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Biomedical subjects

D R Carter

Publications and source records attributed to D R Carter.

At least 73 records · Page 4Linked to original sources
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↗
PubMed1990

An approach for time-dependent bone modeling and remodeling-application: a preliminary remodeling simulation.

In a companion paper, we presented a time-dependent theory for bone modeling and remodeling in response to a daily loading history. This paper represents a preliminary attempt to use the theory to determine the distribution of bone density within the adult proximal femur under an assumed normal loading history. Subsequent functional adaptation of the internal structure due to changes in the loading history are then determined. Throughout this preliminary study, the external geometry of the proximal femur is considered to be fixed, i.e., changes in the external shape are neither stimulated nor allowed. Linear and trilinear (dead-zone nonlinearity) rate remodeling laws were compared. Computer emulations using two-dimensional finite element models were successful in creating a normal-appearing distribution of bone tissue when remodeling was initiated from a solid structure of homogeneous bone density. Subsequent reduction in the loading history caused regional bone atrophy. Reinstatement of the normal loading history caused a generalized increase in bone mass but resulted in a slightly different bone distribution than was calculated for a constant loading history. These results demonstrate the utility of the remodeling theory and are consistent with the hypothesis that similar stress-related phenomena are responsible for both normal morphogenesis and functional adaptation in response to changes in the bone loading.

Animals↗
PubMed1990

The influence of fixation peg design on the shear stability of prosthetic implants.

The variety of fixation peg designs existing on prosthetic implants indicates uncertainty regarding the optimum design of fixation pegs for the reduction of stress and relative motion at the bone-implant interface. Fixation pegs have a number of important functions on a prosthesis, one of which is to reduce shear stress and shear displacement at the bone-implant interface. This is a parametric study intended to identify trends in the shear stability of prostheses incorporating a range of fixation peg designs. The parameters varied included the number of fixation pegs on a surface, the size of the pegs, and the aspect ratio (length/diameter) of the pegs. Mechanical tests were performed on urethane foam blocks with mechanical properties comparable to trabecular bone. The results indicated the following: (a) Fixation pegs act independently in resisting shearing force if they are spaced sufficiently far apart. (b) For any given shear displacement, smaller pegs generate a greater resistive shear force per unit of peg projected area in the direction of the applied load than larger pegs having the same aspect ratio. (c) Smaller diameter pegs cause the supporting material to yield at lower displacements. (d) Pegs with a high aspect ratio provide high shear stability with a minimum amount of bone removed, but may bend if the aspect ratio becomes excessive. (e) Smaller, slender pegs generate a greater resistive shear force at a given displacement per unit of peg volume than larger, lower aspect ratio pegs.(ABSTRACT TRUNCATED AT 250 WORDS)

Equipment Design↗
PubMed1990

A theoretical model of endochondral ossification and bone architectural construction in long bone ontogeny.

The role of mechanical stresses in the formation of endochondral ossification patterns and the construction of basic bone architecture in human long bones is investigated using a three-dimensional generalized model of long bone development. The distribution of mechanical stress which is created in developing bones as a result of intermittent mechanical loading is calculated using a computer model that mathematically represents the bone's geometry, material properties and loading conditions. The process of endochondral ossification is simulated by iteratively converting cartilaginous regions of the computer model to bone, based on the calculated intermittent hydrostatic and shear stress distributions. Once local regions of mineralized bone have formed, these regions are remodeled according to an algorithm which relates bone density to a mechanical stress stimulus. The results simulated the correct sequence of the appearance of morphological structures which are common to long bones in the human appendicular skeleton. These developmental structures include the site of the first endochondral bone and the secondary ossification center and the tubular nature of long bones. Our results suggest that mechanical loading histories may influence bone morphogenesis beginning from the early stages of endochondral ossification and continuing throughout life. The stress-based algorithms may be part of the 'rules of construction' or 'developmental constraints' which guide limb ontogeny.

Algorithms↗
PubMed1990

Femoral head apparent density distribution predicted from bone stresses.

A new theory relating bone morphology to applied stress is used to predict the apparent density distribution in the femoral head and neck. Cancellous bone is modeled as a self-optimizing material and cortical bone as a saturated (maximum possible bone density) response to stress in the bone tissue. Three different approaches are implemented relating bone apparent density to: (1) the von Mises stress, (2) the strain energy density in the mineralized tissue and (3) a defined closed effective stress (spherical stress). An iterative nonlinear three-dimensional finite element model is used to predict the apparent density distribution in the femoral head and neck for each of the three approaches. It is shown that the von Mises stress (an open effective stress) cannot accurately predict bone apparent density. It is shown that strain energy density and the defined closed effective stress can predict apparent density and that they give predictions consistent with the observed density pattern in the femoral head and neck.

Bone Density↗
PubMed1990

Theoretical stress analysis of organ culture osteogenesis.

In a series of in vitro organ culture experiments by Klein-Nulend et al., intermittent ambient hydrostatic pressure was found to significantly increase the rate of calcification in the diaphysis of mouse rudiments while proteoglycan production was increased in the noncalcified epiphyses. In this study, we have conducted finite element stress analyses of this organ culture system in an effort to better understand the distribution of local tissue stresses which were created. Furthermore, we sought to clarify how these experimental results relate to our theory for skeletal morphogenesis which postulates that endochondral ossification is accelerated in regions of octahedral shear (deviatoric) stress and inhibited in areas of compressive hydrostatic (dilatational) stress. The results of the stress analyses show that externally applied hydrostatic pressure produces significant shear stresses at the cartilage/calcified cartilage interface and pure hydrostatic pressure at the rudiment ends. The accelerated osteogenesis in regions of high shear and the increased synthesis of proteoglycans in regions of high compressive hydrostatic stress are therefore consistent with our theory which predicts the regulation of cartilage maintenance and endochrondral ossification by intermittent tissue stresses.

Animals↗
PubMed1990

Computer predictions of bone remodeling around porous-coated implants.

Computer simulations of bone remodeling in response to mechanical stresses can be used to understand normal growth and development of the skeleton or to predict the remodeling of bone in response to prosthetic devices. Using a previously derived bone maintenance theory, a technique for computing bone density distributions was applied to the proximal femur and tibia using two-dimensional, multiple-loading finite element models. The models initially represented solid, homogeneous structures. Using an iterative bone remodeling technique that relates bone apparent density to loading history, the internal distributions of apparent density and elastic modulus for the normal bones were predicted. The finite element models were then modified to represent bones in which porous-coated femoral surface replacements and tibial tray components had been implanted. The same iterative remodeling method was then applied to predict the distribution of bone around these components. The predicted bone density distributions for the natural femur and tibia agree with previously documented normal bone morphology. The predicted bone density distributions around various implanted prostheses were characteristic of the component under investigation and were consistent with clinical and experimental findings of other investigators. In the femoral head, stress shielding occurred underneath the metal surface replacement cup, resulting in lower densities in the femoral head. The addition of a central femoral cup fixation peg caused bone hypertrophy around the peg. In the tibia, the stress concentrations around the pegs also resulted in denser bone, with a concomitant decrease in bone density at more peripheral locations underneath the prosthetic tray. This remodeling technique has the potential to be an important tool in predicting the possible remodeling consequences of new implant design features.

Biomechanical Phenomena↗
PubMed1989

Role of mechanical loading in the progressive ossification of a fracture callus.

The progressive ossification pattern in a fracture callus was predicted based on a theory that relates the local stimulus for ossification to the tissue mechanical loading history. Two-dimensional finite element analyses of a fracture callus were considered at three different stages of ossification. The sites of callus ossification represented in the initial model were predicted by previous analyses relating mechanical stress and vascularity to the differentiation of mesenchymal tissue in the early callus. The zones of further ossification, bone bridging, and bone consolidation predicted in the present study were found to be similar to the ossification patterns that have been documented by other researchers. The approach used to predict fracture healing is identical to that of previous studies predicting joint morphogenesis, with the exception that fracture healing requires continuous, attached skeletal elements, whereas joint morphogenesis requires discontinuous, articulating skeletal elements.

Bony Callus↗
PubMed1989

Bone creep-fatigue damage accumulation.

Creep and fatigue tests were performed on human femoral cortical bone and the results were compared to a cumulative damage model for bone fracture. Fatigue tests in tension, compression, and reversed loading with a tensile mean stress were conducted at 2 Hz and 0.02 Hz. Load frequency had a strong influence on the number of cycles to failure but did not influence the total time to failure. Bone displayed poor creep-fracture properties in both tension and compression. The fracture surfaces of the tensile creep specimens are distinctly different than those of the compressive specimens. The results suggest that tensile cyclic loading creates primarily time-dependent damage and compressive cyclic loading creates primarily cycle-dependent damage. However, data for load histories involving both tensile and compressive loading indicate lower time to failure than predicted by a simple summation of time-dependent and cycle-dependent damage.

Adult↗
PubMed1989

Relationships between loading history and femoral cancellous bone architecture.

A theory relating bone maintenance to mechanical loading history has been applied to successfully predict the distribution of bone density and trabecular orientation in the adult proximal femur. The loading history was simulated by determining the stress fields in a two-dimensional finite element model exposed to various discrete loading cases and making assumptions about the relative number of loading cycles associated with each load case. The total stimulus to bone maintenance was then calculated by a linear superposition of the stimulus of each loading case. Based on the calculated total stimulus, the apparent density and material properties of each element were changed and the stress solutions were again determined. Using this iterative technique, the bone apparent density and orientation characteristics were predicted. The results indicate that the trabecular morphology of the femur can only be explained by considering the joint loadings from multiple directions. Contrary to the 'trajectorial theory' promoted by Wolff (The Law of Bone Remodelling, 1892), trabecular orientations predicted from our multiple-load analyses are not necessarily perpendicular and do not correspond to the principal stress directions of any one loading condition. Our predicted orientations correspond better to the drawing of bone trabecular morphology by von Meyer (Archs Anat. Physiol. wiss. Med. 34, 615-628, 1867) than to the classic drawing by Wolff and suggest that further study of the trajectorial theory is warranted.

Animals↗
PubMed1989

Scaling of long bone fracture strength with animal mass.

Most long bone fractures are the result of bending and/or torsional loading. To allometrically relate bone torsional and bending strength to animal mass (M), we define the bone strength index SB = J/dl where J = midshaft cross section polar moment of inertia, d = diameter, and l = length. In geometrically similar scaling, one would expect SB alpha M2/3. In this study, long bone geometric parameters were measured for 12 species of Artiodactyls. The relationships determined for length and diameter are similar to those reported by previous investigators (l alpha d3/4, l alpha M1/4). For the Artiodactyls studied, we found that SB alpha M0.82. Data previously collected by Biewener on a wide range of mammals (non-Artiodactyls) showed different scaling characteristics (l alpha d0.89, l alpha M0.31). However, our analysis of his data suggests roughly similar scaling of the torsional and bending strength index, SB alpha M0.77. It therefore appears that, in spite of differences in scaling of length and external diameter, the bending and torsional strengths scale similarly across a broad range of animals.

Animals↗