Transcript Rigid lens principals and products

Rigid lens principals
and products
Week 9
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Rigid gas perm lens is
a hard material that
allows 02 to pass
through the material
to the cornea.
It is smaller than a
soft contact lens.
It is fit differently and
more precisely.
It “reshapes” the
cornea to eliminate
the astigmatism.
Soft CL VS gas perm CL
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Gas perm lenses are
made from SA
(silicone acrylate)
and FSA (fluorinated
silicone acrylate).
These material allow
O2 to permeate the
lens to the cornea,
reducing edema and
increasing wear time
for the patient.
It must be fit more
precisely because it
reshapes the cornea.
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Hard lenses,PMMA
(polymethylmethcryla
te) have poor O2
transmission.
1st generation “hard”
lens.
Long lasting and very
durable.
Rigid lens principals
Dk Value
 Diffusion coefficient is
the value of O2
permeability of the
lens material.
 The higher the Dk
value the higher the
O2 permeation to the
cornea.
 A higher Dk value will
lower the chance of
corneal edema.
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Dk can be expressed
in the center
thickness (L)of the
material by the
equation DkL.
This will define the
O2 transmissibility of
the material through
the center thickness.
Rigid lens principals
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Two corneal shapes
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Spherical
Aspherical (toric)
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Spherical
Aspherical
44.00/44.00@90 46.00/42.00@90
Rigid lens principals
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Spherical Cornea
Implies that this
corneal has minimal
topographic variation.
 The K readings will be
very similar horizontally
and vertically.
 There will be minimal
corneal astigmatism.
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Aspherical Cornea
Implies that the cornea
has a variance in
corneal curvature and
power.
 K readings will be
different horizontally
and vertically. The
more the readings vary,
the more astigmatism
there will be.
 This is corneal
astigmatism.
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Rigid lens principals
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Aspherical corneas
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Aspherical corneas
will has corneal
astigmatism.
With the rule
astigmatism
 Against the rule
astigmatism
 Lenticular astigmatism
 Oblique astigmatism
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Rigid lens principals
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Base Curve (BC)
The primary curve
of the lens.
It has to be
designed to contour
the anterior corneal
surface and tear
film.
Rigid lens principals
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The BC’s
relationship to the
cornea is referred
to as:
Bearing
Alignment
Clearance
These are
adjectives used to
describe how the CL
fits on the cornea.
Rigid lens principals
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Bearing
Suggests that the lens
is fitting flatter on the
cornea than the
keratometric readings.
 If the lens is bearing
centrally, it is flat
centrally, pushing the
tear film to the
periphery.
 If the lens is bearing
peripherally, there will
be no tear film
peripherally.
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Rigid lens principals
Clearance
 Suggests that the
lens is fitting steeper
on the cornea than
the keratometric
readings.
 If the lens has
clearance centrally,
the lens fits steeper
than K centrally and
the tears will pool in
the center.
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Rigid lens principals
Alignment
Suggests that the lens
perfectly contours the
cornea and the tear
film is evenly
distributed on the
corneal surface.
 The lens will not be
bearing or have
clearance.
 Usually used for
patients with little or no
corneal astigmatism, fit
on “K”.
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Rigid lens principals
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The base curve is selected
based on the “K”
readings.
Rigid lenses are fit in
three ways.
On “K”
Steeper than “K”
Flatter than “K”
This is ALWAYS based on
corneal astigmatism, not
refractive astigmatism.
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When fitting any of these,
use the flattest “K”
readings taken for that
eye.
Example……..
OD = 39.00/41.75@90
You want to fitter steeper
than “K”. (even though it
is a flat cornea)
Use 39.00 as the starting
“K” for this eye.
Fit steeper than 39.00
depending on the amount
of corneal astigmatism.
Rigid lens principals
On “K” (alignment)
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Implies that the
curvature is equal to
the flattest “K”
reading.
This method is
appropriate for low
cylinder patients.
Flatter than “K”
Implies using a base
curve flatter than the
flattest”K” meridian.
 It will exhibit a bearing
effect on the corneal
cap allowing increased
lid interaction. Bearing
will cause a minus tear
layer effect requiring a
plus power
compensation.
 (FAP) flatter, add plus.
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Rigid lens principals
Steeper than “K”
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Implies using a base
curve steeper than the
flattest “K”. This allow
central clearance of the
lens. The lens will position
centrally, along with an
increase of tear film.
This tear film will cause a
plus lens effect, requiring
a minus power
compensation.
(SAM) Steeper add minus.
Tear film layer
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The tear film created
by fitting steeper or
flatter than “K” re
shapes the cornea
and the tear layer fills
in the gaps to help
correct for the
cylindrical power not
put into the CL.
Rigid lens principals
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Tear film layer
Fitting Flatter than
“K” causes a minus
tear layer effect.
The tear layer is
pushed to the
periphery creating a
minus lens effect.
This will have to
compensated with
flatter add plus when
figuring the lens
power.
Rigid lens principals
Tear film layer
 Fitting steeper than
“K” will cause a plus
tear layer effect.
 The tear layer will
pool in the center of
the lens and this will
need to be
compensated by
steeper add minus to
the power of the
lens.
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Rigid lens principals
Determining diameter
Horizontal visible iris
diameter
The CL diameter should
be 2.0-2.5 mm smaller
than the visible iris
diameter or 1.2 – 1.5
mm from the limbus to
the edge of the lens.
 Normal rigid lens D is 8
to 10.5 mm, much
smaller than a soft CL.
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Rigid lens principals
Determining diameter
Corneal Alignments fit,
flatter than “K”
Large diameters will be
fit flatter than “K”.
 This is called corneal
alignment fit or Korb
method.
 Corneal alignment fit
rides high on the
cornea and fit UNDER
the upper lid.
Sometimes referred to
lid attachment fit.
 FAP rule applies.
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Rigid lens principals
Determining diameter
Apical fit, steeper than
“K”
Smaller diameter
corneas are fit steeper
than “K”
 This is called an Apical
lens design or
interpalpebral lens
design and fits in
between the lids,
centrally on the cornea.
 SAM rule applies.
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Rigid lens principals
Determining diameter
Corneal alignment fit
 Flatter than “K”
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Large diameter
Don’t worry, there’s a
chart for everything!!!
Apical fit
 Steeper than “K”
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Smaller diameter
Rigid lens principals
Let’s review what you
learned today!!!
Rigid lens principals
Rigid lens principals review
Rigid lens principals review
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Bearing means?
Alignment means?
Clearance means?
FAP means?
Why do we need to
use FAP?
SAM means?
Why do we need to
use SAM?
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What type of fit is
steeper than “K”?
Is that for a large or
small diameter
corneal?
What does on”K”
mean?
Rigid lens principals review
Great job!
See you next week for
rigid lens fitting.
Test in 2 weeks.
Rigid lens principals