Phacodynamics is the collective term for the mechanical principles underlying phacoemulsification and aspiration (PEA). It encompasses both fluidics and ultrasonic power modulation.
Phacoemulsification was invented by Kelman in 1967, and subsequently spread rapidly due to advances in equipment and techniques. Today, nearly all cataract surgeries are performed using this method, and a precise understanding of phacodynamics by the surgeon is a prerequisite for safe and efficient surgery.
The main parameters that constitute phacodynamics include the following:
Proper machine settings improve surgical safety and efficiency. Conversely, inappropriate settings can lead to complications such as anterior chamber collapse, posterior capsule rupture, and corneal endothelial damage. Regardless of skill level, understanding the basic principles is the foundation of safe surgery for all surgeons.
Irrigation fluid (BSS: Balanced Salt Solution) is supplied into the anterior chamber by gravity from the irrigation bottle. Irrigation pressure is proportional to bottle height and can be approximated by the following formula.
Irrigation pressure (mmHg) ≈ Bottle height (cm) × 0.7 Example: At a bottle height of 100 cm, approximately 70 mmHg; at 80 cm, approximately 56 mmHg; at 70 cm, approximately 49 mmHg
Placing the bottle higher than the patient’s eye creates a pressure gradient, causing irrigation fluid to flow into the anterior chamber. Raising the bottle height increases intraocular pressure and deepens the anterior chamber. However, caution is needed in patients with glaucoma or arteriosclerosis due to the risk of elevated intraocular pressure.
In recent years, in addition to gravity systems, forced infusion systems such as VGFI (Vented Gas Forced Infusion) and soft bag pressurization have been developed, allowing for more stable control of anterior chamber pressure fluctuations.
Outflow is primarily generated by aspiration via a pump. Increasing the aspiration flow rate increases outflow and accelerates movement within the anterior chamber. Another outflow pathway is leakage through the incision and side ports.
When inflow and outflow are equal, pressure balance within the anterior chamber is maintained, ensuring a stable anterior chamber.
Effects of Fluid Imbalance:
That is correct. Raising the bottle height increases the intraocular pressure before and after surge, but the amplitude of pressure fluctuation does not change. To suppress surge, reducing aspiration pressure or using low-compliance tubing is effective, rather than adjusting bottle height.
Aspiration flow rate is the volume of fluid moving through the tip opening per unit time (mL/min). In peristaltic pumps, the rotation speed can be set directly.
Aspiration pressure is the negative pressure (mmHg) that determines the hold ability to retain a nuclear fragment at the tip when the tip is occluded.
Peristaltic Type
Principle: Rollers squeeze the tubing to generate negative pressure.
Advantage: Aspiration pressure and aspiration flow rate can be set independently.
Disadvantages: Slow rise of aspiration pressure. Designed with emphasis on safety, suitable for beginners to intermediate users.
Venturi type
Principle: Airflow generates negative pressure inside the cassette (Bernoulli’s principle).
Advantages: Fast rise of aspiration pressure, high followability.
Disadvantages: Fine control is difficult, and aspiration flow rate cannot be set independently (approximately half of the set aspiration pressure becomes the flow rate). Suitable for advanced users.
In recent years, the disadvantages of both pumps have been improved, and there are hybrid systems that can switch between the two methods in a single unit.
Surge is a phenomenon in which, immediately after the release of a chip occlusion by a nuclear fragment, the accumulated negative pressure is suddenly released, causing rapid outflow of fluid from the anterior chamber and temporary instability of the anterior chamber.
In a study using enucleated human eyes by Georgescu et al., the fluctuation distance of the anterior chamber during surge was reported to be 0.04 to 2 mm.
When surge occurs, the posterior capsule or iris is drawn toward the chip, risking posterior capsule rupture or iris damage.
Measures to reduce surge:
Ultrasound frequency is defined as 20 kHz or higher, and phacoemulsification devices typically use around 40 kHz (approximately 28.5–40 kHz). The fragmentation power of the tip is based on the following two mechanisms.
Jackhammer effect
Mechanism: Mechanical impact where the tip physically collides with the lens nucleus.
Characteristics: As the distance to the target increases, acceleration rises and power increases. This is the main fragmentation mechanism of longitudinal vibration.
Cavitation effect
Mechanism: A rapid pressure drop during tip retraction generates microbubbles, which implode upon forward movement, releasing energy.
Characteristics: Implosion generates temperatures of approximately 13,000°F (7,200°C) and shock waves of 75,000 psi. It may also cause tissue damage.
Ultrasound power is expressed as the stroke length of the tip, with maximum amplitude set as 100%. Increasing power enhances fragmentation but also increases heat generation and the risk of wound burns. Larger amplitude also increases the repulsive force on the nucleus (chattering).
In addition to conventional longitudinal vibration, a method using torsional vibration has been developed. Because the nucleus is shaved by both forward and backward movements, heat generation is low and fragmentation efficiency is high. By combining traditional and torsional modes and adding pulse settings, safe treatment of even hard nuclei is possible.
Characteristics vary depending on the bevel angle (0–60 degrees) at the tip.
| Bevel Angle | Occlusiveness | Use |
|---|---|---|
| Large (45–60°) | Low (less likely to occlude) | Sculpting (grooving) |
| Small (0–15°) | High (prone to occlusion) | Chopping and fragment removal |
| Mode | Characteristics | Recommended Scenarios |
|---|---|---|
| Continuous | Continuous oscillation | Soft nuclei |
| Pulse | Repeated on/off | Heat control / hard nuclei |
| Burst | Short pulse oscillation followed by pause | Chopping technique |
Pulse mode has pauses between ultrasonic oscillations, which reduces chatter and enables efficient nuclear fragmentation while minimizing heat generation.
Grooving (Open Fragmentation):
Fragmentation after Nucleus Division (Occluded Fragmentation):
Foot Pedal Operation:
In bimanual phacoemulsification, separating the irrigation chopper and the sleeveless phaco probe enables closed-chamber phacoemulsification through a submillimeter incision. In difficult cases such as subluxated lenses, the irrigation port helps stabilize the capsule 1).
For beginners, low settings for aspiration flow, ultrasonic power, aspiration pressure, and irrigation pressure are recommended. This minimizes pressure differences in the anterior chamber and reduces risks such as posterior capsule rupture, iris laceration, and vitreous prolapse. Switching settings according to the situation is a shortcut to safe surgical learning.
In recent ultrasonic cataract surgery devices, feedback control systems using anterior chamber pressure sensors are being introduced. By sensing intraocular pressure in real time and automatically adjusting irrigation flow, it is expected to minimize pressure fluctuations and surges, reducing the risk of posterior capsule rupture.
As alternatives to gravity-based irrigation, forced irrigation methods such as VGFI (gas pressurization inside the irrigation bottle) and soft bag compression using a plate have been put into practical use. These can increase irrigation flow more quickly than gravity systems, contributing to a safer surgical environment with less fluctuation in anterior chamber pressure.
