128 PHACODYNAMICS: STEP BY STEP

15 Phacodynamics

JEROME JEAN BOVET (SWITZERLAND)

A clear understanding of the physical and mechanicalprinciples that govern phacoemulsification can facilitateusage of this technique for effective and efficient cataractremoval

It is essential to understand the basic principle ofphacodynamics. This is even more relevant to bimanualmicrophaco and is mandatory for the evolution of thistechnique.

Phacodynamics is defined as the study of the funda-mental principles of inflow rates, outflow rates, vacuum,phaco power modulation along with microsurgicalmaneuvers with different types and grade of cataract.

A true understanding of this will help in logical settingof the machine parameters in adaptation to differentsurgical techniques.

Even the most modern machine will not give adequateresults as compared to an older machine if the principlesof phacodinamics are not well understood.

Most of the surgeons use their equipment withoutreally understanding their machines or the basic valueswhich allow them to operate in great safety.

The most important parameters in microphaco-emulsification, in order to be performed well requires agood understanding of fluid mechanics.

Basically the dynamics of phaco in both conventionaland microphaco remain the same In microphacoirrigation is through the side port rather than throughthe irrigation slevees (Fig. 15.1).

With this one of the major problems encountered isthat not enough fluid is going into the eye through theside port irrigating chopper. In conventional phaco thesilicon sleeve delivers up to 65 cc/min into the eyewhereas the best irrigating choppers of 20 G cannotdeliver more than 45 cc/min. keeping this basic fact inmind one has to work around it in term of surgicalmanoeuvers, fluid management, ultrasonics and type andgrade of cataract.

Some of the obvious advantages of bimanual micro-phaco are decreased incision size, less astigmatism, goodanterior chamber maintenance if we understand the logicof phacodynamic.

Because irrigation is separated from aspiration theadded advantage theoretically is that all fluids comes inthrough one incision and exit through the other.We havealways to take in account some leak from the irrigationsideport and from the aspiration sideport that’s wedemonstrate in our experimental set-up.

The irrigation fluid in conventional phaco has atendency to push the fragments away from the aspirationtip.

With bimanual microphaco we are improvedfollowability, we are able to manipulate tissue with theincoming stream of fluid and we have a much more stablechamber because all of the fluid is entering through oneside of the eye and leaving through the other side so wedo not have competing currents of fluid aroud the phaco

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needle. Of course bimanual irrigation and aspirationsysteme of the cortex has been known for a long time tobe advantageous.

The fundamental principle of every cataract operationis to improve the vision of the patient after surgery ofthe cataract and not to diminish it.

The purpose of the stability of the anterior chamberis to distance the endothelium and the posterior capsulefrom the tip of the probe.

Consequently, in order to avoid any lesion of thecorneal endothelium, the consequence of which isconsiderable corneal oedema with a risk of the need toperform a graft, it is imperative to control the stabilityand depth of the anterior chamber.

We must always bear in mind that the principal causeof corneal grafts is the cataract operation.

The most important heading in phacodynamic thatwill be discuss below are:

• Fluidic• SurgePhacoemulsification is a cataract surgery in a close

chamber, in a such close chamber ,one must monitor theinflow (irrigation), the outflow (aspiration) and the leak.

There are three components of fluidics.

Irrigation

For most machines irrigation is passive, i.e. it is deter-mined by the height of the perfusion and the diameter ofthe tube. Most of these machines do not possess apressure control or a control of the flow rate of the liquidentering the eye. Such a control would make it possibleto prevent the collapse of the anterior chamber. This isthe weak point of all of the phacoemulsification machinesand the bimanual microphaco technique.

There are two ways of increasing the flow rate of theliquid in the anterior chamber:• By increasing the pressure

Many authors have described theirs own methods toincrease the flow by increasing the pressure everybodyhave forget the Poiseuilles law increasing the pressureincrease also the pressure inside the eye and affect theendothelium• By increasing the diameter of the perfusion probe

utilising a Duet handpieces design by Larry.Lacks ofMicrosurgical technology in which the choppinglumen of a 19 G tip deliver at least 60cc/min with apressure of 33 mm Hg

• An other way to increase inflow volume is to add ananterior chamber maintener as has been described

FIGURE 15.1: Phaco overview

130 PHACODYNAMICS: STEP BY STEP

by Michael Blumenthal, this device may be use inaddition to or instead of an irrigating second instru-ment,altough it involves a third incision for accessto the anterior chamber.

Aspiration

outflow Rate

Outflow rate is defined as the volume of fluid by unitthat exit the anterior chamber that governs the speed ofthe procedure and the danger to aspirated the iris or theposterior capsule

Vacuum is what grips the nucleus which is occludingthe phacotip, this grip allow to decrease ultrasound andaids chopping

Leak

In a conventional phaco the leak is a composant of thesurge and the instability of the anterior chamber. In abimanual microphaco with 19 G irrigation needle (thatmean at least a 60 cc/min of irrigation). The controledleak is to cool done the phacotip without induce aninstability of the anterior chamber (Fig. 15.2).

The inflow rate has to be higher than the outflowrate even with the maximum of vacuum with every typeof pump to avoid the surge.

FIGURE 15.2: Aspiration-irrigation leak: Microbimanual 19 G

Surge

This phenomenon is produced when the phaco tip isoccluded by nuclear masses. The increase in the negativepressure occurs up to the limit values that have beenpredetermined. The greater the negative pressure and thelonger the time it is maintained, the larger will be thevolume of liquid to be replaced along the length of thetubes at the time of the decompression. The vacuum willbe propagated up to the anterior chamber leading to itscollapse (Fig. 15.3).

The best way to avoid the problem is to increase theflow rate upstream of the anterior chamber. In the caseof bimanual surgery, it is necessary to increase thediameter of the irrigation probe to 19 G tip.

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FIGURE 15.3: Surge: Vacuum with a peristaltic pump

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