SIA: a practical plan in cataract surgery

At a teaching hospital, a student and resident want to predict refractive outcomes based on surgically-induced astigmatism. It was a teaching moment.

By Anjali Devgan and Anthony Joseph, MD

Cataract surgery has always induced astigmatism due to the incisions made during the procedure. As surgeons have moved to smaller and smaller incisions over the years, this effect has lessened, but it still plays an important role in our ability to deliver a predictable refractive outcome and better vision for our patients.

Still, we wanted to measure this effect, so we attempted to answer these questions: Could we find a way to predict what the induced astigmatism would be by determining the surgeon’s surgically-induced astigmatism average? And by doing so, how close could we get to the desired refractive outcome?

We didn’t get what we thought. But we did learn it’s all about location, location, location.

Trig lesson

Astigmatism can be thought of as a vector since it has both magnitude and direction. Simple addition and subtraction cannot determine the net result of an incision; we need to go back to high school trigonometry to properly add and subtract vectors. To determine the surgically-induced astigmatism of an incision on the cornea, we need to know three primary sets of data:

• Keratometry, pre- and postoperative

• Location of the corneal incision.

From these data, we can use vector math to calculate the surgically-induced astigmatism of our incision. If we place our cataract incision on either the steep or flat axis, the magnitude of the astigmatism will change but the direction will remain constant. Making the incision off-axis will induce a change in both the magnitude and direction of the final corneal astigmatism.

Looking at the data

At Olive View-UCLA Medical Center, we looked at surgically-induced astigmatism in residents’ cataract surgery cases. Patients were measured using the same keratometer before surgery and then at least one month after surgery. While the person taking the measurements was not always the same, the device used was an automated keratometer, therefore minimizing the operator’s influence. We excluded patients who experienced complications and those who needed a stitch to close the incision. Hundreds of eyes were measured.

In the residents’ cases, there was a large degree of variability in SIA between different residents and even among cases performed by the same resident. The standard deviation of the SIA exceeded the mean SIA for most surgeons, and the data did not follow the typical bell-shaped curve. There were some patients in whom more than 1 D of SIA was noted. From our data, we created an “average SIA” for each surgeon, but since the data had such a wide spread, this mean SIA value did not offer much predictive value.

We also looked at the results from cataract surgery performed by an expert cataract surgeon who used the same incision size in his private practice population. When comparing resident SIA data to the results of the experienced attending-level cataract surgeon, there was more SIA and a larger standard deviation in the resident cases. However, there was also a significant degree of variability in the attending data. It became clear that the average SIA was not a reliable predictor of SIA of the next case, even in the hands of an expert cataract surgeon. Hundreds of eyes were measured in the expert surgeon population as well.

Figure. Placing our phaco incision on the steep axis (top frame) is the best solution since it will decrease the pre-existing astigmatism and keep any residual astigmatism at the same meridian. The second best option is to place the phaco incision on the flat axis (middle frame) since it will not change the direction of the astigmatism, however it will increase its magnitude which can then be addressed by a toric IOL. The least desirable option (bottom frame) is to make the incision askew from steep and flat axes since this will alter both the amount of astigmatism as well its meridian, thereby making orientation of a toric IOL more challenging.

There are limitations to our study and our data. First, we only measured the anterior cornea, which is not the sole determinant of the eye’s total astigmatism. We have all seen patients who have no astigmatism in their anterior keratometry but still have significant astigmatism in their manifest refraction. The opposite holds true with patients having significant anterior keratometric astigmatism yet perfectly spherical refractions.

Our ultimate goal is to neutralize the eye’s total refractive astigmatism, which will give the patient the best possible vision after cataract surgery. We need to incorporate the effect of the posterior cornea as well as any astigmatism induced by the IOL, as sometimes occurs with a slight tilt of the optic.

The usual factors

Many factors affect surgically-induced astigmatism after cataract surgery. The incision size and the architecture change the shape of the cornea. With inexperienced residents, there is increased variability of incision construction compared to a more experienced surgeon. Although it only comes from years of experience, the most important thing is to be consistent with the incision. Keep the incision architecture reproducible and, with respect to the limbus location, the same from case to case.

The location of the visual axis, because it is not centered on the cornea but is nasally displaced, means that a temporal incision has a smaller astigmatic effect compared to a superior one. Additionally, the corneal diameter, often measured as the white-to-white length, affects the SIA. For a patient with a corneal diameter of 11.5 mm, a 3-mm incision has a smaller impact than on a patient whose diameter is 9 mm. For the 11.5-mm cornea, the incision is about 30° wide, but for the smaller 9-mm cornea the same 3-mm incision encompasses almost 40° and will therefore have a larger astigmatic effect.

Another important contributing factor is patient age. The same incision on a younger person will have less of an astigmatic effect as compared to its effect on an older patient. Pachymetry can also affect the cornea’s reaction to the incision made for the cataract. Incisions placed farther from the cornea’s center tend to have less of an astigmatic effect. This is why residents are encouraged to make gray-line incisions, barely nicking the limbal vessels instead of making clear corneal incisions.

The practical approach to SIA

While we can control some factors that will affect the surgically-induced astigmatism in cataract surgery, it is difficult to accurately account for the impact of other factors such as patient tissue, age and healing response.

Our three recommendations are:

1. Be consistent with your incision architecture and placement with respect to the limbus. With time and more cases completed, you will achieve more consistency.

2. Use a temporal incision since it is the farthest from the visual axis and it has less astigmatic effect compared to other locations.

3. Make the incision on the steep axis if possible, or the flat axis if needed, but never make the incision askew to these axes. While we prefer to keep the incision on the steep axis, even placing it on the flat axis will not change the direction of the astigmatism, though it will increase its magnitude.

This last recommendation is the most important but also perhaps the most misunderstood. We want to keep the axis of the astigmatism the same so that it facilitates orientation of a toric IOL, which is considered one of the best ways to neutralize astigmatism. Even though the SIA is not consistent enough to be predictable, by making the incision on the steep or even flat meridian, we are certain not to change the direction of the astigmatism. We can then choose the ideal toric IOL power and simply align it with the steep axis, instead of trying to adjust the toric IOL axis to predict where the new steep axis will lie.

Because toric IOLs are exquisitely sensitive to proper alignment with the correct meridian, we can achieve a higher level of accuracy by keeping the axis of astigmatism consistent. This type of surgical planning will ensure the best visual outcomes for our patients. OM

About the Authors
	Anjali Devgan is a student and a volunteer at Olive View-UCLA Medical Center, UCLA School of Medicine. AnjaliD4123@gmail.com.
	Anthony Joseph, MD, is in practice at Ophthalmic Consultants of Boston and Tufts University School of Medicine. He was formerly a resident at Olive View-UCLA Medical Center, UCLA School of Medicine. tonyjosephmd@gmail.com.