PRK, or photorefractive keratectomy, is an operation that allows you to permanently change the shape of the cornea, to reduce or eliminate myopia, hyperopia or astigmatism. The procedure is similar to LASIK (LASer-assisted In Situ Keratomileusis) surgery in that it involves the use of an excimer laser.
LASIK for short
LASIK surgery (laser assisted keratomileusis in situ) is a mixed technique, which involves the mechanical creation of a flap of corneal tissue, using a microkeratome or a femtosecond laser. This flap is lifted to expose the underlying corneal layer, which is subsequently treated with an excimer laser (it produces pulses of light in the ultraviolet region). At the end of the surgery, the corneal flap is repositioned in place, where it spontaneously heals. , during the healing process.
it is surgically corrected, it allows to better focus the light on the retina, providing a much clearer vision.
During photorefractive keratectomy (PRK), the laser removes by ablation (ie by vaporization) tiny pieces of tissue from the corneal stroma, in the front of the eye, just below the epithelium. Unlike the LASIK technique, the surgeon does not need to create a flap of tissue, but is able to apply the laser directly to the anterior surface of the eye to achieve the desired effect. PRK is best suited to patients with thin corneas or who have particular corneal abnormalities, where using a microkeratome may not be the best choice.
LASEK
Laser Epithelial Keratomileusis (LASEK, LASer Epithelial Keratomileusis) is a procedure similar to PRK, but involves the use of alcohol to remove the corneal epithelium. A laser is then used to change the shape of the cornea. After a few days, the surface layer of removed cells grows back naturally.
. The shape of the anterior surface of the eyes is mapped with an instrument called a corneal topographer, while the thickness of the cornea is measured with a pachymeter
Before the PKR
On the day of surgery, a few drops are instilled into the patient's eyes: a topical antibiotic prevents any possibility of infection, while an anesthetic slightly numbs the corneal surface. A speculum is gently placed between the eyelids to prevent the subject from blinking. , after which the patient is asked to look at a reference light. Before ablation, the surgeon removes a thin layer of the outermost tissue that covers the cornea (corneal epithelium). After surgical remodeling of the cornea, this epithelium completely regenerates within 3-5 days.
During the surgery
To correct myopia, hyperopia and / or astigmatism, the surgeon will use an excimer laser, programmed based on calculations made after the preliminary eye examination. Therefore, laser surgery offers the possibility to correct the refractive defect in an exact way, by sending beams of light, which last only a few billionths of a second, in pre-established coordinates on the cornea. The technique also uses a monitoring system, which tracks the patient's eye position from 60 to 4000 times per second and allows you to effectively follow eye movements, accurately redirecting pulses. Most modern laser tools automatically center the visual axis. patient, then stop ablation if the eye moves and resume when it is back in position. The procedure takes about 5-10 minutes, but can vary depending on the complexity of the correction needed. Most people do not feel pain during PRK, but the patient may feel slight pressure around the eyes.
Recovery
When the operation is finished, the doctor can instill some eye drops, to give the patient greater comfort and protection to the eyes. In addition, a special soft contact lens can be placed, to facilitate the first healing phase, which should take place within the first 3-4 days after PRK During this period, the patient may experience irritation, a foreign body sensation in the eye, sensitivity to light and, in some cases, pain. The healing process continues for months after surgery, but the discomfort tends to gradually subside within a few days and the patient may experience a noticeable improvement in vision. PRK can be performed on one eye at a time. Activities that require one eye. good binocular vision may be suspended during postoperative recovery between surgeries and during particularly prolonged healing periods
Prescriptions and drugs
To facilitate post-operative recovery, special attention should be paid to the doctor's detailed instructions. The ophthalmologist customizes the treatment regimen according to individual post-operative needs, but generally anti-inflammatory eye drops and a topical antibiotic are prescribed. Artificial tears may be necessary to limit the effects of dry eyes, up to one year later. the procedure or in the long term.
it is the most common complication of PRK. In more advanced cases, recurrent erosions may occur, due to the adherence of the corneal epithelium to the upper eyelid, during the night's rest. Many patients experience photophobia, the perception of halos of light or glare while driving in the evening, especially immediately after treatment. These consequences are rarely serious. During the stabilization period, other vision changes may occur, but in most cases, these effects subside within six months after surgery. In rare cases, excessive thinning of the corneal wall can give an unstable shape to the surface of the eye. eye (ectasia).
Severe vision loss is very unusual, but some patients may need further surgical correction or hard contact lenses to restore their full vision.
Some of the possible complications of PRK include:
- Dry eyes
- Ache;
- Glare, halos or luminous aberrations;
- Ocular sensitivity;
- Increased sensitivity to light;
- Under- (more common) or over- (rarer) correction of refractive error;
- Recurrence of myopia;
- Scars;
- Infection;
- Reduced acuity in low light conditions.
A potential PRK candidate must meet a number of basic criteria:
- Over the age of 18;
- Stable refractive error (no noticeable changes over the last year);
- Myopia diopters ranging from -1.00 to -12.00;
- Corneal abnormalities unsuitable for LASIK surgery;
- Pupil size> 6 mm;
- Absence of moderate-severe dry eye, eye irregularities, cataracts, allergies, degenerative and autoimmune diseases.
Some pre-existing conditions can complicate or prevent treatment:
- Vascular collagen disease (which can cause, for example, ulceration of the cornea);
- Eye disease (for example: dry eye, keratoconus or glaucoma);
- Systemic diseases (for example: diabetes, rheumatoid arthritis, etc.);
- Side effects from steroids;
- Type II granular corneal dystrophy.
PRK is a less used procedure than the LASIK technique, but it is still applied when the latter is not the best choice.
Difference between LASIK and PRK
Both procedures use an excimer laser to reshape the cornea and correct refractive defects. During the PRK, the laser is used to reshape the cornea by acting directly on its surface, while in the LASIK technique, it is applied after the creation and lifting of a flap of corneal tissue. LASIK surgery is the most popular procedure, generally used for medium and high visual defects, but also valid in mild forms; however, it is important to follow the surgeon's guidance and judgment to determine the intervention that potentially yields better results.
The following table exposes the main differences between PRK and LASIK eye surgery:
Myopia
Hyperopia
Astigmatism
<= -12.00
<= 5,00
<= -4.00
<= -14.00
<= 5,00
<= -5.00
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