This article is more than 5 years old. Information may no longer be current.
Safer phaco technology: Four pearls for the 4+ brunescent nucleus
 David F. Chang |
It is with brunescent nuclei that surgeons are most interested in improved phaco technology because these cases pose the stiffest test of our skills and equipment. Virtually every surgical step is more difficult and complication prone.
Five challenges of the 4+ nucleus
- The poor red reflex makes the capsulorrhexis more difficult to visualize and complete. This also increases the risk of the phaco tip or chopper inadvertently tearing the continuous edge during nuclear emulsification.
- Hydrodissection is most likely to cause capsular block syndrome with a dense nucleus and a small diameter capsulorrhexis. As the bulky nucleus elevates into the capsulorrhexis, capsulolenticular block may ensue. If the balanced salt solution cannot escape as the injection continues, the capsular bag will be forcefully inflated until the posterior capsule ruptures.
- Incision burns are more likely to occur with brunescent lenses. The thicker nuclear emulsate can admix with a highly retentive ophthalmic viscosurgical device (OVD) to form a viscous plug that clogs the phaco tip or aspiration line. If fluid outflow is blocked, then gravity-fed irrigation inflow will cease as well. With neither inflow nor outflow of fluid to cool it, a phaco tip in continuous mode will instantaneously heat up causing an incision burn.
- Endothelial cell loss is greater with brunescent nuclei. The extra size and density of the nucleus increases the phaco time and energy required to emulsify it. In my opinion, the most important cause of greater endothelial cell loss is the increased particle turbulence that occurs with brunescent nuclear fragments. Rigid nuclear pieces do not mold and conform as well to the phaco tip opening. This and the added stroke length characteristic of higher ultrasound power settings increase chatter and turbulence of nuclear particles.
- Posterior capsule rupture is more common with rock-hard nuclei for several reasons. The added rigidity and girth of the nucleus more directly transfers instrument forces to the capsule and zonules, and there may be insufficient epinucleus to cushion movements of the endonucleus. These lenses often have lax or deficient zonules and a thin or absent epinucleus that also permit the posterior capsule to trampoline more as the final fragments are removed. Finally, sharp-edged fragments can more easily puncture the capsule.
Configuring phaco technology — four strategies
Certainly, advanced phaco techniques such as chopping enable us to reduce endothelial trauma and the risk of capsular rupture. Generous use of a highly retentive OVD is also critical. What features of phaco technology can be used to particular advantage in these lenses? Here are four strategies to increase the margin of safety when emulsifying brunescent nuclei.
1. Hyperpulse
One of the most important improvements in phaco technology was the introduction of hyperpulse power modulation with the Sovereign (Advanced Medical Optics [AMO]) in 2001. Shortening the pulse duration allowed surgeons to significantly increase the frequency of ultrasound pulses. In addition, the ability to reduce the duty cycle produced a major reduction in cumulative ultrasound time. These changes significantly reduced heat production and total ultrasound energy delivered by the phaco tip. As so nicely shown in Teruyuki Miyoshi, MD’s ASCRS award winning videos using ultra high-speed digital photography, alternating each ultrasound pulse with longer rest periods of “off” time dramatically reduces the repelling forces of the vibrating phaco tip. This in turn reduces the chatter and turbulence of small lens particles at the phaco tip that would otherwise bombard the corneal endothelium. Both the latest Alcon and Bausch & Lomb phaco platforms incorporate hyperpulse technology, whose benefits are now universally acknowledged. Any technology, such as Alcon’s Ozil torsional handpiece, that replaces longitudinal phaco needle movement with a side-to-side motion also reduces repelling forces at the phaco tip. This further enhances nuclear followability and reduces fragment chatter.
2. Micro phaco tip
Going from a 19-gauge to a 20-gauge phaco tip is one strategy that all surgeons can use to enhance safety regardless of their brand of phaco machine. This single modification reduces surge, lessens the chance of accidentally aspirating the iris or capsule, and makes it easier to pluck thin or crumbling nuclear fragments from the capsular fornices. The latter advantage stems from the fact that the smaller tip occludes without having to penetrate too deeply into the nucleus. The narrower lumen restricts flow, reduces surge, and prevents material from rushing in as fast as through a standard diameter needle shaft. Like using a smaller irrigation and aspiration (I&A) tip opening, a micro phaco tip also provides greater control over which tissue is or is not aspirated. Slowing things down in this way helps when one wants to guard against snagging the capsule, such as when aspirating epinucleus or thin nuclear pieces abutting the peripheral capsular bag.
|
Counterbalancing these advantages are several tradeoffs. Micro phaco tips tend to increase nuclear chatter because of the smaller “mouth.” They take longer to remove a bulky nucleus — like drinking a milkshake through a small straw. Finally, the smaller surface area of the tip’s opening reduces the effective holding power for any given vacuum level. Fortunately, surgeons can solve the chatter/followability problem through the use of hyperpulse power modulation and by chopping the nucleus into smaller pieces. Improved pump technology enables surgeons to safely use higher aspiration flow and vacuum to compensate for the other factors. This makes it possible to reap the benefits of a smaller phaco tip regardless of whether one performs coaxial or bi-axial phaco and this is the most overlooked safety modification that surgeons can make.
The Dewey Radius Tip (Microsurgical Technologies) is another tip option that can reduce the risk of a tear should the posterior capsule be aspirated. Surprisingly, brunescent nuclei can be effectively emulsified despite rounding the sharp edge contour of the standard phaco needle.
3. Sequentially changing memory settings
Another important strategy is customizing machine memory settings for each sequential stage of the procedure. Although it is tempting to seek the simplicity of one set of parameters that can be used for every type of nucleus and for each step of the case, doing so sacrifices control. The fluidic and power objectives are very different when a surgeon sculpts (maximize cutting efficiency so as to avoid pushing the lens against the bag), chops (maximize holding power to fixate, separate, and then elevate chopped segments), or removes loose fragments (maximize followability, reduce chatter and turbulence, and avoid any momentary surge once the posterior capsule is exposed).
For this reason, surgeons should preprogram ultrasound and vacuum parameters that best prioritize the specific objectives of each step in nuclear emulsification. For example, memory settings with different “packages” of parameters could be set up for sculpting, for chopping, for fragment removal, and for epinucleus aspiration. As a general principle, higher aspiration flow and vacuum are desirable early on to optimize holding power for chopping, separating, and aspirating the initial fragments, when enough remaining nucleus obscures or blocks the posterior capsule. However, high flow and vacuum later become a liability as the final pieces are removed and the posterior capsule is exposed to the phaco tip. Flow and vacuum should be significantly reduced to avoid surge at this stage of the case.
The WhiteStar Signature phaco system (AMO) provides the ultimate capability of stepwise customization. Unlike the Sovereign system, each memory setting on the Signature is assigned the name of a procedural step (eg, chop, quadrant, fragment, epinucleus) and is controlled by forward or reverse foot pedal switching with voice confirmation. All of the programmed parameters can be further adjusted according to four different grades of nuclear density. For example, if the nucleus is denser than anticipated, a touch screen panel adjustment converts every memory setting (eg, chop, quadrant, fragment, epinucleus) to more aggressive vacuum and ultrasound parameters that were preselected for a 4+ nucleus.
The Signature platform introduces a unique dual pump, which can function either as a peristaltic or a venturi pump. More importantly, the surgeon will be able to switch back and forth between the venturi and peristaltic systems at any time without having to change the tubing cassette. This will allow surgeons to alternate the speed and efficiency of a venturi pump with the safety of a peristaltic flow-based pump, depending on the stage of the procedure. For example, I could have a venturi system programmed for my chop memory setting and a peristaltic system for my fragment memory setting. I could choose a venturi system for the epinucleus and cortex so that unoccluded live vacuum would draw the epinucleus toward the tip, rather than my having to aspirate it peripherally. I could also assign a peristaltic function for chopping softer nuclei, but then switch to a venturi system for a dense nucleus to accelerate its otherwise sluggish passage through a 20-gauge phaco tip. I could immediately switch back to a peristaltic system to remove the last few fragments to maximize safety in conjunction with the micro phaco tip.
When selecting a phaco tip and programming fluidic or ultrasound parameters, surgeons are always balancing speed against safety. Such instantaneous switching and customization, which will now include using different pump systems, means that surgeons can finally enjoy the best of all worlds.
4. Antisurge algorithm
A final important safety feature is what I call an antisurge algorithm. AMO first introduced the occlusion mode feature with their Diplomax machine. This allowed surgeons to program changes in ultrasound or fluidic parameters to occur once the phaco tip became occluded or unoccluded. I believe that postocclusion surge is still the most common cause of posterior capsule rupture occurring during nuclear emulsification. I always thought, therefore, that it would be much better if surgeons could quickly reduce vacuum immediately before an occlusion break. When using higher vacuum levels, this would significantly reduce surge and improve chamber stability.
In response to this suggestion, AMO’s Fusion Fluidics pump technology includes an antisurge algorithm to accomplish just this result. The pump’s onboard computer recognizes occlusion and proactively reverses the pump to actively step-down the vacuum before the occlusion break occurs. I recommend programming the foot pedal so that the surgeon can turn this antisurge algorithm on or off during the case. If a surgeon experiences chamber instability due to slight postocclusion surge, the algorithm can be turned on. Like activating ABS brakes on a car, the algorithm automatically drops the vacuum level after a predetermined interval to prevent surge as fragments are being evacuated. The antisurge algorithm tries to automate and duplicate what an experienced surgeon could do with a dual linear foot pedal once the tip becomes occluded. High vacuum is used to maximize holding power, but the surgeon then lowers the vacuum with the foot pedal before delivering phaco power to clear the phaco tip.
Continuing advances in phaco technology have improved a surgeon’s ability to manage the most challenging cataracts. While such sophistication comes at the expense of simplicity, understanding and properly configuring the technology delivers improved performance and safety. This is the compelling payoff for mastering the latest phaco technology.
Dr. Chang is a clinical professor at the University of California, San Francisco, and in private practice in Los Altos, CA.