R. Huiskes

Stresses in the local collagen network of articular cartilage: a poroviscoelastic fibril-reinforced finite element study

Osteoarthritis (OA) is a multifactorial disease, resulting in diarthrodial joint wear and eventually destruction. Swelling of cartilage, which is proportional to the amount of collagen damage, is an initial event of cartilage degeneration, so damage to the collagen fibril network is likely to be one of the earliest signs of OA cartilage degeneration. We propose that the local stresses and strains in the collagen fibrils, which cause the damage, cannot be determined dependably without taking the local arcade-like collagen-fibril structure into account. We investigate this using a poroviscoelastic fibril-reinforced FEA model. The constitutive fibril properties were determined by fitting numerical data to experimental results of unconfined compression and indentation tests on samples of bovine patellar articular cartilage. It was demonstrated that with this model the stresses and strains in the collagen fibrils can be calculated. It was also exhibited that fibrils with different orientations at the same location can be loaded differently, depending on the local architecture of the collagen network. To the best of our knowledge, the present model is the first that can account for these features. We conclude that the local stresses and strains in the articular cartilage are highly influenced by the local morphology of the collagen-fibril network.

Preclinical testing of total hip stems. The effects of coating placement.

The long-term fixation endurance of noncemented hip stems in total hip arthroplasty is subject to incompatible design goals. To reduce stress shielding and periprosthetic bone loss, proximal fixation and load transfer are indicated. However, to prevent interface motion and promote interface-bonding security, fixation preferably should be maximized over the entire stem surface. In this study, the authors questioned whether hydroxyapatite coatings could be applied in patterns that reduce bone resorption, while maintaining safe interface stress levels. For that purpose, strain-adaptive bone-remodeling theory was applied in 3-dimensional finite element models, to simulate the long-term postoperative bone resorption process. During the process, the adaptation of interface stresses was monitored, and its effects on interface failure probability evaluated. This analysis was done for a fully coated stem, a 1/3 proximally coated stem, a smooth uncoated, press-fitted stem, and a stem with 5 proximal patches of circumferential stripes. The uncoated stem reduced bone loss dramatically, but promoted interface motions and distal pedestal formation. In all cases, the gradual bone-remodeling process increased the interface security of the coated stems. Bone loss and interface failure probability were not very different for the fully and 1/3-coated stems. Stripe coating reduced bone resorption considerably, while increasing long-term interface failure probability only slightly. The investigators concluded that the initial stability and the ingrowth potential of such a stem design are likely to be inadequate.

Architectural 3-D Parameters and Anisotropic Elastic Properties of Cancellous Bone

Using 3-D reconstruction, micro-FEM and 3-D architectural analyses, we have studied the relation between various measures of architectural anisotropy, the influence of connectivity and trabecular material anisotropy on the elastic anisotropy of cancellous bone. We have reached the following conclusions: 1) the elastic anisotropy of the trabecular hone material seems only to have marginal influence on the anisotropic elastic properties of cancellous bone, 2) different architectural anisotropy measures are highly correlated, and methods studied are all highly related to the elastic anisotropy, and 3) connectivity has only marginal influence on elastic properties.

Mechanical stability of trabecular bone morphology as a measure for osteoporosis

A new technique for assessing the mechanical stability of trabecular bone was introduced. This technique uses a full three dimensional reconstruction of a trabecular bone specimen and a finite element model to calculate the local stress distribution within the trabeculae. A little bone was artificially removed in the model at highly loaded locations and the changed stress distributions were determined. The changes in these distributions are indicative for the mechanical stability (change in fracture risk) of the trabecular architecture with respect to small changes in mass. The authors propose that this method can be used to measure the mechanical efficacy of a trabecular architecture in terms of fracture risk, thereby defining osteoporosis in a quantitative way.

Complete assessment of elastic properties of trabecular bone architecture from 3D reconstruction images

A method is presented that allows for a complete mechanical evaluation of trabecular bone architecture directly from three-dimensional computer reconstruction images. With this method, the reconstruction images are used as a basis for microstructural FE-analyses. From the results of these analyses the full stiffness matrix of bone specimens is obtained, using a standard mechanics approach. An optimization procedure is then used to find the best orthotropic representation and principal directions of this matrix. The method is demonstrated here relative to two trabecular bone specimens. With the development of in vivo reconstructions and the methods demonstrated here, even in vivo measurements will be possible.