The aim of this study was to determine any differences in the
collagen molecular and fibrillar packing, or the arrangement of the
proteoglycans along the fibril axis, in the stroma of
keratoconus and control corneas. High and low-angle x-ray diffraction patterns from the fibrillar and molecular packing of
collagen in
keratoconus and control corneas were obtained using a
synchrotron radiation source. The results indicate no difference in interfibrillar spacing between
keratoconus and control corneas at normal physiological hydration, or over a range of hydrations (H = 1-11). This unambiguously demonstrates that the thinning of the stroma that occurs in
keratoconus is not a result of closer packing of the
collagen fibrils in the stroma. Intermolecular spacings were shown to be significantly (P less than 0.001) lower in
keratoconus corneas at normal physiological hydration and over a range of hydrations (H = 1-11). Meridional patterns from the axial distribution of electron density along the
collagen fibrils were obtained from untreated control and
keratoconus corneas and from the corneas after their
proteoglycans were stained with
cupromeronic blue. Analysis of the integrated intensities of the first nine orders of these reflections show there is a difference in the staining behavior of
collagen-associated
proteoglycans in control and
keratoconus corneas. Determination of the electron density vectors along the
collagen fibrils of
cupromeronic blue-stained corneas by the use of Patterson functions indicates that the
keratoconus corneal stroma has a specific, ordered
proteoglycan that is present in lower numbers along the
collagen fibrils, and that it stains less with
cupromeronic blue or is in a more disordered arrangement than in the controls.