A principal limiting factor in the selection of high levels of vacuum and/or flow is the development of surge. When the phaco tip is occluded, flow is interrupted and vacuum builds to its preset level. Emulsification of the occluding fragment then clears the occlusion. Flow immediately begins at the preset level in the presence of the high vacuum level. In addition, if the aspiration line tubing is not reinforced to prevent collapse (tubing compliance), the tubing will have constricted during the occlusion. It then expands on occlusion break. The expansion is an additional source of vacuum production. These factors cause a rush of fluid from the anterior segment into the phaco tip. This fluid may not be replaced rapidly enough by infusion to prevent shallowing of the anterior chamber; therefore, there is subsequent rapid anterior movement of the posterior capsule. This abrupt forceful stretching of the bag around nuclear fragments may be a cause of capsular tears. In addition, the posterior capsule can be literally sucked into the phaco tip, tearing it. The magnitude of the surge is contingent on the presurge settings of flow and vacuum.
Surge is therefore modified by selecting lower levels of flow and vacuum. The phaco machine manufacturers help to decrease surge by providing noncompliant aspiration tubing. This tubing will not constrict in the presence of high levels of vacuum. The following are some examples of new technologies:
- Sovereign - Microprocessors sample vacuum and flow parameters 50 times a second, create a "virtual" anterior chamber model. At the moment of surge, the machine's computer senses the increase in flow and instantaneously slows or reverses the pump to stop surge production.
- Millennium - The dual linear foot pedal can be programmed to separate both the flow and vacuum from power. In this way, flow or vacuum can be lowered before beginning the emulsification of an occluding fragment. The emulsification therefore occurs in the presence of a lower vacuum or flow so that surge is minimized.
- Legacy - The ABS tips have 0.175-mm holes drilled in the shaft of the needle. During occlusion, the hole provides for a continuous alternate fluid flow. This will cause dampening of the surge on occlusion break.
Phaco Technique and Machine Technology
A patient will have the best visual result when total phaco energy delivered to the anterior
segment is minimized. Additionally, phaco energy should be focused into the nucleus. This will
prevent damage to iris blood vessels and endothelium. Finally, proficient emulsification will lead
to shorter overall surgical time. Therefore, a lesser amount of irrigation fluid will pass through
the anterior segment. The general principles of power management are to focus phaco energy into the
nucleus, vary fluid parameters for efficient sculpting and fragment removal, and minimize surge.
Divide-and-Conquer Phaco
Sculpting
To focus cavitation energy into the nucleus, a 0°, 15°, or 30° tip turned bevel
down should be used. Zero or low vacuum (dependent upon the manufacturer's recommendation)
is mandatory for bevel down phaco to prevent occlusion. Occlusion, at best, will cause
excessive movement of the nucleus during sculpting. At worst, occlusion occurring near
the equator is the cause of tears in the equatorial bag early in the phaco procedure while
occlusion at the bottom of a groove will cause phaco through the posterior capsule. Once the
groove is judged to be adequately deep, the bevel of the tip should be rotated to the bevel
up position to improve visibility and prevent the possibility of phaco through the posterior nucleus and capsule.
Quadrant and Fragment Removal
The tip selected, as noted above, is retained. Vacuum and flow are increased to reasonable limits subject
to the machine being used. The limiting factor to these levels is the development of surge. The bevel of the
tip is turned toward the quadrant or fragment, and low pulsed or burst power is applied at a level high
enough to emulsify the fragment without driving it from the phaco tip. Chatter is defined as a fragment
bouncing from the phaco tip due to aggressive application of phaco energy.
Epinucleus and Cortex Removal
For removal of the epinucleus and cortex, the vacuum is decreased while flow is maintained. This allows
for grasping of the epinucleus to the anterior capsule. The low vacuum will help the tip hold the epinucleus
on the phaco tip without breaking off chunks due to high vacuum, so that it scrolls around the equator and
can be pulled to the level of the iris. Here, low-power pulsed phaco is used for emulsification. If cortical
cleaving hydrodissection has been performed, the cortex will be removed concurrently.
Stop and Chop Phaco
Groove creation is performed as noted above, under divide-and-conquer sculpting techniques. Once the groove is adequate, the phaco tip and chopper are placed in the depth of the groove and separated, creating a crack. Vacuum and flow are increased to improve the holding ability of the phaco tip. The nucleus is rotated 90°, the tip is then burrowed into the mass of one heminucleus using pulsed linear phaco. The sleeve should be 1 mm from the base of the bevel of the phaco tip to allow adequate exposed needle length for sufficient holding power. Excessive phaco energy application is to be avoided, because this will cause nuclear material immediately adjacent to the tip to be emulsified. The space created in the vicinity of the tip is responsible for interfering with the seal around the tip as well as the capability of the vacuum to hold the nucleus. The nucleus will then pop off the phaco tip, making chopping more difficult. With a good seal, the heminucleus can be drawn toward the incision, and the chopper can be inserted at the endonucleus/epinucleus junction. The pie-shaped piece of nucleus thus created is removed with low-power pulsed phaco, as discussed in the Divide-and-Conquer section. Epinucleus and cortex removal is also performed as noted above.
Phaco Chop
The phaco chop requires no sculpting. Therefore, the procedure is initiated with high vacuum and flow and linear pulsed phaco power. For a 0° tip, when emulsifying a hard nucleus, a small trough may be required to create adequate room for the phaco tip to borrow deep into the nucleus. For a 15° or 30° tip, the tip should be rotated bevel down, to engage the nucleus. A few bursts, or pulses, of phaco energy will allow the tip to be buried within the nucleus. It then can be drawn toward the incision to allow the chopper access to the endonucleus/epinucleus junction. If the nucleus comes off the phaco tip, excessive power has produced a space around the tip, impeding vacuum holding power as noted above. The first chop is then produced. Minimal rotation of the nucleus will allow for creation of the second chop. The first pie-shaped segment of nucleus is mobilized with high vacuum and elevated to the iris plane. There, it is emulsified with low linear power, high vacuum, and moderate flow.
When looking at trends in complications for each of the four major IOL groups over the past 3 years of the explantation surveys, several changes can be seen. In the three-piece monofocal silicone IOL group, incorrect lens power is still the leading indication for explantation followed by dislocation/decentration. However, in the three-piece acrylic IOL group, incorrect lens power has now surpassed glare/optical aberration as the primary reason for explantation of these lenses. Patients with one-piece, plate silicone IOLs still have decentration/dislocation as the primary reason for removal. Incorrect lens power is now the second most common reason. Finally, glare/optical aberrations remain the most common reason for removal of three-piece multifocal silicone IOLs during the 3-year period of the survey.
Irrigation and Aspiration
Similar to phaco, anterior chamber stability during irrigation and aspiration (I&A) is due to a balance of inflow and outflow. Wound outflow can be minimized by using a soft sleeve around the I&A tip. Combined with a small incision (2.8 mm to 3 mm) a deep and stable anterior chamber will result. Generally, a 0.3-mm I&A tip is used. With this orifice, a vacuum of 500 mm Hg and flow of 20 cc/min is excellent to tease cortex from the fornices. The linear vacuum allows the cortex to be grasped under the anterior capsule and drawn into the center of the pupil at the iris plane. There, in the safety of a deep anterior chamber, the vacuum can be increased and the cortex aspirated.
Vitrectomy
Most phaco machines are equipped with a vitreous cutter, which is activated by compressed air or electric motor. As noted previously, preservation of a deep anterior chamber is dependent upon a balance of inflow and outflow. For vitrectomy, a 23-gauge cannula or chamber maintainer inserted through a paracentesis provides inflow. Bottle height should be adequate to prevent chamber collapse. The vitrector should be inserted through another paracentesis. If equipped with a Charles sleeve, this should be removed and discarded. Utilizing a flow of 20 cc/min, a vacuum of 250 mm Hg, and a cutting rate of 250 to 350 cuts per minute, the vitrector should be placed through the tear in the posterior capsule, orifice facing upward, pulling vitreous out of the anterior chamber. The vitreous should be removed to the level of the posterior capsule
Conclusion
It has been stated that the phaco procedure is a blend of technology and technique. Awareness of the principles that influence phaco machine settings is requisite for the performance of a proficient and safe operation. Additionally, often during the procedure, there is a demand for modification of the initial parameters. A thorough understanding of fundamental principles will enhance the capability of the surgeon for appropriate response to this requirement.
It is a fundamental principle that through relentless evaluation of the interaction of the machine and the phaco technique, the skilled surgeon will find innovative methods to enhance technique. "The road to success is always under construction."
Reference
- Cimino WW, Bond LJ. Physics of ultrasonic surgery using tissue fragmentation Part II. Ultrasound in Medicine and Biology. 1996;22(1):101-117.