Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

By contrast, an aberrated eye will show the white spots with different intensities and patterns or a distorted wavefront, indicating deviations of the wavefront from plano. The deviations from plano

are calculated based on the image captured by CCD camera, and the actual wavefront pattern is depicted graphically in color-coded maps (Figure 5).

By measuring the displacement of each spot from its corresponding lens let axis, we can deduce the slope of aberrated wavefront when it entered the corresponding lens let.

Wavefront can be derivate into Zernike polynominals sphere (1st order Zernike polynomial), cylinder and axis of cylinder (2nd order Zernike polynomial), coma (3rd order Zernike polynomial), spherical aberration (4th order Zernike polynomial) and higher order aberrations can be separated (Figure 6).

The wavefront of an ideal optic system is a plane. Any aberration deforms the wavefront in a specific pattern that limits the visual acuity. As a standard treatment only reduce 2nd order aberrations

Section IV: Aberrations and Aberrometer Systems

the visual acuity will still limited by aberrations of 3rd and higher orders. The Zywave aberrometer not only helps to diagnose aberrations but also to reduce all aberrations when using Zyoptix treatments based on Zywave and Orbscan IIz measurements.

ZYWAVE II

The Zywave Aberrometer program goes through a self calibration during each start. This process takes a few seconds. The device is ready to take measurement as soon as the main menu window appears on the screen (Figure 7).

Figure 6. Zernike polynominals.

Figure 7. Zywave main menu

WAVEFRONT DISPLAY

Results Summary

In the picture, the upper right image shows the higher order aberrations. The upper left image shows the entire wavefront (Figure 5). The circle at the bottom shows the simulated PSF (point of spread function). It demonstrates how a patient would see a point light source that is very far away (e.g. a star in the sky).

The statistical data on the lower left side gives a summary of important parameters.

-PPR values for 3.5 mm and full, measured pupil size

-Zernike RMS (Root Mean Square) for 6 mm pupil size.

-Higher order Zernike RMS for 6 mm pupil size

-Higher order Zernike RMS without Zernike coefficient 400 for 6 mm pupil.

The values shown for Predicted Phoropter Refraction (PPR) are calculated for a Back Vertex Correction (BVC) of 15 mm. The PPR is the refraction displayed in sphere, cylinder and axis of cylinder which takes into account all measured aberrations, as higher aberrations like coma or spherical aberration are translated into sphere and cylinder.

2D Plot

The 2D plot shows the wavefront map in µm. The pyramid icon in the left corner opens the select Zernike polynomials window in which the individual Zernike coefficients are displayed. Clicking on the icons will change the map in such a way that the respective coefficients are no longer included (Figure 8).

This diagram shows the PPR sphere (blue), cylinder (purple) and axis (green) for different pupil sizes (Figure 9).

The display shows the simulated PSF (point spread function), but only for higher order aberrations (Figure 10).

This map shows a 3D view of wavefront. In this display individual Zernike coefficients can be displayed as well (Figure 11).

This screen displays the magnitude of all 2nd to 5th order Zernike coefficients in µm. The green area represents the "normal" Zernike coefficients as taken from a random population of approx. 200 preop candidates (Figure 12 and 13).

Time Development View

This portion enables the user to view different examinations and evaluate the development over

time. The two lower windows represent the difference maps between exams A and B and exams B and C (Figure 14).

Introduction

Aberrometers that provide total eye wavefront measurements, used in combination with corneal topography maps, promise to add an important dimension to the practice of refractive surgery. The VISX WaveScan device produces a wavefront print of a patient’s eye that can be used for diagnostic and treatment purposes.

Wavefront-guided refractive surgical treatments are based on data taken directly from patients’ eyes, rather than on manifest refraction (MR). For the first time, surgeons are not limited to correcting sphere and cylinder, but may have the means to assess and treat higher-order ocular aberrations as well.1,2,3

In addition to producing a WavePrint, the WaveScan software converts the data into 2 treatment tables. One table applies to calibration plastifor production of the PreVue lens and the other to corneal tissue. Patients preview their results through lenses, made from calibration plastic, which have been ablated based on treatment table specifications.

With adaptive optics, such as the PreVue lens, surgeons have a way to validate the objective wavefront readings with patient feedback on the proposed treatment.

In general, new computational methods have made possible a laser delivery system that combines the features of small spot scanning with fixed-beam lasers, keeping pulse count low while having the flexibility to accurately recreate a customized treatment table.

When the patient is treated, variable spot scanning (VSS) enables the laser system to reproduce the intricate shape by scanning pulses of varying sizes and shapes onto the cornea. The VISX ActiveTrak system eye tracker ensures that the eye and laser are well matched throughout the surgical procedure, a necessity because of the magnified need for precision when creating customized ablations.

Basics of WaveScan

The VISX WaveScan system is an ophthalmic diagnostic instrument that uses a HartmannShack wavefront sensor to measure the refractive error and wavefront aberrations of the human eye. It is the core device in the VISX WavePrint system, which includes VSS excimer laser energy delivery and the PreVue lens.3 The Hartmann-Shack aberrometer uses an array of lenslets, or small lenses, arranged in a hexagonal array to measure the wavefront emerging across the pupil, and arranges it as a

spot pattern. The aberrometer is independent of subjective patient analysis and provides an objective basis for wavefront measurement4 (Figure 1). One of the Hartmann-Shack system’s advantages is that it can be used to measure and display higher-order aberrations based on algorithms that make hundreds of thousands of calculations using Zernike polynomials (Figure 2).

Section IV: Aberrations and Aberrometer Systems

To capture the data required to create a target ablation shape, the WaveScan device focuses a measurement beam onto the patient’s retina and reflects a wavefront back through the patient’s eye. As the wavefront exits, it is distorted by the optics of the eye. This wavefront is collected and analyzed by the WaveScan device, which generates information used to create wavefront maps5 (Figures 3 & 4). One map