Eight steps to better corneal topography

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 …

Modelling corneal shape

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 …

Descriptors of elliptical shape

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 …

Calculating sag height from an ellipse

Sadly, some topography software doesn’t offer the option to calculate sag at a measurement chord, in which case the next best option is to use the ellipse model that, as far as I am aware, is provided by all corneal topography software. This isn’t quite as accurate a determinant for sag corneal height at the peripheral lens bearing point as …

Types of OrthoK lens

Regardless of design all OrthoK lens designs all aim to achieve the same outcome. That is for your patient to ultimately end up with a lens that they can continue to wear on a nightly basis, which will give them optimal vision during the waking day. They just go about this in slightly different ways. Empirical designs will be calculated as a …

Which OrthoK lens should you use?

All things considered, there is no right or wrong design. Each has its advantages and disadvantages as just described. Empirical lens fitting offers a low-cost entry point into OrthoK lens fitting. Recent improvements to computer modeling have also led to ever-increasing first-time success rates with these designs, which offsets their potential inconvenience for the delay in organizing a replacement if needed at …

Empirical OrthoK systems

Empirical lens fitting is controlled entirely by the lab which makes it analogous to the ‘auto’ or ‘P’ button that has become common on digital cameras. In other words, a good outcome will be achieved most of the time, but like the auto camera setting, it will do a bad job if you fall outside of its normal range of …