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
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.
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:
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:
Step 3 - Estimate corneal power in diopters after refractive surgery
Using the above information, the estimated corneal power is approximately30.54 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: