3D finite element analysis of stem-cement interface under cavity effect

Acrylic cement is used to fix replacement hip implants in the bone. Problems of loosening occur often as a result of fatigue damage or debonding between cement and implant due to the presence of cavity or voids. High stress concentrations can occur in the cement which is often the cause of the initiation and the propagation of cracks leading irremediably to loosening. In this work, we carried out a 3D finite element analysis of equivalent distribution of stresses in damaged orthopedic cement. The damage is due to the presence of cavity of different diameters under three cases of maximal dynamic loads (walking, climbing up stairs and climbing down stairs). The cavity presence with a diameter greater than or equal to 0.3 mm in the cement at the cement-implant interface generates high stresses near the cavity for the three dynamic activities. High interface stresses create microrupture zones around the cavity at the proximal cement region. The probability of debonding is evaluated by a fracture index calculated at the cement-metal interface using the Hoffinan criterion. The results show that debonding occur near the cavity. Micro-debonding zones appear around the cavity. Then, the debonded sites expand along the interface towards the distal part of the cement until interface gets loose.