|Year : 1997 | Volume
| Issue : 4 | Page : 241-249
Physical and mechanical principles of phacoemulsification and their clinical relevance
L Yow, S Basti
Alcon Laboratories, Irvine, California, USA
Alcon Laboratories, Irvine, California
| Abstract|| |
A clear understanding of the physical and mechanical principles that govern phacoemulsification can facilitate usage of this technique for effective and efficient cataract removal in a variety of clinical situations. This article addresses separately, concepts pertaining to the three essential components of phacoemulsification, namely, irrigation, aspiration and emulsification. Machine settings are suggested for the various techniques presently in use. Finally, alternative approaches for lens removal that are currently being investigated are briefly discussed.
Keywords: Phacoemulsification, principles, fluidics, learning curve, transition
|How to cite this article:|
Yow L, Basti S. Physical and mechanical principles of phacoemulsification and their clinical relevance. Indian J Ophthalmol 1997;45:241-9
|How to cite this URL:|
Yow L, Basti S. Physical and mechanical principles of phacoemulsification and their clinical relevance. Indian J Ophthalmol [serial online] 1997 [cited 2014 Mar 11];45:241-9. Available from: http://www.ijo.in/text.asp?1997/45/4/241/14991
Benefits of phacoemulsification (phaco) namely, decreased induced astigmatism, controlled intraocular maneuvers, and relative stability of postoperative refraction, are well recognized. This procedure is hence rapidly gaining popularity and is likely to be a commonly employed technique of cataract removal in most parts of the world in the near future.
Most surgeons agree that there is a definite learning curve while making the transition from manual extracapsular cataract extraction and intraocular lens implantation to phaco. This essentially stems from the fact that phaco is a surgical procedure requiring eye-hand-foot coordination and is a bimanual surgical procedure. Both of these can be easily mastered and several reports have shown that the transition to phaco can be made with good results and minimal complications. In addition to the above mentioned factors, a third factor makes phaco a surgical procedure that is significantly different from extracapsular cataract extraction. This pertains to the fact that several physical and mechanical factors principles govern phaco. Their understanding is as important to the neophyte phaco surgeon as it is to the surgeon conversant with phaco but wanting to optimize surgical performance. This article seeks to address these principles and draws clinical correlates that illustrate their relevance.
| I. Principles of Components of Phaco Machines|| |
Three components constitute the heart of all phaco systems. These are irrigation, aspiration and ultrasound. As lens removal techniques using phaco have evolved in the last 15 years, there have been certain demands placed on phaco systems for the successful performance of surgery with these techniques. Equipment manufacturers have responded to these with great innovations, bringing phaco technology to the refined levels that we find it in today. The principles underlying the main components of phaco machines are discussed below.
| A. The ultrasonic handpiece|| |
| Basics|| |
The ultrasound generating mechanism of the phaco handpiece causes the tip attached to it to vibrate rapidly back and forth. Tip excursion or stroke length is defined as the distance the tip displaces in the longitudinal direction at maximum power. Stroke length varies for different machines and normally ranges from 1.5-3.75 milli-inches. All phaco machines permit the user to alter phaco power and this is usually indicated as a percentage. Whenever the phaco power is set at 100 percent the stroke length is the maximum permissible for that machine. When the power is decreased by a given percentage, the stroke length also decreases.
Although ideally the stroke length for a given machine should not change, small variations may be noted as the phaco tip encounters materials of different density such as a hard nucleus or soft cortex. Some systems have a feedback from the handpiece to maintain a certain stroke length. Typically, the system maintains stroke length by increasing current, voltage and/ or frequency.
The frequency of a given handpiece is usually indicated in kilohertz (KHz). The frequency used most commonly is 40 KHz. In such a handpiece, the tip vibrates in a longitudinal direction 40,000 times per second. Typical frequencies of crystals used in phaco machines vary between 27-60 KHz.
The frequency for a given machine is fixed and cannot be altered. Handpieces with a higher frequency are likely to be more efficient. However, heat generated is also higher when the frequency is high. Although low frequency handpieces are safe, this safety is at the cost of a lower level of efficiency. A 40 KHz frequency is a good compromise between the two and is hence a popularly used frequency by manufacturers.
| Handpiece options|| |
There are two types of handpieces that are capable of providing a vibrating tip to emulsify the lens. A majority of the handpieces are made with piezoelectric crystals and the rest from magnetostrictive materials.
| Piezoelectric handpieces|| |
The term 'piezoelectric' refers to the property of ceramic and other crystals to expand or contract when an electric force is applied to it. This quality is based on the molecular structure of the material. Handpieces were originally designed with two piezoelectric crystals, these handpieces were prone to reliability problems because the stroke length dropped dramatically when one crystal degraded. Some manufacturers have now developed 4 crystal handpieces which maintain stroke length well even if one of the crystals becomes dysfunctional. The crystals are adversely affected by temperature. The point at which the crystal loses its piezoelectric characteristics is called the Curie temperature. Piezoelectric crystals loose their properties when they exceed their Curie temperature and must be repolarized to gain that quality back.
| Magnetostrictive handpieces|| |
These are a class of handpieces that have a tip excursion by way of an externally applied magnetic field. A piece of ferrite material in a magnetic field has properties that gets longer or shorter. The field may be applied by a direct current coil or a separate permanent magnet. The lengthening effect decreases in magnitude with increasing temperature. Heat is produced in the transducer core because of mechanical and magnetic hysteresis and Eddy currents. These handpieces are usually less than 50% efficient. Surgical Design (Long Island, New York) currently has the only magnetostrictive handpiece on the phacoemulsification market.
A typical handpiece [Figure - 1] has a hollow core which allows the aspiration line to draw lens material into it. Crystals or magnetostrictive materials are sealed into the handpiece. Phaco handpieces are designed to withstand numerous autoclavings. Typical failure modes of handpieces usually result from a compromise of the seal on the handpiece from multiple autoclaves. The other failure mode is from the connector end of the strain relief [Figure - 2].
| Phaco tips|| |
These come in many different angles and configurations [Figure - 3]. Selection of the appropriate one depends on the type of lens removal technique. Phaco tips act like chisels and straws that carve and aspirate lens material. The bevel at the end of standard tips can range from 0-60 degrees. More complex tips may have compound angles. End configurations can be round or ellipsoid, bent or flared. Many mid-shaft designs have been created to enhance irrigation and cooling to prevent corneal burns.
| Clinical aspects|| |
| Mechanism of nucleus removal by the phaco tip|| |
Considerable debate surrounds the actual mechanism by which nuclear material is removed by the phaco tip. Theories put forth are the mechanical theory, the ultrasonic energy and the microcavitation theory. The mechanical theory proposes that nuclear material gets chiseled away by the vibrating tip. The second theory relates nucleus removal with phaco to be a function of ultrasonic energy. The microcavitation theory probably explains most if not all of how work is done by the phaco tip. Cavitation is the formation of bubbles from solution. A common example of this phenomenon is when the plunger of a syringe immersed in water, is pulled back too quickly. Tiny bubbles form when the solution is under vacuum, and collapse back when the pressure in the syringe is equilibrated. When the front end of the tip vibrates back and forth at its normal frequency, microcavitation bubbles form when the tip retracts from its original position (to a distance determined by stroke length and the power setting). Cataract removal occurs due to the collapse of these bubbles causing a micro-implosion in the local area. The design of the tip should try to maximize the amount of cavitation near the tip. Designs like the Kelman tip and the Cobra tip [Figure - 3] have enhanced cavitation properties because they have more cross-sectional area than straight tips.
| The phenomenon of macrocavitation|| |
Annoying bubbles that are generated when the ultrasonic mode is activated by the footswitch during surgery, can originate from two sources: due to macrocavitation or from the irrigation line. As elucidated above, rapid forward and backward movement of the phaco tip creates cavitation bubbles whose size is proportional to the cross sectional area of the surface making this motion. While microbubbles are created by the tip itself, large cavitation bubbles (or macrocavitation bubbles) are generated at the hub of the phaco tip if the hub and the tip are at right angles to each other. These are greatly reduced when the hub is tapered [Figure - 4]. Bubbles found within the irrigation line are easily visualized in the translucent tubing and can be eliminated by proper priming and irrigation.
The sound associated with activation of ultrasound by the footswitch may not be a true indication of the quality of the phaco handpiece. Ultrasonic energy is actually outside the audible range of frequency. The play between the handpiece and the tip is responsible for the grating sound that comes about as the phaco mode is activated. Presence of air bubbles between the sleeve and the sides of the tip tend to perceptually make the handpiece sound louder. This louder noise must not be construed to be suggestive of a greater stroke length. The insertion of a straw-like sleeve [Figure - 5] between the phaco tip and the regular sleeve compartmentalizes this space thereby not letting large air bubbles to collect and variably alter the sound made by the phaco handpiece.
Occludability is the tendency of the tip to get occluded, giving rise to a buildup of vacuum. Smaller tip angles tend to have higher occludability. Sharpness of the tip is directly proportional to the tip angle. Tip selection is dependent on the lens removal technique and the hardness of the lens. Larger angles (45-60 degrees) are desirable for sculpting whereas smaller angles (0-15 degrees) are preferred for steps that need vacuum such as quadrant removal or chopping. Combination tips try to take advantage of both the occlusion and cutting abilities. Epsilon tips are oval tips which are used like a sharpened spoon to remove the lens. Bent tips have good cavitation but are harder to visualize.
[TAG:2]B. The aspiration system[TAG:2]
| Basics|| |
Two factors cause material within the anterior chamber to leave or attempt to exit the aspiration flow rate (AFR) and/or vacuum. Both of these are largely a function of the pump of the phacoemulsification system which can be one of two categories: peristaltic type and vacuum transfer type of devices. In vacuum transfer type of pumps (common examples being venturi or diaphragmatic pumps) AFR and vacuum are directly linked to each other and cannot be separated. In peristaltic pumps however they are independent entities. AFR will hence be discussed in the context of peristaltic pumps only.
Vacuum is a suction force created by the pump at the phaco tip. It serves to hold nuclear pieces in apposition with the phaco tip, facilitating their emulsification. As this occurs, the negative pressure in the tubing also sucks the emulsate out of the anterior chamber. The vacuum in a peristaltic pump is measured just prior to the pumphead. Most systems have a transducer which measures the pressure directly in the fluid line. One company uses a non-contact vacuum sensing technique that measures the deflection of the tubing and translates it to a vacuum level. In vacuum transfer type of devices, vacuum is measured in the cassette. The vacuum limit specifies the upper limit to which vacuum can be built up. The actual vacuum is the level of vacuum in the aspiration tubing at a given time. It is dependent on the type of material being removed and the extent to which it is occluded. When the vacuum in the line holding the particle reaches the vacuum limit, vacuum will cease to build (either the pump will stop turning or the system will microvent). Some systems will have an occlusion tone to signal the user the vacuum is at its limit.
| Pump options|| |
| Peristaltic pumps|| |
These are designed with rollers over which tubing is stretched and locked in place [Figure - 6]. As the rollers of the pump gently roll over the tubing, compression waves are created that push a specific amount of fluid away from the handpiece. This creates a negative pressure in the aspiration tubing which, depending on the level of the negative pressure, draws fluid out of the eye. Peristaltic pumps usually have 4-6 rollers. Aspiration flow rate is the rate at which fluid and emulsified nuclear particles are removed from the eye when the phaco tip is unoccluded. AFR is measured in units of cc/min. In a peristaltic system, when the AFR is increased the pump rotates faster. Vacuum and aspiration are separate components in a peristaltic system and can work hand-in-hand. Aspiration draws intracameral fluid and structures to the tip and as these partially or completely cause occlusion of the tip, vacuum builds up and tends to draw these into the aspiration tubing.
A scroll pump is very similar to a peristaltic pump. Instead of rolling over the tubing, the pump scrolls in direct contact with the fluid. Each rotation removes a specific amount of fluid and gives rise to a buildup of some vacuum.
| Vacuum transfer devices (VTD)|| |
These create a vacuum somewhere within the system and transfer the vacuum to another place within the tubing system [Figure - 7]. Examples of such pumping systems are the Venturi system, diaphragmatic and rotary vane pumps. The difference between vacuum transfer devices and peristaltic pumps is the direct relationship on the movement of fluid in the fluidic system. When the peristaltic pump turns, a certain amount of fluid is moved. With the VTD the vacuum generating device creates a vacuum which in turn moves the fluid in the fluidic system. The irrigation head pressure causes fluid to flow towards the area of low pressure (generated in the cassette and reflected into the aspiration tubing). As the vacuum pressure difference tries to equilibrate itself there is flow. This flow is controlled only by the vacuum difference and cannot be directly changed. These systems typically have a rigid cassette for fluid collection. As the collection chamber fills, the fluid dynamics will change slightly due to the smaller air volume in the cassette. When the cassette is full the aspiration will stop unless the waste material is transferred or emptied. In a vacuum transfer system, the "ASP" setting on the panel of the machine corresponds to aspiration pressure in mm Hg on the transfer side.
| Clinical aspects|| |
For their optimal usage, it is mandatory to understand some important aspects of pumps. These are the following:
| Rise time|| |
This is the amount of time it takes for the system to reach the maximum vacuum setting, once an occlusion takes place [Figure - 8]. The rise times for vacuum transfer type of systems is faster than that for peristaltic pumps since it is totally independent of the occluding material and extent of occlusion taking place. The rise time in peristaltic systems is inversely proportional to the AFR. Manufacturers have their own algorithms for controlling rise time. It is important to note that shorter rise times translate to a rapid occurrence of events in the anterior chamber.
Typical rise times for the phaco mode in a peristaltic system at 25cc/min at a vacuum limit of 80mm Hg is of the order of 0.25-1 sec; in a vacuum transfer system it could be <0.1 sec. Human reaction time is usually in the vicinity of 200 msec. It is hence prudent for the neophyte phaco surgeon not to use flow rates >25 cc/ min which invariably are associated with shorter rise times.
| Holdability|| |
This is the ability of the phaco tip to hold onto the material occluding its tip. This is a function of vacuum and hence is dependent on the preset vacuum limit.
| Followability|| |
This is the tendency for structures within the anterior chamber to move towards the phaco tip. When the AFR is high there is a rapid turnover of fluid from the eye. This tends to make mobile anterior chamber structures such as the iris to tend to move towards the port of the phaco tip. It is hence important to use low AFR in small pupil cases or in situations where the iris is already flabby due to accidental engagement of the iris/pupil by the phaco tip.
| C. The irrigation system|| |
| Basics|| |
Irrigation maintains intraocular pressure, maintains fluid flow to remove lens particles from the eye, and cools the handpiece. Irrigation typically is gravity fed to provide adequate anterior chamber depth. Under normal conditions the bottle is raised to 65 cm above the patient eye level (pressure head created: 47.5 mmHg). When such a bottle height is maintained, fluid flows into the anterior chamber at a rate proportional to the rate at which fluid leaves the anterior chamber. The latter can occur due to vacuum, AFR, and fluid leak from the incisions (main and the sideport). The higher the bottle, the deeper the chamber. Care must be taken to account for natural anatomy of eyes as a short eye would require less depth and a long eye might use more depth. Disease states must also be taken into account since pseudoexfoliation patients may have compromised zonules and hence be less tolerant to the increased pressure head generated in the anterior chamber by an excessively elevated infusion bottle.
| Options for the irrigation system|| |
Fluid can be fed into the irrigation tubing with one or two infusion bottles. Most systems use one infusion bottle. The stated purpose of using two systems is to have the second infusion bottle provide infusion when there is a surge, thus preventing mini-collapse of the anterior chamber. Since surge usually lasts a few milliseconds, the ability of the second bottle to counter it appears to be questionable in clinical practice. Another benefit of using two bottles is to have a second feeder to the irrigation system so that fluid flow is maintained even if the first bottle is exhausted.
| Sleeve for the phaco tip|| |
Sleeves are a standard accessory for the phaco tip and are designed to insulate the wound and provide a fluidic pathway for irrigation. This fluid insulation is an important safety mechanism for prevention of excessive heating of the region of the wound. As surgeons transition to smaller and smaller incision sizes in their quest for eliminating induced astigmatism, wound insulation becomes more difficult to accomplish with conventional sleeve designs since tighter incisions are associated with more friction between the sleeve and the phaco tip. Newer designs have sleeves that are stiffer or have been designed with other special features such as ribbing or texturing. These features permit adequate fluid flow even if close contact is maintained between the phaco tip and the incision.
| Clinical aspects|| |
| Basic fluidics|| |
Fluidic balance is the key to successful phaco surgery. Ocular complications are held at a minimum when the irrigation, aspiration, vacuum, and venting can maintain pressurization of the globe. Turbulence in the anterior chamber occurs when the fluidic balance is not optimized. It is of cardinal importance for the anterior chamber volume to be maintained throughout the surgical procedure. An essential prerequisite for this to occur is balancing fluid input and output. Repeated minicollapses of the anterior chamber and iris flutter during nucleus removal are indicators of an imbalance between vacuum and fluid flow. Since the infusion is gravity fed, keeping the bottle at an adequate height (65 cms above the patients eye) permits the outflow during most steps of phaco to be compensated almost instantaneously. One critical step that can cause some degree of collapse of the anterior chamber is when there is an occlusion break during nucleus removal. The increased vacuum at this time in the aspiration line tends to draw fluid rapidly out of the eye (called surge), predisposing to some degree of collapse of the anterior chamber. A system for venting is hence integrated into every phaco machine. Venting allows vacuum levels to be equilibrated to air or fluid into the aspiration line. Venting also occurs when the surgeon abruptly releases the footpedal from an aspiration state. In this situation, it brings residual vacuum to atmospheric pressure.
There are two mechanisms by which venting is achieved by phaco systems: fluid venting and air venting. Fluid venting usually opens a solenoid for irrigation fluid to enter the aspiration line. Air venting systems open the aspiration line momentarily to the atmosphere. The lower pressure in the aspiration line draws air into it. Since air can get compressed (as opposed to fluid), the compliance of the air venting system is more [Figure - 9]. Examples of air venting systems are Chiron (Boca Raton, Florida) Phacotron Gold, Storz (St Louis, Missouri) Premier, Alcon (Irvine, California) Universal, Mentor (Santa Barbara, California) Odyssey, and the AMO (Irvine, California) Diplomax. The AMO PhacoPlus has a microventing scheme at the vacuum limit. When the vacuum has reached its limit, tiny vent pulses prevent the vacuum limit from being exceeded while allowing the pump to continue to turn. The noncompressible nature of fluid makes fluid venting systems less prone to fluctuations.
Two aberrations can occur with improper venting [Figure - 8]. Regurgitation occurs when the vent pulse pressure is greater than the irrigation pressure and is visualized by return of the aspirated material into the anterior chamber. This commonly occurs when the phaco system cassette or reference level is placed above the eye level (such as in a cart placed over the patient). Residual vacuum occurs when the venting does not release all the vacuum in the line, which is demonstrated by the material not completely released from the tip. These observations are more prominent with air venting systems due to the compliance of the air. Since venting is fixed for each system, careful choice of the irrigation bottle height, vacuum setting, and aspiration setting will give the user the most control over the anterior chamber. Selecting the appropriate settings such that lens material can be drawn to the tip adequately and be held by the vacuum is the key to successful fluidic balance.
All phaco systems have a mechanism for disengaging an inadvertanly held anterior chamber structure. This is acheived using the reflux pedal which is located in the footswitch. In a peristaltic system, reflux occurs by a reversal of the direction of rotation of the pump. This reverses fluid flow in the aspiration tubing, releasing the structure. In vacum transfer devices, a mechanical system causes a bolus of fluid to be released. Some systems also provide a reflux bulb in the aspiration line to be manually released.
| Small diameter tubing and tips|| |
Most small incision tips and tubings are approximately 33% smaller than the 1.1 mm standard tips. This fact has an important clinical implication. If we consider the holding force of the particle at a certain vacuum, the holding force would be proportional to the vacuum and the cross-sectional area of the tip. Therefore if the cross-sectional area decreases, the vacuum must increase in order to obtain the same holding force. When using small incision tips and tubing, vacuum settings hence need to be increased. An advantage of using smaller tubing is that when an occlusion breaks with the smaller tubing there is less volume within the tubing to be replaced by the irrigation system. Thus when the occlusion breaks with the small diameter tubing there is less dimpling of the anterior chamber.
| Corneal burns|| |
Corneal burns occur when the heatsinking capabilities of the tissue and the phaco system are exceeded. Typically, continued phacoemulsification in the presence of decreased fluid flow is the cause of many fishlipped incisions. The heating develops at the interface between the phaco needle and the silicone sleeve at the point where the wound incision compresses against the sleeve. The movement of aspirant flow from the internal surface of the phaco tip can greatly decrease the amount of heat generated. Removal of air bubbles near the incision site is also a measure recommended since fluid can dissipate any heat better than air. Extremely long phaco times (>10 minutes) regardless of phaco power sometimes results in corneal burns. The "lens milk" sign is an important indicator of inadequate flow. This is a collection of liquified lens emulsate in the anterior chamber causing its clouding. This sign should prompt the the surgeon to stop phaco and review and rectify the wound tightness, AFR, and /or bottle height before further proceeding with phaco.
Manufacturers have sought different ways to eliminate the problem of corneal burns. Microflow tips (Storz Ophthalmics) have deep grooves carved along the barrel to avoid occlusions of the irrigation flow to prevent corneal burns. The Mackool tip/system (trademark owned by Richard J. Mackool, MD, Astoria, New York) relies on a polyimide insulator which shuttles back and forth to dissipate the heat generated during phaco. The coefficient of friction of the titanium tip against the polyimide is much less than the tip against the silicone sleeve. Heat that would build up in the incision is transferred to the polyimide insulator and be turned into work of shuttling the sleeve. A small ridge is built into the tip to prevent the polyimide from falling off a straight tip. The clinical advantages of such a system is the use of smaller tighter incisions which reduces incision leakage and better chamber maintenance. Clinically when using the Mackool system, the bottle may need adjusting as the incision is made more tight and leakproof. This causes deepening of the anterior chamber. Other manufacturers have sought to improve on the sleeve to prevent total occlusion of the irrigation. The Turbosonics sleeve (Alcon Laboratories, Fort worth, Texas) is randomly textured to prevent a total occlusion of the irrigation flow. The Cobra Seal infusion sleeve (Surgical Design) has an internal spiral silicone rib to create extra spacing from the friction.
| II. Settings for Steps of Phacoemulsification|| |
The phaco neophyte needs to remember some absolute values that constitute standard, high and low values for each of the components of phaco. Once this is done and the above mentioned principles of phaco understood, adjusting the parameters to suit various clinical situations, is a logical derivation of the physical and mechanical changes that are taking place during that step. This obviates the need to memorize various sets of values for various clinical situations. Discussed below are some situations commonly encountered while performing phaco. It must be emphasized that the values suggested herein are suggestions made by the authors based on their experience. These must not be construed to be the only set of values to be used. Needless to mention, the surgeon has the liberty to use various sets of parameters provided he/she understands and is prepared to handle the consequences thereof.
| A. Cracking procedures|| |
| Central sculpting and trench digging|| |
Desirable: Removal of nuclear material
Not desirable: Engaging nuclear material with the phaco tip, clouding of the anterior chamber by the sculpted material. "Chattering" of the nucleus during sculpting.
Suggested settings: AFR 20-25 cc/min, vacuum 0-15 mm Hg, power 60-70%, bottle height 65 cms above eye level, tip angle 30-45 degrees.
Comments: A popular practise of users of peristaltic systems is to use Zero Vacuum during sculpting and trench digging. The vacuum limit is set close to zero and as long as the phaco tip is not occluded, the pump continues to turn at the desired flow rate. Fluid flow occurs because the peristaltic pump is turning. The fluid that is turning over clears the emulsified lens material with it. This setting is desirable when sculpting grooves and working close to the posterior capsule. When the phaco tip occludes, the vacuum rises and the pump stops turning. The advantages of zero vacuum phaco are less lens chatter, a safer procedure near the capsule, and the ability to shave the nucleus without occlusion engaging nucleus into the phaco tip. Vacuum transfer systems cannot use the zero vacuum setting because it would mean there is no pressure difference in the system; hence there will be no flow in such systems. The hydrostatic pressure difference alone (between the bottle height and the cassette) is not normally adequate to permit enough flow to allow safe emulsification of the nucleus with such systems.
| Chopping|| |
Desirable: A strong hold of the nucleus by the phaco tip
Not desirable: Lack of stability of the engaged nucleus
Settings: AFR 25-30 cc/min, vacuum 100-200 mm Hg, power 50-60%, tip 0-30 degrees, bottle height 75 cms above eye level
Comments: Tips with multiple angles such as the Kelman tip are not ideal for chopping as they do not provide a stable chopping block for nuclear chopping.
| B. Quadrant removal|| |
Desirable: Aspiration forces to do majority of the work, emulsification to serve as an adjuct to facilitate this.
Not desirable: Boring through the quadrant
Settings: AFR 25-30 cc/min, vacuum 100 mm Hg, power 50%, tip 30 degrees, bottle height 65cms above eye level.
Comments: Proper use of the second instrument is of cardinal importance to acheive efficient and safe quadrant removal. The second instument can help to constantly reposition the quadrant so as to have the largest portion of the quadrant in line with the tip. It can also serve to perform a stuffing motion facilitating passage of the quadrant through the tip with predominantly vacuum itself. This latter maneuver is useful in cases of brunescent cataracts. Pulsing a phaco tip is a technique that can overcome some of the limitations of the fluidics system and poor phaco power. As a nuclear chunk is held by vacuum, application of continuous phaco energy can drive the particle away from the tip. By pulsing the phaco tip, the off time allows the particle to be regrasped by the tip before the next burst of phaco energy is delivered. This is thought to permit nucleus quadrant removal in a more controlled fashion. Another potential advantage is that lower phaco energy is used since the phaco power is off intermittently between the pulses. Current techniques however call for reducing the phaco power and increasing the vacuum for better holding. This often obviates the need for pulsing since vacuum is predominantly used to remove the nuclear quadrant.
| C. Epinucleus removal|| |
Desirable: A natural tendency for the epinucleus to move towards the phaco tip.
Not desirable: Use of phaco power.
Settings: AFR 30-35 cc/min, Vacum 70-100 mmHg, power 10%, bottle height 75 cms above eye level, tip 30 degrees. Another choice is to use the 0.5 mm I/A tip for epinucleus removal.
Comments: The second instrument can play an important role in ensuring safe epinucleus removal. During the initial part of the manoeuvre removal, the epinucleus at the 6 o'clock position is engaged by the phaco tip and lifted away from the posterior capsule. At this stage, the second instrument can perfrom a flipping movement facilitating epinucleus removal enmasse. Also, when the last part of the epinucleus is being sucked out, it is useful to place the second instrument below the phaco tip can to act as a mechanical barrier to the posterior capsule being drawn to the phaco tip by the surge. The above mentioned principles also govern removal of the nucleus after relaxing nucleotomies in the SPRING surgery technique for soft cataract.
| D. Cortex aspiration|| |
Desirable: A natural tendency for the cortex to be drawn towards the tip.
Not desirable: Having to raise vacum to high levels to combine removal of epinucleus and cortex.
Settings: AFR 25-30 cc/min, vacuum limit 400 mm Hg, 0.3 mm I/A tip, bottle height 65 cms above eye level.
Comments: The following steps are useful to follow during cortex aspiration: engage the cortex using AFR predominantly. Rotate the port to engage a bulky protion of the cortex. Once the port is engaged build up more vacum. As this begins to occur, turn the port to face forwards and generate high vacum by pressing the footswitch fully. The approach to two problem situations that can be encountered are summarized in the algorithms shown in [Figure - 10] and [Figure - 11].
| III. Emerging Directions in Phaco Technology|| |
There are two main areas that phaco manufacturers are looking into: phacotmesis and laser assisted cataract surgery (LACS). These systems have yet to receive complete approvals from the United States FDA. Phacotmesis, introduced by Aziz Anis, is a technique that uses both longitudinal and rotational motion at the handpiece tip to remove the cataract. It combines a peizoelectric handpiece with a hollow shaft motor that creates motion that carves the lens out against a backstop similar to a vitreous cutter. In laser phaco, the laser energy is used in place of the vibrating phaco tip. Laser energy in the form of erbium:yag or nd: yag is used. In the erbium energy system (Premier Laser Systems, Lake Forest, California) laser energy is directed through a fiber and is placed in direct contact to the lens to breakdown the lens. Irrigation and aspiration are used to maintain the chamber and to remove lens fragments. In the nd:yag system (Paradigm Medical Industries, Salt Lake City, Utah), shock waves are created by the laser to break down the lens in the same manner kidney stones are broken down. The laser handpiece probe provides irrigation and aspiration as well as a backstop for the laser energy. One stated advantage of LACS is minimized heat generation. Although these techniques appear promising reports on clinical evaluation in large number of subjects are awaited.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11]