Predictive characterisation of biomedical wires: from atomic structure and thermomechanics to practice

Metallic medical wires are routinely used. Although criteria of an ideal wire are established, no single wire satisfies all these requirements. Towards guiding product selection and importantly marketing, most laboratory studies on commercial products focus on resolving their response to the relevant physical/chemical changes, which normally takes the form of mechanical and corrosion resistance tests that supposedly mimic clinical situations. However, great variation in reported dental literature test data exists, due to varying but unknown thermal―mechanical history. In order to provide understanding of the influence material alloying and production (especially cold―work) on the property/performance, novel combination of neutron scattering and conventional laboratory surface characterisation, thermal analysis and mechanical testing was applied in the systematic physical―chemical―mechanical determination of commercial wires used in orthodontic practice of three most widely used alloy types: stainless steel (SS), Nickel―titanium (NiTi) and beta―titanium (β―Ti). The acquired elemental composition, crystal structure (extent of strain―induced martensite transformation in SS samples), phase transition temperatures and transition entropies in NiTi samples and surface characteristics, helped reveal material processing, hence the thermal―mechanical history. By correlating thermal―mechanical history with the determined mechanical properties we advanced the understanding of how processing influences performance. Through testing the composition―nanostructure steered property predictions with mechanical test results and achieving agreement, the composition―structure―property relationship was elucidated. It is thus envisioned that the insight gained herein may help equip practitioners with the understanding of the structure―property relationship, towards rational material selection and manipulation. The approach and supporting methodology facilitated the work on relating fundamental physical principals with practical aspects of in-service material performance. The approach can be extended to the study of other metallic biomaterials used in medicine, dentistry and beyond, towards raising understanding of the impact of material processing on practical properties, providing performance predictions and rational guidance in appliance design.