Astigmatism Surprise After Refractive Surgery

Misalignment of the surgical treatment is the major source of refractive surprise in relation to astigmatism. Sources of misalignment include cyclotorsion from the seated to supine position, a physical turning of the patients head or intentionally placing a cataract incision on a meridian other than the steepest corneal meridian due to ergonomic factors or to more accurately neutralise the corneal astigmatism using a toric IOL. Corneal incisions, no matter how small, should be analysed vectorially to determine what effect, if any, they have had on the preoperative corneal astigmatism. Refractive cataract surgeons employing a technique to correct astigmatism at the time of surgery (toric IOLs, LRIs, etc.) need to consider the effect of the phaco incision on the remaining astigmatism; otherwise, the IOL or LRI will be misaligned and/or undercorrected. The forces acting to change the corneal structure in a misaligned treatment are flattening (or steepening) and torque. These result in a reduction (or increase) of astigmatism at the intended meridian and also a change (rotation) in the meridian of the astigmatism. Furthermore, placing the toric IOL at an axis that is not the steepest corneal meridian or the toric IOL rotating over time. Vector analysis is a useful tool to calculate the effects of a misaligned treatment on the remaining astigmatism.