Below you'll find a list of all posts that have been tagged as “Topography”
Early successes in correcting myopia with OrthoK lenses were thought by these practitioners to be due to the cornea being bent into a different shape in response to the flat PMMA lenses that they were fitting at that time. Fortunately the pioneering research of Prof Helen Swarbrick and her team in 1998 revealed that this was not the case when they were …
Most corneal topographers in clinical practice use Placido disk image capture technology and typically come in two different versions: either bowl type like the Oculus Keratograph; or cone type like the Medmont E300. There are other approaches to measuring topography, like slit scanning (B&L Orbscan) and Scheimpflug (Oculus Pentacam) systems, each with their own differences in methodology but essentially, they …
To optimize corneal topography image capture your aim is to image as much of the corneal surface as you can, which can be difficult as eyelids and eyelashes get in the way. Here it is worth pointing out that the image quality scores that some topographers provide can be misleading in this regard. It is possible to gain what might …
As much as possible is the correct answer, but as a rule, you should aim for at least 4.5mm of available curvature data from the center in all directions. If this is too difficult to achieve in the vertical meridian you should be fine with 3mm of data above and below the center, although this will depend on the orientation …
Corneal topography is your main diagnostic tool for Orthok lens fitting. Get this right and everything else becomes so much easier. Do it badly, especially at first fit, and the rest of the journey will become harder. Fortunately, it’s not that hard a skill to learn and once you have the hang of it you will easily be able to …
Each corneal topography image holds a vast array of data measurement points that need to be broken down into a more user-friendly form. By example, the Medmont E300 captures data at around 6,000 locations and displays each location in four different formats. I think it’s fair to say that any lens design requiring manual entry for each of these locations …
The different terminology used to define ellipse shape can seem confusing at first. ‘Eccentricity’ is the preferred mathematical term, but the terms ‘Shape Factor’ and ‘Asphericity’ are also in widespread use. So which should you use? When it comes down to it this will usually depend on what is specified by the design you are using so from your topography …
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 …
This is where we turn to the multiple topography maps that in an earlier post I suggested you capture, because through averaging we can obtain a more reliable measure and remove potentially poorly captured maps. Hopefully your corneal topography software will do this for you, otherwise, it is not too hard to work out by adding up the individual K values …