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Refractive Surgery | May 2009

Basic Guide to Fluidics

The phaco tip you choose will determine your phaco fluidics settings.

The combination of advanced cataract surgical techniques with innovative phaco equipment has resulted in a safer and easier procedure for both surgeon and patient. We are now at a stage where many surgeons are moving toward smaller incisions because of these advances. However, adopting these new techniques and technologies is only beneficial for your patients if you truly understand the concepts of phacoemulsification.

Regardless of your level of surgical expertise—whether you have been in practice for 30 years or just completed your residency—everyone can benefit from reviewing the two fundamentals of phacoemulsification: fluidics of lens removal and ultrasound power. Mastering these parameters will undoubtedly improve the safety and efficacy of your technique. This article concentrates on the basics of fluidics and how this relates to tip technology. David Allen, FRCOphth, of the United Kingdom, focuses on the basics of ultrasound in his article on page 32.

The purpose of fluidics during phacoemulsification is to control the inflow and outflow, appropriately balancing them to ensure safe and effective surgery. When fluidics is well maintained, the result is a stable anterior chamber that allows a proper working space and easy manipulation of the cataract material toward the phaco tip. Maintaining this balance may become more difficult with smaller incisions without suitable adjustments as less fluid enters the eye.

OPTIMIZING FLUIDICS
Optimizing phaco fluidics settings is essential; however, the specifics vary widely depending on the surgeon's technique and choice of instrumentation. Although it usually takes trial and error to customize your fluidics settings, having a few ground rules is advisable. Perhaps the best place to start is with your phaco tip. There are numerous phaco tips and designs available today. Let's answer a few simple questions.

Why do you need one? It is the rapid movement at the phaco tip that provides enough energy to disrupt the nuclear material. But it is the shape of the tip and the internal diameter of the shaft that determines the way nuclear material is removed from the eye.

Phaco tips vary in size, shape, curvature, internal and external diameter, restriction of internal architecture, angulation and position of the bevel, leakage aperture, and sharpness. Currently, 19-, 20-, 21-, and 22-gauge tips are available from many manufacturers. Standard needles are straight with 30° bevel, but flared and mini-flared tips are also available. However, the internal diameters and architecture of phaco tips vary with differences in design. Furthermore, some needles are curved as well as having a bevel and a flare.

Flared tips are said to be better for chopping because they provide a greater surface area. But this also makes it more difficult to get occlusion. Mini-flared tips, supplied by Alcon Laboratories, Inc. (Fort Worth, Texas), are for use with torsional ultrasound. In a dense nucleus, there is often clogging unless a 45° bevel is used.

What determines the settings for a given phaco tip? First, the amount of irrigation fluid entry into the eye is a key factor. Other factors include the amount of leakage from the eye, most often through the sideport. The internal diameter of the phaco needle at its thinnest point is crucial to fluidics settings because of the physical principle of Poisseiulle's Law, which determines the resistance to flow through a cylinder. Small increases in the cross-sectional diameter will increase the area of the cylinder by greater amounts. Therefore, the amount of resistance decreases as the diameter increases; as the diameter decreases, the resistance increases.

Why is tip design important? Different tip designs enhance the way the nucleus is broken down, with the size of the tip influencing the way tissue is removed. Add-itionally, the shape of the needle affects the efficiency of cutting. For example, the movement of the curved Kelman tip makes it a better cutter than a straight needle, particularly with dense cataracts.

Optimal tip design also depends on the type of energy being used—longitudinal, torsional (OZil; Alcon Laboratories, Inc.), or transversal (Ellips; Abbott Medical Optics, Inc., Santa Ana, California). With transversal energy, for example, a straight tip creates enough movement to remove tissue as it moves. Torsional energy requires a curved needle. The backward and forward movement of a straight needle also removes tissue, but not as efficiently due to repulsion of nuclear material.

Another factor when deciding on tip selection is your technique. If you are chopping, you want to try to hold the nucleus as firmly as possible without pulling nuclear tissue through the tip. You should be in foot position 2. If you use divide-and-conquer (which requires a lot of sculpting), a wider, open-bevel tip, such as the 45° mini-flared tip from Alcon, will cut more efficiently. As already mentioned, with torsional, clogging may be eliminated by using a tip with a sharper bevel; however, this may make chopping more difficult because of the more open bevel.

TIP DESIGNS
The majority of tips have the bevel of the tip pointing upward; however, a few models feature a downturned bevel. For example, the tip used with the Infiniti (Alcon Laboratories, Inc.) is a 30° bevel-down phaco tip with 12° angulation. This tip penetrates the nucleus from above, with good occlusion during chopping.

There are phaco tips designed for sub—2-mm incisions. The Devine needle, designed for use with the Stellaris (Bausch & Lomb, Rochester, New York), fits through a 1.8-mm incision. Oertli has a needle with a 53° bevel for use through a 1.6-mm incision. They both have restricted central diameters that can slow down surgery and require high vacuum to work efficiently.

There are also differences in sharpness of the phaco tip. In the past, only sharp needles were used because they penetrated all cataract types easily. Rounded tips seem to break up cataracts just as well as sharp tips. For example, the Dewey Radius (MicroSurgical Technology, Redmond, Washington) has a smooth surface and a rounded tip. This tip can remove hard nuclei even though the end is not sharp.

If you take tip style to the other extreme, the Akahoshi Knuckle Tip (ASICO, Westmont, Illinois) has a ball on the end of the tip with a hole in the middle. This tip works well with the prechop technique. It is a fluidics-driven phaco tip. Akahoshi has also designed a square tip for use with torsional ultrasound.

OTHER FACTORS
Vacuum, bottle height, and pump flow rate also affect fluidics. Surgeons often disclose that they use a vacuum of 600 mm Hg; however, few disclose what needle they use. More than likely, it is because they prefer a narrow phaco needle. In the absence of high vacuum, it would be impossible to clear the nuclear material from the eye with a narrow tip. Alternately, the vacuum need not exceed 200 mm Hg with a 19-gauge needle. Phacoemulsification should be fluidics-driven rahter than energy-driven. Energy is required to assist the fluidics to draw the nuclear material through the phaco needle.

The bottle should be adjusted to the height that will allow adequate inflow. Bottle height is the only regulator of inflow. Remember that as you increase the bottle height, the pressure and rate of inflow also increase. The inflow of fluid should always be greater than the outflow, which is regulated by a combination of pump speed and wound leakage. With peristaltic pumps, speed slows as vacuum rises. When occlusion breaks with either type of pump (peristaltic or venturi), there is a return to previous pump speed, which may cause the chamber to shallow if the balance of inflow and outflow is skewed.

CONCLUSION
Although the list of tips mentioned in this article is by no means exhaustive, the most important thing to take from this article is to choose the tip that is appropriate for your phaco technique and incision size. If you are using the wrong needle and are not optimizing fluidics, you are not performing phacoemulsification to your full potential. Understanding phaco fluidics will make you a better surgeon and produce better outcomes.

Richard Packard, MD, FRCS, FRCOphth, practices at the Prince Charles Eye Unit, King Edward VII Hospital, Windsor, England. Dr. Packard states that he is a consultant to Alcon Laboratories, Inc., Abbott Medical Optics, Inc., Bausch & Lomb, and MicroSurgical Technology, Inc. He may be reached at e-mail: mail@eyequack.vossnet.co.uk.

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