Ocular Residual Astigmatism (ORA),
Vectorial difference between refractive cylinder (at the corneal plane) and corneal astigmatism,
refractive cylinder and corneal astigmatism mismatch
The differences between corneal and refractive parameters have been discussed throughout the book and termed Ocular Residual Astigmatism or ORA. How prevalent is ORA and what are the normal ranges of this parameter?
That’s a good question -we have done two studies on that – one in 1997 which showed an ORA of 0.73 D and the other in 2001 with an ORA of 0.81 dioptres, so it is in that range. However, another study which we did in 2007 on 45 keratoconic eyes the average or mean ORA in that group was 1.34 which is more than 50% more than healthy normal astigmatic eyes, So there is no doubt that in the presence of corneal irregularity such as keratoconus the ORA is increased.
Some people have termed ORA as lenticular astigmatism. How do you interpret that?
I just think it comes down to terminology again. No doubt the crystalline lens has astigmatism of a greater or lesser degree but that is not the total explanation for differences between corneal and refractive values. So I think ORA is a good term because it measures the difference between the cornea and the refractive cylinder all the way from the spectacles all the way back to the occipital cortex
What is the importance of calculating the ORA routinely before surgery?
By calculating the ORA the doctor can then predict ahead of time that the patient is going to end up with excess astigmatism remaining and advise them on lower expectations.