Flattening, Steepening and Torque, Incision flattening, steepening, rotation, Corneal astigmatism, flat corneal meridian, steep corneal meridian The terms of flattening, steepening and torque. Please describe those for us as far as incisional surgery goes. The SIA is a composite parameter that includes how much the astigmatism has been reduced by flattening effect or how much it has been increased by steepening at the intended meridian or axis of astigmatic treatment. It also includes torque, how much it has been rotated in a clockwise or anti-clockwise sense. In a practical sense, if you get a keratometer and measure at the intended meridian and then you do the surgery, and measure at that point again with the same keratometer it shows you how much flattening or steepening happened at that axis. So the flattening effect is how much affect the SIA is having at the treatment meridian and what’s happened to the rest of the SIA which is the torsional effect. So SIA is a composite of flattening and torsion How much reduction in effect is achieved when an astigmatic treatment is misaligned? Using vector analysis and not just comparing magnitudes, an astigmatic treatment that is 15 degrees off-axis loses approximately 15% of the flattening effect and 30 degrees off axis results in a 50% loss of effect. It is not until the misalignment is 45 degrees that there is a 100% loss in flattening effect. This was shown in a recent paper by Dr Tognetto in the JCRS.