Phototherapeutic keratectomy (PTK) is a procedure that uses an excimer laser (wavelength 193 nm) to remove superficial corneal opacities, irregularities, and deposits. It is positioned as a bridge between medical and surgical treatments for corneal diseases and is used for both therapeutic and refractive purposes.
From the late 1980s to the early 1990s, the excimer laser was applied to photorefractive keratectomy (PRK) and LASIK. PTK was approved by the US FDA in 1995 for the treatment of anterior segment corneal diseases.
The excimer laser operates on the principle of photoablation, breaking intermolecular bonds and vaporizing corneal tissue. Approximately 0.25 µm of tissue is removed per pulse, allowing precise control of ablation depth. Opacities up to 150 µm from the surface, including the epithelium (about 50 µm), can be removed. Compared to manual keratectomy, PTK results in less irregular astigmatism and shorter treatment time.
PTK is a therapeutic procedure to remove superficial corneal opacities and irregularities. PRK is a procedure to change the curvature of a normal cornea to correct refractive errors. Both use the same excimer laser (193 nm), but the laser irradiation profile and purpose differ.
PTK is best indicated for cases with opacities in the anterior 10–20% of the cornea and no significant thinning 1).
Corneal Dystrophy
BIGH3-related dystrophies: Granular type I and II (including Avellino), lattice, Reis-Bücklers, Thiel-Behnke.
Gelatinous drop-like corneal dystrophy: Remove corneal protrusions by scraping or PTK.
Macular corneal dystrophy: Used to remove superficial opacities.
Corneal Degeneration and Others
Band keratopathy: After EDTA chelation of calcium deposits, reshape with PTK 1).
Salzmann nodular degeneration: Effective for smoothing subepithelial fibrosis 1).
Recurrent corneal erosion: A last resort for refractory cases resistant to conservative treatment.
Bullous keratopathy: Used for pain relief when visual recovery is not expected.
Superficial opacities confined to the anterior 10–20% of the cornea (within about 150 µm from the surface) are the best candidates. Deep stromal scars require excessive ablation, leading to haze and hyperopic shift, and are not indicated. A residual stromal bed of at least 250 µm is required.
The surgical treatment selection according to the depth of opacity is shown below1).
| Layer of Lesion | Representative Disease | PTK Indication |
|---|---|---|
| Epithelium | Irregular epithelium | × (Epithelial debridement) |
| Subepithelial | Salzmann nodular degeneration | ○ |
| Bowman’s layer | Band keratopathy, Reis-Bücklers | ○ |
| Anterior to mid stroma | Granular dystrophy | ○ (ALK/DALK also possible) |
| Mid to posterior stroma | Scar | × (DALK/PK) |
Performed under topical anesthesia (4% xylocaine or 0.5% proparacaine hydrochloride). General anesthesia may be used in children. A lid speculum is placed to begin the procedure.
Performed either by manual removal with a hockey-stick knife or by transepithelial PTK using an excimer laser.
The patient fixates on a fixation light, or the laser is manually centered for ablation. If the surface is rough, a masking agent (hydroxypropyl methylcellulose [HPMC] 0.7–2%) is applied to smooth it, ensuring the laser hits only the elevated areas.
When 70–80% of the target ablation depth is reached, confirmation is performed using a slit-lamp microscope.
Corrected visual acuity improves due to reduction in opacity density and irregular astigmatism. When flap creation and PTK are combined, corrected visual acuity significantly improves at 2, 6, and 12 months postoperatively1).
However, central ablation flattens the cornea and induces hyperopic shift. A 6 mm diameter, 100 µm depth ablation causes approximately 1.5 D of hyperopic shift.
For a 6 mm diameter, 100 µm ablation, approximately 1.5 D (diopters) of hyperopic shift is expected. The deeper the ablation, the greater the hyperopic shift. In some cases, PTK and PRK are combined to adjust the refractive change.
The excimer laser (ArF laser, wavelength 193 nm) is a far-ultraviolet laser. The photon energy at this wavelength exceeds the dissociation energy of carbon-carbon and carbon-nitrogen bonds, directly cleaving intermolecular bonds in corneal tissue and causing vaporization. Thermal damage is minimal, and the effect on surrounding tissue is minimized.
The reason PTK is uniquely effective in band keratopathy is that calcium deposits are ablated faster than the surrounding corneal tissue 1). However, this differential ablation may create surface irregularities, making proper use of masking agents important.
| Complication | Management/Notes |
|---|---|
| Recurrence of primary disease | Especially common in corneal dystrophies |
| Haze (corneal opacity) | Can be suppressed with MMC application 1) |
| Corneal ectasia | When ablation exceeds anterior 1/3 or residual thickness <250 µm 1) |
| Herpes simplex virus reactivation | Prophylactic antiviral therapy in patients with history of herpes 1) |
| Delayed epithelial healing | Managed with autologous serum eye drops or amniotic membrane1) |
| Infectious keratitis | Risk due to loss of epithelial barrier |
| Hyperopic shift | Unavoidable with central ablation. Can be reduced by correcting the ablation margin |
Corneal cross-linking (CXL) requires epithelial removal, and transepithelial PTK using PTK is attracting attention as a combined technique with corneal cross-linking2).
A study comparing PTK (Cretan protocol) and mechanical epithelial removal for epithelial removal during corneal cross-linking reported that the PTK group showed better visual and refractive outcomes2). PTK not only removes the epithelium but also has the effect of reshaping the irregular corneal surface.
Simultaneous combination (PTK/PRK + corneal cross-linking) has been reported to be more effective than sequential combination (PRK 6 months after corneal cross-linking)2), and further accumulation of evidence is expected.
