The patient is 70 years old and the corneal scar of the left eye is about 80% depth within the stroma and fortunately, it is outside of the direct visual axis. Corneal topography shows the induced astigmatism from this injury is about 2 diopters with a steep axis of about 85 degrees. The symmetry of the astigmatism is not perfect and there is some irregularity to it. The measurement of aberrations shows significant corneal distortion values.
Historically, the patient has worn a prescription in glasses of -1.00 -2.25 x 175 for many years. From old records, this refraction resulted in 20/25+ vision 10 years ago. The cataract has caused his acuity to slip to 20/50 best corrected.
Corneal tomography shows similar values to the topography with about 2 diopters of corneal astigmatism steep at 84 degrees with both the SimK and Total Corneal Power (Ray Traced) methodologies.
You have signed up the patient for cataract surgery. The question is do you opt for a toric IOL to help address the corneal astigmatism or do you stick with a spherical (non-toric) IOL and have the patient go back to spectacles to correct the astigmatism?
Either option is reasonable. In this case, I have elected to use the toric IOL which will help address the astigmatism even though it is not a perfect match for the cornea. An ideal candidate for the toric IOL would have corneal astigmatism that is: symmetric, stable, and regular. While this patient has stable corneal astigmatism based on old records, it is not perfectly symmetric and not perfectly regular.
What gives me the confidence that the toric IOL will give the patient excellent vision? First there is congruence between the corneal topography, tomography, and spectacle refraction. Second, we know from old records that spectacles were able to correct the patient to 20/25+ vision despite the corneal aberrations. Third, the astigmatism within the central 3 mm of the cornea (the pupil zone) is reasonably regular and symmetric. And this central zone is the most important — far more important than the peripheral corneal measurements.
This patient received a toric IOL and he ended up with a refraction of near plano (0.00 -0.25 x 85) which resulted in 20/25+ vision without spectacles. He was very happy with the results.