Photorefractive Keratectomy (PRK) & Mitomycin C (MMC): Bowmans Layer Ablation

In LASIK, cutting a $110\ \mu\text{m}$ flap weakens corneal tensile strength by 15% to 25%. In PRK (Surface Ablation):

1. The $50\ \mu\text{m}$ epithelial layer is removed (via 20% ethanol, an Amoils rotating brush, or completely touchless via TransPRK / SmartPulse lasers).
2. The 193 nm ArF excimer laser ablates the target diopters directly into Bowman's layer and anterior stroma.
3. Result: Zero flap is created! Zero risk of flap dislocation, zero risk of traumatic flap tears, and maximum residual stromal bed thickness preserved.

Unlike LASIK, PRK exposes the naked stromal bed to tear film cytokines:

$$\text{Tear TGF-}\beta_1 + \text{PDGF} \longrightarrow \text{Quiescent Keratocytes} \longrightarrow \mathbf{\text{Refractile Myofibroblasts}}$$

Myofibroblasts secrete disorganized, irregular extracellular matrix (abnormal glycosaminoglycans and fibronectin). This disordered collagen grid scatters incoming photons, generating a milky, frosted-glass cloudiness across the central cornea (Late Sub-Epithelial Haze) that ruins visual acuity.

In modern PRK, corneal haze has been virtually eradicated by the intraoperative application of Mitomycin C (MMC 0.02% / 0.2 mg/mL) for 12 to 60 seconds:

• MMC is an antineoplastic agent that cross-links DNA base pairs between guanine residues, preventing cellular replication.
• It selectively halts keratocyte proliferation and suppresses myofibroblast differentiation without inducing endothelial cytotoxicity.
• Combined with strict postoperative UV400 protection (such as ELLASUV certified UV lenses) for 6 months (solar UV light triggers late myofibroblast reactivation), haze incidence drops from 10% to under 0.2%!