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. Author manuscript; available in PMC: 2009 Jun 5.
Published in final edited form as: Exp Eye Res. 2006 May 23;83(4):709–720. doi: 10.1016/j.exer.2006.03.015

Figure 2.

Figure 2

Major biomechanical loading forces in the cornea and a model of biomechanical central flattening associated with disruption of central lamellar segments. A reduction in lamellar tension in the peripheral stroma reduces resistance to swelling and an acute expansion of peripheral stromal volume results (Dupps and Roberts, 2001; Roberts, 2000; Roberts, 2002). Interlamellar cohesive forces (Smolek, 1993) and collagen interweaving (Komai and Ushiki, 1991), whose distribution is greater in the anterior and peripheral stroma and is indicated by grey shading, provide a means of transmitting centripetal forces to underlying lamellae. Because the central portions of these lamellae constitute the immediate postoperative surface, flattening of the optical surface occurs, resulting in hyperopic shift. The degree of flattening is associated with the amount of peripheral thickening (Dupps and Roberts, 2001). This phenomenon is exemplified clinically by PTK-induced hyperopic shift but is important in any central keratectomy, including PRK and LASIK. Simultaneous elastic weakening of the residual stromal bed may occur (Guirao, 2005), and the threshold for inducing irreversible (plastic) or progressive (viscoelastic) steepening (or ectasia) is a matter of great clinical concern.