Cartilage fissures, surface fibrillation, and delamination represent early signs of hip

Cartilage fissures, surface fibrillation, and delamination represent early signs of hip osteoarthritis (OA). were determined. Mesh convergence analysis demonstrated that five trilinear elements were adequate through the depth of the cartilage for precise predictions. The EFD model had the stiffest response with increasing strains, predicting the largest peak stresses and smallest peak strains. Conversely, the neo-Hookean model predicted the smallest peak stresses and largest peak strains. Models with neo-Hookean cartilage predicted smaller transchondral gradients of maximum shear stress than those with Veronda Westmann and EFD models. For FE models with EFD cartilage, the anterolateral region of the acetabulum had larger peak maximum shear stress and first principal strain than all other anatomical regions, consistent with observations of cartilage damage in disease. Results demonstrate that tension-compression nonlinearity of a continuous fiber distribution 14279-91-5 IC50 exhibiting strain induced anisotropy incorporates important features that have large effects on predictions of transchondral stress and strain. This population of normal hips provides baseline data for future comparisons to pathomorphologic hips. This approach can be used to evaluate these and other mechanical variables in the human hip and their potential role in the pathogenesis of osteoarthritis (OA). given by Fig. 2 Uniaxial stress response of the three constitutive models. Experimental data are shown. At small strains (stretch values near 1.0), there were minimal differences between the three models. At larger tensile strains, there were drastic differences. The … is the first deviatoric invariant of the right Cauchy deformation tensor, is the Jacobian of the deformation, is the shear modulus in the limit of small deformations, and is the bulk modulus. This model was selected as a baseline constitutive model both because of its simple quasilinear stress-stretch relationship Rabbit Polyclonal to IKK-gamma (phospho-Ser85) and because it has been used previously in FE models of the human hip joint [25C32]. The second 14279-91-5 IC50 constitutive model was an uncoupled version of the isotropic, hyperelastic Veronda Westmann (VW) model [53,55]: is the second deviatoric invariant of the right Cauchy deformation tensor, the coefficient scales the overall response, the coefficient controls the exponential response, and is the bulk modulus. Although the VW model is isotropic, it captures strain-dependent material nonlinearity [53]. The final constitutive model was an uncoupled version of the ellipsoidal fiber distribution (EFD) model, with a neo-Hookean ground matrix [49,55,56]. The fiber strain energy for the EFD model was in the form [49,55,56] is the square of the deviatoric fiber stretch and is the unit vector along the fiber direction in the reference configuration. The integral is evaluated over the unit sphere spanned by all directions is the unit step function ensuring that only fibers under tension contribute to the strain energy density. The material coefficient scales the fiber response and controls the nonlinearity of the fibers. Both and may vary with according to an ellipsoidal distribution with respective semiaxes (for integer values above 2.0. Thus, the value of was set to 4.0 since this integer value resulted in a closed-form equation that could be used for curve 14279-91-5 IC50 fitting. Although cartilage was only tested in compression, data were fit to the tensile response of the fibers in the EFD model. This was possible because an isochoric response in unconfined compression also produces tensile strains. For the prescribed compressive stretch of is the first eigenvalue of the GreenCLagrange strain tensor and represents the largest tensile strain at each point. was sampled at the articular surfaces and transchondrally at the location of the peak on the articular surface. max was evaluated at the osteochondral interface and transchondrally at the location of the peak max on the osteochondral interface. The locations for sampling transchondral and max were selected to provide a systematic approach to evaluating results. Results were analyzed on the femoral head and in six anatomical regions on the acetabulum.