Bone Shear Research Articles

A modified Coulomb's law for the tangential debonding of osseointegrated implants.

Cementless implants are widely used in orthopedic and dentalsurgery. 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 macroscale, there is a lack of data and reliable models. The aim of this work was 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 modeIII cleavage of a coin-shaped, titanium implant(Mathieu et al. in J Mech Behav Biomed Mater 8(1):194-203, 2012). Overall, the results show aclose 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.

Progressive post-yield behavior of human cortical bone in shear

Bone fragility depends on its post-yield behavior since most energy dissipation in bone occurs during the post-yield deformation. Previous studies have investigated the progressive changes in the post-yield behavior of human cortical bone in tension and compression using a novel progressive loading scheme. However, little is known regarding the progressive changes in the post-yield behavior of bone in shear. The objective of this short study was to address this issue by testing bone specimens in an inclined double notch shear configuration using the progressive loading protocol. The results of this study indicated that the shear modulus of bone decreased with respect to the applied strain, but the rate of degradation was about 50% less than those previously observed in compression and tension tests. In addition, a quasi-linear relationship between the plastic and applied strains was observed in shear mode, which is similar to those previously reported in tension and compression tests. However, the viscous responses of bone (i.e. relaxation time constants and stress magnitude) demonstrated slight differences in shear compared with those observed in tension and compression tests. Nonetheless, the results of this study suggest that the intrinsic mechanism of plastic deformation of human cortical bone may be independent of loading modes.

Mechanical characterization in shear of human femoral cancellous bone: torsion and shear tests

In order to investigate and compare the mechanical behaviour of human cancellous bone during different shear loading modes, two tests were performed to characterise human femoral cancellous bone in shear: a torsion test until failure and a shear test using a sharpened stainless steel tube. Paired cylindrical samples were core drilled from 12 human femoral heads, symmetrically with respect to the coronal plane and along the primary trabecular direction. The distal part of the sample was assigned to a torsion test and the shear test was performed on the proximal part along two perpendicular anatomical directions. Apparent densities and tissue densities were measured on both torsion and shear specimens. The mean torsion properties were shear modulus G, 289 (183) MPa, ultimate stress τ torsion, 6.1 (2.7) MPa, ultimate strain γ ultimate, 4.6 (1.3)%, yield stress τ yield, 4.3 (1.9) MPa and yield strain γ yield, 1.8 (0.3)%. Strong correlation was obtained between G and τ torsion ( r′=0.853, p<0.001). These torsion properties were correlated with apparent density of torsion specimens showing, respectively: r′=0.713, p=0.005 and r′=0.671, p=0.008. Properties from the shear test were invariable with regard to the two tested directions then isotropic ultimate shear stress and isotropic elementary shear stress, which represent the mean values of the two tested directions were, respectively, τ shear, 10.0 (4.5) MPa and τ elem, 18.8 (6.1) MPa. Both shear stresses were correlated with apparent density of shear specimens: τ shear, r′=0.564, p=0.045 and τ elem, r′=0.636, p=0.024. Apparent densities for shear specimens were superior than for torsion specimens ( p=0.06) and the comparison was the opposite for tissue densities ( p=0.028), showing strong density gradients of cancellous bone in the femoral head. These torsion and shear tests which permit the evaluation of cancellous bone behavior under two different types of shear loading, may be performed on different human sites and the measured shear properties may be compared to structural properties of cancellous bone.

Biomechanical considerations in osseointegrated prostheses.

On the basis of the previous discussions, several conclusions may be drawn. 1. The close apposition of bone to the titanium implant is the essential feature that allows a transmission of stress from the implant to the bone without any appreciable relative motion or abrasion. The absence of any intermediate fibrotic layer allows stress to be transmitted without any progressive change in the bond or contact between the bone and implant. 2. The use of a threaded screw provides a form of interlocking with the bone on a macroscopic scale that allows full development of the strength of the bone in shear or compression. A smooth, cylindrical implant may require an adhesive bond for satisfactory performance, but a screw shape is able to work as long as the apposition of bone and implant is close, whether or not a true adhesive bond is developed. 3. The distribution of a vertical or lateral load applied to a fixed partial denture depends on the number, arrangement, and stiffness of abutment fixtures used, as well as the form and stiffness of the fixed prosthesis itself. In general a stiff fixed partial denture will distribute loads to several fixtures more effectively. A flexible prosthesis may be adequate if the strength developed by each fixture is able to carry the full load that is applied. Cantilevered ends of a fixed partial denture increases the loading on the first screw nearest the cantilevered end. Moderate overhangs may be tolerated if the fixtures are sufficiently strong. 4. A tight connection of the fixed partial denture to fixtures provides a combined structure that can act in concert with the bone to provide a greater strength than that of the fixture or the jaw bone alone. 5. The osseointegrated implant provides a direct contact with bone and therefore will transmit any stress waves or shocks applied to the fixtures. For this reason it is advisable to use a shock-absorbing material such as acrylic resin in the form of acrylic resin artificial teeth in the fixed partial denture. This arrangement allows for the development of a stiff and strong substructure with adequate shock protection on its outer surface.