Ординатура / Офтальмология / Английские материалы / The Art and the Science of Cataract Surgery_Boyd, Barraquer_2000
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aspiration line and actually determines the force with which the material is fixated onto the orifice in the phaco tip. This is known as fixation power or grasp and depends on the aspiration force (Figs. 59-60, 61). The higher the aspiration pressure, the more rapid the aspiration flow, and the less the amount of time it takes to obtain the maximum vacuum power. If the occlusion at the tip is broken or interrupted, due to the negative pressure in the aspiration line, fluid is rapidly sucked out of the eye. This may lead to collapse of the anterior chamber with risk of damage to the corneal endothelium as well as the posterior capsule. This is known as the
Surge Phenomenon (Figs. 61-65).
How to Program the Machine for Optimal Use
We have already discussed the phacoemulsificator’s settings which include the ultrasonic power, the aspiration flow, which is the power of attraction and the vacuum, which is the grasping power.
In order to perform a rational phaco, we must know how to program or calibrate the "memory" of the machine. There are three memories in the machine. Memory 1 is for sculpting the nucleus( Figs, 55, 56), Memory 2 is for fragmentation, mobilization and emulsification of the nuclear fragments (Figs, 67, 68) and Memory 3 is for removal of the epinucleus, when this exists (Fig. 69).
In Memory 1: nuclear sculpting, we need high ultrasound power with low flow and low vacuum since at this stage we do not need any fixation or attraction power. In
Memory 2: nuclear fragmentation, however, we need low ultrasound or phaco power
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in order to keep the nucleus fragments close to the phaco tip and prevent the vibrating effect from repelling the fragments from the tip opening. We need a higher flow of aspiration to bring the fragments of the nucleus to the tip of the handpiece and make the procedure faster.. In this Memory 2, we also need higher vacuum since here we need to have good grasping power to hold the fragments against the phaco tip so that we can proceed to emulsify them. Memory 2 is the memory for fragment mobilization and emulsification.
In Memory 3: removal of epinucleus, all the parameters are lowered considering that the epinucleus is soft. Memory 3 is specifically for the epinucleus, whenever it exists.
Fluid Dynamics During Phaco
Michael Blumenthal, M.D., has made profound studies on this most important subject. Its understanding really makes a difference between success and failure in small incision cataract surgery, particularly in phacoemulsification. There are two factors specifically involved: 1) the amount of inflow and 2) the amount of outflow during any given period of the surgery. Fluid dynamics are responsible for the following intraocular conditions during surgery: a) fluctuation in the anterior chamber depth; b) turbulence; c) intraocular pressure.
Blumenthal has pointed out numerous times that zero fluctuation is the target to be achieved in surgery, insuring that intraocular manipulations are most effective and accurately performed as well as keeping steady and natural the intraocular architecture and relationship between various tissues (Figs. 57-60).
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Figure 56: Use of Different Phacomachine Parameters to Sculpt the Nucleus for Making Quadrants - Memory 1 - Divide and Conquer Technique
A linear vertical furrow is made in the nucleus from 6 to 12 o'clock. A second furrow in the lens is made perpendicular to the first using the phacoemulsifier probe. The phaco probe (P) and manipulator (M) engage opposite sides of the furrow inferiorly. Force is applied with the instruments in opposing directions (arrows) to crack (C) the nucleus along the length of the furrow. Additional manipulations of this type further lengthes and deepens the crack. The lens is rotated 90 degrees within the capsular bag and a crack is made in the second furrow in the same manner (not shown). (The incision during transition should be limbal based. Corneal incision shown here is for advanced surgeons.) The parameters of the machine used to create the furrows in the lens are shown in the figures within the rectangular table immediately above this figure. At this stage, the surgeon uses Memory 1whichis shown digitally inthemachineas1. ThedigitalfigureunderU.S.refers totheultrasoundpowerutilized at this stage in order to create the furrows in the nucleus. ASP refers to the aspiration flow rate, and the VAC shown on the machine refers to the amount of vacuum. These parameters are identified in the rectangle next to Fig. 56.
By cracking the lens furrows at their base, the surgeon creates four separate quadrants of nuclear material. Manipulationofeachquadrantforindividualremovaliscarefullyguidedbyuseofflowandvacuum. Flowisused to move a quadrant to the phaco tip (P). Once engaged, vacuum is used to impale and manipulate the quadrant for safe removal.
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Fluctuation in the anterior chamber depth is the consequence of the following conditions: the amount of outflow exceeds the amount of inflow in a given period. As a result, the anterior chamber is reduced in depth or collapses (Figs. 62 and 63). When the amount of outflow is reduced below the amount of inflow, the anterior chamber depth is recovered (Fig. 65). This phenomenon, when repeating itself, increases fluctuation. When fluctuation occurs abruptly, as in the sudden release of blockage of the phaco tip in aspiration, this is called Surge (Figs. 61-65).
Fluidics and Physics of
Phacoemulsification
Barry S. Seibel, M.D., in his classic book Phacodynamics, presents perhaps the
most complete study on the physics on phacoemulsification and the fluid dynamics involved. This must reading for anyone who wants to delve more deeply into this subject.
Seibel points out that phacoemulsification surgery is essentially the integration of two basic elements: 1) you use ultrasound energy in order to emulsify the nucleus; 2) you utilize a fluidic circuit in order to remove the emulsified material through a small incision while maintaining the anterior chamber depth integrity. This fluidic circuit is provided by an elevated bottle of BSS that produces not only the volume of fluid within the circuit but also provides the pressure in order to maintain the anterior chamber hydrodynamically and hydrostatically. When outflow and inflow are balanced, the pressure of the anterior chamber is proportional to the height of the bottle (Figs. 49-A, 49-B).
Figure57:FluidDynamics-Balance of Flow When the Phaco Tip Is Unoccluded - Hydrodynamic Balanced System
When the phaco tip is unoccluded (D), the outflow rate of fluid from the eye (blue arrows) is determined by the rate (G) of pumping action of the peristaltic pump (F) under surgeon control. In the unoccluded "hydrodynamic" balanced system, inflow (red arrows) from the infusion bottle (B) will replace (C) the aspirated fluid at the same rate, to maintain the constant intraocular pressure determined by the height of the bottle above the eye. In this unoccluded case, the rates of inflow and outflow are equal.
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This fluidic circuit is regulated by a pump which not only washes the emulsified substances but also provides a highly useful clinical purpose. When the tip of the phaco handpiece is not occluded, the pump produces certain currents within the anterior chamber, which are measured in milliliters per minute, which are responsible for attracting the nuclear fragments towards the phaco tip. When a fragment completely occludes the phaco tip, the pump provides a vacuum which is measured in mm Hg, which holds the fragments firmly against the phaco tip (Figs. 57-60).
There are two main types of pumps utilized during phaco: The Flow pump and the Vacuum pump.
The Flow pump, responsible for the direct control of flow, physically regulates the fluid within the aspiration line by direct contact between the fluid and the mechanism of the pump. Even though the scroll pump is the latest type of flow pump, the one traditionally known as the peristaltic pump is the more commonly utilized. One of its important characteristic is the capacity to control the flow of fluid as well as the vacuum. This allows the aspiration flow to be independent of the height of the bottle of fluid. Nevertheless, it is dependent on the degree of occlusion of the phaco tip. Aspiration flow diminishes when the degree of occlusion at the phaco tip increases and aspiration stops completely when the occlusion at the phaco tip is total (Figs. 59, 60).
These pumps have in common a drainage cassette adapted to the aspiration line. The pumps are connected to the cassette and produce a suction which in turn propor-
Figure 58: Fluid Dynamics - Balance of Inflow and
Outflow During Phacoemulsification - Tip
Unoccluded - Hydrodynamic Balanced System
This view is a close-up complement of what is illustrated in Fig. 57. The anterior chamber during phacoemulsification is a closed system in which there isbothintakeandoutputofliquidandwhere thepressure must be controlled. With nothing occluding the tip of the phaco handpiece (P), vacuum pressure is zero (table point 1), At this point, the inflow (green arrow) equals theoutflow(redarrow)ofthephacoemulsificationprobe, and the pressure in the eye is maintained and constant (table levels 2 and 3).
tionally regulates the flow of aspiration when the port of aspiration is not occluded. When the port of aspiration is occluded, the flow ceases and the suction is transferred to the cassette by means of the aspiration line to the occluded tip (Figs. 57-60).
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Figure59(aboveleft): FluidDynamics-Balan- ce of Flow When the Phaco Tip Is Occluded withLensMaterial-HydrostaticClosedSystem
When a piece of nuclear material (N) is drawn to and blocks (occludes) the aspiration port of the phaco tip, fluid balance is still maintained within the eye. Although the pump (F) is still running, it can no longer providing fluid outflow
(D) because the system is blocked, but it is now providing vacuum pressure, holding the occluding fragment. In the balanced "hydrostatic" closed system, inflow (C) ceases at the same time since it now has nowhere to move. Controlled intraocular pressure is maintained via the inflow line to the level determined by the height of the bottle (B) above the eye. Equal zero rates of inflow and outflow is revealed by no drainage (G) from the occluded yet balanced system.
Figure 60 (below right): Fluid Dynamics -
Balance of Inflow and Outflow During
Phacoemulsification-TipOccludedWithLens
Material - Hydrostatic Closed System
Thisviewisaclose-upcomplementofthe fluid dynamics shown in Fig. 59. When the tip of the phacoemulsification probe is occluded with nuclear material (L), the vacuum pressure rises to a level to which the machine is set (table - arrow - 1), and the inflow and outflow rates go down (table 2 and 3 - green and red arrows). With the aspiration port occluded, no fluid can enter or exit the eye.
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Importance of and Understanding the Surge Phenomenon
The Surge phenomenon occurs when a fragment of nuclear material is suddenly displaced from occlusion through the aspiration tip at the handpiece of the phaco machine,
thereby giving rise to a sudden elevation of the output of fluid from the anterior chamber (Fig. 61). The output of fluid suddenly becomes larger than the input of fluid. This differential results in sudden collapse of the anterior chamber and can lead to serious complications (Figs. 62, 63).
Figure 61: Mechanism of the
UndesirableSurgePhenomenon
One problem area of the closed phaco system occurs during abrupt dislodging of an occluding piece of lens material so othat it no longer occluds the aspiration port of the phaco tip. A sudden drop in intraocular pressure occurs as the fluid rate into theeyefailstoimmediately match thesuddenfluidrateoutoftheeye. This is known as the Surge Phenomenon. (A) Shows a piece of lensmaterialoccludingtheaspiration port of the phaco tip and is held in place by vacuum pressure createdbytheoperatingpump(D). (Notethereisnodrainage(E)from theblockedsystem.) Infusionfrom theirrigatingbottle(C)hasceased, but is still providing controlled intraocular pressure due to its elevated position above the eye. With sufficient vacuum pressure from the pump and/or emulsification from the ultrasonic energy, the nuclear piece will abruptly enter the aspiration port and the fluid system will onceagainopen
(B). Because the plastic infusion/ aspiration lines and the eye walls are flexible in absorbing the sudden inflow-outflow pressure differential, there occurs a moment when the infusion fluid (G-small arrow) does not effectively enter
the eye fast enough to replace the fluid suddenly moving out of the unblocked system (F-large arrow). Outflow rate from the force of the pump is momentarily greater than the replacing infusion rate. This out of balance system (out of balance in not providing constant intraocular pressure) in which the eye momentarily absorbs the inflow/outflow rate differential, may traumatically collapse the eye for a short period. (See Figs. 62 and 63).
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Figure 62 (above left): Physical Problems
Caused by Surge
During the Surge Phenomenon when a nuclear piece (F) is abruptly aspirated from the eye, the anterior chamber may collapse due to a sudden loss of intraocular fluid. The cornea
(C) may cave in, resulting in possible endothelial cell damage if it comes near the phaco probe. The posterior capsule (D) may also be damaged from anterior displacement toward the instrument. The fluid outflow rate must be broughtundercontrol,andtheinflowrate(small red arrow) and outflow rate (large blue arrow) are again equalized with the eye repressurized, to reestablish a balanced system with constant, controlledintraocularpressureisnotmaintained.
Figure 63 (below right): Problem of Surge
During Phacoemulsification
This view is a close-up complement of what is shown in Figs. 61 and 62 and explained in their respective figure legends. Here we perceivemoreclearlythecomplicationswithin the eye caused by the outflow surge phenomenon. Surge may occur after a fragmented piece of lens nuclear material is suddenly no longer occluding the aspiration port. Aspirationoccursabruptlyandthevacuum usually goes to 0 (table point 1 - blue arrow). Thissuddenaspirationoftoomuchliquidfrom the eye (large red arrow within the a.c.) is greater than the rate at which the inflow can replace the liquid aspirated (small green arrow in phaco probe). Notice that the table shows the outflow rate is large at this stage (table - point 3 red cube and arrow) and the inflow rate (table - point 2 green cube and arrow) has not caught up with it. This differential causes the posterior capsule to move forward (E) and the cornealendotheliumtomoveinward(D),which can result in severe complications.
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When the phaco tip is not occluded, excess vacuum is zero (0), (Fig. 58) but the flow of aspiration is very high with a large quantity of flow going in and out from the anterior chamber. Note the distinction between the normal suction, or vacuum, pressure which always exists in Positions 2 & 3, and which must exist to produce the normal aspiration flow we speak of, with the extra "vacuum" pressure which builds up when there is tip occlusion. When the phaco tip is occluded with nuclear material, the outflow of fluid stops and the vacuum rises to the maximum level to which the machine was originally calibrated and which we previously described (Fig. 60). This high vacuum aids the rapid emulsification of the nuclear fragment with or without ultrasound. When there is much more sudden outflow of fluid from
the anterior chamber than the inflow, the chamber collapses with possible rupture of the posterior capsule and damage to the endothelium (Figs. 61-65).
Lessening Intraoperative Complications from the Surge
As emphasized by Centurion, the latest generation of phacoemulsification machines make surge control possible (Figs. 64, 65). With these machines it is possible to work with a high vacuum of more than 300 mm while maintaining a steady flow rate. When the last part of the nuclear material goes through the phaco tip, a sensor located at the aspiration line signals a micro processor to slow the rate of the pump. Sometimes there is some reflux in the process of maintaining the
Figure 64: Technical Solution to Prevent the Undesirable Surge Phenomenon
One technical solution for eliminating the surge phenomenon involves the use of a high-tech microprocessor. (Fig. A) When a nuclear piece (F) occludes the aspiration port and then suddenly (B) is aspirated (F-arrow) by the vacuum pressure of the pump (P), a sensor (E) located on the aspiration line signals a microprocessor (G) in the unit that an abrupt surge in aspiration flow has begun to take place. Within milliseconds,themicroprocessordirectsthemotor of the pump (P) to slow down. The reduction in aspiration rate resulting from the slowed pump occurs before the eye can collapse from any volume differential encountered between sudden inflow and outflow rates. The potentially dangerous surge phenomenon is avoided. This elimination ofthesurgephenomenonallowsthesurgeon tosafelyusehighervacuumrates(necessary in some situations) with a reduction in the needtouse potentiallydamaginghighultrasonicpowersettings. Surgerybecomes safer and faster.
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same intraocular pressure. This high speed mechanism insures that the pressure is always the same inside the eye.
As emphasized by Barry Seibel, the surge phenomenon occurs in positions 2 or 3 of the foot pedal when a nuclear fragment totally occludes totally the phaco tip. Vacuum builds up in the aspiration line, the lens material is emulsified sufficiently so that it is quickly drawn within the phaco tip, the occlusion is broken, and there is a sudden surge of aspiration, emptying the anterior chamber.
The surge phenomenon is more of a concern when you utilize a conventional tip with the 0.9 port with high vacuum and flow of aspiration. It is less of a problem when you utilize the irrigation-aspiration tip with the smaller opening (0.3 mm). In addition, it is possible to diminish the propensity for surge during phaco by utilizing a more resistant type of tip such as the Microflow or the Microseal or with the systems ABS which we describe in Chapter 8, (Fig. 84).
Figure 65: Advances in Equipment Technology to Prevent the Surge During Phaco
This is a close-up view of the anterior segment showing what is illustrated and explained in Fig. 64 and its figure legend. The latest generation of phacoemulsification machines make surge control possible. During the problem period when the last part of the nuclear material is aspirated through the phaco tip, a sensor signals a microprocessor to slow the rate of the vacuum pump. As a consequence, when the nuclear material no longer occludes the phaco tip and the sensor detects that the vacuum pressure is dropping suddenly (table point 1 blue arrow and block), the sensor instantly sends a signal to the pump to slow the outflow rate (broken red arrow next to phaco tip). The outflow rate (table point 3 - broken red arrow and block) is thereby moderated to allow the inflow rate time to catch up (table point 2 green arrow and block ). This control of the pump action allows inflow and outflow rates increase together in a more equal fashion during this moment of potential negative surge. This makes surgery much safer, quicker and easier.
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NUCLEUS REMOVAL
APPLICATION OF PHACO
FRACTURE AND EMULSIFICATION
This is really when the surgeon begins to utilize the ultrasound energy in the phaco machine and apply it within the patient's eye. During the transition period, this is a step that should be preceded by a good number of hours of practice in the experimental laboratory until the surgeon is confident in the application of the ultrasound energy. It implies that he or she
has been able to successfully perform all the previous steps over and over again in different patients. This experience will serve the surgeon as the requisite basis for success in the emulsification and removal of the nucleus in the present patient.
Inremovingthenucleus thesurgeonfirst attempts to divide the nucleus by fragmenting it into smaller portions that in due time will then be emulsified individually (Figs. 55, 56, 66, 67, 68). If the fracture or division of the nucleus has been incomplete and has resulted in large pieces or incomplete fractures, the surgeon will not be able to perform the pha-
Figure 66: The Role of Cavitation in Breaking the Cataract Inside the Bag
There are two forces involved in emulsifying a cataract. One is the mechanical force of the ultrasound as shown in Figs. 55 and 56 and explained in their respective figure legends; and 2.) the mechanism of cavitation. The magnified section of cataract presented here shows that as the phaco tip makes its tiny ultrasonicmovements,theenergyreleases bubbles (B) inside the nucleus creating cavities(C).Thebuild-upofbubblesinside the nucleus creates new hollow spaces (C) in the lens structure, the phenomenon of cavitation. This cavitation facilitates the break-up and destruction of the cataract.
Some of the new phaco tips as shown in Fig. 51 are designed to produce more cavitation. The one shown in this figure is one of the best, designed by Kelman for the Alcon phaco machines. It has a very thin tip with a 30 degree bend. It is particularly effective in hard nuclei because of its enhanced cavitation.
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