As a result of my conversations with surgeons who perform microphaco, I have determined that the primary reason they experience failures with a surgical technique is due to the instruments they use. The presence of the second instrument in microphaco is much more important than in standard phacoemulsification, something which may not seem logical to physicians who are unfamiliar with this technique.
One point of difficulty with microphaco is achieving sufficient flow through the second instrument. Neglecting to pay attention to the flow will ensure surgical failure. I see design flaws in many microphaco instruments, including constriction in the irrigation cannula's tubing, tubing through the handle that is not sufficiently thin-walled to maximize flow, or openings that are too small to permit enough flow from the instrument to maintain the anterior chamber (which can cause anterior-chamber fluctuations that make the procedure dangerous). Problems with instrument design such as these will induce complications in microphaco and defeat the reason for performing the technique.
One of the key elements of success in regard to ensuring sufficient flow is to match the irrigating instrument and the phaco needle, regardless of the type of tubing you use. For instance, using a 19-gauge bare phaco needle and a 20-gauge irrigating instrument will not work; there will be too much outflow and too little inflow. Although you can adjust the amount of flow, the gap on the outside of a larger needle will create more flow. The gauge of the instruments must match. You must enhance the flow so that whatever is constricted is as short and thin-walled as possible, and the opening in the irrigation cannula you have in place should be as large as possible. Preferably, you should use wound openings that are large enough to maintain the integrity of the instruments and are oval-shaped to enhance the flow.
I prefer an instrument with a single opening at its end because it offers the least-constricted flow with thin-walled tubing. This instrumentation also helps to direct the flow as though it were an additional, invisible instrument. You can direct the flow to the side of particles to move them out of the fornix or off the capsule. You can also use this stream to open up the capsular bag. An instrument with a single opening at its end is a valuable tool with which you can chase material without touching it. I am convinced that this type of instrument will enhance the safety of microphaco.
The primary concern with a single opening at the end of thin-walled tubing is that there is not much strength with which to attach a chopper; this weakness means most instruments are closed on the end. You must then create two openings that are as large as possible. The only manufacturing company that has been able to perfect the single-opening instrument to my knowledge is MicroSurgical Technology (Redmond, WA) in their Duet system. They have a vertical chopper with a single opening that has held up very well
Every microphaco surgeon should test the flow through his instrument. To do this, position the bottle at the height you normally hold it, and then position the handpiece with the second irrigating instrument at the approximate level where you normally operate. Let the flow proceed freely. If the instrument does not produce at least 40 mL of flow per minute with the bottle fully extended, it is dangerous to use that instrument in surgery. Keep in mind that 40 mL of flow per minute is the minimum; 50 mL is even better. An extender to raise the bottle will increase the flow, but 40 mL is still the absolute minimum. I am surprised at both the number of instruments that go untested and the number that do not pass this test.
I do not recommend setting high flow rates in order to speed the surgery. If the instrument will only produce 40 mL per minute of irrigation flow but the aspiration flow rate is set to 50 mL of flow per minute, the chamber will collapse. Generally speaking, I set the flow rate at no more than half of the actual flow. For instance, although I may achieve 50 mL of flow per minute, I will not set it any higher than 25 mL per minute; for 40 mL of flow per minute, I would set it no higher than 20 mL per minute. Many surgeons may think 20 mL of flow per minute seems awfully slow, but, because the instrument is separated and does not short-circuit the flow (much of the coaxial irrigation passes by the aspiration area and travels back up the tubing), the effective speed is about 50% greater than that of traditional phacoemulsification. In this way, 20 mL per minute will feel more like 30 mL per minute, and so forth.
Once the flow is under control, do not overlook other issues of concern in microphaco instrumentation.
For instance, because the reduced amount of flow in this technique requires relatively tight, unforgiving incisions, protuberances on instruments designed to crack or manipulate nuclear material must be able to pass through these incisions without stretching or tearing them. I find some protuberances to be so short that they cannot successfully reach the intraocular area but invade the cornea instead. Upon inserting the instrument, if you begin to turn it before it has entered the anterior chamber, it will turn into the corneal stroma rather than the eye. Instruments that are much larger than the size of the tubing will require too large an incision, which in turn will induce other problems such as an unstable chamber.
I do not recommend using a flared-tip phaco needle. Although some surgeons consider a flared tip useful in microphaco, I disagree because the thinness of the shaft compared with the width of the tip will allow too much leakage around the wound, diminishing the chamber stability and perhaps even causing complications. The mouth of the needle and the barrel of the instrument must be similarly sized to appropriately fill the wound. I prefer to use a phaco needle designed by MicroSurgical Technology, the shaft of which is slightly larger than the barrel of the phaco tip by approximately 0.05 mm. This design allows the needle to move from side to side without significantly increasing the leakage and without inducing corneal folds. Moreover, while I have always preferred performing phaco chop with a 0º tip, it simply will not slide into a microphaco wound that is sized to decrease leakage. I have found that a slightly angled tip, such as 30º, enters the wound nicely and then easily passes into the eye.
There are additional benefits to maintaining a moderately tight wound. Not only will it enhance the flow (flow is generally considered the weakest force in microphaco), but it will also decrease wound leakage, which in turn will force any free-floating particles toward the phaco needle. Leakage around the wound will attract these particles and render them out of reach of the phaco tip. If this happens, you will have to pull the instrument back out of the wound to try to reach the particles.
Currently, my favorite microphaco instrument is the vertical chopper of the Duet System that features a single opening at its end. The company used an innovative design for their instruments: The handles are sold separately, and the instruments themselves are interchangeable with the handles. The handles have a large opening that offers plenty of flow. When you are ready for I/A, you unscrew the chopper and affix a straight irrigation tip. Rather than buying completely separate instruments, you only have to buy the individual instrument tips. The company uses the same design for I/A instruments; you even use the same handles that you use during phaco.
Randall J. Olson, MD, serves as Director of the John A. Moran Eye Center, as well as Chairman at the University of Utah Department of Ophthalmology in Salt Lake City. He holds no financial interest in any product mentioned herein. Dr. Olson may be reached at (801) 585-6622; email@example.com.