X-ray diffraction and polarizing optical microscopy investigation of the structural organization of rabbit tibia.

X-ray diffraction and polarized optical microscopy investigations were carried out on thin sections of rabbit tibia in order to study the morphological organization of the structural components of this tissue, which often is utilized to test bone response to implants. In the optical microscope, the lateral face as well as the lateral portion of the caudal face exhibit a lamellar structure with an alternation of dark and bright lamellae running parallel to the long axis of the tibia. In contrast, both in the medial face and in the medial portion of the caudal face there are numerous osteonic structures. In spite of the complexity of this morphological organization, the results of small- and high-angle X-ray diffraction analyses indicate that the structural relationship between collagen fibrils and inorganic crystals is quite similar to that observed in single osteons and allows evaluation of the orientation of the two main structural components. Both collagen fibrils and apatitic crystallites are preferentially oriented parallel to the long axis of the tibia. The degree of orientation is greater in the thickness than in the plane of the lamellae, suggesting that collagen fibrils and inorganic crystallites lie preferentially in the plane of the lamellae, where they follow an oblique course. The degree of orientation of the apatitic crystallites is higher in the lateral face than in the medial and caudal faces, in agreement with the optical microscopic images. The results provide information that must be taken into account when evaluating the structural modifications of bone due to the insertion of a prosthetic device.