Module 18 Section 2

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Module 18:

Clinical Optics

Part 2

Section 2:

The slab-off is a lens grinding technique used to neutralize an unwanted prismatic effect when looking through a bifocal correction.  The unwanted prismatic effect is caused by anisometropic lens corrections.  Anisometropia is a condition in which there is a significant difference in the refractive errors between the two eyes.  The following would be examples of anisometropic lens corrections.

1) OD +4.00 Sph, OS +1.00 Sph

2) OD -6.00 Sph, OS -2.00 Sph

3) OD +2.00 Sph, OS -2.00 Sph

When looking through a bifocal, we are only concerned with the power of the lenses in the 90 degree meridian.  It is possible to have a lens correction that has the anisometropic effect in only  the 90 degree meridian.  An example would be OD plano-4.00x90, OS plano-4.00x180.  At first glance there appears to be no problem with anisometropia, but evaluation of the lens power in the 90 degree meridian reveals a 4 D difference.

Keeping in mind that a plus lens is like two prisms base to base and a minus lens is like two prisms apex to apex, we can visualize the problem.

Our patient looking through the bifocal in the example above has images moving in opposite directions because he is looking through the lower edge of a minus lens in one eye and a plus lens in the other.  The effect of the prism correction is compounding (added together) because the prism bases are aligned in opposite directions.

The patient depicted above does not have a problem because the lens powers are the same and the prisms have a canceling effect.  In other words, they are aligned in the same direction, so the image shift is in the same direction.

The patient can still be affected by an unwanted prismatic effect, even when the lens/prism orientation has a canceling effect.  As pictured above, this may occur when there is a sufficient power difference between the lenses.

Anisometropia in the bifocal region of the glasses lenses is only a problem if the patient has good binocular vision.  In other words, the patient has good vision with each individual eye and the brain fuses the images.  For this patient, the induced prismatic effect causes either vertical double vision at near, or eyestrain caused by the stress and strain on the extraocular muscles as they struggle to maintain fusion in the face of divergent optics.  If the patient has poor vision in one of his eyes, then the brain may be able to ignore (suppress) the divergent image from the eye with poor vision and there is no problem with double vision or eyestrain.

Symptoms are also affected by the degree of prismatic difference between the eyes and the ability of the patient to tolerate a given degree of difference.  The number most often seen is 1.5 diopters of induced prismatic effect.  The idea is that induced prism above this amount will likely cause symptoms in most patients and induced prism below this amount will likely not cause symptoms in most patients. 

If our slab-off recommendation is to be guided by some numerical cut-off point, then we will need a method of computing the amount of induced prism in the bifocal region of any given glasses correction.

Since induced prism is involved, the formula that will help us is the Prentice Rule:

Induced prism = diopter value of lens x deviation in centimeters

We will modify the formula slightly for our purposes.  First of all, let us consider how much deviation is involved.  This depends upon how far down the eyes are rotating.  In the drawing on the left below, the eye is rotating much farther down than the eye on the right, so the deviation from the optical center would be greater.

You can actually measure this deviation for each patient if you like, but it is much more practical, and it is reasonably accurate, to assume that the deviation is close to 10 mm in most situations.  Of course, your estimation can be revised downward (to perhaps 8 or 5 mm) if there does not appear to be normal rotation downward in a particular patient.

Therefore we will plug in a constant of 1 centimeter for our deviation in centimeters.  This is very convenient, because it reduces our equation for evaluating induced prism in the bifocal to:

Induced prism = diopter value of the lens

What remains of our problem is to figure the power of each lens in the 90 degree meridian and to determine if the prismatic effect is compounding or canceling.

Figuring the power of the lens in the 90 degree meridian

We will use a power cross to visualize the power of the lens in the 90 degree meridian.  Let's work on an example: OD +2.00+3.00x50

The first step is to transpose the prescription:

+2.00+3.00x50   and    +5.00-3.00x140

+2.00 and +5.00 will be our diopter powers, and 50 and 140 will be our axis and meridian powers.  The power cross is drawn as follows:

Remember, the axis is 90 degrees to the power meridian.  The -3.00 diopter cylinder power has an axis of 140, so the +2.00 power is at 50.  We can see from this cross that the power at 90 will be somewhere between +5.00 and +2.00, and it will be slightly closer to +2.00 than to +5.00.  The power equally between the two would be +3.50 ([5 +2] / 2).  Since we see that axis 50 is slightly closer to 90 than 140 is, we estimate the power at 90 to be +3.25.  For a discussion on how to compute the exact power at 90, see Module 19.  Wouldn't it be nice to have a calculator that would do this for you?  Sure it would.  Here it is.  The calculator even has a slab-off calculator that does all of the calculating for you.  Be sure to read the instructions that come with the calculator.

Let's say that the left lens power is +3.00-3.50x180.  We won't need to draw a power cross to visualize this one.  Since the -3.50 cylinder power has an axis of 180, that means the total cylinder power is at 90.  Therefore, +3.00 - 3.50 = -.50 D is the power of this lens in the 90 degree meridian.

In this example, we have the following lens powers at 90:

Rx =  OD +5.00-3.00x140

          OS +3.00-3.50x180

OD +3.25 D at 90

OS -0.50 D at 90

The last step is to determine the total prismatic effect by determining a compounding or a canceling situation.  If the signs of the powers is opposite, as it is in this case, the powers are compounding and are added together to give the total effect:  +3.25 + O.50 = +3.75 D of induced prism.

If the powers are the same sign, then the effect is canceling and the difference is taken between the diopter powers.  Our example would yield +3.25 - .50 = +2.75 D.

According to our standard of 1.5 D, this patient would need slab-off grinding or some other adjustment to avoid reading difficulty.

How does a slab-off work?

We know from the principles of compounding and canceling, that a one diopter base up prism combined with a one diopter base down prism, when viewed with the same eye, will have a canceling effect.  (If one was viewed with the left eye and the other with the right eye, they would have a compounding effect.)

The slab-off grind can be readily identified by the tell-tale line extending across the reading area of the more minus lens.

The same canceling effect can be achieved with what is sometimes called a "reverse slab-off".  This technique involves molding base down prism into the reading portion of the more plus lens, with the same result as the slab-off grinding technique.

There is limit to how much induced prism can be canceled by a slab-off.  The limit is about 6 diopters of prism power.

Are there alternatives to a slab-off?

With large anisometropic differences (e.g. OD +10.00, OS -1.00), the patient would be fit with contact lenses to minimize the effects of anisokonia (image size differences).  The power of the contact lenses can easily be adjusted so that that the patient can wear a "normal" pair of bifocals for reading if indicated.

In less spectacular cases, a common alternative is the use of single vision readers.  We know that there is no prismatic effect when a patient looks through the optical center of a lens.  When fit with single vision readers, the patient is instructed to move her head so that she is always viewing at, or very near, the optical centers of the lenses.  With these glasses it is more practical to elevate the reading material rather than to try to bend the head down.  Another disadvantage of this technique is that distance and reading glasses are required instead of one pair of bifocals.  If the total induced prism is 6 diopters or more, there is no other choice.

Calculation Review

Here are the steps needed to determine the need for a slab-off:

1) Figure the power of each lens in the 90 degree meridian.

2) Determine if the prismatic effect is compounding or canceling

3) Figure the total amount of induced prism present for reading.

4) If the amount is above 1.5 D, a slab-off may be indicated.  If the amount is above 6 D, a slab-off will not work.  An alternative would be single vision readers.

A quick estimation technique

The most difficult part of the slab-off calculation is figuring the power of the lens in the 90 degree meridian.  You could install the Eyetec.net Clinical Optics Calculator on a computer at work and easily get an exact answer. But the calculator may not always be available.  Here is a quick, reasonably accurate technique for the 90 degree computation:

Below is a graphic of the optical protractor.  This is the same "dial" that is used on the phoropter to set the axis of the cylinder.

If our prescription is +2.00+3.00x30, we would take 2/3 of 3.00, which is 2.00, and add that number to the sphere power to get the lens power at 90, which would be +4.00 D.

Example Quick Calculation:

Figure the induced prism in the reading area for the following prescription:

OD +1.00-2.00x100

OS plano-3.00x45

Our power gauge for the right lens would be:

The gauge is very close to zero, so we can estimate the power to be about 10%.  The next step is to take 10% of the cylinder power (-2.00 x .10 = -.20) and algebraically add it to the sphere power (+1.00 - .20 = +.80).  So we can estimate the power of this lens at 90 to be about +.75 D.

Our power gauge for the left lens would be:

The gauge is at 50% power, so we take 50% of the cylinder power (-3.00 x .5 = -1.50) and add it to the sphere power (0 -1.50 = -1.50).  The power of this lens at 90 is -1.50.

The induced prism at 90 is:

OD +.75

OS -1.50

The next step is to determine if the prism powers are compounding or canceling.  If the signs are different, the situation is compounding and the prism powers are added together.  If the signs are the same, the difference between the prism powers gives us the total induced prism.

In this case, the signs are different, so there is a total of 2.25 D of induced prism in the reading portion of these glasses.  If the sign were the same, the total induced prism would be .75 D.