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Biomedical subjects

T P Harrigan

Publications and source records attributed to T P Harrigan.

At least 19 recordsLinked to original sources
PubMed2001

Poroelastography: imaging the poroelastic properties of tissues.

In the field of elastography, biological tissues are conveniently assumed to be purely elastic solids. However, several tissues, including brain, cartilage and edematous soft tissues, have long been known to be poroelastic. The objective of this study is to show the feasibility of imaging the poroelastic properties of tissue-like materials. A poroelastic material is a material saturated with fluid that flows relative to a deforming solid matrix. In this paper, we describe a method for estimating the poroelastic attributes of tissues. It has been analytically shown that during stress relaxation of a poroelastic material (i.e., sustained application of a constant applied strain over time), the lateral-to-axial strain ratio decreases exponentially with time toward the Poisson's ratio of the solid matrix. The time constant of this variation depends on the elastic modulus of the solid matrix, its permeability and its dimension along the direction of fluid flow. Recently, we described an elastographic method that can be used to map axial and lateral tissue strains. In this study, we use the same method in a stress relaxation case to measure the time-dependent lateral-to-axial strain ratio in poroelastic materials. The resulting time-sequenced images (poroelastograms) depict the spatial distribution of the fluid within the solid at each time instant, and help to differentiate poroelastic materials of distinct Poisson's ratios and permeabilities of the solid matrix. Results are shown from finite-element simulations.

Elasticity↗
PubMed1998

Limiting models for calcification in fibrous tissues adjacent to orthopedic implants: variational indicator functions and influences of implant stiffness.

Calcification and eventual integration of orthopedic implants into bone is important to many load-bearing devices, and the influence of load and implant stiffness on this process are assessed in this mathematical modelling study. Three research questions are posed in this study. First, can limiting material models provide useful information on the overall behavior of the tissue adjacent to a loaded orthopedic implant? Second, can the limiting models lead to optimization criteria? Third, can an optimization approach be used to differentiate between the four prospective remodeling rate equations which are proposed? The answers are yes, yes, and no, respectively. A two degree of freedom lumped parameter model for axial loading of an intramedullary implant is considered. Two limiting composite material models are used, and the strain energy density in the calcified and non-calcified phases are assessed as stimuli for calcification. The rate equations posed here assume that the calcified material volume fraction decreases at high strain-energy densities, and increases at small strain-energy densities. In all four cases (both models, both phases) the steady states for these rate equations find equilibrium points of indicator functions which are a weighted sum of total strain energy and the mass of calcified tissue in the layer considered. The weights on strain-energy density and mass differ in each case. This shows that for appropriate choices of parameters, all four models can yield the same results, and it also shows that an optimization approach does not uniquely determine the appropriate rate equation in these cases. The rate equations showed complicated dynamic behavior and a phase-plane analysis was used which led to upper bounds on load, which depended on implant stiffness and distal support. The predictions of the four cases studied are compared.

Bone and Bones↗
PubMed1997

Mechanical model for critical strain in mineralizing biological tissues: application to bone formation in biomaterials.

A simple theoretical model for the role of strain energy density in the initial mineralization of soft tissues is presented and used to derive a limit of the allowable strain in tissue engineered biomaterials. The model incorporates the mechanical energy in calcified tissue due to time-varying loads into the more commonly used energetic arguments for mineralization. By using the Voight (equal-strain) and Reuss (equal-stress) composite material models to relate the volumetric density of calcified tissue to overall material modules, two models were developed to assess the effect of an imposed overall material strain on mineralization. A rate equation based on strain energy was used to model the kinetics of mineralization, and the stability of the rate equation was assessed, leading to a limit on overall material strain based on the specific energy for mineralization of soft tissues. The result depended on the stiffness of the material in series with the mineralizing tissue. Taking the stiffness of the material in series with the tissue as infinite lead to a prediction of critical strain for mineralization in the calcifying biological tissue which was the same on the Reuss and Voight models. The interaction of this theoretical model with biological factors and some clinical implications of the model are discussed.

Biocompatible Materials↗
PubMed1996

Bone remodelling adjacent to intramedullary stems: an optimal structures approach.

The internal parameters in bone remodelling theories often are not clearly related to the bony structure which results from the simulations in which they are implemented. For a restricted class of bone remodelling theories, we have previously found a connection between overall structural optimization and the parameters within a continuum-level remodelling rule. In this study, we assess whether a simplified analytical formula based on structural optimization can predict the behaviour of a large-scale finite element bone remodelling simulation. The analytical formula predicts when bone will remain around an intramedullary implant. The predictions of the formula are borne out in the numerical results. This leads to a physical interpretation of one of the two parameters in the remodelling rule used. The results also show some characteristics which are clinically relevant. This study extends earlier results due to Huiskes for internal remodelling around intramedullary implants by using a different, numerically stable remodelling algorithm based on optimization. The study also shows a direct practical application of the optimizing remodelling theory the authors have developed previously.

Biomechanical Phenomena↗
PubMed1995

Effect of porous coating and loading conditions on total hip femoral stem stability.

An examination of femoral bone-prosthesis interface behavior under different load types is undertaken using finite-element analysis. Three-dimensional finite-element models are made of two designs of hip prostheses after implantation in a femur. Femoral geometry was determined by computed tomography scans. The models were loaded in one-legged stance and stairclimbing configurations. The implants were modeled as both smooth surfaced and porous coated. The amount of contact and the relative motion between bone and implant were calculated. It is shown that torsional loads such as occur during stairclimbing contribute to larger amounts of implant micromotion than does stance loading. Contact at the bone-prosthesis interface is more dependent on load type than on implant geometry or surface coating type.

Biomechanical Phenomena↗
PubMed1994

Fit of the uncemented femoral component and the use of cement influence the strain transfer the femoral cortex.

To determine whether the strain patterns produced in the femoral cortex after uncemented femoral arthroplasty are influenced by the fit of the component and whether these patterns are different from those of cemented components, cortical surface strains of cadaveric femurs subjected to loads simulating single-limb stance were measured before and after the insertion of uncemented, collared, straight-stemmed femoral components. The effects of press fit, loose fit, and precise fit of the components were evaluated and were contrasted to the strain patterns occurring after insertion of cemented femoral components. Strains varied markedly, depending on the fit of the stem of the uncemented femoral component within the isthmus. Nearly normal patterns of femoral strain were produced when the femoral stem was fit precisely at the isthmus, and the proximal femoral strains were similar to those of the intact state. In contrast, press fit and loose fit at the isthmus altered the strain patterns. The proximal medial axial strains were significantly reduced with press fit, to a mean of 39% of normal (p < 0.05), and increased with loose fit, to a mean of 141% of normal (p < 0.05). The prostheses fixed with cement showed a mean reduction in proximal medial axial strains to 33% of normal, which was comparable with press fit uncemented components even though the collar was well seated. Thus, our findings indicated that, in the immediate postoperative period, femoral strain patterns can be influenced by the fit of an uncemented component within the isthmus and by the use of cement.

Aged↗
PubMed1994

Bone remodeling and structural optimization.

Bone remodeling has been viewed both as a process which adapts bone tissue to the mechanical environment at each point in the structure, and as a process which optimally adjusts the tissue distribution within bones to bear the loads placed on them. We have developed a connection between these two views of bone remodeling, in a restricted sense. We start with a remodeling rate equation based on strain energy density. We then define an indicator function which is a weighted sum of total strain energy and a measure of bone mass, and we show that finding bone density distributions in which the remodeling rate equation predicts no changes with time is the same as finding density distributions in which the indicator function is insensitive to small changes in density. The set point in the remodeling rate equation corresponds to a parameter in the indicator function which determines the relative importance of bone mass and strain energy in the optimization indicator function. We have not assessed whether the density distributions which make the density rate of change zero are actually local or global minima for the indicator function in this study, but a related study shows that there is a single unique minimum for the indicator function developed here, implying that a unique solution exists for the bone remodeling rate equations considered in this study.

Biomechanical Phenomena↗
PubMed1994

On the sufficiency conditions for the stability of bone remodeling equilibrium.

In this technical note a sufficiency condition is established for the stability of a strain-energy-based bone remodeling theory in the special case of a beam loaded by an axial force and a bending moment. In a previous report the same condition was shown to be a necessary condition for stability in the same situation. The remodeling scheme is one characterized by a remodeling stimulus equal to the strain energy density divided by the bulk or apparent density raised to an exponent m as well an elastic modulus proportional to bulk or apparent density raised to an exponent n. In order for a remodeling scheme to be stable for an elastic beam loaded by an axial force and a bending moment, it is established that the condition that m must be greater than n is not only necessary, but also sufficient.

Algorithms↗
PubMed1993

Bone strain sensation via transmembrane potential changes in surface osteoblasts: loading rate and microstructural implications.

A model is developed in which osteoblasts can sense the strains applied to a small region of bone through electrical coupling between adjacent cells. The stress-generated potentials within bone are assumed to occur through streaming potentials, and the coupled network of osteocytes is assumed to act in a manner similar to the classical cable model for nerve cells. In a one-dimensional model, the linear poroelastic equations for motion of the fluid are solved analytically for sinusoidally varying imposed strains, and the streaming potentials are predicted from the fluid flow. The changes in the osteocyte and osteoblast transmembrane potential are given by an analytical solution to the governing equations, and the dependence of the transmembrane potential changes (TPC) on position, loading rate, manner of loading (compression versus bending), and on the degree of cellular coupling is discussed. The model correctly predicts the rate dependence of remodelling established by other investigators. The influence of the electrical parameters within the model indicate that further study of the cellular coupling in bone can yield important new information on bone remodelling.

Animals↗
PubMed1992

A finite element study of the initiation of failure of fixation in cemented femoral total hip components.

In order to study initial mechanisms of failure in cemented femoral total hip components, an anatomically accurate three-dimensional linear finite element model was constructed and verified against experimental strain measurements in the cement mantle. Good agreement was found between predicted and measured strains. The likelihood of failure initiation due to cement-prosthesis debonding and crack initiation at voids was studied for loading conditions simulating both one-legged stance and stair climbing. The "out of plane" forces involved in stair climbing appear to be the greatest threat to the fixation of total hip replacements. In stair climbing, cement-prosthesis debonding and pore crack initiation were probable in the proximal anteromedial region of the cement mantle, and near the distal tip of the implant. The proximal stresses in stair climbing were higher than the distal stresses in either stair climbing or one-legged stance.

Bone Cements↗
PubMed1992

An analytical and numerical study of the stability of bone remodelling theories: dependence on microstructural stimulus.

The origin of unstable bone remodelling simulations using strain-energy-based remodelling rules was studied mathematically in order to assess whether the unstable behavior was due to the mathematical rules proposed to characterize the processes, or to the numerical approximations used to exercise the mathematical predictions. A condition which is necessary for the stability of a strain-energy-based remodelling theory was derived analytically using the calculus of variation. The analytical result was derived using a simple elastic model which consists of a long beam loaded by an axial force and a bending moment. This loading situation mimics the coupling between local density and global density distributions seen in vivo. A condition necessary for a stable remodelling scheme is arrived at, but the conditions necessary to guarantee a stable remodelling scheme are not. In this remodelling scheme, the elastic modulus is proportional to volumetric density raised to an exponent n, and the microstructural stimulus is taken as the strain energy density divided by volumetric density raised to an exponent m. In order for a remodelling scheme to be stable in this loading situation, m must be greater than n. Finite-difference time-stepping is used to verify the predictions of the analytical study. These numerical studies appear to confirm the analytical studies. Physiologic interpretation of the behavior found with n greater than m indicates that this type of unstable behavior is unlikely to be observed in vivo. Since numerical approximations are not made in deriving this stability condition, we conclude that the mathematical rules proposed to characterize bone remodelling based on strain energy density should meet this condition to be relevant to physiologic bone remodelling.

Bone Density↗
PubMed1991

A three-dimensional non-linear finite element study of the effect of cement-prosthesis debonding in cemented femoral total hip components.

A three-dimensional non-linear finite element analysis of a cemented femoral component in which the component was partially debonded from the cement mantle was used to assess the effects of debonding on stresses in the cement. Three cases of partial cement-metal debonding were modelled with debonding of the proximal portion of the implant down to a horizontal plane which was 35, 62.5, or 82.5 mm below the prosthesis collar. Each situation was studied under loads simulating both gait and stairclimbing. Also, complete debonding between the implant and the surrounding cement mantle was modeled for loads simulating gait. Under stair climbing loads with partial cement-mental debonding, hoop stresses of 13-18 MPa were observed in the cement at the cement-metal interface at the proximal postero-medial corner of the implant. Similarly, in stair climbing, the maximum principal stresses in the cement were also adjacent to the proximal postero-medial region of the implant. These stresses were compressive and increased from 15 MPa with fully bonded interfaces to 48 MPa with debonding down to 82.5 mm below the prosthesis collar. Under gait loads, complete debonding caused high compressive stresses up to 34.9 MPa in the cement distal to the prosthesis tip. Thus, cement failure subsequent to prosthesis debonding is likely in the proximal region in a partially debonded implant due to stair climbing loads and is likely below the prosthesis tip in a fully debonded implant due to gait loading.

Body Weight↗
PubMed1991

A finite element study of the effect of diametral interface gaps on the contact areas and pressures in uncemented cylindrical femoral total hip components.

Uncemented femoral total hip components rely entirely on contact with the prepared femur for their initial fixation. The contact areas and stresses between a straight tubular bone and a metal cylindrical prosthesis 12.5 cm long and 13 mm in diameter were calculated in a finite element model which includes uniform diametral gaps varying from 20 to 500 microns, using transverse loads from 100 to 2000 N. Frictionless three-dimensional contact elements were used between the bone and the prosthesis. Contact stresses were high and irregular in all cases, and the contact areas were small. Two regions of contact were apparent for lower loads and larger gaps. A third region of contact occurred near the distal tip of the implant at higher loads. This region of contact markedly increased the contact stresses at the distal tip of the prosthesis. A 20 microns overlap between bone and implant was modelled to assess a slight interference fit. The contact stress distribution in this case was markedly different from the stress distribution with a 20 microns diametral gap. The data collectively indicates that gaps of less than 20 microns between bone and implant can substantially change contact stress distributions.

Bone Cements↗
PubMed1990

The influence of support conditions in the loading fixture on failure mechanisms in the push-out test: a finite element study.

The usefulness of the push-out test as an indicator of interface strength was evaluated using finite element models of intact and partially failed cylindrical push-out specimens loaded against a rigid annular support. The irregular stress distributions that were found in intact specimens depended more on interface conditions at the loading fixture than on a 35% increase in interface area. The maximum stress at the interface was a tensile stress. Critical energy release rates for interface failure were calculated for flawed specimens in which flaw size was either 10 or 100 microns, and for boundary conditions at the loading fixture that were either fixed or slipping in the radial direction. The critical energy release rates depended heavily on the support boundary conditions. Thus, the results of parametric push-out tests can be reasonably compared only for specimens that are very similar in geometry and that are loaded in very carefully controlled fixtures.

Bone Cements↗
PubMed1990

Porosity of various preparations of acrylic bone cements.

The total porosity and mean pore sizes of various bone cement preparations were measured using image analysis. The porosity in different commercial bone cements varied from 5% to 16% when these cements were prepared in the usual fashion. Centrifugation for 30 seconds resulted in a substantial reduction in the overall porosity of Simplex P, AKZ, Zimmer Regular, and CMW bone cements by reducing both the mean pore size and the number of pores per unit area. In contrast, the porosity of LVC, Palacos R, and Palacos R with gentamicin bone cements was not significantly decreased by centrifugation. Chilling the monomer before mixing resulted in higher porosity of both the centrifuged and uncentrifuged Simplex P, Zimmer Regular, and CMW bone cements. Simplex P mixed with chilled monomer and centrifuged for 120 seconds has one of the lowest porosities of the various cements, while retaining good handling characteristics and excellent fatigue strength.

Acrylic Resins↗
PubMed1990

A double-blind study on the effects of a capacitively coupled electrical field on bone ingrowth into porous-surfaced canine total hip prostheses.

The effect of a capacitively coupled electrical field on bone ingrowth into titanium fiber mesh porous-surfaced canine total hip arthroplasties (THAs) was investigated in a double-blind experiment. The electrical field was induced by an external source delivering a 60-kHz 5-6-V peak-to-peak sinusoid voltage through skin electrodes. No significant increase in the ingrowth of bone into the porous coating occurred at the end of six weeks of electrical stimulation. The amount of bone that grew into the porous surface, the areal density of bone within the available pore space, and the extent of the prosthesis surface area with bone ingrowth or apposition were not significantly different in the control and stimulated groups. This particular form of electrical stimulation does not improve bone ingrowth into porous-surfaced canine THAs by six weeks.

Animals↗
PubMed1989

The effect of centrifuging bone cement.

We have tested the porosity and fatigue life of five commonly used bone cements: Simplex P, LVC, Zimmer regular, CMW and Palacos R. Tests were conducted with and without centrifugation and with the monomer at room temperature and, except for LVC, at 0 degrees C. We found that the fatigue life of different specimens varied by a factor of nearly 100. It did not depend on porosity alone, but was more influenced by the basic composition of the cement. Simplex P when mixed with monomer at 0 degrees C and centrifuged for 60 seconds had the highest fatigue life and was still sufficiently liquid to use easily.

Bone Cements↗