As a physiological principle, single gas bubbles join to form fewer, but bigger gas bubbles if they are close enough. These big bubbles are deforming the surrounding tissue.
Consequently the upper cut is created within this deformed tissue.
After tissue relaxation and absorption of the gas bubbles, the intended layer of the cut will deviate from the real cut. Extracting such a “deformed” lenticule would result in huge iatrogenic aberrations and in a lower quality of visual outcomes.
To avoid this, the volume of gas bubbles must be kept as low as possible while the distance between the single bubbles should remain both small enough for an effective separation and still large enough for preventing tight junctions.
The first step to shorten the time between the laser cut between the two interfaces (the “bottom” and the “roof”) in the central cornea (the optical zone) was achieved by Shah simply by changing the cut direction. This is the reason why the current scan direction is “spiral-in” followed by “spiral-out” .
We decided to go further: The goal was to find the best combination of energy levels and spot and tracking distances to enhance the cut quality and the ease of tissue separation.
In order to find the right management of laser settings, we started with basic research.
The first trials have been done on pig eyes to find out the limits of energy and spot distances:
Table 11.1 shows how to find outer limits for the three given variables. Please also note: VisuMax laser uses so-called index for laser energy settings. For the purpose of simplicity we keep this “index” as a unit VisuMax users are used to. However, one can keep in mind that one index unit approximately corresponds to 5 nJ.
3 groups of different energy settings per area on pig eyes
Flap energy/lenticule energy
Track distance/spot distance (μm)
−5 D/96 μm
−10 D/166 μm
−5 D/96 μm
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