Achieving custom ablation

First, wavefront sensing devices or aberrometers measure the wavefront distortions created by all the structures in the eye. [6] The complete optical aberration of the eye is the cumulative sum of the aberrations created by all the eye structures. It is not known which structures give rise to which aberration component, but the total aberration of the eye is measured.

Mathematically, the aberrated wavefront can be described by Zernike polynomials. The weighted-sum of a number of polynomials may be used to reconstruct the aberrated wavefront. The higher the number of polynomials used, the better it approximates the actual wavefront. Second, the information generated by the wavefront analyzer is used to guide excimer laser treatment to counteract the eye's aberrations. Third, the excimer laser uses a small spot scanning beam to precisely place the custom ablation profile onto the cornea. This laser should have a fast eye tracker to null all eye movements. Without a fast eye tracker, saccadic eye movements may degrade the effect of the precise laser ablation profile.

Adaptive optics

Wavefront technology and adaptive optics were first used by astronomers in designing land-based telescopes to reduce unwanted wavefront distortions. Stars twinkle because when starlight passes through the earth's atmosphere, the light rays are bent by air of different temperatures and densities (turbulence). Without adaptive optics, the randomly distorted starlight entering a telescope will be magnified and will appear as scintillating blobs.

In the Gemini telescope, a separate column of starlight is funneled into a wavefront sensor. The wavefront sensor separates this column of light and samples each zone to calculate how the atmosphere distorts the light rays. The information from the wavefront sensor is fed back to a deformable mirror capable of changing shape hundreds of times per second to counteract the distortions caused by the atmosphere. In other words, the distorted light rays from the distant stars can be undistorted by combining wavefront analysis and adaptive optics.

Three types of wavefront analyzers or aberrometers

Aberrometers measure optical aberrations by different methods. Ray-tracing devices and Tscherning aberrometers measure aberrations as light enters the eye. Hartmann-Shack wavefront analyzers measure aberrations as light exits the eye.

The Dresden Wavefront Analyzer uses the Tscherning aberrometer's method to calculate optical aberration. A frequency-doubled Nd:YAG emits laser through a mask to create 128 parallel light rays onto the retina. If the eye was a perfect optical system, these rays would appear as an array of perfectly spaced dots on the retina. In an imperfect biological optical system, aberration would cause these rays to appear as deformed dots and an irregular array on the retina. A charge-coupled device (CCD) camera, via indirect ophthalmoscopy, captures the image of dots on the retina. Then, the aberrated wavefront is reconstructed by using Zernike polynomials.

Theo Seiler, MD, used the Dresden Wavefront Analyzer to measure the precise aberrations in the eye, and then performed custom ablation with the Wavelight Allegretto laser. In the 2000 America Academy of Ophthalmology meeting, he presented data comparing custom ablation results with standard LASIK results. The follow-up period was 3 months. Of custom ablation patients, 15% achieved best spectacle corrected visual acuity (BSCVA) of 20/10 compared to 5% with standard ablation. In the custom ablation group, 45% were between 20/16 and 20/12 compared to 30% of those who received standard ablation. In the custom ablation group, 30% were 20/20 compared to 50% in the standard ablation group.

At this stage, Seiler emphasized that the main use of custom ablation is to repair previous irregular ablations, and the potential of achieving super vision is yet to be realized. In an article published by Mrochen in 2001, 3 months follow-up data were shown after wavefront guided LASIK using the Allegretto laser on 35 eyes. [7] Uncorrected visual acuity (UCVA) of 20/20 or better was achieved in 93.5% of eyes, and BSCVA of 20/10 or better was achieved in 16.0% of eyes.

Ray tracing aberrometer (Tracey) is similar to the Tscherning aberrometer because it measures aberration when light enters the eye. Instead of light beams entering simultaneously, they enter sequentially into the eye. After each light ray reaches the retina, the photodetector records and computes its aberration. The advantage of this method over other aberrometers is that in the presence of a highly aberrated eye, criss-cross of light rays is avoided, ie, different light rays, when fired sequentially rather than simultaneously, are less likely to be confused by the photodetectors.

One of the goals of custom ablation is to repair eyes that underwent suboptimal older generation refractive procedures that increased the eyes' optical aberrations. High aberration eyes may cause confusion when the Tscherning and the Hartmann-Shack aberrometers are used, but, theoretically, the Tracey aberrometer will be able to measure large aberrations. Another advantage of the Tracey aberrometer is that it functions at higher speeds than other systems; therefore, it is not affected by saccadic eye movement. The prototype delivers 320 light rays in 20 milliseconds over a 6-mm zone.

In the Hartmann-Shack Wavefront analyzer, an eye-safe laser beam enters the eye. If the eye is emmetropic, the light that scatters back from the retina exits the eye as parallel rays. Aberrations in the eye cause deviations in the exiting light rays, which are measured by a wavefront sensor. The Autonomous (Summit Technology, Inc, Waltham, Mass), VISX (Santa Clara, Calif), and Bausch & Lomb Surgical (Munich, Germany) analyzers rely on a Hartmann-Shack Wavefront analyzer.

Autonomous, in partnership with Zeiss Humphrey Systems, developed the CustomCornea Accessory Device. The eye is aligned with a fixation target, an eye-safe probe laser, a video camera, and a wavefront sensor. The probe laser emits an eye-safe laser into the eye, and, after the light is reflected back, a wavefront sensor measures the directions of the distorted light rays.

At the 2000 International Society of Refractive Surgery meeting, Marguerite McDonald, MD, reported data on 23 bilateral LASIK and 13 bilateral PRK surgeries. One eye of each patient was selected randomly for CustomCornea treatment, while the other received conventional LADARVision ablation profile.

In the myopic subgroup that received LASIK, at 6 months postoperatively, 85% of the eyes treated with custom ablation achieved an uncorrected vision of 20/25 or better, and 92% treated with conventional ablation profile achieved an uncorrected vision of 20/25 or better. In the hyperopic subgroup, 80% of the eyes treated with custom ablation achieved an uncorrected vision of 20/40 or better, and 90% treated with conventional ablation profile achieved an uncorrected vision of 20/40 or better. This suggested that custom ablation created less perfect results compared to conventional LASIK.

In the myopic subgroup that received PRK, at 6 months postoperatively, 85% of the eyes treated with custom ablation achieved an uncorrected vision of 20/20 or better, and 65% treated with conventional ablation profile achieved an uncorrected vision of 20/20 or better. Higher order aberrations decreased by 46% using customized PRK and decreased by only 26% using customized LASIK.

VISX introduced the WaveScan Wavefront Analysis System in 2000, which uses the Hartmann-Shack analyzer principle. The sensor consists of adaptive optics mounted on microchips to create a matrix of lenslets (micromirrors) that refract thousands of light rays. The analyzer focuses the light rays onto a CCD chip and reconstructs the wavefront entering the lenslet array and calculates the aberrations using Zernike polynomials. The process of wavefront analysis and data presentation for each patient's eye is termed waveprint, as it depicts the "fingerprint" of the eye.

VISX went one step ahead to use the wavefront data to ablate a plastic lens called the PreVue lens, so that the patient can wear this lens on a trail frame to mimic the effect of what they will see after receiving custom ablation. If patients were able to see better than 20/16 with the PreVue lens, then they were candidates to be included in the VISX custom ablation trial. In the 2001 ASCRS meeting, Kraft reported data from 17 eyes that were treated with custom ablation. Preoperative BSCVA of 20/12.5 was 6%, 20/16 was 59%, and 20/20 was 100%. Postoperative UCVA of 20/12.5 was 29%, 20/16 was 94%, and 20/20 was 100%. These results seem to indicate that custom ablation increased the likelihood of super vision, but the same pattern of results were obtained when patients were treated with the conventional ablation pattern using the VISX S3 laser with 6.5-mm treatment zone and a blend zone.

The Zywave aberrometer (Bausch & Lomb Surgical, Munich, Germany) uses the principle of a Hartmann-Shack sensor. The Zyoptics system links the data from the Zywave system that performed wavefront analysis with the Orbscan IIz, which provided anatomical customization to the Technolas 217Z laser.

At the American Academy of Ophthalmology Refractive Surgery Interest Group meeting (October 2000), Michael Knorz, MD, reported prospective data on 32 patients in whom one eye received the regular LASIK treatment, and the other eye received Zyoptix custom ablation treatment. [8] Out of the 32 patients, 21 patients experienced an increase in best-corrected visual acuity, 7 patients showed no difference, and 4 patients had worse outcome than the Planoscan software. These represent early data when nomograms have not been perfected yet because best-corrected visual acuity data were presented rather than uncorrected visual acuity data.

In the Zyoptics study performed at TLC Windsor, Probst reported that in 100 eyes with 3-months follow-up, postoperative UCVA of 20/15 or better was 45.0%, 20/20 or better was 89.0%, and 20/40 or better was 99.8%. These compared favorably with those completed with conventional ablation, which showed postoperative UCVA of 20/15 or better was 29.0%, 20/20 or better was 84.0%, and 20/40 or better was 99.5%.