You have heard about post lasik IOL power calculation difficulties being discussed but never really understood the challenges involved in its essence. If so, this article will take you step by step, the challenges in IOL power calculation in post lasik eyes and how to overcome those challenges
Figure 1, shows us the four broad challenges. All Keratometers, whether auto Keratometry including optical biometry or manual keratometry, do not measure the posterior corneal power directly ( with the exception of IOL Master 700 TK). Instead they depend on a single keratometry index (usually 1.3375, but can differ ) to give the power of anterior cornea.
This works only when the cornea has an average anterior/posterior radius of curvature ratio (82%). But this ratio is different in patients undergoing myopic Lasik or hyperopic Lasik, depending on the extent of laser ablation undergone. Thus if traditional keratometry is done, this may result in a wrong measurement, since only anterior cornea is measured with such keratometry devices. On the other hand the IOL Master700 with TK ( see here how it works: https://www.quickguide.org/post/what-is-iol-master-tk ) or the Pentacam ( see article Understanding Different Corneal maps of the Pentacam ) or other tomography machines may help.
The second challenge is the measurement of corneal diameter that is inside of the central 1 mm zone of the cornea, as this is the area that is important especially in myopic lasik patients. To understand this, we need to have an understanding of how a keratometry device works. An IOL Master500 measures 2.5 mm of the cornea, while the Lenstar would measure 1.65 and 2.3mm of cornea. Manual keratometry machine generally measures 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 area of cornea, rather than a zone measured by a corneal topography device.
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 standard keratometry device. 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 and a hyperopic outcome postoperatively, as the central 1mm zone of the cornea may be flatter for a myopic lasik patient than we estimate with a traditional keratometer. Figure 3 from Pentacam EKR report demonstrates how the central 1mm of the cornea in a myopic lasik patient may significantly differ from other areas around the visual axis. It is important to note that even ordinary corneal topography 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.
The third challenge (Figure 2) with regard to post lasik patients is the choice of IOL formula. Standard two variable formula like Hoffer Q, SRK T, or Holladay I may not work for such patients. These formulae partly estimate the post operative IOL position (Effective Lens Position) based on the corneal power, that is, a steeper cornea will have a higher postoperative/pseudophakic anterior chamber depth (pACD), while in a flatter cornea this may be the opposite. But since in a post lasik patient, the anterior corneal curvature is altered, this may lead a false assumption of pseudophakic ACD. Thus a formula that is especially designed to take into account this important aspect of post Lasik cornea is a need. Several post lasik IOL calculation formula are available:
ASCRS post refractive IOL calculator (https://ascrs.org/tools/post-refractive-iol-calculator)
Barrett True-K toric calculator (https://ascrs.org/tools/barrett-true-k-toric-calculator)
Ocular MD post lasik IOL calculator (http://iol.ocularmd.com/)
However, a standard IOL power calculation formula like SRK T may be used if you are using the Holladay EKR from the Pentacam. For a detailed understanding of how Holladay EKR works and how it could be used with standard IOL calculation formula in post myopic lasik patients refer to my article 'Guide to Pentacam Holladay Report' (https://www.quickguide.org/post/holladay-ekr-report) in this blog.
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. Accurracy of any calculation and good postoperative outcome depends on the data being input and this is particularly true for post Lasik patients. 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.Thus there are guidelines laid down to help you distinguish a regular from an irregular cornea. My own biometry sessions do cover this aspect extensively.
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