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Biomedical subjects

A M Weinstein

Publications and source records attributed to A M Weinstein.

At least 73 records · Page 4Linked to original sources
PubMed1982

The mechanical behavior of normal and osteoporotic canine femora before and after hemiarthroplasty.

Nutritional secondary hyperparathyroidism (NSH) was induced in adult beagle dogs through nutritional control by feeding them a diet containing high phosphorus (1.2%) and low calcium (0.12%). A control group was fed a diet containing 0.42% phosphorus and 0.54% calcium. An osteoporotic condition was produced by NSH, which was verified with histologic, histomorphometric, and biochemical analyses. A significant increase in i-PTH level was measured at both ten and 20 weeks after initiation of the diet in experimental animals when compared with control animals. A 25% reduction in volume fraction cancellous bone and increased osteoclastic activity was found in histologic sections from the femoral heads of the NSH animals when compared with control animals. No abnormally large osteoid seam was found in the cancellous bone of the experimental animals. The results of energy dispersive radiographic analysis and bone ash analysis revealed no significant differences in calcium-to-phosphorus or mineral-to-matrix ratios between the two groups. The results of in vitro mechanical testing indicated significant differences in the strain state for the NSH and control animal femora, with the highest strains being observed in the calcar region of the NSH femora. The presence of a Co-Cr-Mo alloy femoral head prosthesis was found to alter the strain distribution significantly in both the control and NSH femora.

Animals↗
PubMed1981

Models of coupled salt and water transport across leaky epithelia.

A general formulation is presented for the verification of isotonic transport and for the assignment of a degree of osmotic coupling in any epithelial model. In particular, it is shown that the concentration of the transported fluid in the presence of exactly equal bathing media is, in general, not a sufficient calculation by which to decide the issue of isotonicity of transport. Within this framework, two epithelial models are considered: (1) A nonelectrolyte compartment model of the lateral intercellular space is presented along with its linearization about the condition of zero flux. This latter approximate model is shown to be useful in the estimation of deviation from isotonicity, intraepithelial solute polarization effects, and the capacity to transport water against a gradient. In the case of uphill water transport, some limitations of a model of fixed geometry are indicated and the advantage of modeling a compliant interspace is suggested. (2) A comprehensive model of cell and channel is described which includes the major electrolytes and the possible presence of intraepithelial gradients. The general approach to verification of isotonicity is illustrated for this numerical model. In addition, the insights about parameter dependence gained from the linear compartment model are shown to be applicable to understanding this large simulation.

Animals↗
PubMed1981

Interface mechanics of porous titanium implants.

The interfacial shear properties of bone tissue growth into porous coated Ti-6-A1-4V femoral implants have been examined as a function of the pore size of the porous surface. Three particle size range powders (297 microns, 420-500 microns, 595-707 microns) were used to fabricate cylindrical implants which were inserted into the femoral medullary canal of dogs for 6 months. Push-out tests on the removed femurs are reported and reveal: (i) that those implants residing in cortical bone exhibited significantly higher shear properties than the equivalent implants in cancellous bone and (ii) that the interfacial shear strength and stiffness decreased with increasing pore diameter within the range 175-325 microns. The extent of bone ingrowth into the surface of the implants was measured using quantitative optical microscopic techniques. This indicated that the percentage of bone which had grown into the surface was inversely proportional to the square root of the pore size and that further the shear properties of the interface were proportional to the extent of bone ingrowth.

Animals↗
PubMed1981

An evaluation of ion-textured aluminum oxide dental implants.

A study was undertaken to evaluate the ion-beam texturing of aluminum oxide as a means of providing a surface which will produce a biological prosthetic attachment. A wafflelike pattern of surface contours 150 x 75 x 35 microm deep was produced on cylindrical dental implants. The textured surfaces were compared to the as received surfaces in adult mongrel dogs. Implants were inserted into surgically modified healed extraction sites and were left in place for six months. Post-sacrifice mechanical testing was used to quantify the displacement response of the implants. The clinical, radiographic and mechanical testing evaluations did not reveal any statistically significant differences in the performance of the dental implants. However, it was observed that anatomical site and mandibular geometry with respect to implant size play a significant role in affecting implant retention.

Aluminum Oxide↗
PubMed1981

The influence of implant elastic modulus on the stress distribution around LTI carbon and aluminum oxide dental implants.

A three-dimensional finite element analysis (FEA) has been used to determine the effect of implant elastic modulus on stresses in tissues around LTI carbon and aluminum oxide dental implants. The finite element model was constructed to represent a baboon mandible containing a blade type dental implant. A three unit fixed bridge was modeled connecting the dental implant to a natural molar. The results of the study indicate that stress levels of approximately a factor of 3 lower in the crestal region can be expected for aluminum oxide implants when compared to the LTI carbon implants. It was also observed that the use of LTI carbon and aluminum oxide dental implants as an abutment in a fixed bridge results in a reduction of stresses in tissues around the natural tooth when compared to normal physiological stress levels.

Aluminum↗
PubMed1981

Coupled water transport in standing gradient models of the lateral intercellular space.

A standing gradient model of the lateral intercellular space is presented which includes a basement membrane of finite solute permeability. The solution to the model equations is estimated analytically using the "isotonic convection approximation" of Segel. In the case of solute pumps uniformly distributed along the length of the channel, the achievement of isotonic transport depends only on the water permeability of the cell membranes. The ability of the model to transport water against an adverse osmotic gradient is the sum of two terms: The first term is simply that for a well-stirred compartment model and reflects basement membrane solute permeability. The second term measures the added strength due to diffusion limitation within the interspace. It is observed, however, that the ability for uphill water transport due to diffusion limitation is diminished by high cell membrane water permeability. For physiologically relevant parameters, it appears that the high water permeability required for isotonic transport renders the contribution of the standing gradient relatively ineffective in transport against an osmotic gradient. Finally, when the model transports both isotonically and against a gradient, it is shown that substantial intraepithelial solute polarization effects are unavoidable. Thus, the measured epithelial water permeability will grossly underestimate the water permeability of the cell membranes. The accuracy of the analytic approximation is demonstrated by numerical solution of the complete model equations.

Animals↗
PubMed1981

Carbon dioxide permeability of rabbit proximal convoluted tubules.

CO2 kinetics were studied under conditions in which CO2 partial pressure and either the bicarbonate or hydrogen ion concentrations were unequal in perfusate and bath. A glass pH microelectrode and microcalorimeter were used to measure pH and total CO2 in perfused and collected fluids. Luminal appearance or disappearance of CO2 was determined from the change in concentrations of CO2 and acidic moieties of the buffers between perfused and collected fluids. The pH was not measurably affected by the tubule processes of metabolism, hydrogen ion secretion, and buffer transport because of relatively high flow rates. Since CO2 appearance and disappearance were directly related to CO2 partial pressure differences across the tubule (r = 0.94), this technique provides a valid estimate of transepithelial CO2 flux in response to the driving force of a CO2 partial pressure difference. From flux per unit of driving force and from estimates of resistances to CO2 diffusion in both internal and external unstirred layers, we obtained a transepithelial CO2 permeability of approximately 10(-4) cm3 . s-1 . cm tubule length-1. This corresponds to a diffusion coefficient through the tubule epithelium about half that through an equivalent thickness of water. We conclude that rabbit proximal convoluted tubules are so highly permeable to CO2 that even if all the filtered bicarbonate were reabsorbed by the generation of CO2 in the lumen, the tubular fluid CO2 partial pressure would exceed that of the peritubular blood by less than 4 mmHg.

Absorption↗
PubMed1981

Retrieval and analysis of intramedullary rods.

UNLABELLED: We examined ten intramedullary rods of similar design after routine retrieval from patients. Of these ten rods, four were found to exhibit cracking around their proximal third. This behavior could not be attributed to the surgical techniques employed or to the length of time in vivo of these rods. Rather, the cracking was a function of both the metal alloy used and the method of manufacture, which occasionally allowed a weld zone to be located at the point of maximum stress with the result that cracking occurred. A change in alloy composition to a low-carbon form of 316 stainless steel probably would reduce the risk of cracking. CLINICAL RELEVANCE: In the treatment of orthopaedic disorders, it is important for the operating physician to appreciate the problems that may be encountered in using implants. One of the most important of these problems is the possibility of implant failure. The present report illustrates how a combination of both metallurgical and fabrication factors may cause such an event to occur.

Adolescent↗
PubMed1980

Implant-bone interface characteristics of bioglass dental implants.

Three-dimensional finite element analysis (FEA) has been used to determine the implant-bone interface characteristics of bioglass dental implants. The results of the FEA were verified by comparison with the results of mechanical testing performed on animal implant specimens. The results of the study showed that the assumption of a discontinuous change of elastic properties at the bone-implant interface was a poor assumption for the bioglass implants. Interface elastic moduli of 354.0, 155.0, and 47.0 MPa for conditions of 25, 50, and 100% tissue attachment were determined for the bioglass implants.

Animals↗
PubMed1980

The influence of design parameters on calcar stresses following femoral head arthroplasty.

A two-dimensional finite element model (FEM) was used to investigate the effect of various prosthesis design parameters on stresses along the proximal medial aspect of the femur. Strain calculations were compared to reported experimentally measured strains for an intact femur and femur containing a prosthesis in an effort to verify the FEM results. The results of the study indicated that stress levels in the medial aspect of the femur approaching those of the intact femur are possible with a device having an elastic modulus similar to bone. It was found that acrylic bone cement had little effect on stress profiles in the medial aspect when compared to stresses for devices utilizing a direct bone to implant attachment mechanism. It was also found that reduction in implant stem length and elastic modulus of tissue present at the prosthesis--bone interface could be used to increase stresses in the medial aspect; however, their effects were not as dramatic as reducing the elastic modulus of the stem material.

Arthroplasty↗
PubMed1979

Electrolyte transport across a simple epithelium. Steady-state and transient analysis.

A simple transporting epithelium is represented as a cellular compartment, compliant in all dimensions, and a paracellular channel, of arbitrary shape, between well-stirred mucosal ans serosal baths. The equations for mass balance, Poiseuille flow, and the Nernst-Planck equation are used to describe the continuous behavior of the system along cell and channel, whereas passive transport across membranes is given by the relations of Kedem and Katchalsky. Time-dependent terms are retained to permit study of transient phenomena. Boundary conditions at the baths demand only mass conservation and specify no a priori estimates of the system variables. A numerical model containing Na+,K+,Cl-, and impermeant cellular anions is formulated with membrane parameters taken from the literature on Necturus gallbladder. The differential equations are represented as a finite difference scheme and solved using Newton's method. It appears that apical cellular NaCl cotransport is necessary to obtain a reasonable cell chloride concentration. Investigation of the osmolality of the transepithelial flow shows that at steady state a leaky epithelium cannot separate baths of substantially different tonicity, although this does not guarantee isotonic transport between equiosmolar media. Changes in bath pressure, application of transepithelial electrical potential, and simulation of ion-substitution experiments are performed to understand the role of membrane permeabilities in determining the dynamic behavior of the epithelium.

Animals↗
PubMed1979

Clinical, radiographical, and histological evaluation of porous rooted polymethylmethacrylate dental implants.

Forty-three porous rooted polymethylmethacrylate (PMMA) dental implants were inserted into twenty-four dogs. Successful implants were maintained for up to three years. Histological sections with the implants in situ of implants revealed a soft tissue-implant interface similar to natural teeth. Bone and fibrous tissue ingrowth into the pores attached the implant to the bone. Failures were attributed to mechanical weakness of the implant, thin buccal cortical bone, and excessive implant-gingiva interface.

Alveolar Process↗