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Surgeon explains approach for topography-assisted astigmatic axis alignment
A simple technique helps surgeons find the correct axis for toric IOLs and limbal relaxing incisions.
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 Daniel H. Chang |
In order to achieve the optimal visual outcome for the patient with cataracts and astigmatism, it is imperative to align the toric IOL and/or limbal relaxing incisions along the proper astigmatic axis. Any rotational error reduces the corrective effect and may even induce astigmatism in a different axis. Therefore, accurate identification of the astigmatic axis at the time of surgery is critical.
A fundamental challenge to treating the proper axis of astigmatism lies in our ability to translate the axis measured in clinic to the orientation of the eye in the operating room. Because we are comparing an axis of astigmatism identified in the seated position at the keratometer and topographer to the supine position during cataract surgery, cyclorotation or head tilt can significantly alter the positioning of the axis. The surgeon’s judgment, in this case, is guesswork at best. That is hardly ideal, given the precision with which we approach every other element of modern cataract surgery.
Preoperative marking of the cornea while the patient is seated does provide some advantages, but it also has its limitations. Primarily, preoperative marking assumes that the patient’s head and eye positions are the same during measurement and marking. If a patient’s head was tilted in the keratometer, holding the head straight during marking would actually be counterproductive. Preoperative marking can also present logistical and patient flow challenges at the surgery center. Additionally, there are limitations to the resolution and accuracy of the ink mark, particularly if a re-mark is needed or the ink diffuses. How often do the 0° and 180° marks end up lying superior or inferior to the principal meridian? Finally, marks on the peripheral cornea create a significant parallax when viewed relative to the IOL plane.
At the other end of the spectrum, there are some very high-tech approaches to astigmatic axis alignment. For example, Robert Osher, MD, has developed a method that uses high-definition iris photography in concert with an overlay on the red-free image from the Lenstar optical biometry device (Haag-Streit) to identify the axis. The ORange intraoperative wavefront analysis system (WaveTec) measures astigmatism intraoperatively and guides the surgeon to adjust the IOL or incision placement in real time. These approaches demonstrate great potential, but they require a significant technology investment that may not be feasible for every surgeon.
Topography-based approach
My personal approach lies somewhere in between. For several years, I have been using topographic images to guide my intraoperative identification of the astigmatic axis. It is a simple technique that can be used with any placido-disk-based corneal topographer.
In addition to IOLMaster (Carl Zeiss Meditec) measurements, I obtain corneal topography on all patients undergoing cataract surgery with astigmatic correction. I utilize the topographic axial map (in conjunction with IOLMaster measurements) to determine the primary axis of astigmatism that I wish to treat. Every topographer generates the axial map from an image of the eye superimposed with the placido disk rings. Because the axial map is generated from the eye image map, the axis of astigmatism can easily be transferred onto an image of the eye, which by definition has the same orientation. The eye image map cannot be tilted or cyclorotated relative to the axial map, regardless of patient head position, because they originate from the same image. This eye image map typically contains iris structures, limbal contours and other markings that can be used to orient the eye on the operating table with the topographic printout.
After identifying the astigmatic axis that I want to treat, I draw a line on the eye image map printout. If I plan to perform limbal relaxing incisions, I then draw radial lines corresponding to the arc angle that I plan to treat. If the particular topographer that I am using does not display axis degree numbers on the eye image map, I first draw my line on the axial map, and then I trace it onto the eye image map. The axis-marked eye image map is then held by my circulating nurse in the operating room. I compare the iris crypts or identifying features on the map to what I am seeing through the operating microscope. This allows me to identify the proper axis of astigmatism on the patient’s eye regardless of head tilt or eye cyclorotation.
This is not a perfect technique, of course. Even with iris structures, there is some small parallax with the IOL plane. Additionally, depending on the structure and color of the iris, the degree of dilation, and the quality of the printed topographic image, it may not be possible to match specific iris characteristics. When this is the case, I simply look at the contour of the limbus, which is rarely perfectly circular. This gives me the 0° and 180° axes, and I can identify the axis of astigmatism from that reference point.
Relying on iris features and/or limbal contours ensures that changes in head or eye position do not affect the accuracy of the astigmatic alignment.
Growing demand
We are now attempting to correct astigmatism for an increasing number of cataract patients. I typically perform limbal relaxing incisions in patients who opt for presbyopia-correcting IOLs. I prefer to implant toric IOLs in patients with 2 D or more of astigmatism. I am looking forward to having toric IOLs with rotational stability and a clear, glistening-free lens material, because the transmission of the full spectrum of visible light provides the greatest postoperative “wow factor” for my patients — particularly in the demanding younger patient whose cataracts may not have had significant yellowing in the first place.
With the current trend toward more accurate surgical outcomes, the need to identify the correct axis of astigmatism will only grow. My topography-assisted technique is a simple, reasonable and efficient approach that offers enhanced accuracy in astigmatic alignment.
- Daniel H. Chang, MD, can be reached at Empire Eye and Laser Center, 4101 Empire Drive, Suite 120, Bakersfield, CA 93309; 661-325-3937; email: dchang@empireeyeandlaser.com.
- Disclosure: Dr. Chang is a paid consultant for Abbott Medical Optics but has no direct financial interest in any products mentioned in this article.