Phacodynamics

Nakhleh E. Abu-Yaghi, MD

Phacodynamics

Studying the fundamentals of :

- Inflow rates- Outflow rates- Vacuum- Phacopower modulation- Micorsurgical maneuvers in dealing with different

types/grades of cataract

Charles Kelman (1930-2004)

• Devised the cryo-probe (1962)

• Introduced extracapsular cataract extraction

• Introduced phacoemulsification (1967)

• Most influential IOL designer (1975)

• Lasker award (highest award in medical science)

Overview• The machine

• The ultrasound energy

• The fluidics

Understanding the machine

The phaco machine• The console

• The foot pedal

• The hand piece

• The connections

The console• Computer controlling functions

• Setting and parameters

• Display foot pedal signals and input information and display modes

The hand piece• Tip• Sleeve• Body• 3 ports (Aspiration, irrigation, Ultrasound)

• The body contains piezoelectric crystals (acting as a transducer) that contract or expand converting electric energy into mechanical energy causing movement of the tip.

• This is temperature dependent (Curie temperature)

• Silicone sleeve for phaco tip:

• Insulates corneal wound and provides fluidic pathway for irrigation

• Infusion flows between the tip and sleeve and exits from two openings perpendicular to the tip bevel

Tip• Outer diameter: wound size (1.1) (0.9- microtip)

• Inner diameter: aspiration flow rate (0.9 mm) (0.7mm- microtip)

• G?

• The tip is from titanium• Angle of standard tip is from 0-60 degrees• The less the angle: the better the holding but the poorer the

cutting action• Tip selection depends on hardness of nucleus and technique• 30 degree tip is a good compromise between trenching and

chopping• End configurations: round, ellipsoid, bent or flared• Occludability: is the tendency of the tip to get occluded• The smaller the tip angle the higher the occludability• Your garden hose is zero angle

Variations on the tips

• Microseal, microflow tips (to reduce incisional burns)

• Kelman and Kobra tips (to improve cutting efficiency)

I/A port• Straight• 45 degrees• 90 degrees

Foot pedal

Excursions and dentations

• Feel the resistance at dentations/positions (mode changes)

• Tactile feedback

• Auditory feedback

Excursion I• Irrigation is on

• No gradient in this step

• Going back to excursion 1 and stopping at D1 prevents collapse of AC

• Nuclear rotation/manipulation of nuclear fragments, require a formed AC without aspiration

Excursion IA• A linear control of vacuum and flow

• Top of foot position 2 provides less vacuum or flow than middle or bottom

• The vacuum and flow effect is created by a peristaltic or venturi pump

• Sources of fluid outflow during phaco:1. Aspiration tube2. Leakage from incisions

Excursion IAP• IAP0= phaco energy is zero

• IAP max= energy at maximum preset

• Phaco power is linear in surgeon and pulse mode

• In panel or burst mode, maximum preset energy is delivered

• In IAP, irrigation is on and aspiration is at maximum preset

Foot gradient (FG)• The excursion of foot pedal in mm to produce unit power of

phaco energy

• If total foot excursion (from IAP0 to IAPmax is 10 cm (100mm),and the maximum preset phaco energy is 100%, hence FG:

FG= 100mm/100=1 unit power per 1mm of excursion

If maximum preset phaco is changed to 50%:FG=100/50= 1unit power per 2mm of excursion more FG more foot controlPhaco maximum should be set at the minimum power required for that particular step

Side kick function• Reflux function

• Not a linear function

Continuous infusion• Foot is off pedal but irrigation is still on

Fluidics

Main concepts• Maintain space (prevent collapse of AC by balancing inflow

and outflow rates)

• Create currents (bring cataract pieces to phaco probe and remove them)

• Keep it cool (prevent thermal injury)

Fluidics parameters• MechanicalInfusionAspiration and leakageVacuum

• Clinical Incision sizeVitreous pressure

Inflow/infusion• BSS flows from bottle to irrigation port

• Infusion is passive by gravity

• Bottle height above patient creates a pressure gradient

• Approximately 11 mmHg (above ambient atmospheric pressure produced intraocularly) for every 15 cm of bottle height above the patients eye

outflow• Aspiration and leakage (ml/min)

• Affected by diameter of phaco tip, tubing and vacuum

• Wound leakage

Aspiration flow rate (AFR)• Measured in ml/min• Is determined by speed of pump

• As flow increases :

current in AC increases: this determines how well particulate matter is attracted to the tip

• Optimum rate is 20-36 cc/min

Followability• Tendency for structures within the AC to move towards the

phaco tip

• It is a function of AFR

• Positive pressure of infusion and negative pressure of aspiration create pressure gradient at the tip that leads to Eddy currents from the infusion orifice to the phaco tip. This area is known as zone of followability

Vacuum• Create holding power to keep material in phaco tip

• Created by a pump:Peristaltic pump: vacuum develops slowlyVenturi pump: rapid rise in vacuum

Trenching: low to no vacuumSegment removal: moderate vaccumChop: high vacuum

Holdability• Ability of the phaco tip to hold onto the material occluding its

tip

• It is a function of Vacuum

• 1st generation machines: max vacuum 120mmHg

• 2nd generation machines: max vacuum 250mmHg

• 3rd generation vacuum up to 650mmHg

AFR Vs Vacuum• AFR is the rate at which fluid and emulsified nuclear particles

are removed from the eye when the phaco tip is not occluded

• Vacuum is the negative pressure that builds up when the tip is occluded

• They are separate components that work hand in hand during outflow

• At low flow rates: gradual build up of vacuum (safe but slow)• At high flow rates: faster vacuum build up (but less safe)

Pump• Main function is to move fluid through the aspiration tubing

• Pump settings control rate of movement of fluid

• Peristaltic pump (Flow based)Allows independent control of aspiration rate and vacuum level

• Venturi pump (Vacuum based)Allows direct control of only vacuum level. Flow is dependent on vacuum level setting

Peristaltic pump

• Rollers move• Compress the outflow tubing in a peristaltic manner• “milking” action on fluid column• The machine can control the flow level (flow based)

• A preset vacuum level is achieved once there is occlusion of outflow line (at low speed of rotation)

• By increasing the flow rate, vacuum is produced in the aspiration line without occlusion

• To build up vacuum without occlusion, you need to increase the flow rate (flow based pump)

Venturi pump• Vacuum is created within a rigid drainage cassette connected

to the aspiration tubing• No milking of the aspiration line phaco tubing can be made

rigid with low compliance

• Main advatage is the ability to create the preset vacuum level without occlusion of the phaco tip needle

• As the surgeon depresses the pedal, the preset vacuum is immediately created (vacuum based)

• Venturi and diaphragm pumps have inherently higher flow rates high build up of vacuum

• Only the vacuum can be controlled. The flow rate is fixed and is a function of the vacuum

• Vacuum is directly transmitted from a closed chamber to the tip ensuring a better followability

• Peristaltic pump Venturi pump• Flow based vacuum based• Vacuum created on occlusion instantanousof phaco tip

Flow constant until occlusion Flow varies with vacuumDrains into a soft bag Drains into a rigid cassette

• The slow flow rate is useful for beginners (high safety margin in case of sudden capsule occlusion in the port)

• Slow rise time allows time to come back to position 1 or even relax

• At a moderately high flow rate it is a good compromise between safety and efficiency

Vacuum rise time• The amount of time taken by the system to reach maximum

vacuum setting once occlusion takes place

• Peristaltic pumps have a slower rise time (can be made faster by increasing the rotation of the wheel)

Other pump modalities• Concentrix (millennium)

(flow mode and Vacuum mode)

• Rotary vane- Chiron’s catalyst(vacuum based)

• Diaphragm(vacuum based)

Clinical Parameters• Incision size: affects: LeakageInfusionAC stabilityDepends on external diameter of phaco tip

• Small incision causes infusion flow obstruction: squeezing of infusion sleeve AC instability and build up of heat at the tip

• Big incision more leakage and more astigmatism

• Vitreous pressure

Patient dependentAffects AC depth during phacoCompensate with increased bottle height

AC stability• Inflow=outflow• AC pressure has to be greater than vitreous pressure and

atmospheric pressure (positive IOP)

• Under pressurization : collapse of AC forward movement of iris and lens and posterior capsule rupture

• One indicator of AC pressure imbalance is bouncing movement of iris and lens

• Over pressurization can lead to deepening of AC and zonular stress

Compliance and Surge• Property or ability of the tubing to collapse or deform under

pressure

• High compliance means more surge

• Surge is a temporary fluid imbalance:When the phaco tip is occluded negative pressure builds in the tubing collapse of the aspiration tube occlusion breaks after eating the nucleus piece tube returns to original shape surge (sudden sucking effect of fluid with AC shallowing (until infusion fluid compensates)

Surge• Here the outflow fluid from the eye exceeds the inflow fluid

(even for a split second)

How to reduce surge?• Lower levels of flow and vacuum• Rigid (low compliance) aspiration tubing• Venting• Increased bottle height• Reduce tip size• Aspiration bypass port (ABS)

Venting• The machine has a sensor that detects occlusion breaks and

releases fluid/air into the system fill the volume for the re-expanding tubing, preventing outflow of fluid from the AC

• (vent valve senses a maximum vacuum level)

• Fluid venting is better (less surge) because air bubbles contract more when expansion of tubing reverses

• Fluid expands less and contracts less than air

Aspiration bypass port• A small hole in the metal part of phaco hand piece that

functions when the tip is occluded. It also has a cooling effect

Central safe zone• A conceptual area within the capsulorhexis margin bounded

vertically by the cornea on top and the posterior capsule from the bottom

• In contrast to the peripheral unsafe zone (corneal curvature is in play)

Ultrasound power

Ultrasound power• It is the conversion of electrical energy into mechanical one by

ultrasound vibrations of the Quartz crystal in the transducer/ vibrator of the hand piece

• Depends on frequency (29-60Hz) and Stroke length (2-6mm)

• The higher the frequency the more the cutting action and heat• The longer the stroke length the greater the action and the

heat

• PHACO POWER= STROKE LENGTH X FREQUENCY

• Frequency: number of longitudinal vibrations per second

• Stroke length: distance the tip displaces in the axial direction during phaco

• Phaco power is indicated as a percentage: at 100% the stroke length is the maximum permissible for the machine. When the power is decreased, the stroke length decreases (frequency is fixed per machine)

Piezoelectric crystals• Converting electrical energy to mechanical energy

Mechanism of phacoemulsification

• The Jackhammer effect: physical striking of needle against nucleus by to and fro movement (transverse or elliptical)

• Cavitation effect: formation of micro-bubbles which at moment of implosion create 7204 C temperature and shock wave of 75000psi which can be directed away from cornea according to angle of tip.

Phaco power variables• Too little power: pushes nucleus instead of carving it zonular

stress and extension of posterior capsular (PC)tears

• Too much power: pierces nucleus rent in PC

• Safest phaco is with appropriate power not with low power

• This depends on :Nuclear densityAmount of tip that is engagedLinear velocity of tip during phaco

Control and delivery of power• Linear (surgeon mode): progressive pressing of foot pedal

control leads to gradual rise in phaco power from 0 to preset level

• Panel mode: parameter reaches the preset panel maximum on pressing the foot pedal without any linear pedal control

(useful in very hard cataract with uniformly hard nucleus)

Phaco mode• Continuous: power is delivered constantly and is either linear or

panel controlled(more ultrasound power- good for sculpting)

• Pulse mode: power is delivered as pulses of energy followed by a gap of equal duration of pulse free period

(there is a pause in ultrasound) (duty cycle concept)The more the pedal is pressed, the higher the power in linear mode

• Burst mode: maximum power is delivered at intervals which vary with amount of depression of foot pedal

(burst width is less the more the surgeon steps on the pedal, full pedal gives continuous burst i.e zero burst width)

Q• Will phaco energy increase by using more pulses per second?

• No

• Each short phaco pulse is followed by a short phaco-off time

• Duration of on and off time can be set by operator and is called duty cycle

• This can reduce heat generation and increase followability

• This is all programmable and affects smoothness and precision of power delivery

Duty cycle• Phaco on time/ phaco on time +phaco off time

• Higher duty cycle results in better cutting power but increased heat generation

• Lower duty cycle allows more fluidic aspiration of nuclear fragments while minimizing heat and phaco power

tips• Pulse mode:

Holding the lensChoppingBringing material to central area

• More pulses per second: sculpting and removing quadrants

Burst• Wide burst time: epinucleaus (when you want to hold lens

material you need an off phaco mode)

• Narrower: quadrants

• Full burst: sculpting

Different settings• Trenching (hi power, low vacuum)• Chopping (hi vacuum, moderate power)• Quadrant removal (hi vacuum, moderate power)• Epinucleus removal (high vacuum, no or low phaco)• Cortex aspiration (high vacuum)

Final words• Understand your machine to become a more efficient driver

• Proper knowledge harnesses hidden advantages

• Check your machine

Basic principles?

There are none!