Correcting astigmatism with OrthoK

When it comes to astigmatism, the research shows that approximately 40% of the initial astigmatism remains post treatment when using spherical OrthoK lenses.1 Following this research, -1.25DC is likely to end up at -0.50DC which, in most cases would be considered clinically acceptable. By the same conversion -1.50DC would give a predicted outcome of -0.60D, which, while not much different, is beginning to push …

What makes an OrthoK lens?

While there are variations across different OrthoK lens designs, there are some components that are shared across all designs. All myopic OrthoK lens designs have a back-surface optic base curve that is fitted flatter than corneal curvature, some form of reverse curve to bring the lens back into contact with the cornea, and peripheral alignment curves to provide a comfortable edge lift and tear …

What is the Jessen Factor?

George Jessen was the first to publish on OrthoK in his 1962 paper on Orthofocus Techniques that described the concept of using rigid PMMA to temporarily correct myopia, hyperopia and astigmatism.1 The concepts Jessen introduced evolved to the OrthoK designs and approach to fitting used today. It is the calculation of the base curve for an OrthoK lens described in this …

Sag based approach to lens fitting

Sagittal (sag) height describes the height of the cornea at a given point. If you imagined resting a completely flat surface onto the front of the eye there would be no gap between the flat surface and the cornea at the apex, as this is where it is resting on the eye. Move 2mm away from the apex though and …

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 …

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 …