When measuring the HVID it can be difficult to know exactly where to measure from.
The placido image in the medmont topographer displays the cornea as dark, with a transition zone, moving to light grey.
We promote measurement from the edge of the dark part of the cornea. The transition zone is ultimately limbal tissue which is always flatter, and more irregular. Often you can see this in the outermost placido ring.
When delivering your EyeSpace lenses ensure that they do not overlap the limbus. Ultimately for Ortho-K you want the lens to be as large as possible without the lens bearing or overlapping the limbal tissue, especially in the vertical meridian.
In cases where a patient has a break in the limbal structure ( an example is a pterygium) we measure from the edge of the outermost regular placido ring.
HVID Measurement example showing the measurement tool being used to determine the HVID as horizontally as possible.
HVID example for the measurement when designing for a corneal lesion such as pterygium.