Updated: Mar 22, 2019
Recently I was involved in a discussion on different makes of Keratometers by different companies. Many of the participants voiced their concern on the difference of reading between many different Keratometers, leading to a confusion as to which data to be relied on. On a separate article named " What to do when your K readings are different from different Keratometers " this issue has been addressed. This article however would concentrate on why do different Keratometers, optical or otherwise, fail to have consistency amongst the readings. In answering so, I would also take you briefly through the science behind the calculation of the modern optical biometers.
As we understand, different Keratometry devices read different rings of the cornea. While the manual Keratometers would typically measure at central 3.0 to 3.2 mm of cornea, the optical Biometry machines like IOL Master would measure 2.5 mm of the cornea. The Lenstar measures two separate rings of 1.65 and 2.3 mm approximately. The Verion from Alcon measures a peculiar central part of .8 to 1.2 mm of the cornea while it takes the axis from a broader ring beyond this area. As you can see, none of the devices measure a fixed area, and it varies between the make of the devices. One big reason for the differences of the readings is heterogeneity of measurement areas.
Another factor in the difference of the reading is the Keratometry Index ( K Index ) employed by the devices. A point to be noted here is that a Keratometer does not measure in diopters. It measures the Radius of Curvature in mm, which is then converted back to diopters through a formula ( D=n-1/r ) where n is the Keratometry Index. The European and US make devices do not have the same K Indiex. This would therefore lead to further differences in the final K reading between the devices.
The last but not the least important is to understand how does the modern optical devices measure the cornea ? The Key here is to understand the Purkinje Reflexes from the cornea. None of these devices measure the Posterior Cornea ( with the exception of Zeiss Total Keratometry recently launched ). The IOL Master version V would throw six spots of light on the anterior cornea in a hexagonal pattern in the diameter of 2.5 mm. So the distance between each spot is 1.3 mm to the visual axis in a perfectly symmetrical and spherical cornea. The position of each spot reflecting back from the cornea and relative to the other light spots would be picked up and analyzed by the computer. Light reflections closer to each other would signify a steeper cornea and vice-versa.
The radius of curvature is determined by perpendicular imaginary lines that extend to the center of the cornea. If a cornea was absolutely spherical and a circle, only two spots of light would have been enough to determine the curvature. But a cornea is rarely spherical and has some amount of astigmatism in it. This therefore demands more spots of light projected on the cornea. The lines that extends from the light spots on the cornea to the center of the circle helps us to determine the radius of curvature of the cornea in mm. As said earlier, this is converted back to diopter through the K Index and the formula.
Different Keratometers measure different diameter of rings of the cornea. That being said, more the device measures the central portion of the cornea, the more it may report a steeper cornea. This is because a cornea is generally prolate in shape and hence a steeper cornea is reported when a smaller diameter is measured.
However, the exception may be the VERION from Alcon. I have been often asked why the VERION reports a flatter cornea when it measures a very small central portion of the cornea.
My answer is that this is because the flattest portion of a hill is not in the middle but at its top. If you go beyond a certain area and measure too centrally, you may get more flatter cornea measured and reported than other devices. While this may be good for post myopic Lasik cases, this may not be very ideal for average patients, and even in eyes with central pathology. Keep in mind, the cone in a Keratoconus patient is not in the center, but inferiorly displaced or decentered (-Y axis ).
That being said, this article is not to bring out the advantage and disadvantage of different devices in the market, but understand the science behind it.
Keep an eye on this blog for more interesting articles in the future.