Analysis of the debonding of the stem-cement interface in intramedullary fixation using a non-linear fracture mechanics approach

Implant loosening is one of the most important cause of long-term failure of total hip replacements. Fatigue failure of the stem-cement interface and the bulk cement may cause aseptic loosening of the femoral stem. In this work, both mechanisms of failure were simulated using finite elements. The stem-cement interface failure was modelled by means of the cohesive surface theory that was implemented into a specific interface element, while damage accumulation and creep in bone cement were formulated through the theory of Continuum Damage Mechanics. Model parameters, such as, the mechanical characteristics of the interface, damage accumulation rules both for the cement and the interface, crack closure effect and friction evolution law, were determined to simulate the subsidence patterns of the stem in the cement mantle from experimental tests. A parametric analysis was also performed observing how each parameter of the model influences the micromotions and damage accumulation in the cement mantle. Results of this study support the importance of simulating the progressive debonding of the stem-cement interface on the prediction of long-term implant loosening.