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CONTINUING EDUCATION AND TRAINING
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ghosting. Grossly abnormal topography
would not be expected after refractive surgery,
but some patients remain intolerant of
corneal lenses, in which cases ScCLs
could be tried.
Normal topography with high
refractive errors
ScCLs may be indicated when high power
rigid corneal lenses cause intractable problems
of excessive mobility or poor centration.
They may be used whatever the
ametropia if there is intolerance to corneal
or hydrogel lens wear in high myopia, or
hypermetropia or significant non-pathological
corneal astigmatism.
Therapeutic or protective
applications
ScCLs uniquely retain a pre-corneal fluid
reservoir providing corneal hydration in serious
dry eye conditions, such as Stevens
Johnson Syndrome or ocular cicatricising
pemphigoid. A ScCL in situ may give an
improved environment for corneal healing
where newly formed epithelium is continually
sloughed away by the action of the lids,
or may prevent tear film evaporation with
poor lid closure or lid absence. The lens
retains a fluid reservoir which maintains
some degree of corneal hydration and
protects the cornea from trichiasis and lid
margin keratinisation, and the front surface
provides hugely improved refracting surface,
giving a considerable visual benefit.
There is also an application in less severe
dry eye or tears dysfunction pathology.
Sometimes the patient may describe very distressing
and disproportionate ocular discomfort
with very little visible corneal disruption.
Mucus filaments adherent to the cornea are
very painful when they pull on the epithelium
or finally detach, but wave freely in the precorneal
fluid reservoir behind a ScCL, and
appear to float off into the fluid pool with little
discomfort. ScCLs may be used as a prop
for some cases of ptosis.
Cosmetic shells
A realistic iris can be encapsulated into
PMMA scleral shells to mask unsightly blind
eyes, or to relieve intractable diplopia or
glare in cases of aniridia.
Recreational or occupational
applications
Particles behind rigid corneal lenses in dusty
work place environments are very troublesome.
ScCLs eliminate this problem almost
entirely. Most recreational activities are satisfactorily
managed with the use of hydrogel
lenses, but ScCLs are still indicated on
occasions for contact or water sports.
ScCL fitting options
Ventilation by some means is a prerequisite
for PMMA lenses whether impression or preformed,
but non-ventilated designs are the
preferred option for RGP. Some limitations
still remain for irregular ocular topography
with RGP preformed sclerals, but considerably
more irregular eyes can be fitted using
preformed RGP sclerals because of the tear
reservoir compared to fenestrated PMMA.
Impression lenses of some kind can be used
virtually irrespective of irregular topography.
All the following alternatives are possible,
and have different applications.
RGP
RGP sclerals should now be considered the
first choice for the great majority of ScCL
cases. Some long-term wearers have worn
the lenses for over 50 years, so these should
be considered a long-term option for most
new referrals. As the great majority of ScCL
referrals are for PCE or corneal transplant, it is
crucial to minimise the risk of corneal hypoxia.
A transplant may have to be a future management
option for any PCE referred for ScCL
fitting. If so, corneal neovascularisation, while
not necessarily sight-threatening in its own
right in the early stages, may increase the risk
of transplant rejection.
Preformed non-ventilated RGP can be
used for the great majority of cases, irrespective
of corneal topography. There are more
potential problems with the scleral topography,
but if the scleral zone is well enough
sealed, a pre-corneal fluid reservoir is
retained without bubbles and with minimal
settling on the globe. The optimum clearance
at the visual axis is approximately 0.25mm,
but can be more than twice that value at the
limbus in some sectors without causing any
problems provided it remains air free.
The principal problem with preformed
non-ventilated RGP ScCLs is that they have
to be inserted filled with saline. This requires
keeping the lens horizontal at the moment
of insertion, so it is necessary also to have
the patient’s head horizontal and facing the
floor. This is a more difficult task for
patients, and sometimes for the practitioner
as well, compared to the relatively simple
procedure for inserting a fenestrated lens,
which can be inserted with the head in the
normal upright posture.
Preformed fenestrated RGP is indicated
if a fenestration is beneficial, for
example, if retention of the pre-corneal fluid
reservoir at the moment of insertion is
impossible. However, there is a serious limitation
because air bubbles are admitted into
the pre-corneal fluid reservoir and cross the
visual axis if the pre-corneal fluid reservoir
depth is greater than 0.1mm. It is unlikely
that a tear pool of uniformly shallow depth
can be maintained with even just a moderately
irregular corneal topography.
Impression non-ventilated RGP is an
option if there is an intractable problem
retaining an air free pre-corneal fluid reservoir
with a non-ventilated preformed
design. The ultimate fitting target is a flush
back surface for the scleral zone and an
optic zone clearance similar to that for a
non-ventilated preformed RGP. As the scleral
zone is fabricated from an eye impression,
a very good alignment can be expected with
effective sealing on the scleral zone, hence
there is scope for increasing the optic zone
clearance with a good prospect of retaining
a pre-corneal fluid reservoir.
Impression fenestrated RGP is a major
undertaking because it is a departure from
mainstream manufacturing and, in
addition, requires very precise fabrication of
the back surface to keep a uniform depth of
the pre-corneal fluid reservoir. However,
there may be a better chance than with a
fenestrated preformed RGP lens because
the optic zone is individually fabricated from
the exact shape of the eye.
PMMA
PMMA ScCLs still have a role to play but
there is an acknowledged long-term threat of
hypoxic corneal changes, so these would not
normally be the first choice for new ScCL
wearers. However, if a PMMA lens has been
worn successfully for some years without significant
sequelae, there is no reason for a
change is to be made. Most impression
PMMA lenses for moderate or advanced
corneal ectasias were ventilated, hence the
initial clearance at the visual axis could not be
much more than 0.1mm. After a period of
settling on the globe, an almost universal
occurrence with a ventilated lens, the result
was apical contact. The author’s observation
is that contact is better tolerated in PMMA
than with RGP materials. The probable explanation
is the increased co-efficient of friction
28 | October 20 | 2006 | OT