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Keratoconus treatment. Intrastromal rings
Historically speaking, intrastromal rings were created to correct
refractive defects and they began to be researched by Dr. Barraquer
in the 1950’s. Since then many ophthalmogists have
continued to make contributions until reaching its present form.
1 or 2 semicircular segments of variable thickness
are implanted in the interior of the cornea - stroma. The various forms, thickness
and curvature of these intrastromal segments will modify corneal curvature.
They reduce the corneal astigmatism, reduce
coma aberrations and halt the advance of keratoconus.
Essentially, they avoid the progression of keratoconus and improve vision quality.
The main indications of
intrastromal rings are: Primary corneal ectasia
like keratoconus with an intolerance to contact lenses, progressive
keratoconus and pellucid marginal degeneration
Ectasias owing to postlasik or PRK treatments, irregular
astigmatisms after deep penetrating keratoplasty and
irregular astigmatisms after irregular radial keratectomy.
And treatments for post-traumatic irregular cornea.
The contraindications for intrastromal rings are:
acute keratoconus with K´s exceeding 65 diopters,
central clouding, lendothelial cells counts of less than 1000
cells per square millimetre and high expectations of
uncorrected emmetropia. The action mechanisms
of intrastromal rings are: moulding the cornea by means of an additive technique
which conserves corneal integrity, displacement of the corneal apex
towards the centre of the pupil, evening out of the corneal surface
which allows a reduction in aberrations and an improvement in tolerance
to contact lenses and allowing correction with glasses
and a reduction in the refractive error, improving visual acuity and corneal
stabilization, delaying or doing away with the need for a transplant.
There are a wide range of patterns for implanting the intrastromal rings
described in the SA.ANA classification for
implants in personalized fashion. Before starting
a keratoconus, a high astigmatism occurs which will be reflected in the
topography. It will also induce a high
coma aberration. Intrastromal rings are intended
to reduce the astigmatism and reduce the coma.
The first thing is to determine the astigmatism through its steepest axis
and the flattest. And determine the coma which will be identified through
the bisector, imaginary line that separates the steepest hemisphere
from the flattest hemisphere and the coma axis, its perpendicular.
The next thing to consider is, ¿what segment to
choose and where to implant it? We have to choose what
nomenclature we are going to use to determine it. We can do it by
measuring the steepest axis and the bisecting line of the coma. Or by measuring
the flattest axis and the coma axis. We are going to choose
the steepest axis and the bisecting line of the coma, since we think that it is more intuitive.
Next step is to identify the thinnest point
and classify it according to its distance to the pupillary centre:
Central from 0 to 1 mm, paracentral
from 1 to 2 mm and pericentral greater than 2 mm.
In this way we will know where to implant the ring.
At 5 mm in central ectasias and at 6 mm
in paracentral and pericentral ectasias.
We should know that 80% of keratoconus are
paracentral. There is a wide range of possibilities
for ring implants: asymmetric and axial,
symmetric and non-axial, symmetric and axial and symmetric
and non-axial. We will start explaining
central ectasias.The ectasia is central
when the steepest point is less than 1 mm from the pupillary axis.
They have a low coma and high spherical aberration. We have to
determine the astigmatism steepest axis. It is called
bow-tie pattern. We will carry out a
symmetric and axial treatment implanting two 5mm segments
facing the most curved axis which can be of
90 and 90 degrees, emmetropic, hyperopic or
pure cylinders, of 120 and 120 degrees, in myopias
between 2 and 6 diopters, and between 4 and 6 diopters of astigmatism
and of 150 and 150 degrees in myopias higher
that 6 diopters with an astigmatism under 6 diopters.
Another pattern is called ***
For this ectasia we will choose
an asymmetric and non axial treatment and a 5mm and 210 degrees
segment located inferior or inferior temporal depending on the coma,
and 5mm from the pupil axis.
Paracentral ectasias. We are looking at a paracentral
ectasia when the steepest point is 1 or 2 mm from the
pupillary axis. They will be coincident if the steepest topographic axis
matches the bisecting line of the coma, or if there is a difference
between 0 to 30 degrees. In a 43% of cases
it will be classified as a croissant pattern.
In these cases we will carry out an asymmetric
and axial treament, with a 210 degrees segments for high
comas and low astigmatisms between 0 or 2 diopters. And
150 degrees segments for average comas with medium astigmatisms
in between 3 and 6 diopters. A 120 degrees
for low comas, and high astigmatisms of 6 diopters.
Or else 2 segments facing each other, one of 150 degrees
and other one of 90 degrees for low comas and high astigmatisms
between 6 or 8 diopters. One of 120 and another one of
90 degrees for low comas with high astigmatisms
between 8 and 9 diopters. When the steepest
topographic axis fails to coincide with the bisecting line of coma or there is a difference of
between 30 and 75 degrees, this will constitute an
oblique axis paracentral ectasia. Only 53%
of cases will be defined by a duck type pattern.
In these cases we will carry out an asymmetric and axial treatment.
A segments of 150 and 210 degrees from the
steepest topographic axis surrounding the cone. It will induce
an astigmatism and it will even out the surface but it will not improve the coma.
It will also be possible to implant two segments,
one of 150 and other one of 90 degrees but it does not give good results.
Another option would be to carry out an asymmetric
and non axial treatment through a tecnique in which
we draw an imaginary axis between the bisecting line of the coma and the steepest axis.
A 150 degrees segment is implanted parallel to the imaginary axis
to flatten the cornea, but this can increase the corneal astigmatism.
When the steepest topographical axis is perpendicular
to the bisecting line of the coma or there is a differente between 75
and 100 degrees we say that they are perpendicular. This represents a 4%
of all cases and this pattern is called snowman.
In these cases we have to consider whether to reduce the coma or the
astigmatism. We will carry out an asymmetric
and non axial treatment with a 210 degree segment parallel
to the bisecting line of the coma when there is a high coma. The
astigmatism might increase but the corrected visual acuity can improve.
Or else a symmetric and axial treatment, with two
90 degrees segments, emmetropic, hyperopic or pure cylinder.
Or two 120 degrees segments for myopias between 2
and 6 diopters and 4 to 6 diopters of astigmatism.
Or two segments of 150 degrees in myopias over 6 diopters
and astigmatisms under 6 diopters. This would be considered
if there is a low coma and the main focus is to treat the astigmatism.
A treatment could be carried out to
reduce both the astigmatism and the coma, the implant type would be
asymmetric and non axial and 3 segments would be implanted. One of
210 degrees parallel to the bisecting line of the coma, at
the highest coma acting on the astigmatism, and two 90 degrees
segments and 5 mm optical zone to reduce the
high astigmatism. In order to
choose the segment thickness we need to consider the patient´s age.
The younger the patient there will be more corneal elasticity. For K´s
between 45 and 50 diopters at the steepest area
we will choose a 150 µ segment. From 50 to 55
diopters we will choose a 200 µ segment. From
50 to 60 diopters, a 250 µ one
only if the minimum pachymetry is of 420 µ. And over
60 diopters, 300 µ if the minimum pachymetry is of
500 µ . And the summary table of that documented
may be the following depending on the type of ectasia: The central
ectasias have two patterns: bow-tie and ***,
where the most curved axis and the bisecting line of the coma behave in variable fashion
and the coma is usually low. The type of implant
with a bow-tie pattern would be asymmetric and axial. On the ***
non-asymmetric and non-axial. As regards
the paracentral ectasia, the first pattern is of the croissant type
in which the most curved axis and the bisecting line of the coma coincide
and the implant would be in asymmetric and axial shape.
A second duck type pattern where the most curved axis and the bisecting line
of the coma are oblique and the implant type would be asymmetric
and axial. And a third pattern called snowman where
the most curved axis and the bisecting line of the coma would be perpendicular and the implant type
would be asymmetric and non-axial.
And finally, pericentral ectasias. They are endowed with a croissant type pattern
where the most curved axis and the bisecting line of the coma coincide
and hence the implant would be asymmetric and axial.
Techniques for implanting intrastromal rings
Firstly, we calculate the depth of
the tunnel. 70% of the minimum corneal thickness at
5 or 6 milimeters. Tunnel creation
plus intrastromal ring segment implantation through manual technique with trephine,
or else tunnel creation and implantation through
femtosecond laser. After ring segment implantation
a crosslinking can be carried out.
This is a combined treatment where the segment improves the visual quality and the
crosslinking stabilises the progression.
Intrastromal ring segments
are an effective treatment for keratoconus and for pathologies
that lead to corneal deformities and irregular astigmatisms
and they bring corneal stability improving the
topographic and aberrometric corneal patterns, and improving the
corrected visual acuity.
Keratoconus treatment:
Corneal transplant. Corneal transplant would be suited to
advanced cases of keratoconus with larger K´s at 55 or 60
diopters, fine pachymetries of less than 200 µ or
corneal transparency losses owing to Vogt’s striae and / or
Fleischer rings. This is a complex surgery in which there is
a high probability of elevated residual astigmatism.
We can use the following techniques: penetrating keratoplasty
using trephine or Femtosecond laser or a deep anterior
lamellar keratoplasty in those cases without any endothelial damage and with
the integrity of the Descemet membrane. Deep anterior
lamellar keratoplasty would be a good option for moderate advanced keratoconus
with outcomes similar to penetrating keratoplasty for visual acuity,
refraction and contrast sensitivity and great aberrations.
The loss of corneal endothelial cells
in patients treated with deep anterior lamellar keratoplasty
occurs at a lower speed, making it
advantageous to the young patients selected compared with
penetrating keratoplasty.