Zirconia Toughened Alumina Femoral Head and Acetabular Socket: Process optimization, Designing, Fabrication and Properties

Abstract

Despite several decades of research for new materials for articulating joints in orthopedic applications, the efforts to develop patient-specific prototype of such biomaterial devices are rather limited. While addressing this aspect, the present work demonstrates an integrated manufacturing approach how to fabricate zirconia toughened alumina femoral head and acetabular socket initiation with composition and process optimization, and designing of prototypes. The properties and performance of a such ceramic components significantly depend on the microstructue (grain size) and sinter density, it is therefore important to optimize both the process parameters (sintering temperature, sintering time) and material parameters (sinter–aid addition and reinforcement content) to obtain tough and strong materials. Based on considering the fundamental densification-grain size relationship and using the predictive linear, quadratic or interactive response among the process and material parameters, the adopted response surface methodology (RSM) approach is shown to provide excellent capability to predict sinter density and grain size with significant statistical correlation between experimental and predicted values. Summarising, the optimization study establishes that sintering of 5 wt.% zirconia toughened alumina sintered with 800 ppm MgO sinter-aid at 1600oC for 6h can exhibit a great combination of relative density, compressive strength (1100 MPa), tensile strength (200 MPa) and SEVNB fracture toughness (4.3 MP m1/2). In order to assess the cytocompatibility properties, C2C12 mouse myoblast cells were grown on the ZTA composite having the best combination of mechanical properties. The results of MTT assay reveal an increase in the number of mitochondrially active cells with time in culture for a period of up to 3 days. The fluorescence microscopic observations also confirmed good cell attachment and cell-to-cell contact with cellular bridge formation. In view of the importance of the wear resistance properties in the performance and durability of prototypes in total hip joint replacement application, the unlubricated sliding wear experiments with commercial cubic zirconia, and stainless steel counterbody reveal that a combination of steady state COF of 0.5 and 0.42 and wear rate of 10-9 mm3/N m observed for the optimized ZTA composite with having the best combination of mechanical properties. The wear mechanism is dominated by abrasive wear and cracking induced delamination of tribolayer. In commensurate with the computer aided-design (CAD) of prototypes after optimization of process and properties of ZTA, the custom made modular steel-die mould assembly was fabricated to produce high strength green powder compact of Al2O3-5 wt% ZrO2 (3 mol %Y2O3)-800 ppm MgO without any geometric distortion at uniaxial pressure of 18 - 22 ton. In line with design consideration, green compact of the femoral head / acetabular socket was presintered at 1200oC in air for 2h in a conventional sintering furnace and subsequently computer numerical control (CNC) machined to a limited extent. The final stage of prototype development involved the multi-step sintering of the compact at 1600oC for 6h in air, followed by polishing using tailor made arrangement. The process quality was closely monitored by measuring dimensional changes at each manufacturing stage as well as the circularity measurement of final polished prototype. The microstructure as well as the physical properties in terms of hardness, indentation toughness, and burst strength is also reported. Taken together the present manufacturing approach appears to be a scalable and commercially viable fabrication strategy to make bioceramics based femoral head and acetabular socket biomedical devices.