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D. G. Dawson, T. P. O'Brien, S. R. Dubovy, J. B. Randleman, B. E. McCarey, H. E. Grossniklaus, H. F. Edelhauser; Post-Lasik Ectasia: Histopathology, Ultrastructure, and Corneal Physiology. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3513.
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To study the histopathology and ultrastructure of post-LASIK ectasia corneal buttons and to describe the cohesive tensile strength of normal corneas at three stromal levels (anterior, mid, and posterior thirds) obtained from eye bank donors.
The histology and ultrastructure of 6 post-LASIK ectasia buttons was evaluated using light microscopy and transmission electron microscopy. The cohesive tensile strength from 5 normal eye bank donors was measured in the anterior third of the corneal stroma. This result was then compared to measurements taken in the mid or posterior third of the corneal stroma.
All post-LASIK ectasia corneal buttons (6 out of 6) had the thinnest residual stromal bed thicknesses less than 300 µm and all buttons had at least one full-thickness Bowman’s layer break over the region of ectasia. Ultrastructurally, all buttons had thinner--< 1 µm--than normal--2 to 3 µm--corneal lamellae in the ectatic region of the residual stromal bed. The keratocyte density in ectactic region of each button was not significantly different from normal post-LASIK corneas, but overall was 5 to 15% less than normal eye bank donor corneas. The corneal stromal tensile strength was highest in Bowman’s layer (50 g/mm) and the peripheral anterior third (40 g/mm) followed by the central anterior third (35 g/mm) and peripheral posterior two thirds (30 g/mm) of the corneal stroma. The weakest region of the corneal stroma was the central posterior two thirds (20 g/mm).
Our results show that excimer laser-based refractive surgery can result in sufficient weakness of the corneal stroma to result in corneal ectasia. Similar to keratoconus, early stages can be observed on light microscopy at Bowman’s layer, but it is markedly less severe in degree than keratoconus cases. Transmission electron microscopy suggests that elastic strain (stretching) on collagen fibrils in corneal lamellae and possibly shearing changes between the lamellae in the residual stromal bed may be the earliest changes found in post-LASIK ectasia cases. Our corneal physiology studies, in general, show that the central anterior third of the corneal stroma supports 45% of the eye wall stress. LASIK surgery approximations suggest a reduced central tensile strength on average of 27%; advanced surface ablative reduces the tensile strength on average 13%.
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