Modeling the biomechanics of the mandible: a three-dimensional finite element study.

Three-dimensional finite element models of a partially edentulated human mandible were generated to calculate the mechanical response to simulated isometric biting and mastication loads. The level of mesh refinement was established via a convergence test and showed that a model with over 30,000 degrees of freedom was required to obtain analysis accuracy. The functional loading cases included muscle loading based on an algorithm that assigns muscle forces in accordance with muscle cross-sectional area, while maintaining static equilibrium. Results were found for isometric application of unilateral and bilateral bite and mastication loading, and two different sets of displacement boundary conditions were imposed at the condyles. The mechanical response is shown in terms of displacements, principal strains, and a new measure called the 'mechanical intensity scalar'. For each load case studied, there was substantial bending in the molar region of the corpus and high tensile strains in the anterior portion of the ramus.

Supercomputer use in orthopaedic biomechanics research: focus on functional adaptation of bone.

The authors describe two biomechanical analyses carried out using numerical methods. One is an analysis of the stress and strain in a human mandible, and the other analysis involves modeling the adaptive response of a sheep bone to mechanical loading. The computing environment required for the two types of analyses is discussed. It is shown that a simple stress analysis of a geometrically complex mandible can be accomplished using a minicomputer. However, more sophisticated analyses of the same model with dynamic loading or nonlinear materials would require supercomputer capabilities. A supercomputer is also required for modeling the adaptive response of living bone, even when simple geometric and material models are use.

Thermodynamic restrictions on the elastic constants of bone.

The thermodynamic restrictions on the elastic coefficients of linear orthotropic elasticity and linear transversely isotropy elasticity are recorded and it is shown that previously reported data for the elastic orthotropic constants of bone satisfy these thermodynamic restrictions.

The mechanical properties of immature osteopetrotic bone.

The elastic properties of the longer bones of the fore and hind limbs of an osteopetrotic Angus calf were measured using an ultrasonic technique. The same bones of a second Angus calf with unaffected bone tissue were also studied. The osteopetrotic bones were found to exhibit statistically significant lower values of density and mineral content than did the unaffected bones (P = .0054 and P = .0492, respectively). This result is contrary to what has been reported in the literature. The variations of the elastic properties and the density around the circumference of both the immature osteopetrotic femur and the unaffected femur were found to be similar to those previously measured on normal adult bovine femora.

A continuous wave technique for the measurement of the elastic properties of cortical bone.

A continuous wave technique is described for measuring the nine independent orthotropic elastic coefficients from a single cubic specimen. The side dimensions of this cubic specimen are on the order of 5 mm. Because of the small size of the specimen, the spatial resolution of material inhomogeneity using this technique is quite good. Although it is possible to apply this technique to any elastic material such as woods or metals, the elastic properties of human and canine cortical femora are presented here. The orthotropic elastic coefficients and the variation of these coefficients are presented as a function of anatomical position.

Bone remodeling due to continuously applied loads.

It has long been known that the stress history of bone tissue influences its structure; however, the nature of this relationship remains largely uncharacterized. The objective of this work was to induce a quantifiable change in the stress history of in vivo bone tissue and examine subsequent changes in structural and material properties that might occur. Continuous compressive loads were applied to the diaphysis of adult mongrel dogs for 2 months. The loads, ranging from 12-130 N, were superposed on the normal activity of the animals by implanting spring loading devices on the diaphysis of the femur. After the animals were sacrificed, mid-diaphysial specimens were subjected to compression testing to determine a structural bulk stiffness. The cross-sectional areas of original bone tissue and new bone deposition were then determined. The ash weights of selected specimens were also determined. The results indicate that a positive correlation between the increase in cross-sectional area and the superposed stress does exist. The new bone apposition was found almost entirely on the periosteal surface. Very little evidence of internal remodeling or endosteal movement was observed. The new tissue was found to have a lower ash weight and appeared to have a disorganized microstructure. Mechanical testing also suggests that the newly deposited tissue is far less stiff than the mature original bone.

Ultrasonic measurement of orthotropic elastic constants of bovine femoral bone.

Using a pulse transmission ultrasound method, we have determined the elastic properties of bone samples taken from along the length and around the periphery of a bovine femur. Twenty specimens, in the form of 5-mm cubes, were tested. All nine of the orthotropic elastic constants were determined for each specimen. Analysis of our data indicate that there are statistically significant variations from the usual assumption of transverse isotropy.

Experimental measurement of the stiffness of the cupula.

An experimental procedure is described which consists of cutting the canal duct, inserting a micropipette and administering known volumetric displacements to the cupula. The cupula is made visible by dying the endolymph. Known displacements are administered to the cupula, and the time constant of the return to its equilibrium position is measured. With this information, the stiffness of the cupula is calculated. The experiment was successfully carried out on five White King pigeons. The mean stiffness found in somewhat less than other results reported in the literature, and reasons for this discrepancy are noted.