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LASIK surgery using the Star S4 IR™ excimer at US National Naval Medical Center Bethesda
LASIK or Lasik (Laser-Assisted in situ Keratomileusis), commonly referred to as laser eye surgery, is a type of refractive surgery for the correction of myopia, hyperopia, and astigmatism. The LASIK surgery is performed by an ophthalmologist who uses a laser or microkeratome to reshape the eye's cornea in order to improve visual acuity. For most patients, LASIK provides a permanent alternative to eyeglasses or contact lenses. Major side effects include halos, starbursts, night-driving problems, keratoconus (corneal ectasia), and eye dryness.
LASIK is most similar to another surgical corrective procedure, photorefractive keratectomy (PRK), and both represent advances over radial keratotomy in the surgical treatment of refractive errors of vision. For patients with moderate to high myopia or thin corneas which cannot be treated with LASIK and PRK, the phakic intraocular lens is an alternative.
Barraquer's early work 
In the 1950s, the microkeratome and keratomileusis techniques were developed in Bogotá, Colombia by the Spanish ophthalmologist Jose Barraquer. In his clinic, he would cut thin (one hundredth of a mm thick) flaps in the cornea to alter its shape. Barraquer also investigated how much of the cornea had to be left unaltered in order to provide stable long-term results. This work was followed by that of the Russian scientist, Svyatoslav Fyodorov (1920-2000), who developed the Radial keratotomy (RK) in the 1970s and designed the first posterior chamber implantable contact lenses (phakic intraocular lens) in the 1980s.
Medical laser 
In 1968, at the Northrop Corporation Research and Technology Center of the University of California, Mani Lal Bhaumik was developing the carbon-dioxide laser. This was the origin of the excimer laser, the cornerstone of laser eye surgery. At a May 1973 meeting of the Denver Optical Society of America, in Denver, Colorado, Bhaumik announced his success with laser and later patented his invention.
Application of medical laser to refractive surgery 
In 1980, Rangaswamy Srinivasan, at the IBM Research laboratory, discovered that an ultraviolet excimer laser could etch living tissue, with precision and with no thermal damage to the surrounding area. He named the phenomenon "ablative photo-decomposition" (APD). Five years later, in 1985, Steven Trokel at the Edward S. Harkness Eye Institute, Columbia University in New York city, published his work using the excimer laser in radial keratotomy. He wrote,
- "The central corneal flattening obtained by radial diamond knife incisions has been duplicated by radial laser incisions in 18 enucleated human eyes. The incisions, made by 193 nm far-ultraviolet light radiation emitted by the excimer laser, produced corneal flattening ranging from 0.12 to 5.35 diopters. Both the depth of the corneal incisions and the degree of central corneal flattening correlated with the laser energy applied. Histopathology revealed the remarkably smooth edges of the laser incisions."
Introduction of LASIK 
On 20 June 1989, Gholam A. Peyman was granted a US patent for LASIK (US4840175). It was,
- "A method and apparatus for modifying the curvature of a live cornea via use of an excimer laser. The live cornea has a thin layer removed therefrom, leaving an exposed internal surface thereon. Then, either the surface or thin layer is exposed to the laser beam along a predetermined pattern to ablate desired portions. The thin layer is then replaced onto the surface. Ablating a central area of the surface or thin layer makes the cornea less curved, while ablating an annular area spaced from the center of the surface or layer makes the cornea more curved. The desired predetermined pattern is formed by use of a variable diaphragm, a rotating orifice of variable size, a movable mirror or a movable fiber optic cable through which the laser beam is directed towards the exposed internal surface or removed thin layer."
Implementation in USA 
The LASIK technique was implemented in the USA after its successful application elsewhere. The Food and Drug Administration (FDA) commenced a trial of the excimer laser in 1989. The first enterprise to receive FDA approval to use an excimer laser for photo-refractive keratectomy was Summit Technology (founder and CEO, Dr. David Muller). In 1992, under the direction of the FDA, Pallikaris introduced LASIK to ten VISX centres. In 1998, the "Kremer Excimer Laser", serial number KEA 940202, received FDA approval for its singular use for performing LASIK. Subsequently, Summit Technology was the first company to receive FDA approval to mass manufacture and distribute excimer lasers. VISX and other companies followed.
Pallikaris suggested a flap of cornea could be raised by microkeratome prior to the performing of PRK with the excimer laser. The addition of a flap to PRK became known as LASIK.
Further developments 
Since 1991, there have been further developments such as faster lasers; larger spot areas; bladeless flap incisions; intraoperative corneal pachymetry; and "wavefront-optimized" and "wavefront-guided" techniques. However, use of the excimer laser risks damage to the retina and optic nerve. The goal of refractive surgery is to avoid permanently weakening the cornea with incisions and to deliver less energy to the surrounding tissues.
Experimental techniques 
- "plain" LASIK: LASEK, Epi-LASIK,
- Sub-Bowman’s keratomileusis (thin flap LASIK),
- Wavefront-guided PRK,
- advanced intraocular lenses.
- Femtosecond laser intrastromal vision correction: using all-femtosecond correction, for example, Femtosecond Lenticule EXtraction, FLIVC, or IntraCOR),
- Keraflex: a thermobiochemical solution which has received the CE Mark for refractive correction. and is in European clinical trials for the correction of myopia and keratoconus.
- Technolas FEMTEC laser: for incisionless IntraCOR ablation for presbyopia, with trials ongoing for myopia and other conditions.
The procedure involves creating a thin flap on the eye, folding it to enable remodeling of the tissue beneath with a laser and repositioning the flap.
Preoperative procedures 
Contact lenses 
Patients wearing soft contact lenses are instructed to stop wearing them 5 to 21 days before surgery. One industry body recommends that patients wearing hard contact lenses should stop wearing them for a minimum of six weeks plus another six weeks for every three years the hard contacts have been worn. The cornea is avascular because it must be transparent to function normally. Its cells absorb oxygen from the tear film. Thus, low-oxygen-permeable contact lenses reduce the cornea's oxygen absorption, sometimes resulting in corneal neovascularization—the growth of blood vessels into the cornea. This causes a slight lengthening of inflammation duration and healing time and some pain during surgery, because of greater bleeding. Although some contact lenses (notably modern RGP and soft silicone hydrogel lenses) are made of materials with greater oxygen permeability that help reduce the risk of corneal neovascularization, patients considering LASIK are warned to avoid over-wearing their contact lenses. Usually, it is recommended that they discontinue wearing contact lenses days or weeks before the LASIK eye surgery.
Pre-operative examination and education 
In the USA, the FDA has approved LASIK for age 18 and over. More importantly the patient's eye prescription should be stable for at least one year prior to surgery. The patient may be examined with pupillary dilation and education given prior to the procedure. Before the surgery, the patient's corneas are examined with a pachymeter to determine their thickness, and with a topographer, or corneal topography machine, to measure their surface contour. Using low-power lasers, a topographer creates a topographic map of the cornea. The procedure is contraindicated if the topographer finds difficulties such as keratoconus The preparatory process also detects astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and the location of corneal tissue to be removed. The patient is prescribed and self-administers an antibiotic beforehand to minimize the risk of infection after the procedure and is sometimes offered a short acting oral sedative medication as a pre-medication. Prior to the procedure, anaesthetic eye drops are instilled.
Operative procedure 
Flap creation 
A soft corneal suction ring is applied to the eye, holding the eye in place. This step in the procedure can sometimes cause small blood vessels to burst, resulting in bleeding or subconjunctival hemorrhage into the white (sclera) of the eye, a harmless side effect that resolves within several weeks. Increased suction causes a transient dimming of vision in the treated eye. Once the eye is immobilized, the flap is created. This process is achieved with a mechanical microkeratome using a metal blade, or a femtosecond laser that creates a series of tiny closely arranged bubbles within the cornea. A hinge is left at one end of this flap. The flap is folded back, revealing the stroma, the middle section of the cornea. The process of lifting and folding back the flap can sometimes be uncomfortable.
Laser remodelling 
The second step of the procedure uses an excimer laser (193 nm) to remodel the corneal stroma. The laser vaporizes the tissue in a finely controlled manner without damaging the adjacent stroma. No burning with heat or actual cutting is required to ablate the tissue. The layers of tissue removed are tens of micrometres thick. Performing the laser ablation in the deeper corneal stroma provides for more rapid visual recovery and less pain than the earlier technique, photorefractive keratectomy (PRK). During the second step, the patient's vision becomes blurry, once the flap is lifted. They will be able to see only white light surrounding the orange light of the laser, which can lead to mild disorientation. The excimer laser uses an eye tracking system that follows the patient's eye position up to 4,000 times per second, redirecting laser pulses for precise placement within the treatment zone. Typical pulses are around 1 millijoule (mJ) of pulse energy in 10 to 20 nanoseconds.
Repositioning of the flap 
After the laser has reshaped the stromal layer, the LASIK flap is carefully repositioned over the treatment area by the surgeon and checked for the presence of air bubbles, debris, and proper fit on the eye. The flap remains in position by natural adhesion until healing is completed.
Postoperative care 
Patients are usually given a course of antibiotic and anti-inflammatory eye drops. These are continued in the weeks following surgery. Patients are told to rest and are given dark eyeglasses to protect their eyes from bright lights and occasionally protective goggles to prevent rubbing of the eyes when asleep and to reduce dry eyes. They also are required to moisturize the eyes with preservative-free tears and follow directions for prescription drops. Occasionally after the procedure a bandage contact lens is placed to aid the healing, and typically removed after 3–4 days. Patients should be adequately informed by their surgeons of the importance of proper post-operative care to minimize the risk of complications.
Wavefront-guided LASIK 
Wavefront-guided LASIK is a variation of LASIK surgery in which, rather than applying a simple correction of focusing power to the cornea (as in traditional LASIK), an ophthalmologist applies a spatially varying correction, guiding the computer-controlled excimer laser with measurements from a wavefront sensor. The goal is to achieve a more optically perfect eye, though the final result still depends on the physician's success at predicting changes that occur during healing and other factors that may have to do with the regularity/irregularity of the cornea and the axis of any residual astigmatism. In older patients, scattering from microscopic particles (cataract or incipient cataract) may play a role that outweighs any benefit from wavefront correction. Therefore, patients expecting so-called "super vision" from such procedures may be disappointed.
Australian ophthalmologist Noel A. Alpins, who developed a vector analysis method to analyze astigmatism in cataract, refractive, and corneal surgery, has long advocated combining "vector planning" with wavefront-guided LASIK. Alpins' contention is that the purely refraction-based approach represented by wavefront analysis contradicts corneal surgical experience developed over many years. Refractive surgeons have long known that corneal regularity is the foundation of superior visual outcomes. Because refractive and corneal topographic astigmatism do not always align, corrections for internal optical errors surgically sculpted onto the cornea may increase corneal irregularity. Alpins believes that the pathway to "super vision" requires a greater customized reduction of corneal astigmatism than is usually attempted, and that any remaining astigmatism ought to be regular (as opposed to irregular), which are both fundamental principles of vector planning overlooked by a purely wavefront-guided treatment plan. Alpins' observation was confirmed in a prospective study of LASIK patients, which found a greater reduction in corneal astigmatism and better visual outcomes under mesopic conditions using wavefront technology combined with vector analysis (the Alpins method) than using wavefront technology alone, and also found equivalent higher-order aberrations.
No good data can be found that compare the percentage of LASIK procedures that employ wavefront guidance versus the percentage that do not, nor the percentage of refractive surgeons who have a preference one way or the other. Wavefront technology continues to be positioned as an "advance" in LASIK with putative advantages; however, it is clear that not all LASIK procedures are performed with wavefront guidance.
Still, surgeons claim patients are generally more satisfied with this technique than with previous methods, particularly regarding lowered incidence of "halos," the visual artifact caused by spherical aberration induced in the eye by earlier methods. A meta-analysis of eight trials showed a lower incidence of these higher order aberrations in patients who had wavefront-guided LASIK compared to non-wavefront-guided LASIK. Based on their experience, the United States Air Force has described WFG-Lasik as giving "superior vision results".
LASIK surgery results 
The planning and analysis of corneal reshaping techniques such as LASIK have been standardized by the American National Standards Institute, an approach based on the Alpins Method of astigmatism analysis. The FDA website on LASIK states,
- "Before undergoing a refractive procedure, you should carefully weigh the risks and benefits based on your own personal value system, and try to avoid being influenced by friends that have had the procedure or doctors encouraging you to do so."
Patient satisfaction 
Surveys of LASIK find rates of patient satisfaction between 92 and 98 percent. In March 2008, The American Society of Cataract and Refractive Surgery published a patient satisfaction meta-analysis of over 3,000 peer-reviewed articles from international clinical journals. Data from the prior 10 years revealed a 95.4 percent patient satisfaction rate among LASIK patients.
Controversies in safety and efficacy 
The reported figures for safety and efficacy are open to interpretation. In 2003, the Medical Defence Union (MDU), the largest insurer for doctors in the United Kingdom, reported a 166 percent increase in claims involving laser eye surgery; however, the MDU averred that some of these claims resulted primarily from patients' unrealistic expectations of LASIK rather than faulty surgery. A 2003 study, reported in the medical journal, Ophthalmology, found that nearly 18 percent of treated patients and 12 percent of treated eyes needed retreatment. The authors concluded that higher initial corrections, astigmatism, and older age are risk factors for LASIK retreatment. In 2004, the British National Health Service's National Institute for Health and Clinical Excellence (NICE) considered a systematic review of four randomized controlled trials before issuing guidelines for the use of LASIK within the NHS. Regarding the procedure's efficacy, NICE reported,
- "Current evidence on LASIK for the treatment of refractive errors suggests that it is effective in selected patients with mild or moderate short-sightedness,"
- "Evidence is weaker for its effectiveness in severe short-sightedness and long-sightedness."
Regarding the procedure's safety, NICE reported that,
- "There are concerns about the procedure's safety in the long term and current evidence does not appear adequate to support its use within the NHS without special arrangements for consent and for audit or research."
Some refractive surgeons in the United Kingdom and United States, including at least one author of a study cited in the report, proposed that NICE relied on information that was severely dated and weakly researched. Revised guidelines (IPG164) were issued by NICE in March 2006, stating,
- "Current evidence suggests that photorefractive (laser) surgery for the correction of refractive errors is safe and efficacious for use in appropriately selected patients."
On October 10, 2006, WebMD reported that statistical analysis revealed that the infection risk for contact lens wearers is greater than the infection risk from LASIK. Daily contact lens wearers have a 1-in-100 chance of developing a serious, contact lens-related eye infection in 30 years of use, and a 1-in-2,000 chance of suffering significant vision loss as a result of infection. The researchers calculated the risk of significant vision loss consequence of LASIK surgery to be closer to 1-in-10,000 cases.
On 25 February 2010, Morris Waxler, former Food and Drug Administration (FDA) official in charge of approving laser vision correction (LASIK) devices, stated concerns about the risk of serious side-effects from LASIK and the original FDA approval process. His concerns about the safety of LASIK were discussed in an interview on Good Morning America. On 6 January 2011, Waxler requested that,
- "the Commissioner of Food and Drugs withdraw FDA approval (PMA) for all LASIK devices and issue a Public Health Advisory with a voluntary recall of LASIK devices in an effort to stop the epidemic of permanent eye injury caused by lasers and microkeratomes used for LASIK eye surgery."
Waxler alleged that,
- "The FDA was deprived of knowledge of the full extent of LASIK injuries prior to and during FDA reviews of documents submitted in support of the safety and effectiveness of LASIK devices under 21 CFR 812 and 21 CFR 814. In addition, LASIK manufacturers and their collaborators withheld safety and effectiveness information from their investigational device exemption (IDE) reports to the FDA. In addition, they hid LASIK injuries from FDA within the context of out-of-court settlement of innumerable lawsuits. Clinic-sponsored IDE studies cherry-picked, withheld, and hid data from FDA that clearly showed LASIK with excessive adverse event rates (greater than 1%). These activities were an industry-wide effort, organized wholly or in part by the manufacturers and their collaborators in order to circumvent FDA law and regulation. I will submit confidential information on these matters separately to FDA’s Office of Criminal Investigation."
Patient dissatisfaction 
Some patients with perceived poor outcomes from LASIK surgical procedures report a significantly reduced quality of life because of vision problems or physical pain associated with the surgery. Patients who have suffered LASIK complications have created websites and discussion forums where prospective and past patients can discuss the surgery. In 1999, "Surgical Eyes" was founded in New York city as a point of resource for patients with complications of LASIK and other refractive surgeries by RK patient, Ron Link, "Surgical Eyes" has since been superseded by the "Vision Surgery Rehab Network" (VSRN). No patient advocacy organization has changed its official position in regard to refractive surgery, despite the allegations of Morris Waxler, PhD concerning criminal misconduct during the approvals process. Between 1998 and 2006, the US FDA received 140 "negative reports relating to LASIK".
Higher-order aberrations 
The term "higher-order aberrations" are visual problems that require special testing for diagnosis and are not corrected with normal spectacles (eye glasses). These aberrations include 'starbursts', 'ghosting', 'halos' and others. Some patients describe these symptoms post-operatively and associate them with the LASIK technique including the formation of the flap and the tissue ablation. The advancement of the LASIK technology has reduced the risk of clinically significant visual impairment after surgery. There is a correlation between pupil size and aberrations. This correlation may be the result of irregularity in the corneal tissue between the untouched part of the cornea and the reshaped part. Daytime post-LASIK vision is optimal, since the pupil size is smaller than the LASIK flap. However, at night, the pupil may dilate such that light passes through the edge of the LASIK flap which gives rise to aberrations. LASIK and PRK may induce spherical aberration if the laser under corrects as it moves outward from the centre of the treatment zone, especially when major corrections are made. Others propose that higher order aberrations are present preoperatively. They can be measured in micrometers (µm) whereas the smallest laser beam size approved by the FDA is about 1000 times larger, at 0.65 mm. In situ keratomileusis effected at a later age increases the incidence of corneal higher-order wavefront aberrations. These factors demonstrate the importance of careful patient selection for LASIK treatment.
Other side effects 
- Surgery induced dry eyes
- Over-correction or under-correction.
- Vitamin D deficiency from sun sensitivity and hence avoidance.
- Fluctuation in Visual acuity.
- Halos and starbursts occur around bright lights at night At night, the pupil may dilate to be larger than the flap leading to the edge of the flap or stromal changes causing visual distortion of light that does not occur during the day when the pupil is smaller. The eyes can be examined for large pupils pre-operatively and the risk of this symptom assessed.
- Ghost images or
- double vision
- Light sensitivity.
- Large pupils
- lid edge irritation
- Striae (flap wrinkles)
- Decentered ablation
- Debris or growth under flap
- Thin or buttonhole flap
- Induced astigmatism
- Corneal Ectasia
- Epithelium erosion
- Posterior vitreous detachment
- Macular hole occurs at an incidence of 0.3 percent.
Dry eyes 
The most common patient complaint due to refractive surgery is dry eyes. It is reported by some patients in both the immediate and the late post-operative period. In 2001, in Finland, an incidence of 48% at six months post LASIK was reported. In 2006, in USA, an incidence of 36% at six months was reported. Underlying conditions with dry eye such as Sjögren's syndrome are considered contraindications to Lasik.
The incidence of persistent dry eye has been estimated to be as high as 28% in Asian eyes and 5% in Caucasian eyes. Approximately 90% of corneal sensory nerves are severed in LASIK. Sensory nerve fibers in the cornea are important for stimulating tear production. One year after LASIK, sub-basal nerve fiber bundles remain reduced from normal by more than half. At 5 years after LASIK, subbasal nerves for the first time return to densities that are not significantly different from densities before LASIK. Some patients also experience reactive tearing, in part to compensate for chronic decreased basal wetting tear production.
Treatments include artificial tears, prescription tears and punctal occlusion. Punctal occlusion is accomplished by placing a collagen plug in the tear duct, which normally drains fluid from the eye. Some patients complain of ongoing dry eye symptoms despite such treatments and dry eye symptoms may be permanent. Patients with non-Sjogren's based dry eye have been able to receive Lasik with application of autologous serum and punctal occlusion prior to surgery.
Complications due to LASIK have been classified as those that occur due to preoperative, intraoperative, early postoperative, or late postoperative sources:
Intra-operative flap complications 
The incidence of flap complications is about 0.244%. Flap complications (such as displaced flaps or folds in the flaps that necessitate repositioning, diffuse lamellar keratitis, and epithelial ingrowth) are common in lamellar corneal surgeries but rarely lead to permanent loss of visual acuity. The incidence of these microkeratome-related complications decreases with increased physician experience.
Slipped flap 
A "slipped flap" is a corneal flap that detaches from the rest of the cornea. The chances of this are greatest immediately after surgery, so patients typically are advised to go home and sleep to let the flap adhere and heal. Patients are usually given sleep goggles or eye shields to wear for several nights to prevent them from dislodging the flap in their sleep. A short operation time may decrease the chance of this complication, as there is less time for the flap to dry...
Flap interface particles 
"Flap interface particles" are a finding whose clinical significance is undetermined. Particles of various sizes and reflectivity are clinically visible in about 38.7% of eyes examined via slit lamp biomicroscopy and in 100% of eyes examined by confocal microscopy.
Early post-operative complications 
Diffuse lamellar keratitis 
Diffuse lamellar keratitis (DLK) is an inflammatory process that involves an accumulation of white blood cells at the interface between the LASIK corneal flap and the underlying stroma. It is known colloquially as "sands of Sahara syndrome" because on slit lamp exam, the inflammatory infiltrate appears similar to waves of sand. The USAeyes organisation reports an incidence of 2.3% after LASIK. It is most commonly treated with steroid eye drops. Sometimes it is necessary for the eye surgeon to lift the flap and manually remove the accumulated cells. DLK has not been reported with photorefractive keratectomy due to the absence of flap creation.
The incidence of infection responsive to treatment has been estimated at 0.4%.
Keratoconus is a genetic condition that disposes the cornea to thinning after surgery. Although this condition is screened for in the preoperative examination, it is possible, in rare cases, (about 1 in 5,000) for the condition to appear later in life (the mid-40s). If this occurs, the patient may need rigid gas permeable contact lenses, Intrastromal Corneal Ring Segments (Intacs) Corneal Collagen Crosslinking with Riboflavin or a corneal transplant.
Subconjunctival haemorrhage 
Late postoperative complications 
Epithelial ingrowth 
The incidence of epithelial ingrowth has been estimated at 0.1%.
Late traumatic flap dislocations 
Cases of late traumatic flap dislocations have been reported up to seven years after LASIK.
Micro folding 
Glaucoma diagnosis 
After LASIK, measurements of intraocular pressure (used to diagnose and treat glaucoma) can be more difficult. The changes also affect the calculations used to select the correct intraocular lens implant for cataract surgery. This is known as "refractive surprise." Preoperative, operative and postoperative measurements can assist.
Rare events 
- Retinal detachment: the incidence of retinal detachment has been estimated at 0.36 percent.
- Choroidal neovascularization: the incidence of choroidal neovascularization has been estimated at 0.33 percent.
- Uveitis: the incidence of uveitis has been estimated at 0.18 percent.
Mountain climbers 
Although the cornea usually is thinner after LASIK, because of the removal of part of the stroma, refractive surgeons strive to maintain the maximum thickness to avoid structurally weakening the cornea. Decreased atmospheric pressure at higher altitudes has not been demonstrated as extremely dangerous to the eyes of LASIK patients. However, some mountain climbers have experienced a myopic shift at extreme altitudes.
Effect on corneal leukocytes and keratocytes 
Onset of presbyopia 
Myopic (nearsighted) people who are close to the age (mid- to late-forties) when they will require either reading glasses or bifocal eyeglasses may find that they still require reading glasses despite having undergone refractive LASIK surgery. Myopic people generally require reading glasses or bifocal eyeglasses at a later age than people who are emmetropic (those who see without eyeglasses), but this benefit may be lost if they undergo LASIK. This is not a complication but an expected result of the physical laws of optics. Although there is currently no method to completely eradicate the need for reading glasses in this group, it may be minimized by performing a variation of the LASIK procedure called "slight monovision." In this procedure, which is performed exactly like distance-vision-correction LASIK, the dominant eye is set for distance vision, while the non-dominant eye is set to the prescription of the patient's reading glasses. This allows the patient to achieve a similar effect as wearing bifocals. The majority of patients tolerate this procedure very well and do not notice any shift between near and distance viewing, although a small portion of the population has trouble adjusting to the monovision effect. This can be tested for several days prior to surgery by wearing contact lenses that mimic the monovision effect. Recently, a variation of the laser ablation pattern called PresbyLASIK, has been developed to reduce or eliminate dependence on reading glasses while retaining distance vision.
Age considerations 
New advances in eyesight corrective surgery are providing patients with greater choices. Patients in their 40s or 50s may also be candidates for implantable lenses, especially if there are signs of early cataract development.
Lasik versus photorefractive keratectomy 
There have been a number of studies comparing Lasik to photorefractive keratectomy, or PRK. Lasik is associated with decreased inflammation and quicker recovery, but at the cost of decreased expression of nerve growth factor (NGF) at the surgical bed. PRK, on the other hand, has longer recovery time with more postoperative discomfort and irritation, but rates of ocular dryness are less, as only the epithelium is denuded in this approach. Lasik, on the other hand, involves creation of a corneal flap. The medical literature is not uniform, however, regarding their comparative effects on ocular dryness, although the majority of research supports PRK as causing less reduction in the tear film. Surgical technique has improved with time, and a more recent study has shown no difference in ocular dryness in Lasik or PRK at 12 months, as compared to pre-operative baseline, although interval assessments at months 1, 3 and 6 did show dryness in both groups. Quantitative changes occurring at the eye surface are more pronounced with Lasik, but more irritation, pain and eyelid sticking are felt with PRK, which could be related to increased denervation with Lasik. A Cochrane Systematic review comparing the LASIK and PRK concludes that LASIK may have faster recovery time and lower pain in comparison to PRK.
See also 
- Samuel E. Blum
- Automated lamellar keratoplasty
- Femtosecond laser intrastromal vision correction
- Refractive error
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- FDA information page on LASIK Includes detailed animation of the procedure
- University of Illinois page on LASIK Educational videos and full-text peer-reviewed journal articles on LASIK
-  U.S. National library of medicine.
-  Lasik Problems (organisation).
- "The Irish Times: Laser Eye surgery"