Tissue addition theory of radial keratotomy: a geometric model.

We examined the use of a geometric model to explain the effects of radial keratotomy (RK) on corneal topography based on wound gape of radial incisions. Histopathologic studies of healed human RK incisions revealed incision gape in Bowman's membrane and anterior stroma, with intervening epithelial ingrowth or fibrous scarring. For six human RK incisions examined centrally, mid-peripherally, and peripherally, the linear separation of the cut ends of Bowman's membrane averaged 0.012 mm, 0.018 mm, and 0.027 mm, respectively. The effect of postoperative corneal flattening of the "addition of tissue" within incisions was mathematically predicted by calculating the effects of inducing radial gape on a spherical shell. The predicted change in corneal curvature agreed closely with data from a human case report. The "tissue addition" theory may partly explain the change in corneal topography following RK and subsequent wound healing.