Editing Phacoemulsification (section)

==Mechanism==
<!-- How it actually works.-->
The phacoemulsification system comprises three sub-systems: Ultrasound, aspiration, and irrigation.<ref name="Yow and Basti 1997" />

===Ultrasound===
The ultrasound component is used to break the lens down into particles small enough to be aspirated through the suction passages around the tip, which allows a very small incision for access. The incision is small enough that sutures are not needed for closure, and very little astigmatism is caused by healing of the wound in the cornea.

The phacoemulsification handpiece has a tip which vibrates longitudinally at a frequency in the range of 27 to 60&nbsp;kHz, with a stroke length of 60 to 150 micrometres. Power is adjustable by the operator as a percentage of full power, and indicates a variation in nominal stroke length. Actual stroke length may vary slightly depending on the density of the material it contacts, though some instruments use feedback to maintain nominal stroke by adjusting current, voltage or frequency. Nominal frequency is not adjustable. Both efficiency and heat generation are increased with higher frequency, and 40&nbsp;kHz is considered a good compromise and is in common use.<ref name="Yow and Basti 1997" />

Most handpieces use [[piezoelectric crystal]]s and the rest use [[Magnetostriction|magnetostrictive material]]s to generate the vibration.<ref name="Yow and Basti 1997" />

The handpiece is hollow and usually accommodates an aspiration line, and the vibratory transducer components are sealed into it. The handpiece is designed and constructed to be autoclaved between uses.<ref name="Yow and Basti 1997" />

The phaco tip is available in a variety of configurations, including a selection of tip angles to suit lens removal technique. Standard tip angles range between straight and 60 degrees, and more complex tips may have [[compound angle]]s. The end of the tip may be round, ellipsoid, bent or flared. A variety of designs are intended to enhance cooling and irrigation, and to prevent burns.<ref name="Yow and Basti 1997" />

There are three hypothesised mechanisms of how the nuclear material is emulsified. One proposes that the tip acts as a chisel and removes material on the forward stroke, another proposes that ultrasonic energy is somehow involved,{{clarify|how is "ultrasonic energy" hypothetically "involved"?|date=May 2023}} and the third proposes that the tip causes [[Cavitation|microcavitation]] bubbles on the retraction stroke, which collapse to exert high pressures on the materials very close to the bubble, which cause them to disintegrate.<ref name="Yow and Basti 1997" />

===Aspiration===
The aspiration system is used to remove the emulsified lens tissue as it is broken down by the tip. This may be done through the handpiece, with the inlet orifice around the vibrating tip. or through a separate aspiration tip, inserted through a smaller incision.<ref name="Yow and Basti 1997" />

The pump of the phacoemulsification system can be a [[Peristaltic pump|peristaltic type]] or a vacuum transfer type. In peristaltic pumps aspiration flow rate and vacuum are independent. Vacuum is the suction force which holds cataract nuclear fragments against the phaco tip so that they can be emulsified, and draws the emulsion into the tip.<ref name="Yow and Basti 1997" /> Vacuum is the relative low pressure generated by the pump removing liquids and gas from the suction side, and the pressure difference between the vacuum pump reservoir and the ambient pressure at the inlet to the tip of the handpiece draws fluids through the aspiration ducting. When the inlet is occluded by solid material, such as a cataract fragment, the pressure difference holds the solid in contact with the tip while the ultrasonic vibration breaks up the solid to fragments small enough to pass into the aspiration ducting and be carried away by a current of ambient fluid, which must be replaced as fast as it is removed, to retain internal pressure and shape of the eye.

===Irrigation===
The three purposes of irrigation are to maintain intraocular pressure, carry lens particles out of the eye in the aspiration system,and to cool the phaco handpiece. Gravity feed of 650mm water column (75.5mm Hg) is typical. At this supply pressure, fluid enters the anterior chamber at a rate proportional to the rate at which it leaves due to aspiration and leakage. The pressure head is adjusted to suit anatomical variations and the health of the eye. Complications are less likely if the volume and pressure of the globe are maintained during surgery. This requires a balance between fluid input and output, which is a balance between irrigation, aspiration and leakage. Repeated partial collapses of the anterior chamber, and iris fluttering during removal of the nucleus are signs of inadequate fluid supply, which can be adjusted by changing the height of the gravity feed supply bottle. A height of 650mm above the eye is usually enough to compensate outflow almost immediately. During emulsification, the abrupt variations in flow at the start and end of emulsification of each fragment can cause fluctuations in volume and pressure, which can be corrected by control of the aspiration foot pedal.<ref name="Yow and Basti 1997" />

Sleeves for the phaco tip are standard accessories to insulate the wound surface from heat generated by the ultrasonic energy, and provide a route for irrigation.<ref name="Yow and Basti 1997" />