While the terrible disease keeps spreading, it appears there is a sparkle of light in some parts of the world. It isn’t gone by any means and although it is too soon to draw final conclusions, it appears that in the Netherlands at least we seem to have reached some kind of plateau. That is what they have been calling #flatteningthecurve. We can only hope that this is true and that the rest of the world will follow soon.
From Curves to Sagittal Height
I can’t help it – but every time I see the ‘Flattening the Curve’ phrase, I simply have to think about contact lenses. Call me a contact lens maniac if you want. All we ever did when we were fitting corneal (R)GP lenses was ‘fattening the curve’ a tiny bit in comparison to the curves of the cornea to achieve a good lens fit. This flattening and steepening terminology we transferred into soft lens fitting when they got on the market exactly 50 years ago. So, the ‘flattening’ and thinking in curves has its heritage in good old hard lenses – but it doesn’t make sense actually to do so. In fact: in soft lens fitting instead of going flatter, we actually have to go steeper or (better) deeper than the corneal shape. Let me explain why that is.
Soft lens fitting is all about the overall sagittal height of the lens. What we know from recent studies, is that the typical soft lens has to have a somewhat larger sagittal height than the sagittal height of the ocular surface over the same chord. That difference is called the delta-SAG and is actually represented by the little air bubble we typically see when we apply a soft lens to the ocular surface. The lens will then ‘drape’ or ‘flex’ to align with the shape of the eye. Without that delta-SAG, a lens would by definition move too much, or is too ‘flat’ if you want, and is not a comfortable lens. There is no magic number to give for the delta-SAG amount – it depends on many factors (mostly material related) but it is expected that the average lens has to be 100-300 micron more (or deeper) than the ocular surface. But in any case, it represents a ‘deeper’ appearance of the lens on the ocular surface, not a flatter one.
When we change the base curve on a soft lens, let’s say we flatten it, it is not about flattening the curve so much that changes the lens fit on-eye: it is the change in overall sagittal height of the lens caused by the change in base curve that has the desired effect. Same with lens diameter: an increase in lens diameter, quite drastically, increases the overall sagittal height of the lens – with a potential profound effect on lens behaviour (supposedly a tighter lens fit in the case of lens diameter increase). In other words – and I quote Fiona Stapleton here:” both base curve and diameter are surrogate measures for sagittal height”.
Flattening the Curve on Dropouts
Dropout is a major problem in contact lens practice. There are many factors that contribute to this. But it is pretty obvious that if we want to flatten the curve on dropouts – the first thing is to look closer at the lens fit and how we approach this. See the editorial ‘When was the last time you fitted a soft lens’ in Contact Lens & Anterior Eye with Lyndon Jones and James Wolffsohn on the topic. It is about time – at the celebration of the 50th birthday of soft lenses to change the way we approach its fitting. And lets now hope regarding corona we are indeed #flatteningthecurve. Although I have to say – it is more about lowering the overall sagittal height of the curve than about the flatness of it as such in all honesty.
This Eef@online series is kindly supported by an educational grant from Contamac