PRK involves removal of the outer layer of the cornea, the epithelium, a rapidly regenerating layer of cells about 6 cells thick in contact with the tear film that can completely replace itself from peripheral limbal stem cells within a few days with no loss of clarity. The deeper layers of the cornea referred to as the stroma are composed of hydrated protein collagen fibers which give the cornea its strength, elasticity and clarity. The stroma is the layer of tissue exposed in both PRK and Lasik that is reshaped by precise tissue ablation(removal)with the laser. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate after the surgery. The procedure is distinct from Lasik where a partial horizontal cut through the corneal stroma is made either with a mechanical microkeratome or femtosecond laser creating a hinged flap. The procedures have many similarities but each has distinct advantages and disadvantages. The visual results at 6 months are similar for both PRK and Lasik. Lasik and PRK are usually painless procedures requiring only eyedrops for anesthesia.
The intra-operative risks for Lasik are greater than PRK, because Lasik involves an extra intra-operative step, flap creation. On rare occasions with either the laser or the microkeratome flaps can be imperfect. Most flap related complications can be handled without significant visual loss but there are times where the defects can contribute to permanent visual loss.
Postoperatively patients prefer Lasik over PRK for its typical lack of pain or discomfort and rapid visual recovery. After PRK most patients report mild irritation and foreign body sensation, however, on some occasions patients can have significant discomfort. The pain can usually be controlled with eye drops or systemic medications. Lasik patients benefit with faster visual recovery compared to PRK. Most patients can see fairly well the day after lasik while the vision with PRK is usually blurred for a week or sometimes longer.
Postoperative complications due to the laser ablation itself are rare with modern lasers, but occur with similar rates with either procedure. These include decentered ablations, overcorrections and undercorrections. The superb tracking and automatic pupil centration of the Wavelight Allegretto laser helps to avoid decentered ablations. The Perfect Pulse Technology and 400 HZ speed of the Wavelight Allegretto helps to reduce overcorrections and undercorrections.
There are postoperative risks that are unique to Lasik
These include transient light-sensitivity syndrome (TLSS), diffuse lamellar keratitis (DLK), flap wrinkles (striae),flap dislocation and epithelial ingrowth.
Transient light-sensitivity is a inflammatory condition unique to the femtosecond laser and requires treatment with an intensive course of topical steroids. This complication can last for weeks or months but usually has no permanent affect on the vision.
DLK or diffuse lamellar keratitis is a condition where an inflammatory stimulus causes the infiltration of white cells in the flap interface. This inflammation can be mild or severe. If the diagnosis is rapidly made and treated promptly the condition usually resolves without visual sequelae.
Epithelial ingrowth is a condition where epithelium grows beneath the flap causing irregular astigmatism and in severe cases, melting of the overlying corneal tissue resulting in corneal scarring.
Flap dislocation happens when the flap gets mechanically dislocated from its normal condition either spontaneously or from trauma. The flap can be repositioned surgically but higher rates of striae and epithelial ingrowth can occur with flap dislocation.
Lasik results in greater weakening of the mechanical strength and rigidity of the cornea since by its nature of creating a flap results in a thinner residual corneal bed than PRK. Thin corneal beds increase the risk of developing a condition called corneal ectasia where the cornea becomes distorted due to its weakened condition. Fortunately, the risk of ectasia has been reduced with the advent of making thinner corneal flaps known as thin-flap lasik. Careful pre-operative screening with corneal topography can also reduce the risk of corneal ectasia.
There seems to be a higher incidence and degree of post-operative dry eye syndome with lasik as compared to PRK since a deeper treatment interrupts the nerve supply more. Most cases of dry eye improve over time as the corneal nerves regenerate, however, on rare occasions the dry eye is permanent.
A postoperative complication more likely to occur after PRK than Lasik is the development of corneal haze.
Corneal haze has a higher incidence with higher corrections and deeper ablations. Ablations deeper than 80 microns or 18% of the preoperative corneal thickness are more likely to cause corneal haze. In these cases, the intra-operative application of Mitomycin-C to the corneal bed has been very effective in preventing corneal haze and multiples studies have shown its use to be safe except in pregnant women who shouldn't be having surgery anyway. All woman of childbearing age should have a pregnancy est before being treated with mitomycin.
Infections are very rare after Lasik or PRK, however, infections occur more frequently after PRK. Infections usually respond to topical antibiotics.