Understanding the Phaco Machine

37   Understanding the Phaco Machine


Barry S. Seibel and William J. Fishkind


Modern phaco machine technology upgrades the phaco handpiece to the role of our most diverse and elegant handheld instrument. It can be used to prevent tears in the capsule during phaco, and it can be reprogrammed to minimize further damage if the capsule should rupture.


Overview


The patient will have the best visual result when the phaco energy delivered to the anterior segment is minimized. Additionally, phaco energy should be directed into the nucleus. This avoids injury to iris blood vessels, trabecular meshwork, and endothelium. Finally, skillful emulsification leads to shorter overall surgical time. Therefore, a lower amount of irrigation fluid passes through the anterior segment.


Normally, all phaco procedures have two phases: first, creation of fragments, which requires sculpting or chopping; second, the removal of the fragments in a controlled approach. Occlusion is mandatory to move fragments to the iris plane. Fragment removal is assisted by partial to complete occlusion phaco.


All phaco techniques are preceded by capsulorrhexis, cortical cleaving hydrodissection, and removal of the anterior cortex and epinucleus to expose the endonucleus.


Complications related to the phaco machine can generally be grouped into two categories. First, the inappropriate use of machine parameters may result in direct tissue damage through such mechanisms as chamber collapse, corneal wound burn, iris incarceration, and capsule rupture. Second, failure to optimally adjust machine parameters so as to facilitate efficient surgery will result in the surgeon having to compensate by more frequent and exaggerated manual maneuvers that can increase the possibility of tissue damage. Although some surgical complications are inevitable by-products of preexisting anatomy, many can be avoided by judiciously examining the relationship of the machine technology and the fundamentals of the microsurgical techniques to modern phaco surgical methods. Many aspects of the machine technology’s contribution to surgical complications can be understood by first simplifying the role that the machine plays in a routine case. That role is to create and maintain a fluidic circuit that starts at the elevated irrigating bottle, passes through the eye and then the phaco pump, before draining into a collection chamber.


If a cataract has a soft, gel-like density, then the nuclear material can propagate along the fluidic circuit as the phaco needle’s aspiration port is placed against the nucleus, aspirating it. As the nuclear density increases, the pump must create more vacuum to deform the nuclear material sufficiently for aspiration. If the nucleus has still greater, crystalline-type density, then an appropriate amount of ultrasound must be titrated so that the material can be emulsified sufficiently to enable pump vacuum and flow to deform it into and through the phaco needle and aspiration line. The ultrasonic vibrating phaco needle tends to push material away from it with traditional longitudinal non-torsional ultrasound.


During sculpting, the nucleus is held in place by the capsule and zonules, but during fragment emulsification, fluidic parameters of vacuum, flow, and bottle height must be adjusted to counteract the ultrasonic repulsion so as to aspirate the nuclear material into the phaco needle.


The anterior chamber (AC) must be maintained during all phases of cataract surgery; its fluid pressure, therefore, must be greater than the ambient atmospheric pressure. Any vacuum created in the fluidic circuit by the pump is optimally located between the aspiration port and the pump; any vacuum (pressure lower than atmospheric pressure) in the eye portion of the fluidic circuit would result in a chamber collapse, leading in turn to potential damage to the capsule, zonules, and corneal endothelium. One of the main determinants of intraocular pressure (IOP) is the height of the irrigating bottle, which yields an IOP of 11 mm Hg (above ambient atmospheric pressure) for every 15 cm (6 inches) of elevation above the eye. This relationship is accurate for hydrostatic pressures in pedal position 1. In pedal positions 2 and 3, the IOP decreases (but remains greater than atmospheric pressure) with induced flow in proportion to the commanded pump strength, as well as the degree of aspiration port occlusion.


New modifications in the Centurion (Alcon, Fort Worth, TX) have “active fluidics” where the surgeon sets the target IOP during surgery and software in the unit maintains that IOP throughout the surgery, thus enhancing AC stability. The Stellaris (Bausch and Lomb, Rochester, NY) has surgeon-controlled pressurized infusion to maintain AC stability during phaco.


Chamber collapse may potentially occur if the surgeon uses a standard bottle height for most cases but neglects to appropriately raise it if the flow rate is subsequently increased to enhance followability. The converse is also true. For example, a surgeon may lower the bottle height to achieve a lower IOP for such conditions as weak zonules or a posterior capsule tear, but chamber instability will result if the flow rate is not correspondingly adjusted to a lower setting that is appropriate for the lower bottle height. Once again, bottle height must be titrated not only for hydrostatic pressures in pedal position 1 but also hydrodynamically to a given pump strength for positions 2 and 3. Needless to say, the highest elevation of the bottle will be inadequate if the irrigating fluid is completely depleted; operating room staff vigilance is required in this area for longer procedures. The potential for this problem is increased if the surgeon has the room lights dimmed partly or completely during cataract surgery. In those machines with pressurized infusion, it is obvious that the infusion rate must be adjusted to balance the aspiration rate.


Pump Types


To create a vacuum, a pump of some type is necessary. Classically, pumps have fallen into two categories. The flow pump, the best example being a peristaltic pump, enables the surgeon to directly program both flow and vacuum parameters. The vacuum pump, for example the Venturi pump, enables direct surgeon control over vacuum only. The amount of flow is dependent on the vacuum setting and cannot be set by the surgeon. Recently, pumps have evolved such that flow pumps are so responsive that they can be programmed to respond as if they were vacuum pumps. Additionally vacuum pumps can be manipulated to act as if they were flow pumps. These modern pumps are therefore considered as “hybrid pumps.”


Irrespective of pump selection, flow, measured in cubic centimeters per minute (cc/min), is the force that brings material toward the phaco tip. In general, the higher the flow, the faster events will occur within the AC. Vacuum setting, measured in millimeters of mercury (mm Hg), will hold material on the phaco tip, once occlusion has occurred.


Fluidic Problems: Flow Management


The flow rate (cc/min), being an important factor in determining IOP, can be increased directly on a flow pump by increasing the commanded flow rate or indirectly on a vacuum pump by increasing the commanded vacuum; in both cases, the actual flow rate is dependent on the degree of aspiration port occlusion. The actual flow rate is also affected by the fluidic circuit’s resistance, which is in turn determined by the internal diameters of the phaco needle as well as the fluidic tubing (especially the aspiration line). In addition, flow rate is affected proportionately by bottle height, but only when using a vacuum pump. One potential arrangement that may lead to complications is using a vacuum-priority pump for a high-vacuum technique. A high commanded vacuum setting produces a high flow rate with the unoccluded phaco tip. The surgeon may compensate for this with an elevated bottle height to maintain adequate IOP in the face of the high flow rate. However, the higher bottle height produces an even higher flow rate, with the aspiration port unoccluded, which not only diminishes the effectiveness of increasing the IOP but also produces potentially dangerously fast intraocular currents that can uncontrollably attract and incarcerate unwanted material such as iris and capsule.


The induced flow rate from a high commanded vacuum can be limited to a safer level by the use of a high-resistance phaco needle with a small internal diameter, such as a flare tip needle. In addition, the surgeon should titrate the amount of commanded vacuum during phaco with linear pedal control according to the clinical application and the status of the aspiration port. Appropriately high vacuum may be used safely when the aspiration port is occluded, for example, when gripping a hemi-nucleus in preparation for chopping. However, when anticipating an occlusion break at the end of a chop, the surgeon should linearly titrate vacuum to a lower, safer, and more appropriate level.


Aspiration Line Obstruction


Another potential area of complication related to IOP maintenance and flow concerns is the presence of an obstruction between the aspiration port and the pump. Although kinked aspiration line tubing can produce this effect, it is most often caused by the localized accumulation of nuclear emulsate, clogging the aspiration line, especially when sculpting dense, mature cataracts. This type of obstruction does not occur regularly, but it can significantly impair the effectiveness of the phaco machine by limiting the pump’s ability to transfer its force (either via flow or vacuum) past the obstruction to the phaco tip where it is needed. The surgeon can recognize this problem when free (e.g., chopped) nuclear fragments fail to be effectively drawn to the phaco tip when in pedal position 2 and using a flow setting that usually is effective in this situation. Similarly, an aspiration line obstruction is suspected when faced with insufficient grip of an occluding nuclear fragment when using a vacuum setting that usually achieves a good grip. An aspiration line clog is also a strong possibility when observing intraocular flocculence (“lens milk” or “phaco dust”) during sculpting, indicating the inability for pump-induced flow to effectively clear the AC of the ultrasonically induced emulsate.


Any of these scenarios must prompt the surgeon to interrupt the surgery so that the problem can be isolated and rectified. Verification of a clog is achieved by placing a test chamber over the visibly unobstructed phaco needle and engaging pedal position 2 while observing inadequate or absent activity in the irrigating bottle’s drip chamber. The accumulation of emulsate can sometimes be visualized in the aspiration line, often at the junction of the aspiration tubing/handpiece junction, or at the aspiration tubing/cartridge connection; in these cases, digital massage of the tubing at this area often breaks up the obstruction. Sometimes, very high commanded flow and vacuum along with high ultrasound will free a nonvisualized obstruction; remember to perform this maneuver extraocularly with a test chamber over the phaco needle. If the clog is within the handpiece, forcibly irrigating it with a balanced salt solution (BSS) syringe is usually an adequate solution.


The greatest danger of an aspiration line obstruction is the surgeon’s failure to recognize the situation and rectify it. The most benign outcome of such a failure is the impairment of the machine’s effectiveness in producing the desired intraoperative flow and vacuum. However, a greater danger occurs if, as a result of the subsequently impaired followability and grip, the surgeon chases after nuclear fragments into the periphery of the AC rather than maintaining the phaco needle in a safer, more central position and having the machine fluidics attract fragments to and into the aspiration port. With the needle in a peripheral position, an aspiration line obstruction might spontaneously clear, inducing a surge that can incarcerate and damage the juxtaposed iris or capsule. Furthermore, if the obstruction does not clear spontaneously, the probability of a corneal wound burn becomes progressively greater as more ultrasound energy is engaged without sufficient cooling flow.


Vacuum Settings


The appropriate setting of the machine’s vacuum parameter, measured in millimeters of mercury (mm Hg), is another key element in avoiding complications. As discussed previously, adjusting the commanded vacuum on a vacuum priority pump (e.g., Venturi) proportionately adjusts the flow rate when the phaco tip’s aspiration port is not occluded. But when the phaco tip is occluded, adjusting the commanded vacuum (vacuum priority pump) or the vacuum limit preset (flow priority pump) proportionately adjusts the grip and deformational force that is applied to the material that is occluding the aspiration port. The amount of grip for a given amount of pump vacuum is proportional to the surface area of the phaco needle’s aspiration port; the surgeon, therefore, should anticipate the need for increasing the vacuum from the usual levels when changing to either a smaller gauge or less beveled phaco needle. As with any parameter, the vacuum should be adjusted appropriately for a given surgical function; a higher adjustment would needlessly compromise the operation’s safety margin.



For example, a high vacuum level of 350 mm Hg might be required during a chop maneuver to grip and pull the engaged hemi-nucleus centrally so as to facilitate the peripheral placement of the chopping instrument. However, once the hemi-nucleus is mechanically fixated between the chopper and the phaco tip, high vacuum is no longer required. Indeed, it can subsequently become a liability if maintained beyond the completion of the chop, with breaking of the vacuum seal between the phaco tip and the nucleus. When using a vacuum pump, the high induced flow from the high vacuum level with an unoccluded tip can produce shallowing or collapse of the AC. Therefore, after higher vacuum has been applied when needed with complete aspiration port occlusion, the vacuum level should then be lowered in anticipation of using pedal position 2 or 3 along with partial or complete unocclusion of the aspiration port.


Partial Occlusion Phaco


To better identify the concept of partial occlusion we must analyze the events surrounding occlusion. It is important to understand that partial occlusion phaco occurs during fragment removal.


When removing a fragment, the fragment is pulled toward the phaco tip by the flow of fluid drawn into the phaco needle. When it reaches the phaco tip, it occludes the needle orifice and vacuum holds it in place. At this moment the flow stops and the vacuum rises to its preset maximum limit when using a flow pump, or equilibrates with the commanded vacuum on a vacuum pump. This is occlusion (Figs. 37.1 and 37.2). It represents a specific moment in time. During occlusion, the vacuum rises, the aspiration line tubing constricts, and the fragment is held more and more firmly to the phaco tip. When the surgeon activates phaco energy, the emulsification of the fragment instantaneously permits flow to begin (Fig. 37.3). The flow volume increases to its preset maximum exceptionally rapidly based on pump speed and due to expansion of the vacuum tubing. The inflow of the fragment particulates and fluid into the phaco needle momentarily exceeds the inflow from the irrigation line, and the AC shallows. This abrupt forceful flow of fluid, beyond the steady-state flow with an unoccluded aspiration port, is defined as surge, and results in simultaneous anterior movement of the posterior capsule as well as collapse of the cornea. The event may be violent enough to tear the capsule by itself, tear a preexistent tear of the AC at the equator, or tear around a sharp edge of partially emulsified hard nucleus. It also can be aspirated into the phaco tip and breached (Fig. 37.4).




If we deem that the moment of occlusion symbolizes a specific instant in time, we can partition the emulsification of fragments into three divisions: preocclusion, occlusion, and postocclusion. Obviously, surge is undesirable. However classically we have performed phaco by using occlusion to hold fragments and postocclusion to emulsify them. Thus we unconsciously inhabited the world of unwelcome surge!


Partial occlusion phaco is the method by which we break the cycle of occlusion and surge! During fragment emulsification, if the fragment is brought close to the phaco tip orifice, but never completely occludes it, there is never full occlusion. Thus, a distinct new term, partial occlusion, describes the monumental change. If there is no occlusion there cannot be surge (Table 37.1).


The fragment emulsification occurs in the interval between preocclusion and occlusion. Therefore, if we never have occlusion during sculpting due to low vacuum settings, and we never have occlusion during fragment removal due to partial occlusion, the surgeon never has to encounter the unnerving occlusion/surge event. The incidence of torn posterior capsules and unplanned vitrectomy lessen. Patients experience superior surgery and outcomes.



Table 37.1 Stages of Partial Occlusion Phaco
















Preocclusion


Occlusion


Postocclusion


       ↑


       Partial Occlusion




Surge


Partial occlusion occurs when the fragment is in close proximity to, but not occluding, the phaco tip (Fig. 37.5). If no occlusion occurs, there will be no surge. The question then becomes, How do we shift the equation to the preocclusion side? The answer resides in the understanding of the elements of phaco energy.


Phaco Energy1


There are three types of phacoemulsification energy in evidence at the phaco tip: jackhammer energy, low-frequency cavitation energy, and high-frequency cavitation energy.


Jackhammer energy is created by the mechanical striking of the phaco needle against cataractous material. It is a powerful force.


Low-frequency cavitational energy is created by the vibration of the phaco tip. It has a relatively long wavelength and penetrates into tissues a great distance from the phaco tip. The manufacturer of the machine determines the frequency. This energy creates both transient and sustained cavitational energy.


High-frequency cavitation occurs during fragment emulsification and facilitates removal of the fragment. This energy also discharges from the phaco tip, injuring surrounding tissues.


Transient Cavitation


When there is adequate fuel (in this case aqueous fluid/BSS) at the phaco tip, and it is energized, the backward movement of the tip pulls dissolved gases out of solution and creates micro-bubbles. The forward movement of the tip compresses the bubbles repeatedly until they implode. The implosion causes a shock wave of 75,000 pounds per square inch, which discharges from the tip in the direction of the bevel of the needle. The equivalent process occurs along the needle barrel and at any point of change in diameter of the needle. Therefore, cavitation is enhanced at the narrowing of the shaft of a flared tip, the angulation of the Kelman tip, or at the needle hub. Transient cavitation is shortlived, lasting only 2 to 4 ms as fuel is rapidly depleted.


Sustained Cavitation


After 2 to 4 ms, when fuel is depleted, the needle continues to vibrate, but bubbles just vibrate without imploding. This is useless and wasted energy.


Modifications of Phaco Power


Two modifications of phaco energy release are instrumental in shifting the procedure away from occlusion phaco and in the direction of partial occlusion phaco: micro-pulse energy production and partial occlusion phaco mechanics.


Micro-Pulse Energy Production

Micro-pulse (hyper-pulse) phaco can be delivered by one of the three machines that are most available in the United States that have this modification; it is called by different names. However, in all cases, changes in machine software and handpiece piezoelectric crystal inertia enable exceedingly short bursts of phaco energy coupled with exceedingly short periods of aspiration only. The duration of energized times, as well as aspiration only time, are independently adjustable. The result of this important modification is to maximize the use of transient cavitation associated with jackhammer mechanical energy. The power bursts are so short that all the cavitational energy generated is powerful transient cavitation. The needle vibration stops before fuel is burned and stabilized cavitation begins.



There are three important consequences of microburst energy generation28:



  1. Energy misuse, produced by periods of stabilized energy expenditure with its worthless injury to endothelium, iris blood aqueous barrier, and trabecular meshwork, is curtailed.
  2. There is a dramatic decrease in heat production and energy delivery without any loss of phaco efficiency.
  3. The micro-pulse phaco initiates partial occlusion phaco.

Partial Occlusion Phaco Mechanics

In this scenario, the fragment is drawn toward the phaco tip orifice by fluid flow. It almost occludes the tip when micro-pulse phaco is energized. The fragment is emulsified by the short powerful bursts of transient cavitational energy, in harmony with the jackhammer effect. This combined energy drives the fragment away from the phaco tip. However, 4 ms later, energy production pauses, aspiration brings the fragment back toward the orifice, and just as it is about to occlude the tip, energy is again resumed, and on and on the cycle repeats. The fragment is extraordinarily close to the tip but never entirely occludes it. Thus, micro-pulse phaco is the generator of partial occlusion phaco (Fig. 37.5).


The philosophy of the Bausch and Lomb Stellaris is to employ micro-pulse coupled with a lower frequency cavitation generator. The lower the frequency, the larger the cavitation bubble produced. In fact, the cavitation bubble at 28.5 KHz is 73 µm, whereas that of 40 KHz is 52 µm, for a difference of 71%. The larger bubble, upon implosion in a micro-pulse environment, must give off more powerful cavitational energy. The partial occlusion is enhanced both by removing larger chunks of the fragment and by not requiring as close proximity of the fragment to the tip for emulsification, as with smaller bubble formation.


Nonlongitudinal Phaco

The discussion above has referenced the movement of the phaco needle in a longitudinal, or forward and backward movement. An innovation in design enables the tip to move in nonlongitudinal directions. The importance of this change to nonlongitudinal power is the augmentation of the shaving characteristic of the phaco needle. Longitudinal power cores material so that cavitational energy can emulsify it. Nonlongitudinal energy shaves fragments of cataractous material, further enhancing partial occlusion phaco while additionally improving followability (Fig. 37.6).911 Two manufactures have adopted this style of power generation.


Alcon Laboratories (Fort Worth, Texas) has created OZil torsional power generation. By utilizing an angled Kelman phaco tip driven with an oscillatory movement, a zone of cavitational energy is created around the angled tip. The torsional needle movement enhances the jackhammer effect, which predominantly shaves and removes fragments. Cavitational energy does not play much of a role. However, the shaved fragments are often larger than the cored fragments created by longitudinal phaco. In an effort to further emulsify the shaved fragments, Alcon has developed Intelligent Phaco (IP). This software creates an occlusion threshold that is surgeon selected. When it is reached, longitudinal movement of the needle replaces the torsional movement. This instantly produces cavitational energy throughout the needle barrel and at the hub. Thus, trapped fragments are emulsified, clearing the needle, allowing vacuum to decline, and beginning torsional movement once again.


May 13, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Understanding the Phaco Machine
Premium Wordpress Themes by UFO Themes