In essence, LASIK or PRK is sculpting the curvature of the would-be-correcting contact lens onto the inert layer of the cornea. The curvature change is permanent, re-focusing the eye, and leaving behind only normal tissue. The technology to do this borrows from space-age tracking systems, fine laser sculpting in silicone chip manufacturing and more.
Light rays from an object approach the eye and are bent and focused onto the retina by the cornea and natural lens. Both of these structures are transparent and converge light rays.
Myopia, hyperopia, and astigmatism
As we discussed in the What Is LASIK page, refractive surgery like LASIK can address three types of vision problems known as “refractive errors”: nearsightedness, farsightedness, and astigmatism.
In a myopic or near-sighted person, the convergence is too strong, bending the rays too much, causing the focus point to be in front of the retina.
In a hyperopic or far-sighted person, the convergence is too weak, and the focus point is past the retina.
With astigmatism, the convergence is different along one axis than that 90 degrees away.
Glasses or contact lenses have a curve and diverge or converge light rays to compensate for the mis-focusing.
With laser eye surgery, an excimer laser is used to vaporize microns of corneal tissue in the shape of the compensating contact lens.
Keep in mind that the cornea can assume only one curvature. In patients over 45 years of age, an additional problem overlies the underlying myopia or hyperopia. It’s called presbyopia, due to loss of flexibility of the internal lens. This impairs the ability to adjust from far to near. LASIK eye surgery cannot cure this. But you can address this problem by leaving one eye a little near-sighted, giving “casual near ability” without sacrificing significant far vision.
The LASIK procedure
Creating the corneal flap
The corneal sculpting needs to be done on the inert middle layer of the cornea called the stroma. The stroma is similar to the cartilage in the nose, but is transparent. Sculpting this layer is permanent as it does not grow back. On the other hand, the most superficial layer is a skin, called epithelium, which does grow back if removed. In LASIK, to access the stroma, we need to create a flap in the cornea to expose the stroma. This is done by using a femtosecond laser to create a wall of micron-sized interconnected bubbles just beneath the skin, including a very superficial thin amount of stroma. More bubbles are stacked at the edge of the flap, except at a hinge, resulting in a swinging door. It’s like creating a blister on your skin with a hinged attachment and flipping it over, exposing the tissue beneath. In the case of PRK, the skin is removed with a vibrating instrument. It conserves some corneal tissue in that the flap with LASIK includes some stroma.
Once the stroma is exposed and cleaned, the eye is aligned with the sculpting laser, called an excimer laser. Borrowing from space-ship docking technology from NASA, the visual axis of the eye (close to the center of the pupil) and the axis of the excimer laser are perfectly aligned. And they are re-aligned 500 times a second, compensating for any eye movement. The compensations occur in all 3 dimensions.
Adjusting the shape of your cornea
Based on previous measurements of the eye, a unique customized pattern of laser pulses (Contoura) has been formulated in the laser computer. Each pulse of laser lifts off a few microns of tissue, working at the molecular level. The ultraviolet laser vibrates the bonds between carbon, oxygen and other molecules and literally converts a tiny spot of solid corneal stromal tissue into gas. This is called ablation or vaporization. Five hundred tiny pulses per second are placed in the pattern dictated by the patient’s measurements until the shape of the would-be-correcting contact lens has been applied to the stroma. It is precise and accurate. Most importantly, after the last time the laser hits the stroma, what’s left is normal tissue. There is no collateral damage. No burning or tissue shrinkage.
In the case of LASIK, the flap is replaced back onto the stroma and aligned. Because the tissues are basically normal, less healing is required. The flap is generally adherent in 3 minutes and strongly adherent by the next morning, resulting in good vision. In the case of PRK, the skin has to grow back over the exposed stroma, which takes about a week. A bandage contact lens facilitates this.
It’s been called a “modern-day miracle.”
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