High-definition Wavefront Aberrometry Provides Encouraging Early Results

“Out-of-the-box” accuracy compares well to previous results with nomogram adjustment.

By Sondra Black, OD

After opening a new refractive surgery center in Burlington, Ontario, we recently began using a new high-definition wavefront aberrometer for our refractive surgery patients.

The iDesign aberrometer (Abbott Medical Optics) relies on Hartmann-Shack technology, much like its predecessor, the WaveScan. But, compared with the WaveScan’s 240-spot lenslet array, the newer aberrometer captures about five times more data points: up to 1,200 Hartmann-Shack spots, depending on pupil size. This increases precision and accuracy and was a key point in our decision to acquire the iDesign and Visx laser (Abbott Medical Optics) for the new center. We use a Schwind Amaris 750 Hz laser system at our main clinic in Toronto.

The new aberrometer’s interface has been refined so that fewer keystrokes are required, and the device incorporates multiple measures into one, including full-gradient topography, autorefraction, pupillometry, and keratometry. Although we do not expect that it will replace advanced topography in our practice, the ability to measure corneal curvature and automatically incorporate keratometry into the treatment design is a nice first step in simplifying the pre-operative testing and treatment design process.

EARLY RESULTS

Early results with iDesign have been very good. Thus far, we have used iDesign aberrometry as the basis for treatment of 75 eyes, of which about one-quarter were PRK treatments. This is a relatively high rate of PRK and was likely due to a backlog in challenging cases (older age, thinner corneas) held by local referring optometrists in anticipation of our new center opening.

All low myopes (-3.00 D or less) who underwent LASIK achieved 20/20 visual acuity or better at 1 month. Of the moderate myopes (-3.00 to -6.00 D) who underwent LASIK, 90% were 20/20 or better and 100% were UCVA 20/25 or better on postoperative day 1. By the 1-month visit, 75% were 20/20 UCVA and all were at least 20/30 UCVA or better (Figure 1).

Figure 1. One-month results for low to moderate myopes treated at Crystal Clear Vision. Modest undercorrections will be addressed in future nomogram refinements.

We were highly conservative with our first group of patients because we were concerned about overcorrection, as we would be with any new laser. It is not uncommon in this group to deliberately target mild hyperopia (+0.25 to +0.50) in anticipation of mild regression, but we elected not to do so with this initial group.

One month is too early to assess the results of PRK, and there have been too few high myopes treated so far to say anything about results in that group, but we look forward to providing additional data as follow-up continues.

Figure 2. Two images illustrate how the Hartmann-Shack spot quality deteriorates with ocular surface dryness. In one image (A), the spots are sharp, regular and distinct, indicating a good tear film. In the second image (B), taken after the subject’s eye was kept open until tear film breakup, the spot pattern is wavy, blurred, and indistinct.

There was no loss of BCVA in any group. Our anecdotal impression is that the results seem more stable, with less regression, but it is still too early to say this with complete confidence.

These are excellent results “out of the box” and compare favorably to our results with CustomVue procedures using the Visx Star S4 laser and WaveScan wavefront analyzer (Abbott Medical Optics) with highly refined nomograms. As previously noted, we started with a deliberately conservative nomogram in order to avoid overcorrections. Our approach will likely have to become more aggressive, especially for moderate to high myopes. As with any new laser system, we are still developing nomograms and expect results to continue to improve as the nomograms are refined.

ASTIGMATIC ACCURACY

Our results with the WaveScan aberrometer were already very good, so one might wonder whether an improvement in accuracy would even be clinically relevant. I would argue that it would be relevant, especially in high myopes and astigmats. It is easy to make a patient with -2.00 D sphere happy, but we also want to get as close to a perfect result as possible in patients with more challenging refractions.

Moreover, patients with high astigmatism levels are overrepresented in the refractive surgery population. These are patients who have never enjoyed great correction with glasses or contact lenses, and so they self-select for refractive surgery.

The accuracy of aberrometry in identifying the magnitude and axis of astigmatism is a bit humbling. As optometrists, we tend to think we can refract more accurately than any device, but when it comes to astigmatism, aberrometry outperforms the best of us. It is not unusual for an aberrometer to read 50% more cylinder than a patient’s spectacle prescription. I used to assume the aberrometer was wrong and change the treatment plan to match the amount of cylinder I determined subjectively. I quickly learned that was not the right approach. I found that, if I put the aberrometer-measured cylinder into the phoropter as the starting point for a refraction, patients typically accept and prefer it over their spectacle correction.

So far, I have found the high-definition aberrometer to be extremely accurate in eyes with astigmatism, regarding both magnitude and axis. Steven Schallhorn, MD, has reported similar early results with this aberrometer in a large series of eyes treated at Optical Express centers in Europe (data on file with Abbott Medical Optics). In that study, astigmatic patients treated based on iDesign aberrometry had fewer outlier results and a tighter correlation coefficient compared with standard aberrometry, and less chance of an axis shift. They also had better patient-reported outcomes, including high rates of satisfaction.

OTHER ADVANTAGES

Another significant advantage of high-definition aberrometry is the ability to capture a wavefront map in eyes with small pupils. Previously, we routinely had difficulty capturing some eyes with a pupil size of 4.0 to 5.0 mm, but those have been no problem with iDesign. Better capture ability increases our confidence in offering same-day surgery if needed and also means we can treat more patients using Custom technology.

This aberrometer also has a greater dynamic range, with the potential to capture more highly aberrated eyes. Our present study included only normal eyes of patients presenting for refractive surgery, but the potential to use it to evaluate those with aberrated corneas, corneal scars, or even keratoconus is intriguing.

Finally, I am enthusiastic about the ability to better image the tear film during wavefront capture with this device. Over the years, we have learned that a healthy tear film is essential to success with any refractive procedure, from LASIK to corneal inlays to multifocal IOLs. As the iDesign captures readings, I can visualize spot quality dynamically, which makes it easier to quickly identify ocular surface problems (Figure 2). If the spots are blurry or indistinct, I know it is important to treat aggressively with tears, punctal occlusion, and topical cyclosporine before surgery to ensure that surgical results are not compromised by ocular surface problems.

Although the results described above are early, I anticipate that highly accurate ablation patterns, combined with the iDesign’s ability to capture information in small pupils and to identify ocular surface problems, will improve results and simplify postoperative management of patients undergoing laser vision correction. n

Sondra Black, OD
• Vice president and clinical director at Crystal Clear Vision Canada, Toronto, Ontario.
• (416) 928-0777 or (416) 988-8495; sondra.black@crystalclearvision.com
• Financial disclosure: none acknowledged