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 …

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 …

How corneal eccentricity impacts refractive change from OrthoK

In general, corneas with greater eccentricity offer greater potential for OK induced refractive change than more spherical corneas, which was shown quite nicely by John Mountford in his landmark paper published in 1997.1 John established a regression formula to associate induced refractive power change to change in corneal eccentricity that had a high correlation. He also stated that the endpoint of …

How cornea curvature influences outcomes from OrthoK

In a previous post, I covered how the degree of corneal eccentricity needs to be taken into consideration when evaluating the extent of refractive change can be achieved from Orthok in an eye. In this post, I am going to cover how corneal curvature also has its part to play. The generally accepted wisdom is that steeper corneas tend to …

How corneal toricity influences OrthoK lens fit

Before digging deeper into this topic, clarity is needed on the difference between corneal toricity and corneal astigmatism. When using the term toricity we are referring to the physical shape of the cornea and specifically how corneal curvature differs between the flat and steep meridians that are perpendicular to each other. In general, this will be the difference in curvature …

OrthoK lens selection at initial fit

By now you will have your patient’s spectacle prescription to hand, measured their corneal topography and have conducted an anterior eye exam to make sure it is safe to proceed, assuming of course that you have already recently assessed the rest of their eye health to ensure that all other parts of their visual system are healthy and in good …

Teaching OrthoK lens insertion

The approach here is much the same as for standard rigid lenses, which is to place the lens directly on the cornea in whichever manner the patient feels comfortable and is most proficient. You also need to demonstrate how to handle a lens that doesn’t end up where intended and displaces onto the conjunctiva, and before leaving your office your …

Initial OrthoK lens fit assessment

Depending on how much you have already read or what advice you have received, you may be under the impression that you should assess the lens fit before proceeding to overnight wear. While I agree with the potential benefits this offers, in practice, I have found mixed success over what would have been achieved from just using the predicted best …