How to decrease surgically induced astigmatism in cataract surgery
Click Here to Manage Email Alerts
Cataract surgery has always induced astigmatism due to the incisions created during the procedure.
As we 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.
Astigmatism can be thought of as a vector because it has both magnitude and direction. Simple addition and subtraction are insufficient to determine the net result of our incision. We need to go back to high school trigonometry to properly add and subtract vectors. In order to determine the surgically induced astigmatism (SIA) of an incision on the cornea, we need to know three primary sets of data: the preoperative keratometry, the postoperative keratometry and the location of the corneal incision. From this data, we can use vector math to calculate the SIA 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 be constant. Making the incision off-axis will induce a change in both the magnitude and direction of the final corneal astigmatism.
Source: Source: Uday Devgan, MD
Comparing beginner surgeons with experienced ones
In the beginner surgeon cases, there was a large degree of variability in SIA between different trainees and even among cases done by the same surgeon in training. 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 because the data had such a wide spread, this mean SIA value did not offer much predictive value.
When comparing resident SIA data with the results from the experienced attending-level cataract surgeon, there was more SIA and a larger standard deviation in the beginner 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. There are limitations to looking at just the anterior keratometry as we need to incorporate the effect of the posterior cornea and any astigmatism induced by the IOL that can occur with a slight tilt of the optic.
Factors that affect SIA
There are many factors that affect SIA after cataract surgery. The incision size and the architecture are crucial because they change the shape of the cornea. With inexperienced residents, there is increased variability of incision construction compared with a more experienced attending surgeon. Although it only comes from years of experience, the most important thing is to be consistent with the incision. Being consistent includes keeping the incision architecture reproducible and the location with respect to the limbus the same from case to case.
The visual axis is not actually in the center of the cornea; it is nasally displaced, which means that a temporal incision has a smaller astigmatic effect compared with a superior one. Additionally, 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 for a patient with a smaller diameter of 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, with the same incision on a younger patient having less of an effect compared with an older patient. Pachymetry can also affect the cornea’s response to the cataract incision. Incisions placed further from the center of the cornea tend to have less astigmatic effect. This is why residents are encouraged to make gray-line incisions barely nicking the limbal vessels instead of purely clear corneal incisions.
The practical approach to SIA
As we have listed, there are many factors that will affect SIA in cataract surgery. While we can control some of them, such as incision architecture, width and placement, it is difficult to accurately account for the impact of other factors such as patient tissue, age and healing response. We also found that both resident and attending surgeons have a variability in the SIA of their incisions, and the average SIA was not a reliable predictor of SIA of the next case.
Three simple recommendations
1. Be consistent with your incision architecture and placement with respect to the limbus. With time and a large volume of cases completed, more consistency will be achieved.
2. Use a temporal incision because it is the farthest from the visual axis and it has less astigmatic effect compared with other locations.
3. Make the incision on the steep axis if possible, or the flat axis if needed, but try not to 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 although 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 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 placing on the steep or even flat axis, 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 (Figure 1). This type of surgical planning will ensure the best visual outcomes for our patients.
For a video of these concepts, please visit https://cataractcoach.com/category/Astigmatism/.
- For more information:
- Uday Devgan, MD, in private practice at Devgan Eye Surgery and a partner at Specialty Surgical Center in Beverly Hills, California, can be reached at devgan@gmail.com; website: www.CataractCoach.com.
Collapse
Read more about
Click Here to Manage Email Alerts