Christopher A. Kuntz

You can estimate how much astigmatism you will have after cataract surgery in a couple of ways, depending on what information you have about your eyes. 

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1.  Refraction Method:  

 

If you have a recent refraction, the second number describes the amount of astigmatism in your glasses.  Your contact lens prescription does not provide this, only your glasses prescription.  

 

Here is a sample of a glasses prescription:

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OD    -1.00 + 1.50  x  175
 

OS    -2.25  + 0.50  x  034

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This patient has 1.50 Diopters of astigmatism in the right eye (OD), and 0.50 Diopters of astigmatism in the left eye (OS).  Sometimes you will see a (+)  before the astigmatism number, as in this case, and sometimes you will see a (-).   You can ignore this.  Only the size of the number matters for our current purposes.

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This provides a rough estimate of how much astigmatism you can expect to have after cataract surgery.  In general, the higher this number, the more likely you are to benefit from astigmatic management during cataract surgery.  However, it is possible that this number could be very low and your surgeon might still recommend a toric intraocular lens.  

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Why is this method only a rough estimate?

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2. Corneal Curvature Method:

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This gives you a much better estimate of what you can expect after surgery.  Your "K values" are the numbers that describe your anterior corneal curvature.   Obtaining these values is a standard part of any general eye exam, and your surgeon or Optometrist will be able to provide these for you. Your K values might look like this:

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OD   K1 =   44.25 @ 085   K2 =   42.00 @ 175

 

OS    K1 =   44.00 @ 010  K2 =   43.00 @ 100

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The K values describe the steepest and flattest anterior corneal curvature, and the orientation of each curvature.  These two values are always 90 degrees apart. 

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Reading the K values above:

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The right eye (OD) has a steep curvature of 44.25 Diopters at 85 degrees and a flat curvature of 42.00 Diopters at 175 degrees.  The left eye (OS) has a steep curvature of 44.00 Diopters at 010 degrees and a flat curvature of 43.00 Diopters at 100 degrees.

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Estimating the amount of astigmatism you can expect to have after cataract surgery requires two steps.  

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Step 1:  Subtract the smaller flat value from the larger steep value.  In the example above:

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OD  44.25 - 42.00 = 2.25 Diopters @ 085

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OS   44.00 - 43.00 = 1.00 Diopters @ 010

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Step 2:   Adjust for the axis according to the following table:

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Axis                               Adjustment

 

0-30 degrees:        Increase by 0.25 Diopters

30-60 degrees:      No change

60-120 degrees:    Decrease by 0.25 Diopters

120-150 degrees:  No change

150-180 degrees:  Increase by 0.25 Diopters

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Why is this adjustment necessary?

 

Applying this adjustment to the example above:  For the right eye, the steep axis is between 60-120 Degrees, so we subtract 0.25 Diopter.  For the left eye, the steep axis is between 0-30 degrees, so we add a 0.25 Diopter:

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OD:   2.25 - 0.25 = 2.00 Diopters

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OS:   1.00 + 0.25 = 1.25 Diopters

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