I Calculate That?”
Having been a dispensing optician for over 15 years, there
are very few things that surprise me anymore. Constant turn
over in the optical business, patient complaints, doctor
complaints, managed care, lens spoilage, etc., etc., etc.
I’ve worked in retail optical, an optical lab and finally as
office manager of an optometry practice. I’ve been around
the blocker. Pardon the optical humor.
One thing that really amuses me is when the patient has
vertical imbalance and we need to recommend bicentric
grinding -- otherwise known as slab-off. My amusement stems
from the fear in the eyes of the average optician when they
need to calculate slab-off. It is certainly a complicated
topic, but not one that should be overwhelming. Since it is
a topic that we will all encounter, and since we do not want
anyone else to do our calculations for us, what follows is a
review of the vertical imbalance subject. After all, this is
one of those areas of opticianry that seems to fly by a lot
of good people in the profession.
To understand why slab-off is sometimes required, a little
background information is needed. It’s important to remember
viewing an object is a team effort. Working in pairs, the
eyes produce a fairly extensive visual field, fuse an image
stereoscopically (in 3-D) and converge when looking at an
object closer than twenty feet. This is simply known as
binocular vision or stereopsis. When a patient sees
normally, the images focused on the retina appear
approximately the same size, shape and location. However,
when the prescription that is required for each eye has a
significant difference problems ensue.
When the patient has the same refractive error in both eyes
but with a significant difference in power it is a condition
known as anisometropia. In optical terms, anisometropia is
typically a difference in refractive value of 1.00D or more.
These patients will generally present one of three ways:
congenitally, post-trauma or after cataract surgery. A
similar condition is known as antimetropia. Antimetropia
occurs when there is a substantial difference in which one
eye requires a concave, or “minus” prescription, while the
other is corrected using a convex, or “plus” prescription.
When the prescription is the same or similar between the two
eyes the patient will experience no prismatic effect as they
move their eyes behind their lenses. The prismatic effect
that one eye gets as the eye moves away from the optical
center will be canceled out by the prismatic effect that the
other eye receives. For instance, if one gets base out the
other will get base in and the net result is no prism and no
problem for the patient. However, if there is a large
difference in power between the two eyes one eye will
develop much more prism than the other eye and now the
patient will have displacement of the image. When they are
pre-presbyopic the patient simply learns to turn their head
to view objects. This keeps their eyes at the optical center
of their lenses and they will not induce any prism. Once
they become presbyopic though, we now advise them to move
their eyes down into the segment and the prism returns!
Now that they have a segment for their presbyopia they can
no longer rely on looking through the optical centers in
order to avoid the prismatic problem. Now they must drop
their eyes into the segment to get the benefit of the
reading prescription. This simple act creates the prism and
the result for the patient is a splitting of the image as
they try to read. The patient will also feel uncomfortable
trying to read for any prolonged period of time. They will
frequently feel strained, sometimes nauseous and
occasionally get headaches from the prism.
Our mission is to correct the vertical imbalance in their
prescription so that they will be comfortable in their new
multifocals. We have six methods of correcting this
imbalance: Two pairs of glasses – one for reading and
another for distance, dis-similar segments, r-compensated
segments, Fresnel press-on prisms, prism segments and of
Slab-off is also known as bicentric grinding. Bicentric
grinding is a type of lens fabrication process in which
base-up prism is ground on the lower portion of the lens to
redirect light as a patient gazes downward. This technique
allows the patient to fuse the images and prevent prism as
the eye travels down the vertical meridian of the lens. The
term slab-off came about because the technique makes the
finished lens look as if someone removed a small section of
the bottom portion. Of all the ways to correct vertical
imbalance, slab-off is by far the most common technique used
---but it is not always the best understood.
Patients with anisometropia or antimetropia and who have
presbyopia, needing bifocals or trifocals to see up close,
will require slab-off in order to see comfortably up close.
Since they require multifocal lenses, they must drop their
gaze from viewing objects at a distance to utilizing the
multi-focal area of their lens. Therein lies the problem and
the need for slab-off. Slab-off can be ground on the front
or back of the lens or molded into the front in the case of
reverse slab-off. Regular slab-off is ground base up on the
most minus or least plus lens in the 90th meridian by the
lab. This is done to correct vertical prismatic imbalance
that results in diplopia when looking at something at the
near point. Reverse slab-off is molded base down on the
least minus or most plus lens in the 90th meridian at the
segment line of the multi-focal lens. Since the slab-off
correction is already in the lens blank, a lab will only
need to cut the back curve for distance which is something
that can be done easily and is less prone to error. Reverse
slab off is used more extensively than regular slab off for
this reason. Typically, slab-off is done on a flat top
bifocal because it provides reasonably good cosmetics.
How Much Slab-Off Do I Need?
Never leave this question to the lab! You are the trained
professional and you are the one responsible for what is
dispensed! NEVER trust anyone to calculate anything for your
patient. Calculating the amount of slab-off required is
relatively simple. Since we’re most concerned with the
vertical balance of the lens, we need to determine how much
power resides in the 90? meridian. Consider the following
OD -4.00 -2.00 x 030
OS -1.00 -2.50 x 045
We need to find the power at 90?. You’ll need to follow
these steps to determine powers in the 90° meridian for each
Subtract the axis for each lens from 90. For the right
eye, we have 60. For the left, 45.
Now you need to find the sine of each angle. For the
right, we have .8660
For the left, 0.7071.
Squaring these results with the x² function, this will
yield .75 for the right, 0.4999 (or 0.5) for the left.
Taking this result, we multiply it by the amount of
cylinder in each lens yielding
-1.50 D for the right and -1.25 D for the left.
We will now add these amounts to our spherical power and
yield -5.50 D for the right and -2.25D for the left. These
are the powers of each lens at 90°.
Since most labs use reverse slab-off, we want to use the
lens with the least amount of minus power, or most plus.
This criteria, designates our left lens as the “carrier”
lens -- the lens taking or “carrying” the slab-off.
Next, we need to take into account the reading depth of the
lens. This is determined by simply using 8 mm. Why 8 mm?
This has been studied many times over the years and the
average amount that a human being drops their eyes to read
is 8 mm.
Now we have the power and the reading depth for each eye. We
will need to apply Prentice’s Rule to determine the amount
of prism for each eye. We determined the right eye had a
power of -5.50 D at 90° and the left -2.25 D. First we
subtract the difference between the two eyes and then apply
Prentice’s Rule, we yield the following:
3.25 D X 8 mm/10 = 2.6 ? base down OD = the imbalance
between the two eyes. If we can provide slab-off of the same
amount in the opposite direction we will eliminate this
prismatic imbalance and the patient will be able to read
comfortably for any length of time they wish. Or as in the
case of reverse slab-off, we will put the same amount of
prism and base direction in the other eye and this to will
balance the two eyes. For this example we would order 2.5 ?
base down in the OS in reverse slab-off. Note: Reverse
slab-off is only available in certain quantities (1.5, 2,
2.5, 3, 3.5, 4 & 5) which is why we rounded this answer from
2.6 to 2.5.
Hopefully, the next time your patient needs slab-off, you
will feel more comfortable performing the calculation. Not
only is that rewarding in and of itself, but you will also
have a better understanding as to why it’s needed to help
our patients see their best and maintain their comfort. With
a little questioning of your patients you will be surprised
how many of them have issues with vertical imbalance that
have gone uncorrected in the past.
With contributions from: Brian A. Thomas, P.h.D, ABOM