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PubMed2004

Effect of loading rate on bond strength.

The purpose of this study was to examine the effect of loading rate on the bond strength of brackets bonded to enamel. Forty premolars were used and brackets were bonded with a chemically cured or a light-cured adhesive, mounted on a testing machine, and debonded under shear stress at two loading velocities: a standard 1 mm/min, and a fast 200 mm/min which better approximates the actual jaw velocity during chewing. Bond strength results (N) were analyzed with two-way analysis of variance (loading rate, adhesive) (alpha = 0.05). An increased loading rate resulted in decreased bond strength, probably due to the induction of a stiff body response and elimination of the viscoelastic properties of the resin. The results indicated a similar effect on both chemically cured and light-cured adhesives, thus emphasizing the need for standardized test conditions in bond strength protocols. The implication of loads generated during chewing must be considered when estimating the long-term survival of the bond.

Adhesiveness↗
PubMed2001

Combined effects of diets with reduced calcium and phosphate and increased fluoride intake on vertebral bone strength and histology in rats.

Ingested fluoride is incorporated into bone apatite and can affect the structural integrity of bone. Fluoride absorption in the gut and incorporation into bone is affected by the presence of other ions, including calcium. We hypothesized that a low calcium phosphate diet combined with high fluoride intake would have independent detrimental effects on vertebral bone strength. We measured the vertebral biomechanical properties and bone histology for rats given controlled diets with varied fluoride and calcium phosphate intakes for either 16 or 48 weeks. Diets were designed to produce mild calcium deficiency while keeping Ca:P ratio constant. Hence calcium deficiency was accompanied by a proportional phosphate deficiency. Calcium phosphate deficiency (25% of normal) significantly diminished vertebral strength only in younger rats. As the rats grew older, the effects of calcium phosphate deficiency were mitigated. Calcium phosphate deficiency reduced vertebral strength in young rats primarily by reducing the trabecular bone volume. The highest fluoride intake (50 mg/L) significantly diminished vertebral strength, but there was no effect of lower fluoride intakes on bone strength. In contrast to calcium phosphate deficiency, high fluoride intake had no effect on trabecular bone volume but instead increased the amount of unmineralized osteoid, particularly in older rats. This impairment of mineralization by fluoride appeared to be the primary cause of the diminished vertebral strength. The detrimental effects of fluoride on vertebral bone strength were not amplified by calcium phosphate deficiency.

Animals↗
PubMed1996

Dentin shear strength: effects of tubule orientation and intratooth location.

OBJECTIVES: Dentin has a highly oriented tubule structure, and the tubule number density and area fraction of intertubular dentin vary with distance from the pulp. This investigation sought to determine the influence of tubule orientation on shear strength of dentin from samples derived at various intratooth locations. METHODS: Third molars were sectioned and prepared to provide samples from two locations (center and cusp) and with one of three specific tubule orientations. In series 1, matched pairs of midcoronal samples were tested using two tubule orientations. A paired t-test was used for statistical analysis. In series 2, three samples from central and cuspal areas were tested using three different tubule orientations. A two-day ANOVA was used for statistical analysis. Each sample had dimensions of approximately 1 x 1 x 5 mm and was tested in a hydrated state by a single plane lap shear method. RESULTS: The paired dentin samples of the midcoronal dentin in the two orientations had shear strengths of 72.4 +/- 15.6 MPa and 78.4 +/- 13.2 MPa, and were not significantly different (p > 0.05; paired t-test). In the second series, samples from the center location with tubules parallel to the shear plane with applied force in directions rotated by 90 degrees did not exhibit a significant difference (p > 0.05), with an average value of 53.5 +/- 9.5 MPa. Samples oriented with tubules along the long axis of the specimen and tested with shear force applied perpendicular to the tubule direction had significantly higher (p < 0.05; two-way ANOVA) shear strength (78.0 +/- 8.5 MPa). The specimens from the cusp area did not exhibit a statistically significant difference (p > 0.05; two-way ANOVA) with respect to the three orientations (83.6 +/- 8.4; +/- 13.8; 91.8 +/- 12.7 MPa). Cuspal areas were stronger than central areas in two of the three orientations tested. SIGNIFICANCE: Results indicated that the shear strength differs in central and cusp areas and is dependent on dentin tubule orientation in the central area. Shear strengths were much larger than values reported in shear bond strength tests. This suggests that dentin shear strength is far in excess of dentin bond strengths using shear tests, and that fractures through dentin in such result from flaws or stress concentration in the dentin.

Analysis of Variance↗
PubMed2002

4-META use in dentistry: a literature review.

4-META (4-methacryloyloxyethy trimellitate anhydride) was invented and patented in Japan in January 1979 and patented in the United States in April 1979. For the first 8 years, 4-META dental products (manufactured by Sun Medical Co, Kyoto, Japan) were not readily available in North America. Most of the early research therefore was done in Japan and reported in Japanese. A second wave of products (manufactured by Parkell, Farmingdale, N.Y.) became available in the United States in 1987, which precipitated research in English. This article is a chronological review of the literature published between 1978 and 1998 on 4-META dental adhesive materials. Thirteen commercial products are reviewed in terms of their physical strength; research on the mechanisms of 4-META action and the biocompatibility of 4 META was excluded.

Acrylic Resins↗
PubMed2001

Dependence of yield strain of human trabecular bone on anatomic site.

Understanding the dependence of human trabecular bone strength behavior on anatomic site provides insight into structure-function relationships and is essential to the increased success of site-specific finite element models of whole bones. To investigate the hypothesis that the yield strains of human trabecular bone depend on anatomic site, the uniaxial tensile and compressive yield properties were compared for cylindrical specimens from the vertebra (n=61), proximal tibia (n=31), femoral greater trochanter (n=23), and femoral neck (n=27) taken from 61 donors (67+/-15years). Test protocols were used that minimized end artifacts and loaded specimens along the main trabecular orientation. Yield strains by site (mean+/-S.D.) ranged from 0.70+/-0.05% for the trochanter to 0.85+/-0.10% for the femoral neck in compression, from 0.61+/-0.05% for the trochanter to 0.70+/-0.05% for the vertebra in tension, and were always higher in compression than tension (p<0.001). The compressive yield strain was higher for the femoral neck than for all other sites (p<0.001), as was the tensile yield strain for the vertebra (p<0.007). Analysis of covariance, with apparent density as the covariate, indicated that inter-site differences existed in yield stress even after adjusting statistically for density (p<0.035). Coefficients of variation in yield strain within each site ranged from only 5-12%, consistent with the strong linear correlations (r(2)=0.94-0.98) found between yield stress and modulus. These results establish that the yield strains of human trabecular bone can differ across sites, but that yield strain may be considered uniform within a given site despite substantial variation in elastic modulus and yield stress.

Adult↗
PubMed2005

Osteoprotegerin treatment impairs remodeling and apparent material properties of callus tissue without influencing structural fracture strength.

The influence of osteoprotegerin (OPG) treatment on callus formation, callus tissue structural strength, apparent material properties, and histology of tibia fractures in rats was investigated after 3 weeks and 8 weeks of healing. OPG was given intravenously (10 mg/kg twice weekly) during the entire observation period, and control animals with fractures received vehicle only. When compared with control fractures after 3 weeks of healing, OPG treatment reduced the number of osteoclasts in the callus tissue (93%, P < 0.001) and hampered resorption of genuine cortical bone in the fracture line; OPG treatment did not influence callus dimensions, callus bone mineral content (BMC), fracture structural strength, or callus tissue apparent material properties. When compared with control fractures after 8 weeks of healing; OPG treatment reduced the number of osteoclasts in callus tissue (92%, P < 0.001), augmented callus dimensions (anteriorposterior diameter: 12%, P = 0.034, mediolateral diameter: 13%, P = 0.013), and increased callus BMC (50%, P = 0.007); OPG treatment hampered deposition of new woven bone at the fracture line of the genuine cortical bone (new woven bone present in all vehicle animals, but only in 13% of the OPG-treated animals (P < 0.001)); OPG treatment did not influence structural strength of the fractures, but decreased apparent material properties of the callus tissue (ultimate stress: 51%, P < 0.001; elastic modulus: 42%, P = 0.033). The experiment demonstrates that OPG treatment does not influence the early callus expansion and fracture strength. However, during the subsequent period of remodelling, OPG treatment impairs the normal remodeling and consolidation processes.

Absorptiometry, Photon↗
PubMed2005

Raloxifene and vitamin K2 combine to improve the femoral neck strength of ovariectomized rats.

We evaluated the skeletal effects of two osteoporosis therapies in an ovariectomized rat model, raloxifene and vitamin K2, as well as the vitamin K2 plus raloxifene (K + Ral) combination. In two studies, 6-month-old rats were ovariectomized, except for sham-ovariectomy controls (Sham), and dosed orally with vehicle, 30 mg/kg vitamin K2, 1 mg/kg raloxifene, or the combination of K + Ral for 6 weeks following surgery. Vitamin K2 had no effect on serum estrogen, low-density lipoprotein cholesterol (LDL-C), or urinary deoxypyridinoline levels, but slightly increased osteocalcin levels compared to Ovx. Raloxifene lowered total cholesterol, LDL-C, osteocalcin, and urinary deoxypyridinoline levels to below Ovx levels, while having no effect on estrogen levels. Raloxifene, but not vitamin K2, prevented ovariectomy-induced loss of bone in the distal femoral metaphysis and proximal tibial metaphysis, as did the K + Ral combination. Raloxifene, but not vitamin K2, partially prevented, loss of vertebral bone mineral density (BMD), whereas K + Ral had BMD greater than that of Ovx. Vitamin K2 increased bone formation rate to above Ovx, whereas raloxifene and K + Ral reduced bone formation rate to Sham levels. Vitamin K2 had no effect on eroded surface compared to Ovx, while raloxifene and K + Ral reduced eroded surface to Sham levels. Groups were not different in the BMD of femoral midshaft; however vitamin K2 was observed to increase periosteal mineralizing surface of the tibial shaft to above Ovx, while raloxifene reduced periosteal mineralizing surface toward Sham levels. Femoral neck strength was not different between groups, indicating no significant beneficial effect of either raloxifene or vitamin K2 at this site. However, K + Ral had reproducibly greater femoral neck strength than Ovx or Sham. Raloxifene, but not vitamin K2, partially prevented loss of lumbar vertebra strength; but K + Ral was not different from Sham or Ovx. Therefore, raloxifene and vitamin K2 had complementary effects on bone resorption and formation activities, respectively, resulting in a reproducible, significant improvement of femoral neck strength. These rat data suggest interesting therapeutic possibilities that may require clinical verification.

Administration, Oral↗
PubMed2006

Robustness testing of a tablet formulation using multivariate design.

A total of 45 experiments were carried out to evaluate the robustness of two similar tablet formulations--a product of two strengths--with respect to normal batch-to-batch variation of the excipients and the active pharmaceutical ingredient. The formulations consist of 10 ingredients. Because of the differing amounts of active pharmaceutical ingredients, the two formulations also differ in the amounts of two of the diluents and one of the binders. The excipients and active pharmaceutical ingredient were characterized in terms of multiple variables, and principal properties were calculated with principal component analysis. A Plackett and Burman design was applied to the principal properties. The relationships between the design factors and two responses, mean disintegration time and mean crushing strength, were evaluated by using regression methods. Both formulations were found to be robust under controlled conditions.

Chemistry, Pharmaceutical↗
PubMed2006

Freezing as a path to build complex composites.

Materials that are strong, ultralightweight, and tough are in demand for a range of applications, requiring architectures and components carefully designed from the micrometer down to the nanometer scale. Nacre, a structure found in many molluscan shells, and bone are frequently used as examples for how nature achieves this through hybrid organic-inorganic composites. Unfortunately, it has proven extremely difficult to transcribe nacre-like clever designs into synthetic materials, partly because their intricate structures need to be replicated at several length scales. We demonstrate how the physics of ice formation can be used to develop sophisticated porous and layered-hybrid materials, including artificial bone, ceramic-metal composites, and porous scaffolds for osseous tissue regeneration with strengths up to four times higher than those of materials currently used for implantation.

Biocompatible Materials↗
PubMed2001

Mechanical strength of the isolated carotid artery in SHR.

We have previously reported an adaptation of arterial wall elasticity in spontaneously hypertensive rats (SHR) that involves an increase in both fibronectin/alpha5beta1-integrin complexes and smooth-muscle elastic lamellae connections. We examined the mechanical strength (MS) of the carotid artery in relation to its elastic properties, its elastin/collagen content, and the structure of the internal elastic lamina. MS was defined as the in vitro intraluminal pressure and wall stress that produces rupture of the vascular wall. Intact carotid arteries from 3-month-old normotensive rats (Wistar-Kyoto, WKY) and SHR were cannulated on a specially designed device and adjusted to their in situ length. A slowly increasing static pressure was applied until wall rupture occurred to determine the static mechanical behavior and MS. Static elasticity was similar in SHR and WKY, as were the rupture pressure (2740+/-90 versus 2740+/-40 mm Hg) and wall stress at rupture (11.5+/-1.0 versus 12.8+/-0.4 MPa), indicating equivalent MS in both groups. Histological examination showed several wall ruptures and dissociation of lamellar units that did not differ significantly between the 2 groups. Confocal microscopy showed that the size of fenestrations of the internal elastic lamina and the fraction of area occupied by them were reduced 3-fold in SHR. We have demonstrated that static elasticity of the arterial wall and mechanical strength are similar in carotid arteries from SHR and WKY.

Animals↗
PubMed1996

Effect of repetitive impact on the mechanical strength of pyrolytic carbon.

Impact loading is experienced by mechanical heart valve components when the leaflet comes into contact with the housing orifice as the valve opens and closes during the cardiac cycle. Since the human heart beats approximately 40 million times per year, a large number of impact loading cycles are imposed on the carbon valve components. Contact loading can produce high mechanical stress at the contact area, leading to potential material damage and fracture. This paper describes a procedure for characterizing the effect of repetitive impact on the mechanical strength of pyrolytic carbon. The results shed light on the basic understanding of the damage formation and its strength degradation effect. In particular, fatigue crack growth was identified as the primary mechanism which causes the observed strength degradation. Design procedures based on fracture mechanics and contact mechanics are used in mechanical heart valve design to prevent structural failure associated with impact fatigue.

Biocompatible Materials↗
PubMed2005

Tubule orientation and the fatigue strength of human dentin.

In this study the influence of tubule orientation on the strength of human dentin under static and cyclic loads was examined. Rectangular beams were sectioned from the coronal dentin of virgin extracted molars (N=83) and then loaded in quasi-static 4-point flexure or 4-point flexural fatigue to failure. The flexure strength, energy to fracture and fatigue strength were evaluated for specimens with the dentin tubules aligned parallel (theta=0 degrees ) and perpendicular (theta=90 degrees ) to the plane of maximum normal stress. Results from monotonic loading showed that both the flexural strength and energy to fracture of dentin specimens with theta=0 degrees were significantly greater than those with theta=90 degrees . Furthermore, the apparent endurance strength of dentin with theta=0 degrees (44MPa) was significantly greater than that of the dentin with theta=90 degrees (24MPa). The ratio of apparent endurance strength (for fully reversed loading) to the flexure strength for theta=0 degrees and theta=90 degrees was 0.41 and 0.28, respectively. Although the influence of tubule orientation was most important to mechanical behavior, the flexure strength and energy to fracture also decreased with an increase in tubule density. According to differences in the fatigue strength with tubule orientation, restorative practices promoting large cyclic normal stresses perpendicular to the tubules would be more likely to facilitate fatigue failure in dentin with cyclic loading.

Compressive Strength↗
PubMed2001

The effect of the addition of surface treated chopped and continuous poly (methyl methacrylate) fibres on some properties of acrylic resin.

Poly (methyl methacrylate), the material most commonly used in the construction of dentures, is not without limitations particularly in terms of mechanical strength, such as flexural and fatigue strength. The Dental Practice Board for England statistics show that many dentures fail as a result of fracture. This study investigated the self-reinforcement of acrylic resin through the addition of surface treated poly (methyl methacrylate) fibres in chopped and continuous form. The effect of the addition of fibres on transverse and impact strengths was investigated. The addition of surface treated chopped or continuous fibres to acrylic resin did not improve the transverse or impact strengths. The addition of surface treated poly (methyl methacrylate), in the present form, to acrylic resin cannot be recommended as a method of reinforcement.

Acrylic Resins↗
PubMed2002

Strength/endurance training versus endurance training in congestive heart failure.

PURPOSE: The aim of this study was to compare the effects of endurance training alone (ET) with combined endurance and strength training (CT) on hemodynamic and strength parameters in patients with congestive heart failure (CHF). METHODS: Twenty male patients with CHF were randomized into one of two training regimens consisting of endurance training or a combination of endurance and resistance training. Group ET had 40-min interval cycle ergometer endurance training three times per week. Group CT combined endurance and strength training with the same interval endurance training for 20 min, followed by 20 min of strength training. Left ventricular function was assessed at baseline and after 40 training sessions by echocardiography and radionuclide ventriculography. Work capacity was measured with cardiopulmonary exercise test (CPX) and lactate determination. Strength was measured with an isokinetic dynamometer. RESULTS: After 40 sessions, the ET group improved functional class, work capacity, peak torque, and muscular endurance. However, peak O2 remained unchanged. Left ventricular ejection fraction (LVEF) and fractional shortening (FS) decreased, whereas left ventricular end-diastolic diameter (LVED) increased. The CT group improved NYHA score, working capacity, peak O2, and peak lactate; peak torque and muscular endurance, LVEF, and FS increased, whereas LVED decreased. Compared with ET, CT was significantly (P < 0.05) better in improving LV function. CONCLUSION: Combined endurance/strength training was superior to endurance training alone concerning improvement of LV function, peak VO2, and strength parameters. It appears that for stable CHF patients, a greater benefit can be derived from this training modality.

Adult↗
PubMedDate not supplied

[Determining factors of bone mechanical resistance].

Bone strength is determined by bone geometry, cortical thickness and porosity, trabecular bone morphology and the intrinsic properties of bony tissue. The major complication of osteoporosis, i.e. fracture, is due to a lower bone strength. Thus, any treatment of osteoporosis implies an improvement in bone strength. Bone strength is indirectly estimated by bone mineral density (BMD) using Dual-Energy X-ray Absorptiometry (DXA). Since, DXA-measured BMD accounts for part of the variance in bone strength, some important factors are not captured by DXA, in terms of the effects of anti-osteoporotic treatment and the progression of osteoporosis. Geometry and trabecular microarchitecture have also to be taken into account. Thus, the assessment of the intrinsic mechanical quality of bony tissue should provide a better understanding of the role of tissue quality in determining bone strength. The careful investigation of all the determinants of bone strength (bone tissue included) should be considered in the pathophysiology of osteoporosis and in the mechanisms of action of anti-osteoporotic drugs.

Absorptiometry, Photon↗
PubMed1998

Role of microstructure on contact damage and strength degradation of micaceous glass-ceramics.

OBJECTIVES: This study examines the hypothesis that microstructure plays a critical role in the accumulation of strength-degrading damage in dental ceramics. A series of micaceous glass-ceramics crystallized from a common glass composition, using heat treatments to increase the diameter and aspect ratio of mica platelets, is used as a model ceramic system. METHODS: Damage modes are investigated by Hertzian contact testing. Four-point bend tests on indented specimens quantify the influence of single-cycle and multi-cycle damage on strength. RESULTS: Two competing damage modes are observed: fracture, by tensile-induced cone cracking at the macroscopic level; and quasi-plastic deformation, by shear-induced yield at the microscopic level. The quasi-plastic mode becomes more dominant as the microstructures become coarser and more elongate. Bend tests show severe strength losses in the finer grain structures where cone cracking dominates, but relatively small losses in the coarser grain structures where quasi-plasticity dominates. Whereas natural strengths decline with increasing crystallization temperature, the strengths after indentation damage attain a maximum at intermediate crystallization temperatures. Multiple-cycle contact loading reduces strengths even further, and at relatively low indentation loads, indicating susceptibility to fatigue. Finite element modelling is carried out to evaluate the stress components that drive the damage modes. SIGNIFICANCE: Microstructure is confirmed to be a controlling factor in determining the nature and degree of strength-impairing damage accumulation in dental ceramics. The Hertzian test provides a means of characterizing such damage in the context of clinical function.

Aluminum Silicates↗
PubMed1999

Contact damage resistance and strength degradation of glass-infiltrated alumina and spinel ceramics.

All-ceramic crowns are coming into widespread use because of their superior esthetics and chemical inertness. This study examines the hypothesis that glass-infiltrated alumina and spinel core ceramics are resistant to damage accumulation and strength degradation under representative oral contact conditions. Accordingly, Hertzian indentation testing with hard spheres is used to evaluate damage accumulation in alumina and spinel ceramics with different pre-form grain morphologies and porosities. Indentation stress-strain curves measured on fully infiltrated materials reveal a marked insensitivity to the starting pre-form state. The glass phase is shown to play a vital role in providing mechanical rigidity and strength to the ceramic structures. All the infiltrated ceramics show subsurface cone fracture and quasi-plastic deformation above critical loads P(C) (cracking) and P(Y) (yield), depending on sphere radius, with P(Y) < P(C). Strength degradation from accumulation of damage in Hertzian contacts above these critical loads is conspicuously small, suggesting that the infiltrated materials should be highly damage-tolerant to the "blunt" contacts encountered during mastication. Failure in the strength tests originates from either cone cracks ("brittle mode") or yield zones ("quasi-plastic mode"), with the brittle mode more dominant in the spinels and the quasi-plastic mode more dominant in the aluminas. Multi-cycle contacts at lower loads, but still above loads typical of oral function, are found to be innocuous up to 10(5) cycles in air and water, although contacts at 10(6) cycles in water do cause significant strength degradation. By contrast, contacts with Vickers indenters produce substantial strength losses at low loads, suggesting that the mechanical integrity of these materials may be compromised by inadvertent "sharp" contacts.

Aluminum Oxide↗
PubMed2004

Fatigue strengths of particulate filler composites reinforced with fibers.

The aim of this study was to evaluate the dynamic fatigue strengths at 10(5) cycles and the strains of particulate filler composite resins with and without reinforcing fibers. An UHMWPE (Ribbond), a polyaromatic polyamide fiber (Fibreflex), and three glass fibers (GlasSpan, FibreKor, Vectris Frame) were used to reinforce the particulate filler composite resins. The fatigue properties were measured in three-point bending mode using a servohydraulic universal testing machine at a frequency of 5 Hz, until failure occurred or 10(5) cycles had been completed. The fatigue strengths at 10(5) cycles were determined by the staircase method. The fractured aspects of specimens were evaluated by an optical and scanning electron microscope. The fatigue strengths of particulate filler composite resins were 49-57 MPa, and those of fiber-reinforced were 90-209 MPa. Unidirectional glass fibers showed higher reinforcing effects on the fatigue strengths of composite resins. The strain of UHMWPE-reinforced composite was largest.

Analysis of Variance↗