Wavefront Technology and Custom Vue Wavefront guided Ablation

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Custom Vue Wavefront guided Treatments, registration and centering

Astigmatism, trefoil, coma, tetrafoil and high order astigmatism all have orientation, or an axis. Usually all aberrations have axial symmetry centered on the pupil. This is why it is extremely important to register (orient) and center the wavefront information on the eye when doing Custom Vue Wavefront guided Ablation with the excimer laser.

Capture the wavefront

Register and center (Match)

Treat

The figures above represent an eye with the high order aberration called tetrafoil. The red line represents the axis of the tetrafoil. The yellow line in the figure to the left represents the center of the eye and the wavefront. The light blue line in the figure in the center and right represents centering the wave front and treatment algorithm (blue with red contour lines) on the center of the eye. The centering function on the WaveScan System is done while capturing the wavefront by having the person fixate the eye on the special fixation light in the instrument. The centering function is done in the laser by special auto centering hardware and software in the laser. Special infrared cameras that sense and track the center and edge of the pupil do this. The registration is now automatic. The registration (orientation or axis) is done by first placing ink marks on the eye at the 3:00 and 9:00 position on the eye while sitting upright at a slit lamp microscope (see left figure below). When a person reclines (lays down on their back) the eyes often rotate slightly (cycloduction) so that the ink marks are no longer aligned at the 3:00 and 9:00 position (see center figure below). When the person is placed under the laser, the person's head is rotated to align these ink marks to the horizontal line on the ridicule (red) of the laser microscope (see right figure below). This is the coarse alignment done by hand. Then the auto registration iris recognition softwear is engaged and the computer notes the torsional and translational eye position. The treatment then proceeds with auto tracking.

At the slit lamp microscope

under the laser

under the laser after rotating the head

You can see from the figure below that the amount of correction lost for a misalignment error of ten degrees is 30% for simple 2nd order astigmatism. The amount of correction lost for a misalignment error of ten degrees is 50% for trefoil and 65% for tetrafoil.

This is why it is extremely important to register (orient) and center the wavefront information on the eye when doing CustomVue wavefront guided treatments or conventional LASIK or PRK with the excimer laser.

It's the first fully automated method of aligning and registering wavefront corrections for CustomVue treatments. Here is how it works. It's normal for the pupil to shift positions from the point of wavefront measurement to the point of treatment. Iris Registration Technology centers the treatment correctly, independent of changes in the pupil center from measurement to treatment. It also torsional (clockwise or counterclockwise) aligns the treatment automatically. Auto registration relies on computer programs and infrared cameras that take pictures of the iris when the wavefront measurement takes place in the office setting. In addition the VISX S4 IR laser has infrared cameras and computer programs to match the VISX S4 IR laser captured iris image to that of the wavefront machine's previously taken in the office. This assures that when a person lies down under the laser when there eye cyclo torts (rotation of the eye in the socket clockwise or counterclockwise) that the laser head optically rotates to realign perfectly with the eyes position present in the office wavefront measurement setting. With auto tracking if you move your eye while the laser (VISX S4 IR) is on the laser head follows the movement. With auto centering the laser (VISX S4 IR) treatment is automatically aligned with the center of the pupil that was measured at the time when the person had the wavefront measurement in the office setting, This is why I said, "With the developments of auto centering, auto tracking and iris registration any more refinements would not be as significant".

Previously I said "It's normal for the pupil to shift positions from the point of wavefront measurement to the point of treatment." More about this now! The center of the pupil, measured at the time when the wavefront measurement was done is not necessarily the center of the pupil when the person lies down under the laser. This is because, under different lighting conditions and different physiologic tone of the pupil, the pupil is a different size and shape. When the pupil dilates or constricts the center of the pupil shifts ever so slightly, because the iris fibers do not contract symmetrically. This results in .1 to .5 mm. shifts in the center of the pupil. Because of these pupil shifts and the movement of the iris it is necessary to reference to a fixed and unchanging structure. This reference is chosen as the limbus. The limbus is the outer most part of the cornea where it meets the white of the eye (sclera). The computer in the laser calculates the centroide shift relative to the limbus. To do this the laser computer compares the center of the pupil when at the wavefront measurement setting to the center of the pupil when under the laser (VISX S4 IR). All of these measurements are referenced to the limbus. Then the laser's computer program shifts the laser head the correct amount and in the right direction and location so treatment is perfectly matched to the eye's measured wavefront and centered perfectly (this is part of auto registration). Then treatment begins.