Take-home message: Since making the switch to a markerless toric alignment system, Bryan S. Lee, MD, JD, explains how the technology has eliminated ink marking.
By Bryan S. Lee, MD, JD; Special to Ophthalmology Times
From an optical standpoint, toric IOLs are of benefit to patients with astigmatism, but their performance is dependent on proper lens selection and positioning. Surgeons who implant toric IOLs have to consider the concept of “stackable” sources of error.
Compared with a standard monofocal IOL, there are several additional steps—identifying the correct axis of astigmatism, marking the reference axis, marking the steep axis, and aligning the IOL—that must all be done correctly. Although the risk of a mistake is low at each step, small errors can add up and contribute to an undesirable outcome.
Though traditional marking and alignment techniques can work well, I prefer to eliminate as many potential sources of human error as possible. To that end, I have found that switching to a markerless toric alignment system (Callisto Eye, part of the Zeiss Cataract Suite) has eliminated ink marking altogether. This device is an easy transition for users of other elements of the suite (including the IOLMaster and the OPMI Lumera microscopes).
The registration process
Here’s how it works: A reference image is automatically acquired by a camera integrated with optical biometry (IOLMaster) during routine preoperative biometry. That image can be manually transferred via USB drive or sent by a secure connection to the surgery center.
The markerless alignment system compares the preoperative reference image with the eye during surgery, using limbal blood vessels to register the images in real time.
Much like iris registration in laser vision correction, this helps to correct for unpredictable cyclorotation. It also superimposes key information on the surgeon’s view through the microscope, including the reference (0 to 180 degree) axis and the steep axis.
If desired, the markerless alignment system can show virtual marks for the capsulorhexis and identify the visual axis to aid in multifocal IOL centration. The system continuously tracks the image and adjusts the overlay to maintain accurate registration throughout the case.
A typical toric case
We do not make any ink marks now. The patient is brought to the operating room, the eye is prepped and draped, and the lid speculum inserted. On the screen, an assistant selects the correct patient and eye to pull up the reference images. As soon as the surgeon has centered and focused the microscope, the assistant starts the registration process.
Through the scope, I will see the 0 to 180 degree axis and the planned toric axis. I then turn on the intraoperative aberrometer (ORA, Alcon Laboratories) and orient the ORA reticle with the reference axis on the system.
I turn everything off to remove the cataract, then pressurize the eye, perform the aphakic ORA reading, and adjust the spherical power if necessary (I rarely change the cylinder based on ORA). I select the IOL and inject it as usual, aligning it with the steep axis on the system overlay.
Three parallel lines mark the intended axis of correction, so the surgeon can be very precise in aligning the IOL identification marks in one of the two “lanes” created by the axis overlay (Figure 1).
Next, I re-pressurize the eye and do a pseudophakic ORA reading to check the residual cylinder. With the clear acrylic Tecnis Toric (Abbott), I can make microadjustments in either direction (clockwise or counterclockwise). ORA is often immediately “no rotation recommended” because the system has the IOL on the planned axis. When an axis refinement is needed—to shift from an axis of 160 to 165 degrees, for example—the system quickly adjusts the overlay to the new axis.
Finally, as I remove the viscoelastic at the end of the case, the computer-aided system helps to ensure that the lens stays where I want it. Because the axis can be seen in real time, I can easily prevent the lens from moving off-axis as viscoelastic is removed and the eye is refilled with balanced salt solution, which reduces the need to make positional adjustments.
The real-time, continuous overlay is helpful in challenging cases where there is a small pupil or floppy iris. In such a case, the toric IOL marks may not be clearly visible through a large pupil, but I can still push the iris back and have the advantage of the overlay as the alignment on each side of the IOL is checked.
Gains in precision, efficiency
Computer-aided registration is more accurate than manual ink marks, which can themselves be several degrees wide and are dependent on the patient remaining still.
Daniel Black, MD, previously reported (at the 2015 meeting of the American Society of Cataract and Refractive Surgery) that more than 99% of his patients had ≤0.5 D of residual refractive cylinder using this system. He compared the refractive accuracy in 507 toric IOL cases with ink marking with that of a cohort of 161 eyes undergoing Callisto-guided, markerless surgery. In the ink marking group 6.0% had >0.5 D of astigmatism compared with only 0.6% in the Callisto group.1
After using the markerless toric alignment system for more than a year, I have not had any cases that failed to register successfully. Great confidence has been developed in its accuracy thanks to the postoperative results.
Computer-assisted alignment and registration have also provided important efficiency gains. Eliminating the need to take the patient to the slit lamp or use a variety of ink marking instruments has dramatically improved workflow.
Less time is also spent taking ORA measurements and making axis adjustments intraoperatively because there is a more accurate starting point. In addition, less time is spent checking and adjusting the IOL at the end of the case because the surgeon has the overlay to keep it properly aligned during viscoelastic removal.
It remains important for the toric IOL surgeon to identify the correct axis and magnitude of astigmatism preoperatively and select the correct IOL power. Once this preoperative plan is in place, the markerless toric alignment system greatly increases surgeon confidence that it can be correctly executed during surgery, with optimal visual results.
1. Black D. Evaluation of markerless alignment system for toric IOLs. Paper presented at: ASCRS/ASOA Annual Symposium & Congress; April 20, 2015; San Diego, CA