Topographic mapping helps validate keratometry numbers before surgery
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Autokeratometry and topography together can be used to validate measurements when planning toric IOL placement and alignment, according to Warren E. Hill, MD.
To obtain accurate measurements of corneal astigmatism, the correct orientation of the steep and flat meridians needs to be determined, then the power difference between the two meridians is measured, Hill said at the virtual American Academy of Ophthalmology annual meeting.
“The numbers from the keratometry are really just that. They’re a set of numbers,” Hill said.
To help ensure predictable outcomes using those numbers, Hill obtains a standard topographic axial curvature map, looking at the topographic power distribution within the central 4 mm, and determines whether the power distribution is suitable for the toric IOL.
“Ideally, it should be both regular and symmetrical,” Hill said, meaning a line can be drawn through each of the two astigmatic lobes on the topographic map while passing through the center of the corneal vertex.
“To be symmetrical, we like to see basically the same power on either side of the corneal vertex,” Hill said.
Hill next finds the orientation of the steep meridian.
“That’s a lot of information from just this humble topographic axial curvature map,” Hill said.
Hill then obtains a set of keratometry measurements to ensure agreement with the manually determined steep meridian.
“We want to confirm that there is agreement between what we know to be correct and the autokeratometry,” Hill said.
Once the steep meridian is validated, it is more likely that the measured power between the meridians is likely to be correct, Hill said.
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Hill WE. Toric IOL planning and alignment. Presented at: American Academy of Ophthalmology annual meeting; Nov. 13-15, 2020 (virtual meeting).
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