Fig. 3.8 Venturi pump AMO signature venturi pump showing flow based mechanics

WhiteStar SignatureŖ

FusionŖ Dual Pump Pack (OPO71)

flow, which attracts fragments toward the phaco tip, and vacuum, which holds the fragments on the phaco tip. Generally low flow slows down intraocular events, and high vacuum speeds them up. Low or zero vacuum is helpful during sculpting of hard or large nucleus, where it is less likely to occlude the tip with equatorial capsule or iris when high energy is being delivered resulting in significant damage to these structures.

3.6 Surge

A fundamental limiting factor in the selection of high levels of vacuum or flow is the development of surge. When the phaco tip is occluded, flow is interrupted and vacuum builds to its preset level (Fig. 3.9). Vacuum and partial emulsification of the occluding fragment then clears the tip. Flow begins instantaneously at the preset flow rate in the presence of a high vacuum. In addition, if the aspiration line tubing is not reinforced to prevent collapse (a function of tubing compliance) the tubing will constrict during occlusion and then expand on occlusion break. This rebound of the compliant tubing increases outflow. These factors trigger a rush of fluid from the anterior segment into the phaco tip. The fluid in the anterior chamber may not be replaced rapidly enough by infusion, to

prevent shallowing of the anterior chamber. Therefore sudden volume reduction in the anterior segment is succeeded by the rapid anterior movement of the posterior capsule. This abrupt forceful stretching of the bag around the nuclear fragments (especially if the fragment is hard with jagged edges) may be a cause of capsular tears. In addition, the posterior capsule can be literally sucked into the phaco tip, thereby tearing it. The phaco machine manufacturers help to decrease the surge by providing noncompliant aspiration tubing. This will not constrict in the presence of high levels of vacuum. The following, more important new technologies are noteworthy:

1.CASE: AMO Sovereign/Signature—Microprocessors sample vacuum and flow parameters 50 times a second creating a “virtual” anterior chamber model. At the moment of occlusion the computer senses the decrease in flow and instantaneously slows the pump to reduce the vacuum and suppress the surge. The Alcon Infinity works in a similar manner (Fig. 3.10).

2.Dual Linear: Bausch & Lomb Millennium/ Stellaris—The dual linear foot pedal can be programmed to separate the vacuum, and therefore the flow, from power. In this way, vacuum can be lowered before beginning the mobilization and the partial emulsification of an occluding fragment. Therefore the surge is minimized.

a

b

Fig. 3.9 (a) Occlusion: vacuum increases to pre set maximum, flow decreases toward zero, tubing collapses. (b) Occlusion break: vacuum drops to zero. Flow rapidly increases to preset. Tubing expands. Outflow exceeds inflow. Anterior chamber begins to shallow. Photo courtesy of Thieme Publishers, New York City

3.Aspiration Bypass System (ABS) Alcon Infiniti/ Legacy—The Aspiration Bypass System (ABS) tips have 0.175 mm holes drilled in the shaft of the needle. During occlusion, the hole provides for a constant alternate fluid flow. This will cause dampening of the surge on occlusion break.

3.6.1Non-Longitudinal Phaco: Modification of Fluid Control by Power Modulations

Three significant trend setting technologies have revolutionized the way the power is modulated. When employing these power modulations, the duration of the power application and the motion of needle movement affect the fluid flow and occlusion. These modulations have an effect on the fluidic balance during phaco which is as important to chamber maintenance and the ease of nuclear fragments removal, as that of the preset vacuum and flow.

1. Micro-Pulse Phaco

As discussed previously, the rapid 4-ms power on cycles maximizes the development of transient cavitational energy. All cavitational energy in the 4-ms burst is capable of emulsifying the tissue. The ensuing 4-ms period of aspiration replenishes the fluid at the phaco tip and cools it. The use of micro-pulse phaco is necessary to create the shift in phaco technique from postocclusion phaco to partial occlusion phaco.

Fig. 3.10 If occlusion should occur CASE changes the pump speed and changes the commanded vacuum at a preset “up” level, “down” level or time

2. Torsional Phaco (Alcon Infiniti Ozil Handpiece)

Classic phaco has utilized a phaco tip that moves forward and backward, or longitudinally. Torsional phaco is defined as the 32-kHz oscillatory movement of an angled (Kelman) phaco tip which shears nuclear material at the phaco tip. This can be alternated with longitudinal movement of the needle at 44 kHz. The torsional component is linear and the longitudinal component can be micro pulse. The potential flexibility of this system is enormous (Fig. 3.11).

3. Elliptical Phaco (AMO Signature)

In this system the longitudinal movement of the phaco tip at 38 kHz is combined with a transversal motion at 26 kHz. The resultant movement of the needle can be

Fig. 3.11 (a) Alcon Infinity. Ozil Torsional Handpiece 32 kHz. Oscillatory Movement 44 kHz. Linear Movement. Torsional and longitudinal combined in different ratios. (b) Torsional energy spreads laterally like a bow tie shaving the nucleus. Photo courtesy of William J. Fishkind, MD

described as prolate-spheroid. (Shaped much like an egg cut in half) (Fig. 3.12).

While the longitudinal phaco cores the nuclear material, the non-longitudinal phaco shaves the nuclear material (Figs. 3.11 and 3.12) This mode of needle movement is a noteworthy variation from other technology, since by its very movement, it generates partial occlusion phaco and therefore lessens the risk of surge.

3.6.2Partial-Occlusion Phacoemulsification

The way to avoid surge is to prevent total occlusion entirely. By definition, a surge requires total occlusion.

Fig. 3.12 (a) AMO elliptical. Phaco tip moves longitudinally, and in a prolate spheroidal manner to shave the nucleus. (b) AMO elliptical creates energy in the shape of a prolate spheroid shaving the nuclear material. Photo courtesy of William J. Fishkind, MD

In partial-occlusion phaco, micro pulse phaco is the catalyst. The nuclear fragment is brought close to the phaco tip with a 4-ms period of aspiration until the fragment partially occludes it. With the onset of a 4-ms burst of phaco energy, the fragment is mobilized and is partially emulsified before it can totally occlude the phaco tip. Therefore, the flow never falls to zero and the vacuum never builds to maximum, and the surge is avoided. This appears to be an exceptionally effective process of emulsification. It allows for fragment removal with minimal energy intensity and duration, and results in a deep and controlled anterior chamber.

Torsional (Ozil) technology (Alcon), and Ellips (AMO) also generate pre-occlusion phaco. The oscillatory movements of the phaco tip automatically knock the fragments off the phaco tip. Unlike longitudinal phaco where the removal of the tissue is described as coring, the removal with non-longitudinal phaco is described as shaving. Since the oscillatory movement