Design Limitations of Current Wavefront-Guided Laser Systems
Wavefront technology sounds like a very good thing. Customized treatments capable of optimizing vision based upon individual variations of each eye seems almost too good to be true. In my opinion, it behooves us as surgeons to separate marketing hype from scientific reality. Wavefront technology may be 'the next wave' in treatment technology, but it may not happen as fast as we all might wish. One very significant problem relates to how all current wavefront imaging devices interpret and display their readings.
Aberrations of any optical system can be described in mathematical terms first developed about 100 years ago by an astronomer named Zernike. In the early 1900's, lathes were first widely used to mass-produce lenses (see 'lathe-cutting' described above). One of the features of a lathe-cut lens is that it has virtually perfect radial symmetry (which the human cornea typically does not have). Zernike coefficients are the mathematical representations that describe 'lower-order' and 'higher-order' aberrations of an optical system such as a lathe-cut lens or combination of lenses. However, wavefront imaging alone is not sufficient to identify and treat certain obvious corneal imperfections of the human cornea including keratoconus, decentered prior laser treatment, or asymmetric astigmatism.
Wavefront analysis measures the unique optical characteristics including subtle optical aberrations of the whole eye. This information can then be used to guide customized laser treatment to optimize visual performance for each eye. Think of a tailor or dress-maker custom-tailoring a garment to the particular measurements of each client, compared to buying something 'off the rack' and you will appreciate the difference between wavefront-guided treatment and methods that preceded this capability.
Wavefront guided treatment does cost somewhat more than conventional treatment, much as custom-tailored garments typically cost more than items off the rack. You might ask, "If clothes off the rack fit well, why pay extra for custom tailoring?" While it is true that not everyone needs customized, wavefront-guided treatment, it is also true that quality of vision (overall sharpness, better night vision, reduced or absent glare, halo, better contrast sensitivity) is incrementally better in people that receive wavefront-guided treatment as compared to people with similar prescriptions that receive conventional (non-wavefront) treatment. Also, we tend to be very demanding, particular and finicky about having sharp vision; and perhaps the criteria here are tighter or more exacting than for a garment such as a shirt or a dress. At LA Sight, we recognize this and employ wavefront mapping to guide our recommendations.
Most of us involved in vision correction, including manufacturers and the surgeons who use their lasers, would like nothing more than to offer the best technology to our patients; and have everyone achieve great visual outcomes. We must be careful, however, not to over-sell present or future capabilities. We need to be candid about advantages and limitations of the technology we use. If imaging systems based on Zernike mathematics are potentially imperfect, we need to acknowledge that and find a better way to solve the problem. Optical engineers and laser companies know this already, and are working hard to develop better imaging devices and better mathematical analysis tools, but the best and latest current systems still fall short of being able to render optically perfect treatments in every single case.
The Visx wavefront imaging system (referred to as "WaveScan") measures wavefront information and treats without pupil dilation. This instrument uses data only from the central 4.5mm diameter of the cornea, so cannot correct aberrations outside this zone. The Alcon LADAR wavefront analyzer (called LADARWave) is intended to capture wavefront data after dilation of the pupil with drops, so measures typically with a pupil size of ~8.0mm. Therefore, it captures much more data than the Visx WaveScan, and can apply the treatment to a larger treatment diameter. This is one of the main reasons why Dr. Wallace chose to transition from using the Visx laser (which we had in our office until late '03) to using Alcon's LADARVision laser thereafter.
In all vision-correcting laser systems, wavefront-guided treatment sometimes requires sculpting to a greater depth than standard treatment. Treating to this extra depth is not always safe or advisable if the corneal thickness is not adequate. Dr. Wallace will review the particulars of treatment depth (also called "ablation depth") with you if appropriate at the time of your consultation.
Another important limitation inherent in every corneal laser system currently available has become known as the "cosine correction problem." Ophthalmic lasers are all calibrated by measuring energy absorption on a flat plastic test surface. When excimer laser energy strikes the (perpendicular) test plastic, 100% of the energy is absorbed. The real cornea is curved, however, not flat. In treating a real cornea, the energy delivered everywhere but the very top of the corneal dome will strike on a down-sloping angle, away from the perpendicular. The energy absorbed is proportional to the angle of the downslope, hence the "cosine" reference. Most laser systems are not able to measure, anticipate and correct for this appropriately, therefore some treatment error can be introduced, and this error cannot be corrected even by fancy "wavefront" systems. One of the main reasons for bringing the Allegretto laser into our practice is that this laser is the only one engineered to anticipate and correct for the cosine-offset effect. In eliminating this (small but non-zero) additional source of treatment error, the Allegretto system is able to achieve better treatment accuracy.