Chapter 3: Incisional versus Laser Ablative approaches

Cataract incisions, refractive laser ablations, Astigmatic Keratotomy, Ocular residual astigmatism (ORA), Wavefront and Vector Analysis How does the treatment of astigmatism differ when using incisional procedures compared to laser ablations? Interesting question because during the eighties I was doing Radial Keratotomy and then moved on to doing astigmatic keratotomy, , with incisions you always use the topography or the keratometry to operate at the steepest meridian. So the steepest meridian was the midpoint of your incisions. In 1991 along came the laser and all of a sudden, the whole paradigm changed because when you perform laser surgery with Excimer laser you operate according to the spectacles and the spectacle astigmatism cylinder is often on a different axis, a different magnitude, than the cornea. Professor Alpins, can you describe this difference between refractive cylinder and corneal astigmatism a little more? Differences between corneal astigmatism and refractive cylinder are very common. This can be in magnitude, axis or both. I have termed this difference the ocular residual astigmatism (ORA) and is expressed in dioptres. An ORA greater than 0.75D is significant. Did the introduction of wavefront treatments address this refractive corneal difference or ORA? No as the wavefront aberrometry that is measured is based on refractive parameters and does not consider the refractive corneal differences in astigmatism. It is important to also remember that the second order sphero cylinder measured using aberrometry does not incorporate any cortical component that is measured using manifest refraction.