Toric IOLs allow us to surgically treat corneal astigmatism at the time of cataract surgery thereby providing our patients with better vision without spectacles. The key is to have a toric IOL that is matched to the eye and aligned in an appropriate manner.
In the pre-operative evaluation, we need to determine the total corneal astigmatism prior to cataract surgery. Corneal topography, tomography, and ketatometry are all helpful and will hopefully be quite similar. Note that of these, only corneal tomography will allow direct measurement of the posterior corneal surface. When the degree of total corneal astigmatism is determined, we must also find the steep and flat axes. We also want the corneal astigmatism to be regular, symmetric, and stable.
The phaco incision that we create will also affect the astigmatism, so we must factor that into the calculations. Remember that astigmatism is a vector which means it has both a magnitude and a direction. With our phaco incision we will affect the magnitude but we can minimize any change in direction (or axis).
Imagine an airplane flying with a direct headwind or a perfectly aligned tailwind: the speed of the plane will either speed up (with a tailwind) or slow down (with a headwind), but the plane will not be blown away from its intended direction. If we have a crosswind which hits the plane at an angle then both the speed and direction of the plane will change.
This also applies to astigmatism: if our phaco incision is on the steep axis or the flat axis then the total astigmatism will either increase (if the incision is on the flat axis) or decrease (if the incision is on the steep axis), but the direction (axis) of the astigmatism will not change. This is why I make my phaco incision on the steep axis in this surgery.
The four black dots at the limbus are intended to mark the cardinal meridians (0º, 90º, 180º, and 270º) and then the Mendez degree gauge is used to find a best-fit for these marks. It is very close but not perfectly exact, which is acceptable because these toric IOLs come in half-diopter steps for both spherical and toric power.
The patient in this case started with:
- K: 43.00 x 108 / 45.00 x 018 (average K = 44.00, 2 D of corneal astigmatism)
- MRx: +0.50 +1.75 x 020 gives 20/60 vision (20/100 uncorrected vision)
After surgery, the results are:
- K: 43.25 x 108 / 44.75 x 018 (average K = 44.00, 1.5 D of corneal astigmatism)
- MRx: -0.25 +0.25 x 020 gives 20/20 vision (20/20 uncorrected vision)
Click below to learn a complete Toric IOL case from start to finish:
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