Biomedical subjects

D R Carter

Publications and source records attributed to D R Carter.

At least 19 recordsLinked to original sources

PubMed1995

Developmental mechanics determine long bone allometry.

Evolutionary and developmental factors responsible for the scaling relationships observed in animal skeletons are poorly understood. We have created a mathematical model for long bone cross-sectional development which incorporates both intrinsic growth and extrinsic, adaptive bone modeling in response to changes in bone mechanical strains during ontogeny. The model successfully simulates the developing morphology in individual animals and the bone geometric allometric relationships among adults across many species (range from mouse to elephant in size). Our results suggest that long bone scaling characteristics are not a result of intrinsic genetic factors but are the results of highly conserved, extrinsic biophysical processes whereby bone tissue strains modulate skeletal morphogenesis.

Biomechanical Phenomena↗

PubMed1995

Hindlimb suspension diminishes femoral cross-sectional growth in the rat.

Growth, functional adaptation, and torsional strength were examined in the femora of 39-day-old male Sprague-Dawley rats subjected to hindlimb suspension for 0, 1, 2, 3, or 4 weeks and were compared with measurements for age-matched control animals. Our goal was to understand the effect of reduced loading on the normal age-related changes in femoral properties during growth. The control animals exhibited growth-related increases in all geometric and torsional properties of the femur. The mean body mass and femoral length of the hindlimb-suspended rats were similar to those of the controls throughout the experiment. Over 4 weeks, the femoral cross-sectional and torsional measurements from the hindlimb-suspended rats demonstrated increases in comparison with the basal values (+33% cross-sectional area, +64% polar moment of inertia, +67% ultimate torque, and +181% torsional rigidity), but the age-matched controls showed significantly greater growth-related increases (+71% cross-sectional area, +136% polar moment of inertia, +127% ultimate torque, and +367% torsional rigidity). The differences in femoral structural strength between the hindlimb-suspended animals and the age-matched controls were attributable to differences in altered cross-sectional geometry.

Adaptation, Physiological↗

PubMed1995

Bone strain gage data and theoretical models of functional adaptation.

The in vivo implantation of strain gages on the surface of bones has proven to be a very useful technique for studying the relationship between in vivo loading and bone growth and adaptation. However, data from such experiments have yet to be well incorporated within the context of theoretical models of bone adaptation. Methods for analyzing bone rosette strain gage recordings within the framework of strain energy density-based computational modeling/remodeling theories are presented. A new strain energy density based parameter, energy equivalent strain, is proposed as a single scalar measure of cyclic strain magnitudes and the concept of a daily strain stimulus is also introduced. As an illustrative example, the approach is applied to analyze previously reported in vivo data from the anteromedial human tibia (Lanyon et al., 1975, Acta orthop. Scand. 46, 256-268).

Adaptation, Physiological↗

PubMed1995

Numerical instabilities in bone remodeling simulations: the advantages of a node-based finite element approach.

Long bone structure occurs in two distinct forms. The bone mass near the joint is primarily found in a distributed, porous trabecular structure, while in the diaphyses a tubular cortical structure is formed. It seems likely that these two observed morphologies come about, at least in part, as a mechanical adaptation to the different mechanical demands in the two regions. Mathematical formulations of this dependency have been proposed, thus facilitating numerical simulations of bone adaptation. Recently two types of discontinuities have been observed in these simulations. The first type (near-field) appears in areas near distributed load application and is characterized by a 'checkerboard' pattern of density wherein adjacent remodeled elements alternate between low and high density. The second type of discontinuity (far-field) appears remote from the load application and is characterized by strut or column-like regions of elements which become fully compact bone while adjacent regions are fully resorbed. In fact, the far-field discontinuity is an accurate representation of bone physiology and morphology since it is consistent with the appearance of cortical bone in the diaphysis. On the other hand, the near-field discontinuity, appears in a region where continuous distributions of intermediate apparent densities (trabecular bone) are expected. This finding may cause some to question whether a single continuum formulation of bone remodeling can predict both discontinuous far-field behavior and continuous near-field behavior. We describe a node-based implementation of current continuum bone remodeling theories which eliminates the spurious near-field discontinuities and preserves the anatomically correct far-field discontinuities, thus indicating that a single biological process may be at work in forming and maintaining both far-field and near-field morphologies.

Algorithms↗

PubMed1995

Computational method for determination of bone and joint loads using bone density distributions.

Because bone structure is influenced by mechanical loading during ontogeny, the geometry and density distribution of bones contain information about their loading histories. Based on a mathematical theory relating stress history to bone remodeling, we have developed a method to determine dominant bone loading conditions using an optimization procedure. We applied this load determination method using a simplified two-dimensional bone-end finite element model, for which a standard density distribution had been calculated under a given set of loading conditions. With this density distribution, the optimization procedure was used to determine the original loads from a broad set of many plausible basic load distributions and locations. The optimization procedure adjusted the magnitude of each basic load to achieve the desired tissue level attractor stress stimulus throughout the model. The results show that the density-based bone load determination method yields accurate results for basic test cases and, thus, may have potential for estimating in vivo bone loads for both extant and extinct animals.

Animals↗

PubMed1995

Long bone geometry and strength in adult BMP-5 deficient mice.

Bone morphogenetic proteins (BMPs) play a critical role in early skeletal development. BMPs are also potential mediators of bone response to mechanical loading, but their role in later stages of bone growth and adaptation has yet to be studied. We characterized the postcranial skeletal defects in mature mice with BMP deficiency by measuring hind-limb muscle mass and long bone geometric, material, and torsional mechanical properties. The animals studied were 26-week-old short ear mice (n = 10) with a homozygous deletion of the BMP-5 gene and their heterozygous control litter mates (n = 15). Gender-related effects, which were found to be independent of genotype, were also examined. The femora of short ear mice were 3% shorter than in controls and had significantly lower values of many cross-sectional geometric and structural strength parameters (p < 0.05). No significant differences in ash content or material properties were detected. Lower femoral whole bone torsional strength was due to the smaller cross-sectional geometry (16% smaller section modulus) in the short ear mice. The diminished cross-sectional geometry may be commensurate with lower levels of in vivo loading, as reflected by body mass (-8%) and quadriceps mass (-11%). While no significant gender differences were found in whole bone strength or cross-sectional geometry, males had significantly greater body mass (+18%) and quadriceps mass (+15%) and lower tibio-fibular ash content (-3%). The data suggest that adult female mice have a more robust skeleton than males, relative to in vivo mechanical demands. Furthermore, although the bones of short ear mice are smaller and weaker than in control animals, they appear to be biomechanically appropriate for the in vivo mechanical loads that they experience.

Analysis of Variance↗

PubMed1994

The effect of dowel lubrication on retention of resin-cemented dowels.

Threaded dowels were cemented into prepared canals of extracted human teeth by use of unfilled Bis-GMA resin cement. One half the dowels were lubricated before cementation, unscrewed partially, and reseated to original length after cement had set, to assure retrievability. There was no significant difference in mean tensile force required to dislodge the dowels cemented with or without prior lubrication. Lubrication of dowels makes them predictably and easily retrievable with no short-term effect on tensile strength of the cemented lubricated dowels. Long-term effects of the lubricant on the resin are unknown.

Bisphenol A-Glycidyl Methacrylate↗

PubMed1992

New approaches for interpreting projected bone densitometry data.

Bone densitometry using dual-photon absorptiometry (DPA) or dual-energy x-ray absorptiometry (DXA) has become a standard method for assessing bone mineral content in the spine and other skeletal regions. A projected areal density, referred to as bone mineral density (BMD,g/cm2), is normally calculated to assess regional bone density and strength. We demonstrate that this measure can be misleading when used to compare bones of different sizes due to inherent biases caused by bone thickness differences. For example, assuming that volumetric bone density remains constant and bony linear dimensions are proportional to height, a 20% increase in height would result in a 20% increase in both the thickness and the BMD of any bone. We describe new analysis methods to reduce the confounding effect of bone size, and we introduce a parameter, bone mineral apparent density (BMAD, g/cm3), that better reflects bone apparent density. Using this parameter, we calculate a quantity that serves as an index of bone strength (IBS, g2/cm4) for whole vertebral bodies. These analyses were applied to lumbar spine (L2-4) DXA measurements in a population of women 17-40 years old and appear to offer advantages to conventional techniques.

Absorptiometry, Photon↗

PubMed1992

Effects of resistance and endurance exercise on bone mineral status of young women: a randomized exercise intervention trial.

A substantial body of cross-sectional data and a smaller number of intervention trials generally justify optimism that regular physical activity benefits the skeleton. We conducted an 8 month controlled exercise trial in a group of healthy college women (mean age = 19.9 years) who were randomly assigned to a control group or to progressive training in jogging or weight lifting. We measured the following variables: bone mineral density (BMD) of the spine (L2-4) and right proximal femur using dual-energy x-ray absorptiometry, dynamic muscle strength using the 1-RM method, and endurance performance using the 1.5 mile walk/run field test. A total of 31 women completed the 8 month study. For women completing the study, compliance, defined as the percentage of workout sessions attended, was 97% for the runners (range 90-100%) and 92% (range 88-100%) for the weight trainers. Body weight increased by approximately 2 kg in all groups (p less than 0.05). Weight training was associated with significant increases (p less than 0.01) in muscle strength in all muscle groups. Improvement ranged from 10% for the deep back to 54% for the leg. No significant changes in strength scores were observed in the control or running groups. Aerobic performance improved only in the running group (16%, p less than 0.01). Lumbar BMD increased (p less than 0.05) in both runners (1.3 +/- 1.6%) and weight trainers (1.2 +/- 1.8%). These results did not differ from each other but were both significantly greater than results in control subjects, in whom bone mineral did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptiometry, Photon↗

PubMed1992

Skeletal development and bone functional adaptation.

The role of in vivo mechanical loading histories in normal skeletogenesis is related to the process of adaptive, stress-regulated bone remodeling in the adult. The results of many previous computer models for endochondral ossification and bone modeling and remodeling are reviewed. These studies support the view that simple stress-related mathematical algorithms or "construction rules" can be used to emulate normal skeletal development and architectural construction. Such mathematical rules presumably represent the net result of biophysical phenomena influencing cell metabolism and biosynthetic activity. These rules are also successful in describing the adaptation of adult bone to changes in tissue stresses. The findings suggest that stress-related functional adaptation in mature bones may be merely the adult manifestation of the same mechanical construction rules that guide and constrain normal development.

Adaptation, Physiological↗

PubMed1992

Rabbit knee immobilization: bone remodeling precedes cartilage degradation.

This study analyzed processes underlying osteoporosis and osteoarthrosis after short-term immobilization of the right hind limb of postadolescent (2.8 kg) and mature (4.0 kg) rabbits. After 3 weeks, the lateral posterior aspect of the lateral tibial plateau and the lateral femoral condyle of the immobilized limb exhibited prominent subchondral vascular eruptions. Femoral metaphyseal bone density decreased 27 and 18% in the immobilized limbs of postadolescent and mature rabbits, respectively. Calcein green fluorescence increased 1.9-fold (p less than 0.001) in the metaphyseal trabeculae of immobilized femurs. With immobilization, sulfate incorporation into femoral cartilage glycosaminoglycan increased, although total cartilage glycosaminoglycan and hydroxyproline levels were unchanged. Thymidine incorporation into DNA increased four- to fivefold in tibial and femoral cartilage of the immobilized limb. In this study, bone loss and remodeling preceded erosive cartilage degradation.

Animals↗

PubMed1992

In vitro study of initial stability of a conical collared femoral component.

This in vitro experimental study compared the initial stability of an uncemented conical collared femoral component to that of the same component with the collar removed. The two configurations examined simulated joint resultant forces encountered in single leg stance and stair climbing. For the simulated single leg stance loads, the data do not allow any inferences about relative component stability. With the exception of one collarless control, all micromotion for single leg stance loading was under 150 microns, measured approximately 1.5 cm below the resection line. For scaled stair-climbing loads, however, the conical collared component group was significantly more stable than the collarless control group in transverse (primarily rotational) micromotion. The overall average measured transverse motion for the collarless control group was more than 3.7 times greater than that of the conical collar group at scaled stair-climbing loads. The two conical collared components loaded to full peak stair-climbing load (2,100 N) exhibited micromotion under 160 microns. The results suggest that the conical collar may improve stability of an uncemented prosthesis under loads that include an out-of-plane (rotational) component.

Biomechanical Phenomena↗

PubMed1991

Effects of voluntary exercise on bone mineral content in rats.

We used a voluntary running model to explore the relationship between average daily running distance and bone mineral status of rats. A total of 60 male Sprague-Dawley rats were randomly assigned at 6 weeks of age to a sedentary control group (n = 22) or to a group with unlimited access to a running wheel (n = 38). The running distance of exercising rats was monitored daily, and steady-state running levels ranged from 3.2 to 18.1 km/day. At the end of the experimental period, femora and tibiae were dissected and bone mineral content (BMC, g/cm) and bone mineral density (BMD, g/cm2) were measured by single-photon absorptiometry. Cross-sectional morphometry was examined by taking a transverse section of the femoral middiaphysis. Hindlimb percentage fat was significantly higher in controls than in runners (20.0 +/- 1.2 versus 11.1 +/- 0.6, p less than 0.001), and soleus mass was greater in runners than in controls (371 +/- 8.1 versus 320 +/- 0.8 mg, p less than 0.001). Femoral and tibial lengths, weights, and volumes were significantly higher in runners than in controls (p less than 0.005). BMC and BMD were higher in runners than in controls at all sites apart from the distal femur. Cross-sectional areas at the femoral midshaft were greater in running rats than in sedentary controls (6.26 +/- 0.1 versus 5.45 +/- 0.3 mm2, p less than 0.02), as was the polar moment of inertia (15.6 +/- 0.6 versus 12.7 +/- 0.2 mm4, p less than 0.05). No positive correlation was found between distance run and BMC, BMD, cross-sectional area, or polar moment of inertia.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1991

Computational methods for analyzing the structure of cancellous bone in planar sections.

Conventional stereologic methods for expressing the orientation of anisotropic materials are limited to materials assumed to possess orthogonal directions of orientation. In many substances, including cancellous bone, this assumption is unsubstantiated. Presented here are two simple methods for characterizing the orientation of any anisotropic material within a plane. By modeling the substance as a series of lines oriented in particular directions, it is possible to arrive at either a "phase distribution" that expresses the degree of orientation distributed over a range of angles or a series of "primary orientations" that express the degree of orientation at a select number of angles, with an additional measure of the degree of isotropy. This characterization of anisotropy is highly dependent on such parameters as feature size, sample size, test line spacing, and test line width. Given the careful selection of these parameters, the new methods provide simple measures of orientation, which may prove useful in testing Wolff's trajectorial theory of the relationship between mechanical stresses and the orientation of cancellous bone.

Animals↗

PubMed1991

Musculoskeletal ontogeny, phylogeny, and functional adaptation.

Physical forces applied to connective tissues may cause significant changes in cell metabolism and gene expression. Theoretical investigations indicate that mechanical loading histories beginning very early in skeletal development may guide endochondral ossification patterns and the initial architectural construction of bones. Developmental patterns and structures of bones can be emulated using mathematical algorithms or "rules of construction" which relate developmental processes to tissue stress (or strain) histories. Skeletal forms and tissues are well-designed for their mechanical function primarily because their histomorphological construction has been guided by mechanical loading during growth and development. Construction rules of developmental mechanics can also be used to describe many of the histological and morphological adaptations of mature skeletal tissues to changes in customary physical activity. Over many generations, changes in the heritable genetic information occurs by mutation and genetic variability. The range of skeletal forms that are possible in evolution due to such variations, however, is constrained by the developmental rules of construction that reflect biophysical processes associated with the tissue mechanical loading.

Adaptation, Physiological↗

PubMed1991

Clinical and anthropometric correlates of bone mineral acquisition in healthy adolescent girls.

We studied the acquisition of bone mineral in 45 healthy prepubertal and pubertal girls and related changes in bone mass to age, body mass, pubertal status, calcium intake, and exercise. A subgroup of 12 girls was followed longitudinally. Bone mineral content (BMC) of the lumbar spine, whole body, and femoral neck was measured by dual energy x-ray absorptiometry and that at the midradius by single photon absorptiometry. For comparison, spine and whole body mineral contents were also measured by dual photon absorptiometry. Bone mass was expressed in conventional terms of BMC and area density (BMD). However, we show that BMD fails to account for differences in bone thickness. Since bone size increases during adolescence, we present a new expression, bone mineral apparent density (BMAD), which is BMC normalized to a derived bone reference volume. This term minimizes the effect of bone geometry and allows comparisons of mineral status among bones of similar shape but different size. BMC increased with age at all sites. These increases were most rapid in the early teens and plateaued after 16 yr of age. When bone mineral values at all sites were regressed against age, height, weight, or pubertal stage, consistent relationships emerged, in which BMC was most strongly correlated, BMD was correlated to an intermediate degree, and BMAD correlated only modestly or without significance. Dietary calcium and exercise level did not correlate significantly with bone mass. From these relationships, we attribute 50% of the pubertal increase in spine mineral and 99% of the change in whole body mineral to bone expansion rather than to an increase in bone mineral per unit volume. In multiple regressions, pubertal stage most consistently predicted mineral status. This study emphasizes the importance of pubertal development and body size as determinants of bone acquisition in girls. BMAD may prove to be particularly useful in studies of bone acquisition during periods of rapid skeletal growth.

Absorptiometry, Photon↗

PubMed1990

Effects of fluoride treatment on bone strength.

Bone mass and architecture in appendicular and most axial sites is controlled primarily by the tissue-loading history. We introduce a conceptual framework for understanding how fluoride treatment alters this control and can cause systemic increases in bone mass. Due to possible adverse influences of fluoride on the mineralized tissue physical characteristics, however, the increase in bone mass does not necessarily result in an increase in bone strength. Using engineering analyses of bone trabeculae, we calculate the losses in trabecular strength which can be caused by the presence of hypomineralized or hypermineralized fluorotic tissue. Significant increases in bone volume fraction and bone mass may be required to overcome these strength deficits.

Biomechanical Phenomena↗

PubMed1990

An approach for time-dependent bone modeling and remodeling--theoretical development.

A time-dependent approach for emulating bone modeling and remodeling in response to the daily loading history is presented. We postulate that genotype, systemic metabolic conditions, and local tissue interactions establish the level of local tissue mechanical stimulation (attractor state) appropriate for the maintenance of bone tissue. The net daily rate of apposition or resorption on a bone surface is determined by the difference between the actual stimulus and the tissue attractor state and can be modulated by other biologic factors. In calculating the net change in local bone apparent density, the technique takes into account the bone surface area available for osteoblastic and osteoclastic activity. Endosteal, periosteal, haversian, and cancellous bone modeling and remodeling are thereby treated in a consistent, unified fashion.

Animals↗