Ch 5 Plus - Cylinder Power Refinement

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Go to Chapter 6: Sphere Power Refinement

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Chapter 5: Cylinder Power Refinement

The third step is refining the cylinder power.

The purpose of this step is to establish the amount of astigmatism power needed to focus both meridians of an image on the retina. You recall that when we established the sphere power in the first step we created a situation where two images were being formed at an equal distance from the retina. Now, as we refine the cylinder power, each step will move these images closer and closer until they both focus on the retina.

To do this we’ll first turn the JCC so that the axes are no longer straddling the refractor cylinder and are instead aligned with it.

As in the last step, we’ll identify the first choice as one,

..then flip the JCC and say “two”,

…and note which choice the patient prefers.

In a plus cyl refractor, we’re going to continue “chasing the white dot”. If the patient likes the choice where the white dots are in parallel with the cylinder axis, ..like it is now, we’re going to add cylinder power. If, on the other hand, the patient prefers the option where the red dots are aligned with the cylinder axis, ..like this, we’ll take away cylinder power. In a nutshell, white dots ..mean.. increase cyl power, ..red dots ..mean ..take it away.

Let’s go back to our patient.

“Joe, Which one is better, one

….or two?”

(Joe says two)

Since he said “two”, and because this is the option where the white dots were aligned with the cylinder axis, we’re going to add +0.50 to the cylinder power.

Now here’s the tricky part. We’re going to do something called “maintaining the spherical equivalent”. The easiest way to explain the purpose of this is to go back and look at the optics inside the eye.

If we simply add astigmatism power, we’re only going to change the power in one of these meridians. As a consequence we’ll move one image closer to the retina but the other one will stay where it is.

However, to get the most accurate refraction we want these two images to remain the same distance from the retina.

To fix this, every time we add plus cylinder power we’re going to compensate by changing the sphere power too.

We’ll do this by changing the sphere power by half the amount we changed the cylinder power, and in the opposite direction.

For example, if we add a half diopter of plus cyl, …we’ll compensate by adding a quarter diopter of minus sphere power.

Note how both meridians on the image were pushed back and are now an equal distance from the retina.

On the other hand, if we take away a half diopter of plus cyl we’d also remove a quarter diopter of minus sphere power.

Eventually this becomes second nature: Add 2 clicks of cyl, turn the sphere wheel up. …subtract two clicks of cyl, turn the sphere wheel down.

Why don’t we go back and start at the beginning…

Let’s flip the JCC and ask our question again.

“Joe, which is better, One…or …two”

And Joe says he likes one.

Since the patient likes the white dots we’ll add a half diopter of cyl, …remembering to maintain the spherical equivalent by adding a quarter diopter of minus sphere power.

We’ll ask again, “which is better, one ..or ..two”

And Joe says he likes one again again.

Since he likes the red dots this time we’ll switch directions and remove .25 diopters of cyl. Since we’re only changing the cyl power by .25 diopters we won’t worry about maintaining the spherical equivalent at this point.

Like the cylinder axis refinement step, we’re kind of “narrowing in” on the best cyl power. When the patient keeps asking to change the cylinder power in one direction, we change it in .50 steps. …When the patient switches directions, we start changing the power in .25 steps. When you get to a point where the patient cannot choose between the two options or is vacillating between two cyl powers you’ve reached your endpoint and it’s time to go on to the next step.

To summarize, the cylinder power refinement part of refraction involves

… positioning the JCC’s axes so they’re aligned with the astigmatism axis

… flipping the JCC and noting which choice the patient likes better

…adding plus cyl when the patient likes the white dots, decreasing plus cyl when the patient likes the red dots.

…And lastly, don’t forget to maintain the spherical equivalent by changing the sphere power by half the amount you changed the cyl power and in the opposite direction.