March 4, 2016 | POSTED BY | Clinical Pearls
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During my contact lens rotation, I learned a neat trick to quickly estimate a contact lens prescription that requires vertexing. This method works great in a clinical/practical setting when you don’t have a calculator and you need to quickly determine a working contact lens prescription from the patient’s spectacle prescription. Oh, I almost forgot, you don’t need to memorize the vertex formula. Here it is…

First of all, just take a look at the simple chart below. Since we already know that lens power of 4.00 D or greater requires vertexing at each meridian, so we start with 4.00 D = add vertex factor of +0.25 D. For every 2 D power change, you will require a vertex factor change of +0.25 D.

Power of lens Vertex Factor (VF)
4.00 D +0.25 D
6.00 D +0.50 D
8.00 D +0.75 D
10.00 D +1.00 D
12.00 D +1.25 D
14.00 D +1.50 D
16.00 D +1.75 D

Example #1:
OD: -10.00 – 5.00 x 180
OS: +5.00 – 1.00 x 180

Given: Spectacle Rx for OD: -10.00 – 5.00 x 180. What’s the contact lens Rx?

1) Sphere power = -10.00 D –> -10.00 + vertex factor of +1.00 D = -9.00 D at 180th meridian

2) Sphere + Cylinder power = -10.00 – 5.00 = -15.00 D –> -15.00 + VF of +1.50 = -13.50 D at 90th meridian

3) Adjusted Cylinder power = -5.00 D + VF of +1.50 = -3.50 D

4) Estimated Rx = -9.00 – 3.50 x 180 (Need to ADD -0.25 to cylinder power to obtain the Final Rx)

5) Final Rx = -9.00 – 3.75 x 180

Given: Spectacle Rx for OS: +5.00 – 1.00 x 180. What’s the contact lens Rx?

1) Sphere power = +5.00 D –> +5.00 + vertex factor of +0.25 D = +5.25 D at 180th meridian

2) Sphere + Cylinder power = +5.00 – 1.00 = +4.00 D –> +4.00 + VF of +0.25 = +4.25 D at 90th meridian

3) Adjusted Cylinder power = -1.00 D + VF of +0.25 = -0.75 D

4) Estimated Rx = +5.25 – 0.75 x 180 (Need to ADD -0.25 to cylinder power to obtain the Final Rx)

5) Final Rx = +5.25 – 1.00 x 180

Useful Notes:

a) Spectacle plane –> Contact lens plane
(Remember: “CAP” = Closer ADD Plus!!!)

b) Minus spectacle Rx –> contact lens Rx (CLRx) = expect contact lens Rx to be LESS minus.
Ex: SRx = -4.50 D –> CL Rx = -4.25 D

c) Plus spectacle Rx –> contact lens Rx = expect contact lens Rx to be MORE plus.
Ex: SRx = +5.00 D –> CLRx = +5.25 D