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Astigmatic Fitting Guide
 

 

     Bi-Toric Design From “K” & Rx
         Example: K = 41.00 / 45.00 x 85 Rx = +0.50 -4.75 x 175

  » Determine the amount of corneal cylinder.
     Answer: 4.00D Corneal Cylinder

  » Use the following chart for your base curve:

Corneal Cylinder Base Curve  
  (Flat Meridian) (Steep Meridian)
1.50 to 1.87D Fit on K Fit on K
2.00 to 2.37D Fit on K 0.25D Flatter
2.50 to 2.62D Fit on K 0.50D Flatter
2.75 to 5.87D 0.25D Steeper 0.50D Flatter
6.00D or greater 0.50D Steeper 1.00D Flatter
  Answer: We will fit our base curve 0.25 steeper than the flattest meridian and 0.50 flatter than the steepest meridian.

       Base Curve: 41.25 / 44.50

  » Put the spectacle Rx on the optical cross.

 

Vertex both meridians (using 12mm vertex distance).
Flat Meridian +0.50 x 175 = +0.50
Steep Meridian -4.25 x 085 = -4.00

   » Using SAMFAP, determine the power in both meridians. The flat meridian        usually has the most plus or least minus in the prescription.

Flat Meridian +0.50 Steep Meridian -4.00
Steepened Flat Meridian By -0.25 Flattened Steep Meridian By +0.50
Power in Flat Meridian +0.25 Power in Steep Meridian -3.50
   
      Meridianal Power = +0.25 / -3.50
Cylinder Power = +0.25 -3.75

  » Choose the diameter the same way you would for a spherical design using       the average of the two base curves.

Base Curve Recommended Diameter
39.00 to 43.87 9.6
44.00 to 45.87 9.3
46.00 to 48.37 9.0
48.50 to 50.00 8.7


FINAL PRESCRIPTION:  
Base Curve 9.6: 41.25/44.50
Power: +0.25 -3.75
Diameter: 9.6