PubMed Health⌕ Search

Biomedical subjects

S S Kohles

Publications and source records attributed to S S Kohles.

At least 19 recordsLinked to original sources
PubMed2005

Dynamic matrix composition in engineered cartilage with stochastic supplementation of growth factors.

Dynamic extracellular matrix (ECM) synthesis is explored in a hypothesized engineered cartilage construct. Growth (alpha) and decay (beta) rate parameters are developed from a previous engineered cartilage model. The presented mathematical model was constructed from the parameterized experimental data using a deterministic and stochastic examination of ECM synthesis based on a negative feedback control mechanism. A growth factor supplementation is incorporated in a probabilistic mathematical approach. The growth factor component modified an initial deterministic model through a Gaussian white noise fluctuation. As the primary constituents of ECM, the mathematical tool is intended to characterize the probable steady state distribution of glycosaminoglycan (GAG) and collagen molecules as well as mean mass accumulation at homeostasis. Computer simulation of the models is applied to reported data from four similar chondrocyte-polymer construct culture systems. The range in rate ratios reflect the differing nature of GAG and collagen synthesis (alphaGAG/betaGAG = 4.2 to 148.6; alphacollagen/betacollagen = 8.1 to 2590.4). This technique reduced the influencing synthesis factors to a few key descriptive parameters. Additional anabolic and catabolic factors may further be built into the models.

Animals↗
PubMed2001

Direct perfusion measurements of cancellous bone anisotropic permeability.

More extensive characterization of trabecular connectivity and intertrabecular space will be instrumental in understanding disease states and designing engineered bone. This project presents an experimental protocol to define the directional dependence of transport properties as measured from healthy cancellous bone when considered as a biologic, porous medium. In the initial design phases, mature bovine bone was harvested from the femoral neck (n=6 cylinders) and distal condyle (n=4 cubes) regions and used for "proof of concept" experimentation. A power study on those results led to the presented work on 20 cubic samples (mean volume=4.09cm(3)) harvested from a single bovine distal femur. Anisotropic intrinsic permeabilities (k(i)) were quantified along the orthogonal anatomic axes (i=medial-lateral, anterior-posterior, and superior-inferior) from each individual cubic bone sample. Using direct perfusion measurements, permeability was calculated based upon Darcy's Law describing flow through porous media. The maximum mean value was associated with the superior-inferior orientation (4.65x10(-10)m(2)) in comparison with the mean anterior-posterior (4.52x10(-10)m(2)) and medial-lateral (2.33x10(-10)m(2)) direction values. The results demonstrate the anisotropic (p=0.0143) and heterogeneous (p=0.0002) nature of the tissue and encourage the ongoing quantification of parameters within the established poroelastic models.

Animals↗
PubMed2000

Elastic and physicochemical relationships within cortical bone.

The purpose of this study was to examine the relationships that exist between the elastic properties and the physicochemical properties of cortical bone in two groups of experimental animals. The animal model was the immature mutant dwarf rat, and the groups consisted of rats treated and not treated with recombinant human growth hormone (rhGH). The objective was to establish and broaden the quantifiable link between the three-dimensional form and function of bone beyond the typical unidirectional measures. This study was based on previously reported work that refined the ultrasonic elasticity technique for use with small specimens (<1.0 mm) and determined that the administration of rhGH can counter the degenerative effects produced by hormone-suppressed downregulation on the elastic and physicochemical characteristics of cortical bone. Ultrasonic wave propagation and density measurements were used previously to determine the three-dimensional (orthotropic) material properties of rat femoral cortical bone. X-ray powder diffraction, microscopic, morphometric, and biochemical analysis techniques have been used to describe physicochemical properties, including mineral crystal size, cortical porosity, mineral and nonmineral content, and microstructural characteristics. In this study, mathematical relationships between the local physicochemical (independent variable) and elastic (dependent variable) properties were formulated via linear and nonlinear regression analyses. In general, apparent density was found to have the highest level of correlation with most of the longitudinal and shear moduli (R(2) = 0.300 to 0.800). Concomitantly, mineral crystal width and cortical porosity offered the best correlations with the Poisson's ratios (R(2) up to 0.600). Wilcoxon t tests verified a significant decrease in the elastic properties in dwarf rat cortical bone after rhGH treatments (p < 0.05). Physicochemical measures of bone quality (density, crystal size) generally decreased while measures of bone quantity (cortical area, moments of inertia) generally increased (p < 0.05) after rhGH treatments. Some mineral and nonmineral properties were unchanged. This study presents a quantifiable link between cortical bone elasticity and its composite construction as measured across two dramatically different experimental groups.

Animals↗
PubMed2000

A morphometric evaluation of allograft matrix combinations in the treatment of osseous defects in a baboon model.

Recent and ongoing research efforts have been made to increase the efficacy of biomaterials as structural fillers during in vivo bony reconstructions. Although the selection of the possible material choices has grown, a biomaterial that can be physically molded to the defect/void space as well as offer biomimetic tissue regeneration has yet to be made available. With the potential success of demineralized freeze-dried bone allografts (DFDBA) combined with tendonous collagen as an effective filling material, further research should help to elucidate its use. The purpose of this study was to evaluate the regenerative healing response of five allograft mixtures via the morphology of filled, periodontal defects. Critical size mandibular and maxillary osseous defects were surgically created in six adult baboons. The filling response of four combinations of DFDBA and tendon collagen was compared with an all-collagen graft after 3 months of implantation. The overall results indicate that all combinations of DFDBA and collagen provided a better fill response than the all-collagen matrix (P < 0. 05). Statistically, however, all of the combinations were similar (P > 0.05) with a 60:40 collagen to DFDBA mass ratio resulting in the largest defect fill response.

Alveolar Ridge Augmentation↗
PubMed2000

Applications of an anisotropic parameter to cortical bone.

An equational description of the extent of the anisotropy in cortical bone is presented from both the perspective of plane stress (two-dimensional stress state) and plane strain (three-dimensional stress state). The orthotropic elastic properties that are incorporated in these states are used to provide a more thorough and refined description of planar and volumetric anisotropy in comparison to the commonly used ratio of elastic moduli. The resulting anisotropic parametric equations (eta(sigma) and eta(epsilon)) are applied to the elastic material properties measured from cortical bone within rats, dogs, cows and humans as reported in 12 previous studies. The resulting calculated parameters reduce the typically nine independent properties down to three parameters which in turn represent the degree of anisotropy within the three orthogonal planes of symmetry as are common in cortical bone. It was found that no statistical difference existed between the plane stress versus plane strain parameter in all but two studies (p > 0.10). Planar and volumetric anisotropies were compared to the isotropic condition (eta(sigma) = eta(epsilon) = 1.0) for all of the included studies. All of the studies reported cortical bone properties that were volumetrically anisotropic (p < 0.05), however, a common plane of isotropy was noted in the radial-circumferential (1-2) plane (p > 0.05). Future use of these parametric equations will allow further illucidation of the issue of mesomechanical and micromechanical levels of anisotropy within other tissues and materials of interest.

Journal Article↗
PubMed1998

A device for measuring relative angular displacement.

A simple, inexpensive, and accurate way to measure relative segmental rotations resulting from torsional loadings locally is described. To measure these rotations, we fabricated a planar spatial linkage (open-loop kinematic chain) requiring only one rotational displacement transducer. This paper describes this device, defines its kinematics, and examines its accuracy.

Biophysics↗
PubMed1997

Interstitial fluid flow in tendons or ligaments: a porous medium finite element simulation.

The purpose of this study is to describe interstitial fluid flow in axisymmetric soft connective tissue (ligaments or tendons) when they are loaded in tension. Soft hydrated tissue was modelled as a porous medium (using Darcy's Law), and the finite element method was used to solve the resulting equations governing fluid flow. A commercially available computer program (FiDAP) was used to create an axisymmetric model of a biomechanically tested rat ligament. The unknown variables at element nodes were pressure and velocity of the interstitial fluid (Newtonian and incompressible). The effect of variations in fluid viscosity and permeability of the solid matrix was parametrically explored. A transient loading state mimicking a rat ligament mechanical experiment was used in all simulations. The magnitude and distribution of pressure, stream lines, shear (stress) rate, vorticity and velocity showed regular patterns consistent with extension flow. Parametric changes of permeability and viscosity strongly affected fluid flow behaviour. When the radial permeability was 1000 times less than the axial permeability, shear rate and vorticity increased (approximately 5-fold). These effects (especially shear stress and pressure) suggested a strong interaction with the solid matrix. Computed levels of fluid flow suggested a possible load transduction mechanism for cells in the tissue.

Animals↗
PubMed1997

Thermographic strain analysis of the proximal canine femur.

Thermographic strain analysis (TSA) was used to measure the surface strain distribution of cyclically loaded canine femora. Eleven canine femora were cyclically loaded at 20 Hz in compression at 600 N (+/-200 N). After calibration with measured local strain data, it was possible to quantify the full field patterns measured from the proximal, anterior and medial cortex. The average of each TSA signal normalized by the coincident strain data (0.996) was very near to 1.0 (p = 0.999). The thermographical scans iterate the maximum compressive strains carried by the femur within the region just distal to the femoral neck. Further understanding of the strain distribution in this region is critical in the design of components that attempt to mimic anatomical load transfer after total hip arthroplasty. TSA appears to offer a promising technology as a full field experimental strain analysis method for use with biomechanical issues.

Animals↗
PubMed1997

Ultrasonic wave velocity measurement in small polymeric and cortical bone specimens.

A system was refined for the determination of the bulk ultrasonic wave propagation velocity in small cortical bone specimens. Longitudinal and shear wave propagations were measured using ceramic, piezoelectric 20 and 5 MHz transducers, respectively. Results of the pulse transmission technique were refined via the measurement of the system delay time. The precision and accuracy of the system were quantified using small specimens of polyoxymethylene, polystyrene-butadiene, and high-density polyethylene. These polymeric materials had known acoustic properties, similarity of propagation velocities to cortical bone, and minimal sample inhomogeneity. Dependence of longitudinal and transverse specimen dimensions upon propagation times was quantified. To confirm the consistency of longitudinal wave propagation in small cortical bone specimens (< 1.0 mm), cut-down specimens were prepared from a normal rat femur. Finally, cortical samples were prepared from each of ten normal rat femora, and Young's moduli (Eii), shear moduli (Gij), and Poisson ratios (Vij) were measured. For all specimens (bone, polyoxymethylene, polystyrene-butadiene, and high-density polyethylene), strong linear correlations (R2 > 0.997) were maintained between propagation time and distance throughout the size ranges down to less than 0.4 mm. Results for polyoxymethylene, polystyrene-butadiene, and high-density polyethylene were accurate to within 5 percent of reported literature values. Measurement repeatability (precision) improved with an increase in the wave transmission distance (propagating dimension). No statistically significant effect due to the transverse dimension was detected.

Animals↗
PubMed1997

Stability of proximal femoral grafts in canine hip arthroplasty.

In a canine model, the fixation stability of a prosthesis and proximal bone graft composite were measured relative to the distal femur. One group had the prosthesis graft composite cemented into the distal femur. The second group had the prosthesis graft composite press fit into the distal femur for biologic ingrowth. Displacements of the proximal femoral grafts relative to the host bone in each group were measured after ex vivo (acute with graft) implantation and 4 months after implantation. A third group with no osteotomy (acute intact) simulated perfect graft to host bone union. Relative displacements representing 6 degrees freedom (translation and rotation) were calculated from the displacement values measured by 9 eddy current transducers. Measurements of displacement were used to test the hypothesis that distal press fit fixation equals distal cement fixation at 4 months after implantation. In all cases the measured translations and rotations of the graft to implant construct were small and of a magnitude that should encourage bone ingrowth (< 0.05 mm and < 0.1 degree, respectively). The stability of the press fit group at 4 months was not significantly different from the cemented group in axial and transverse displacement during axial and transverse loading, respectively. There was no difference in stabilities at 4 months between distal press fit and cemented fixation in hip replacements requiring a proximal femoral graft.

Animals↗
PubMed1997

Finite elasticity formulations for evaluation of ligamentous tissue.

The variety of techniques used to measure the cross-sectional area of soft connective tissues during mechanical testing lead to inconsistencies in elastic descriptions. This study compares the numerical differences between finite elasticity (Eularian and Lagrangian formulations) and infinitesimal elasticity when considering stress, strain and elastic modulus of ligamentous tissue. Our results found stress differences (Cauchy versus Kirchhoff) of 22.4%, strain differences (engineering versus Green versus Almansi) as large as 14% and elastic modulus differences (Eularian versus Lagrangian) of 44% from ligament tissue sampled from rats. It is therefore critical to maintain consistent (energy conjugate) elastic formulations for reporting mechanical evaluation of soft hydrated tissue.

Algorithms↗
PubMed1996

Effect of a hypergravity environment on cortical bone elasticity in rats.

There is considerable interest in determining whether hypergravity can be used as a countermeasure for microgravity-induced bone loss. This study was conducted on 20 immature male rats in order to investigate possible elastic adaptations of cortical bone in rapidly growing rats exposed to chronic hypergravity. Ten rats were continuously centrifuged for 14 days at twice gravitational acceleration (2G) on a 12.75 foot radius centrifuge and 10 rats concurrently acted as stationary controls. The effect of hypergravity on the elastic characteristics of cortical bone was quantified via ultrasonic wave propagation. Propagation velocities of longitudinal and shear waves were measured through cubic cortical specimens from the posterior femoral diaphyses. Density was measured with an Archimedes' technique. The orthotropic elastic properties were calculated and used to compare the difference between groups. Results showed an average increase in both the Young's moduli (Eii, + 2.2%) and shear moduli (Gij, + 4.3%) with a statistically significant increase only in G12 (+15.7%, P = 0.046). The ratio of transverse to axial strain (Poisson's ratio, nuij) demonstrated statistically significant changes in nu12, nu21, nu13, and nu31 (P < 0.05). These findings suggest that although slight elastic changes were incurred via a hypergravity environment, the treatment level or duration in this study do not dramatically perturb the normal elastic behavior of cortical bone and that dramatic biomechanical differences noted in previous studies were due more to structural changes than material elasticity changes. Hypergravity applied post facto to a microgravity environment would offer further illucidation of this method as treatment for a degenerative spaceflight experience.

Animals↗
PubMed1996

Fixation of femoral allograft/prosthesis composites after 25%, 50% and 75% resection.

The relative linear and angular displacements of proximal femoral reconstructions were compared within six different replacement techniques during ex vivo axial compression, mediolateral bending, and axial torsion in dogs. Each femur was osteotomized at 25%, 50%, or 75% of its length and the proximal portion subsequently replaced using one of six techniques. The reconstruction techniques included various combinations of proximal and distal fixation methods (graft fixation/distal fixation): (1) an allograft/prosthesis composite (APC) press-fit proximally and cemented distally (press-fit/cement); (2) APC cemented proximally and distally (cement/cement); (3) APC cemented proximally and the host bone/graft interface double plated (cement/plates); (4) APC cemented proximally and secured distally with bicortical screws (cement/screws); (5) APC secured proximally and distally with bicortical screws (screws/screws); (6) Segmental proximal femoral replacement cemented into the distal femur without an allograft (no graft/cement). For axial compression and mediolateral bending, the combined resection lengths revealed no differences in linear and angular displacements, respectively, between reconstruction methods. During axial torsion, the cement/cement technique allowed larger angular displacements than all but the press-fit/cement technique which had larger displacements than the cement/screws, screws/screws, and no graft/cement groups (p < 0.0001). Overall, the measured implant stability was solid and consistent as evidenced by small amounts of relative displacement and small error values.

Animals↗
PubMed1994

Mechanical evaluation of six types of reconstruction following 25, 50, and 75% resection of the proximal femur.

The structural stiffness and the stiffness of the osteotomy site after six types of reconstruction of the proximal femur were compared by testing in axial compression, mediolateral bending, and axial torsion in a canine model. An osteotomy was carried out for 25, 50, or 75% of the length of each femur, and the proximal portion was replaced by one of five allograft/endoprosthetic composites or a segmental replacement. The reconstructions included (a) a composite press-fit proximally and cemented distally, (b) a composite cemented proximally and distally, (c) a composite cemented proximally and fixed with two plates at the allograft-host bone interface, (d) a composite cemented proximally and secured distally with bicortical screws, (e) a composite secured proximally and distally with bicortical screws, and (f) a segmental prosthesis cemented into the distal femur. The results showed that the segmental reconstruction and the reconstruction with double-plate fixation and a cemented endoprosthesis were structurally stiffer and had greater stiffness of the osteotomy site than the other reconstructions. In comparison, reconstructions that involved cement alone or cement and press-fit techniques generally were more compliant than the others, both structurally and at the osteotomy site.

Animals↗
PubMed1994

Ultrasonically determined elasticity and cortical density in canine femora after hip arthroplasty.

Effects of canine hip replacement (with a porous-coated femoral component) on the material properties of surrounding cortical bone were evaluated. The hypotheses were: (1) after four months of implantation, mechanical properties of the cortex would change, and (2) a collared implant would be associated with smaller changes than a collarless design. Unilateral total hip arthroplasty was performed in 15 mixed-breed dogs. Nine received a collared and six received a collarless femoral component. Four months after implantation, longitudinal ultrasonic wave propagation velocities and bone mineral densities (from dual energy X-ray absorptiometry) were measured in harvested femora and used to calculate the axial elastic constitutive coefficients for the cortex surrounding the implants. Results showed no difference in bone elasticity or bone density between collared and collarless designs. Significant velocity decreases from control values (p < 0.0001) were noted in all implanted femora at four months. Bone mineral densities also displayed decreased values after four months of implantation (p < 0.0145). Elastic coefficients were consistently less after four months of implantation when compared to control values (p < 0.0001). This alteration in material properties would affect load transfer into the implanted femur via the increased disparity between implant and bone stiffnesses regardless of the component design. Significant differences in the elastic coefficients between implanted and control femora support hypothesis 1. However, no group differences were found between collared and collarless implantations; thus, the study does not support hypothesis 2.

Alloys↗
PubMed1994

Mechanical properties of long bones in dogs.

Basic research in canine mechanics must be performed to better understand the forces and moments the appendicular skeleton must withstand. This type of research may allow surgeons to make substantial advances in total joint replacement and fracture fixation design and may enhance our understanding of bone remodeling and fracture occurrence in relation to exercise and trauma. In our study, craniocaudal bending stiffness, mediolateral bending stiffness, axial compressive stiffness, and torsional stiffness of the humerus, femur, radius, and tibia of dogs was determined, using nondestructive bending, compression, and torsional tests. Entire diaphyseal and middiaphyseal properties of these long bones were evaluated. Bones also were tested to failure in torsion to quantify the failure properties of these long bones. Left to right variability was examined to validate the use of contralateral limbs as the control condition for experimental studies. There were no significant right to left differences in entire diaphyseal mechanical properties for any of the long bones, except for compressive stiffness of femurs. Homotypic differences in entire diaphyseal mechanical properties, if present, ranged from 8.0 to 35% for the 4 long bones (power = 0.8). For middiaphyseal mechanical properties, there were no significant right to left differences in the 4 long bones, except for craniocaudal bending stiffness of tibias. The homotypic differences in middiaphyseal mechanical properties, if present, ranged from 7.2 to 62% for the 4 long bones (power = 0.8). In all bones and loading modes, middiaphyseal stiffness was greater than entire diaphyseal stiffness (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Fixation stability of femoral components in a canine hip replacement model.

A canine hip replacement model was used to compare fixation stability in cemented and cementless femoral components. Parameters of comparison were the load-induced positional changes of each prosthesis relative to its proximal femoral cortex, hereafter called relative displacements. Identical femoral components, with the proximal third of their stem porous-coated, were implanted in the right femurs of 10 large, mixed-breed dogs. Five were tightly fit to allow porous ingrowth, and five were cemented into the medullary canal. Four months after implantation, all femurs were harvested. A prosthesis was implanted in the left (normal) femur of each dog ex vivo with fixation identical to the contralateral limb to simulate acute postoperative fixation. Eddy current transducers measured relative displacements under application of static loads, serially applied in the axial, mediolateral, and craniocaudal directions. Thereafter, the femurs were transversely sectioned and morphologically analyzed to correlate bony apposition at the implant surface with relative displacements. We observed no difference in relative displacements between acute and 4-month-cemented groups (e.g., 0.0059 +/- 0.0021 vs. 0.0060 +/- 0.0048 mm, respectively, for 100-N axial loading measured at midstem). With cementless implantation, relative displacements of the acute group were significantly larger (p = 0.007) than those of the 4-month group (e.g., 0.236 +/- 0.257 vs. 0.097 +/- 0.129 mm, respectively, for 100-N axial loading measured at midstem). Cementless components implanted for 4 months were not significantly different than cemented components, but a trend suggested that they were still not as stable as cemented components, particularly for craniocaudal loads. Relative displacements of the 4-month, porous ingrowth group were approximately proportional to the percentage of bony apposition raised to the -1.44 power (r = 0.94).

Animals↗