Glucose transport in a model of the rat proximal tubule epithelium.
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Animals↗
Biomedical subjects
A M Weinstein
Publications and source records attributed to A M Weinstein.
The overshoot phenomenon in cotransport.
Based on simplified equations, the overshoot curve experimentally observed with Na+-linked cotransport of neutral substrate (sugars or amino acids) has been simulated by computer. The approach is in principle similar to that of previous approaches (Weiss, S.D., McNamara, P.D. and Segal, S. (1981) J. Theor. Biol. 91, 597-608), but more general; in particular, it includes the effect of electrical membrane potential difference, and the quantitative relationship between height of peak and certain transport parameters, such as maximum rate, dissociation constant of ternary complex, electric charge of translocator, respectively. In addition, it tests two alternative models with respect to the rate-determining step: the translocation, on the one hand, and the association/dissociation of the ligands at the translocator site, on the other. The major findings are the following: (1) An overshoot can be obtained similar to that usually found experimentally, provided that maximum rate and affinity between translocator and transport of solute exceed certain minimum values. (2) The overshoot effect with Na-linked cotransport is enhanced by a negative membrane potential (inside relative to outside) and decreased by a positive potential. In the first case, the peak is higher and occurs faster. In the latter case, the peak is lower and delayed. (3) The effect of an electric potential difference on the overshoot curve does not depend appreciably on the charge of the empty translocator, except if the translocation of the latter is strongly rate-limiting. (4) To obtain an overshoot curve, it is not necessary that the translocation step be rate-limiting, contrary to what has been postulated previously (Läuger, P. (1980) J. Membrane Biol. 57, 163-178).
Amino Acids↗
Transport by epithelia with compliant lateral intercellular spaces: asymmetric oncotic effects across the rat proximal tubule.
Mathematical models of the proximal tubule are considered in which the lateral intercellular spaces distend in response to increased interstitial pressures and basal outlet permeabilities increase as a result of interspace widening. An approximate analytical model of the interspace reveals the possibility that such compliance may introduce an asymmetry to the effect of protein oncotic forces on transepithelial volume flow. Peritubular oncotic forces close the interspace, enhance interspace hypertonicity, and thus substantially increase volume reabsorption (enhanced intraepithelial solute-solvent coupling). The model also predicts a decline in epithelial water permeability (Lp), salt reflection coefficient, and salt permeability, with the application of peritubular protein. When parameters are chosen so as to represent the rat proximal tubule, the predicted effect on solute permeability is comparable to the observed changes in electrical resistance of the epithelium. However, when the luminal solution is slightly hypotonic to blood and proximal reabsorption has become isosmotic, the models show relatively small protein effects, which are dependent upon cell and tight junction permeabilities and are little influenced by interspace compliance. The capability of such models to represent the peritubular protein enhancement of isosmotic salt and water reabsorption by the proximal tubule in vivo is questioned.
Animals↗
Pharmacotherapy of asthma.
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Adrenal Cortex Hormones↗
Quantitative histologic evaluation of LTI carbon, carbon-coated aluminum oxide and uncoated aluminum oxide dental implants.
The response of mandibular bone to identical geometry LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide blade-type dental implants in baboons for 2 years was evaluated using histologic, microradiographic, and scanning electron microscopic methods. In addition, a quantitative histologic analysis was performed identifying the type, amount, and distribution of tissue surrounding the dental implant systems. This is the final phase of a study investigating the effect of implant elastic modulus and implant surface chemical composition on the performance of dental implants. Previous studies have utilized clinical and radiographic evaluations, postretrieval mechanical testing, and finite element stress analysis to evaluate the dental implant performance. The results of the histologic study revealed a direct implant-bone interface with no intervening soft tissue in 16 of the 21 implants (76%). A fibrous tissue interface was observed in 5 of 21 implants (24%). Quantitative histologic results for the implants with a direct implant-bone interface showed statistically larger crestal cortical plates (p less than 0.05) and greater area fraction crestal cancellous bone (p less than 0.05) in the LTI carbon implant compared to the carbon-coated and uncoated aluminum oxide implants. The carbon-coated and uncoated aluminum oxide implants demonstrated statistically greater area fraction cancellous bone at the inferior region of the implant (p less than 0.05) and thinned and reduced crestal cortical plates when compared to the LTI carbon implants. The results indicate that significant stress shielding of the crestal bone had occurred with the rigid carbon-coated and uncoated aluminum oxide implants when compared to the LTI carbon implants which had a material elastic modulus similar to cortical bone. Based upon the histologic results, it appears that the LTI carbon implants with the direct implant-bone interface exhibited a greater potential for long-term successful performance compared to the aluminum oxide substrate implants.
Aluminum↗
The retention mechanics of LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide dental implants.
The in vitro mechanical behavior of identical geometry LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide blade-type dental implants has been evaluated using rosette type strain gauges and a LVDT system. The implants served as a distal abutment for a three-unit fixed prosthesis and functioned for a period of 2 years in female baboons. The comparison of the LTI carbon implants to the carbon-coated aluminum oxide implants allowed for a study of the effect of implant elastic modulus on the mechanical behavior, while the comparison of the carbon-coated and uncoated aluminum oxide implants allowed for a study of the effect of chemical composition at the tissue-implant interface. The results of the mechanical testing indicate that the implant displacement response of the LTI carbon implants was greater than that of the carbon-coated and uncoated aluminum oxide implants. Little difference in displacement response was observed for the carbon-coated and uncoated aluminum oxide implants. No clear trend in the strain response of the buccal mandibular bone was observed for the three implant systems. The greatest differences in strain response, however, was observed between the uncoated aluminum oxide implants and the LTI carbon and carbon-coated aluminum oxide implants.
Aluminum Oxide↗
Nonequilibrium thermodynamic model of the rat proximal tubule epithelium.
The rat proximal tubule epithelium is represented as well-stirred, compliant cellular and paracellular compartments bounded by mucosal and serosal bathing solutions. With a uniform pCO2 throughout the epithelium, the model variables include the concentrations of Na, K, Cl, HCO3, H2PO4, HPO4, and H, as well as hydrostatic pressure and electrical potential. Except for a metabolically driven Na-K exchanger at the basolateral cell membrane, all membrane transport within the epithelium is passive and is represented by the linear equations of nonequilibrium thermodynamics. In particular, this includes the cotransport of Na-Cl and Na-H2PO4 and countertransport of Na-H at the apical cell membrane. Experimental constraints on the choice of ionic conductivities are satisfied by allowing K-Cl cotransport at the basolateral membrane. The model equations include those for mass balance of the nonreacting species, as well as chemical equilibrium for the acidification reactions. Time-dependent terms are retained to permit the study of transient phenomena. In the steady state the energy dissipation is computed and verified equal to the sum of input from the Na-K exchanger plus the Gibbs free energy of mass addition to the system. The parameter dependence of coupled water transport is studied and shown to be consistent with the predictions of previous analytical models of the lateral intercellular space. Water transport in the presence of an end-proximal (HCO3-depleted) luminal solution is investigated. Here the lower permeability and higher reflection coefficient of HCO3 enhance net sodium and water transport. Due to enhanced flux across the tight junction, this process may permit proximal tubule Na transport to proceed with diminished energy dissipation.
Animals↗
Prognostic patterns in acute renal failure: the New York Hospital, 1981-1982.
Sixty four patients who developed acute renal failure at The New York Hospital between July 1981 and June 1982 were studied. The average age was found to be 59.5 years. The overall mortality rate was 62.5%. Patients with non-oliguric renal failure had a lower mortality rate (25%) than those with oliguric renal failure (79%). Those patients with non-oliguric renal failure were more likely to have a discrete cause of renal failure (drugs) and to be in a more stable cardiovascular status. Tachycardia, hypotension, respiratory failure, and documented (or presumed) sepsis all adversely affected prognosis.
Acute Kidney Injury↗
Femoral head stresses following cortical bone grafting for aseptic necrosis. A finite element study.
A two-dimensional finite element model was used to compare the femoral head stress distribution of an intact femur with that of a femur following core biopsy and biopsy with cortical grafting. There was a decrease in superior and lateral femoral head stresses of up to 50% by an incorporated and properly placed cortical graft. Core biopsy and cortical grafting short of the subchondral plate produced stress-concentrating effects. A medially placed cortical graft did not stress-relieve the superolateral femoral head. Thus, if cortical grafting is to be undertaken, the graft should extend from the lateral cortex to the subchondral plate, transfixing the length of the necrotic segment. Because of deleterious effects on femoral head stress distribution, core biopsy without cortical grafting should be discouraged or only very cautiously used.
Bone Transplantation↗
The influence of implant geometry on the stress distribution around dental implants.
A three-dimensional finite element stress analysis has been used to investigate the influence that variations in the infrastructural geometry of a blade-type dental implant have on the stress distribution around LTI carbon and aluminum oxide implants. The finite element model was constructed based upon an analysis of serial sections of a retrieved implant specimen. In addition to the implant, the finite element model contained a three-unit fixed bridge connected to a natural molar with periodontal membrane. The removal of the bridge allowed for the study of freestanding implants and molar. Variations of the implant blade geometry were found to produce significant changes in the stress distributions around bridged and freestanding aluminum oxide implants. Very little effect, however, was observed around the LTI carbon implants. A comparison of the stresses around the freestanding molar and the stresses around the bridged and freestanding implants was made to determine the implant design that came closest to reproducing the stress state around the modeled molar. The LTI carbon system that best achieved this stress state was found to be a full-blade implant used in conjunction with a tooth as an abutment in a fixed bridge. The aluminum oxide system that best achieved this stress state was found to be of the post or short-blade design used as a freestanding implant.
Aluminum Oxide↗
Parameters affecting the stress distribution around LTI carbon and aluminum oxide dental implants.
Three-dimensional finite element stress analysis was used to study the effects that implant neck geometry and the tissue properties at the implant-bone interface have on the stress distribution around free-standing blade- and post-type LTI carbon and aluminum oxide dental implants. Implants having neck flares of 13 degrees and 26 degrees were studied. In addition, to simulate fibrous encapsulation of the implant as opposed to a direct bone apposition retention mechanism, a soft tissue interposing layer between implant and bone was also modeled. The results of the study indicate that a reduction in neck flare from 26 degrees to 13 degrees was a positive design change for blade- and post-type LTI carbon implants and blade-type aluminum oxide implant but not for post-type aluminum oxide implant. The results of the study indicate the presence of fibrous tissue surrounding the implants may be indicative of a failing system and may be the result of either hypophysiological stress (aluminum oxide implants) or hyperphysiological stress (LTI carbon implants).
Aluminum↗
A clinical comparison of LTI carbon, alumina, and carbon-coated alumina blade-type implants in baboons.
The clinical performance of LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide blade-type dental implants was studied in baboons. The objective of the study was to determine the effect that implant material elastic modulus and surface composition have on implant performance. Clinical parameters of mobility, sulcus depth, soft tissue characteristics, and radiographic appearance were used in the evaluation. The implants were placed in healed extraction sites in adult female baboons and were used as a distal abutment for a three-unit fixed gold prosthesis. The restorations were allowed to assume normal occlusal function and were left in situ for a period of two years. The radiographic and sulcus depth measurements appeared inferior for the LTI carbon implants and best for the carbon-coated aluminum oxide implants. No differences in mobility or soft tissue characteristics were noted for the three implant systems. Two implants both in the same animal--one LTI carbon and one uncoated aluminum oxide--were definite clinical failures. The results of the study indicate that an elastic modulus mismatch between implant and bone is not an a priori cause of implant failure and that the implant surface composition had little apparent effect on the clinical and radiographic performance of these implant materials.
Aluminum↗
The influence of the bone-implant interface stiffness on stress profiles surrounding Al2O3 and carbon dental implants.
Dental implants have been used and studied for the replacement of missing teeth for many years. Finite element stress analysis (FESA) has previously been used in their evaluation to study the effect of various design parameters on induced stresses. A two-dimensional FESA was used to evaluate the effect that the implant-bone interface elastic modulus has on the stress distribution around LTI carbon and aluminum oxide dental implants. The results of this investigation indicate that a soft tissue interface between implants and bone negates the effect of implant elastic modulus and results in stress profiles that were almost identical for the LTI carbon and aluminum oxide implants.
Aluminum↗
A three-dimensional finite element analysis of a porous rooted Co-Cr-Mo alloy dental implant.
A three-dimensional finite element model was used to investigate the biomechanical response of a porous rooted Co-Cr-Mo alloy implant. The load displacement responses calculated from the finite element model were verified with experimentally-determined values for three-cylindrical, porous rooted Co-Cr-Mo alloy dental implants retrieved from canines after two years in function. Models were developed based on histological analyses of tissue surrounding the implants. The results of the study indicate that the assumption of a direct bone-to-implant interface (i.e., an ankylosed implant) may not be a good representation for a porous rooted implant retained by bone ingrowth.
Animals↗
A model for the implant-bone interface characteristics of porous dental implants.
The use of computer-aided design techniques, such as finite element stress analysis, to evaluate implant designs requires that the mechanics at the interface between the implant and the bone be characterized. This problem is accentuated when implants designed to achieve stabilization by tissue growth into porous coatings are used. A model has been developed to represent the mechanical properties of the porous material-bone interface. The model, which accounts for the mechanics of bone tissue grown into a porous implant surface, was also developed so that it could be incorporated into a three-dimensional finite-element analysis. The analytical results from a three-dimensional finite-element model of a cylindrical porous rooted dental implant system incorporating the interface model were compared to the results of mechanical tests performed on similar implants which had resided in canines for two yr. The results from the finite element analysis using the interface model were found to be in better agreement with the experimental results than when a direct bone-to-porous-material interface was assumed.
Animals↗
The effects of intramedullary implants on bone strains and remodeling in the femur.
Bone remodeling and strain distribution in the femur was studied after implantation of LTI pyrolytic carbon, bioglass coated Co-Cr-Mo alloy, and carbon coated porous Co-Cr-Mo alloy intramedullary plugs. Mechanical testing and finite element analysis has shown that the intramedullary stems significantly alter the strain pattern in the femur. However, no statistically significant differences were observed among the implant groups. The analytical and experimental results were found to be in good agreement. Radiographically, differences were observed in the bone remodeling around the carbon implant compared to either of the Co-Cr-Mo based implants. Both internal and external bone remodeling takes place when an intramedullary implant is present.
Animals↗
Stress distribution in the proximal femur after surface replacement: effects of prosthesis and surgical techniques.
The increasing number of reported femoral neck fractures following surface replacement indicates a potentially serious problem in this procedure. A comparison of the stress levels in the proximal femur before and after hip surface replacement was made using the finite element technique. A proposed surgical bone graft procedure utilizing a strut of cortical bone placed through the femoral head into the neck to help support the surface replacement was also studied as an adjunct for use in avascular necrosis. The findings indicate that stress magnitudes and distributions in the proximal femur are markedly altered after surface replacement when compared to a femur without a prosthesis. Stress levels in the femoral head with surface replacement were found to be significantly lowered suggesting future problems with stress shielding. Lateral neck stresses were found to be equal to or greater than stress levels in the femur without a prosthesis. Stress orientation in the femoral neck is also markedly altered. Surface replacement design and recommended surgical positioning were important variables in determining stress levels. Femoral neck stresses were found to be significantly lowered for all designs of surface replacement when the bone graft modification was utilized. The results of this study explain the major mechanisms of failure of the femoral component. The proposed bone graft modification is demonstrated by the finite element model to decrease the possibility of femoral neck fracture.
Femur↗
Retention characteristics of porous rooted Co-Cr-Mo alloy dental implants.
The interface characteristics of porous rooted cobalt-chromium-molybdenum alloy (Co-Cr-Mo) dental implants which had been in free standing function in canine mandibles for a period of two years were investigated. The displacement of the implants and of points on the adjacent mandibular cortex were determined by mechanical testing. Bone ingrowth was quantified, and the structure of the bone-implant interface and mandibular cortex were characterized using histologic and microradiographic analyses. Displacement characteristics were correlated with determinations of the tissue structure adjacent to and growth within the implant to provide information about the biological attachment. A correlation was found between the thickness of the buccal and lingual cortical plates and implant displacements; implants having the greatest displacement response were in mandibles with the thinnest cortical plates. A relationship could not be established between the implant displacement response and the quantitative tissue structure data. Differences observed in the displacement response of the implant by mechanical testing were not observed by clinical measurements of mobility. It was concluded that implant retention mechanical behavior results from both interfacial displacement and deflection of the adjacent mandibular structures.
Animals↗