Article Date: 12/1/2004

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Determining IOL Power for Post-Refractive Surgery Patients

BY BUDDY RUSSELL, FCLSA, COT, AND THERESA M. KLEIN, COMT

Corneal refractive surgery has become common, and the various procedures continue to undergo changes to improve patient outcomes. Most patients who undergo refractive surgery are happy with the results and experience good visual outcomes.

But, what happens when one of these patients develops a cataract? Do problems arise in visual outcomes following cataract removal and lens implantation in these patients? If so, then what are the problems and what are we doing to remedy them?

Closeup on Cataracts

Cataracts are opacities and/or changes in the crystalline lens that interfere with vision. We classify them in the following four categories:

1. Senile -- normal, age-related changes in the crystalline lens

2. Congenital -- approximately 25 percent of this type has a genetic component. Other factors can include prematurity, birth trauma, infection, medications and metabolic disease

3. Traumatic -- cataracts that occur as a result of injury

4. Secondary -- cataracts that occur as a result of entities such as metabolic diseases or medications (such as steroids)

Calculating IOL Power

For those patients who develop a cataract, surgeons remove the crystalline lens and implant an intraocular lens (IOL) in the same procedure, unless this is contraindicated.

To determine IOL power, you need the size of the eye and the postoperative target power (such as emmetropia or slightly near sighted). You can determine the size of the eye by measuring the corneal curvature and the axial length. Manual keratometry most commonly measures corneal power. You can determine the axial length of the eye by A-Scan biometry, which we'll describe in more detail below. You can then enter these calculations into a computer program that determines the IOL power.

Using A-Scan Biometry

Correctly measuring the eye's axial length is a primary factor in determining the correct IOL power. A-Scan biometry measures the distances among ocular structures as well as the total distance from the anterior cornea to the macula along the visual axis. It then calculates the length of the eye by measuring the amount of time it takes for sound signals to travel through ocular tissues of known sound velocity. The anterior cornea, anterior lens, posterior lens and the vitreo-retinal junction represent the four primary echoes from which we calculate such measurements.

The echoes produced during normal A-scan ultrasonography represent the acoustic interfaces where the sound velocity changes. By knowing the sound velocity and the time required for the echoes to return to the probe, you can calculate the distance (in millimeters) that the sound wave has traveled.

Keratometry Problems

Manual keratometry samples four points on a cornea approximately 3mm apart from each other. The flatter the corneal curvature, the longer the radius, the larger the image mire and the lower the power. The steeper the corneal curvature, the shorter the radius, the smaller the image mire and the higher the power. The average corneal curvature is approximately 44 diopters. Myopic eyes may have steeper corneal curvatures, while hyperopic eyes may have flatter corneal curvatures.

A manual keratometer assumes that a cornea has a prolate shape (steeper centrally) and extrapolates information based on a normal curvature relationship. One problem with manual keratometry is that many patients who have undergone corneal refractive surgery have a resulting oblate corneal shape (flatter centrally). Thus, manual keratometry tends to overestimate central keratometry following refractive surgery for myopia. It extrapolates approximately the central 4.5mm of surface power in these flatter corneas. While this information is consistent, it's consistently wrong. The average postoperative outcome if clinicians use such keratometric readings to determine IOL power can be approximately +3.00D to +5.00D of relative undercorrection.

Finding an Alternative

We now recognize that alternative methods are necessary to determine the appropriate IOL power for patients who have undergone corneal refractive surgery. The two most widely accepted methods are the refractive history method and the contact lens over-refraction method.

Refractive History Method Start by obtaining the preoperative keratometry (average K measurement before refractive surgery), preoperative refraction (spherical equivalent before refractive surgery) and the stable postoperative refraction (spherical equivalent after refractive surgery). This assumes that the refractive change results from the change in pre- and post-op keratometry. Thus, you can determine the effective keratometry for IOL calculations based on the amount of refractive change.

Potential problems arise if you can't obtain or if you receive unreliable preoperative data. In addition, the developing cataract can influence the post-refractive surgery refraction.

Contact Lens Over-Refraction Method Place a contact lens of a known base curve and power on a patient's postoperative eye and perform an over-refraction. You can then calculate the true power that the cornea had before refractive surgery. This method uses a known curve to determine an unknown curve.

Compare the patient's refraction without the contact lens to the contact lens over-refraction. Then algebraically add the amount of difference between the manifest refraction without the contact lens and the contact lens over-refraction (plano lens) to the known base curve of the contact lens. For example:

Manifest refraction without CL = ­5.00 +2.00 x 170
­4.00 (Spherical Equivalent)
Contact lens over-refraction = ­8.00
­8.00 (Spherical Equivalent)
Contact Lens Base Curve = 38.00 diopters
Difference: ­4.00 to ­8.00 = ­4.00
38.00 + (-4.00) = 34.00

Using this method, the average central keratometry of the post-refractive cornea is 34 diopters in curvature. It's important to note the direction of the change in the refraction compared to the over-refraction when making this calculation.

For example:

Manifest refraction without CL = ­5.00 +2.00 x 170
­4.00 (Spherical Equivalent)
Contact lens over-refraction = +1.00
+1.00 (Spherical Equivalent)
Contact Lens Base Curve = 38.00 diopters
Difference: ­4.00 to +1.00 = +5.00
38.00 + (+5.00) = 43.00

Using this method, the average central keratometry of the post-refractive cornea is 43 diopters in curvature. Therefore, use 43.00 as the average keratometry measurement along with the axial length measurement in calculating the IOL power.

It's worthy to note that researchers have determined that vertex distance calculations aren't necessary when using the contact lens over-refraction method.

When using this method, we suggest that you use large (10.8mm) diameter lenses in plano power manufactured from PMMA plastic. Eye Scan Consulting markets such kits with an easy-to-follow flow sheet to help with proper calculations. The lenses in this kit are also color-coded for easy identification to help avoid erroneous data.

Maintaining Uncorrected Visual Acuity

Patients who have undergone corneal refractive procedures may some day need cataract surgery. Unfortunately, we don't yet have a single method to determine the IOL power that is 100 percent successful for such patients.

Many surgeons use the methods we've described and compare the results before lens implantation. Remember, this patient population consists of individuals who don't want to wear corrective lenses. A surgeon may call upon you to help determine what IOL power will allow the patient to remain correction-free.

The authors have no proprietary interest in Eye Scan Consulting.

Mr. Russell is an associate with the Contact Lens Service at Emory University Eye Center in Atlanta, GA.

Ms. Klein works in a private practice in Minneapolis, MN. She has more than 20 years of experience in ophthalmology and works regularly in clinical and surgical settings.

 


Contact Lens Spectrum, Issue: December 2004