  Cementless implants are widely used in orthopedic and oral surgery. However,
debonding-related failure still occurs at the bone-implant interface. It
remains difficult to predict such implant failure since the underlying
osseointegration phenomena are still poorly understood. Especially in terms of
friction and adhesion at the macro-scale, there is a lack of data and reliable
models. The aim of this work is to present a new friction formulation that can
model the tangential contact behavior between osseointegrated implants and bone
tissue, with focus on debonding. The classical Coulomb's law is combined with a
state variable friction law to model a displacement-dependent friction
coefficient. A smooth state function, based on the sliding distance, is used to
model implant debonding. The formulation is implemented in a 3D nonlinear
finite element framework, and it is calibrated with experimental data and
compared to an analytical model for mode III cleavage of a coin-shaped,
titanium implant (Mathieu et al. 2012). Overall, the results show close
agreement with the experimental data, especially the peak and the softening
part of the torque curve with a relative error of less than 2.25 %. In
addition, better estimates of the bone's shear modulus and the adhesion energy
are obtained. The proposed model is particularly suitable to account for
partial osseointegration, as is also shown.
