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CONTINUING EDUCATION AND TRAINING
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Figure 9 The same eye as seen in
Figure 8 with a significant increment in the
optic zone projection illustrating full
corneal clearance.
comparison to the corneal thickness, as
shown in Figure 10. The fluorescence can
be seen perfectly well with white light;
cobalt blue filters reduce the brightness too
much for the corneal thickness to be seen
well.
Apical clearance between 0.2mm and
0.3mm is a satisfactory target for a nonventilated
ScCL, approximately half a normal
corneal thickness, with the pre-corneal
fluid reservoir extending just beyond the
limbus. The exact measurement of corneal
clearance is not critical, although if excessive
it is more difficult to maintain an
air-free pre-corneal fluid reservoir or for the
patient to insert the lens without an air
methods consequent to the introduction of
RGP materials. There are many fitting
nuances which could not be covered, and
there are significant problem areas which
have only briefly been touched upon.
However, it is emphasised that the fitting
processes have become straightforward and
predictable in most cases, with a rapid
arrival at the end-point mostly using nonventilated
preformed RGP designs.
ScCLs may be lens-of-choice when a
result is needed more than at any other
time. There are no conditions for which they
cannot be considered, so it is clearly necessary
to preserve the clinical and manufacturing
skills required . Since the introduction of
is defined according to two axially measured
parameters: the OZP from the extrapolation
of the scleral curve measured at the apex
and at the limbus. These increments are just
clinically significant enough to keep the
number of lenses in the fitting system
manageable. OZP is a function of the BSR,
hence if the BSR is flattened, the OZP needs
to be increased to compensate.
The OZS or OZP for an initial trial lens is
selected from an assessment of the overall
corneal profile with the naked eye. If inaccurately
estimated, there is no major loss as
the first lens inspected in situ gives a clear
indication for subsequent lens selection.
Keratometry and corneal topography are of
limited value as neither provides any information
about the peripheral cornea or the
projection from the sclera.
The parameters are assessed simultaneously
with the lens in situ, but the initial decision
must be to determine the optimum
scleral zone specifications as the scleral zone
has an impact on the optic zone clearance.
The lens is inserted filled with saline and fluorescein
so that areas where the lens is clear
of the surface can be easily distinguished
from areas where the lens is in contact.
Figures 8 and 9 demonstrate the effect of
increasing the OZP to reduce corneal contact.
Slit lamp biomicroscopy
assessment of the optic zone
If fluorescein is added to the saline prior to
insertion, inspection of the optic zone using
a cobalt blue filter shows any corneal
contact zones. A slit lamp optical section
can be used to measure the optic zone
clearance. A pachometer is not necessary
but the depth of the pre-corneal reservoir
can be estimated with sufficient accuracy by
Figure 10 Slit lamp optical cross section demonstrating the pre-corneal fluid reservoir.
The lens, the reservoir and the cornea are all clearly seen in cross section. The depth of
the reservoir is just less than the corneal thickness at the visual axis, therefore
approximately 0.35mm to 0.4mm. By comparison, the cornea is 0.5mm to 0.6mm in
thickness. The reservoir is thinner superiorly, and thicker inferiorly, indicating a degree of
downward displacement of the lens. This is a normal feature with non-ventilated RGP
ScCL fitting and does not usually have any detrimental effect (Courtesy of Scott Hau)
bubble. If there is insufficient clearance,
corneal contact zones may reduce comfort
and tolerance. Increasing the OZS or OZP by
0.25mm alleviates a compressive central
contact zone to give corneal clearance.
Conclusion
This is paper intended to provide an introduction
to modern scleral contact lens practice,
principally to outline changes in fitting
RGP materials their application can be made
at all grades of pathology where benefits
can be seen. There remain some problems,
and commitment to clinical practice is a
prime requirement. The pathology may be
active or progressive, so close liaison with
ophthalmology is essential. A small number
of practitioners are needed to maintain a
functional service, but those who are not
directly involved should also be aware of the
significant developments in recent years.
31 | October 20 | 2006 | OT