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

Abstract

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 μm), 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 μm. 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.