John R. Cotton

Computational models of hair cell bundle mechanics: III. 3-D utricular bundles

Six utricular hair bundles from a red-eared turtle are modeled using 3-D finite element analysis. The mechanical model includes shear deformable stereocilia, realignment of all forces during force load increments, and tip and lateral link inter-stereocilia connections. Results show that there are two distinct bundle types that can be separated by mechanical bundle stiffness. The more compliant group has fewer total stereocilia and short stereocilia relative to kinocilium height; these cells are located in the medial and lateral extrastriola. The stiff group are located in the striola. They have more stereocilia and long stereocilia relative to kinocilia heights. Tip link tensions show parallel behavior in peripheral columns of the bundle and serial behavior in central columns when the tip link modulus is near or above that of collagen (1x10(9) N/m(2)). This analysis shows that lumped parameter models of single stereocilia columns can show some aspects of bundle mechanics; however, a distributed, 3-D model is needed to explore overall bundle behavior.

A finite element method for mechanical response of hair cell ciliary bundles

This paper describes the development of a methodology for performing a mechanical analysis of hair cell ciliary bundles. The cilia were modeled as shear deformable beams, and interconnections were modeled as two-force members. These models were incorporated into software, which performs a finite element analysis of a user-defined bundle. The algorithm incorporates aspects of the bundle such as geometric realignment and buckling of compressed side links. A sample bundle is introduced and results of modeling it are presented.

Computational models of hair cell bundle mechanics: II. Simplified bundle models

Simplified versions of hair cell bundles are mechanically modeled. The influence of various geometric and material combinations on bundle stiffness, link tensions and deformation shape are examined. Three models are analyzed within this paper: two stereocilia connected by one link, two stereocilia connected by a biologically realistic set of links, and a column of stereocilia connected by realistic links. Stereocilia are modeled using a distributed parameter model [J. Biomech. Eng. 122, 44]. Some fundamental rules for linking bundles emerge from these tests: (1) Links must have a threshold stiffness value for the bundle to deform as a whole. Beyond this value, the stereocilia are perfectly linked and variations in link stiffness do not significantly effect the bundle stiffness or link tension. (2) Decreasing the relative heights of successive stereocilia may increase link tension while decreasing bundle stiffness. (3) When lateral links exist, the top most lateral links carry the majority of tension. Lower links in single column model appear mechanically insignificant. (4) Extending the length of the bundle in a column does not increase the stiffness once the column reaches a certain length.

Multibody dynamics model of head and neck function in Allosaurus (Dinosauria, Theropoda)

We present a multibody dynamics model of the feeding apparatus of the large Jurassic theropod dinosaur Allosaurus that enables testing of hypotheses about the animal’s feeding behavior and about how anatomical parameters influence function. We created CTand anatomical-inference-based models of bone, soft tissue, and air spaces which we use to provide inertial properties for musculoskeletal dynamics. Estimates of bone density have a surprisingly large effect on head inertial properties, and trachea diameter strongly affects moments of inertia of neck segments for dorsoventral movements. The ventrally-placed insertion of m. longissimus capitis superficialis in Allosaurus imparted over twice the ventroflexive accelerations of a proxy control insertion lateral to the occipital condyle, the latter being its position in nearly all other theropods. A feeding style that involved defleshing a carcass by avian-raptor-like retraction of the head in Allosaurus is more probable than is lateroflexive shake-feeding, such as that seen in crocodilians and inferred for tyrannosaurids. Eric Snively. Department of Mechanical Engineering, Russ College of Engineering, 249 Stocker Center, Ohio University, Athens, OH 45701, USA es180210@ohio.edu John R. Cotton. Department of Mechanical Engineering, Russ College of Engineering, 249 Stocker Center, Ohio University, Athens, OH 45701, USA cotton@ohio.edu Ryan Ridgely. Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA ridgely@ohio.edu Lawrence M. Witmer. Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA witmerl@ohio.edu

Computational models of hair cell bundle mechanics: I. Single stereocilium.

A distributed parameter model for describing the response of a stereocilium to an applied force is presented. This model is based on elasticity theory, plus the geometry and material properties of the stereocilium. The stereocilia shaft above the taper is not assumed to be perfectly rigid. It is assumed to be deformable and that two separate mechanisms are involved in its deformation: bending and shear. The influence of each mode of deformation is explored in parametric studies. Results show that the magnitude of tip deflection depends on the shear compliance of the stereocilium material, the degree of base taper, and stereocilium height. Furthermore, the deformation profiles observed experimentally will occur only if there are constraints on the geometry and material properties of the stereocilium.

Simulation of creep in non-homogenous samples of human cortical bone

Characterising the mechanisms causing viscoelastic mechanical properties of human cortical bone, as well as understanding sources of variation, is important in predicting response of the bone to creep and fatigue loads. Any better understanding, when incorporated into simulations including finite element analysis, would assist bioengineers, clinicians and biomedical scientists. In this study, we used an empirically verified model of creep strain accumulation, in a simulation of 10 non-homogeneous samples, which were created from micro-CT scans of human cortical bone of the femur midshaft obtained from a 74-year-old female cadaver. These non-homogeneous samples incorporate the presence of Haversian canals and resorption cavities. The influence of inhomogeneity on the response and variation in the samples in both creep and stress relaxation tests are examined. The relationship between steady-state creep rate, applied loads (stress relaxation and creep tests) and microstructure, that is bone apparent porosity, is obtained. These relations may provide insight into damage accumulation of whole human bones and be relevant to studies on osteoporosis.

The Tarsometatarsus of the Ostrich Struthio camelus: Anatomy, Bone Densities, and Structural Mechanics.

Background The ostrich Struthio camelus reaches the highest speeds of any extant biped, and has been an extraordinary subject for studies of soft-tissue anatomy and dynamics of locomotion. An elongate tarsometatarsus in adult ostriches contributes to their speed. The internal osteology of the tarsometatarsus, and its mechanical response to forces of running, are potentially revealing about ostrich foot function. Methods/Principal Findings Computed tomography (CT) reveals anatomy and bone densities in tarsometatarsi of an adult and a young juvenile ostrich. A finite element (FE) model for the adult was constructed with properties of compact and cancellous bone where these respective tissues predominate in the original specimen. The model was subjected to a quasi-static analysis under the midstance ground reaction and muscular forces of a fast run. Anatomy–Metatarsals are divided proximally and distally and unify around a single internal cavity in most adult tarsometatarsus shafts, but the juvenile retains an internal three-part division of metatarsals throughout the element. The juvenile has a sparsely ossified hypotarsus for insertion of the m. fibularis longus, as part of a proximally separate third metatarsal. Bone is denser in all regions of the adult tarsometatarsus, with cancellous bone concentrated at proximal and distal articulations, and highly dense compact bone throughout the shaft. Biomechanics–FE simulations show stress and strain are much greater at midshaft than at force applications, suggesting that shaft bending is the most important stressor of the tarsometatarsus. Contraction of digital flexors, inducing a posterior force at the TMT distal condyles, likely reduces buildup of tensile stresses in the bone by inducing compression at these locations, and counteracts bending loads. Safety factors are high for von Mises stress, consistent with faster running speeds known for ostriches. Conclusions/Significance High safety factors suggest that bone densities and anatomy of the ostrich tarsometatarsus confer strength for selectively critical activities, such as fleeing and kicking predators. Anatomical results and FE modeling of the ostrich tarsometatarsus are a useful baseline for testing the structure’s capabilities and constraints for locomotion, through ontogeny and the full step cycle. With this foundation, future analyses can incorporate behaviorally realistic strain rates and distal joint forces, experimental validation, and proximal elements of the ostrich hind limb.

Creep Simulation of a Micro-CT Based Finite Element Model of Porcine Cancellous Bone

Creep and fatigue behavior of cancellous bone are thought to be important in senile fractures [1, 2], bone remodeling [3], and implant subsidence [4]. Mechanical tests of cancellous bone samples have explored permanent deformation over time in both creep and fatigue loading. For example, Bowman, et al. [1] investigated creep behavior of bovine trabecular bone showing strong relationships between the applied stress and both time-to-failure and steady-state creep rate.Copyright

FINITE ELEMENT COMPARISON OF CRANIAL SINUS FUNCTION IN THE DINOSAUR MAJUNGASAURUS AND HEAD-CLUBBING GIRAFFES

Majungasaurus crenatissimus is a spectacularly preserved carnivorous dinosaur from latest Cretaceous Madagascar. Computed tomographic (CT) scans reveal unusual internal anatomy of the dinosaur’s cranium [1,2; Figure 1]: the nasals form a large hollow chamber traversed with bony struts, and a unicorn-like projection of the frontals is also hollow. The wall thickness and struts within these sinuses recall sinuses of giraffes, which strike each other with a median projection (ossicone) above a frontal sinus and lateral ossicones of the parietals [3]. Giraffe-like cranial sinuses, and large attachments for neck muscles [4], raise the hypothesis that Majungasaurus could engage in giraffe-like head strikes to each other’s necks and flanks.Copyright

Finite Element Analysis Demonstrates Splinting in the Porcine Mandibular Condyle Causes Changes in Bone Volume Fraction and Stiffness Anisotropy

Currently about 10 million Americans report signs and symptoms of TMJ dysfunction. One form of treatment for TMJ dysfunction is dental splints which reorient the jaw during mastication. This presumably changes the direction, magnitude and location of mechanical loads on the mandibular condyle of the temporomandibular joint (TMJ). The precise nature of load changes and their effect on the underlying condylar trabecular bone have not been reported.© 2012 ASME