Article Date: 2/1/2004

Understanding the Science of Wavefront-Guided Correction
Here's a straightforward guide to the fundamentals of this promising technology.
By Rick Potvin, M.A.Sc., O.D., Orlando, Fla.

You may not realize it, but you're already measuring wavefront error. Every time you perform a refraction, you determine several components of a patient's wavefront error -- the lower-order aberrations of sphere and cylinder.

Wavefront technology represents a more sophisticated way to understand the eye's optical properties. It enables us to measure errors that normally would go undetected, and may explain why some patients are difficult to refract (for example, those with irregular astigmatism). Wavefront sensing may transform the way eyecare professionals measure and, coupled with the excimer laser, correct for optical error.

In this article, I'll explain how wavefront technology combined with the excimer laser allows surgeons to capture aberrations, register the captured wavefront measurements to the ablation and apply the appropriate ablation profile. This results in a wavefront-guided, customized refractive correction.

First, let's take a closer look at how wavefronts are measured.

The ideal wavefront

A point source of light from a distant object produces effectively parallel rays. This corresponds to a series of plane waves that meet the eye. (A wave is constructed by joining all the rays with perpendicular lines.) In an eye with no aberrations, these plane waves would travel through this perfect optical system and form a point focus on the retina, limited only by diffraction.

If an eye has aberrations, a plane wave entering the eye will never come to a distinct point focus. The resultant pattern on the retina is known as the "point-spread function," and its size and shape indicate the eye's aberrations. The rays entering the eye each would have a slightly different point of focus on the retina.

 

In an eye with no aberrations (top), plane waves form a point focus on the retina. If an eye has aberrations (bottom), plane waves entering the eye form a "point-spread function" on the retina.

Measuring aberrations

Three techniques have been adapted for use in measuring the eye's optical aberrations:

The Tscherning Principle. This technique involves projecting a regular dot or grid pattern onto the retina. A CCD fundus camera then captures the image of this grid on the retina. A sophisticated algorithm compares the observed dot pattern to the theoretical expectation, producing a map of the eye's wavefront error. This is similar to ray-tracing, but ray-tracing determines the wavefront error only one location at a time.

Scheiner's Principle. Sensors based on Scheiner's Principle may be manual or automated. In the manual version, two small light beams with known separation are directed into the eye, and a patient adjusts his or her relative position until they overlap. The degree to which the beams must be moved indicates the wavefront error. Some neural effects may also be captured, as it is the patient's subjective impression that determines the end point. This is a time-consuming test. An automated variant involves the equivalent of point-by-point retinoscopy, removing the subjective component and speeding data collection.

The Shack-Hartmann Principle. Sensors based on this principle are the most common of the three. All laser systems approved for wavefront-guided treatment in the United States incorporate Shack-Hartmann technology.

This principle is based on the theory of light reversibility. A point source of light bounced off the fovea passes through the eye's optical components and is collected on a detector. This detector focuses distinct small areas of the resultant wave using a lenslet array. The difference in the location of the resulting focal points relative to the theoretical location from a plane (perfect) wave determines the wavefront error in each region. Mathematical algorithms then determine the combined wavefront error.

In the past, Shack-Hartmann wavefront sensors have been criticized for an inability to measure a large range of aberrations accurately. The LADARWave CustomCornea device has overcome this deficiency by incorporating a proprietary "proportional array technology," giving it the largest dynamic range commercially available.

The Zernike polynomial classifies optical aberrations by radial order and angular frequency. The polynomial breaks down a wavefront into identifiable and measurable components.

Classifying aberrations

Wavefront sensors use a complex geometric formula to break down a wavefront into systematically identifiable components. Currently, the polynomial of choice is the Zernike polynomial, first used to classify the aberrations in optical systems such as telescopes.

The diagram shown here breaks down the first 14 optical aberrations identified in the Zernike polynomial by radial order (top to bottom) and angular frequency (center to periphery). The components of the second radial order (astigmatism, defocus) represent the spherocylindrical refraction we measure now and are known as low-order aberrations. Terms beyond the second order, known as higher-order aberrations, include such things as spherical aberration (the central fourth-order aberration).

While all Zernike terms combine to recreate the wavefront error in an eye, it can be instructive to examine the effects of specific aberrations. For example, if spherical aberration is simulated as the only wavefront error in an optical system, the potential outputs look as they do in the above diagram.

The image on the upper left shows the image produced by a lenslet array from a Shack-Hartmann aberrometer, while the wavefront aberration map (lower left) shows variation from the ideal using 3D and color coding. Spherical aberration, an annular pattern, creates a map that looks like a sombrero.


Upper left: The image produced by a lenslet array from a Shack-Hartmann aberrometer. Upper right: Variation from the ideal wavefront. Bottom left: The wavefront aberration map showing variation from the ideal. Bottom right: The effect of spherical aberration on image quality.

The image on the top third from the left is the point-spread function, which shows the effect of spherical aberration on a single point of distant light. The image has a central focus but spreads out in all directions. The bottom-right diagram shows the effect of spherical aberration on image quality, using a high-contrast Snellen acuity chart.

Actual patient images and diagrams, of course, are generally far more complex than this.

Correcting aberrations

One goal of measuring the eye's wavefront error is to use that information to correct a patient's vision more accurately. Eyeglasses, contact lenses and conventional refractive surgery correct only low-order aberrations. A wavefront-guided correction also treats higher-order aberrations. Such a correction requires a more complex ablation profile for the laser.

To correct a wavefront accurately, the wavefront first must be measured precisely. Because sequential wavefronts generally are measured to generate ablation patterns, it's useful to register the position of the eye as each image is captured. Then, if the eye has moved at all, the generated wavefront can be corrected for eye position, resulting in a more accurate composite measurement. The LADARWave device is the only aberrometer that addresses this need to register the wavefront.

Once a wavefront error has been determined accurately, a customized ablation is arrived at by first inverting the measured wavefront.

Knowing the relative position of the wavefront on the eye is also important at the time of surgery. Since the LADARVision 4000 Excimer Laser System provides for eye registration, the operator can accurately determine the wavefront's relative position on the eye. The system compensates for cyclotorsion, a common eye rotation that occurs when patients move from sitting to lying down.

Once the wavefront error has been determined accurately, a customized ablation profile involves "canceling" the error. The customized ablation profile is determined by first inverting the measured wavefront. Then the desired correction is adjusted to account for the cornea's index of refraction. Finally, adjustments are made for the laser's relative effectiveness when on-axis and in the periphery. These sophisticated calculations can neutralize optical surface aberrations to produce an ideal wavefront.

Tying it all together

Wavefront-guided refractive surgery relies on the precise measurement of optical aberrations (both high- and low-order), proper registration of the wavefront to the eye both at the time of measurement and the time of surgery, and accurately generating a related ablation pattern. Attention to these details may be why Alcon's CustomCornea refractive surgery system is currently the only wavefront-guided procedure shown to significantly affect higher-order aberrations compared with results from conventional LASIK.

 

The Limits of Human Vision

 

Eliminating aberrations through wavefront-sensing technology may not lead to "super vision" in most patients, but it can lead to a higher quality of vision. That's because there are both anatomical and neural limits to human vision.

Photoreceptors in the foveola, which are about 2 microns wide, are the basic unit of the eye's detection system. To be seen, an image and its background must stimulate enough receptors to send the appropriate electronic message to the brain. Photoreceptor spacing in the eye, then, limits potential visual acuity to 20/10 or 20/8 in most people.

Neurally, if you put an image onto the retina with spacing finer than the inter-receptor spacing, the eye physically can't separate the image's components. They're averaged over the single photoreceptor. When that happens, the brain uses the incomplete data from the photoreceptors to interpret the image, often producing a false image in a phenomenon called aliasing.

The LADARVision System is changing the way LASIK surgery is performed. In the CustomCornea FDA clinical trials, 79% of patients said their post-op vision was better than what they experienced with eyeglasses or contact lenses.

The CustomCornea platform consists of the LADARWave wavefront device (aberrometer) and the LADARVision 4000 excimer laser. It's the only system with an FDA label stating that its wavefront-guided treatment improves optical quality compared with conventional laser treatments. It also has the only tracker claim of improved accuracy for corneal shaping. This system appears to be the most advanced wavefront-guided system yet approved by the FDA.

Retrospective studies show that conventional laser treatments often induce increased higher-order aberrations. The LADARVision CustomCornea procedure induces significantly fewer aberrations, and 38% of patients show reduced higher-order aberrations post-surgery vs. 14% of conventional LADARVision patients.

Leading-edge technologies

Recent advances in wavefront sensing have brought this technology into the field of eye care. While not likely to change your practice immediately, wavefront sensing has considerable potential to affect both the diagnosis and treatment of refractive errors. Keeping up with this technology will be key to providing optimal patient care in the future.

Dr. Potvin is a member of the Research & Development team in Alcon's Orlando Technology Center, home of the LADARWave wavefront sensing device and the LADARVision 4000 Excimer Laser System.

 

 


Optometric Management, Issue: February 2004