Astigmatism-related fitting questions from fitters and replies by Paul Rose
last updated 2020.12.29
Q: If I incorporate some residual astigmatism onto the front of the lens, will this have any effect on the way the lens fits or performs?
A: Incorporating residual astigmatism on the front of the lens will NOT affect the fit for Rose K2 XL. However, for corneal lenses particularly in smaller diameters it can cause the lens to ride lower so only incorporate residual astigmatism when it makes a significant improvement to the VA as often with keratoconus corneal lenses tend to ride low because of a decentered apex and this just exacerbates the problem.
Q: Is it possible to find the right contact lens, with high astigmatism and irregular cornea, at the first application?
A: Not often but this certainly can happen. The fitting guide is ONLY AN INDICATION FOR THE FIRST TRIAL LENS. Accurate assessment of the fluorescein pattern is the only way to assess the best fit.
Q: How much of astigmatism can we ignored, by front toric?
A: Usually 0.5D cyl is not warranted to correct. However, my rule is that it is not worth correcting unless it makes a significant improvement to the vision. You can verify this by adding it over the spherical powered trial lens.
Q: How to estimate the degree of toricity required without topography?
A: There is no easy way. I recommend initially using the standard amount of toricity and order the lens warranted so you can increase or decrease the toricity with the second lens order.
Q: How can we calculate FST: only over refraction ?
A: I assume you are referring to a bitoric here. The lab can automatically calculate this for you based on the back surface toricity. This is called a compensated toric. However this does not take into account any internal astigmatism so the most accurate way is using a toric trial lens made with a spherical power on the front and do an overrefraction.
Q: Does peripheric toric work on oblic?
A: Yes absolutely, the flat and steep axis must be 90 degrees apart. Ex: If the flat axis is at 30/210 the steep axis will be at 120/300. Note the lens will orientate itself to the correct axis on insertion.
Q: What to do when the astigmatism is ATR?
A: If you are fitting a TP then the TP will orientate correctly so the flattest axis will sit around 90 not 180. The lens on insertion will orientate itself correctly. However ATR astigmatism often cause corneal lenses to ride lower so if this is an issue fit XL.
Q: How to write to supplier when you consider ordering both toric edge and front toric prescription lens?
A: You will need to state on your order the flattest axis on the cornea which is where the flattest axis on the lens will align. The lab will need to know this so they can then calculate the axis between this axis and the axis of the astigmatism on the front of the lens.