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66 records · Page 4Linked to original sources
PubMed1990

Effect of vacuum mixing on the mechanical properties of antibiotic-impregnated polymethylmethacrylate bone cement.

Polymethylmethacrylate bone cement, containing either no added antibiotic, 0.5 g of Vancomycin, 1.0 g of Vancomycin, or 1.0 g of Tobramycin, was mixed either in air or a vacuum chamber. Following storage in a water bath at 37 degrees C for 48 h, the specimens were tested in four-point bending. The porosity of the specimens was assessed radiographically, and their antibacterial activity was monitored for 21 days. The bending strength of the vacuum mixed specimens containing no antibiotic was 40% greater than that of similar air-mixed specimens. However, there were no significant differences in the bending strength of either the air- or vacuum-mixed specimens when any of the antibiotic dosages were added. The bending modulus of the vacuum-mixed specimens, containing no antibiotic, was significantly greater than the moduli of all the other specimen groups which did not differ from each other. Vacuum mixing reduced the apparent porosity of the specimens fivefold, and while the addition of antibiotic did not effect porosity of the air-mixed specimens, that of the vacuum-mixed specimens was doubled. Although initial rapid decreases were seen, leaching of antibiotic from the cement and antibacterial activity continued through the 21-day monitoring period.

Biomechanical Phenomena↗
PubMed1986

The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins.

The goal of this study was to determine the effects of resin formulation variables, such as diluent concentration, catalyst type and concentration and cure mode, on the degree of conversion of carbon double bonds and mechanical properties of dental restorative resins. Diametral tensile strength, compressive strength, hardness, flexural modulus and strength, and dynamic mechanical properties were tested, and the results were correlated to the degree of conversion results obtained by infrared analysis. The results showed a significant correlation between increased mechanical properties and higher degrees of conversion. Enhanced conversions were achieved by incorporating higher diluent and lower inhibitor concentrations into the resins. Ambient temperature properties were similarly enhanced by lower inhibitor concentrations, but were not enhanced by higher diluent concentration. Dynamic mechanical properties testing at oral and elevated temperatures elucidated possible differences in resin microstructure and network quality. The storage moduli decreased over the dental temperature range and was lower at all temperatures for resins with lower conversions. The glass transition temperature was also lower in resins with poorer conversions, suggesting that these resins may be more unstable at oral temperatures than more highly converted resins. Dynamic mechanical properties were most closely correlated to degree of conversion in these polymeric systems.

Acrylic Resins↗
PubMed1991

[Biomechanical properties of heat and irradiation treated spongiosa].

Cylindrical specimens of trabecular pig bone were tested to uniaxial compressive strain levels of 30% to study the influence of various sterilization techniques and methods of HIV-inactivation on the mechanical properties characterized by compressive modulus, yield point, energy absorption and maximum stress. Heat inactivation at 60 degrees C (Lactated Ringer, 1 h) showed no effect; 80 degrees C (Lactated Ringer, 1 h) resulted in a diminution of the yield point and the maximum stress (p less than 0.005), while energy absorption and compressive modulus were not affected. No reduction in the stability was seen when ethanol was used instead of Lactated Ringer. At a temperature of 100 degrees C, all measured parameters were reduced to approximately 60% compared with the control group. A decrease to 13% to 25% was seen after autoclavation (120 degrees C, 2 bar, 20 min and 134 degrees C, 3 bar, 12 min). Irradiation (60Co) with 3 respectively 10kGy did not impair the stability, whereas a dose of 25 kGy led to a reduction to 61% to 69%. No additional effect was seen when irradiation was followed by storage at -80 degrees C for one week. These effects on bone stability should be considered when choosing a method of bone preparation to obtain HIV-inactivated bone grafts. Autoclavation should be used with caution when stability of the bone graft is essential. In this case, irradiation seems to be a safe method of sterilizing bone grafts ensuring both a high degree of safety and stability.

Animals↗
PubMed1979

Thermoelasticity of red blood cell membrane.

The elastic properties of the human red blood cell membrane have been measured as functions of temperature. The area compressibility modulus and the elastic shear modulus, which together characterize the surface elastic behavior of the membrane, have been measured over the temperature range of 2-50 degrees C with micropipette aspiration of flaccid and osmotically swollen red cells. In addition, the fractional increase in membrane surface area from 2-50 degrees C has been measured to give a value for the thermal area expansivity. The value of the elastic shear modulus at 25 degrees C was measured to be 6.6 X 10(-3) dyne/cm. The change in the elastic shear modulus with temperature was -6 X 10(-5) dyne/cm degrees C. Fractional forces were shown to be only on the order of 10-15%. The area compressibility modulus at 25 degrees C was measured to be 450 dyne/cm. The change in the area compressibility modulus with temperature was -6 dyne/cm degrees C. The thermal area expansivity for red cell membrane was measured to be 1.2 X 10(-3)/degrees C. With this data and thermoelastic relations the heat of expansion is determined to be 110-200 ergs/cm2; the heat of extension is 2 X 10(-2) ergs/cm2 for unit extension of the red cell membrane. The heat of expansion is of the order anticipated for a lipid bilayer idealized as twice the behavior of a monolayer at an oil-water interface. The observation that the heat of extension is positive demonstrates that the entropy of the material increases with extension, and that the dominant mechanism of elastic energy storage is energetic. Assuming that the red cell membrane shear rigidity is associated with "spectrin," unit extension of the membrane increases the configurational entropy of spectrin by 500 cal/mol.

Elasticity↗
PubMedDate not supplied

Bond strength of fiber-reinforced composite resin restorations.

Severe tooth wear is common in older dentate individuals, with one treatment option being composite resin restorations reinforced with a suitable matrix. This study evaluated the use of high modulus polyethylene (Celanese) fibers as a reinforcing matrix for composite resin. Human mandibular incisor teeth were sectioned to simulate severe tooth wear. Sectioned surfaces were measured, the teeth paired and assigned to control or test sample groups, and further assigned to be tested with a labial or lingual shearing force. A phosphorylated dentin bonding adhesive was applied to the abraded dentin surface. Labial and lingual intra-enamel bevel preparations were cut on each specimen. A piece of Celanese fabric was bonded onto the acid-etched labial and lingual bevels of the test specimens using an enamel bonding agent. Class IV composite resin restorations were then placed onto the test and control specimens. Following water storage, the specimens were subjected to shearing forces. Bond strengths for test specimens were significantly greater (p < 0.03) than the controls. Specimens with a labially applied force also had significantly higher bond strengths (p < 0.001). SEM analysis revealed adhesive bond failures over dentin surfaces, with cohesive bond failures within the composite resin. Celanese fibers maintained the restorations on the teeth, although adhesive failures were seen between the fibers and enamel bonding agent.

Analysis of Variance↗
PubMed1991

[Comparison of mechanical qualities of cortical bone preserved by various freezing methods].

The purpose of this study was to compare the mechanical properties of the human cortical bone following three deep-freezing techniques: addition of D.M.S.O. (di-methyl-sulfoxide) and staged decrease of temperature with immersion into liquid nitrogen, immersion into nitrogen without cryoprotection, deep freezing in an electric device, down to -80 degrees C. Axial loading tests performed on two femurs (88 samples) allowed to determine the strain of failure fmax and the Young's modulus E. Torsion tests were undertaken on four femur (66 samples) to obtain the strain failure tmax and the Young's modulus G. We have taken several samples from the cortex, that we randomly divided into four groups, three of them being deep-frozen with one of the methods studied, the fourth being tested "fresh", after standard sample manufacturing. The results obtained at the fresh state were comparable with those published in the literature (fmax = 158 MPa, E = 9000 MPa, tmax = 60 MPa, G = 4000 MPa). The results obtained after deep freezing both in axial loading and in torsion, demonstrated significant differences, ever though of low value. The surgeon may use one of either freezing method, without fearing any deterioration of the mechanical properties that could threaten the primary stability of the graft. However, only liquid nitrogen permits a long lasting storage, and only the use of cryoprotection can give the hope of preservation of the chondrocytes in case of diaphyseal-epiphyseal grafts.

Bone Transplantation↗
PubMed1990

Three-axial strain controlled testing applied to bone specimens from the proximal tibial epiphysis.

Reproducibility of the determination of Young's modulus and energy absorption along the three axes of trabecular bone cubes was analysed by non-destructive compression to 0.5% strain using different testing protocols. These protocols included testing with and without pre-conditioning to a viscoelastic steady state, and different orders of test directions. Reproducibility of conditioned tests was generally better than that of non-conditioned tests. No major effect of changing the order of the test direction was found. Three-axial conditioned testing of cubes from the proximal tibial epiphysis of five humans revealed a global transverse isotrophy while most cubes showed orthotropy. The ratio between stiffness along the long axis of the tibia and the stiffness in the transverse plane was 3.7 +/- 0.4 (mean +/- 2 SE). The corresponding ratios for elastic energy storage and viscoelastic energy dissipation were 2.5 +/- 0.2. There was no difference between the relative energy loss during a testing cycle (loss tangent) in the three axes.

Adult↗
PubMed1986

Mechanical properties of some pigmented and unpigmented aqueous-based film coating formulations applied to aspirin tablets.

The Brinell hardness and Young's modulus of pigmented and unpigmented films of hydroxypropyl methylcellulose alone, and in combination with either polyethylene glycol 400 (plasticizer) or polyvinyl alcohol, which were applied to aspirin tablets, have been measured. Generally hardness and modulus data showed similar trends. The hardness and modulus of hydroxypropyl methylcellulose fell in the presence of polyethylene glycol 400 as a result of its plasticizing action. On the other hand, the hardness and modulus of the film former rose slightly when polyvinyl alcohol was initially incorporated, probably due to the crystalline phase of the additive, and then decreased when the level of the additive was further raised. Hardness and modulus were higher in films pigmented with talc than in those containing titanium dioxide because of the plate-like shape of talc and its greater interaction with the polymer systems. Some correlation was found between the Young's moduli of the applied films and those of the corresponding free films, with the moduli of the latter two 2-5 times greater. Ageing at 37 degrees C and 75% r.h. was found to cause a decrease in the mechanical properties of the unplasticized film coating systems probably as a result of decreased molecular order and enhanced polymer chain mobility.

Aspirin↗
PubMedDate not supplied

Viscoelastic properties of very dilute paramyosin solutions.

The storage and loss shear moduli, G' and G'', have been measured for dilute solutions of paramyosin from the clam Mercenaria mercenaria in water and glycerol--water mixtures containing potassium chloride and phosphate buffer. The Birnboim--Schrag multiple-lumped resonator was used in the frequency range from 150 to 8100 Hz; the concentration range was 0.7 to 2 X 10(-3) g/mL and the temperature range was 0.0 to 6.0 degrees C. The intrinsic moduli were obtained by extrapolation to infinite dilution. When compared with predictions of Yamakawa for a rigid cylindrical molecule, they agreed at low frequencies but diverged at high frequencies. Excellent agreement was obtained with calculations for a hybrid model whose relaxation times are attributed to rigid-body end-over-end rotation together with some internal modes of motion, probably flexural. The rotational relaxation time agreed rather well with that determined by DeLaney and Krause from electrical birefringence measurements. From the ratio of the rotational to the longest flexural relaxation time, the flexural rigidity and Young's modulus of the paramyosin molecule were estimated by relations derived by Wada and collaborators; the modulus was 1.2 X 10(10) dyn/cm2.

Elasticity↗
PubMed1982

Flexibility of myosin rod determined from dilute solution viscoelastic measurements.

The frequency dependencies of the storage and loss shear moduli, G' and G", of myosin rod solutions at 1.0 and 7.0 degrees C were measured by use of the Birnboim-Schrag multiple lumped resonator apparatus in solvents with and without glycerol. The infinite dilution moduli were determined and compared with theoretical models for a rigid rod and a freely jointed trinodular rod and with an empirical model for a semiflexible rod. Only the latter could fit the data. A rotational relaxation time of 25 mus and a slowest bending time of 3.1 mus, both reduced to water at 20 degrees C, were determined from the fit. A persistence length of about 130 nm was obtained from either the bending time, the rotational relaxation time, or the intrinsic viscosity. The average thermal excursion of the end of subfragment 2 was estimated to be 26 nm, more than sufficient to span the gap between the thick and thin filaments in muscles at all sarcomere lengths. Thus, a hinge between heavy meromyosin and light meromyosin does not appear necessary for myosin-actin contact. Young's modulus of about 1 x 10(9) N/m2 also makes it unlikely that subfragment 2 can be the elastic element in the Huxley-Simmons model of muscle contraction.

Animals↗
PubMed1975

The energetics of the jump of the locust Schistocerca gregaria.

The anatomy of the metathoracic leg is redescribed with particular reference to storage of energy in cuticular elements and the way in which the stored energy is used in jumping. The jump of adult male locusts requires an energy of 9 mJ and that of the female requires 11 mJ. The semilunar processes of each metafemur store 4 mJ at a stress of 15 N, and the extensor tibiae apodeme stores a further 3 mJ at the same stress. The total stored energy in both metathoracic legs is 14 mJ. The extensor tibiae muscle produces a maximum isometric force of over 15 N at 30 degrees C and, when loaded with the extensor apodeme and semilunar processes, attains this force in 0.3 sec with a strain of 0.8 mm. The peak power output is 36 mW or 0.45 W.g-1. The peak isometric force is attained when the tibia is fully flexed and the force falls as the tibia extends. The extensor tibiae muscle A band is 5.5 mum long and the peak force is over 0.75 N.m-2. The peak velocity of shortening is 7 mm.sec-1 or about 1.75 lengths/sec at 30 degrees C. The tensile strength of the extensor apodeme is 0.6 kN.mm-2 and Young's modulus is 19 kN.mm-2. The safety factor does not exceed 1.2 and the safety factor of the semilunar processes and tibial cuticle is little higher. The jump impulse lasts 25-30 msec. A velocity of 3.2 m.sec-1 is reached after a peak acceleration of 180 m.sec-2. The peak power output is 0.75 W at close to maximum velocity. Energy losses in rotating the femur and tibia are small and it is shown that the leg is able to extend at 7 times the normal rate with losses of about 20%. Most of the stored energy is converted to kinetic energy as the animal jumps. A model is based on the relaxation of a spring that has the properties of the elastic elements of the locust leg into a lever with the same kinematics as the locust leg produces a force-distance curve similar to that measured for locust jumps. The major part of the jump energy is stored before the jump.

Animals↗
PubMed1990

1990 Volvo Award in experimental studies. The dependence of intervertebral disc mechanical properties on physiologic conditions.

In vivo creep-recovery and disc pressure measurements were performed on the lumbar spine of immature and mature swine. The creep-recovery measurements were performed using a custom materials testing apparatus designed to apply static or dynamic loads to the spine of anesthetized animals. A series of three separate experiments were performed to assess the effects of: (I) animal death, (II) graded injury to the disc anulus, and (III) respiratory mechanics on the biomechanical response of the porcine L1-L3 vertebral unit (VU). In Experiments I and II, creep rate, modulus, and viscosity parameters were computed using a three-parameter solid rheological analysis of the displacement-time response recorded during the application of a 300-N load. In Experiment III, the effects of respiratory volume and frequency changes on disc pressure were assessed in the unloaded, statically loaded, and immobilized porcine VU. Our results indicated that the adult VU tended to be stiffer, deform or creep more slowly, and had a significantly higher viscosity than the VU of immature pigs. The results of Experiment I demonstrated that the biomechanical response for the VU was significantly altered by the death of the animal; the VU of the living animal (adolescent or mature) was more compliant and deformed at a faster rate than the VU of the same animal after death. Disc injury produced changes in stiffness, viscosity, and creep rate analogous to that of aging, and on the basis of the graded injuries created in this study, it appears that a small defect in the annulus is just as deleterious as removing a large section of anular material. The results of Experiment III indicated that respiration plays an important role in the normal, in vivo mechanical and nutritional behavior of the porcine VU. Altogether, these results demonstrate that, in the absence of normal physiologic conditions, one may not be able to reliably predict the mechanical response of the lumbar spine, and suggest that standards for the testing, handling, and storage of biologic tissue should be established.

Animals↗