Corneal transplants usually use donor corneas with no regard to their orientation with respect to the remaining host material. It is not known why some of these transplants are not successful but it may in some cases be due to the collagen orientation being wrong. A preliminary clinical trial in America suggeste that donor to host alignment may play a role in the magnitude of post- surgical astigmatism [1]. A major clinical trial is now underway based in Manchester. Weakening and elongation of the sclera is implicated in the formation of myopia [2] and the mechanical properties of the sclera in different directions will depend greatly on the fibril orientation in the tissue. Changes in the collagen orientation in different parts of the sclera during the development of myopia have not been studied in detail.
Using synchrotron x-ray diffraction Meel et al. [3] found two preferred directions in human corneal stroma, inferior-superior and medial-lateral, and found that the preferred directions were more pronounced in the posterior stroma than the anterior stroma. We are currently extending this work to give quantitative orientation distributions with greater depth resolution across the cornea and sclera.
Normal human cornea and sclera were supplied by UKCTS Eye Bank in Bristol and x-ray diffraction patterns collected at station 7.2. A pattern was collected from the centre of an intact full thickness cornea. A thin layer was then dissected from the anterior surface and patterns collected both from the resulting lenticule and from the remaining tissue. A further layer was then removed and further patterns collected. In all the cornea was dissected into 4 approximately equal thickness layers.
The sequence of patterns shows the change from an approximately isotropic distribution in the anterior cornea to a biaxial distribution in the posterior cornea.
Preliminary work on full-thickness sclera was also carried out. A globe was opened out into four segments and patterns collected at points spaced by 2mm along a transect from the limbus to the optic nerve.
The preliminary work has shown that it is possible to obtain orientational information from diffraction patterns taken from corneal lenticules of around 100 microns thickness and that x-ray diffraction can also demonstrate changes in scleral colagen orientation around the ocular globe. We are in the process of carrying out further work to map in detail the fibril orientation with depth around the entire ocular globe.
References:
[1] Rapuano, C.J., Dana, M.R. Gomes, J.A.P. and Laibson, P.R. (1995) Inv.
Ophthal. Vis. Sci. 36(4), S653
[2] Curtin, B.J., Iwamoto, T. and Renaldo, D.P. (1979) Arch. Ophthalmol. 97,
912-915
[3] Meek, K.M., Blamires, T. Elliot, G.F. Gyi, T.J. & Nave, C. (1987) Current
Eye Research 6(7), 841-846