Why recalibration for EKR devices may be crucial

May 11, 2016

Further calibrations for the equivalent keratometry readings (EKR) obtained by the Pentacam High Resolution (HR) (Oculus, Germany) should be conducted before they are used for total corneal power assessment in post-LASIK eyes, researchers said.

Further calibrations for the equivalent keratometry readings (EKR) obtained by the Pentacam High Resolution (HR) (Oculus, Germany) should be conducted before they are used for total corneal power assessment in post-LASIK eyes, researchers said.

Related: What you should know about SMILE and wavefront guided LASIK

“Compared to the total corneal power obtained by the clinical history method, the equivalent keratometry values generally overestimated the total corneal power in post-LASIK eyes,” wrote Yanjun Hua from Shanghai Jiao Tong University and colleagues.

The researchers published their findings in PLOS One.

In order to predict IOL power after corneal refractive surgery, clinicians must accurately assess total corneal power.

Recent: Learn how curiosity propels astigmatism innovations

The clinical history method and the double-K methods both require preoperative data. The Maloney method, the Shammas formula, and the BESSt formula do not require preoperative data. The Holladay’s equivalent keratometry readings obtained by the Pentacam HR also doesn’t require preoperative data, but it’s precision for assessing total corneal power after corneal refractive surgery is not well established.

The researchers wanted to assess the repeatability of the equivalent kearatometry readings in all central corneal zones.

Recent: How IOL platform designs are achieving quality of vision

They enrolled 100 untreated eyes (from 47 men and 53 women) with a mean manifest spherical equivalent refraction of -3.23 D. They also enrolled 71 post-LASIK eyes (from 24 men and 19 women) with a mean manifest spherical equivalent refraction of -5.01 before the surgery and 0.03 after the surgery. The mean amount of refraction change in dioptres before and after the surgery was 5.05.

In both the untreated group and the post-LASIK group, all the Sw values were less than 0.22 D, the 2.77Sw values were less than 0.62 D and the CVw values were less than 1%. All the intraclass correlation coefficients were higher than 0.978.

Significant differences

 

The researchers found high repeatability of all the equivalent keratometry readings, SImK, and Knetvalues from both untreated eyes and post-LASIK eyes using the Pentacam HR.

Among all the EKRs in the untreated group, the researchers found, the EKR2 was the smallest at 43.39 D. It was not significantly smaller than ERK1 at 43.4 D (P = 1.000), but it was significantly smaller than all the other ERKs and SimKs, and significantly higher than Knet (P <0.001).

Related: Keratoconic eye experience with a long-arc intrastromal corneal ring

SimK was 43.48 D in the untreated group, which was higher than EKR1, EKR2 and EKR3, but lower than EKR4, EKR5, EKR6 and EKR7.

In the post-LASIK group, the researchers found a decrease from EKR1 to EKR4, which was the minimum at 39.05, then an increase to EKR7. The clinical history method yielded a value of 38.40 D.

This trend differs from two previous studies, one by Falavarlani et al of 35 eyes after PRK, and one by Savini et al after myopic laser correction of 16 eyes. In these studies, the values gradually increased from EKR1 to EKR4.5. The researchers speculated that the difference might be explained by the various surgical approaches and laser systems used.

Related: Clinical experience with a mechanical pupil dilator

The clinical history method, first presented by Holladay et al in 1989, has been seen as the gold standard for measuring and calculating the total corneal power after corneal refractive surgery. Still, there is no consensus on the best device for this purpose.

In this study, the clinical history method produced smaller values than all the EKRs. It was 0.65 D smaller than EKR4, the smallest of all the EKRs (P < 0.001).

More: Are extended-depth-of-focus IOLs hitting the visual sweet spot?

The Km obtained by the IOLMaster was equal to EKR4 and similar to SimK. This was higher than the values obtained by Savini et al, but lower than those obtained by Falavarjani et al. The researchers speculated that this difference could be attributed to the preoperative refraction of the subjects.

In this study, the mean preoperative spherical equivalent was -5.01D. In Savini et al, this value was -5.10 D, and in and Falavarjani et al it was -3.46 D. A higher refractive error will lead to a larger amount of surgical correction in the event of similar mean corneal power. This leads to a flatter cornea with smaller postoperative corneal power.

Based on all three studies, the researchers supposed that the EKR overestimates the corneal power in post-LASIK eyes.

Recent: What NASA astronauts can teach us about glaucoma

They offered two observations in support of this proposition. First, they noted that in their study the EKR4, which was the smallest among all the EKRs, was 0.6 to 0.7 D higher than the gold standard, the clinical history method.

Second, the SimK, Km, EKR4, and EKR4.5 in this study were very close to each other, and both Savini et al. and Falvajani et al obtained the same results. The researchers calculated SimK and Km by the same formula, which explains why they found no statistical significance observed between SimK and Km.

More: How sleep patterns may affect MGD, dry eye

Although these researchers could obtain accurate results using this formula for untreated eyes, the corneal power was overestimated after corneal refractive surgery because of the change of ratio between the posterior and anterior corneal curvature. In this study, EKR4 – the lowest value of the EKRs – was equal to Km, and similar to SimK.