Abstract
Phacoemulsification is now the most common method used in
cataract surgery in the United Kingdom, and brings significant advantages to
the great majority of patients. New surgical techniques have extended the range
of clinical problems that can be dealt with using phacoemulsification surgery,
and new types of instrumentation and machinery have made this technique a safe
and reliable procedure.
Keywords
Phocoemulsification, cataract surgery, intra-ocular lens, complications.
Introduction
Phacoemulsification refers to a method of cataract surgery in which energy delivered by a probe inserted through a small self-sealing wound is used to break up the lens and allow its removal as small fragments. The probe is usually an ultrasonic probe, although laser probes also exist. Prior to the development of phacoemulsification the technique routinely performed was extracapsular cataract extraction (ECCE) which involved manual expression of the nucleus of the lens in one piece via a much larger incision. The increased availability of improved phacoemulsification machines from 1990 meant that phacoemulsification surgery became available to a broader surgical audience.
Advantages of phaco
Phacoemulsification allows surgery to be carried out under a closed system, using a small wound with reliable intraocular lens (IOL) implant positioning within the bag. The small size of the instruments used and the fact that the IOLs can be folded for insertion means a small self-sealing tunnel wound, typically 2.5mm to 3.5mm wide, can be used. These wounds are very much stronger than the wounds used in manual expression surgery and are very resistant to rupture with blunt injury. The small wounds also cause very little induced surgical astigmatism (typically less than 0.75 D) as they have a minimal effect on the shape of the cornea and do not require sutures.
The instruments used during the procedure fit in the small wound precisely thus preventing fluid movement in or out of the eye via the wound. This "closed system", with no imbalance between the amount of fluid entering or leaving the eye, maintains the volume and shape of the eye and the intraocular pressure at near physiologic levels throughout surgery. Tissues within the eye, including iris and posterior capsule, do not move excessively, aiding post-surgical recovery, since the stimulus for intraocular inflammation is reduced. In comparison, extracapsular techniques do not maintain positive intraocular pressure throughout surgery and there can be significant movement of fluid from inside the eye to the external surface of the eye and back again which may allow greater opportunity for intraocular contamination. The low intraocular pressure may increase blood aqueous barrier breakdown and post-surgical inflammation. Another major benefit with the closed system is that whenever the instruments are removed from the eye the wound would self-seal. The ocular volume and intraocular pressure are stabilised, which confers an important element of safety in cases of sudden patient movement or coughing during the operation or in rare cases of suprachoroidal haemorrhage.
A further advantage of phaco emulsification is the capsulorrhexis (the circular hole created by a continuous tear in the anterior capsule) which is very stable. This preserves the capsule as a single intact sheet forming a stable bag and controlled environment during surgery. An intraocular lens can be implanted into this stable bag without the risk of decentration or dislocation out of the bag into the sulcus.
Finally, after surgery the quick recovery allows a rapid return to normal
activities for the patient.
Disadvantages of phaco
In comparison to manual expression techniques of cataract extraction phacoemulsification is expensive with costly machines and high running costs. The surgery itself requires a different range of surgical skills and manoeuvres to be learnt by the surgeon and there is a "learning curve" during which complications and adverse surgical events are higher than usual. In addition phacoemulsification machines are complex instruments, with many variables in the settings and require great care in their preparation for use.
Aside from the complications that are common to any type of cataract extraction, phacoemulsification has specific complications. For example, if excessive levels of energy are used, a corneal wound burn or corneal endothelial damage may result, and failure to completely remove the viscoelastic used in the procedure may result in raised intraocular pressure.
How does a phacoemulsification machine work?
Phacoemulsification machines maintain eye volume and pressure during surgery
by having an irrigation system that infuses fluid into the
anterior chamber and an aspiration system that simultaneously
removes it in a controlled fashion. They also have a system
for delivering ultrasound energy into the cataract in order
to break up the lens material into smaller fragments that
can then be removed with the aspiration system. Energy is
delivered via the phacoemulsification hand-piece which contains
piezoelectric crystals that generate ultrasound energy in
the range of 28,000 to 60,000 Hz. The energy travels down
the centre of a metal probe and material is aspirated back
up the same probe. Fluid (balanced salt solution) is constantly
infused into the eye via a plastic sheath around the metal
probe. The surgeon controls the irrigation, aspiration and
delivery of ultrasound energy by manipulation of a foot pedal.
In order that energy is not wasted or delivered unnecessarily
into the eye, there have been recent developments in computer
software to tailor the delivery of ultrasound energy into
the cataract. For instance ultrasound energy can be delivered
in very short bursts or pulses of varying levels, which allows
the energy to be delivered in a very focused way. Some machines
will also automatically detect when material is completely
blocking the ultrasound probe and the computer will automatically
switch to a different setting allowing high amounts of energy
to be delivered. Automated systems make surgery simpler, more
efficient and safer, since they reduce the scope for human
error (Figure 1).
The phacoemulsification procedure
The basic components of phacoemulsification surgery are as follows:
Small self-sealing wound
These wounds can be in the sclera, at the limbus or in clear cornea and are
usually placed superiorly or temporally. Rather than the incision directly entering
the eye it is formed as a tunnel. The fundamental principle of the self-sealing
wound is a "three step" approach which involves an initial incision perpendicular
to the surface of the eye, followed by a change in direction to dissect along
the plane of the scleral/corneal tissue towards the central cornea for 2mm to
3.5mm and then finally entering the anterior chamber through the posterior cornea.
The width of the incision is usually governed by the IOL that is to be used.
The length is dependent on the width, the wound being most stable if the length
of the tunnel is equal to or greater than the width of the tunnel. This configuration
of the wound allows the physiologic intraocular pressure to push the internal
lip of the wound against the external lip and so compress the tunnel and seal
the wound. For this reason these wounds do not usually need suturing although
occasionally sutures are required, e.g. if the tunnel is short or unusually
broad or if a corneal wound burn has occurred resulting in distorted tissues
around the wound. Scleral-based wounds allow for a longer tunnel and produce
less astigmatism and endothelial loss than a clear corneal wound. However, they
are slightly more time consuming to do and require some form of conjunctival
dissection or exposure, which can reduce the success rate of glaucoma filtering
surgery should this be required at a later date.
Capsulorrhexis
The circular hole in the anterior capsule is formed by initially breaching
the anterior capsule with a sharp instrument (usually a modified needle) and
then continuing the tear to form a completed circular hole in the capsule, utilising
the red reflex from the fundus to visualise the edge of the tear during the
procedure. The depth of the anterior chamber is maintained throughout by the
use of a viscoelastic material injected into the anterior chamber. It can be
a difficult procedure in patients with white, mature cataracts because there
is no red reflex to aid visualisation. This problem has been solved by staining
the anterior capsule with a blue dye called Vision Blue (methylene blue dye)
prior to beginning the capsulorrhexis. It can also be difficult to carry out
a capsulorrhexis in patients with any fibrosis of the anterior capsule (e.g.
mature cataracts or in eyes with previous uveitis). Subluxed or dislocated cataracts
may also present significant difficulties due to the instability of the lens
during the procedure.
Hydrodissection
Dr Howard Fine introduced this simple step as a method of simplifying the subsequent
steps in phacoemulsification and making the process safer. A gentle stream of
fluid (balanced salt solution) is injected underneath the anterior capsule in
at least three or four different directions throughout the peripheral lens to
separate the cortical fibres from the capsule. This frees up the lens so that
it can be rotated easily within the capsule bag aiding nucleus and cortical
removal. This process also removes peripheral cortical fibres and reduces the
incidence of capsule opacity after cataract surgery.
Nucleus removal
This step is often the most challenging and difficult part of the entire procedure
and many alternative techniques have been described. Howard Gimbel introduced
the 'divide-and-conquer' method that has been widely adopted. This method involves
repeated grooving with the phaco-probe in order to create a deep channel approximately
95% of the thickness of the nucleus. The nucleus is then rotated round and another
groove sculpted to form a "Maltese Cross" pattern. Two instruments are then
placed at the bottom of the grooves and spread sideways to break the nucleus
into pieces. The resulting four quadrants can be more easily manipulated in
the eye and are individually fragmented and removed. Although one of the more
simple techniques to perform, a significant disadvantage is that with dense
cataracts it can be time consuming and requires high levels of ultrasound energy
very close to the posterior capsule which risks posterior capsule rupture, either
directly from instrument touch or indirectly from transmitted energy. The other
popular technique is the 'chop' method, initially introduced by Dr Nagahara,
with many surgeons subsequently adding refinements. The nucleus is gripped with
the phaco-probe and a second instrument is used to mechanically split or chop
the nucleus into pieces. The amount of ultrasound energy delivered in to the
eye is reduced, since much of the fragmentation of the lens is performed mechanically
rather than by ultrasound energy. It is therefore the technique of choice for
dense cataracts.
Removal of lens cortex and lens epithelial cells
A probe that, like the phaco probe, provides irrigation, but does not deliver
ultrasound energy, removes the thin layer of cortex that remains after nucleus
removal. This process allows controlled removal of cortical fibres with much
less risk of damaging the very delicate posterior capsule. The same probe is
also used to remove lens epithelial cells from the inside of the capsule, a
procedure that reduces the rate of posterior capsule opacity.
Insertion of the intraocular lens implant
The capsular bag is then expanded with a viscoelastic agent
(a clear gel substance which gently maintains space inside
the eye) allowing the safe insertion of an IOL. Acrylic or
silicone lenses can be folded and inserted directly or via
an injection system. IOLs unfold in the capsular bag, with
the haptics stabilising the lens within the bag (Figure
2).
Final steps
The viscoelastic agent (usually a hyaluronic acid polymer) is removed from the eye and the stability of the wound is checked. An injection of antibiotic (either subconjunctivally, or into the anterior chamber) is administered to reduce the risk of endophthalmitis. According to surgeon preference subconjunctival steroid may also be given to reduce post-operative inflammation.
Difficulties that may be encountered
Small pupils
Good access to the cataract is required for phacoemulsification
- it is difficult to carry out the procedure safely through
a very small pupil. It is not uncommon to encounter patients
with small, eccentric or abnormal pupils, either from prolonged
meiotic treatment, previous inflammation causing posterior
synecheiae, previous trauma or surgery, diabetes or pseudoexfoliation.
In these situations the pupil will not enlarge with the use
of intensive eye drops and even after posterior synecheiae
are dissected free from the anterior capsule the pupil may
remain very small. Solutions to this problem have included
physically stretching the pupil with instruments, making small
cuts in the iris sphincter muscle (sphincterotomies) or the
use of iris retractors. These are flexible hooks that are
placed through tiny incisions (under 1mm) in the peripheral
cornea and which hook onto the pupil margin to stretch and
hold the pupil open (Figure
3).
Unstable lenses
Some eyes have unstable crystalline lenses due to missing or weakened zonules
from previous blunt trauma, pseudoexfoliation, Marfan’s syndrome
or high myopia. In these cases a capsule tension ring (Figure
4) may be employed. This flexible hoop is placed through
the capsulorrhexis into the capsular bag, expanding the bag
in the region of the equator and stabilising it. It can be
inserted before phacoemulsification is begun or at any time
during the procedure should instability become evident. Not
only does the increased stability of the lens facilitate the
operative procedure, it helps prevent decentration or dislocation
of the lens implant and capsular phimosis (shrinkage of the
capsulorrhexis after surgery). If the capsular bag is deemed
to be very unstable, with a high possibility of the IOL dislocating
then a tension ring with two little loops 180 degrees apart
may be used. This type of ring, devised by Robert Cionni,
allows sutures to be passed through the loops and on through
sclera in order to stabilise the capsular bag (by suturing
the ring to the sclera).
High refractive errors
Foldable IOLs are available in a range of +10 to +30D. Some manufacturers have
extended the range of foldable lenses down to zero for myopic patients, but
in the case of high hypermetropia two lenses may need to be placed in the eye,
one in front of the other (piggyback lenses). As an example, an 18D implant
can be placed with another 18D implant on top of it to yield approximately 36D
correction. However, this approach is not without problems and it is now recognised
that an opaque membrane of proliferating tissue (interlenticular opacification)
may occur between the two IOLs when they are both located in the capsule bag
This can lead to a hypermetropic shift in refraction. The risk of this complication
can be minimised by careful choice of IOL type and by placing the second implant
in the sulcus.
Previous intraocular surgery
Previous surgery alters the environment in the eye and can make phacoemulsification
challenging. For instance, in an eye with a previous vitrectomy there is reduced
support for the lens during surgery, and an increased risk that the posterior
capsule has been previously breached.
Other considerations
Astigmatism
Pre-existing corneal astigmatism can be reduced in cataract
surgery by using toric IOLs (Figure
5) which currently only come in 2 powers (+2 and +3.5
DC) and because of their design, can sometimes rotate off
axis after surgery. An alternative method to reduce astigmatism
is to use arcuate keratotomies which are arc shaped relaxing
incisions placed in the cornea at the time of surgery, typically
to a depth of 550 to 610 µ.
Accommodation
With the removal of the crystalline lens any remaining accommodative
power is lost. In order to address this problem, multifocal
lenses with graduated lens power have been developed (Figure
6). These have the disadvantage of reducing contrast sensitivity.
Some newer lens designs may have the ability to use physiologic
accommodative forces.
Prevention of posterior capsular opacity
Posterior capsule opacity (PCO) post cataract surgery is the second most common condition requiring treatment in the USA, after cataract surgery itself. It occurs as a result of lens epithelial cell proliferation, resulting in fibrous membranes that grow across the posterior capsule and disturb vision. It may present within months to years after cataract surgery. The PCO rate over a five-year period is between 30 to 50% in patients who receive rigid PMMA IOLs, but has reduced to approximately 20% with most foldable IOLs and now appears to be less than 10% with several newer types of foldable IOL (acrylic lenses or new generation silicone lenses). Using lenses with a square or truncated edge to the optic may reduce the rate further.
Despite advances in IOL design (see Percival, this issue) probably the most
important steps for reducing PCO are adequate removal of lens epithelial cells
and lens cortex at the time of surgery and ensuring the capsulorrhexis is a
little smaller than the size of the lens optic (so that the anterior capsule
overlaps onto the edge of the lens optic).
What can go wrong?
The most common intra-operative complications are posterior capsule rupture or vitreous loss. The National Cataract Surgery Survey demonstrated that these two complications occur in 4.4% per cent of all cataract operations carried out in the United Kingdom. These complications can have serious adverse effects. Capsule rupture without vitreous loss can result in an increased incidence of cystoid macular oedema (CMO) and late onset IOL decentration. Vitreous loss may result in CMO, retinal detachment, intraocular infection or suprachoroidal haemorrhage any one of which can be sight threatening.
Conclusion
Phacoemulsification is a predictable, safe procedure giving excellent visual results and allowing quick rehabilitation of the patient. It has many advantages over extracapsular cataract extraction, in particular a stable wound and IOL, minimal induced astigmatism and rapid recovery. It is also a more controlled procedure to perform due to the benefits of closed system surgery. However, there is a recognised complication rate, the surgery may be challenging in eyes with co-existent ocular disease and it is not necessarily the best technique for every patient with cataract.
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