Technological requirements for custom ablation

Now that wavefront-guided ablation has evolved from the drawing board to the clinic, it has become even more important to understand the science behind the technology.

Raymond Applegate OD, PhD reviewed the basic tenets of wavefront science and then used that as a foundation to explain the essential real world technological requirements needed to perform customised ablations.

Wavefront measurement picks up where manifest refraction leaves off, revealing detailed information about the optical errors of the eye that go beyond cylinder and sphere. The wavefront one hears so much about refers to light described in terms of waves rather than rays.

"If we take light coming from optical infinity, as the light approaches the eye it will form a bundle of rays which are parallel to one another (see Figure B**). If we want to convert those rays to wavefronts we draw small lines perpendicular to each ray at the same point in time and connect all the small segments to form a wavefront. After refraction by a perfect lens or a perfect eye, light rays will come to a focus, as will the wavefront. After refraction rays of light can be converted to wavefronts exactly as before refraction. Visualise small line segments being drawn on each ray and connecting them. In an aberration free optical system, perfectly spherical wavefronts will be formed. The curvature of each wavefront will get steeper and steeper until it is infinite at the focal point. While the ideal eye will bring rays of light to a perfect focus, the aberrated eye will not," he explained.

The wavefront error in the eye is simply the difference between the ideal wavefront and the actual wavefront as a function of location within the pupil (see Figure C). Wavefront measurements are important for several reasons. The wavefront error details how much tissue needs to be removed to correct refractive errors involving both lower and higher order aberrations (See Figure D).

Wavefront error is also a metric for postoperative analysis. That is, it tells you whether or not you achieved your goal.

"The wavefront error defines the optical defects of the eye, and consequently the change in corneal shape needed to correct the optical defects. That is, it tells us exactly how much tissue to remove at every location. The wavefront measurement tells you how much tissue to remove at each corneal location, but it doesn't tell you how to do it."

After many years of basic and clinical research, surgeons now have a better idea of 'how to do it' thanks to tools like those offered by the LADARVision® 4000 system. But any tools that will be used for customised vision correction must meet a daunting list of technical requirements.

First of all, the wavefront system must provide measurements which are both accurate and precise. Accuracy is not the same as precision. One could have measured values which are very precise (repeatable) but not accurate because of calibration errors.

Experience has shown that wavefronts must be accurately measured and centred on the pupil, extending at least 1.0 mm beyond the boundary of the patients largest physiologic pupil. This means that a method for registering the wavefront measurement is essential. Registration refers to methods for fixing ocular landmarks to make sure the planned correction goes exactly where it is intended.

"One can have an accurate wavefront measurement, but if that measurement is not properly registered with the eye during the actual procedure, the outcome will be in error. If you have a beautifully precise and accurate measurement and put it in the wrong place you have created more errors than you have corrected."

The accuracy required for a registration system depends on several factors including the magnitude of the eye's optical errors. The registration has to be extremely accurate when contemplating a correction in a highly aberrated eye with a high refractive error. This has become an important issue now that systems are available which are capable of correcting these kinds of errors.

An effective wavefront-guided system also requires a well-defined and controlled small spot scanning laser platform. The laser has to be able to deliver the desired beam energy and profile to all corneal points being treated. The beam has to be constructed to remove the smallest amount of tissue necessary to correct the corneal shape. In order to correct optical aberrations of the eye up through the fourth order, the laser beam needs to be smaller than 1.0 mm.

Accurate registration is essential, but it is not enough by itself to guarantee precise treatment. Because the eye is constantly moving, laser vision correction systems require an eye-tracking component. To compensate for potential small measurement errors contained in any single measurement, such systems need to sample the eye position two to four times faster than the system is sending information to the correcting mirrors.

Finally, any wavefront laser system intended for vision correction also should include a method to track outcomes. These can provide feedback data that can be used to adjust algorithms and nomograms used to optimise treatments.

"Not all parameters are under technological control. The cornea is not a piece of plastic; each cornea is a little bit different. Tools that are not part of the wavefront measurement or the laser platform can have different effects. For example, different microkeratomes are used in different ways by different surgeons. Environmental factors such as altitude, temperature, humidity, patient age, sex and hundreds of others exist that can affect outcomes. As a result, we need a very good way to track outcomes so that you can adjust them as necessary to tune up the system to get the exact results you want."

There are several competing LASIK systems now on the market, many of which offer some type of wavefront measurement system. No two systems are alike. Omar J. Hakim MD has had the opportunity to use many of the systems now available to refractive surgeons. Drawing on his real-world clinical experience, he discussed what he considered to be the essential elements required by a wavefront system that can be used to provide custom ablations.

"I've had an opportunity to use several of the systems now out there. Each system approaches the problem differently. To judge them we need to look at the technology that drives them, and ultimately at the results," he commented.

With the advent of wavefront sensing, the options for helping patients have increased. Surgeons can now consider issues of quality of vision, including higher order aberrations, contrast sensitivity, and low contrast visual acuity. Surgeons also want to use this technology to treat symptomatic eyes previously operated on that have inadequate optic zones, central islands, irregular astigmatism or other problems.

"Simply put, we need two things in order to perform custom ablation. One, we need to be able to measure the wavefront. Two, we need to be able to treat the wavefront. We have to be able to measure many orders of aberrations, especially if we want to measure highly aberrated eyes such as symptomatic post-LASIK or keratoconic eyes," he noted.

The various wavefront systems now available use different means to measure ocular aberrations. Most aberrometers are Hartman-Shack based because of the optical stability and the reliability of this design. However, crossover effects can limit some Hartmann-Shack aberrometers. This can limit their ability to measure higher order aberrations.

"We know that when we get a wavefront coming into a Hartmann-Shack lenslet array we get the deviation of the point based on the slope of the wavefront. If those points cross over, we may not get a reliable wavefront. Fortunately, Alcon has optimised the lenslet array design and the spacing between the video camera and that array to minimise crossover. By doing that with the LADARWave™ System we still get the robustness of the Hartmann Shack design yet can measure up to eighth order aberrations. It is the best of both worlds."

The LADARWave™ System is a second-generation Hartmann-Shack based device. It includes advanced software and processing capabilities. It has a very large dynamic range, with a refractive range from -14 D to +8.0 D with 6.0 D cylinder. The LADARWave™ can measure up to 240 points over a 7.0 mm diameter pupil.

"The large dynamic range means we can measure large variations across the wavefront, so if we have somebody who has a highly aberrated or keratoconic eye we can measure those without the need to use compensating lenses."
The LADARWave™ system also features automatic fogging. This is important, as surgeons are now recognising the importance of accommodation and its effects on the wavefront.
This instrument has a fixation target that starts to fog the patient. It takes a series of measurements of the wavefront. When the wavefront stops changing, the patient is fogged and the wavefront measurements which will be used to drive the treatment are taken.

"It is really not enough just to measure wavefront because the wavefront doesn't take into consideration biomechanical effects from creating the flap, or the healing response that occur following surgery. We want to take the initial wavefront data and transform it, based on previous, predictable changes we know occur due to creating the flap and the healing of the cornea, to create an ablation profile that will compensate for these surgical factors."

Once an ablation profile, based on the wavefront and transformed based on predictable surgical effects, is obtained it needs to be translated into treatment. Clinical experience has also taught the profession what is required on the laser side of the equation. That is, the optimal system needs small spot ablation. The LADARVision™ system uses a 0.8mm, 60Hz flying scanning spot with non-sequential pulse placement. It has a Gaussian beam profile, which compared to a 'top hat' profile gives a much smoother ablation, which is particularly important in customised ablations.

It is also vital that the system places each pulse of the laser ablation in exactly the correct location. This is where the tracking system and registration comes in.

"The difference between the LADAR tracker and any other is that the LADAR system uses a laser-based tracking system rather than an infrared or video based tracking system. This allows sampling of 4000Hz vs 250Hz for the best video-based trackers. Sampling rate really is important. The tracking mirrors don't start moving until the system knows the eye has moved. Sampling is how often the system knows that the eye has moved. The more often the system knows where the eye is, the sooner it starts to respond to that movement and the more accurately it can compensate for that movement. So, we are looking at almost 20x faster with this system. The LADAR system is the only one with an FDA claim that the tracking actually improves the accuracy of corneal shaping."

An effective custom ablation must include the ability to match the ablation to the measured wavefront, i.e. registration. This is really critical. It doesn't matter how perfect the wavefront measurement is. It doesn't matter how great the laser and tracking system is unless the ablation goes in the exact location where the wavefront was measured. The LADARVision™ system addresses this challenge by the creation of two ink marks at the limbus. These ink marks are used for reference when the wavefront is measured and again when the ablation is performed.