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Beyond Base Curve
Understanding Soft Lens Fitting
We have better soft lenses than ever before – in terms of material and replacement frequency. The adage ‘one-size-fits-most’ applies to these lenses, but that also means ‘not to all’. The concept of soft lens fitting based on central keratometry values plus an additional something to calculate the base curve of a soft lens has proven to be somewhat of an illusion: there appears to be no direct relationship between central keratometry values in isolation and the way a soft lens behaves on the eye. Worse: it gives the complete wrong message. As opposed to rigid corneal lenses, soft lenses are not fitted flatter to the eye, but deeper! Without any type or form of grip, the lenses would either move excessively, decenter dramatically and/or simply slide off.
Still, ‘the flatter than’ is what most of us use in contact lens education around the world to teach our students. Is there a better way to understand soft lens behaviour on the eye – and to teach this to our students – beyond being limited to base curve alone?
This webiste provides provides an overview of everything we know – with all ingredients for a fruitful discussion on the topic with participants. For starters there are articles on understanding ocular surface shape of there chord (diameter) of where the soft lens lands, let’s say 15mm. This is our workbench. There some other articles dive into the terminology and understanding of soft lens shape – beyond base curve – which proves to be an arbitrary value. For this, the working theory is using sagittal height, or SAG. An overview of commercially available lenses and their SAG values are given. Then – if we combine information about the ocular surface height with the height of the lenses: what would be the optimal lens fit? For this, one has to define the optimal delta-SAG (the GAP between the lens and the ocular surface SAG). One way of looking at this, is by using on-eye diameter increase. How this works can be found in the latest addition to the library titled ‘DEEP thoughts on soft lens fitting’.
Stay tuned – there is more to come on this. Hopefully this way we can get some ‘grip’ on soft lens fitting, improve comfort and vision – and reduce drop-outs.
Survival of the Fitting
https://softspecialedition.com/survival-of-the-fitting.html
As always with articles in this section comments are welcomed by posting here https://www.eyedropsdatabase.co.uk/eddb-the-forum/discussions-on-guest-articles
