WHAT A REVELATION IT IS FOR
PATIENTS TO HAVE A SMALL SURGICAL
PROCEDURE WITH GREAT EFFECT IN
THE MAJORITY, ALLOWING THEM TO
DISPENSE WITH (OR AT LEAST REDUCE
THE BURDEN OF) DAILY EYE DROP
ADMINISTRATION.
These days, many disciplines of surgery
access the target areas of the body
where surgery is required, using smaller
and smaller incisions than in the past. Modern
day access to the remote control of instruments
and the availability of minute video cameras, has
allowed real time viewing access to the tissues
under scrutiny. Just these factors have permitted
the reduction in incision size which have, in
turn, permitted a significant reduction in healing
time and discomfort for patients.
Ophthalmology employs a number of terms to
denote these incision size changes. Terms vary
from ‘small’ to ‘mini’ then on to ‘micro’ in so
many of the different fields in eye surgery.
Let’s start with a common one:
CATARACT SURGERY
There was a time when eye surgeons had to
cut half way across the base of the cornea to
get a cataract out. The corneas of most eyes are
11-12mm in diameter – the mathematics of that
suggest that the circumference of the coloured
portion of the eye is around 36mm. Half the
corneal base therefore is about 17mm. We
needed an incision this long to access the lens
of the eye to remove the cataract in days gone
by. If the whole lens was removed we called this
“intra-capsular” cataract surgery. We learnt very
quickly that we could separate out the different
components or contents of the lens of the eye
into nucleus and cortex, and remove the smaller
nucleus by expression (squeezing out) and the
cortex by aspiration (suction). Removing the
lens of the eye in this manner is called “extra-capsular”
cataract surgery and can be executed
through incisions of between 8-10mm in the
peripheral cornea.
An instrument called a ‘phacoemulsifier’
then came along allowing emulsification or
dissolution of the hard nucleus of the lens of the
eye. This instrument could be inserted into the
eye through a 2.5mm incision allowing further
reduction in incision size as the aspiration
component of the operation could be done
through the same incision size.
But, at this point in the advancement of
cataract surgery, we knew that we could enhance
outcomes of cataract surgery by inserting an
appropriate intra-ocular lens into the capsular
bag (the remainder of the lens of the eye after
removal of the nucleus and cortex). These lenses
needed at least a 6mm size incision for insertion
into the eye. Development of lens technology
was slower than the cataract removal technology
and some years elapsed before we could fold the
lens into a shape that could enter the eye through
a smaller incision and then reconstitute correctly
once inside the eye to provide an accurate
enough focus to keep an eye spectacle free.
As expected lens technology progressed to
the point where we could eventually match the
incision size required for the phacoemulsifier
and pressure was again on the phacoemulsifying
instrument to be made thinner, reaching the
levels of today at 1.8 to 2.0mm .
With this level of incision size achieved, the
profession coined the term “Micro-Incision
Cataract Surgery” – the acronym MICS.
At this current state of the art of MICS, both
the phacoemulsifier and the cortical aspiration
instrument can fit though the incision, as well
as the new intra-ocular lens, and still unfold into
optimal form inside the eye.
What is also important to know is that the
control of astigmatism (one of the optical
aberrations of the eye) can also be partially
corrected by the siting and size of the incisions in
the cornea. The corneal tissues are actually under
tension in a normal eye, so when an incision is
made, the adjacent tissues spring outward causing
astigmatism if placed in the wrong place, but
correcting astigmatism if placed in an optimal
position. These corrective incisions are usually
between 2.0mm and 3.5mm in size and to be
effective are placed in a certain radial orientation
to reduce astigmatism. The same incisions are
then used for the phacoemulsification, cortical
washout and lens insertion.
The average incision today is 2.2mm in size
for most cataract surgery, hence the topic for
the discussion above. Quite an astounding
achievement.
WHAT ABOUT REFRACTIVE
SURGERY?
The aforementioned process is now accurate
enough, with the appropriate intra-ocular lenses,
to be used as a refractive surgical procedure.
However, the place that we associate with
refractive surgery is usually the cornea. The
procedure most often thought of as refractive
surgery is LASIK. This procedure has a flap
of cornea that is lifted with a retaining hinge
of approximately 5mm in length. If we do our
mathematics again and look at a flap of 8.5mm
in diameter, we are making an incision into the
38 | Pindara Magazine ISSUE 13 | 2018