The first LASIK problem is the flap making process itself. This is done with a mechanical instrument known as a microkeratome, which is a touchy and delicate instrument. It can cause problems like irregular flaps, buttonhole flaps, and free flaps. Even with a skilled and experienced surgeon, and with the latest generation of microkeratomes, the occasional patient still cannot complete the procedure. The flap making process needs to be abandoned, and the process needs to be repeated after a couple of months. Usually, the eye is fine in the interim, but no one likes their surgery to be postponed midway through the procedure.
The second LASIK problem is the fact that the corneal nerves get cut during the process of making the flap, and return to normal only after several months after the process. During this time, corneal sensation is dulled, and as a result the eye does not produce any many tears that it needs. So some patients experience dryness in the eye in the immediate few weeks after LASIK. While we prescribe tear supplements to help the patient, some patients with previously dry eyes need a lot of help and patience.
The third LASIK problem with is that very rarely, the cornea gets weakened beyond a point. Remember, that corneal reshaping means removing tissue from the cornea. For the treatment of myopia, the maximum tissue gets removed in the central cornea. When you remove tissue from the cornea, the cornea gets weakened. This happens with PRK, with LASIK, and with ReLEx. However, the problem is particularly acute with LASIK because the flap does not provide any structural strength to the cornea. So in LASIK, the cornea is weakened both by the flap creation process, and the tissue removal process. In PRK, it is weakened only by the tissue removal process.
There are well established guidelines about how much tissue to leave behind after LASIK, minus the flap. Usually if a surgeon would follow these guidelines, there is really no problem. In the absence of the following the guidelines, the cornea gets weakened, and the pressure from within the eye forces the cornea to bulge out, and the cornea starts thinning and bulging out progressively. This complication is known as ectasia.
Sometimes, even a surgeon who follows the guidelines can get patients with ectasia. The problem is this: The flap does not contribute to the structural strength of the cornea. Suppose a doctor has targeted a flap thickness of 130 microns. Microkeratomes have a standard deviation of flap thickness of about 20 microns. This means that there is a 5% chance that the flap is two standard deviations (or about 40 microns) away from the desired thickness. Therefore a doctor can go very wrong, if he assumes that the flap is only 130 microns thick.
There is a minor cosmetic issue with microkeratome LASIK. While cutting the flap in LASIK with the microkeratome blade, suction is applied on the eye. Not only does the patient experience a lot of pressure in the eye and loses vision briefly (20 seconds or so), there is also some minor bleeding of the blood vessels in the white of the eye (known as sub conjunctival hemorrhages). These red spots appear around the cornea, and while they pose no problems to the eye or vision, they do appear as a cosmetic irritant for about 2 weeks, till the blood gets absorbed.
Finally, there is an issue with LASIK because a flap is created. Since the flap is held in place only by “epithelial glue” at the edge of the flap, there is a risk that by rubbing the eye severely in the first few weeks, or because of grave injury at some later point, the flap can get displaced from its place.
To overcome some of the problems with microkeratomes, a femtosecond laser was developed. A femtosecond laser is a class of lasers known as ultrashort lasers. They have the property that when the laser spot is shot into the cornea, it gets very tightly focused, and a small amount of corneal tissue (around 1 micron) gets converted into a gas bubble. Since these lasers fire at a very high frequency (in case of the lasers at New Vision, 500 Kilo Hertz), millions of shots can be laid down in a precise plane, wherein much of the tissue in a 1 micron thick layer gets converted into gas bubbles. Shots can even be stacked, one on top of the other, which allows the creation of a side cut incision. Thus, instead of using a high speed oscillating blade, as in a microkeratome, the laser is used to create the corneal flap.
This type of LASIK is also called blade free LASIK or femto-LASIK or bladeless LASIK or intralase.