A surgeon has many instruments on the surgical tray. They are carefully chosen. There is often an affinity for these instruments. Moreover, the surgeon spends a great deal of time learning how best to manipulate each of them to achieve outstanding outcomes.
Somehow, the phaco machine itself does not get the same intensity of consideration. Surgeons evaluate different machines prior to purchase and get comfortable with the settings programmed by the manufactures’ representatives. But do they habitually scrutinize their procedure and endeavour to redefine those parameters that help improve efficiency and safety?
This study is intended to provide background information on the science that allows the machine to perform emulsification and elevate it to the most sophisticated instrument on the surgical tray.
The three machines available in the USA are the ones that will be referenced throughout this study (Table 1).
TRENDS IN PHACOEMULSIFICATION
In all likelihood, the most significant trend is an evolutionary change in the way anterior segment surgeons perform phaco. It is the shift from occlusion-based phaco to partial occlusion phaco.
Although the importance of this change does not garner many articles or analyses, the manufacturers, recognizing the importance of the method of phaco, and with advice from cataract surgeons, have consistently modified the machine engineering to produce this change.
A second major trend is the finding that the cavitational energy at the phaco tip must be harmonized with jackhammer energy. Both these forms of energy work synergistically to emulsify lens material. Both also create the microenvironment of partial occlusion phaco .
PARTIAL OCCLUSION PHACO
To better identify the concept of partial occlusion, we must analyse the events surrounding occlusion. It is important to understand that partial occlusion phaco occurs during fragment removal. During sculpting of any kind, occlusion is avoided by employing low vacuum settings.
When removing a fragment, the fragment is pulled towards the phaco tip by the flow of fluid drawn into the phaco needle. When it reaches the phaco tip, it covers the needle orifice and vacuum holds it in place. At this moment, the flow stops and the vacuum rises to its preset maximum. This is Occlusion. It represents a specific moment in time. During Occlusion, the vacuum raises, the tubing collapses 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. The flow volume increases to its preset maximum exceptionally rapidly, based on both pump speed and 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 anterior chamber (AC) shallows. This abrupt forceful flow of fluid, evacuating the AC, 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. The capsule itself can be aspirated into the phaco tip and breached.
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 phase
- Postocclusion phase
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. 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.
The question then becomes, how do we shift the equation to the preocclusion side?
The answer to this question resides in the understanding of the elements of phaco energy.
There are three types of phacoemulsification energy in evidence at the phaco tip. They are 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.
When there is adequate fuel (Fluid) at the phaco tip, and it is energized, the backward movement of the tip pulls dissolved gases out of solution and creates microbubbles. The forward movement of the tip compresses the bubbles repeatedly until they implode. The implosion causes a shock wave of 75 000 Lbs/sq.in., 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 short-lived, lasting only 2–4 μs as fuel is rapidly depleted.
After 2–4 μs when fuel is depleted, the needle continues to vibrate, but bubbles just vibrate without imploding. This is useless and wasted energy (Fig. 1).
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
The first is micro pulse phaco. The three machines have the modification and call it by different names. However, in all cases, changes in machine software and handpiece piezoelectric crystal inertia allow for exceedingly short bursts of phaco energy coupled with exceedingly short periods of aspiration only. The duration of energized time, as well as aspiration only time, is 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 two important consequences of microburst energy generation. The first is that energy misuse produced by periods of stabilized energy expenditure with its worthless injury to endothelium, iris blood aqueous barrier and trabecular meshwork are curtailed.
The second, and most important, is that micro pulse phaco triggers partial occlusion phaco.
In this scenario, the fragment is drawn towards 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 μs afterwards, energy production pauses, aspiration brings the fragment back towards 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. 2).
The philosophy of the Bausch & Lomb Stellaris (Bausch & Lomb (Global Surgical Headquarters) 30 Enterprise, Suite 450 Aliso Viejo, CA 92656, USA) is to employ micro pulse coupled to 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 μ while that of 40 KHz is 52 μ or 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 (Fig. 3).
The discussion above has referenced the movement of the phaco needle in a longitudinal, or forward and backward movement. An innovation in design allows 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 follow ability (Fig. 4) .
Two manufactures have adopted this style of power generation.
Alcon Laboratories 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 predominately 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. This software creates an occlusion threshold, which 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.
Abbott Medical Optics has chosen a different approach. They use a standard tip driven in an elliptical motion. The needle path resembles the shell of an egg. This modification also shaves cataractous fragments but is less prone to clogging, as it emits a high level of cavitational energy.
Nonlongitudinal phaco, similar to micro pulse phaco, gives rise to partial occlusion phaco. Therefore, when using nonlongitudinal phaco, it is not necessary to use micro pulse phaco. Nonlongitudinal phaco effectively shaves material reducing overall time of phaco energy release. It is so efficient at generating partial occlusion phaco that it seems to the surgeon that fragments hug the phaco tip and are effortlessly removed in a quiet and stable AC.
Pumps and fluids
Although partial occlusion phaco is the central driver of efficient and safe phaco, the procedure is carried out in a balanced fluidic cavity.
The AC is maintained by a balance between inflow and outflow. Inflow is determined by the bottle height above the patient's head. Recently, Bausch & Lomb Stellaris has introduced pressurized infusion. Sometimes bottle height is limited by ceiling restrictions. Pressurized infusion solves this problem. In addition, it is possible to maintain the chamber with small bore inflow needles and keep up with higher aspiration flow rates.
Outflow is governed by a fit between phaco sleeve and incision, phaco needle diameter, machine setting of flow and/or vacuum, and tubing diameter, compliance and restrictions. A major determinant of outflow is the mechanism for generating flow and vacuum, the pump.
All manufactures incorporate software and hardware to design algorithms, which create a pseudo AC on the basis of measurements derived from the actual AC. This might be envisaged as a virtual AC. Multiple sensors gather the information to create this imaginary chamber and multiple software programs respond to its analysis to calculate and intercede, implementing changes to flow and vacuum designed to stabilize the AC dynamics. Phaco pump activity is precisely coupled to these diagnostic programs .
There are two fundamental types of phaco pump.
- The flow-based pump, which is a peristaltic pump.
- Flow (ml/min) and vacuum (mmHg) are independent variables.
- The vacuum-based pump, which is some variety of venturi pump.
- Vacuum (mmHg) is the only adjustable variable. Flow is dependent upon vacuum.
These pumps are designed as rotating cams, which sequentially compress tubing. The speed of the pump is surgeon adjustable. Some pumps will reverse direction under command of the software. All machines have occlusion thresholds, which modify pump speed, or power modulations, when a preset vacuum value is exceeded. Thus, the surgeon adjusts the flow and vacuum as desired. Occlusion thresholds are also selected and postocclusion parameters are programmed. All these parameters must be carefully adjusted to allow partial occlusion phaco to occur. Too high a vacuum, for example, captures material too strongly to the phaco tip hindering partial occlusion and promoting occlusive phaco.
These pumps use venturi systems to create vacuum in a rigid container. Vacuum is determined by the surgeon and flow results from fluid passing through the needle into the reservoir. These machines now incorporate software to control not only absolute vacuum but also how quickly the vacuum rises. This is commonly called vacuum rise time.
The task of the pump and the controlling software is to avoid surge. Partial occlusion phaco is the driving force for surge-free phaco. However, intermittently occlusion does occur. When it does, machine sensors in the virtual AC are required to respond instantly to slow the pump and decrease vacuum by venting. All manufacturers are producing stiffer or less compliant aspiration tubing. This prevents the expansion of the tubing at occlusion break, which amplifies the magnitude of surge. Air in the aspiration line disturbs the capability of the pump to react to changes in vacuum or flow. All manufacturers pay heed to preventing the accumulation of air within the aspiration line by removing bubbles as they are aspirated.
Finally, specific programmes have been designed either to prevent surge or to recognize approaching surge and stop it.
One of these methods, automatic bypass system, has been used by Alcon Laboratories for a long time. A 0.175-mm hole is drilled in the shaft of the needle. Should the tip occlude fluid flows through this hole into the needle to equalize vacuum upsurge and thwart surge.
AMO has developed CASE Software, which has an adjustable threshold vacuum setting. When this level is reached, the pump instantly slows, and in some cases moves backwards, until vacuum is mitigated. The phaco then proceeds without surge.
B&L StableChamber tubing increases resistance in noncompliant tubing to accomplish the same goal.
The intention of this article has been to provide the scientific theory, which governs the design of those phaco machines readily available to us [5▪]. These concepts help us purchase and devise preliminary settings. Over time, modification of incision size, phaco needle tips and our personal technique will unquestionably necessitate changes in the machine parameters. Femtosecond laser will be incorporated into the surgical armamentarium, but phaco will persist and be integrated into this procedure. Sophisticated modern instrument technology can manage any change and situation we require of it. We need only to make the inquiry!
I wish to thank Abbott Medical Optics and Bausch & Lomb for providing background information.
Conflicts of interest
I am a consultant for Abbott Medical Optics and LensAR.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 82).