November 02, 2016
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Managing excessive residual astigmatism after toric IOL implantation

Two methods of back calculation help to ensure ideal alignment of a toric IOL.

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Even though 90% of our toric IOL cases should be within 0.5 D of residual astigmatism, there will be cases in which the outcome is intolerable for the patient and something must be done. The following discussion explains the proper management.

The first step is to reassure the patient that an adjustment can easily be made and to make sure that the refraction is stable. Toric IOLs rarely rotate after implantation, but the wound is not stable until at least 3 weeks after surgery with a 2.5-mm temporal incision and will fade another 0.25 D against-the-rule (ATR) between 3 weeks and 6 months. If the preoperative astigmatism was not exactly at 90° or 180°, then the fade may also affect the axis of the residual astigmatism. A stable refraction is the final answer.

Once the refraction is stable, the exact meridian of the toric IOL must be measured. This is most commonly done with the biomicroscope (slit lamp) using a reticule with meridians labeled from 0° to 180°. In addition to refraction and IOL orientation, you must also have postop keratometry, spherical equivalent power and toricity of the IOL and axial length to allow the calculation of the ideal alignment. There are two methods for the calculation: 1) postoperative keratometry and refraction and 2) observed IOL meridian and postoperative refraction — the latter being more accurate because postop keratometry readings often have an irregular component, reducing the accuracy.

Figure 1 illustrates the two calculations from the Holladay IOL Consultant Software. The postop refraction (–2.50 + 3.75 × 5°) and postop keratometry (42.37 @ 85° and 44.62 @ 175°) are shown along with the spherical equivalent power (20 D), toricity (3 D) and observed meridian (110°). Method 1 (from postop keratometry and refraction) determines that the IOL should be rotated 113° clockwise and the residual refraction will be –0.85 + 0.45 × 175°. Method 2 (from observed IOL meridian and refraction) determines that the IOL should be rotated 118° clockwise and the residual refraction will be –0.78 + 0.32 × 172°. Notice the ideal placement axis, rotation and residual astigmatism are slightly different. As stated above, method 2 is almost always better because the observed meridian can be determined to within 1° whereas the keratometry readings with irregular astigmatism are far less accurate. It is still important to perform both methods for a sanity check. Also, method 1 calculates the toricity and axis of the IOL (2.77 D @ 108°), which should be close to the actual toricity of the IOL (3 D in this example). Two online toric postoperative back calculators are currently available: http://www.hicsoap.com/ under the “calculators” tab and http://astigmatismfix.com/ from John Berdahl and David Hardten.

If the predicted residual refraction after rotation to the ideal meridian is more than 0.5 D, then the current toricity of the IOL is not ideal. If the residual astigmatism in plus cylinder is at the same axis as the steep keratometry and ideal IOL meridian (172° in this case), then more toricity is needed in the IOL. If the axis in plus cylinder is flipped (rotated by 90°), then the current toricity is excessive.

Figure 1. Two methods of back calculation of ideal toric alignment for a 2.25 D toric IOL and expected residual astigmatism.

Images: Holladay JT

Figure 2. Two methods of back calculation of ideal toric alignment for a 6 D toric IOL and expected residual astigmatism. Notice that with 6 D of toricity with “ideal” placement there is still more than 1 D of residual astigmatism, and the two methods have slightly different recommended rotations.

In Figure 2, the IOL has 6 D of toricity, and even when aligned ideally, the residual astigmatism exceeds 1 D. Also, the recommended rotation is slightly different with the two methods (4° in method 1 vs. 6° in method 2). Again, the second method using the observed meridian of the toric IOL is usually more accurate. The precision with which the toric IOL must be aligned is directly related to the toricity of the IOL. With a high degree of preoperative astigmatism (more than 3 D), intraoperative aberrometry is helpful for primary alignment. Secondary procedures to fine-tune the orientation of the high toric IOL are frequent. With these patients, additional chair time should be taken to explain their situation and prepare them for a possible secondary procedure. Once the postoperative refraction is stable, laser refractive surgery is usually preferred because rotating a high toric IOL to within a few degrees is almost impossible and the outcome is less predictable.

If rotation is all that is necessary, most surgeons feel comfortable rotating the IOL even though it is an intraocular procedure. However, when the predicted residual astigmatism is excessive with the current IOL and exchange is necessary, laser refractive surgery is a good alternative and the orientation of the IOL is irrelevant as long as the refraction is stable. As mentioned above, the cataract wound will fade another 0.25 D ATR between 3 weeks and 6 months after surgery, so targeting 0.25 D of with-the-rule is advisable to anticipate the expected fade with time.

Following this protocol for the small percentage of patients with excessive residual astigmatism after toric IOL implantation will result in all patients having successful and happy outcomes.

Disclosure: Holladay reports he is a consultant to Abbott Medical Optics, AcuFocus, Alcon Laboratories, ArcScan, Calhoun Vision, Carl Zeiss, Elenza, Oculus, Visiometrics and WaveTec.