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 exchange (see image below). You don’t really need to know any more than that, because essentially the central and peripheral parts of the lens are no different to a standard rigid contact lens design, and the amount of ‘reverse’ in the reverse curve is calculated for you within the lens design.
The principal difference between a standard rigid contact lens and an OrthoK lens, is that the best fit rigid lens is one that closely aligns with the cornea. Consequently, the lens is typically created to have close to the same central curvature as the cornea and the lens gets progressively flatter towards the periphery to align with how the cornea also flattens towards the periphery. The last curve at the very edge of the standard rigid lens is calculated to be flatter still to create a ski lip type profile that prevents the edge from digging into the cornea and allows tears to easily slide under the lens. So thinking back to your contact lens fitting 101 from those heady university days, you would typically measure corneal curvature using a keratometer and match the back surface curvature of the lens to the keratometer reading. Standard rigid lenses follow a curve based fitting philosophy.
The same is not the case for OrthoK lenses, as here we are fitting the optic zone of the the lens to be flatter than kerotmoetry because we explicitly want this zone of the lens to flatten the cornea. The degree of flattening is typically calculated following a formula known as the Jessen Factor. A problem this excessively flat optic creates is that the lens will now gradually stand away from the cornea towards the periphery leading to unstable lens fit. In fact, with one blink it is most likely to pop straight out. This is where the reverse curve comes in, because as its name suggests, this part of the back surface of the lens surrounding the back optic zone is manufactured to have steeper curvature than the central back optic zone. The steeper reverse curve causes the lens to return towards the cornea as we move further towards the periphery. What’s more, we can control how quickly the lens returns to the cornea by altering the steepness of the reverse curve. The steeper we make it, the shorter the distance it will take for the lens to come back in contact with the cornea. Who could think that designing contact lenses isn’t fun!
All great so far, but the problem we now face is that the reverse curve, to do its job, must be steeper than the cornea, so if we don’t flatten it out at some point the lens will eventually dig into the eye. This is where the alignment curves come in as these are designed to realign the lens with the underlying cornea out in the periphery. In summary, to make an OrthoK lens fit a cornea and provide a corneal flattening effect, from the centre out it needs to be fitted flatter than the cornea, then steeper, then the close to the same as the cornea. All be amounts calculated to match the same overal sagittal height as the cornea leading to the fact that OrthoK lenses follow a sag based fitting philosophy. Pretty complicated hey, and certainly more complicated than for your standard rigid lens where you only need to align the lens with the keratometry reading.
Fortunately, the solution to all of this is quite straight forward, which is that you need to think of OrthoK lens fitting as being sag rather than curvature based – a topic that I’ve covered in a separate post.