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. For many phacoemulsification platforms fluid inflow is based on gravity fluidics, and the infusion pressure is directly related to the bottle height. 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] Some newer generation phacoemulsification platforms utilize forced infusion pumps that maintain a preset intraocular pressure during surgery which may provide improved anterior chamber stability.

Outflow

Outflow is the fluid that leaves the anterior chamber. Most fluid leaves through the phaco machine pump . This can be increased by increasing the aspiration flow rate. Another source of fluid loss is through wound leakage.

Fluid Balance

The bottle height or intraocular pressure is set so that the infusion pressure is adequate to balance the outflow. This balance maintains a stable anterior chamber by keeping the pressure in the anterior chamber fairly constant during surgery.

Fluid Imbalance

If the balance of inflow and outflow is altered, the anterior chamber can be under or over-pressurized. Under-pressurization can lead to shallowing and/or collapse on the anterior chamber. This causes 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 or intraocular pressure set 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

Aspiration flow rate is the volume of fluid flowing through the tubing at any given time. This is often reported in cubic centimeters per minute (cc/min). With a peristaltic pump, flow is determined by the speed of the pump. Increasing flow rate improves the attraction of particulate material to the phaco tip and speeds events in the anterior chamber.

Pump

The part of the phaco machine which moves fluid through the aspiration tubing. The pump settings control the aspiration flow rate. The two main types of pumps in phaco systems are flow pumps (peristaltic) and vacuum pumps (venturi). There are also hybrid systems that incorporate both types.

Flow Pump

The primary example of the flow pump is the peristaltic pump. These pumps allow for independent control of aspiration rate and vacuum. Peristaltic pumps control the outflow of fluid through the movement of a series of rollers over flexible tubing. The compression of the tubing moves fluid through the tubing and creates a vacuum. Vacuum is controlled by the amount of time the rollers run over the tubing when occluded, thus holding material to the tip of the phaco.  

Vacuum Pump

The primary example of the vacuum pump is the venturi pump. This pump type allows direct control of only vacuum level. The flow of liquid or compressed gas across an opening creates vacuum proportional to the rate of flow. Flow is a result of and dependent upon the vacuum level, and a preset vacuum level can be obtained without occlusion of the phaco tip.

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 

In regards to phacoemulsification, compliance describes the change in volume of tubing when subjected to negative pressure. Highly compliant tubing has a tendency to collapse on itself when subjected to negative pressure, while rigid, low compliance tubing does not have such a tendency to change shape.

Surge

Surge occurs during phacoemulsification when outflow exceeds inflow, and a sudden rush of fluid from the anterior chamber occurs post-occlusion. When the tip of the phaco needle is occluded, flow is instantly interrupted and negative pressure builds within the tubing. When the occlusion clears, aspiration rate builds. If the tubing has high compliance and collapses during the subjected negative pressure, it rapidly returns to its original shape post-occlusion, thus increasing vacuum and leading to a rapid exit of fluid from the anterior chamber. Higher compliance tubing leads to a greater surge amplitude following occlusion break. Low compliance tubing, decreased flow and vacuum, micro-pulse phaco, venting and variable rise time all can decrease surge.

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 on (no aspiration, no ultrasound)
  • Position 2  irrigation on, aspiration on, (no ultrasound)
  • Position 3  irrigation on, aspiration on, ultrasound on

UltraSound Power

Fixed Machine setting

Each parameter during phacoemulsification surgery (vacuum, aspiration, ultrasound) can be selected as fixed or variable. If a parameter is fixed, the preset value for that parameter is activated when the foot pedal enters the appropriate position. The parameter remains constant during the excursion of the foot pedal in that position.

Surgeon Controlled or Variable Machine setting

Variable settings increase linearly through continued excursion of the foot pedal in a given position.

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

Power generation at the phaco tip is dependent on the frequency of the needle movement and stroke length.

Frequency

The speed of the needle movement. It is determined by the manufacturer of the machine. Most 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 2-6 mils (thousandths of an inch). 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. Selection of the appropriate needle depends on lens removal technique. 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. The phaco needle, moving at ultrasonic speeds, creates 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. This implosion 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 are available that maximize features of both types. 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 effectively emulsify the nucleus, the surgeon needs to balance the attraction and repulsion forces at the phaco tip. The aspiration flow pulls material towards the phaco tip and the ultrasound movement of the phaco tip pushes material away. Vacuum holds nuclear fragments on the phaco tip. Surgeons can optimize their settings to improve this balance.

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. Adjustments to duty cycle, which is the combination of phaco and rest time, and power modulation with pulsed, burst and hyperpulsed phaco power can optimize efficiency during surgery.

Ultrasound Power Settings to limit energy damage

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

Examples of power settings include:

For Sculpting:

Linear control instead of fixed control allows more control over the amount of energy being applied.

Pulse instead of continuous phaco reduces the energy applied.

For Chopping: In order to chop it is necessary to first make a purchase of the nucleus by applying ultrasound power, embedding the phaco needle into the nucleus, and then building vacuum in position 2, prior to chopping the lens into fragments. Utilizing burst mode phaco power, with a burst width from 40-80ms, followed by fixed vacuum is the most efficient for this method of nuclear disassembly.

For Quadrant removal: Pulse phaco is the most efficient method for removal of disassembled nuclear material. This provides "on" time with phaco power applied to the nucleus for emulsification, followed by "off" time to allow aspiration of emulsate and to hold the nucleus to the tip. Power can be efficiently titrated using linear control.



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.