The Role of Stress Concentrations in the Bone Cement Mantle

Acrylic bone cement is a porous biomaterial with many applications across both the medical and dental fields. In orthopaedics, it is used in the fixation of artificial implants where it forms a mechanical bond between the implant and the surrounding tissue. Bone cement is prepared during surgery by mixing a polymer powder and a liquid monomer. The mixture is then inserted into the body in a dough-like state, setting around the implant. Due to the manner in which the cement is prepared and inserted, the material tends to contain defects. Porosity arises in the cement due to trapped air and evaporation of the monomer during the mixing process. Larger defects of various shapes occur as a result of trapped debris, incomplete filling of the space, premature solidification and shrinkage: these are all similar to causes of defects in castings and injection-mouldings [1]. Even without these defects, stress concentrations can arise due to protrusion of bone into the cement as well as the geometry of the hip implant. High stresses can occur in the cement close to the femoral stem, the magnitudes of which are dependant of the cross-section of the stem and other design features [2].