Acute tissue deformation of the human cornea after radial keratotomy

The regional deformation pattern of the cornea after radial keratotomy, which is essential for understanding the mode of action of the procedure, has not previously been studied in detail.

Up to 90 tiny mercury droplets were placed from center to limbus on the epithelial and endothelial corneal surfaces of eight eviscerated human donor eyes with four radial keratotomies (depth 100% of central corneal thickness, 3.5-mm clear zone). From digital images obtained under pressure loads ranging from 2 to 100 mm Hg, the distances between the fixed droplets were measured with an accuracy of 1 micron. After transforming the data to polar coordinates, regional meridional and circumferential strain patterns were calculated. Regional meridional and circumferential radii of curvatures were calculated from corneal profile images obtained at different pressure loads before and after keratotomy.

Increasing the intraocular pressure from 2 to 100 mm Hg induced: an epithelial side wound gape of 44 mm; epithelial side circumferential tissue compression between incisions; considerable epithelial side meridional tissue elongation at and between incisions; little endothelial side circumferential strain across incisions; and little endothelial side meridional strain at and between incisions. The radial keratotomy induced 2.30 diopters (D) of central corneal flattening at an intraocular pressure of 2 mm Hg. The degree of central flattening correlated linearly with the amount of wound gape. In the physiological pressure range of the central cornea flattened 0.05 D for each millimeter-of-mercury increment in intraocular pressure. Pronounced meridional steepening was induced corresponding to the middle and peripheral parts of the keratotomy incisions.

Our study suggests that the peripheral "tissue addition" seen after radial keratotomy is a net result of wound gape and circumferential tissue compression. Local bending of intact stromal tissue below the incisions plays an important role for the generation of the wound gape at the corneal surface. These data may help verify finite-element computer models of the human cornea.