Phacoemulsification is a cataract surgery method in which the internal lens of the eye which has developed a cataract is emulsified with the tip of an ultrasonic handpiece and aspirated from the eye. Aspirated fluids are replaced with irrigation of balanced salt solution to maintain the volume of the anterior chamber during the procedure. This procedure minimises the incision size and reduces the recovery time and risk of surgery-induced astigmatism.
It is best suited to relatively soft cataracts, where the ultrasonic energy required is moderate, and insertion of foldable intraocular prosthetic lenses, which take advantage of the small incision possible. It is the most common procedure for cataract removal in the developed world, with an excellent prognosis in uncomplicated cases.
Etymology The term originated from phaco- (Greek phako-, comb. form of phakós, lentil; see lens) + emulsification.
Contraindications
The same general contraindications for cataract surgery apply. Specific contraindications for phacoemulsification include hard or dense cataracts where phacoemulsification is likely to cause permanent damage to the cornea.
Mechanism
The phacoemulsification system comprises three sub-systems: Ultrasound, aspiration, and irrigation.
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 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 kHz is considered a good compromise and is in common use.
Most handpieces use piezoelectric crystals and the rest use magnetostrictive materials to generate the vibration.
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.
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 angles. 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.
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, and the third proposes that the tip causes 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.
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.
The pump of the phacoemulsification system can be a 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. 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.
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.
Preparation and precautions
Anaesthesia Proper anesthesia is essential for ocular surgery. A facial nerve block may occasionally be performed to reduce lid squeezing. General anesthesia is recommended for children, traumatic eye injuries with cataract, for very apprehensive or uncooperative patients and animals. Cardiovascular monitoring is preferable in local anesthesia and is mandatory with general anesthesia.
Either topical, sub-tenon, peribulbar, or retrobulbar local anaesthesia is used, usually causing the patient little or no discomfort. Topical anaesthetics are most commonly used; these are placed on the globe of the eye as eyedrops before surgery or in the globe during surgery. Local-anaesthetic injection techniques include sub-conjunctival injections and injections behind the globe (retrobulbar block) to block regional nerves and prevent eye movement. Intravenous sedation may be combined with the local anaesthetic.
Site preparation Proper sterile precautions are taken to prepare the area for surgery, including use of antiseptics like povidone-iodine. Sterile drapes, gowns and gloves are employed. A plastic sheet with a receptacle helps collect the fluids during phacoemulsification. An eye speculum is inserted to keep the eyelids open. The operation site is prepared by disinfection of the area around the eye and any other exposed area of the face, and exposure of the eyeball using an eyelid speculum;
Surgical technique
The surgical procedure is typically performed under an operating microscope.
Incision Before the phacoemulsification can be performed, one or more incisions are made in the eye to allow access for the surgical instruments. Entry into the eye is made through a minimal incision; the incision for cataract surgery has evolved along with the techniques for cataract removal and IOL placement. In phacoemulsification, the size depends on the requirements for IOL insertion. A more-posterior incision simplifies wound closure and decreases induced astigmatism but is more likely to damage blood vessels. With foldable IOLs, it is possible to use incisions smaller than . The shape, position, and size of the incision affect the capacity for self sealing, the tendency to induce astigmatism, and the surgeon's ability to manoeuvre instruments through the opening. One or two smaller side-port incisions (paracentesis) at 60-to-90 degrees from the main incision augment access to the anterior chamber with additional instruments.
Coaxial Coaxial phacoemulsification uses a single probe to irrigate, emulsify and aspirate, which is operated through a single incision.
Bimanual Bimanual phacoemulsification uses one probe to emulsify and aspirate, and a second that is only used for irrigation. This technique was first used when more modern phacoemulsification handpieces made much less heat and therefore could be used without an irrigating sleeve. The sleeveless handpiece could be put in the eye with a smaller incision, potentially reducing induced astigmatism. The downside to the procedure when first adopted was that there were no IOLs available in the US which would fit through the smaller incision.
Maintaining the eye shape Ophthalmic viscosurgical devices (OVDs), also known as viscoelastics, are injected into the anterior chamber to support, stabilize, and protect the eyeball to help maintain eye shape and volume during the procedure, and to distend the lens capsule during IOL implantation.
OVDs are used to protect the corneal endothelium from mechanical trauma and to maintain volume and form of the intraocular space during an open incision. The OVD is introduced into the space by syringe through a cannula.
Intraocular pressure is maintained by the irrigation with BSS, which is either done through the phaco handpiece, or through a separate cannula. The tendency is for a single phaco handpiece to be used, containing the ultrasonic tip, irrigation nozzle and aspiration intake clustered at the tip. The tip is inserted through the OVD, which reduces the loss by outflow at the incision, and this is further limited by small clearance around the tip where it passes through the cornea. Pressure is controlled by elevation of the saline bag above the level of the eye.
Capsulorhexis
To remove a cataract by extracapsular techniques, the capsule of the lens must be opened. In earlier intracapsular cataract extraction, the whole lens and capsule was removed at the same time. This was done to prevent the inflammatory response to leftover lens material. Since it was all removed en-bloc, there was no residual lens material. With effective aspiration practically all the material can be removed while leaving the posterior capsule membrane intact. This provides a barrier between the front and back chambers of the eye, and prevents the vitreous from moving forwards. It also provides the intraocular lens implant with the ideal place to be located in the eye, away from contact with other structures yet securely held in place.
The surgeon removes the anterior face of the capsule that contains the lens inside the eye, tearing a circular opening in the front surface of the lens capsule to access the lens within. A continuous curvilinear capsulorhexis is usually used to create a round, smooth-edged opening through which the lens nucleus can be removed or emulsified and the intraocular lens implant inserted. When done correctly, a CCC does not have any edge notches, and forces applied to the capsule during surgery are better distributed and less likely to result in a tear.
The usual method is to use a bent needle to begin a tear in the capsule, and then guide the edge of the tear around the anterior surface with either the same needle or Utratas forceps. There are advantages and disadvantages to both approaches, and most surgeons will use both instruments as the situation requires.