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Antimetropia is a form of anisometropia and is a condition in which
one eye needs a plus power while the other requires a negative power.
These conditions cause several problems when correction is attempted with
spectacles. As the eye looks through a lens at its visual axis, there is
no prismatic effect, therefore there is no image displacement. As the eye
rotates and moves away from the visual axis, prismatic effect occurs
causing image displacement. In an anisometropic patient, this displacement
is not equal, causing diplopia, which makes fusion difficult. Another
problem encountered when anisometropia is corrected with spectacle lenses
is aniseikonia.
Aniseikonia is a difference in image size between the right and left lens
caused by the difference in magnification or minimization from the lenses.
Aniseikonia is corrected with an iseikonic lens (this will be the subject
of next month’s article). It should be noted that anisometropic patients
tend to lean more towards the use of contact lenses. Contact lenses tend
to be more stationary on the eye, moving with it so that the prismatic
effect is almost nonexistent.
To correct this diplopia, a form of lens fabrication called bicentric
grinding, or slab-off is used. Slab-off is a technique in which the
base-up prism is ground on half the lens in either the most minus or least
plus lens. The finished product looks as if a “slab”, or section of the
lens has been removed, hence the name slab-off. Not every one with
anisometropia needs slab-off. There are those that can easily adapt to the
power difference with no problem; however some patients cannot fuse the
images together. Hyperopic patients tend to show symptoms more so than
myopic patients; symptoms include eye-ache, headache and blurring.
Presbyopic patients tend to need slab-off more so than non presbyopes. A
non-presbyopic patient can learn to tilt their whole head downwards,
keeping vision close to the optical center of the lens, thus minimizing
the amount of prismatic effect. Presbyopic patients on the other hand,must
view much further down the lens in order to make use of the reading
segment. The process of bicentric grinding can be done on both single
vision lenses and multifocals, including trifocals and progressive lenses.
Slab-off or bicentric grinding is much more common on lined multifocals,
but care must be used in the case of trifocals. The visible line that
bisects the lens horizontally must line up between the reading and
intermediate section of the seg, not the top line like in bifocals.
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We
must first determine which eye will go through the bicentric
grinding process. The rule is the eye with either the most minus or
least plus will receive the prism correction. Once the eye is On
occasion we have patients who come in with a prescription showing a
significant power difference between the right and left eyes.
Sometimes this variance is rather significant, around four diopters,
or less significant, at around two diopters. This condition is
called anisometropia, and is often congenital but can also be caused
by cataracts or trauma. Continued on page 16 16 | OPTICOURIER |
JANUARY 2006 determined, it is then necessary to determine the power
of that eye in the 90° meridian. This can be done by using the
oblique meridian formula or with the table. You will most likely
need to round to the nearest 5°. |
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EXAMPLE:
O.D. -.25 -1.00 x 30
O.S. -5.00-.75 x 50 |
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For
the right eye, the axis is 60° away from the 90° axis – showing us
that there is .75% of the cyl power in this meridian – giving us a
cyl power of .75 diopters.Now add this cyl power to the sphere power
with a resulting total power of -1.00 diopter in the 90° meridian.
Now for the left eye, the left axis is 40° away from the 90°meridian
– showing us that there is .14% of the cyl power in this meridian -
giving us a total power of 5.10 diopters at the 90 axis. Once the
power in the 90° meridian is determined, we must now calculate the
reading depth. This is done by adding 5mm to the seg drop. To
determine the seg drop, divide the B measurement and subtract from
the seg height. |
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EXAMPLE:
A frame with a B measurement of 50 and a seg height of 20
will have a reading depth of:
1) 50 ÷ 2 = 25
2) 25 - 20 = 5
3) 5 + 5 = 10 (this is the reading depth in mm) |
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So
now that we have the power and the reading depth, let’s figure out
the amount of prism for each eye. For this we will use Prentice’s
Rule. Multiply the power by the reading depth then divide by 10. |
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EXAMPLE:
O.D. -1.00
O.S. -5.10
1.00 x 10 = 10 10 ÷10=1 Total= 1 diopter
5.10 x 10 = 51 51 ÷ 10=5.1 Total = 5.1 diopters
For the total prismatic effect we subtract the right from the left:
5.1-1=4.1 diopters of correction needed. |
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The actual process of bicentric grinding is pretty involved so
instead I will give just a brief overview. There are several ways to
process these lenses. For glass lenses they can be processed by
grinding the base up prism on the front of the lens. Because the
actual reading segment is inside the lens, it will have no affect
on the seg. For plastic lenses, the front is not an ideal place. As
the reading segment sticks out it would be damaged during the
grinding process. Instead, it is done on the rear ocular surface of
the lens. In the case of a glass lens a plastic lens is cemented on
the front of the lens, allowing only part of the lens to be
processed. Plastic often has a thick layer of resin placed on the
back which is allowed to set, allowing only part of the lens to be
ground at a time.
In 1983 Younger Optics developed a reverse slab-off lens. These
lenses had the slab-off pre-molded on the front of the lens. This
is done by employing base down prism instead of base up.
Because of this above rule, most minus and least plus must be
reversed. The least minus or most plus will be the lens requiring
the slab-off. This reverse slab-off lens was designed to aid in
the difficulties of bicentric grinding. The previous methods had
a tendency to be very time consuming and often there was a
lack of skilled help to adequately process the lenses. Even with
the development of reverse slab-off, the old process still remains
the true test of a laboratory optician. |
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