Organization of collagen fibrils in the corneal stroma in relation to mechanical properties and surgical practice.

We have used synchrotron x-ray diffraction to obtain quantitative information about the precise orientation of the collagen fibrils in the human cornea and sclera, and how these fuse at the limbus. We have shown that the human corneal stroma has preferred collagen orientation in the inferior-superior and nasal-temporal directions. At the limbus, however, the preferred orientation is tangential to the cornea. We demonstrated that these limbal fibrils are in the form of a circumcorneal annulus and quantified how this annulus varies with position. We have also started to unravel how the corneal collagen fibrils fuse with the collagen in the annulus. The results have both mechanical and surgical implications. Keratoplasty is performed without considering the preferred directions of the collagen fibrils. Surgery is increasingly used to correct astigmatism and myopia. The site and direction of an incision during, for example, cataract surgery, can influence the eventual level of astigmatism. X-ray diffraction helps our understanding of the underlying structural and hence mechanical reasons for refractive problems following surgery.