TORIC IOLS

[caption id='attachment_5300' align='alignright' width='300'] An astigmatic corneal topography[/caption]

Accurate toric IOL power calculation for astigmatic eyes requires the use of the correct ratio of corneal to IOL plane astigmatism, and that ratio will vary primarily in accordance with the effective lens position, said Jaime Aramberri MD, San Sebastian, Spain. “Correcting corneal astigmatism with the toric IOL is just as simple as implanting a lens the correct magnitude of cylinder in the correct axis, but we have to perform some calculations in order to get accurate results,†Dr Aramberri told the XXX Congress of the ESCRS.

The surgeon’s first challenge is the calculation of the IOL plane astigmatism necessary to cancel out the corneal plane astigmatism. The standard method is to calculate the power difference between the two corneal meridians and entering that value into a standard formula which will yield ratio of the IOL astigmatism over the corneal astigmatism.

However, the correct approach is to use the average K to calculate the same effective lens position for both meridians. That requires the use of a formula that allows the independent calculation of effective lens position, Dr Aramberri said.

Using this calculation technique provides a more accurate estimation of the ratio of the IOL plane astigmatism to the corneal ratio. The ratio mainly depends on the effective lens position and is not influenced by the eye’s axial length or corneal curvature, he noted.

Dr Aramberri described a study in which he created a pseudophakic astigmatic eye model, with three variable biometric parameters, which were the axial length, curvature and effective lens position. His ray tracing analysis showed that independently varying the axial length or curvature did not substantially change the ratio between corneal and IOL plane astigmatism.

That is, in an eye with an axial length of 20.5mm and a corneal curvature of 44 D to 43 D and an effective lens position of 5.0mm, the ratio of the IOL astigmatism to corneal astigmatism would be 1.39. Increasing the axial length to 23mm or 30mm but leaving the other values unchanged yields precisely the same value. Similarly, in an eye with a corneal curvature of 47 to 46 D and an axial length of 23mm and an effective lens position of 5.0mm, the corneal plane/IOL plane astigmatism ratio would be 1.43. Leaving the other values unchanged but decreasing the corneal curvature to 44 to 43 D decreases the ratio slightly to 1.39. Decreasing the corneal curvature further down to 41 to 44 D decreases the ratio to 1.36.

In contrast, in an eye with an axial length of 23mm and a corneal curvature of 44 to 43 D, changing the effective lens position from 4.0mm from the cornea to 6.0mm to the cornea increased the ratio from 1.29 to 1.5. “Anterior chambers tend to be shallower in shorter eyes and deeper in longer eyes, so axial length plays an indirect role, but this is because of the effective lens position, not because of the axial length per se,†Dr Aramberri said.

He noted that for every one millimetre of error in the effective lens position there will be 0.1 D of error per dioptre of cylinder in the IOL plane. Special care is therefore necessary in eyes where calculating effective lens position is particularly prone to error. Such eyes include short eyes, those that have been treated with LASIK or PRK, those with keratoconus, corneal scars, or zonular pathology, as well as those that have undergone vitrectomy and those with megalocornea.

Dr Aramberri said that online calculators vary regarding how they calculate the IOL cylinder/corneal cylinder ratio. For example, in an eye requiring a 6.0 D IOL, therefore a long eye, which also has 6.01 D of astigmatism at the corneal plane, the Oculentis calculator uses a ratio of 1.36, giving a value of 8.19 D at the IOL plane. However, in a short eye requiring a 30 D IOL and with 6.01 D of corneal astigmatism, the Oculentis calculator uses a ratio of 1.3, giving a value of 7.78 at the IOL plane.

Dr Aramberri said the Rayner calculator gives values closer to those of his own eye model. That is, in the longer eye example the ratio between the IOL and the corneal astigmatism would be 1.44, while in the shorter eye example it would be 1.2. The AcrySof calculator, on the other hand, gives the same corneal/IOL plane astigmatism ratio, 1.46, for both short eyes and long eyes. That means the calculator would provide accurate results for long eyes with deep anterior chambers but would over correct the astigmatism of shorter eyes.

“There are significant differences in the cornea plane/IOL plane cylinder ratios among calculators and this can explain some astigmatic over and under correction,†he said.