Mismatch between flap and stromal areas after laser in situ keratomileusis as source of flap striae

Purpose: To calculate theoretically the magnitude of the excess area between the lower surface of the flap and the underlying ablated stroma. Methods: On the initial assumptions of a nonextensible flap and a spherical cornea, flap and ablated stromal areas were determined as a function of myopic correction in the range of 0 to -12 diopters (D) for typical values of corneal radius (7.8 mm) and flap thickness (160 mum), together with a range of ablation zone diameters (4.0 mm, 6.0 mm, 8.0 mm, and 10.0 mm). Results: Excess flap area increases with the magnitude of the refractive correction and the diameter of the ablated zone. For a -6.0 D correction and an 8.0 mm ablation zone, the excess area is nominally about 1.0 mm(2), giving a potential overlap of the flap at the edge opposite the hinge of about 100 mum. Conclusions: Excess flap area may cause striae because of wrinkling. Although a nonextensible flap is assumed in the model, any stretching or contraction due to cutting the flap will be independent of the refractive correction. Hence, a mismatch in areas must still occur. This geometric effect may have clinical consequences in optical aberration, refractive regression, or impaired wound healing. (C) 2002 ASCRS and ESCRS.