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Ocular Surgery News, July 1, 2004
Improving
CataractSurgery Performances
Optimizing machine parameters
for phaco chop
David F. Chang, MD
I conceptually divide pure chopping techniques that eliminate sculpting into two categories – horizontal and vertical chop. Both rely on manual fracturing techniques to reduce phaco power and time. Of these variations, vertical chop is more dependent on high vacuum because the deeply impaled phaco tip must completely immobilize the nucleus. This not only prevents the nucleus from being dislodged and pushed posteriorly by the vertical chopping motion, but also allows the chopper to generate a shearing fracture force. Having a sharp vertical chopper tip is also crucial, as it ensures that the surgeon will incise into the nuclear pieces, rather than dislodge them.
For any technique, there are two general categories of machine settings to adjust. These are (1) the fluidics – bottle height, aspiration flow rate, vacuum and dynamic rise and (2) ultrasound power and power modulation – e.g., continuous, burst, pulse, hyperpulse and NeoSoniX (Alcon, Fort Worth, Texas) modes. Just as I cannot give a single speed at which a person should always drive a car, I cannot provide settings that will always be appropriate. I can recommend starting points, and surgeons should modify these settings according to their technique, experience and the individual characteristics of each lens. Just as drivers change their speed when the road is winding or wet, surgeons must understand why, and in what way they need to alter their machine parameters depending on the situation.
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One important concept is that the surgeon should be changing settings throughout the course of the case. That is because the fluidic and ultrasound objectives change while proceeding through the successive procedural steps. As a conceptual framework, I like to define four main objectives that change in priority during a phaco procedure – (1) sculpting efficiency, (2) impaling and holding power, (3) followability of fragments and (4) maximum chamber stability.
I recommend creating a lens removal step or memory setting for each objective. Surgeons who rely on pure chopping can eliminate the sculpting memory setting. With vertical chop, objective 2 helps us to secure and chop the nucleus, to laterally separate each hemi-nucleus or hemi-fragment and to lift fragments out of the capsular bag (Figure 1). Objective 3 maximizes the efficiency of removing mobilized pieces with minimal chatter and turbulence. Finally, objective 4 is important for avoiding surge and a trampolining posterior capsule as the last nuclear fragment and the epinucleus are removed (Figure 2).
With a peristaltic pump, the primary aspiration variables are aspiration flow rate and vacuum. Flow determines the speed at which changes are occurring within the anterior chamber and can be raised or lowered depending on whether events are proceeding too rapidly or too slowly. Vacuum determines the holding force of the phaco tip, so it is the key variable for the first objective – holding power. To generate high vacuum, the tip must remain occluded. With denser nuclei, burst mode is especially useful for maintaining a good seal around the tip. At any given vacuum level, the larger the diameter of the tip opening is, the greater the holding power will be.
The risk of post-occlusion surge limits how high a surgeon can safely program vacuum settings, and manufacturers have developed many surge reducing strategies to combat surge. These include the use of stiff-walled, low compliance tubing, smaller diameter lumens for the tubing, phaco tips that reduce flow, smart vacuum-sensing pumps and tip modifications, such as the aspiration bypass system (ABS) (Alcon).
Why is it that using the same equipment and settings, we see more surge in some cases than others? Despite these new devices and technical modifications, variables in surgical technique and nuclear density further affect surge. Because an occlusion break from maximum vacuum levels produces surge, rapid and total tip occlusion is a prerequisite. Partial tip occlusion prevents the maximum vacuum level from ever being reached. Thus, compared to divide-and-conquer, we tend to see more surge during chopping, because we maximally occlude the tip every time we impale the nucleus. This is why surge preventing technology has been so important for phaco chop.
Surge is also more evident with softer nuclear fragments because the “gummy” material will more readily mold to and plug the phaco tip opening. Brunescent mobile fragments have rigid surfaces that cannot mold to the phaco tip. Surge is less evident as these pieces are removed because tip occlusion is usually partial.
As more of the nuclear material is removed and the posterior capsule is exposed, surge becomes a greater concern. The vacuum should be lowered for this phase. Beware of weak zonules, however. Even the lower epinuclear vacuum setting may be too high if the zonules are lax. This is because the posterior capsule is able to “trampoline” toward the phaco tip if the weakened zonules do not keep it taut, and vacuum must be lowered even further in this event.
Determining the maximum “safe” vacuum setting is best accomplished by performing what I call the “surge test.” Start with a “quadrant” vacuum setting using low flow, while most of the nucleus is still present to protect and shield the posterior capsule. Hold a fragment in the center of the eye until maximum vacuum is attained, as indicated by the beeping occlusion sound. At this point, apply ultrasound, and as the occlusion breaks, assess the amount of surge. If there is no surge, increase the vacuum by 20% to 25% and repeat this test. Eventually, there will be an excessive amount of surge. At this point the options are to increase the infusion (increase bottle height or use high infusion sleeve), decrease the aspiration flow rate, lower the maximum vacuum setting or select another phaco tip option.
The Infiniti (Alcon) incorporates a number of features to minimize surge. The FMS (Fluid Management System) pump is a completely enclosed aspiration system that allows rapid sampling of the vacuum and infusion rates during a case in order to automatically adjust pump speed. Surgeons also have options such as a high infusion sleeve, and the ABS and flare tips.
With each successive step of the procedure, changes in power modulations should parallel fluidic parameter adjustments. Continuous mode is effective for sculpting, but does a poor job when the goal is to impale and generate holding power with dense nuclei. By eroding the material surrounding the tip, continuous mode may not create a good seal as brunescent material is impaled. In the absence of full tip occlusion, maximum vacuum cannot be generated. In my hands, burst mode is better for impaling dense nuclei, as single or repetitive bursts preserve the seal around the buried tip. When combined with a fast “dynamic rise” or ramp speed, this modality achieves maximum vacuum almost instantaneously. Burst mode is therefore ideal for objective 2.
For quadrant and fragment removal, pulse mode is preferable to continuous mode for maximizing followability. In pulse mode, the “on” time is 50%, meaning that the “on” periods are followed by equal duration “off” periods. This not only cuts the energy delivery in half, but it also reduces the axial repelling force of the vibrating phaco tip. In contrast to continuous mode, the aspirating fluidic forces have to compete with the ultrasound repelling force for only half the time. This reduces the chatter and turbulence of rigid nuclear particles at the tip. This advantage can be further amplified by using hyperpulse mode, where the duty cycle can be set as low as 5%. With a 25% duty cycle, this means that the rest period is 75% of the cycle, or three times as long as the ultrasound “on” period. Hyperpulse also permits extremely high pulse rates of 50 to 100 pulses per second. By further decreasing ultrasound time and interrupting it more often, hyperpulse reduces tip heat, lowers energy delivery and decreases chatter and particle turbulence. This improves safety with respect to incision burn and endothelial cell loss associated with brunescent nuclei.
Further ultrasound reduction can be achieved through the use of the NeoSoniX handpiece, which allows non-axial oscillation of the phaco tip. With a large brunescent quadrant, this oscillatory motion also seems to improve followability by constantly re-centering material on the phaco tip. Some surgeons will find that this advantage is offset by having to use a much heavier handpiece, however.
For vertical chop, I progress through three sequential memory settings that provide different vacuum levels, rise times and phaco modulations. The following settings illustrate my strategy for a 3+ dense nucleus. For the first objective of impaling and maximizing holding power, high vacuum is needed; for denser nuclei, my preference is 450 mm Hg with a dynamic rise up to +3. With most of the nucleus in front of the posterior capsule, I am less concerned about surge, so my goal is to attain high vacuum as quickly as possible. I use 70% linear power in burst mode with 50-ms bursts for a 3+ nucleus.
To remove the multiple chopped nuclear fragments, objective 2 – followability – becomes the priority. Vacuum should be decreased slightly to 400 mm Hg to allow for an increased flow rate (e.g. 35 cc/min). Dynamic rise is slightly lower at +2. The main alteration is the use of hyperpulse at a 30% duty cycle, and 50 pps, in order to minimize chatter and particle turbulence. While removing the final fragments and the epinucleus, chamber stability becomes the primary objective. Vacuum should be lowered significantly to 250 mm Hg during this phase because holding power is less important. Dynamic rise can be set to 0 to give a relatively slower vacuum rise time. More low-end control of epinuclear aspiration can be achieved by using linear control of both aspiration flow and vacuum in foot pedal position 2. I remain in hyperpulse mode and lower the power to 50%.
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As seen in the above illustration, the Infiniti allows the surgeon to pre-program different power and fluidic settings for each stage of the procedure. The surgeon can quickly and easily shift from one memory setting to the next by using the foot pedal. This allows surgeons to employ very high vacuum at the beginning of the case, when it is most needed, and to avoid surge later by lowering the vacuum as progressively more nucleus is removed. In addition, the foot pedal can also be used to vary our bank of sequential memory settings according to the grade of nucleus. Generally, higher phaco power and vacuum levels are needed for 4+ nuclei, while much less aggressive parameters are used for 1 to 2+ nuclei.
Programming as many as 12 separate memory settings may sound overwhelming, until one understands how the phacodynamic objectives change in priority during the course of the case. Once these three or four phaco objectives are defined, further adjustments can be made according to nuclear density. This versatility it is particularly helpful for the most challenging and brunescent cases we encounter.
References
- Chang DF. Phaco Chop: Mastering Techniques, Optimizing Technology, and Avoiding Complications. SLACK Incorporated, Thorofare, NJ; 2004.
