Article

Advances in surface ablation

Surface ablation is a collective term to describe refractive surgery with corneal epithelium removal in the absence of a repositionable stromal flap, generally referred to as PRK. Although LASIK is still the most common procedure performed by refractive surgeons in the United States, surface ablation may be preferable in some cases, most notably in patients with thin corneas, to avoid ectasia, to enhance biomechanical stability and to prevent flap complications.

METHODS OF DEBRIDEMENT

Several techniques have been developed to debride the corneal epithelium prior to surface ablation. Mechanical debridement using a blunt blade like the Tooke knife, or a blunt “hockey stick,” was the original method, but could be time consuming and create scratches in Bowman’s membrane,1 particularly if a sharper knife were used.2 A rotary brush was developed to facilitate rapid removal with less damage to Bowman’s membrane. Alcohol debridement is commonly performed using an optical zone marker ring to create a well. For 20 to 30 seconds, 20% diluted absolute ethanol solution is applied to the area to be de-epithelialized, then it is reabsorbed with a surgical sponge.2 This method allows the epithelium to be discarded or preserved as a flap; when the epithelial flap is preserved, it is known as laser epithelial keratomileusis (LASEK). Transepithelial phototherapeutic keratectomy (T-PTK) uses an excimer laser to remove the corneal epithelium prior to PRK.3 Advantages include reduced treatment time and no alcohol toxicity. Epi-Bowman keratectomy (EBK) uses a device called the Epi Clear to remove the epithelium above Bowman’s layer and membrane. One study found that EBK results in earlier healing and reduced postoperative pain as compared to mechanical debridement.4

PAIN MANAGEMENT

Although the excimer laser was thought to induce minimal thermal damage to the corneal stroma, corneal temperatures have been shown to rise to as high as 53 degrees C after ablation.5 Chilling the cornea before, during and after PRK — commonly performed with a chilled saline wash — decreases pain, haze and myopic regression.6 Zeng et al. recommend that a cold patch of soft polyvinyl chloride, filled with antifreeze gel, be fastened with Velcro strips to hold the patch in place and worn for 24 hours postoperatively. It was shown to be better than cold saline and lowered the need for painkillers.7

To aid in postoperative pain management and healing, a bandage contact lens can be used. Data show that senofilcon A (Acuvue Oasys) was the most comfortable bandage lens material after PRK.8 Various combinations of medications have been used to treat postoperative discomfort. Topical nonsteroidal anti-inflammatories (NSAIDs) may be toxic and delay healing in excess quantities; their use is often restricted to three days post surgery. Dilute anesthetic drops such as preservative-free tetracaine 0.5% in the first 24 to 72 hours may also help, but require extreme caution because overuse delays healing.

Oral NSAIDs, including cyclooxygenase-2 inhibitors taken before and after the procedure, may be useful and are recommended, particularly when narcotics are contraindicated. The use of minims – and patient education on their side effects – are necessary. Other oral pain management includes narcotics, corticosteroids or gabapentinoids. Antiemetics to prevent pain and medication-associated nausea and vomiting may also be indicated.

ABERRATION CONTROL

Conventional myopic PRK resulted in small blending zones and oblate corneas postoperatively, inducing higher order aberrations.9 Wavefront-guided (WFG) excimer lasers, the gold standard, ablate the cornea based on aberrometry measurements to treat spherocylinder refractive error and higher-order aberrations and are patient specific, centered on the pupil. Wavefront-optimized (WFO) lasers use the patient’s refraction to determine the spherocylinder correction while reducing induced spherical aberration by completing extra pulses in the periphery.1 Studies have shown that although there is no difference in uncorrected vision after three months between WFG and WFO PRK, the WFG group showed fewer higher order aberrations including trefoil and coma.10

Topography-guided treatment (TG-PRK) is beneficial in highly aberrated corneas with irregular astigmatism,11 but it is only approved in the United States to treat myopia and traditional astigmatism. It involves a Placido disc centered on the corneal vertex, approximating the visual axis, benefiting patients with a large angle kappa between the pupil and visual axis.12 It decreases higher-order aberrations by making the cornea more regular, treating variations in astigmatism and creating smoother transition zones, preventing peripheral aberrations. A potential downside to TG-PRK is that it requires a skilled operator to obtain accurate topographic data maps.

Patients who benefit the most from TG-PRK are those with, in order of significance, normal but asymmetric corneas with a cylinder greater than 1D; higher-order aberrations greater than 0.35 µm, (particularly coma and trefoil); and pupils greater than 7 mm. As a primary procedure, it is approved for a spherical equivalent of up to -8D and a cylinder of up to -3D.13 Exclusion criteria include hyperopia, mixed astigmatism and any abnormal anatomy that precludes high-quality topographical measurements, including long lashes, deep-set eyes, keratoconus and dry-eye disease. As a secondary procedure, TG-PRK may be used to treat corneas with a prior refractive surgery or when following penetrating keratoplasty, although these are off-label uses in the United States.

Cyclotorsion of the eye during surface ablation can result in uncorrected or induced refractive error. Iris registration with dynamic intraoperative rotational eye tracking is incorporated in many laser platforms to halt the laser when movement is detected.

Table 1. Variants of laser vision correction
Abbreviation Procedure Key differentiating feature
PRK2 Photorefractive keratectomy Excimer laser used to reshape the de-epithelialized cornea
T-PTK2 Transepithelial phototherapeutic keratectomy Excimer laser used to remove the epithelium followed by PRK
EBK4 Epi-Bowman’s keratectomy A device called Epi Clear is used to remove the epithelium above Bowman’s layer and basement membrane
LASEK2 Laser epithelial keratomileusis with alcohol Removal of the epithelium using alcohol to create an epithelial flap which is repositioned postoperatively
Epi-LASIK2 Laser epithelial keratomileusis with an epi-keratome Creation of a thin epithelial flap using a microkeratome with a soft, blunt applanation plate
LASIK2 Laser in situ keratomileusis Creation of a stromal flap with a microkeratome or femtosecond laser; then excimer laser stromal ablation and flap repositioning
Intralase SBK Sub-Bowman’s keratomileusis Femtosecond laser is used to create a very thin corneal stromal flap just below the level of Bowman’s membrane
SMILE Small-incision lenticule extraction Femtosecond laser intrastromal incisions to remove a lenticule of stromal tissue through a channel in the cornea (no flap is created, and excimer ablation required)

PREVENTING HAZE

The incidence of haze following PRK has decreased dramatically over the years, with the development of flying spot lasers (as opposed to those that use a closing diaphragm) to deliver smoother ablations, and more sophisticated ablation profiles. Haze varies widely and can be graded at the slit lamp or with confocal microscopy. There is a greater risk with deeper ablation and subsequent retreatments. Subepithelial reticular haze usually appears in the first month postoperatively, peaks in density in three to six months and regresses between nine and 12 months. It is attributed to activation of keratocytes. Promoting rapid re-epithelialization may help prevent haze. Smoking cessation and aggressive treatment of dry eye and blepharitis aid in preventing complications. Oral supplementation with vitamins A, C and E and UV protection are also helpful. Most corneal haze is not clinically significant, but the development of late haze is most commonly attributed to myofibroblasts and only occurs in 0.5% to 5% of cases.14

A short duration course of topical corticosteroids with tapering may be prescribed to reduce the risk of haze following PRK but its long-term use is controversial. Side effects include a rise in intraocular pressure (approximately 1/3 of the population are steroid responders) and an increased risk of earlier cataract development in all patients with prolonged use. Late onset haze may be treated with high-dose corticosteroids for several weeks, with a slow taper over several months, but only 15% of eyes will respond, and if they do, it will be within the first week of treatment.15 Other patients may require retreatment for corneal haze, though this is rare. Aggressive treatment of dry eye and surface disease may be helpful in these patients.14

Mitomycin-C (MMC), an antibiotic, is an antineoplastic agent that inhibits synthesis of DNA, RNA and proteins. Prophylactic use of MMC during PRK inhibits haze formation by decreasing keratocyte proliferation in the anterior corneal stroma.16 The dosage may be variable at 0.1 - 0.2 mg/mL for 12 seconds to two minutes. A longer duration of MMC use is necessary if there has been previous corneal surgery. Bevacizumab and rapamycin are being used experimentally to prevent haze formation and may do less damage to keratocytes than MMC.17 Other potential investigational alternative therapies for preventing haze include Trichostatin A, a histone deacetylase inhibitor that prevents myofibroblast formation, PRM-151-recombinant human pentraxin-2 protein, cytokines and growth factors.14

Artificial tears are widely prescribed after surface ablation for comfort and healing. Preservative-free preparations are preferred in the first three months post surgery. Blood-based serum tears, also known as autologous serum eye drops, can help wound healing.18 Eye platelet-rich plasma has a greater concentration of growth factors and cell adhesion molecules than autologous serum, and could also have a role in corneal epithelial wound healing.19

Cryopreserved amniotic membranes have been used to treat nonhealing epithelial defects following surface ablation. They potentially reduce inflammation by preventing inflammatory cell infiltration, decreasing lipid peroxidation, avoiding keratocyte apoptosis and stimulating re-epithelialization.20

Table 2. Surface ablation pros, cons
Advantages Disadvantages
No risk of intraoperative or postoperative LASIK flap complications More postoperative pain
Highly reduced risk of keratectasia Prolonged visual recovery
Results equal to LASIK in low and moderate myopia Increased risk of infection
No flap-induced higher order aberrations Longer use of steroids
Less postoperative dry eye Potential for delayed haze

SURFACE ABLATION COMBINED WITH CXL

Corneal crosslinking (CXL) with riboflavin (vitamin B2) and UVA light delays or halts ectasia progression in keratoconus or post-LASIK ectasia by increasing the strength and rigidity of the corneal collagen bonds.21 It may be used in conjunction with topography-guided surface ablation to improve vision. In a study conducted by Kontadakis et al. of the long-term comparison of TG-PRK with CXL versus CXL alone, those who had PRK in conjunction with CXL had statistically significant improvements in vision and flatter keratometry readings.22

CONCLUSION

Modern-day surface ablation is a safe and effective procedure with recent advances in methods of debridement as well as aberration control. WFG, WFO and TG-PRK lead to improved visual outcomes with fewer aberrations for patients with myopia and astigmatism. Postoperative pain appears to be decreased with chilling of the cornea and is manageable with bandage contact lenses, NSAIDs, topical anesthetic drops and oral analgesics.

MMC significantly reduces haze and appears safe, but long-term results are still pending, with promising alternatives also being studied.

In combination with CXL, surface ablation appears to improve visual outcomes in patients with keratoconus. OM

REFERENCES

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