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Low riding lens
Observation: A
slightly low riding lens is the ideal position upon dispensing.
The lens will then center with the eye closed. Do not
make a change unless the lens is chronically low riding
with eyelid closed (as demonstrated by topography) or
if unacceptable ghosting persists.
Cause:
The cornea becomes flatter from the apex to the periphery.
This degree of corneal flattening is different for everyone,
with some corneas having a greater or lesser degree of
flattening. If the flattening is too great, the alignment
curves will be too steep.
Solution:
Loosen (flatten) the alignment curves by .50
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High
riding lens
Cause:
The high riding lens is usually caused either from the
lens being too loose (AC too flat) or from an asymmetrical
corneal shape.
Solution:
If the lens is too loose, tighten (steepen) alignment
curve by 0.50 D.
Lens decenters nasal/temporal
Cause:
Generally caused by a very spherical cornea or a cornea
with against the rule cylinder.
Solution: Increase
the diameter of the lens by at least 0.40mm. The recommended
diameter would be 11.0mm. This assumes an HVID of at least
11.8mm.
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Vaulting
Observation:
Vaulting occurs when excessive bearing is present in the
peripheral regions (AC too steep) causing reduced central
bearing. This will be obseved as central pooling or increased
presence of fluorescein under the center of the lens.
Cause:
The major cause of central vaulting is an alignment curve
(AC) that is too steep. The more peripheral from the corneal
apex, the more difficult it is to predict the rate of corneal
flattening. When the alignment curve is too steep, the central
portion of the lens will rise up, preventing it from applying
compression to the center of the cornea. A fitting curve
that is too steep can also cause central vaulting but is
much less common.
Solution:
Flatten the alignment curves by 0.50 D. The risk
is that by loosening the alignment curves too much, centering
problems can develop. If the lens is well centered, and
does not appear tight in the alignment curve area, flatten
the fitting curve by 0.10mm.
Under
Responders
Observation:
An under-responder is a patient whose myopia does not
reduce as anticipated. An example is a –3.00,
which was reduced to –1.00 after one month of
wear and then remains unchanged for 3 weeks. You will
be able to refract the patient, without lenses, to 20/20
or better.
Cause:
Typically, the
under-responder will have vaulting in the center. Some
patients will, however, respond slower than others perhaps
due to different cell structure of the cornea. You do
not want to rush into making a change if the exam figures
are correct.
Solution:
Follow the same solutions for vaulting. If no vaulting
is present, recheck the original exam figures. If the
fluorescein pattern looks good, wait a while longer,
e.g. at least two to three weeks to allow for slow responders.
If there is still no further reduction on the unaided
visual acuity, increase the target power by 0.50D to
0.75D.
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Central
Islands
Observation:
Central islands are areas of distortion in the visual
axis that are observed with corneal topography. If you
do not use a corneal topographer in the follow-up exams,
you will observe slightly distorted mires using the keratometer.
This condition differs from the under-responder in that
you will not be able to refract the patient, without lenses,
to 20/20.
Cause:
Generally caused by the fitting curve being too steep,
causing the base curve (BC) to lift off too much from
the central cornea. Another cause is excessive astigmatism.
With corneal astigmatism present, there are unequal bearing
areas where the fitting curve comes into contact with
the cornea.
Solution:
Flatten the fitting curve by 0.05mm to 0.10mm. This action
applies pressure that is more central and will smooth
out the central region. If the central disturbance is
from astigmatism, then flattening the BC will help to
correct this. Target the spherical equivalent of the original
refraction to be plano to +1.00 assuming the patient will
not have any accommodative symptoms.
Central
Staining
Observation:
This is a complication
due to either mechanical irritation or physiological
problems.
Cause:
One major cause of central staining is a coated lens.
Because of the steep Fitting Curve (FC), it is difficult
to clean the central posterior surface of the lens.
This will create an irritating surface, which in turn
causes the staining and a tendency for lens adherence.
If the BC is too flat, the reduced mechanical pressure
can also cause irritation. Reduced oxygen availability
can also cause central staining but this is a rare occurrence.
Solution:
The first thing is to make sure the posterior surface
of the lens is clean. Review the cleaning solution used.
Make sure there are no dry spots. If the staining remains,
steepen the BC by 0.5D.
Air Bubbles
Observation:
Air bubbles are a common occurrence and typically disappear
after wear. Only when staining occurs under a persistent
air bubble does the lens need to be changed.
Cause:
Air bubbles form when not enough solution is under the
Fitting Curve. Usually the upper lids will compress the
lens to the cornea and the bubbles will disappear in the
morning. The fitting curve has a steep configuration,
which is sometimes difficult to fill with tears. Occasionally,
the resultant air bubble can encompass 270 degrees around
the FC. Any staining present is due to the air bubble
where the cornea is not getting the lubrication or oxygen
that it needs.
Solution:
If the air bubble is less than 45 degrees in length upon
insertion, just monitor the next day to see if any staining
occurs. If the air bubble is greater than 45 degrees,
have the patient remove the lens and fill the concave
surface with solution and the reinsert the lens while
looking down. If a large air bubble persists, monitor
the next day to see if it remains and staining persists.
If so, monitor for three days to see if the bubble and
staining recede. If both persist, then flatten the fitting
curve 0.10mm. This will reduce the steepness of the fitting
curve and reduce the air bubble. Air bubbles are usually
a self-limiting condition, which require no change.
Reduced Holding Time
Observation:
This is when the unaided visual acuity after cornea reshaping
does not hold an acceptable amount of time.
Cause:
Generally caused by a lens that is not centered, with
the steep area almost touching the visual axis. When the
cornea normally regresses, the visual axis is impacted
sooner because there is less distance between the visual
axis and the edge of the peripheral steep ring. If some
vaulting has occurred, there will be a smaller central
visual zone with a corresponding wider concentric steep
ring. The cornea can only undergo a limited amount of
change. Usually, the more induced change, the faster the
cornea will regress. Therefore, if you have reduced –5.00
diopters of myopia, you should not expect the unaided
visual acuity to hold all day. As a general rule, the
lower the starting amount of myopia, the greater chance
of holding for all waking hours. Emerald is not recommended
for reducing myopia greater than –5.00 diopters.
Solution:
If the lens is de-centered, make the appropriate modifications
to the design to center the lens better. If vaulting is
present, do what is required to reduce the vaulting (see
“vaulting” above). Flattening the BC by 0.50
D can also prolong the holding time by making the cornea
change more before a decrease in UCVA is noticed. Flattening
the base curve will only be effective for a patient that
is able to accommodate the additional correction early
in the day.
Ghosting at Night
Observation:
Night ghosting is a normal observation. This usually recedes
with time but may always be present to some extent.
Cause:
The main cause
of ghosting is when the reduced illumination at night
causes the pupil to become larger than the central correction
area of the cornea. This might occur even with a well-centered
lens. Patients with smaller pupils will experience this
very little or not at all, compared to patients with very
large pupils. Another cause of ghosting is a decentered
lens. This can present as ghosting reported during the
day. Central islands can also give the same subjective
complaints as ghosting.
Solution:
Time is the answer
for normal ghosting. If the lens is not centered, then
follow the methods described earlier to center the lens.
The optical zone of the lens can also be enlarged from
6.2 to 6.5mm. However, this might lead to a decrease in
the holding time. It is recommended that you wait 1 month
before increasing the size of the optical zone.
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