LASIK surgery (Laser-Assisted in situ Keratomileusis) is a two step procedure. Its first surgical step to be executed by the Surgeon is the execution of an intrastromal cut onto the eye’s cornea, using a microkeratome or a femtosecond laser, to create a permanent corneal flap. Once the cut has been performed, the flap is lifted, and the stroma is photo-ablated with an excimer laser. Finally, the flap is in-situ repositioned. It is a quite lengthy procedure requiring surgical manipulations and some intraoperative risks.
LASIK can treat mild to moderate nearsightedness, farsightedness, and astigmatism.

Several international researchers have published, over the years, a number of scientific papers about LASIK and the real rate of unexpected outcomes and complications induced by LASIK.( Outcome of LASIK for Myopia with FDA-approved Lasers – Cornea, volume 26, number 3 April 2007)

Undercorrection or Overcorrection
Overcorrection or undercorrection may be an unexpected outcome of LASIK. Different patients may present different responses to the laser ablation, therefore unexpected overcorrection or undercorrection may occur.

Flap-related Complications
The first step of every LASIK surgery is the creation of the corneal flap that itself can be the cause of some complications and undesired side effects.
Suction and applanation must be applied on the cornea to create the flap, and these steps may in fact cause the bursting of some small blood vessels, resulting in subconjunctival hemorrhage. Furthermore, applying suction onto the eye may cause a temporary dimming of vision in the treated eye. Subconjunctival hemorrhages, however, normally resolve themselves within a few weeks. (1) (15)
A possible serious complication is the displacement of the flap, an event that may occur as late as several years after the surgery. (2) (3) (5) In fact, the corneal flap never heals completely onto the stroma. The flap may also detach, break or fold in. (3) (4) (6) (7)
The displacement or detachment of the flap has been observed up to 7 years after the LASIK treatment. (5) (7) (8)
A further complication is an undesired epithelial ingrowth inside the stromal interface. (5) (10) This complication, according to its degree of seriousness, may require reopening of the flap, cleaning of the interface and its repositioning. (5)
Particles have been observed inside the stromal interface, visible with a slit lamp or a microscope. Their impact on quality of vision may vary according to the nature and quantity of these particles. (10) (11)
Furthermore, the stromal interface may facilitate the insurgence of an inflammatory process called Lamellar Keratitis. (5) (11) (12) Similarly, it may host infections. (9) (15) (16) The solution to these complications may require cleaning the interface, with lifting and repositioning of the flap.

Ectasia
As the cornea will lose structural strength permanently, due to the LASIK flap, a possible complication called Ectasia may develop. Ectasia is a deformation or bulging of the cornea, often progressive, with a consequent insurgence of myopia and deterioration of the quality of vision. (13) (14) (16)
Ectasia is a very dangerous complication that may lead, at worst, to corneal transplant.

Dry Eyes
After LASIK, patients often report dry eyes. This side-effect occurs immediately after the surgery and may endure for several years after treatment. Dry eyes, causing pain and scratchiness, are the most common LASIK complication. (16) (17) Unfortunately, it has been scientifically demonstrated that dry eyes can also decrease visual acuity.
There is currently no cure for dry eyes, with the exception of the temporary relief coming from using artificial tears. (16) (17)

Visual Aberrations
High-order aberrations can be a serious complication induced by LASIK. As we have seen, high-order aberrations are visual problems that cannot be corrected with eye glasses. The most common problems associated with high-order aberrations are: halos, starbursts, ghosting and blurry vision. (16) (17) (18) (20)
Halos generate a blurry stripe around the edge of an object, while starbursts are light rays that depart radially while looking at light sources at night, for example during night driving.
Ghosting occurs when multiple images, or ghost images, are visible. Ghosting may present itself both, at daytime as well as at night.
Blurry vision, that manifests itself like “out of focus″ vision in all lighting conditions, often will appear more severely in dim light. Sometimes, images will only smear in a preferential direction.
A loss of contrast sensitivity, also linked to LASIK and due to the interface between the flap and the stroma, is also proven. Contrast sensitivity is the ability to distinguish clearly details of an object between its light and dark portions. Contrast sensitivity, therefore, is especially important in dim light or fog conditions, when the contrast between objects and their background often is reduced already. Night driving is one activity where poor contrast sensitivity may impair safety. (16) (20)
A report published in the “Ophthalmology″ medical journal in 2003, following a research carried out at the Department of Ophthalmology of UMDNJ – New Jersey Medical School, points out that nearly 11% of patients needed retreatment following LASIK. (21)
Finally, several studies have pointed out that, despite recent technological advancements, Wavefront ablations did not deliver their promise of superior results in terms of visual acuity and correction of lower order aberrations over standard procedures. Furthermore, Wavefront does not appear to have reduced the rate of incidence of LASIK-induced higher order aberrations. (22) (23)

References

PRK (Photo Refractive Keratectomy )has been the first generation of laser vision correction.
It is a surface ablation surgery executed in two steps. In the first surgical step the Surgeon removes a large area of the corneal epithelium by means of blades, scrubs, alcohol and/or different media. Once the epithelium has been removed, the stroma is photo-ablated with an excimer laser. The epithelial healing process normally requires from three to five days. It is a quite lengthy procedure requiring surgical manipulations and some intraoperative risks.
PRK can treat nearsightedness, farsightedness, and astigmatism.

PRK procedure requires the Surgeon to remove the corneal epithelium, thus inducing possible complications as stromal strias caused by the media used to remove it.
Postoperative vision may fluctuate until complete stabilization that generally occurs within 30 days after surgery.
Some patients, because of healing may experience, rarely, clouded vision to some degree, often referred to as haze. In these cases, haze normally clears within 12 months, with the help of some eyedrops prescribed by your Ophthalmologist. Haze is often linked to UV exposures, for example sunbathing. Therefore, it is recommended to avoid direct sunlight exposure for a minimum of six months after your cTen™ treatment and to wear sunglasses with a total UV-filter.
Other rare visual complications may include halos, a shimmering circle around light sources such as car headlights, glare or increased sensitivity to bright light, and/or double vision (diplopia), described as “ghosting” around an object.

SMILE (Small Incision Lenticule Extraction)is a surgical procedure derived from LASIK and, like LASIK, is executed inside the stroma.
While in LASIK a single cut is performed to prepare the cornea for the excimer ablation, SMILE procedure requires two distinct cuts in order to create a lenticule of stromal tissue, shaped according to the needed correction, which must be manually removed by the Surgeon. It is a quite lengthy procedure requiring surgical manipulations and some intraoperative risks.
SMILE is meant to treat nearsightedness and astigmatism.

As it is relatively new, the long period effect of this procedure have not been cleared yet. According to FDA premarket approval the unexpected outcomes and complications induced by SMILE are listed as follows (https://www.accessdata.fda.gov/cdrh_docs/pdf15/p150040c.pdf)

Undercorrection or Overcorrection
Overcorrection or undercorrection may be an unexpected outcome of the SMILE procedure. Different patients may present different responses to the laser ablation, therefore unexpected overcorrection or undercorrection may occur. SMILE procedure does not allow retreatments of previously surgically induced overcorrection, undercorrection and/or irregular aberration.

Flap-relatedIntrastromal cut Complications
The first step of every LASIK SMILE surgery is the creation of the intrastromal lenticule to be removedcorneal flap that itself can be the cause of some complications and undesired side effects.
Suction and applanation must be applied on the cornea to create the flapintrastromal lenticule, and these steps may in fact cause the bursting of some small blood vessels, resulting in subconjunctival hemorrhage. Furthermore, applying suction onto the eye may cause a temporary dimming of vision in the treated eye. Subconjunctival hemorrhages, however, normally resolve themselves within a few weeks. (1) (15)
A possible serious complication is the accidental suction loss during the intrastromal cut, which may generate an irregular cut. In such a case a different surgical strategy must be pursued.
A possible serious complication is the displacement of the flap, an event that may occur as late as several years after the surgery. (2) (3) (5) In fact, the corneal flap never heals completely onto the stroma. The flap may also detach, break or fold in. (3) (4) (6) (7)
The displacement or detachment of the flap has been observed up to 7 years after the LASIK treatment. (5) (7) (8)
A further complication is an undesired epithelial ingrowth inside the stromal interface. (5) (10) This complication, according to its degree of seriousness, may require reopening of the flap, cleaning of the interface and its repositioning. (5)
Particles have been observed inside the stromal interface, visible with a slit lamp or a microscope. Their impact on quality of vision may vary according to the nature and quantity of these particles. (10) (11)
Furthermore, the stromal interface may facilitate the insurgence of an inflammatory process called Lamellar Keratitis. (5) (11) (12) Similarly, it may host infections. (9) (15) (16) The solution to these complications may require cleaning the interface, with lifting and repositioning of the flap. [less….]a further surgical intervention that cannot be performed using SMILE procedure.

Ectasia
As the cornea will lose structural strength permanently, due to the LASIK flapthe intrastromale lenticule removal, a possible complication called Ectasia may develop. Ectasia is a deformation or bulging of the cornea, often progressive, with a consequent insurgence of myopia and deterioration of the quality of vision.
Ectasia is a very dangerous complication that may lead, at worst, to corneal transplant.

Dry Eyes
After LASIKSMILE, patients often report dry eyes. This side-effect occurs immediately after the surgery and may endure for several years after treatment. Dry eyes, causing often cause pain and scratchiness, are the most common LASIK complication. (16) (17) Unfortunately, it has been scientifically demonstrated that dry eyesand can also decrease visual acuity.

There is currently no cure for dry eyes, with the exception of the temporary relief coming from using artificial tears. (16) (17)

Visual Aberrations
High-order aberrations can be a serious complication induced by LASIKSMILE. As we have seen, hHigh-order aberrations are visual problems that cannot be corrected with eye glasses. The most common problems associated with high-order aberrations are: halos, starbursts, ghosting and blurry vision. (16) (17) (18) (20)

Halos generate a blurry stripe around the edge of an object, while starbursts are light rays that depart radially while looking at light sources at night, for example during night driving.

Ghosting occurs when multiple images, or ghost images, are visible. Ghosting may present itself both, at daytime as well as at night.

Blurry vision, that manifests itself like “out of focus″ vision in all lighting conditions, often will appear more severely in dim light. Sometimes, images will only smear in a preferential direction.

A loss of contrast sensitivity, also linked to LASIK SMILE and due to the interface created by the lenticulebetween the flap and the stroma, is also proven. Contrast sensitivity is the ability to distinguish clearly details of an object between its light and dark portions. Contrast sensitivity, therefore, is especially important in dim light or fog conditions, when the contrast between objects and their background often is reduced already. Night driving is one activity where poor contrast sensitivity may impair safety. (16) (20)