In a previous blog I described how OrthoK lenses are fit using a sag rather than curve-matched philosophy and that to achieve the best fit the sag height needs to be matched between the cornea and lens at the peripheral bearing point of the lens. The best way to achieve this is for your topography software to calculate the sag height at a chord length that matches the lens bearing point, which the manufacturer would have calculated to coincide with the mid-peripheral landing zone of the lens. If your topography software will give you a sag height measurement at this chord then great, but sometimes it will instead display an ‘out of range’ or ‘no data’ error caused by the measurement chord exceeding the limits of the captured data.
In this case your best option is to gradually reduce the measurement chord until a numerical value is displayed and enter this value into the software however, you must then also enter the measurement chord that was used in the lens design software otherwise you will be providing false information and will end up with a badly fitting lens. If your lens design software doesn’t provide this option, then it would instead be better to enter the best fit ellipse measurements provided by the corneal topographer.
Measuring weighted sag height from corneal topography
A further option that some topography software provides is the ability to extract a measurement of weighted height across a chord that takes corneal tilt into consideration. Corneas tend to be flatter in the nasal quadrant, which means that the sag height measured from the corneal apex to the lens peripheral bearing point on the nasal side will typically be less than the sag height measured from the apex to the temporal side.
Because the lens will need to meet both peripheral bearing points the average sag height needs to be established, which is easy to do by adding both together and then dividing by two. E.g. for a right eye with a flat meridian at 12 degrees, the nasal sag will be measured at an axis of 12 degrees, and the temporal sag at 12 + 180 = 192 degrees. For a sag of 1.465mm at 12 degrees and 1.495 at 192 degrees the average would be (1.465 + 1.495) / 2 = 1.48mm.
The weighted average height will do all of this for you and take into consideration other factors that influence sag measurement across a total chord making it the best sag measurement to use if it is available.