Scattering properties and transparency characterization of human corneal grafts

Abstract

The cornea is the single human tissue being transparent. This unique property may be explained by the particular structure of the cornea, but the precise role of each of its constituents remains unsolved. On other matter, prior to corneal transplant, graft must be evaluated during a sorting procedure where a technician assesses of its transparency quality. Nevertheless, this criterion remains subjective and qualitative. This study proposes to combine 3D imagery using Full-Field Optical Coherence Tomography jointly with angular resolved scattering measurement to achieve a quantitative transparency characterization of the cornea. The OCT provides micrometric resolution structural information about the cornea, and we observe the evolution occurring when oedema develops within the tissue. Scattering properties are evaluated and compared parallely, as the transparency of the graft. A close link between the scattering intensity level of the cornea and its thickness is highlighted through this study. Furthermore, the three-dimensional imagery offers a view over the structural modifications leading to a change in transparency, and the combination with scattering properties measurement provides clues over the characteristic scale of scatterers to consider for a better understanding of corneal transparency evolution. Achieving an objective and quantified parameter for the transparency would be helpful for a more efficient corneal graft sorting, and may be able to detect the presence of localized wounds as the ones related to a previous refractive surgery. However, the study of graft nearly eligible for corneal transplant would be needed to confirm the results this study presents. CONTEXT AND PROBLEMATICLight transmission inside the human ocular globe is ensured by a disc with a mean diameter of 11 mm and an average thickness of 600 µm: the cornea. It is the outermost layer of the eye, and the single human tissue with the lens being transparent. It offers an averaged transmission factor superior to 90% in the visible spectral range (400 to 800 nm) 1 , several considerations being involved in such phenomenon.This transparency is firstly due to the avascularization of the cornea, the oxygen and nutriments supply being supported by tears and aqueous. Because of the absence of haemoglobin, the light transmission is not only restricted to the "therapeutic window" (wavelengths from 650 nm to 1 µm) usually available in biological tissues. Nevertheless, the main explanation of this transparency property is linked to the specific and ordered structure of the cornea. The cornea consists in the superimposition of 5 distinct layers, the most important being the stroma which represents 90% of the whole thickness. It is composed of collagen fibrils with a diameter of 20 to 35 nm, disposed in parallel the one to the others with a 40 nm space, within 2 µm thick lamellas 2 . These lamellas are piled up all along the thickness of the cornea with slight angles between them. Several theories were proposed implying the highly ordered structure ...