Currently, orthopedic implants such as the hip prosthesis are made of metallic materials whose stiffnesses are higher than that of human bones. Consequently, bearing loads such as the human body weight are almost fully taken by the implant (stress shielding effect). Bone-resorption around the implant which can lead to the aseptic loosening of the implant, or bone fracture are among the possible consequences. To address this problem, one solution is to develop a new generation of implants containing a porous metallic structure with great potential for mechanical and morphological compatibility. Indeed, the stiffness of the porous implant can be adjusted to match that of the bone by controlling its porosity. Moreover, osseointegration in the porous structure is enhanced, and therefore provided a suitable initial stability of the implant.
This manuscript-based thesis containing three scientific publications presents the works that have led to the modeling, manufacture and morphological and mechanical characterization of such porous metallic structures.
Firstly, an algorithm was developed to generate the morphology of porous structures (or foams). Then, stereology was used to characterize the morphology of foams of various dimensions and porosities in terms of the pore size, shape and distribution. The minimum dimensions of the foam providing a representative morphology were assessed using the representative volume element approach. Finally, the morphology of the porous structures generated by the algorithm was successfully compared to that of porous titanium foams manufactured by the space holder method: the modeling algorithm was therefore validated.
Secondly, a methodological approach was proposed to design, manufacture and test porous tensile specimens in order to determine their mechanical properties. The design phase was based on the finite element method and the representative volume element approach. Such a combination allowed determining the minimal dimensions of the gauge section of the porous samples. Then, tensile specimens of various porosities (30 to 50 %) were generated by the morphology generation algorithm and produced using selective laser melting. Following their additive manufacturing, the porous specimens were subjected to uniaxial tension to failure to allow the determination of their mechanical tensile properties. Finally, the experimental results were successfully compared to those of the numerical model.
Thirdly, a porous femoral stem designed to provide an improved biocompatibility was developed. Based on the results previously obtained, the optimal porosity of the femoral stem allowing obtaining a pore size favoring bone ingrowth and a stiffness matching that of bone was determined. Then, selective laser melting was adopted to additively manufactured the porous stem altogether with its fully dense replica for reference purposes. In the same time, a finite element model was developed in order to predict the mechanical response of such implant. Finally, the porous and dense stems were subjected to mechanical test allowing validating the numerical model, and to demonstrate that the porous stem is much more compliant than its dense counterpart.
Simoneau, C. (Author),
Terriault (Supervisor) &
Brailovski (Co-supervisor),
11 Jan 2017Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering