Chapter 5: The Non-zero Target

Non-Zero Target, Corneal astigmatism target, Refractive cylinder target, Spherical equivalent You mention throughout your book differences between corneal and refractive parameters, particularly with regards to astigmatism. Let’s discuss this difference and what really the surgeon should be aware of. Well the difference between refraction and topography are prevalent and always have been. There are always going to be measurement differences between refraction and topography because there are so many different ways of measuring it. Even with different topographers and keratometers, they are measuring different parts of the cornea and you are going to get different numbers so when you are looking for a Toric implant you are looking at different corneal values to work out which is the one that most closely represents the refractive cylinder. Now you have termed corneal refractive difference the ocular residual astigmatism or ORA. Does it come down to the magnitude of existing astigmatism or does it happen with all magnitudes? The ocular residual astigmatism is a vector because you calculate it so it is quantified in dioptres and in degrees and it is the vectorial difference between the corneal value of astigmatism and the refractive cylinder taken back to the corneal plane. It can happen with any magnitude. It’s surprising how high an ORA can be, even with a lower amount of astigmatism when the brain perceives there to be quite a lot of refractive cylinder and so there is a misconception that you are going to have a high ORA when you have high corneal astigmatism. But that is a misconception because you can have a high ORA with low corneal astigmatism and high refractive cylinder, or low refractive cylinder and high corneal astigmatism. It works either way. It is the difference between the two. The term ‘non-zero target’ – why would a surgeon be targeting something other than zero when treating astigmatism? Well, this again comes back to the ORA where there is a difference between corneal and refractive astigmatism. The surgeon will either target to reduce the astigmatism zero in the refraction or on the cornea but will not be able to reduce both to zero because of the inherent preoperative differences. So there will be a non-zero amount of astigmatism remaining either on the cornea or in the refraction.