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Biomedical subjects

R S Lakes

Publications and source records attributed to R S Lakes.

At least 37 records · Page 2Linked to original sources
PubMed1993

Temperature dependence of the dynamic viscoelastic behavior of chemical- and light-cured composites.

The relationship of temperature to the viscoelastic properties of six composites, three light-cured and three chemical-cured, was studied, using constant dynamic loading over the narrow range of temperatures (20-60 degrees C) which can be encountered in the mouth. The parameters measured were: storage modulus G(1), loss modulus G(2), tan delta, quality factor Q, coefficient of decay alpha, and dynamic viscosity. It was found that the dynamic viscoelastic properties of the tested materials are temperature-dependent, but probably not to a clinically significant degree. Chemical-and light-cured composites of the same filler loading do not exhibit significantly different viscoelastic dynamic properties.

Bisphenol A-Glycidyl Methacrylate↗
PubMed1993

Cusp movement in premolars resulting from composite polymerization shrinkage.

This study examined the effect of cavity size, hydration conditions and type of composite on cusp movement following placement of MOD composite restorations in vitro. A microscope with a micrometer stage was used to measure deflection of cusps for 14 days after initiation of curing. The intercuspal width decreased for the first few minutes after the composites were placed, then increased toward the original dimensions. Teeth with small cavities contracted less than those with large cavities, and recovery was complete or nearly complete only in teeth with small cavities. Hydrated teeth and restorations recovered more rapidly and more completely than those that were dry. Differences in deflection of the cusps were observed with two different restorative materials.

Absorption↗
PubMed1993

Interferometric measurements of cusp deformation of teeth restored with composites.

A Michelson interferometry apparatus was used for measurement of the displacement of the buccal cusps of premolars after restoration of MOD preparations with composites. The effects of composite type, cavity size, and hydration conditions were examined. Interferometry permitted real-time measurement of cusp movement as it occurred. Contraction occurred very rapidly, about 1/3 of the 60-minute amount within the two-minute period of exposure to the curing light. Cusp movement was smooth rather than interrupted, indicating lack of microfracturing at deformations of 11-46 microns. Contraction, 0.94% for Heliomolar and 1.2% for P-50, was similar to the linear polymerization shrinkage of the resins. Less cusp movement occurred in small cavities than in large cavities. Hydrated teeth had less cusp movement than dehydrated teeth.

Acid Etching, Dental↗
PubMed1990

Fracture mechanics of bone with short cracks.

Tensile fracture experiments were performed upon specimens of wet mature bovine Haversian bone, with short, controlled notches. Stress concentration factors were found to be significantly less than values predicted using a maximum stress criterion in the theory of elasticity. Results were also modeled with the aid of linear elastic fracture mechanics. Agreement of experiment with theory was better in this case, however deviations were seen for short notches. Two mechanisms were evaluated for the behavior: plasticity near the crack tip, and effects of the Haversian microstructure, modelled by Cosserat elasticity, a generalized continuum theory. Plastic zone effects were found to be insignificant. Cosserat elasticity, by contrast, predicted stress concentration factors which better approximated observed values. To explore strain redistribution processes, further experiments were conducted upon notched specimens in torsion at small strain. They disclosed a strain redistribution effect consistent with Cosserat elasticity. These microelastic effects were attributed to the Haversian architecture of bone. The implications of the results are that bone resists the effect of stress raisers such as fatigue microcracks and surgical sawcuts to a much greater extent than anticipated on the basis of its elastic or elastoplastic properties.

Animals↗
PubMed1988

Thermomechanical study of Ni-Ti alloys.

A preliminary study was conducted to demonstrate the usefulness of the combined technique of differential scanning calorimetry (DSC) and mechanical testing for the shape memory metals of 54NiTi and 53NiTiCo(3%) alloys. The DSC technique was used to measure precise transformation temperatures and the amount of thermal energy required for the corresponding phase transformation. The degree of plastic deformation by bending and the effect of alloying (such as Co) were studied with combined DSC and mechanical property measurements.

Alloys↗
PubMed1987

Creep of amalgam at low stresses.

The creep and recovery of dental amalgam were studied with a torsional creep apparatus. The purpose was to investigate viscoelastic behavior in a low stress range that might result from normal chewing forces. The creep of specimens up to six months old was also studied. Four types of alloys for amalgam were used: two single-composition sphericals (SCS), an admixture-lathe-cut eutectic (ALE), and a conventional (CON). Constant torque was applied to dumbbell-shaped specimens for three hr and, after the stress was released, recovery was followed for 50 hr. All measurements were made in water at 37 degrees C after storage of the specimens for one week in 37 degrees C water. Creep was also measured at nine intervals on specimens aged from three hr to six months. The SCS amalgams exhibited the least amount of creep, followed by the ALE amalgam. The CON amalgam had the highest creep. The SCS amalgams approached linear viscoelastic behavior in the shear stress range of 0.78-4.10 MPa at 37 degrees C. They did not exhibit steady-state (viscous) creep at stresses less than 4.10 MPa. Recovery was complete but proceeded more slowly than creep. The ALE and CON amalgams exhibited viscous creep at 1.23 MPa and higher. The steady-state creep rate depended on stress, and the stress exponent ranged from 2.3-3.5. All except the ALE amalgam showed essentially constant creep compliance when aged up to six months, once the amalgamation reaction was completed after 24 to 48 hr. The compliance of the ALE amalgam gradually decreased over the six-month period.

Chemical Phenomena↗
PubMed1986

Cosserat micromechanics of human bone: strain redistribution by a hydration sensitive constituent.

Experimental determination of the strain distribution in prismatic, square cross-section bars of human compact bone in torsion disclosed nonclassical effects associated with the microstructure. Specifically, in wet bone at small strain, significant deviations from the classically predicted strain distribution were observed. The measured strain distribution in wet bone followed predictions based on Cosserat (micropolar) elasticity. In dry bone, the strain distribution was very close to the prediction of classical elasticity. The interaction between Haversian osteons and the cement substance between them was hypothesized to be the principal mechanism for the phenomena. To evaluate this hypothesis, additional specimens were subjected to prolonged torsional load and the cement lines were observed by reflected light microscopy. Localized deformation at the cement lines was observed, but it was less than values reported earlier for bovine plexiform bone.

Biomechanical Phenomena↗
PubMed1986

The material properties of bone-particle impregnated PMMA.

The elastic Young's modulus and shear modulus of bone-particle impregnated polymethylmethacrylate (PMMA) has been measured experimentally at room temperature as a function of bone particle concentration. It was found that the moduli increased with increasing bone particle content. This increase was less than the stiffness increase predicted by higher-order composite theory [1, 2] under the assumption of perfect bonding between particles and matrix. It was concluded that a bond existed but that it was not a perfect bond.

Bone and Bones↗
PubMed1985

Creep of posterior dental composites.

The creep of microspecimens of posterior dental composites was studied using a torsional creep apparatus. Shear stresses were maintained for 3 h and recovery was followed for 50 h. Creep curves were obtained at 21, 37, and 50 degrees C and four torque levels. The effect of conditioning the specimens in water up to 8 weeks was studied. The posterior composites exhibited linear viscoelastic behavior at low deformations. They had higher shear moduli and greater resistance to creep than conventional and microfilled composites. In aging experiments, maximum shear moduli occurred when specimens were 48 h to 1 week old. Subsequent softening was attributed to water absorption. Residual strain was highest when the composites were stressed within 24 h of initiating polymerization. Residual strain was very low in specimens 48 h to 8 weeks of age.

Aluminum Compounds↗
PubMed1984

Creep of conventional and microfilled dental composites.

The creep of microspecimens of two conventional and two microfilled composites was studied using a torsional creep apparatus. Small stresses below the materials' proportional limits were maintained for 3 h and recovery was monitored for 30 h. Creep curves were obtained at 21.5, 37, and 50 degrees C, and three torque levels. The composites exhibited linear viscoelastic behavior at low deformations. The magnitude of creep depended on the amount and type of resin in the composites. The materials did not recover their original shape after 30 h after the stress was removed. No transitions were observed in the temperature range employed. The shear moduli decreased with temperature and are in reasonable agreement with literature values obtained with other tests.

Acrylic Resins↗
PubMed1982

Dynamical study of couple stress effects in human compact bone.

Torsional resonance experiments performed on wet human compact bone disclose effects due to couple stress. The characteristic length, which is an additional material coefficient which appears in couple-stress theory, is of the order of the size of osteons and appears to be smaller at high frequencies than at low frequencies. The presence of couple-stress effects implies a reduction in the stress concentration factor around holes, particularly small holes.

Bone and Bones↗
PubMed1981

Evaluation of instrumented force platform as a test to measure residual effects of anesthetics.

Recovery from anesthesia was assessed in a controlled manner in 38 healthy student volunteers, using two psychomotor tests (perceptual speed and tapping board) and an instrumented force platform 1, 3, 5 and 7 h after intravenous injection of 0.3 mg/kg diazepam, 2.0 mg/kg methohexital, 6.0 mg/kg thiopental or saline. Postural stability remained unaltered but the performance on psychomotor tests improved when the tests were repeated after saline injection. Methohexital did not induce any changes in body sway or psychomotor performance at the time periods tested when compared with saline. Postural stability of subjects receiving thiopental or diazepam was imparied (P less than 0.001) for 1 and 7 hours after anesthesia, respectively, when compared with saline. The impairment on performance in the psychomotor tests induced by thiopental or diazepam was of smaller magnitude and for diazepam of shorter duration than balance disturbances measured with the body sway tests. Further clinical studies on the use of the instrumented force platform as a fast and easily interpretable guideline for discharge from hospital after different modes of outpatient anesthesia are warranted.

Adult↗
PubMed1981

Transient study of couple stress effects in compact bone: torsion.

Couple stress theory, which admits an internal moment per unit area as well as the usual force per unit area, is a generalization of classical elasticity. Experimentally we have demonstrated the existence of couple stress by measuring the effect of size on apparent stiffness of compact bone in quasi-static torsion. From these measurements, we obtain the characteristic length for bone in couple stress theory.

Adult↗