Ramana M. Pidaparti

Does microdamage accumulation affect the mechanical properties of bone

It has never been demonstrated that microcrack accumulation in bone leads to impaired mechanical properties. We hypothesized that microdamage accumulation is positively and linearly correlated with a reduction in bones elastic modulus. We also tested the hypothesis that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices. Canine femurs (n = 26) were tested in four-point cyclic bending under load control until they had lost between 5 and 43% of their stiffness. Ten femurs were used as nonloaded controls. The loaded portion of the bone was stained en bloc with basic fuchsin to detect the presence of microdamage. The number of stained microcracks, their lengths and the area of damaged bone were measured under the microscope. Crack numerical density, surface density, mean crack length, and the percentage of damaged area were calculated. Significant microdamage accumulation was not detected until the bone had lost 15% of its elastic modulus. The relationship between crack density and stiffness loss was approximately quadratic, but the relationship between damaged area and stiffness loss was linear. There were significantly more microcracks in tensile cortices, but on average cracks were significantly longer in compressive cortices. We conclude that microcrack accumulation impairs the mechanical properties of bone by reducing its elastic modulus. We also conclude that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices.

Development of a fluorescent light technique for evaluating microdamage in bone subjected to fatigue loading

A new method using fluorescent light microscopy has been developed to visualize and evaluate bone microdamage. We report the findings of two different experiments with a common aim of comparing the fluorescent light technique to the brightfield method for quantifying microdamage in bone. In Experiment 1, 36 canine femurs were tested in four-point cyclic bending until they had lost between 5 and 43% of their stiffness. The loaded portion of the bone was stained en bloc with basic fuchsin for the presence of damage. Standard point counting techniques were used to calculate fractional damaged area (Dm.Ar = Cr.Ar/B.Ar, mm2/mm2) under brightfield and fluorescent microscopy. In Experiment 2, bone microdamage adjacent to endosseous implants, subjected to fatigue loading (150,000 cycles, 2 Hz and 37 degrees C) ex vivo was examined. The bone around the implant was either allowed to heal (adapted specimen) for 12 weeks after placement in dog mid-femoral diaphyses prior to testing or was loaded immediately to simulate non-healed bone surrounding endosseous implants (non-adapted). Crack numerical density (Cr.Dn = Cr.N/B.Ar, #/mm2), crack surface density (Cr.S.Dn = Tt.Cr.Le/B.Ar, mm/mm2) and fractional damaged area were calculated separately by both techniques in the adapted and non-adapted specimens. In both Experiments 1 and 2, significantly more microdamage was detected by the fluorescent technique than by the brightfield method. Also, there was a trend towards higher intraobserver repeatability when using the fluorescent method. These results suggest that the brightfield technique underestimates microdamage accumulation and that the fluorescent technique better represents the actual amounts of microdamage present. The results demonstrate that the fluorescent method provides an accurate and precise approach for bone microdamage evaluation, and that it improves the prediction of stiffness loss from damage accumulation.

Cancellous bone architecture: Advantages of nonorthogonal trabecular alignment under multidirectional joint loading

Wolff proposed that trabeculae align at 90 degrees angles (orthogonal). However, nonorthogonal alignment of trabeculae has been observed near many joints, including the proximal femur. We propose that nonorthogonal alignment is an adaptation to multidirectional joint loads. When the loading direction does not correspond with the trabecular alignment, warping or shear coupling occurs leading to large shear strains within the cancellous structure. Using a simplified continuum model for trabecular bone, we demonstrate that shear coupling caused by multidirectional joint loads is reduced 33-75% when trabeculae are aligned 60 degrees from one another (as is observed in regions of the proximal femur), as opposed to 90 degrees from one another (as was predicted by Wolff). The results suggest that an optimal cancellous structure may appear differently under multidirectional joint loads than the trajectorial organization proposed by Wolff, which was based upon assumptions drawn from unidirectional loading.

En bloc staining of bone under load does not improve dye diffusion into microcracks

Microdamage accumulation in bone has been implicated in the pathogenesis of some bone fractures, and in implant loosening. Standard techniques for staining microcracks may not allow all cracks to be stained. We tested the hypothesis that crack closure in bone cortices after removal of a bending load may prevent diffusion of stain to sites of microcrack nucleation. Following cyclic loading, 26 canine femurs were divided into a group stained en bloc while applying a four point bending load, and another group stained without an applied load. No differences in number or length of microcracks were observed, indicating that crack closure does not prevent diffusion of stain to the crack location. Staining under load is unnecessary.

A viscoelastic material model to represent smooth muscle shortening

The mechanical properties of a contracting smooth muscle can be changed by changing its length. A viscoelastic material model was developed to predict the length-dependent stiffness changes when a constrained muscle is allowed to shorten under a constant external force. Three-dimensional finite element simulations were carried out to estimate the stiffness changes and compared to available experimental data. A good agreement was found indicating that the viscoelastic material model developed gives a valid representation of the length dependent stiffness changes of a smooth muscle. Sensitivity analysis was carried out to determine the relative effects of material constants in the model on the length dependent stiffness.

Bone stiffness changes due to microdamage under different loadings

Stiffness changes due to microdamage in the longitudinal and cross-sectional directions in a dog bone model under different loadings were investigated using three-dimensional finite element analysis. Stiffness changes and severity of both longitudinal and cross-sectional type microcracks were estimated between the damaged and undamaged bone under four-point bending, torsion and tension. Finite element simulation results indicated that longitudinal damage was more severe than cross-sectional damage under axial tension and bending, and the opposite was true for torsional loading. However, for axial tension, the stiffness change due to cross-sectional microcracks remained constant.

A two-dimensional general formulation for the cord-composite plates

Abstract A two-dimensional model that takes into account the 1-D behaviour of the cord, is presented for the analysis of cord-composite plates. The cords are represented by an equivalent geometrical surface having the mechanical characteristics including bending property and the coupling between twist-extension. The non-symmetrical position of the cord within the cord-composite plate is also considered. Examples of cord-composite plates consisting of one central layer, two symmetrical layers, and two non-symmetrical layers are studied, and the results are presented to illustrate the coupling effects on the mechanical behaviour.

Numerical study of crack detection based on microwave-induced heating and thermography

This paper reports on a new methodology for detecting surface cracks in metallic structures by combining a microwave resonant cavity with infrared imaging. The underlying principle is based on crack induced disruptions of microwave wall currents creating localized concentrations of microwave energy. These regions of concentrated energy may, in turn, produce a localized heating in a thin layer of dielectric material placed adjacent to the surface being inspected. Detection of local hot spots via infrared imaging may then be used to infer the presence of a crack or other discontinuity in the surface. This study utilized a numerical simulation of electromagnetic fields within the resonant cavity and the resulting dielectric heating. The objective of the numerical study was to gain insight into fundamental electromagnetic and thermophysical processes on which this NDT scheme was based. The transient 3D Maxwells equations were solved numerically using the method of Finite Difference-Time Domain to determine electromagnetic field distributions. The energy equation was then solved in order to determine thermal energy deposition and temperature fields in the dielectric layer. The sample numerical simulations indicate that combining microwave heating with thermographic imaging could lead to a viable non- destructive testing instrument for crack detection.