One of the main goals of the modern cataract refractive surgeon is to hit the intended refractive target. If we can find a way to do this consistently with toric IOLs, getting our cataract patients within 0.5 D of the residual cylinder, then we can recommend toric lenses for more patients. In a study of more than 23,000 eyes, 74% had greater than 0.5 D of corneal astigmatism, and 36% had greater than 1 D,1 which means the majority of cataract patients in our clinics may be candidates for a toric or toric multifocal lens.
To achieve the best results for these patients, first we need accurate biometry, keratometry, and topography. Our IOL calculations must be correct, based on the right nomograms. We want to select patients whose corneal astigmatism measurements are consistent with their historical astigmatism.
Importantly, we also need to prevent incremental rotation of toric IOLs, which directly affects visual acuity outcomes. Intraoperatively, it is essential to align our axis and position the IOL accurately, because with each degree of rotation, we lose 3% of the cylinder power. At 30 degrees, we lose the cylindrical effect completely. Marking toric IOLs manually is time consuming and can be inaccurate. I now use intraoperative guidance, which offers the precision and overall improved visualization we need to hit our refractive targets with confidence.
DIGITAL OVERLAY FOR PRECISE ALIGNMENT
Traditionally, we’ve marked patients freehand with a surgical pen as they’re sitting at the slit lamp and focusing at distance, so we don’t induce any cyclotorsion. Even my best manual marks weren’t great. Unfortunately, the ink would spread up to 10 degrees, making it very difficult to precisely align the lens (Figure 1).
Today, we have much more precise digital methods for alignment. We use the IOLMaster 700 and Callisto eye (Zeiss). As we take the biometry, the IOLMaster 700 marks limbal blood vessels, and then the digital data is transferred from the biometer to the OR via USB or Forum (Zeiss).
In the OR, we see the IOLMaster view and the live OR view side by side on our large-screen monitor (Figure 2). The software compares the nasal and temporal halves, which it can do accurately even if the pupil is dilated and some blood vessels are constricted because it maps many vessels for alignment.
I use a temporal mark to ensure the incision is at temporal 180 de-grees. This helps me to develop my own personal surgically induced astigmatism nomogram more accurately. During surgery, Callisto eye digitally overlays the correct alignment for the toric lens, giving us much more accurate guidance than even the best manual marks (Figure 3). We can turn the overlay on and off with a toggle of the microscope paddle. I turn on the digital overlay during removal of the viscoelastic, and then rotate the toric lens into position. I find that it’s pretty straightforward.
In our OR, we also have an ORA System (Alcon). I use ORA for intraoperative auto refraction in post-lasik or post-PRK patients, but that may change with the IOL Master 700’s newest software, which incorporates direct measurement of posterior corneal curvature using really robust algorithms. This may be a game changer for us. I don’t use ORA for toric alignment because I find it’s not as accurate as the Callisto eye. In a study of postoperative residual cylinder, 68.6% of Callisto eye patients had less than 0.25 D of residual cylinder, compared to just 31.3% of ORA patients.2
Beyond intraoperative toric alignment, we use the Callisto eye to align our incisions and guide capsulorrhexis. It is very consistent. Even if the patient moves or the head moves, the overlay follows the eye. We use it on cases of de-centered IOLs and IOL repositioning. If the patient has a large angle kappa, it can mark the visual axis real time.
IMPROVING WORKFLOW FOR TORIC CASES
Any technology that can help us significantly improve outcomes, whether it does so by helping us precisely place a toric IOL or by any other means, deserves attention. Unfortunately, I’ve found that some new technologies slow us down a bit, but the Callisto eye actually greatly enhances my OR efficiency.
The workflow with manual marking was slow. Bringing patients out to the holding room, sitting them up, and marking them can slow down a day of back-to-back cases. We tried using ORA for alignment, but it didn’t offer the results we needed. The Callisto eye offered the clinical results we want, combined with an efficient process in which the system captures the reference image in the office while patients are undergoing biometry with the IOLMaster 700.
There’s no worry about cyclotorsion. I don’t have a degree gauge or a plumb bob on my iPhone to line up with the eye. The process with the Callisto eye is simpler and less stressful for me, and the workflow is pretty seamless. This is a great addition to the experience of successfully correcting astigmatism for so many patients, who are now very happy with their visual outcomes. ■
- Hoffmann PC, Hutz WW. Analysis of biometry and prevalence data for corneal astigmatism in 23,239 eyes. J Cataract Refract Surg. 2010;36(9):1479-1485.
- Unpublished data from Jonathan Solomon, MD, Bowie Vision Institute (Bowie, MD).