EFFECT OF BLADE CONFIGURATION, KNIFE ACTION, AND INTRAOCULAR-PRESSUREON KERATOTOMY INCISION DEPTH AND SHAPE

For the same diamond blade extension, uphill (centripetal) radial kera totomy incision direction achieves greater depth and consequently grea ter refractive effect than downhill (centrifugal) incisions. To determ ine which factors may account for this difference, two uphill and two downhill incisions were made with a double-edged diamond blade set to 90% central pachometry in 26 human donor eyes at 15 or 60 mm Hg. Uphil l incisions made with the perpendicular blade had greater mean incisio n depth than downhill incisions made with the oblique blade at 15 mm H g (83.6 +/- 3.9% and 68.2 +/- 5.2%) (p < 0.0005) and at 60 mm Hg (86.3 +/- 3. 1% and 79.7 +/- 1.7%) (p < 0.0005). Uphill and downhill incisi ons both made with the perpendicular blade had equal depth (85.4 +/- 4 .9% and 83.7 +/- 3.5%) (p > 0.1). The perpendicular blade edge created a straight, and the oblique edge an S- or J-shaped, histological inci sion configuration. Corneal profile pictures taken during each incisio n showed the knife to tilt opposite of the incision direction and to m ove at a constant angle to the limbal plane, producing a smaller optic al clear zone (OCZ) in the posterior stroma than intended with uphill incisions. Greater refractive effect with uphill incisions may be expl ained by the perpendicular blade being more effective in incising corn eal lamellae, and the creation of a smaller posterior OCZ. Intraocular pressure variations during surgery may affect achieved incision depth of downhill, but not of uphill, incisions.