Biomedical subjects

D R Carter

Publications and source records attributed to D R Carter.

At least 91 records · Page 5Linked to original sources

PubMed1988

Stresses in plated long-bones: the role of screw tightness and interface slipping.

Using a three-dimensional finite element model of a plated long bone, we studied the influence of screw tightness, sliding frictional interfaces, and loading magnitude on the stresses within the plated bone. The model incorporated frictional interface elements that allowed stress-free separation under tensile loading to occur between the plate and bone and between the screw heads and the plate. The applied loading stimulated both static preloads created by tightening the screws that secure the plate to the bone and physiologic loads created by activity. Initial screw tightening with plate application created regions of bone hydrostatic compressive stress that may be partly responsible for ischemia under the plate. The inclusion of frictional interfaces resulted in a nonlinear relationship between physiologic loan and bone strain that was dependent on screw tightness. This nonlinear response correlated well with the results of previous in vitro studies showing that slippage between the plate and the bone can occur at physiologic load levels. The results showed that the effect of such slippage can be at least as important as plate material, rigidity, and placement in determining the degree of stress shielding. The results also indicated that previous plated bone models that assumed tight interfaces may have overestimated the extent of mechanical stress shielding.

Biomechanical Phenomena↗

PubMed1988

Mechanical stresses and endochondral ossification in the chondroepiphysis.

In 1911, Gebhardt used a photoelastic model to relate mechanical stresses to the ossification pattern of the chondroepiphysis. Pauwels later conducted a photoelastic study using the same model geometry to develop a theory that the secondary ossific nucleus originates at a position of high-magnitude hydrostatic pressure where the shear stresses are zero. We conducted two-dimensional finite element analyses of the model used by Gebhardt and Pauwels. We demonstrate that Pauwels's photoelastic results are correct but are based on the imposition of incorrect boundary conditions. When more realistic boundary conditions were used, the finite element results changed dramatically. These results suggest that (a) the ossific nucleus appears in an area of high shear (deviatoric) stresses; (b) the edge of the advancing ossification front (zone of Ranvier or ossification grove) also experiences high shear stresses; and (c) the joint surface, where articular cartilage forms, is exposed to high-magnitude hydrostatic compression. These findings support the theory proposed by Carter and associates that intermittently applied shear stresses (or strain energy) promote endochondral ossification and that intermittently applied hydrostatic compression inhibits or prevents cartilage degeneration and ossification.

Growth Plate↗

PubMed1988

Effects of ingrowth, geometry, and material on stress transfer under porous-coated hip surface replacements.

Three different interface geometries for porous ingrowth surface replacements of the hip were examined using two-dimensional linear and nonlinear contact finite element analyses. The results indicate that incorporation of a nearly flat prosthesis interface between the surface replacement and the underlying cancellous bone may reduce stress shielding and improve stress transfer from the component. For all designs analyzed, the bone stress shielding was insensitive to component material stiffness when the elastic modulus was greater than 30 MPa. The use of titanium instead of cobalt-chrome (Co--Cr) as the prosthesis material therefore had a negligible effect on stress shielding.

Hip Joint↗

PubMed1988

A biomechanical assessment of plate fixation, with insufficient bony support.

The use of an internal fixation plate in the presence of a bone defect was studied using a theoretical model of an idealized long bone having a circular cross section and loaded using a combination of axial and bending loads. The analysis showed that the "bending-open" loading mode does not occur if, in the normal unplated bone, the line of action of the resultant axial load passes within the outer cortex at the location where the plate is to be applied. In this situation the fracture will deform in a "bending-closed" mode regardless of whether the plate is attached to the tension or the compression side. If bony contact cannot be achieved, lower plate stress is always encountered when the plate is attached to the compression side instead of the tension side. In vivo verification of the model was addressed in a pilot experiment using instrumented metal plates applied bilaterally to the femora of one dog. Bilateral bone defects were created in the midfemoral canine diaphysis. On one leg the plate was applied to the lateral aspect ("tension" side), and on the other leg the plate was applied to the medial aspect ("compression" side). The plate attached to the lateral aspect deformed plastically in the bending-closed mode. The contralateral plate that was attached to the medial aspect (compression side) of the femur did not show signs of plastic deformation. Furthermore, the plate strains were lower in the plate attached to the medial aspect than those in the plate attached to the lateral aspect.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1988

Correlations between mechanical stress history and tissue differentiation in initial fracture healing.

A general theory for the role of intermittently imposed stresses in the differentiation of mesenchymal tissue is presented and then applied to the process of fracture healing. Two-dimensional finite element models of a healing osteotomy in a long bone were generated and the stress distributions were calculated throughout the early callus tissue under various loading conditions. These calculations were used in formulating theoretical predictions of tissue differentiation that were consistent with the biochemical and morphological observations of previous investigators. The results suggest that intermittent hydrostatic (dilatational) stresses may play an important role in influencing revascularization and tissue differentiation and determining the morphological patterns of initial fracture healing.

Animals↗

PubMed1988

The role of mechanical loading histories in the development of diarthrodial joints.

The possible role of mechanical loading history in chondroosseous development at the ends of long bones is explored using two-dimensional finite element models of chondroepiphyses. Loading histories are characterized in terms of discrete loading cases defined by joint contact pressure distributions and an associated number of loading cycles. An osteogenic stimulus throughout the chondroepiphyses is calculated following the theory that cyclic octahedral shear stresses promote endochondral ossification and cyclic compressive dilatational stresses inhibit ossification. The resulting distributions for the osteogenic stimulus predict the appearance of the secondary ossific nucleus and the shape of the developing bony epiphysis. The zone of Ranvier and the formation of articular cartilage and the growth plate are also predicted by the models. These findings are consistent with the hypothesis that tissue stress histories constitute an important influence during skeletal morphogenesis. Further study and testing of the concepts introduced in this study are appropriate.

Biomechanical Phenomena↗

PubMed1988

Influence of physical activity on the regulation of bone density.

Using a mathematical model which relates bone density to daily stress histories, the influence of physical activities on the apparent density of the calcaneal cancellous bone was investigated. Assuming that the mechanical bone maintenance stimulus is constant for all bone tissue, bone apparent density was calculated by a linear superposition of the mechanical stimulus provided by different daily physical activities. An empirical weighting factor, m, accounted for possible differences in the relative importance of load magnitude and number of cycles in each activity. By considering hypothetical variations in body weight and occupational activity levels, the range of probable m values was established. The model was then applied to the results of two previous running studies in which calcaneal density was measured to obtain an estimate of the stress exponent parameter, m. The results indicate that stress magnitudes (or joint forces) have a greater influence on bone mass than the number of loading cycles. We demonstrate that by carefully considering the magnitudes of imposed skeletal forces and the number of loading cycles, it may be possible to design exercise programs to achieve predictable changes in bone mass.

Body Weight↗

PubMed1988

Stress analyses of glenoid component designs.

Metal backing of glenoid components for total shoulder replacements and the use of bony ingrowth surfaces on these components have recently been introduced. In this study, finite element analyses were performed to determine the stress fields in the natural glenoid and to calculate the change in bone stresses after implantation of glenoid components of various designs. The effects of metal backing, keel geometry, and superior constraint on bone stresses indicate that stress distributions on the natural glenoid corresponded to bone morphology. Metal-backing the glenoid component may cause slight improvement in stress transfer to cortical bone. Altered fin geometry better stabilized the glenoid component. Superior restraints on the component intending to prevent subluxation increase stresses and may cause earlier loosening than encountered with unconstrained components.

Humans↗

PubMed1988

Mechanical stress and morphogenetic endochondral ossification of the sternum.

The possible role of mechanical stress in determining the patterns of endochondral ossification in skeletal anlages was explored using stress-analysis computer models of developing human sterna. It has been hypothesized previously that the normal sequence of proliferation, maturation, degeneration, and ossification of cartilage is accelerated in regions of high cyclic octahedral shear stress and inhibited in regions of intermittent compressive-hydrostatic (dilatational) stress. This hypothesis was investigated using two-dimensional, all-cartilage, plane-stress finite-element models of the three basic shapes of human sterna that were identified by Ashley. A mathematical criterion, which combined the opposing influences of the shear and dilatational stresses into a single net stimulus for ossification, was used successfully to simulate the three basic patterns of sternal endochondral ossification that were previously documented. Our findings support the view that mechanical forces may strongly influence skeletal morphogenesis, growth, and development, beginning at a very early stage.

Computer Simulation↗

PubMed1987

Warping of cross sections in the torsion of long bones with internal fracture fixation plates.

Torsion of noncircular beams results in warping of each cross section. When noncircular cross sections are constrained to remain plane, the resulting shear stress distribution is different from what Saint Venant torsion (with warping) would predict. This has practical implications to the stress analysis of plated long bones subjected to torsional loadings. Analyses in which warping is not allowed predict incorrect stress fields in the plate and bone and overpredict the amount of stress shielding associated with fracture plate fixation.

Biomechanical Phenomena↗

PubMed1987

Contact finite element stress analysis of porous ingrowth acetabular cup implantation, ingrowth, and loosening.

Two-dimensional linear and contact finite element analyses were conducted of total hip arthroplasty using metal-backed, porous ingrowth acetabular components. The stress transmission characteristics from the component to the surrounding bone were given special attention. Resultant loads of 20 and 40 degrees medial of vertical were studied, and the influence of adding a metal flange to the rim of the cup was evaluated. The results indicated that when a conventional metal-backed component (without a flange) is initially implanted and subjected to normal loading, these components may experience distraction between the component and the surrounding bone at inferior sites. Compressive stresses in the superior dome cancellous bone, however, will be substantial. If complete porous ingrowth is achieved, the superior dome compressive stresses will be reduced and substantial shear stresses created. In addition, high local bone stresses were found at the component rim. If bone ingrowth is achieved only in specific locations, stress transmission will be dictated by those locations and may differ markedly from the case of complete bone ingrowth. In the event that no porous ingrowth is achieved and a fibrous layer forms around the component, the interface stresses will be similar to those calculated for the natural hip. The addition of a flange to the rim of the cup will reduce the magnitude of the radial stresses transmitted to the cancellous bone superiorly and medially by directly transferring some of the load to the lateral wall of the pelvis. The flange will also help to relieve the high local stresses that are found at the component rim.

Biomechanical Phenomena↗

PubMed1987

Mechanical loading history and skeletal biology.

A comprehensive theory which relates tissue mechanical stresses to many features of skeletal morphogenesis, growth, regeneration, maintenance and degeneration is reviewed. The theory considers the repeated or intermittent mechanical forces which constitute the loading history on the chondro-osseous skeleton. The results of numerous mechanical stress analyses indicate that the local tissue stress history plays a major role in controlling connective tissue biology. The strong influence of mechanical energy in ontogenesis implies a comparably strong influence in phylogenesis. The fact that the mechanical stress histories in skeletal tissues are directly related to the force of gravity suggests that the life forms that have evolved on Earth are closely tied to our gravitational field.

Animals↗

PubMed1987

Trabecular bone density and loading history: regulation of connective tissue biology by mechanical energy.

The method of considering a single loading condition in the study of stress/morphology relationships in trabecular bone is expanded to include the multiple loading conditions experienced by bone in vivo. The bone daily loading histories are characterized in terms of stress magnitudes or cyclic strain energy density and the number of loading cycles. Relationships between local bone apparent density and loading history are developed which assume that bone mass is adjusted in response to strength or energy considerations. Three different bone maintenance criteria are described which are formulated based upon: (1) continuum model effective stress, (2) continuum model fatigue damage accumulation density, and (3) bone tissue strain energy density. These approaches can be applied to predict variations in apparent density within bone and among bones. We show that all three criteria have similar mathematical forms and may be related to the density (or concentration) of bone strain energy which is transferred (dissipated) in the mineralized tissue. The loading history and energy transfer concepts developed here can be applied to many different situations of growth, functional adaptation, injury, and aging of connective tissues.

Biomechanical Phenomena↗

PubMed1987

Acetabular lucent lines and mechanical stress in total hip arthroplasty.

The radiographs of 97 patients (117 hips) who had a straight-stem Muller femoral component and a non-metal-backed acetabular component were reviewed to determine whether the mode of acetabular loosening predicted by finite element stress analysis (FESA) is observed clinically. The follow-up period averaged 3.1 years (range, 2.0-4.6 years). Significantly more lucent lines were present in zones 1 and 3, compared with zone 2 (P less than .01). This finding corroborates the predictions of FESA and suggests that the production of acetabular lucent lines is due in part to chronic mechanical overload.

Acetabulum↗

PubMed1987

Relation of coxarthrosis to stresses and morphogenesis. A finite element analysis.

We calculated subchondral deformations and stresses in the femoral head and acetabulum during weight bearing using finite element models. Areas of high joint contact pressures on the femoral head were shown to correspond to high hydrostatic compression in subchondral bone. The magnitude of the subchondral bone compressive hydrostatic stress correlated with cartilage thickness and was highest in the superior femoral head and moderate at the acetabular roof. The seldom contacting surfaces of the medial-inferior and peripheral areas of the femoral head and the roof of the acetabulum had lower hydrostatic compression and significant subchondral bone tensile strains tangential to the joint surface. Initial cartilage fibrillation and osteophyte formation are often found in these areas. These findings suggest that fluctuating hydrostatic pressure inhibits vascular invasion and the degeneration and ossification of articular cartilage. The generation of tensile strain may promote the degenerative process by direct mechanical mechanisms. Additionally, since tensile strains are associated with a reduction in the compressive hydrostatic stresses in the cartilage and an increase in shear stresses, their presence may permit or promote vascular invasion, cartilage degeneration, and osteophyte formation. These mechanical principles in arthrosis are the same as those that have been previously demonstrated to guide the degeneration and ossification of the cartilage primordium during skeletal morphogenesis. In this sense, arthrosis may be viewed as the final stage in the degeneration and ossification of the cartilage anlage.

Acetabulum↗

PubMed1987

Influences of mechanical stress on prenatal and postnatal skeletal development.

A new theory is introduced to describe some of the influences of mechanical stresses on chondroosseous biology. It is proposed that degeneration and ossification is a normal process for all cartilage in the appendicular skeleton, which is accelerated by intermittently applied shear stresses (or strain energy), and inhibited or prevented by intermittently applied hydrostatic pressure. These concepts were applied using finite element computer models in an effort to predict the ossification pattern of the prenatal and postnatal femoral anlage. The theoretical calculations successfully predicted the key features of skeletal morphogenesis including the development of the primary ossification site, a tubular diaphysis and marrow cavity, metaphyseal and epiphyseal trabecular bone, the location and geometry of the growth plate, the appearance and location of the secondary ossific nucleus, and the existence and thickness distribution of articular cartilage. The results suggest that degenerative joint disease in immobilized or nonload-bearing mature joints may be a manifestation of the final stage in the ossification of the anlage. In nonfunctional joints, the absence or reduction of intermittent hydrostatic pressure in the articular cartilage permits cartilage degeneration and the progressive advance of the ossification front toward the joint surface until the articular cartilage has been ossified.

Cartilage, Articular↗

PubMed1986

Geometric, elastic, and structural properties of maturing rat femora.

Geometric, elastic, and structural properties of growing rat femora were determined from bending and torsion tests followed by bone sectioning and measurement of areal properties. Rosette strain gages bonded to the bone surface measured the strain during testing. A computer generated elliptical cross-sectional representation of the cross section geometry was used for calculation of material and structural properties. All structural and material properties increased with increasing age, exhibiting age-related changes that were best represented by an allometric or "heterauxic" growth pattern (y = axb) up to maturity. The femoral axial, flexural, and torsional rigidity increased 5.7, 10.1, and 14.8 fold, respectively, during maturation from 21 to 119 days of age. The increase in whole bone rigidity during maturation was caused primarily by changes in geometry. The bone tissue tensile longitudinal elastic modulus and shear modulus approximately doubled, and the shear strength increased approximately fourfold over this same period. Following maturity, a much slower increase in bending and torsional properties was noted. The results suggest that bone structural properties are regulated by changes in both geometric and material properties.

Aging↗

PubMed1986

A unifying principle relating stress to trabecular bone morphology.

The relationships between cancellous bone apparent density, trabecular orientation, and stress are developed and a mathematical theory describing these relationships is proposed. The bone is assumed to be a self-optimizing material. Using a continuum model, sufficient conditions are developed which ensure that, for a given stress encountered during normal activity, the theory will predict both trabecular orientation and apparent density. Using two special approaches, one based on optimizing strain energy density (stiffness) and the other on optimizing strength, the relationship between apparent density and stress is derived. This is the first time that a single theory has been advanced to predict both the orientation and apparent density of cancellous bone.

Biomechanical Phenomena↗