The technology behind lasik seems too good to be true! But this isn’t science fiction. This is reality! How is it possible that a laser is able to reshape the cornea in just the perfect way to correct vision and get out of glasses?
Some bright minds back in the day discovered something very cool. With the right laser and the right energy, a laser pulse can be applied to the cornea and cause it to change shape. This laser, called an excimer laser, disrupts the microscopic chemical bonds to remove a very precise amount of cornea tissue. Moreover, it was discovered that the cornea reacts to the excimer laser in a very very predictable fashion. Each laser pulse has the same effect over and over again. Because of this, pulses can be placed around the cornea to cause a very predictable change and correct vision.
To get this effect, the laser treatments must be applied to a certain region just below the surface of the cornea called the stroma. This is where the lasik flap enters the picture. To put it most simply, the lasik flap creates a window to get down to the stroma and allow the excimer laser to do it's job.
How is the lasik flap created?
These days, lasik flaps are created with a laser called the femtosecond laser. (In the past, lasik flaps were created with blades; for many reasons, don't go to lasik providers that still create lasik flaps with blades).
Not all lasers are the same. While some lasers treat whatever it hits (such as how the excimer laser described above). Some lasers can actually focus and treat inside of transparent objects at a highly targeted spot. This is how the femtosecond laser works. The femtosecond laser is actually able to focus within the cornea to do a precise treatment.
Each treatment of the femtosecond laser creates a microscopic air pocket within the cornea. In fact, the femtosecond laser gets its name because each pulse of the laser is only a femtosecond long, which is insanely short: a millionth of a billionth of a second!! This short pulse allows the laser to be incredibly precise and gentle.
Now, one pulse wouldn’t really do anything. So, the next step for the laser is to create another pulse right next to the first one. In this way, these pulses are chained together to create multiple interlocking microscopic air pockets. With programming, these air pockets can take on complex shapes to create the shape of the lasik flap. All that is left is for the surgeon to finish connecting all the air pockets together and fold back the lasik flap for the excimer laser treatment!
Back to how the prescription is treated
So great! We have a lasik flap and we have a laser that can change the shape of the cornea in a very predictable way. How does it actually correct prescription?
Let’s first talk about laser eye surgery for nearsighted individuals. When people are nearsighted, their cornea is too steep; light bends out of focus before it hits the retina. To correct this, the excimer laser is used to flatten the center of the cornea (by removing some microns) to prevent light from being bent out of focus. Perfect vision!
Farsighted individuals are corrected in a similar way. In these cases, their cornea is too flat. Light isn't bent enough to focus on the retina. So the excimer laser is applied to the periphery of their cornea, causing the center to become steeper and light to focus where it needs to go.
Lasik corrects astigmatism too! With astigmatism, the eye has more of a "football" shape. Similar to far-sighted treatments, the excimer laser applies treatments to the flat sides of the football, steepening up the cornea into a basketball shape. Astigmatism gone!
Surprisingly simple concepts behind such powerful advanced lasers! Despite how simple the concepts are, lasik has advanced to be a finely calibrated orchestra to perfectly shape the cornea and provide amazing uncorrected vision.
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