The
most common option for correcting vertical imbalance of an
Anisometropia produces a vertical prismatic effect in the entire
lower half of
one lens only, beginning at the level of the
bifocal segment line. This type of correction is called a
slab-off or bicentric grind. A slab-off
should almost always be used unless the amount of
correction required is less than 1.50 diopters of
prism. At less than that, it is difficult to
control the appearance and placement of the slab
line. Most problems with vertical imbalance do
not occur once it is below 1.50 diopters of prism
anyway.
Before using 6 diopters of slab off
consider using a regular slab-off ( BU ) on one
eye and a reverse slab-off ( BD ) on
the
other. This should create easier acceptance for
the patient.
It is possible
to use a slab-off on any lens, glass or plastic,
including a progressive addition lens
(plastic only). On conventional slab-offs the
lens with the most minus or least plus should
have the slab-off and the opposite is true for a reverse
slab-off.
Use our Slab-off
Calculator to help you determine the amount of correction required
for any prescription. If you have any questions or comments about
how to use our Slab-off Calculator use the Information Request Form
below and we will be happy to assist you.
There are several other
ways to compensate for vertical prism at the near besides a slab-off
or reverse slab-off. When dispensing glasses for correcting vertical
imbalance consider these other options. They may be a little more
economical for some patients but may not be as convenient or
cosmetically attractive as a slab-off
Two pairs of glasses
Dropping the optical center or MRP of a multifocal.
Raising the seg height
Fresnel press on prism
Dissimilar segs
Compensated "R" segs
The amount of slab-off prism present may be verified by comparing
the seg areas of the slab-off lens with its partner lens through
a lensometer. An easier method, however, makes use of the lens
clock. First determine the base curve of the slab-off lens by
placing the contact point horizontally across the distance
portion parallel to the slab-off line. After noting the base
curve, then place the contact points of the lens clock
perpendicular to the slab-off line. The center contact point of
the lens clock should be placed directly on the line. The
difference in these two readings is the amount of slab-off prism
ground in the lens.