Abstract Nickel-titanium shape memory metal Nitinol (NiTi) is a new kind of implant material, which provides a possibility to prepare functional implants activated at body temperature and withstands kinking better than conventional metals. Applications utilizing these unique properties are a target of active research interest. Host reactions to NiTi and to the forces created by functional implants should also be studied. A functional NiTi intramedullary nail, which causes a bending force on the bone, was developed for correcting bone deformities. In the present studies, the action of the device was inverted to induce a bone deformity instead of correcting one, in order to test the hypothesis that bone modelling can be controlled using such functional nail. Implanting the nail into the medullary cavity of rat femur for twelve weeks caused bowing of the bone, retardation of its longitudinal growth, and thickening of the bone and the cortex. In another study the effects of functional and straight nails were compared. Bowing of the bone and significant overall thickening of the bone and the cortex were associated only with the functional nail, while the straight nail induced only minor thickening of the bone. Retardation of longitudinal growth was seen in both groups, and this may have been caused by perforating the distal epiphyseal plate during the nailing. Finite element model of the bone-nail combination was also created. Porous NiTi was studied as a bone graft substitute by filling a bone defect in the distal femoral metaphysis of a rat bone with porous NiTi implants of different porosities. After 30 weeks, porosity of 66.1% (mean pore size (MPS) 259μm) showed the best bone-implant contact (51%). However, porosity of 46.6% (MPS 505μm) with 39% bone-implant contact was not significantly inferior in this respect and showed a significantly lower incidence of fibrosis within the implant and thus seemed to be the best choice for a bone graft substitute, out of the porosities tested here. The porosity of 59.2% (MPS 272μm) showed lower contact values. NiTi tendon suture material was studied by implanting NiTi sutures into rabbit tendon and subcutaneous tissues for two, six, and twelve weeks. NiTi proved to be stronger than polyester, which served as control material. The encapsulating membrane was minimal with both materials, suggesting good biocompatibility in tendon tissue. The implantation did not affect the strength properties of either material. On the basis of the present studies, NiTi provides a possibility to develop new kinds of implants for correcting bone deformities, for filling bone defects in weight-bearing locations and a good candidate for a tendon suture material.

This thesis investigates the variability of ontogenetic maxillary bone modeling patterns in humans (Homo sapiens) and chimpanzees (Pan troglodytes). Along with sutural growth, bone modeling is the microscopic process by which bones grow in size and model their shape. It results from the simultaneous cellular activities of bone formation (produced by the osteoblasts) and bone resorption (produced by the osteoclasts) on bone surfaces. The study of these activities can bring new insights into our understanding of maxillary, and, more generally, facial ontogeny. However, bone modeling variability remains poorly understood. Using surface histology, we developed quantitative methods to objectively compare and visualize bone modeling patterns. In parallel, geometric morphometric methods were used to capture and quantify maxillary shape changes. Both methods were used for the first time together in an integrative approach. A large sample of H. sapiens individuals ranging from birth to adulthood, and originating from three geographically distinct areas (Greenland, Western Europe and South Africa), was used to infer the variation in maxillary bone modeling at the intraspecific level. We found that human populations express similar bone modeling patterns, with only subtle differences in the location of bone resorption. Moreover, differences in developmental trajectories were identified. This suggests that population differences in maxillary morphology stem from changes in timing and/or rates of the osteoblastic and osteoclastic activities. Adult individuals show similar maxillary bone modeling patterns to subadults, with both cellular activities expressed at reduced intensities. All human populations express high amounts of bone resorption throughout ontogeny, and high inter-individual variation. In contrast, we find low amounts of bone resorption and a low inter-individual variation in chimpanzees, which results in the anterior projection of their maxilla. In chimpanzees, resorption is predominant in the premaxilla, which has been found in some species of Australopithecus and Paranthropus. Other similarities in the location of bone resorption, mostly close to the sutures, suggest the preservation of shared ontogenetic patterns between the humans and chimpanzees. The low intraspecific variation in the location of bone resorption found in both species suggests that species-specific bone modeling patterns can be inferred from a limited number of individuals. This will allow future studies to discuss the bone modeling patterns in fossils for which subadult individuals are scarce.

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