IOL Power Estimation after Radial Keratotomy

RK IOL calcs

My technician brought the patient to our biometer (Haag-Streit Lenstar) and it simply would not read the corneal power for this patient. The prior radial keratotomy (RK) made the central cornea so flat and irregular that it confused the machine.

prior RK - biometry
Biometry data showing blanks (—-) for the K values for both eyes.

Since the corneal power changes the IOL power with almost a 1-to-1 ratio, knowing the keratometry is critical for IOL calculations.

prior RK topo and Auto Rx
While the SimKs show values of 34 & 35, the topographic map shows a central K power closer to 30 (the blue zone in the top, middle frame)

The corneal topography shows SimK values of 34 & 35 but the central corneal power on the map shows a value of about 30 diopters. The Placido image also shows distortion of the rings resulting in corneal irregularity and increased higher order aberrations,

prior RK corneal OCT mapping
OCT mapping of cornea showing pachymetry and epithelium mapping

OCT mapping of the cornea shows a net corneal power of about 28 D, with an anterior value of 31.8 and posterior value of -3.9.

prior RK tomography
The Dual Scheimpflug tomography shows a mean corneal power of 31.5 via ray tracing.

The dual Scheimpflug tomography is a way of analyzing many imaging cuts of the cornea which are then pieced together to make a three-dimensional corneal model. With an irregular cornea like this one, this modality may prove to be the most useful.

At this point we have the following corneal power readings:

  • Lenstar: —- (unable to read)
  • Topography:
    • simK 34 / 35
    • via topo image 30
  • OCT: 31.8 anterior surface
  • Dual Scheimpflug Tomography:
    • simK 32.7
    • tangential 37
    • ray traced 31.6

SimK values are attempting to simulate the K values from a traditional keratometer which measures at about a 3.5 mm optical zone. These are going to be inaccurate because the central optical zone of the cornea after RK is very small, less than 3 mm in diameter.  Do not use SimK values here.

The fear in RK eyes is that we get a post-op hyperopic surprise because of using too high of a K value which calculates a lower IOL power. 

To avoid the post-op hyperopic surprise in these patients, we need to use a lower K value, so something in the range of 31 diopters and then on top of that, we need to use an online calculator or an adjustment method to calculate.

In this case, we have estimated that a +28.0 D IOL (Alcon SN60WF) will give us an outcome between plano and -0.50 post-op.

How did we do? About 6 weeks after cataract surgery the patient has a refraction with a spherical equivalent of about -0.25 as show here:

Screen Shot 2018-08-17 at 1.16.17 PM

The auto-refractor picks up a large degree of corneal astigmatism but this is not manifest in the patient’s subjective refraction. Without correction, he sees 20/40 because his central cornea has multiple different zones of refraction. We did not use a toric IOL for this patient because of the corneal irregularities and the patient wishes to go back to his hard contact lens use for best visual quality.

The patient was very happy with the visual results from his cataract surgery because we achieved a good refractive outcome and we also set very reasonable expectations.

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