Keratometry
When looking at eyes that had valid measurements on the both the IOLMaster and the LENSTAR, there were a total of 103 eyes out of the 124 that met the criteria. The average keratometry for IOLMaster was 44.18D (sd, 1.637) and for the LENSTAR was 43.79 (sd, 1.959). As can be seen in the graph below there are 82 out of 103 (80%) eyes that are within 0.25 D, and 91 out of 103 (88%) eyes are within 0.5D. There is a slight trend towards reading the keratometry slightly at a lower number, although the differences are not statistically significant.
When considering the astigmatism or the difference in the keratometric readings in different meridians, designated as the K1 and the K2, the mean astigmatic power with the IOLMaster was 1.28D (sd, 1.512) in the 114 eyes out of 124 and with the LENSTAR, it was 1.15 D (sd, 1.155) in the 110 eyes out of 124.
If one were to crop out the individuals that have a total of 0.75 D or greater of astigmatism as benchmarked by the IOLMaster, the mean astigmatism in the 57 eyes had measurements on both instruments. The IOLMaster mean astigmatism was 1.89 D (sd, 1.745) and with the LENSTAR it was 1.63 D (sd, 1.292). As can also be seen on the graph 33 out of 57 or 58% of the eyes are within 0.25 D and 48 out of 57 or 84% are within 0.5D.
In the group of individuals with equal or greater than 0.75 D of astigmatism, when comparing the median axis of the astigmatism, setting the IOLMaster axis as the benchmark, 27 out of 57 (47%) of the eyes were within 0-4 degrees, 13 out of 57 (23%) were within 5-10 degrees, 12 out of 57 (21%) were within 11-20 degrees and 5 out of 57 (9%) were 21 degrees or more apart on the two instruments.
Disscussion of Keratometry Results:
Keratometry is the second most important factor in IOL calculation and sources of post-operative refractive error. It is impossible to tell which instrument truly measures the correct keratometry. Keratometry is location dependent and IOLMaster measures the K’s at 2.5 mm, yet the LENSTAR measures the K’s at 2.30 mm and 1.65 mm. Another factor is the axis of the keratometry.
Comparing the IOLMaster and the LENSTAR keratometry measurements revealed excellent correlation between the two instruments. The mean K as measured by each instruments showed 80% of the eyes were within 0.25 D. When only considering the amplitude of astigmatism (K1-K2), in the eyes with 0.75 D or more of astigmatism as measured by IOLMaster, 58% were within 0.25 D and 84% were within 0.5 D, with the caveat that LENSTAR typically measured smaller value. Additionally, 70% of eyes with 0.75 D or more of astigmatism had axis measurements within 10 degrees by both instruments.
As a clinician, I wonder, if there is any real clinical difference between the keratometry of the two instruments. Future projects should also consider how the inter-instrument correlation of similar make instruments would be. How would keratometric results of two IOLMasters, or two LENSTARs compare?
There is a tremendous amount of hype out there that one instrument is far superior in keratometry and calculations for TORIC IOLs. The current set of data clearly does not support such hype. Aside from "theoretical claims," I await to see real data and science behind the existing claims!
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Keratometry (Current Page)