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| 1. Focusing Spot. 2. Crosshair |
Six spots of light are projected onto the cornea in a hexagonal pattern with a diameter of about 2.5 mm. The distance of each sport to the visual axis is about 1.3 mm. The position of each pair of reflection spots is detected and measured but the IOLMaster keratometer, the relative positions of each pair are compared to determine corneal curvature and astigmatism as radial measurements.
From the position of the focused spots on the cornea, the IOLMaster keratometer provides information on how curved the cornea is at each spot. The nearness of the spots to one another indicates how steep or flat the cornea is between those points. Flatter corneas cause the spots to be located further from one another, whereas steeper corneas cause the spots to be located closed together.
The IOLMaster keratometry determines the radius - an imaginary line that extends into the eye perpendicular to the corneal surface from the reflection spot. These two lines meet at the center of the imaginary circle. The corneal surface between the two points of reflection is sections of a circle, while the points where the two lines meet are the center of the same circle. An imaginary circle is formed, and the lines to the center are radii of the circle.
The size of the imaginary circle is determined by the reflection points -points that are closer together will produce smaller radii, while points that are further apart will produce larger radii.
The IOLMaster converts the radial measurements into diopters and it determines the total corneal power from the anterior curvature of the cornea using a keratometry index. In the USA the defaults of the keratometry index is 1.3375
Differences between the IOLMaster keratometer and manual keratometer
A manual keratometer measures the corneal curvature at 3.0 - 3.2 mm. The manual keratometer assumes the cornea is spherical and compares 2 measurements spots on two chord diameters, 90 degrees apart from each other. Since the IOLMaster measures at 2.5 mm, the IOLMaster keratometry is often slightly steeper on average than a manual keratometer measurement. On a post-refractive eye, the IOLMaster central Ks should be slightly flatter than manual Ks.
When comparing the IOLMaster and manual Ks, although the numbers may appear different, the averages most times are very similar. For example:
42.00 D @ 180 / 44.00 D @ 90 versus 13.00 D @ 180 / 43.00 D @ 90 degrees. Both readings look different but the average is the same. Formulas use the average K for calculation.
The IOLMaster measures curvature based on the relative position of six spots on the corneal. Manual keratometry measures two reflected mires. Therefore, the IOLMaster provides more central measurement of the cornea at visual axis.
Manual keratometry requires a highly skilled technician. The mires in the eye piece must be careful focused, multiple users often forget to adjust this focus for their own eye, inducing measurement error. The reflected mires on the cornea must also be focused well. Transcription errors are common when manually charting data from the manual keratometer to a computer used for IOL calculations. The IOLMaster keratometry offers less technician dependence and version 5 and 500 eliminates many of the prior versions focusing issues. The IOLMaster keratometry can be delegated to several trained staff members with virtually no inter-operator difference in measurement accuracy.