Photorefractive Keratectomy (PRK)
How PRK Works
This technique is the most simple and straightforward method of laser correction. To perform PRK, the surgeon uses the excimer laser to vapourize away microscopic layers of tissue from the surface of the eye. By modifying the shape of the cornea, light rays can focus correctly within the eye to provide clear vision. PRK is less invasive than LASIK since only the surface tissue of the eye is affected.
For the procedure, the patient sits in a surgical chair which is reclined into a horizontal position. A surgical assistant instills topical anaesthetic drops on the eye to "freeze" it for a painless correction.
The cornea consists of different layers of tissue. The surface layer, known as the epithelium, is a soft, protective layer that regenerates quickly.
Before laser correction, the surgeon removes this outer membrane by gently rubbing it away with an instrument. Within a few days of the surgery, the epithelium regenerates itself, again forming a protective layer over the eye.
The interior part of the cornea, known as the stroma, does not regenerate itself when removed. For PRK, the surgeon programs and positions the laser to remove a small portion of this stromal tissue to permanently change the shape of the cornea. In most cases, only 5 to 15% of the corneal thickness is treated, leaving the structural integrity of the cornea intact. The cool laser beam vapourizes tissue away, one microscopic layer at a time, without burning or cutting. Since the excimer laser light is created at a specified wavelength that does not pass through the cornea, no other part of the eye is affected.
To correct nearsightedness, the laser removes central tissue to reduce the curvature of the cornea so light rays will focus farther back and reach the retina. To correct astigmatism, the laser removes tissue from designated areas to equalize the curvatures of the cornea so light rays from the full scope of vision can focus evenly on the retina.
After the tissue has been removed, the surgeon places a special soft contact lens on the eye to protect it for the first few days of healing. This lens remains on the eye both day and night for up to four days, although it may be replaced on the second or third day.
The laser correction itself usually takes less than one minute per eye, although the patient is in the surgery suite for about 15 minutes.
Who Benefits Most
- People with low and moderate degrees of nearsightedness
- People with low and moderate degrees of astigmatism
After Surgery Expectations
Patients normally require several days of healing to achieve good functional vision after PRK correction. Patients can see immediately after the surgery, but vision is not clear. The protective epithelium layer must heal back and become smooth again to allow clear vision. This healing usually occurs within a week, allowing most patients to then drive and go back to work, wearing no corrective lenses.
- After PRK correction, most people experience some temporary side effects associated with the healing of the eye. These include initial exaggerated results, increased light sensitivity, halos or glare from bright lights at night, decreased clarity in dim light and slightly drier eyes. Four to six weeks after correction, most of these side effects have disappeared, and vision is generally very good. Complete visual stabilization usually occurs within one to four months.
- Patients should expect mild to moderate discomfort for one or two days after the surgery but most do not need more than over-the-counter medications to control it. A small percentage of patients do experience significant discomfort requiring pain medication.
- Antibiotic, anti-inflammatory and moisturizing drops are required for at least a few days after surgery. As the healing process continues, steroid drops may be required to optimize the surgical result.
- The majority of patients receive full correction in one treatment. For those who do not, a second treatment or enhancement is generally possible.
Specific Advantages of PRK
- Over a decade of surgical success
- No incision necessary
- Non-complex corrective procedure
- Permanent tissue modification for lasting correction
- PRK is a very safe procedure
Significant complications are rare, and even if they do occur, they can usually be resolved with medications or further surgery.
- Infection (less than 1/5,000)
Infections are very rare, but they can damage the cornea if not resolved by early treatment. They are identified early during after-surgery checkups and effectively treated with medications.
- Corneal haze or scarring (1-2%)
Microscopic haze, generally unnoticed by the patient, is normal for a few weeks following the surgery. It clears as the eye heals. However, it occasionally appears more noticeable or more lasting than normal. High PRK corrections have a higher incidence of haze problems. Healing time and additional treatments are used to resolve the problem. A slight scarring resulting from an uneven healing process is also possible. A second laser treatment to smooth the cornea generally solves the problem, but complete smoothing is not always achievable.