RESUMEN
We present new results on the iron dynamics in the icosahedral quasicrystal i-AlCuFe and two cubic approximants as well as the non-approximant Al-Cu-Fe cubic B2 phase. Conventional Mössbauer spectroscopy is used as well as, for the i-AlCuFe phase, high Doppler velocity Mössbauer spectroscopy and quasielastic neutron scattering for samples with different Fe isotope contents. We show that in the i-phase the Fe Lamb-Mössbauer recoilless fraction decreases below that predicted for lattice vibrations alone for temperatures above about 550 K. This decrease is correlated with the onset of a quasielastic signal seen in both Mössbauer and neutron backscattering spectroscopy, which indicates the presence above 550 K of Fe jump processes confined in a local cage. The timescale of the Fe jumps (660 ps at 1000 K) and their temperature dependence differ widely from those of Cu jumps in the same i-AlCuFe quasicrystal. From the temperature dependence of the quadrupole splitting of the (57)Fe Mössbauer spectrum, one can distinguish two kinds of Fe jumps, one starting at 550 K and the second above 800 K. In the two cubic approximants, a loss in the Fe recoilless fraction also occurs above 550 K, revealing the same kind of Fe dynamics as in the i-phase but the effect is smaller. On the other hand, no anomalous Fe dynamics (other than lattice vibrations) is detected in the B2-AlCuFe phase. Since the cubic approximants possess similar local configurations as the quasicrystal, we conclude that locally a Penrose tile description is appropriate. This shows that the detected Fe jumps can be interpreted in terms of phason-like local tiling flips.
RESUMEN
Connective tissue adaptation, including the development of cartilaginous anlagen into bones, is widely believed to be related to dynamic, intermittent load and stress histories. Static stresses, on the other hand, are generally believed deleterious in tissue adaptation. Using serial MRI in a natural human experiment (manipulation and corrective casting of infant clubfoot), we have observed casting produces two effects: (1) the well recognized change in relative positions of the hindfoot anlagen; (2) a newly recognized immediate shape change in the anlagen. These changes seemingly enhance the rate of growth of the anlagen and of the ossific nucleus. The shape change or deformation in the anlagen would occur as a result of alterations in the magnitudes and directions of loading from soft tissue attachments and muscle activity and would necessarily be associated with changes in the stress states within the anlagen and, when present, the ossific nuclei. Given the known role of load and stress history in tissue adaptation, we presume the reduced stress histories influence the enhanced growth rates. These observations contradict some current theories of tissue adaptation since static, rather than dynamic stresses play a crucial role in accelerating the growth and development of anlagen in the infant clubfoot.
Asunto(s)
Cartílago/patología , Pie Equinovaro/patología , Fenómenos Biomecánicos , Desarrollo Óseo , Cartílago Articular/patología , Pie Equinovaro/diagnóstico , Tejido Conectivo/patología , Humanos , Recién Nacido , Imagen por Resonancia Magnética , Modelos Biológicos , Osteogénesis , Estrés Mecánico , Factores de TiempoRESUMEN
To explore the potential role that load-induced fluid flow plays as a mechano-transduction mechanism in bone adaptation, a lacunar-canalicular scale bone poroelasticity model is developed and implemented. The model uses micromechanics to homogenize the pericanalicular bone matrix, a system of straight circular cylinders in the bone matrix through which bone fluids can flow, as a locally anisotropic poroelastic medium. In this work, a simplified two-dimensional model of a periodic array of lacunae and their surrounding systems of canaliculi is used to quantify local fluid flow characteristics in the vicinity of a single lacuna. When the cortical bone model is loaded, microscale stress, and strain concentrations occur in the vicinity of individual lacunae and give rise to microscale spatial variations in the pore fluid pressure field. Furthermore, loading of the bone matrix containing canaliculi generates fluid pressures in the contained fluids. Consequently, loading of cortical bone induces fluid flow in the canaliculi and exchange of fluid between canaliculi and lacunae. For realistic bone morphology parameters, and a range of loading frequencies, fluid pressures and fluid-solid drag forces in the canalicular bone are computed and the associated energy dissipation in the models compared to that measured in physical in vitro experiments on human cortical bone. The proposed model indicates that deformation-induced fluid pressures in the lacunar-canalicular system have relaxation times on the order of milliseconds as opposed to the much shorter times (hundredths of milliseconds) associated with deformation-induced pressures in the Haversian system.
Asunto(s)
Matriz Ósea/fisiología , Simulación por Computador , Modelos Teóricos , Animales , Osteón/fisiología , Humanos , ReologíaRESUMEN
To explore the hypothesis that load-induced fluid flow in bone is a mechano-transduction mechanism in bone adaptation, unit cell micro-mechanical techniques are used to relate the microstructure of Haversian cortical bone to its effective poroelastic properties. Computational poroelastic models are then applied to compute in vitro Haversian fluid flows in a prismatic specimen of cortical bone during harmonic bending excitations over the frequency range of 10(0) to 10(6) Hz. At each frequency considered, the steady state harmonic response of the poroelastic bone specimen is computed using complex frequency-domain finite element analysis. At the higher frequencies considered, the breakdown of Poisueille flow in Haversian canals is modeled by introduction of a complex fluid viscosity. Peak bone fluid pressures are found to increase linearly with loading frequency in proportion to peak bone stress up to frequencies of approximately 10 kHz. Haversian fluid shear stresses are found to increase linearly with excitation frequency and loading magnitude up until the breakdown of Poisueille flow. Tan delta values associated with the energy dissipated by load-induced fluid flow are also compared with values measured experimentally in a concurrent broadband spectral analysis of bone. The computational models indicate that fluid shear stresses and fluid pressures in the Haversian system could, under physiologically realistic loading, easily reach the level of a few Pascals, which have been shown in other works to elicit cell responses in vitro.
Asunto(s)
Líquidos Corporales/fisiología , Osteón/fisiología , Modelos Biológicos , Reología/métodos , Adaptación Fisiológica/fisiología , Anisotropía , Huesos/fisiología , Simulación por Computador , Elasticidad , Análisis de Elementos Finitos , Humanos , Porosidad , Presión , Viscosidad , Soporte de Peso/fisiologíaRESUMEN
The American Association of Hip and Knee Surgeons (AAHKS) distributed a survey to its members exploring practice patterns implemented to prevent venous thromboembolic disease (VTED) in patients undergoing total hip arthroplasty (THA) and total knee arthroplasty (TKA). Of 720 (33%) members, 236 responded. Prophylaxis was prescribed for 100% of patients during the course of hospitalization for THA and TKA. Warfarin was the commonest pharmacologic treatment used for THA (66%) and TKA (59%) patients. Low-molecular-weight heparin was used in 16% of THA patients and 18% of TKA patients. The most commonly employed mechanical modality was pneumatic devices in THA (51%) and TKA (50%). Universal acceptance of the need for prophylaxis administration for patients undergoing THA and TKA is shown. The method and duration remain highly variable; although the survey illustrates such variation, it suggests there is no one best method of prophylaxis.
Asunto(s)
Anticoagulantes/administración & dosificación , Artroplastia de Reemplazo de Cadera , Artroplastia de Reemplazo de Rodilla , Vendajes , Complicaciones Posoperatorias/prevención & control , Embolia Pulmonar/prevención & control , Trombosis de la Vena/prevención & control , Anticoagulantes/efectos adversos , Aspirina/administración & dosificación , Aspirina/efectos adversos , Causas de Muerte , Recolección de Datos , Heparina de Bajo-Peso-Molecular/administración & dosificación , Heparina de Bajo-Peso-Molecular/efectos adversos , Humanos , Complicaciones Posoperatorias/mortalidad , Embolia Pulmonar/mortalidad , Sociedades Médicas , Estados Unidos , Trombosis de la Vena/mortalidad , Warfarina/administración & dosificación , Warfarina/efectos adversosRESUMEN
To explore the hypothesis that mechanical excitation-induced fluid flow and/or fluid pressure are potential mechanical transduction mechanisms in bone adaptation, a complementary experimental and analytical modeling effort has been undertaken. Experimentally, viscoelastic tan delta properties of saturated cortical bovine bone were measured in both torsion and bending, and significant tan delta values in the 10(0)-10(5) Hz range were observed, although the nature of the damping is not consistent with a fluid pressure hypothesis. Analytically, micromechanically based poroelasticity models were exercised to quantify energy dissipation associated with load-induced fluid flow in large scale channels. The modeling results indicate that significant damping due to fluid flow occurs only above 1 MHz frequencies. Together, the experimental and analytical results indicate that at excitation frequencies presumed to be physiological (1-100 Hz), mechanical loading of bone generates extremely small pore fluid pressures, making the hypothesized fluid-pressure transduction mechanism upon osteocytes untenable.
Asunto(s)
Huesos/fisiología , Animales , Ingeniería Biomédica , Líquidos Corporales/fisiología , Bovinos , Elasticidad , Modelos Biológicos , Presión , Análisis Espectral/métodos , Estrés Mecánico , ViscosidadRESUMEN
Bone adapts to its mechanical environment, and, since the late 1800s, investigators have presumed that this adaptation relates to strain magnitude. Indeed, overwhelming evidence supports the view that either strain or some strain-related quantity stimulates bone adaptation or remodeling. Virtually all investigators, implicitly or explicitly, assume that the level of strain magnitude responsible for bone adaptation is that measured by strain gauges in vivo (i.e., 100-2500 microstrain) and that bone cells are directly deformed by strained matrix. We present evidence that bone cell deformation in this range does not cause bone adaptation. First, bone cells in vitro typically do not respond to average (continuum) levels of strain magnitude. Second, bone cells in vitro do respond to fluid flow-induced shear stresses in these ostensible physiological ranges. Third, in vivo strain magnitudes presumed to stimulate remodeling reflect only averages, and not local peaks, which are 2-15 times higher. Thus, we hypothesize that sensing cells do not respond to levels of strain presumed to be physiological.
Asunto(s)
Remodelación Ósea , Huesos/citología , Adaptación Fisiológica , Adulto , Matriz Ósea/fisiología , División Celular , Humanos , Osteoblastos/fisiología , Osteoclastos/fisiología , Estrés MecánicoRESUMEN
Many types of cells, when grown on the surface of a cyclically stretched substrate, align away from the stretch direction. Although cell alignment has been described as an avoidance response to stretch, the specific deformation signal that causes a cell population to become aligned has not been identified. Planar surface deformation is characterized by three strains: two normal strains describe the length changes of two initially perpendicular lines and one shear strain describes the change in the angle between the two lines. The present study was designed to determine which, if any, of the three strains was the signal for cell alignment. Human fibroblasts and osteoblasts were grown in deformable, rectangular, silicone culture dishes coated with ProNectin, a biosynthetic polymer containing the RGD ligand of fibronectin. 24 h after plating the cells, the dishes were cyclically stretched at 1 Hz to peak dish stretches of 0% (control), 4%, 8%, and 12%. After 24 h of stretching, the cells were fixed, stained, and their orientations measured. The cell orientation distribution was determined by calculating the percent of cells whose orientation was within each of eighteen 5 degrees angular intervals. We found that the alignment response was primarily driven by the substrate strain which tended to lengthen the cell (axial strain). We also found that for each cell type there was an axial strain limit above which few cells were found. The axial strain limit for fibroblasts, 4.2 +/- 0.4%, (mean +/- 95% confidence), was lower than for osteoblasts, 6.4 +/- 0.6%. We suggest that the fibroblasts are more responsive to stretch because of their more highly developed actin cytoskeleton.
Asunto(s)
Fibroblastos/fisiología , Osteoblastos/fisiología , Adolescente , Adulto , Fenómenos Biomecánicos , Tamaño de la Célula , Células Cultivadas , Humanos , Masculino , Persona de Mediana Edad , Periodicidad , Estimulación Física , Estrés MecánicoRESUMEN
Osteoporosis is characterized by excessive loss of bone mass, while exercise is believed to maintain or enhance bone mass. Since exercise marginally affects osteoporosis, we wondered whether bone cells from osteoporotic patients would fail to respond to strain. Primary human bone-like cultures were obtained from females over age 60 with hip arthroplasty procedures performed for either osteoporotic fracture (n = 8) or non-osteoporotic osteoarthrosis (n = 5). Cultures (96,000 cell/cm2) were strained in rectangular optically clear silastic wells. Three periods of uniaxial substratum strain (1000 micro-strain, 1 Hz, 10,000 cycles, sine wave) were provided every 24 h using a four-point bending, computer-controlled device. Results at a frequency of 1 Hz were compared to cultures exposed to 20 Hz with bone cells derived from one osteoarthritic subject. Alterations in protein level expression of bone-related proteins were determined using a semi-quantitative confocal approach along with enzyme (alkaline phosphatase) activity and enzyme mRNA copy number using cRNA RT-PCR. Strain did not alter levels of bone-related protein levels, enzyme activity, or steady state copy number per cell in response to strain in either group. Strained cultures from osteoporotic patients exhibited little variation from unstrained controls, while individual cultures from osteoarthritic patients exhibited increases in one protein or the other. The results suggest that bone cells from older individuals may not be responsive to continuum levels of strain anticipated with vigorous activity.
Asunto(s)
Envejecimiento/fisiología , Huesos/fisiología , Anciano , Anciano de 80 o más Años , Fosfatasa Alcalina/genética , Fosfatasa Alcalina/metabolismo , Secuencia de Bases , Fenómenos Biomecánicos , Huesos/citología , Células Cultivadas , Colágeno/metabolismo , Cartilla de ADN/genética , Femenino , Humanos , Persona de Mediana Edad , Osteoartritis/patología , Osteoartritis/fisiopatología , Osteonectina/metabolismo , Osteopontina , Osteoporosis Posmenopáusica/patología , Osteoporosis Posmenopáusica/fisiopatología , ARN Mensajero/genética , ARN Mensajero/metabolismo , Sialoglicoproteínas/metabolismo , Estrés MecánicoRESUMEN
A mathematical model is developed for calculating the contact stress distribution in the hip for a known resultant hip force and characteristic geometrical parameters. Using a relatively simple single nonlinear algebraic equation, the model can be readily applied in clinical practice to estimate the stress distribution in the most frequent body positions of everyday activities. This is demonstrated by analyzing the data on the resultant hip force obtained from laboratory observations where a stance period of gait is considered.
Asunto(s)
Marcha/fisiología , Articulación de la Cadera/fisiología , Modelos Biológicos , Acetábulo/fisiología , Humanos , Rotación , Estrés Mecánico , Soporte de Peso/fisiologíaRESUMEN
STATEMENT OF PROBLEM: Dental implant failure rates for osseointegration are greater in the highly atro-phic maxilla. Presuming higher failure rates relate to strain-driven adaptation, an enhanced understanding of formative bone response to loading (modeling) and maintenance of an integrated state (remodeling) should improve treatment. PURPOSE: To understand the role of occlusal loading on long-term osseointegration in areas of compromised cancellous bone, a review of the salient features of adaptive bone modeling and remodeling is presented with an emphasis on cancellous bone responses. CONCLUSIONS: The ability for dental implants to maintain a long-term stable interface in the maxilla lies in the ability of trabecular bone to maintain adequate local material (strength) and architectural (connectivity) properties. In this discussion, an emphasis has been placed on understanding how trabecular bone can respond to the mastication-induced loading environment on an implant.
Asunto(s)
Remodelación Ósea/fisiología , Implantación Dental Endoósea , Implantes Dentales , Oclusión Dental , Maxilar/fisiología , Adaptación Fisiológica/fisiología , Atrofia , Fuerza de la Mordida , Fracaso de la Restauración Dental , Humanos , Masticación , Maxilar/patología , Maxilar/cirugía , Oseointegración , Estrés Mecánico , Soporte de Peso/fisiologíaAsunto(s)
Ortopedia/historia , Austria , Distinciones y Premios , Historia del Siglo XIX , Historia del Siglo XX , Humanos , Estados UnidosAsunto(s)
Articulaciones/inervación , Articulaciones/fisiología , Mecanorreceptores/fisiología , Ligamento Cruzado Anterior/inervación , Lesiones del Ligamento Cruzado Anterior , Marcha , Humanos , Traumatismos de la Rodilla/fisiopatología , Ligamentos/inervación , Mecanorreceptores/anatomía & histología , Músculo Esquelético/inervación , Regeneración Nerviosa , Propiocepción , Reflejo/fisiologíaRESUMEN
Osteoarthrosis of the hip may be treated by osteotomy, but surgeons report variable results, and there is no consensus regarding which method to use in choosing the type of osteotomy. The authors defined three biomechanical measures of hip incongruence (characteristic point locus, joint space, and contact region) and developed a two-dimensional frontal plane model to compute joint incongruence over the joint range of motion during normal activities of daily living. The preoperative measures were calculated for 38 patients who had undergone osteotomy at least 5 years earlier. The authors calculated the measures throughout a functional range of motion after 13 stimulated varus or valgus osteotomies. A logistic regression analysis determined which, if any, of the three measures, in conjunction with other clinical variables, correctly predicted outcome. The average values for the characteristic point locus, joint space, and contact region measures ranged from 0.260 cm to 2.127 cm, 0.963 cm2 to 9.327 cm2, and 0.063 cm to 4.230 cm, respectively. Unimodal behavior between two of the three measures (joint space and contact region) and osteotomy angle were observed, suggesting these two would be the most useful in predicting an optimal osteotomy. The most significant independent variable predicting clinical outcome was the joint space measure. This supports the potential of an optimization approach for determining the best angle for a hip osteotomy.
Asunto(s)
Articulación de la Cadera/patología , Osteoartritis/patología , Osteotomía/métodos , Acetábulo/diagnóstico por imagen , Acetábulo/patología , Acetábulo/fisiopatología , Acetábulo/cirugía , Actividades Cotidianas , Algoritmos , Artralgia/diagnóstico por imagen , Artralgia/patología , Artralgia/fisiopatología , Artralgia/cirugía , Fenómenos Biomecánicos , Simulación por Computador , Cabeza Femoral/diagnóstico por imagen , Cabeza Femoral/patología , Cabeza Femoral/fisiopatología , Cabeza Femoral/cirugía , Estudios de Seguimiento , Predicción , Articulación de la Cadera/diagnóstico por imagen , Articulación de la Cadera/fisiopatología , Articulación de la Cadera/cirugía , Humanos , Modelos Logísticos , Modelos Biológicos , Osteoartritis/diagnóstico por imagen , Osteoartritis/fisiopatología , Osteoartritis/cirugía , Osteotomía/clasificación , Radiografía , Rango del Movimiento Articular , Estudios Retrospectivos , Rotación , Resultado del TratamientoRESUMEN
Locations, magnitudes, and directions of pelvic muscle and acetabular contact forces are important to model the effects of abnormal conditions (e.g., deformity, surgery) of the hip accurately. Such data have not been reported previously. We computed the three-dimensional locations of all pelvic muscle and acetabular contact forces during level gait. The approach first required computation of the intersegmental joint resultant forces and moments using limb displacement history, foot-floor forces, and estimated limb inertial properties from one subject. The intersegmental resultant moments were then distributed to the muscles using a 47-element muscle model and a non-linear optimization scheme. Muscle forces were vectorally subtracted from the intersegmental resultants to compute the acetabular contact forces. While the peak joint force magnitudes are similar to those reported previously for the femur, the directions of pelvic contact forces and muscle forces varied considerably over the gait cycle. These variations in contact force directions and three-dimensional forces could be as important as the contact force magnitudes in performing experimental or theoretical studies of loads and stresses in the periacetabular region.