Birefringent properties of the Human cornea in vivo: towards a new model of corneal structure

The fundamental corneal properties of mechanical rigidity, maintenance of curvature
and optical transparency result from the specific organisation of collagen fibrils in the
corneal stroma. The exact arrangement of stromal collagen is currently unknown but
several structural models have been proposed. The purpose of the present study is to
investigate inconsistencies between current x‐ray derived structural models of the
cornea and optically derived birefringence data.
Firstly, the thesis reviews the current understanding of corneal structure, particularly
in relation to corneal birefringence. It also reviews and develops the different
analytical approaches used to model optical biaxial behaviour, particularly as applied
to predict corneal optical phase retardation.
The second part develops a novel technique of elliptic polarization biomicroscopy
(EPB), enabling study of corneal birefringence in vivo. Using EPB, the pattern of corneal
retardation is recorded for a range of human subjects. This dataset is then used to
investigate both central and peripheral corneal birefringence as well as the corneal
microstructure.

A key finding is that the central parts of the cornea exhibit a retardation pattern
compatible with a negative biaxial crystal, whereas the peripheral corneal regions do
not. Furthermore, within the central regions of the cornea, orthogonal confocal conic
fibrillar structures are identified which resemble the analytically derived contours of
equal refractive index of an ideal negative biaxial crystal.
The third part of this work presents a synthesis of previous published experimental,
anatomical and theoretical findings and the experimental results presented in this
thesis. Based on these findings, a novel corneal structural model is proposed that
comprises overlapping spherical elliptic structural units.
Finally, ensuing biomechanical and clinical consequences of the spherical elliptic
structural model and of the EPB technique are discussed including their potential
diagnostic and surgical applications.