Setting machine parameters for phaco chop. (David F. Chang, MD)

To maximize the benefits from the latest in phacoemulsification technology, it is important to understand how to adjust and customize phaco parameters. Surgeons often question what phaco settings to use when making the transition from cracking to chopping. David F. Chang, clinical professor of ophthalmology at the University of California, San Francisco, reviewed a few principles that can help to facilitate the transition.

Although stop and chop involves chopping, "non-stop" phaco chop is a pure chopping technique, which according to Dr. Chang, eliminates all sculpting. There are two variations of non-stop chopping. He calls the classic Nagahara technique "horizontal" chopping because the chopper tip moves toward the phaco tip in the horizontal plane. The quick chop method is called "vertical" chopping because the two instrument tips move toward each other in the vertical plane. "Although they operate via different mechanisms, both techniques deliver the universal advantages of decreasing phaco power, phaco time, and stress on the zonules and capsule", said Dr. Chang.

Of the two main aspiration variables of a peristaltic pump, flow rate governs the speed of the procedure. Dr. Chang suggests that transitioning surgeons may want to start with a low flow rate, and later increase this later as they become more comfortable. Vacuum determines holding power - the strength with which nuclear material can be gripped at the tip. If pieces keep dropping off the tip, a higher vacuum may be needed.

"Holding power is important for the initial maneuvers of chopping the nucleus into fragments, and pulling the first few pieces out of the capsular bag. Particularly for dense nuclei, this solid purchase can be optimized through the combination of high vacuum settings, and burst mode to maintain a tight occlusion seal around the tip", said Dr. Chang.

"Just how high you can safely set the vacuum is determined by the point at which excessive post-occlusion surge takes place. Modern phaco machines use a number of different strategies involving pumps, tips, and tubing to limit surge," said Dr. Chang. "This allows us to gain the improved control afforded by higher vacuum and holding power".

To determine the maximum safe vacuum level for your machine, Dr. Chang recommends performing a "surge test", which requires that there still be plenty of nucleus in the eye to hold back the posterior capsule. "Starting with your quadrant setting, hold a small fragment of nucleus in the center of the pupil with foot position 2. Deliberately wait for the beeping sound signaling that the maximum vacuum level has been reached. Then gauge the amount of surge that occurs once the piece is suddenly emulsified through the tip", said Dr. Chang. "If no surge occurs, then increase the vacuum by 10%, and repeat the process until the surge eventually becomes unacceptable, signifying that the maximum safe level has just been exceeded."

While high vacuum makes the phaco-assisted aspiration of mobilized chopped fragments more efficient, and less ultrasound energy dependent, it can become a disadvantage toward the end of the procedure. "As the last pieces and the epinucleus are removed, the priority becomes eliminating even the slightest hint of surge, since the posterior capsule is now exposed. Holding power is not important at this point, so the surgeon should have a lower vacuum setting programmed for this final stage", said Dr. Chang. "A common mistake is to try to use a single high vacuum program throughout the entire case. In addition to dropping the vacuum, I increase the flow rate slightly for this stage to enhance followability."

"If we combine the advanced ultrasonic and fluidic technology of the newer machines with a technique such as phaco chop, which eliminates sculpting, we can reduce the amount of phaco energy we put into the eye to the lowest amount possible", Dr. Chang said.