Why You Need To Think Beyond Your Trusted Keratometer

Assume Mr X had LASIK several years ago for -8.00 D of myopia and is now scheduled to undergo cataract surgery...

You have carefully gathered together all of the historical information needed and have even measured the corneal power its exact center with one of the newer, very sophisticated Keratometry system. The measured and historical numbers all line up and you are confident that your estimation of central corneal power is as accurate as possible. Using a post myopic lasik formula, you calculate for an IOL power that seems appropriate.

His surgery goes well, but to your great surprise, the first post-operative week Mr X has a spherical equivalent of +2.00 D. You go back and re-calculate everything, confirm the axial length and still arrive at the same IOL power...

What went wrong ?

 It turns out that there are quite a few limitation that we have in evaluating the cornea and determining IOL power. One of the biggest challenge is determining corneal power with Keratometry.

The Keratometer, irrespective of manual or auto Keratometer, or Keratometry with Optical Biometry, have their own limitation. To understand this, we need to have a basic understanding of how a keratometer work. An IOL Master measures 2.5 mm of the cornea, while the Lenstar would measure 1.65 and 2.3. A manual may measure between the 3.0 mm to 3.2 mm of the cornea. But what is important to note is that they measure only a ring in these particular points rather than a zone measured by a topographer. Thus scheimflug based tomography being equipped with a rotating camera would help us to better understand the central 1mm of the cornea, of which we are blind if we use a keratometer. This is important for post myopic lasik patients as traditional purkinje image based keratometry is blind in measuring the central 1 mm of the cornea. This can lead to often overestimating the corneal power, as the central 1mm zone of the cornea may be flatter than we estimate with a keratometer. It is important to note that even an ordinary topographer cannot measure the central 1mm zone of the cornea, as you would have the Placido based camera projecting the mires on the cornea situated there.

Another basic limitation of a keratometer, no matter how advanced it is, is that, a keratometer cannot determine an irregular cornea. This assumes special significance in case of TORIC IOLs and multifocal lenses. A Keratometer is based on some assumptions and the final data derived is based on extrapolation. As earlier explained, a keratometer only measures a ring, and it cannot determine if there is a irregular cornea outside this ring. Hence, the steep and flat meridians are always given as 90 degree apart. Hence verification of corneal symmetry with corneal topography is more important in cases of planning for a toric IOL or beyond.

That being said, every keratometer would give some underlying indication whereby you can understand that the patient may need more diagnosis beyond the standard keratometry. A key indicator is the standard deviation and high fluctuation of keratometry values between each individual readings. Do not miss the purkinje reflex of the cornea, to evaluate irregularity. Be particularly cautious and refer to topography if the two eye difference is more than a diopter. And, above all keep a special eye on readings outside the normal range.


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