doctor-hill.com IOL Power Calculations

# Contact Lens Method.

The Contact Lens Method, originally outlined by Dr. Holladay, was once considered a helpful way to estimate the average central corneal power following radial keratotomy. This technique required a special PMMA contact lens, of a known base curve and power. We have found the contact lens method to be less accurate following other forms of keratorefractive surgery, such as LASIK, LASEK, PRK, and ALK.

Following all forms of ablative keratorefractive surgery (LASIK, PRK, etc.) a review of the literature now suggests that the hard Contact Lens Method may be less accurate than originally thought. For this reason it is no longer recommended in this clinical setting.

The information here is offered for historical interest only:

In this situation, the clinical relationship:

Cbase  +  Cpower  +  Rcl  -  Rbare = Ktrue

generally holds true, if the following are known:

 Cbase = base curve of the contact lens in diopters,and Cpower = spherical power of the contact lens in diopters, and Rcl = spherical equivalent refractive error with the contact lens, and Rbare = spherical equivalent refractive error without the contact lens, then Ktrue = the estimated corneal power after refractive surgery

To give accurate information, the refractive numbers (Rcl and Rbare ) must retain their corresponding plus (hyperopic) and minus (myopic) signs, and be corrected for vertex distance.

Example:

Before refractive surgery by 16-incision RK, a 48-year old male, with high axial myopia, was found to have a refractive error in the left eye of -16.00 +2.00 x 092 at a vertex distance of 12.0 mm.

Prior to refractive surgery, by both topography (simulated Ks), and standard Javal-Schiötz keratometry, the central corneal power of the left eye was found to be 42.00 x 002 and 44.00 x 092. Four years following RK, the vision in the left eye is 20/20 without correction. The refraction has been stable and is found to be -0.50 +0.50 x 092.

What is the estimated corneal power for this patient four years after radial keratotomy?

Step 1 - Determining base curve of an over-refraction contact lens.

Pick a 9.5 mm, reverse geometry, PMMA, plano contact lens with a base curve somewhat steeper than what you might expect would be the true corneal curvature. Any high quality contact lens laboratory can make a set of these for you ranging from 30.0 D to 45.0 D in one diopter steps. A good general rule of thumb is to use a contact lens base curve that is approximately 95% of the average measured Ks.

For example, if the post-keratorefractive surgery measured Ks are found to be 35.88 x 180 and 33.75 x 090, a good choice for the over-refraction contact lens base curve would be calculated as follows:

Base curve = 0.95 * (0.50 * (K1+K2))

Base curve = 0.95 * (0.50 * (35.88 D + 33.75 D))

Base curve = 0.95 * 34.82 D = 33.07 D  and then round up or down to the nearest whole diopter to arrive at the following final base curve to use for a contact lens over-refraction:

Base curve = 33.00 D  (actual measured base curve is 32.95 D)

Note:  Our contact lens laboratory was instructed to list the measured base curve for this lens to two decimal places. In this case, the measured base curve of the plano, reverse geometry contact lens is 32.95 D. We will use this number in our calculations.

Step 2 - Determine the refraction with the contact lens in place

With the plano contact lens in place, the over-refraction is -2.75 D at a vertex distance of 12 mm. This would become -2.66 diopters at the corneal surface.

A 12.0 mm vertex distance correction is determined as follows:

1/(-2.75 D) = -0.3636 m

-0.3636 m - 0.012 m = -0.3756 m

1/(-0.3756 m) = -2.66 diopters

 Trial frame pinhole occluder with a 4.5 mm central opening.
As recommended by Dr. Holladay, we have found that it is better to do these measurements in the setting of a normal (non-dilated) pupil and with an occluder placed in the black trial frame that has a 4.5 mm opening drilled into its center. We then perform all refractions through this "artificial entrance pupil." In this way, only the central portion of the post- keratorefractive cornea is used to determine the refractive state of the eye. Dr. Holladay is to be congratulated for this very helpful insight, which further enhances the accuracy of these measurements.

Step 3 - Estimate corneal power in diopters after refractive surgery

Using the above information, the estimated corneal power is approximately30.54 diopters.

 C base + C power + R cl - R bare = K true (32.95) + (0.00) + (-2.66) - (-0.25) = 30.54 diopters

Note: Most contact lens laboratories will be happy to custom make a special set of reverse geometry 9.5 mm diameter plano PMMA contact lenses in 1.0 diopter steps, with base curves from 30.0 to 45.0 diopters specially for this purpose.

As described above, the laboratory should be instructed to list the actual measured base curve for each lens. It is not uncommon to order a 36.0 diopter contact lens and have the actual base curve turn out to be something different. As the base curve is not being used for corneal fitting, but instead as part of a mathematical formula, the laboratory will need to carefully measure each contact lens and provide you with this information. Use this actual measured base curve figure when doing your corneal power calculation.

If you do not have a relationship with a contact lens laboratory familiar with manufacturing a set of these special lenses, we recommend the following:

Contex, Inc.
4505 Van Nuys Blvd.
Sherman Oaks, CA 91403

Tel: (818) 788-5836
FAX: (818) 788-5078