Below you'll find a list of all posts that have been categorized as “1. Background Learning”
OrthoK has been around for a long time, from around 1962 in fact which is when the first scientific journal paper on the topic was published. Back then practitioners found they could correct myopia (short-sightedness) by getting patients to wear progressively flatter rigid lenses manufactured in PMMA (Perspex) material. It was well known that wearing rigid lenses caused corneal distortion, …
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 …
In an earlier blog I described how OrthoK remodels the thickness profile of corneal epithelium – so what can this tell us about limits to refractive change that can be created? The average thickness of a person’s anterior corneal epithelium is 50µm, so if we are going to squish it we can’t squish it by more than 50µm (1µm = …
The amount of refractive correction that can be achieved by OrthoK has become such a contentious issue that it warrants more detailed explanation than I provided in my first blog on this topic. Welcome to the world of statistics – but don’t let this put you off, this is basic stats that you will be already familiar with in one way or …
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 …
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 …
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 …
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 …
Except for a few limits really anyone who is myopic and happy and able to wear contact lenses! Well, there are some limits to this rule to find the true key to success, but not many. Refraction Limits of refraction correction were covered in detail in an earlier blog where I made a case for accepting -4.50D with up to …