Ординатура / Офтальмология / Английские материалы / The Art of Phacoemulsification_Mehta, Alpar_2001

32 THE ART OF PHACOEMULSIFICATION

David E Allen

INTRODUCTION

The art of cataract extraction has developed considerably over the past ten years, largely as a result of the increased sophistication of the machines the surgeon now uses to assist the process. This chapter is written with particular reference to four machines of which the author has experience. They are not the only machines to offer some of the features described, and the reader should be able, as a result of the knowledge gained from this chapter, to make appropriate enquiry to the manufacturers of other machines.

THE MACHINE IN PRINCIPLE

A generic phacoemulsification machine consists of two components: a pump which assists in the flow of fluid through the eye, and a probe which helps to reduce the crystalline lens to pieces of a size that can be removed by the pump.

The pump actively controls the volume and rate of removal of some fluid from the eye (other fluid leaves the eye through incisional leakage). Inflow of fluid to the eye to replace that which leaves is passive, and is dependent on gravitational forces, modulated by the inflow resistance of the tubing and handpiece. If the anterior chamber is to be safely maintained, then the maximum potential inflow must exceed the maximum outflow of fluid. Modern machines offer very good control of the active removal of fluid.

Phacoemulsification is achieved by either a magnetostrictive mechanism (the original type of phaco generator still used in some handpieces) or by the piezoelectric

NEW PHACOMACHINES OFFER MORE CONTROL 33

effect of certain types of crystals. Computer-controlled electrical circuits produce vibrations that are transmitted to a tungsten tip. The active mechanisms by which phaco needles work to break down the solid lens include

•Ultrasonic effect of the high speed vibration of the tip

•Cavitation effects

•Particle and fluid pressure waves

•Direct mechanical jack-hammer action.

Modern phaco machines make use of sophisticated computer electronics to control the phaco-tip, and the tips themselves are being designed to enhance the “cutting”

action, particularly using cavitation.

SURGEON’S REQUIREMENTS DURING THE PROCEDURE Efficient Removal of Fluid and Particles from the Eye

As described above, fluid egress from the eye is largely controlled by the machine, and is the most important factor in maintaining the correct balance. Modern pumps allow the rate of flow from the eye through the handpiece to be varied. Peristaltic pumps generate positive displacement of fluid along the handpiece tubing. Varying the pump speed has a direct effect on the rate of fluid displacement. Vacuumbased pumps (venturi or rotary vane) generate flow as a result of the pressure difference between the needle tip in the eye (at IOP) and the vacuum chamber in the machine. Varying the vacuum level in the cassette indirectly affects flow in these machines. A new type of pump (scroll pump) has characteristics that allows it to work in a way that emulates either of these modes and is currently available in the “Concentrix” module as an option in the Bausch and Lomb “Millennium“ machine.

Varying the flow rate allows the surgeon to control the speed with which material is attracted to the tip. Low flow allows the surgeon to safely work close to sensitive structures such as iris or capsule (Fig. 3.1). A high flow rate causes material to be rapidly attracted to the tip, and once the tip is occluded, produces a rapid vacuum build-up. Flow of fluid also, as a secondary effect, tends to cool the phaco tip, both the flow along the lumen, but more importantly the compensatory inflow along the irrigation channel round the outside of the tip.

Efficient Removal of Lens, Reducing the Amount of Phaco Energy Used

This is achieved by using appropriate vacuum settings. In a flow-based system the maximum vacuum generated at the tip can be adjusted independently from the flow rate. In vacuum-based systems the vacuum in the machine vacuum chamber is the maximum vacuum that can be generated at the tip, as well as the factor controlling flow through the handpiece. Modern phaco machines allow the surgeon to select a vacuum setting that is much higher than was safe with older systems. Many modern systems can now safely use maximum vacuum settings between 300 and 500 mm Hg.

34 THE ART OF PHACOEMULSIFICATION

Fig. 3.1: Low flow allows the surgeon to work safely close to sensitive structures such as iris and capsule

High-vacuum settings allow lens material to be removed through the tip with less phaco energy. High vacuum can cause nuclear material to be “molded” through the phaco tip without phaco energy, or allows low phaco power to be assisted by high vacuum in achieving the same effect. Whenever phaco power is applied to nuclear material at the tip the forward movement of the tip repels the material. Material held by a higher vacuum (and/or flow) onto the tip is less likely to be pushed away—increasing the tip’s effectiveness.

The efficiency with which a tip functions to ”cut” lens material is another important factor. The tip and the handpiece (particularly the crystals or magnets it contains) tend to heat up during use, and this can have a significant impact on the functioning of the tip. The most efficient phaco machines have electronic circuits that frequently (or even “constantly”) monitor the performance of the tip and adjust the frequency of vibration to take account of this—“auto-tuning”. Some also adjust the voltage (power) applied to the tip to take account of the loading on the tip—taking account for example of the difference in effective weight of a tip if a large nuclear fragment is impaled on it.

The shape of the tip has been the subject of developments in the past 10 years. The first really different tip was the “Cobra tip” (Fig. 3.2) produced by Surgical Design, and this has now been copied by several manufacturers. This tip has a conventional external diameter at the external opening, but 2 to 3 mm behind this the tip reduces significantly in diameter. The “shoulders” inside the tip increase the surface area moving back and forth, which has two effects. There is an increase in the amount of acoustic shock waves, but more importantly an increase in the cavitation effects of the tip (Figs 3.3A and B). Alcon subsequently developed the “Kelman tip”, which is bent and generates similar increased acoustic and cavitation

to ocular physiology than older techniques. This is partly because of the reduced incision size, but also partly because of increased anterior chamber (AC) stability. AC stability is enhanced when fluid entry and egress to and from the eye are in balance. Modern machines allow this despite the trend, alluded to above, towards higher flow and/or vacuum settings. Modern peristaltic pumps produce a much smoother flow pattern than the versions used on early phaco machines. Modern microprocessor controls make the pump much more responsive to the surgeon’s demands (expressed through the foot pedal).

Higher maximum vacuum settings can now be used as a result of this improved control as well as other improvements. When the phaco tip is occluded, vacuum in the tip and tubing equilibrates and approaches the maximum preset. There is then a pressure difference between the anterior chamber (usually at +30-40 mm Hg) and the lumen of the tubing or vacuum chamber (at the preset maximum, which may be as low as –400 mm Hg). When occlusion breaks, there is a rush of fluid from the AC into the tubing to equilibrate the pressures—postocclusion surge. Soft silicone tubing connecting the handpiece to the machine can tend to collapse when subjected to high vacuum during occlusion, and with the release of that vacuum can be subject to a rebound expansion. This re-expansion of tubing tends to increase the magnitude of postocclusion surge.

Modern machines use a combination of different strategies to deal with postocclusion surge. The first machine to actively combat this problem and allow the use of high vacuums was the Surgical Design “Ocusystem“. This machine uses a combination of relatively rigid tubing and a pressure sensor in the aspiration line. When this sensor detects a rapid rise in pressure in the aspiration line (i.e. occlusion had broken), a pinch valve opens allowing a small volume of fluid from a secondary (higher) bottle to enter the aspiration line neutralizing the pressure differential. Other options range from the use of rigid tubing as well as an extremely low compliance pump (Bausch and Lomb ‘Millennium’), through microprocessor control that delays the uptake of pump rotation and sometimes transient reversal of pump (AMO ‘Sovereign’), to adjustments in the phaco tip that reduce the pressure difference (Alcon ‘ABS tips’). These and similar developments mean that modern machines can use much higher vacuum levels (up to 450 or 500 mm Hg during phaco), while still maintaining stability of the AC. This means the lens is removed more efficiently, there is less risk to the integrity of corneal endothelium and posterior capsule, and reduced breakdown of the blood-aqueous barrier (BAB) as a consequence of reduced AC pressure fluctuations.

Reduced Flow of Fluid through the Eye

Fluid outflow from the eye during phacoemulsification is not just through the aspiration port of the tip. There may be significant incisional leakage through the main incision or any side-port incisions. Traditionally, incisions have been made large relative to the tip, to avoid compression of the silicone irrigation (inflow) tubing. Compression of the inflow tubing has two effects: (i) maintenance of the

through the eye during surgery correlates with the risk of endophthalmitis and endothelial damage. It may also correlate with reduced inflammation and reduced BAB breakdown. Safe and efficient sealing of the incision around the probe along with the factors discussed above (higher vacuum, efficient tips) lead to a much reduced volume of fluid flowing through the eye. These all contribute to the improved outcomes of modern phaco procedures compared to earlier results.

PRINCIPLES OF LENS REMOVAL AND MAKING BEST USE OF MACHINE PARAMETERS

Original phaco techniques were dependent on the use of power to gradually “shave” the nucleus into a smaller and smaller piece. Modern lens removal strategies are based on mechanically breaking the lens into smaller fragments that can then be consumed. If the surgeon uses a technique such as “divide and conquer“ or “stop and chop”, the phaco probe is used to cut one or more grooves through the nucleus. These grooves are normally best cut as deeply as possible, close to the posterior capsule. Often (particularly when dealing with a soft nucleus) the surgeon will also wish to sculpt out close to the equator of the lens. When sculpting close to the posterior capsule, capsulorrhexis edge or iris (Fig. 3.1), the surgeon requires low flow settings on the phaco machine in order to minimize the chances of these structures being drawn into and being damaged by the phaco probe. As well as low flow during nucleus sculpting, the machine should be set with a low vacuum. This ensures that if the wrong structure is accidentally engaged, less damage will result and, for example, the capsule may not be torn if the preset vacuum is, say, 20 mm Hg.

It is during the sculpting phase that the surgeon is most likely to require higher levels of phaco power, as the technique uses phaco power to cut through the lens material. The amount of power used should be titrated to the density of the lens. Once the lens has been broken into pieces (either by sculpting or by chopping/

in a continuous linear fashion in position 3 of the foot pedal. The surgeon can therefore titrate the power used according to the density of the nucleus and according to the particular maneuver in hand. One circumstance when a surgeon may choose panel control of power is when dealing with a hard nucleus. Under these circumstances it is easy for a surgeon to tend to try to physically push the tip into/through the nucleus, rather than allowing the ultrasonic power to achieve this. This inadequate use of power can put considerable strain on the zonules. Setting a fixed power

level (say 50% or 70% for a very hard nucleus) ensures that maximum power is available to “cut” into the nucleus as soon as foot pedal position 3 is entered. Alternatively, the Bausch and Lomb “Millennium“ phaco machine has the ability

to set a minimum as well as a maximum in its linear control of power. A surgeon therefore may choose linear control but power may start at say 35 percent and range up to 70 percent linearly. This retains the advantages of linear control (use of the minimum power level necessary) while reducing the chances of using totally inadequate power levels.

While sculpting, the surgeon requires continuous power while the tip is advancing. For consumption of fragments, however, continuous power is not required, as the aim is to try to mold the fragment into a size and shape that will be aspirated. One of the earliest advances in the control of power, shortly after the concept of linear control, was pulsed phaco. The original rationale for this was the fact that applying phaco power to a fragment tends to push it away from the tip by direct hammer action of, and shock-waves from, the needle. After a short burst of phaco, the fluidics draw the fragment back into contact with the tip and re-establish occlusion in preparation for the next burst. Therefore the original drive was to make the power application more efficient. Now, however, the main reason for surgeons using pulsed power is to reduce the total amount of phaco energy used, by using short bursts to assist the vacuum in molding material into the tip. The second reason is that consumption of material is slowed, making it less likely that rapid consumption will lead to the attraction of unwanted material (such as capsule or iris) into the tip. Most implementations of pulse mode allow power to be varied as normal in foot pedal position 3.

Some recent machines have an additional power variation known as burst mode. The machines can be set to deliver a single burst of power, at a fixed level, and for a set (but adjustable) duration, or can be made to deliver bursts at increasing frequency depending on the foot pedal traverse in position 3. With burst mode, power is delivered at a fixed level. Burst mode is said to be particularly helpful with techniques that employ embedding the phaco tip into the nucleus for some form of chopping/splitting. The use of a very short burst at fixed power prevents lateral spread of acoustic energy from the tip that can lead to the creation of a space around the tip preventing good occlusion. This type of phaco is said to lead to more efficient embedding into a tight-fitting space in the nucleus, and so a better seal is created, leading to better vacuum rise time, etc.

All phaco machines currently in the market allow the surgeon to control the aspiration flow rate. Those surgeons using vacuum-based machines, however, (mainly Venturi) often do not realize that they have control. The principles were mentioned earlier in this chapter. Flow-based machines give the surgeon the ability to directly control the aspiration flow rate, and have a dial or electronic equivalent, that allows the direct setting of a flow rate in cc/min. With an unoccluded tip, flow in a vacuum-

based system is generated by the pressure difference between the anterior chamber and the vacuum chamber in the machine. Therefore as the commanded vacuum in the cassette increases, the pressure gradient along the tubing increases, generating increased flow through the handpiece.

Some machines have linear adjustment of flow rate (or vacuum) during traverse through foot pedal position 2. In addition to this feature all machines in current production have various memory settings which allow the surgeon to set different flow rates for different parts of the procedure, or different types of cataract. This ability to adjust flow rate (directly or indirectly) allows the surgeon to control the flow of material to the tip, or the vacuum rise time when the tip is occluded.

The surgeon should always remember that all flow of fluid out of the eye must be balanced by an equal inflow if the anterior chamber is to remain stable and formed. In addition, the higher the outflow (and therefore inflow), the greater is the potential turbulence in the anterior chamber. This is important in the presence of an unstable capsule caused by either a break in the rhexis rim, or a rupture in the posterior capsule. Under these circumstances the surgeon should reduce the flow rate as well lowering the bottle height, to minimize stress on, and potential extension of, the torn capsule edge.

Control of Vacuum

Control of the maximum vacuum level has only recently become something that has featured in the thoughts of surgeons. This is because earlier generations of machines did not allow safe use of vacuum levels above 75 or 100 mm Hg. For the reasons rehearsed earlier in the chapter, higher vacuum levels are now safely used in some modern machines.

The vacuum set on a flow-based machine is the maximum vacuum the machine is allowed to generate once the tip is occluded. In unoccluded mode, the pump generates flow through the tip. With an occluded tip, the pump continues to turn and generates a vacuum that rises towards the preset maximum. Once that maximum has been reached the pump stops and/or some venting into the aspiration line is allowed so as to maintain that vacuum level. Once the occlusion breaks and the vacuum reduces, the pump begins to turn again. In a vacuum-based machine, the set vacuum is that which is produced constantly in the vacuum chamber, and this indirectly causes flow until occlusion, and then, as in the flow-based system, the vacuum within the tubing and handpiece rises to that same level.