What is refractive surgery
Refractive surgery includes several surgical techniques used to correct vision defects or treat particular conditions of the ocular surface.
PRK, or photorefractive keratectomy, is an intervention that permanently modifies the shape of the cornea, to reduce or eliminate myopia, hypermetropia or astigmatism. The procedure is similar to LASIK (assisted-in-situ Keratomileusis) surgery, as it involves the use of an excimer laser.
In short: the LASIK technique (laser assisted in situ keratomileusis). LASIK surgery is a mixed technique, which involves the mechanical creation of a flap of corneal tissue, by means of a microkeratome or a femtosecond laser. This flap is raised to expose the underlying cornea layer, which is subsequently treated with an excimer laser (produces light pulses in the ultraviolet region). At the end of the operation, the corneal flap is repositioned in place, where it is warmed up spontaneously, during the healing process.
PRK
Photorefractive keratectomy
During photorefractive keratectomy (PRK), the laser removes tiny tissue fragments from the corneal stroma, at the front of the eye, just below the epithelium by ablation (ie by vaporization). 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 front surface of the eye, to achieve the desired effect. PRK is more suitable for patients with thin corneas or those with particular corneal anomalies, where using a microkeratome may not be the best choice.
LASEK. Epithelial laser 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 naturally grows again.
PRK - Procedure
Preliminary assessment
Before refractive surgery it is necessary to undergo a careful re-examination of general and eye health conditions. The shape of the front 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 in the patient's eyes: a topical antibiotic helps prevent any possibility of infection, while an anesthetic slightly numbs the corneal surface. Between the eyelids, a speculum is gently positioned to prevent the subject from blinking, after which the patient is asked to fix 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 regenerates completely 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 of correcting the refraction defect accurately by sending light beams, which last only a few billionths of a second, in coordinates pre-established on the cornea. The technique also uses a monitoring system, which tracks the position of the patient's eye from 60 to 4000 times per second and allows effectively following eye movements, precisely redirecting impulses. More modern laser instruments automatically center the patient's visual axis, then suspend ablation if the eye moves and resume when it is in position again. The procedure takes about 5-10 minutes, but may vary depending on the complexity of the correction required. Most people do not feel pain during PRK, but the patient may feel slight pressure around the eyes.
Recovery
When the surgery is finished, the doctor can instill some eye drops, to give the patient greater comfort and eye protection. Furthermore, a special soft contact lens can be placed to facilitate the first healing phase, which should occur within the first 3-4 days after PRK. During this period, the patient may experience irritation, a feeling of a foreign body in the eye, sensitivity to light and, in some cases, pain. The healing process continues for months after surgery, but the discomfort tends to progressively decrease 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 requiring good binocular vision could be suspended during post-operative recovery between interventions and during particularly prolonged healing periods
Prescriptions and drugs
To facilitate post-operative recovery, special attention must be paid to the doctor's detailed instructions. The ophthalmologist personalizes the therapeutic regimen according to individual post-operative needs, but an anti-inflammatory eye drops and a topical antibiotic are generally prescribed. Artificial tears may be necessary to limit the effects of dry eyes, up to a year after the procedure or in the long term.
Complications
PRK has an excellent safety profile and complications occur in less than 5% of cases. In fact, as in all laser surgery procedures, there is a risk that temporary or permanent side effects may arise. Dry keratoconjunctivitis 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 night rest. Many patients experience photophobia, perception of halos of light or glare as they drive in the evening hours, especially immediately after treatment. These consequences are rarely serious. During the stabilization period, other vision changes may occur, but in most cases, these effects regress 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 (ectasia).
Severe vision loss is very unusual, but some patients may need additional surgical correction or rigid contact lenses to restore their full vision.
Some of the possible complications of PRK include:
- Dry eyes;
- Ache;
- Glare, halos or luminous aberrations;
- Eye sensitivity;
- Increased sensitivity to light;
- Sub- (more common) or supra (more rare) correction of the refractive error;
- Recurrence of myopia;
- Scars;
- Infection;
- Reduced acuity in low light conditions.
Indications
The best candidate for refractive surgery must present:
- Adequate levels of myopia, hyperopia or astigmatism;
- Intolerance related to glasses or contact lenses (ie the patient expresses the desire to reduce or eliminate dependence on vision correction devices);
- Realistic expectations of the final results (with a complete understanding of the benefits, as well as of the possible risks).
PRK is considered safe and effective for people with mild to moderate levels of myopia, hyperopia and / or astigmatism. A positive outcome can also be obtained for high levels of myopia and hypermetropia, but individual clinical cases must be carefully evaluated. The greater the degree of treatment, the greater the risk of manifesting corneal opacity with the healing of the eye.
A potential candidate for the PRK must meet a number of basic criteria:
- Age over 18 years;
- Stable refraction error (no noticeable change over the last year);
- Diopters of myopia ranging from -1.00 to -12.00;
- Corneal anomalies not suitable for LASIK surgery;
- Pupil size> 6 mm;
- Absence of moderate-severe eye dryness, ocular irregularities, cataracts, allergies, degenerative and autoimmune diseases.
Some pre-existing conditions can complicate or prevent treatment:
- Vascular collagen disease (which can cause, for example, corneal ulceration);
- Ocular disease (for example: dry eye, keratoconus or glaucoma);
- Systemic diseases (for example: diabetes, rheumatoid arthritis etc.);
- Steroid side effects;
- 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.
| Advantages and disadvantages of PRK | |
| Pro | Versus |
| Suitable for patients with a thin cornea | Longer time to achieve slower vision and recovery results than LASIK surgery |
| No risk of complications associated with the creation of the corneal flap | Increased risk of post-operative infection, inflammation and corneal opacity |
| Reduced risk of corneal thickness impairment (ectasia) | More discomfort during initial recovery than LASIK surgery |
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 above 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 valid also in mild forms; however, it is important to follow the guidance and judgment of the surgeon to determine the intervention that potentially allows for better results.
The following table shows the main differences between PRK and LASIK eye surgery:
| PRK | LASIK | |
| Intra-operative pain | Nobody | Nobody |
| Post-operative pain | Variable (from minimum to significant) | mild |
| Approved for: Myopia farsightedness Astigmatism | <= -12.00 <= 5.00 <= -4.00 | <= -14.00 <= 5.00 <= -5.00 |
| Performing on thin corneas | Yup | No |
| Running on flattened corneas | Yup | No |
| Running on sunken eyes | Yup | No |
| Execution in case of dystrophy of the corneal epithelium | Yup | No |
| Corneal healing | Stable tensile strength | Reduced tensile strength |
| Risk with contact sports | No | Yup |
| Damage due to increased intraocular pressure | No | possible |
| Induced retinal detachment | No | possible |
| Post-operative eye dryness | Occasional | Frequent |
| Recovery of visual acuity | Slower | Fast |
| Long-term results | More predictable | Less predictable |
| Rates related to possible complications | 1-5% | Up to 25-30% |
| Follow up | Up to about 20 years | Most <10 years |
| Complications associated with the creation of the corneal flap | Nobody | Flap detachment due to trauma; creation of folds that require repositioning; diffuse lamellar keratitis; scar formation, etc. |
Long-term results
The purpose of refractive surgery is to minimize or eliminate the patient's need to wear glasses or contact lenses. PRK and LASIK allow to obtain similar results. Most people can reach 20/20 after photorefractive keratectomy (PRK) surgery, and almost all patients improve their visual acuity. However, the predictability of the correction of the visual defect is not an absolute guarantee: the improvement, which can be obtained after recovery, is not quantifiable, particularly for cases of severe myopia. The outcome of PKR depends on how well the eyes heal. After the procedure, some patients may still need to use glasses or contact lenses, but the prescription for correcting the residual refraction defect can be significantly lower.
