As a method of nuclear fragmentation , phaco chop provides vices numerous advantages over cracking. It decreases total phaco time and energy, decreases stress on the zonules and capsule and confines all of the phacoemulsification to the central 3 mm of the pupil.

After Kunihiro Nagahara, MD, of Japan first introduced the concept of phaco chop in 1993, the "stop and chop" method of Paul Koch, MD, went on to become the most popular chopping technique. Dr. Koch recognized that although the nucleus could be sliced into many fragments, these segments still fit tightly together within the capsular bag, like pieces in a jigsaw puzzle. The immobility of these pieces within the bag makes it relatively difficult to aspirate them out.

The "stop and chop" technique begins with a deep central groove, which is used to crack the nucleus in half. One then stops cracking and chops the remaining sections. The initial wide groove provides enough space within the capsular bag to easily aspirate and maneuver the chopped pieces out with the phaco tip. However, a significant amount of phaco energy and capsular force is still involved in sculpting the central groove.

"Non-stop" phaco chop

I employ a chopping method that eliminates the initial groove and crack. In this way, it can be thought of as a "non-stop" way to do phaco chop. Using a Lieberman microfinger (Karl Ilg, St. Charles, III.) as a chopper and manipulator, the first step is to divide the nucleus in half without any sculpting. The central core of the nucleus is impaled with the phaco tip and held with position 2. The chopping instrument passes peripherally beneath the anterior capsule and hooks the equator of the nucleus. The microfinger actually moves within the epinucleus and thus does not stretch the anterior capsule excessively.

The chopping instrument is pulled toward the phaco tip and, upon contact, the two tips are moved slightly apart. This motion continues the split along a natural cleavage plane through the proximal remainder of the nucleus. Both the phaco tip and the chopper tip must be deep enough so that the central nuclear core is sandwiched and compressed between them during the chopping motion.

Zero degree tip. Having no bevel, the tip is internally sharpened to provide cutting ability. Because it occludes more easily, this tip enhances aspiration performance and followability.

The nucleus is rotated 30 to 45° clockwise, and the opposite heminucleus is impaled with the phaco tip. A small, pie-shaped wedge is now created by the second chop. The firmer the nucleus, the smaller this piece should be. Although it is the most difficult to remove, the first segment usually can be elevated out when firmly grasped by a small phaco tip employing high vacuum. Alternatively, the blunt and curved tip of the microfinger can be used to manually tumble the initial piece out. The nucleus is rotated farther and the next piece is chopped and aspirated out. Each successive piece becomes easier to remove as more and more space is vacated within the bag.

Advantages of phaco chop

Chopping dramatically decreases phaco power and time by eliminating all sculpting Manual energy, generated by one instrument pushing against another, replaces the need for ultrasound energy to subdivide the nucleus. Ultrasound is employed only in the aspiration and evacuation of the fragments. The phaco tip, and consequently all ultrasound, never moves peripheral to the central 3-mm zone.

In addition to requiring less phaco power and time, chopping minimizes the stress placed on the zonules. In order for a phaco stroke to cut through the nucleus, the lens must be immobilized. Like a vise holding a

I employ a chopping method that eliminates the initial groove and crack. In this way, it can be thought of as a "non-stop" way to do phaco chop.

-David F. Chang, MD

piece of wood, it is the zonules and capsule that grip and fixate the nucleus as the groove is cut by the phaco tip. With phaco chop, however, it is the phaco tip against which the chopping instrument pushes. These manual forces are directed centrally inward, rather than outward toward the zonules and capsule as with sculpting and cracking.

These advantages of decreased zonular stress and decreased phaco power define the indications for phaco chop. While I employ this as my primary technique for routine cases, I believe it is the procedure of choice for complicated phaco cases that entail greater risk of posterior capsule rupture - for example, small pupils, loose zonules and mature or brunescent cataracts. It is also advantageous for patients with corneal endothelial disease, where minimizing ultrasound power and time is critical.

Enhanced phaco technology

I am currently using the AMO Diplomax (Irvine, Calif.) phaco machine. Several features of this system are particularly well suited to the "non-stop" method of phaco chop. First, the phaco tip is small, fitting comfortably through a 2.5-mm incision. As an aspirating instrument, a smaller phaco tip is more maneuverable at "digging out" the initial chopped piece of nucleus.

AMO also offers a 0° phaco tip which is particularly nice for this technique, in which ultrasound is needed only for aspiration of nuclear pieces. The more beveled the phaco tip is (such as a 45° tip), the better it is for sculpting, but the harder it is to occlude. Thus, a 15° tip occludes and aspirates much more efficiently than a 45° tip.

A 0° tip has no bevel at all, making it the best aspirating design of all. This is a perfect complement to nonstop phaco chop where there is no shaving or sculpting, and where ultrasound is only used to embed the tip or aspirate nucleus and epinucleus. The speed with which this tip occludes increases the efficiency of grabbing pieces and improves followability. As a result, sizable sections of firm nucleus can be evacuated faster and with less total ultrasound energy. Predictably, a 0° tip is unsurpassed at aspirating epinucleus.

High vacuum provides two significant advantages for most phaco techniques. First it increases the strength with which a piece of nucleus is held by the phaco tip. This is helpful in:

• immobilizing the nucleus during a chop;
• gripping and elevating nuclear fragments created by chopping or cracking; and
• grasping and holding the epinucleus as it is flipped.

Secondly, by increasing the effective force of suction following tip occlusion, it allows one to use less ultrasound in evacuating nuclear material.

Before the advanced fluidics of modern phaco machines, the problem of post occlusion surge always limited the level of vacuum that we could employ. This surge is evident as momentary chamber shallowing once an aspirated piece is cleared. The Diplomax has two features that eliminate any chamber bounce at vacuum levels of 200 to 250 mm.

First, the high vacuum tubing set provides a larger diameter inflow line coupled with a smaller diameter outflow line that has thicker walls. The extra wall thickness increases the rigidity of the outflow line which resists collapse as suction builds behind the tip occlusion. As with a bulb syringe, suction results as the collapsed compliant plastic snaps back to its natural shape. The decreased outflow lumen diameter means that a lesser volume of fluid will surge into the line as the tip occlusion is cleared.

Second, microprocessors allow the machine to be programmed to change parameters automatically once the phaco tip is occluded. As an example, one can initially employ a lower aspiration flow setting to bring material to the tip at a slower, safer rate. After occlusion, the aspiration rate can be programmed to increase so that the maximum vacuum is reached as quickly as possible. This "occlusion mode" setting increases the hold on the material. Once the maximum vacuum setting is achieved, the peristaltic pump immediately stops turning. By dropping the flow rate to zero, there is no further buildup of vacuum within the tubing behind the occluded tip. Thus, as the material is cleared through the phaco tip, there is minimal post occlusion surge of flow.

Occlusion mode phaco

Finally, the Diplomax software gives several unique options for the phaco mode that facilitate phaco chop. In "burst" mode, the machine is programmed to deliver a single burst of phaco, similar to a single pulse when in pulse mode. Individual bursts of phaco energy are useful for impaling the nucleus with the phaco tip. Like a toothpick entering a piece of melon, the tip is better embedded with single solitary strokes Continuous vibrating motion only serves to loosen the tip's purchase by creating a small micro-cavity surrounding the tip. Thus, with firm nuclear material, burst mode generates a better hold and better tip occlusion than continuous mode phaco.

To evacuate large nuclear fragments, another feature is useful. The ultrasound mode can be programmed to switch from continuous into pulse mode once the tip is occluded. Both modes are under linear surgeon control with the foot pedal. Once the occlusion is broken by evacuation of the piece, position 3 shifts back into continuous mode. This gives one the careful control of low power continuous mode phaco to attract and engage a loose piece, combined with the efficiency and better followability of pulse mode to clear large, firm nuclear chunks.

Conclusion

Improvements in phaco machine technology and fluidics have truly enhanced our surgical control and capabilities. These advances in design are particularly helpful in techniques such as "non-stop" phaco chop. Utilizing manual forces and higher vacuum settings results in a marked reduction in phaco power, time and stress on the capsule and zonules. These advantages make phaco chop especially helpful for complicated and higher risk cases.

W First chop performed by pulling chopper toward phaco tip. The nucleus is rotated and the distal heminucleus is impaled with the phaco tip Side view shows position and depth of both the phaco tip and the chopping instrument. Standard l&A tubing (left) and high vacuum l&A tubing (right). With high vac tubing, the irrigation line (blue) has a 10% larger lumen and thicker walls to prevent partial kinking. The high vac aspiration line (red) has a reduced lumen diameter and increased wall thickness. The reduced compliance of this tubing decreases the potential for post-occlusion surge.

Surgical Maneuvers :

Step-by-step illustrated surgical techniques. David F. Chang, MD, is an associate clinical professor of ophthalmology at the University of California, San Francisco. He is in private practice specializing in cataract surgery at 762 Altos Oaks Dr., Ste. 1, Los Altos, CA 94024; (415) 948-9123; fax: (415) 9480563. Dr. Chang has no financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.