The theory and implications of the biaxial model of corneal birefringence

Purpose: A theoretical model of biaxial optical anisotropy is derived and its structural, biomechanical and developmental implications are discussed with reference to known corneal anatomy. Methods: A concise review of the theory of optical crystallography is followed by the derivation of a theoretical model of the optical anisotropic properties of a dome of biaxial birefringent crystalline material. The model is then applied to parameters relevant to the biaxial model of human corneal birefringence. Results: Theoretical distributions of refractive indices and vibration directions for transmitted monochromatic light are derived for the central and paracentral zones of a model human cornea. Contours of equal refractive index (equirefringence curves) are found to have orthogonal confocal spheroconic geometry. Conclusions: A novel model of corneal structure is proposed in which discrete uniaxial positive birefringent fibre-like elements conform to the derived spheroconic geometry. Biomechanical implications and the relationship of the birefringent elements to known corneal anatomy are discussed. The crystallographic conventions are proposed as a standard for further investigations of corneal birefringence.