Our main phaco incision will induce astigmatism and multiple variables affect the degree of this change. Larger incisions have more of an effect and so do incisions with shorter tunnel lengths. The size of the eye also plays a role because an identical incision in an eye with a smaller corneal diameter will have more effect than in an eye with a larger diameter.
The most important lesson that I can teach you with respect to toric IOLs is where to place your incision. The ideal position is on the steep meridian, and we can even place it on the flat meridian, but we do not want to place the incision at an oblique angle to the steep and flat meridians. Why is this?
Remember that astigmatism treatment is the additional and subtraction of vectors, which have both magnitude and direction. The magnitude is the amount of astigmatism in diopters and the direction is the meridian in degrees. Imaging an airplane flying from point A to point B. If there is a perfect headwind or a perfect tailwind, then the airplane will keep the same direction but change in its speed (slower for a headwind, faster for a tailwind). If there is a crosswind instead, then the plane will change direction and speed. The plane will be blown off course and will not head towards point B unless the pilots compensate for the crosswind.
For astigmatism treatment with toric IOLs, we want our incision to be a perfect headwind or tailwind, and not a crosswind. If we make the incision on the flat meridian, then we can compensate by increasing the toric power. If we make the incision on the steep meridian, then we can incorporate that into the toric power calculation or even decrease the toric IOL power required.
Click below to learn the important aspects of surgical planning for this toric IOL case: