Wavefront-guided custom ablations experience and insights

While most of the current wavefront systems have common objectives, the means by which they achieve these objectives vary considerably. In his presentation, Ronald R. Krueger MD explained the quantitative and qualitative differences among the different wavefront devices.

The different systems for measuring wavefront aberrations include outgoing wavefront aberrometers, double-pass aberrometers, retinoscopic or slit skiascopy methods and retinal imaging aberrometry.

By far the most popular way of defining wavefront patterns is outgoing aberrometry via the Hartmann-Shack system. These systems, now used by five companies, shoot the laser into the eye and light scatters back, defining the aberration profile. Remitted light goes through a tiny array of lenslets, which then focus it into a grid pattern on a detection array. The system then compares the results to an ideal reference wavefront.

One thing that distinguishes the different Hartmann-Shack based systems is the resolution provided by the number of spots acquired from the lenslets. The LADARWave™ system, for example, records 240 spots in a 7.0 mm pupil. It then filters out some of the errors in periphery, leaving about 200 spots. This forms the basis for the creation of the wavefront profile.

The Zywave (Bausch), another Hartmann-Shack-based system, records only 70-75 spots in a 7.0 mm pupil area. Yet another device, the WaveScan (Visx) device, records a similar number of spots as the LADARWave™ System. The WASCA system (Carl Zeiss-Meditec) provides the highest resolution, recording 800 spots.

Another wavefront measurement device, the OPD Scan (Nidek), utilises a slit skioloscopy approach rather than a Hartmann-Shack screen. This retinoscopic approach measures the reflections of a series of ingoing slits of light. These are reflected off the retina and detected by an array of photodetectors. The system combines data from 360 lines of analysis to calculate the wavefront profile.

Other systems use one of two retinal imaging aberrometric techniques, Tscherning or, Tracey, to determine wavefront profiles. The Tscherning technique (used by Wavelight and Schwind) uses a laser grid of 169 spots of light projected onto the retina. The Tracey system uses a retinal ray-tracing technique to map 65 spots within two concentric rings.

"Which of the different wavefront technologies is the best? We don't really know. The analysis is underway to determine which is most reproducible and accurate. We are starting to get some clues. We need comparative studies to determine how accurate and reproducible the different systems are. Then we need to find out how it really works in custom ablation," said Dr. Krueger.

Very few comparative studies of the different systems have been conducted. One study by Dan Durrie MD compared the ability of three different systems (Bausch, Tracey and Wavefront Sciences) to measure higher order. The Wavefront Sciences unit appeared to have the highest level of reproducibility.

Dr. Durrie and colleagues also looked at how accurate five wavefront devices were in terms of measuring refraction. They compared the refraction provided by all five devices on one patient with a manifest refraction of +2.5 D. The Wavefront Sciences device was closest, followed by the LADARWave™ system. The Alcon system provided the most accurate result in terms of measuring cylinder.

"The ultimate measure is how well a wavefront system does in terms of actual correction. The Alcon system includes the LADARVision® 4000 laser, LADARWave™ system tracking and registration, small spot, and customised blend zone. Considering these features, the Alcon system is going to give the greatest likelihood of correcting wavefront error and giving us the best visual acuity in the future."