Phacodynamics

From EyeWiki


Phacoemulsification cataract surgery requires the use of complex phaco machines. Understanding the mechanical principles underlying this technology allows the surgeon to optimize machine settings and safely trouble shoot problems encountered during surgery.

All Eye Surgeons, regardless of level of surgical expertise, will benefit from reviewing the fundamentals of Phacoemulsification, including fluidics and ultrasound power modulation. The accomplished surgeon can improve the safety and efficiency of their techniques and the novice surgeon will have a distinct advantage to mastering these skills.

Fluidics

A term used to describe the balance of fluid inflow and outflow during Phacoemulsification cataract surgery.
One of the goals of the surgery should be to maintain a stable anterior chamber. This can be done by making sure the fluid entering the eye is equal to the amount that exits. This will keep the anterior chamber pressurized.

Inflow

The inflow fluid originates in the irrigation bottle and  is a balanced salt solution. The fluid travels from the irrigation bottle through plastic tubing, into the phaco needle and finally into the anterior chamber of the eye. To create a pressure gradient the bottle is placed at a height above the patient. When the pinch valve is open the fluid in the bottle and tubing creates pressure in the anterior chamber. Approximately 11 mm Hg pressure (above ambient atmospheric pressure) is produced intraocular for every 15 cm (6 inches) bottle height above the patient’s eye. [1]

Outflow

The fluid that leaves the anterior chamber. Fluid leaves through the phaco needle into the tubing and into the anterior chamber of the eye . This can be increased by increasing the aspiration flow rate. Another source of fluid loss is through wound leakage.

Fluid Balance

The bottle height is placed so that the pressure gradient is adequate to keep up with the outflow. This balance maintains a stable anterior chamber by keeping the pressure in the anterior chamber fairly constant

Fluid Imbalance

If the balance of inflow and out flow is altered, the anterior chamber can be under or over-pressurized. If under pressurized this can lead to shallowing and/or collapse on the anterior chamber. This will cause forward movement of the iris, lens and posterior capsule. . This may lead to inadvertent rupture of the posterior capsule, due to its movement towards the phaco needle. One indicator of anterior chamber pressure imbalance is the bouncing movement of the iris and lens.
Over pressurization (bottle height too high) can cause misdirection of aqueous fluid or deepening of the anterior chamber with zonular stress.

Both safety and efficiency of Phacoemulsification cataract surgery are directly related to fluidics. Proper settings and use of the machine will improve the safety and efficiency of the surgery. Improper settings can create a dangerous situation
 

Aspiration Flow Rate

The amount of fluid flowing through the tubing. This is reported in cubic centimeters per minute (cc/min). With a peristaltic pump, flow is determined by the speed of the pump. As flow increases the current in the anterior chamber increases how well particulate matter is attracted to the phaco tip.

Pump

The part of the phaco machine which moves fluid through the aspiration tubing. The pump settings control the rate of movement of the fluid.

Flow Pump

The primary example of the flow pump is the peristaltic pump. These pumps allow for independent control of both aspiration rate and aspiration level. 

Vacuum Pump

The primary example of the vacuum pump is the venturi pump. This pump type allows direct control of only vacuum level. Flow is dependent upon the vacuum level setting.

Vacuum

The difference in fluid pressure among two points. Negative pressure measured in millimeters of Mercury (mm Hg). Vacuum determines how well, once occluded on the phaco tip, nuclear material will be held to the tip.(holding power)

Compliance 

Change of the tubing shape and volume when put under negative pressure. When the tip of the phaco needle is occluded, negative pressure will build in the tubing. The higher the compliance the more change in the tube’s volume. When negative pressure is created highly compliant tubing collapses on itself reducing its inner volume’s capacity. When occlusion breaks, the tubing returns to its original shape and a temporary imbalance of exists causing rapid exit of fluid from the anterior chamber. This is called Surge
The higher the compliance the greater the surge amplitude during occlusion break.

Surge

An additional source of vacuum that builds up when the phaco tip is occluded. This is due to the tubing compliance. If a phaco needle is occluded and tubing collapses due to negative pressure. When occlusion breaks the rate of fluid exit from the anterior chamber can be temporarily increased due to the sudden expansion of the tubing. This can cause temporary shallowing until the fluid is replaced by the infusion
Selecting lower levels of flow and vacuum will reduce the amount of surge.
Rigid noncompliant aspiration tubing will also decrease the amount of surge. Another mechanism to reduce surge is venting. Venting allows vacuum levels to be equilibrated to air or fluid into the aspiration line. A system for venting exsits in all phaco machines.

Rise time

The amount of time required to reach a given vacuum preset, assuming complete tip occlusion.

Foot Pedal

The foot pedal can be used to controls of the speed of the pump and or the ultrasound

  • Position 1  irrigation (no aspiration no ultra sound)
  • Position 2  irrigation on, aspiration on, (no ultrasound)
  • Position 3  irrigation on, aspirtation on, ultrasound on

UltraSound Power

Fixed Machine setting

The variable that is controlled is set and when the foot pedal is pushed the machine produces the preset amount of ultrasound power or aspiration flow rate. (this can be used when referring to power and aspiration flow rate.

Surgeon Controlled Machine setting

The amount of ultrasound and or aspiration is controlled by the surgeon by pushing down the foot pedal. Also referrerd to as variable control.

Phaco Needle

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. The frequency of a given handpiece is usually indicated in kilohertz (KHz). The frequency used most commonly is 40 KHz

Power generation

Phaco needle movement creates mechanical power. Total power is a combination of the frequency of the needle movement and stroke length.Power generation

Frequency

The speed of the needle movement. It is determined by the manufacturer of the machine. Phacoemulsification needle move at a frequency of between 35,000 to 45,000 cycles per second (Hz)
This frequency range is the most efficient for nuclear emulsification.
Lower frequencies are less efficient and higher frequencies create excess heat.

Stroke length

The length of the needle movement. This length is generally 2 mils (thousandths of an inch) to 6 mils. Most machines operate in the 2-mil to 4-mil range. Longer stroke lengths are prone to generate excess heat. The longer the stroke length, the greater the physical impact on the nucleus, and the greater the generation of cavitation forces. Stroke length is determined by foot pedal excursion in position 3 during linear control of phaco.

Phaco Needles

The shape and size of the needle will impact the fluidics and the power of the ultrasound delivered to the cataract. There are several choices. 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.

Mechanical energy of the Phaco tip

Two types of energy: Jackhammer effect and cavitation.[1]

The jackhammer effect is the physical striking of the needle against the nucleus.
The cavitation effect is created energy that is released when micro-bubbles implode.
When the phaco needle, moving at ultrasonic speeds, creating intense zones of high and low pressure. Low pressure, created with the backward movement of the tip, literally pulls dissolved gases out of the solution, creating micro bubbles. Forward tip movement then creates an equally intense zone of high pressure. This produces compression of the micro bubbles until they implode.
At the moment of implosion, the bubbles create a temperature of 13000° F and a shock wave of 75,000 pounds per square inch (PSI). 75% of the micro bubbles implode, to create a powerful shock wave radiating from the phaco tip in the direction of the bevel with annular spread. 25% of the bubbles are too large to implode. These micro bubbles are swept up in the shock wave and radiate with it.
The cavitation energy can cause tissue damage and may have limited value in cutting[2]

Occludability

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[3]

Balancing Phaco power and Aspiration flow rate

In order to emulsify the nucleus, the surgeon needs to balance the pull and push effect of the phacomachine. The aspiration flow pulls material towards the phaco tip and the ultrasound movement of the phaco tip pushes material away. The vacuum can hold nuclear fragments on the phaco tip. Surgeons can modify their settings to improve this balance.
Using a pulse mode will allow on time and off time.

Again balancing the attractive forces with the repulsive forces allows the nuclear material to stay at the tip and be emulsified so that it can be aspirated.

Generally, low flow slows down intraocular events, while high flow speeds them up. Low vacuum is helpful during sculpting of a hard or large nucleus, in which the high power intensity of the tip may be applied near the iris or anterior capsule. Zero vacuum should be avoided a small about of vacuum is necessary to remove any clogging of emulsate in the tubing avoid inadvertent aspiration of the iris or capsule, preventing significant morbidity.

Ultrasound Power Settings to limit energy damage

When a surgeon chooses machine settings, it is important to keep in mind the goals of the surgery. A simple summary of the goals is to successfully remove the cataract lens and replace it with an IOL while minimizing tissue injury.

Tissue injury includes wound burn, endothelial cell loss,  iris trama and trabecular meshwork damage.Efficient and skillful use of ultrasound power will limit the amount of energy damage.

Mimimizing energy damage can be accompished by the choosing the most efficient nuclear dissasembly technique and choosing machine settings that efficiently utilize power.

Examples of power setting modifications include.

For Sculpting:

Change from fixed to linear. This will allow the surgeon to titrate the exact amount of energy to use by the foot pedal.

Change from continuous to pulse. Each pulse has a duty cycle which has on time and off time of the phaco power. This adjustment of using pulse will reduce the energy in the eye by the percentage of time designated as off.

For Chopping: In order to chop it is necessary to make a purchase of the nucleus with the phaco needle then to drop back onto foot position 2 to let vacuum build. To make a purchase of the nucleus the surgeon wants a short amount of energy then to drop back into apiration (foot position 2) To chop it is easiest to use a burst mode of a burst width from 40-80ms then to have the vacuum on fixed so that it will build quickly.

For Quadrant removal: To remove the divided pieces of uclear materials, use a pulse setting. This will allow "on "time to allow the nucleus to emulsify the quadrant and off time to allow the the Aspiration to hold the nucleus on the tip of the phaco needle. Titrate the phaco power by using linear.



Power Settings

Ultra Sound Power settings Advantages Image.jpg

Additional Resources

References

  1. 1.0 1.1 Barry S Seibel,Phacodynamics:Mastering the tools and techniques of cataract surgery.third addition, Thorofare, NJ.Slack 1999
  2. Sacharias J. Role of cavitation in the phacoemulsification process JCRS 2008,34,846-852.
  3. Yow L,BastiS.Physical and mechanical principles of Phacoemulsification and their clinical relevance.Indian J Ophthalmol 1997, 45:241-9
  1. Rekas M, Montés-Micó R, Krix-Jachym K, Kluś A, Stankiewicz A, Ferrer-Blasco T. Comparison of torsional and longitudinal modes using phacoemulsification parameters. J Cataract Refract Surg. 2009 Oct;35(10):1719-24
  2. Devgan U. Phaco fluidics and phaco ultrasound power modulations.Ophthalmol Clin North Am. 2006 Dec;19(4):457-68.