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PubMedDate not supplied

Failure mechanisms in human vertebral cancellous bone.

Specimens of human vertebral cancellous bone were compressed to well past mechanical failure (15% strain) in the infero-superior direction. The mechanisms of failure were examined microscopically and histologically. The primary mechanism of failure was shown to be microscopic cracking rather than overt failure of the trabecular elements. The morphology of the cracks was consistent with an hypothesis that they were the result of shear stress (or strain) in the tissue. Complete fracture of trabeculae was confined to elements oriented transversely to the direction of loading. The tissue's ultimate strength and residual strength after compressive failure were both strongly correlated to tissue stiffness (R2 = 0.88 and R2 = 0.71, respectively). It is proposed that cancellous bone strength may be a consequence of the adaptation of bone stiffness to applied stresses. With removal of the load, all specimens recovered at least 94% of their original height. Implications of energy dissipation by microcracking for recovery and maintenance of overall trabecular architecture are discussed.

Aged↗
PubMed2004

The effect of jaw position on measures of tongue strength and endurance.

Assessment of tongue strength and endurance is common in research and clinical contexts. It is unclear whether the results reveal discrete function by the tongue or combined abilities of the tongue and jaw. One way to isolate the movement of the tongue is to constrain the jaw kinematically by using a bite block. In this study, 10 neurologically normal young adults performed tongue strength and endurance tasks without a bite block ("jaw-free") and with bite blocks of various heights (2, 5, 10, and 15 mm for strength; 5 mm for endurance). Data signals included tongue pressure exerted on an air-filled bulb, surface electromyography (SEMG) from the superior tongue blade, and SEMG from 1 masseter. On average, tongue strength (pressure in kPa) was greatest with no bite block and generally decreased as bite blocks increased in height. Pairwise analyses revealed statistically significant differences for all but 3 comparisons (jaw-free to 2 mm, 2 to 5 mm, and 5 to 10 mm). After removing outlying data from 1 participant, tongue endurance at 50% of tongue strength was significantly greater without a bite block than with one. SEMG data did not differ significantly for the strength task across bite block conditions, but inspection of the individual data revealed a tendency for masseter activity to be lower when the jaw was unconstrained. These results suggest that maximal tongue strength and endurance are best assessed with an unconstrained mandible or with a very small bite block.

Adult↗
PubMed1996

Bone mass, areal, and volumetric bone density are equally accurate, sensitive, and specific surrogates of the breaking strength of the vertebral body: an in vitro study.

The validity of the bone mineral density (BMD) measurement depends on its accuracy as a predictor of the breaking strength of bone. As the breaking strength is proportional to the square of the apparent density, a small error in the calculation of BMD may result in a larger error in the predicted bone strength. The aims of this study were (i) to determine whether inaccuracies in the measurement of the dimensions, projected area, and volume of the vertebral body (used to derive the areal and volumetric BMD) result in errors in the predicted breaking strength and (ii) to compare the accuracy, sensitivity, and specificity of bone mineral content (BMC), areal BMD, volumetric BMD, and volumetric bone mineral apparent density (BMAD) as surrogates of bone strength. We measured the BMC (by densitometry), dimensions and volume (using calipers, densitometry, the Carter et al. and Peel and Eastell methods), and breaking strength (using the Instron 1114 apparatus, Newtons, N) of 22 vertebral body specimens. All methods resulted in errors in height, width, and depth between -11.3 +/- 1.0 and 30.4 +/- 1.8% relative to the "gold" standard caliper method. The vertebral body volume (of 38.0 +/- 1.2 cm3) measured by submersion was used as the gold standard to derive the volumetric BMD gold standard (of 0.162 +/- 0.01 g/cm3). All methods, except the Peel and Eastell method, resulted in errors ranging between -10.7 +/- 1.5 and 56.9 +/- 3.4% in vertebral body volume and -35.6 +/- 1.5 to 12.6 +/- 1.8% in volumetric BMD (all p < 0.0005). The same absolute value for volumetric BMD predicted a breaking strength that differed according to the method used to derive BMD. For example, a volumetric BMD of 0.162 g/cm3 predicted a breaking strength of 6208 N (submersion method), 5473 N (caliper method), 6095 N (Peel and Eastell method), 7697 N (DXA method), and 9470 N (Carter et al. method). The mean volumetric BMD derived by each method differed (0.181, 0.165, 0.133, and 0.104 g/cm3, respectively). However, all were accurate; each predicted a similar breaking strength (6177, 6217, 6209, and 6221 N respectively). Likewise, breaking strengths predicted by the mean BMC, areal BMD by calipers, and areal BMD by dual-energy X-ray absorptiometry (DXA) were 6267, 6214, and 6244 N, respectively. The methods were equally sensitive; a 1 standard deviation (SD) decrease in volumetric BMD resulted in a similar decrease in the breaking strength of 1818 (caliper), 2080 (Peel and Eastell), 2001 (DXA), and 1625 N (BMAD by Carter et al). A 1 SD decrease in BMC, areal BMD (using calipers) and areal BMD (using DXA) predicted a decrease in the breaking strength of 2019, 1738, and 1825 N, respectively. All methods were equally specific; the variance in bone strength explained by bone mass did not differ for volumetric BMD (38-61% depending on the method), BMC (58%), or areal BMD (48%). In conclusion, despite errors in the measurement of the dimensions of the vertebral body, bone mass, areal, and volumetric bone density are equally accurate, sensitive, and specific surrogates of the breaking strength of bone in vitro.

Absorptiometry, Photon↗
PubMed2006

Increasing bending strength of tibial locking screws: mechanical tests and finite element analyses.

BACKGROUND: Healing of tibial fractures treated by locked nailing is threatened by locking screw failure. However, the effects of the design factors of the screws on their mechanical strength have rarely been studied. METHOD: Three-point bending tests and finite element analyses were used to investigate the bending strength of five types of commercially available tibial locking screws and two types of specially designed screws. Yielding strength and fatigue life measured in bending tests were correlated to total strain energy and maximal tensile stress computed in finite element analyses. Parametric analysis and design optimization were done according to the Taguchi method. Validation studies to assess the stress rising effect of the threads on the fatigue strength were conducted in two types of new screws made of either stainless steel or titanium alloy. FINDINGS: The yielding strength of the screws was closely related to their total strain energy, and the logarithm of the fatigue life was closely related to the maximal tensile stress with correlation coefficients of -0.95 and -0.90, respectively. Parametric studies indicated that fatigue strength of the screws was affected mainly by inner diameter (contribution, 63.8%) and root radius (27.8%). The yielding strength was determined primarily by inner diameter (88.5%). Titanium screws had a longer fatigue life than stainless steel screws, especially in screws with larger root radii. INTERPRETATION: A screw's strength is closely related to its design factors. Finite element models, which can reliably reflect the mechanical strength of screws can save time and effort during screw design. Larger root radius can effectively improve the fatigue strength, especially for titanium screws as compared with stainless steel screws.

Biomechanical Phenomena↗
PubMed2005

The effect of core:dentin thickness ratio on the bi-axial flexure strength and fracture mode and origin of bilayered dental ceramic composites.

OBJECTIVES: The aim of the current study was to assess the effect of core:dentin thickness ratio on the flexure strength, fracture mode and origin of bilayered dental ceramic composite disc specimens. METHODS: Sets of 30 bilayered composite discs with core:dentin thickness ratio of 2:1, 1:1 and 1:2 were tested in bi-axial flexure with both the reinforcing core and veneering dentin loaded in tension. Mean flexure strengths, standard deviations and associated Weibull Moduli (m) were determined. A combination of optical and scanning electron microscopy was employed for identification of the fracture mode and origin. RESULTS: The core:dentin ratio influenced the bi-axial flexure strength and reliability of the flexure strength data when both the reinforcing core and veneering dentin porcelain were tested in tension. The strength and reliability was increased for a core:dentin thickness of 2:1. The number of fracture fragments, the frequency of occurrence of specimen delaminations, Hertzian cone formations and sub-critical radial cracking in the bilayered dental ceramic composite disc shaped specimens was also dependent on the core/dentin ratio and the surface loaded in tension. CONCLUSIONS: Core:dentin thickness ratio influences the bi-axial flexure strength and fracture mode and origin in bilayered dental ceramic composite specimens.

Ceramics↗
PubMed2003

Effects of combined treatment of insulin and human parathyroid hormone(1-34) on cancellous bone mass and structure in streptozotocin-induced diabetic rats.

The purpose of this study was to test the hypothesis that combined treatment with insulin and human parathyroid hormone (hPTH) is more effective than treatment with insulin or hPTH alone in improving cancellous bone mass, connectivity, and strength in insulin-dependent diabetic rats. Diabetes was induced by intraperitoneal injection of streptozotocin (STZ) in 7-month-old female Wistar rats. The diabetic rats received insulin, hPTH, insulin and hPTH, or hPTH vehicle for 4 weeks, starting 8 weeks after STZ injection. They were compared with baseline controls and normal controls that received STZ alone and STZ vehicle alone, respectively. The rats' proximal right tibias were processed to serve as undecalcified Villanueva-stained bone sections for histomorphometry. Changes in trabecular connectivity were determined through node-strut analysis. The decreased cancellous bone volume (BV/TV) and bone formation in diabetic rats improved in all the drug-treated groups compared with baseline controls. Furthermore, recovery of BV/TV was greater in rats that received the combination of insulin and hPTH than in those that received insulin or hPTH alone. In node-strut analysis, the node-related parameter (N.Nd/TV) significantly increased in rats that received the combination of insulin and hPTH, but did not increase in those that received insulin or hPTH alone. In addition to these results, the combination treatment significantly increased bone mineral density of the femur and bone strength in the femoral metaphysis compared with treatment with insulin or hPTH alone. These results indicate that the doses of insulin and hPTH employed in the combination treatment were more effective in improving not only bone mass but also trabecular connectivity and bone strength than treatment with insulin or hPTH alone in insulin-dependent diabetic rats.

Animals↗
PubMed2000

Comparison of in vitro fracture strength of metallic and tooth-coloured posts and cores.

In root-treated anterior teeth a better aesthetic can be achieved by using translucent and tooth-coloured post and core restorations instead of traditional metal post systems. The aim of this in vitro study was to compare the fracture strength of tooth-coloured ceramic and fibre-reinforced posts as well as titanium posts with clinically proven gold alloy posts as a control. Two all-ceramic, one gold alloy and three combined post restoration systems (titanium and ceramic, respectively, fibre-reinforced posts with composite core) were each applied to upper incisors and after thermal cycling and mechanical loading (TCML) the fracture strength was determined. The variant providing the most reliable fracture strength values and the experimental fibre-reinforced composite (FRC) posts were - in combination with adhesively luted crowns - used to determine the fracture strength of the restoration after thermal cycling and mechanical loading. The median fracture strength of the luted posts and cores fluctuated between 114 and 546 N. Posts with composite cores demonstrate a higher fracture strength in comparison to the all-ceramic and gold alloy systems. Typical failure of metal systems was marked by loosening of the bonding and pulling out of the post in contrast to fracture of the ceramic posts. After the application of the crowns the ceramic system with composite core provided a fracture strength of 338 N and the FRC system a fracture strength of 228 N. Regarding fracture strength, the ceramic posts with composite cores are an alternative to commonly used gold posts and cores. A standardized technical manufacture of FRC posts and cores would provide better fracture strength results of this new dental material. Post and core restorations with tooth-coloured, translucent material offer an improved aesthetic especially in anterior restorations. Because of the high fracture strength prefabricated ceramic posts in combination with composite cores should prove their clinical applicability.

Aluminum Silicates↗
PubMed1996

Bond strength of an adhesive polymerized under nitrogen.

PURPOSE: To evaluate the use of nitrogen, in conjunction with visible light polymerization, for improving composite-to-dentin bond strengths. MATERIALS AND METHODS: Composite cylinders were bonded to dentin with Scotchbond Multi-Purpose. Immediate (Groups 1-4) and 24-hour (Groups 5-8) strengths were determined for the following two adhesive application and two polymerization techniques: Groups 1 & 5: gentle air thinning of adhesive and polymerization with conventional visible light curing (VLC) unit; Groups 2 and 6: aggressive air thinning of adhesive and conventional VLC; Groups 3 and 7: gentle air thinning of adhesive and polymerization in nitrogen environment with modified VLC unit; and Groups 4 and 8: aggressive air thinning of adhesive and polymerization in nitrogen environment with modified VLC unit. RESULTS: The immediate bond strengths (MPa) were: Group 1: 15.1 +/- 1.8, Group 2: 6.1 +/- 2.1, Group 3: 17.8 +/- 3.1, Group 4: 15.8 +/- 2.0. The 24-hour strengths were: Group 5: 26.7 +/- 4.4, Group 6: 13.5 +/- 7.5, Group 7: 24.1 +/- 5.7, Group 8: 26.4 +/- 4.3. Visible light polymerization of a thinned layer of adhesive resin in a nitrogen environment produced significantly higher immediate and 24-hour bond strengths than when compared to visible light polymerization in ambient air.

Air↗
PubMed1998

Optimization of the granulation process for designing tablets.

A computer optimization technique based on surface response methodology was applied to optimize the wet granulation process for designing tablets. Physical properties (mean granule size, granule size distribution, compressibility, granule strength) of a model granule formulation containing ethenzamide were accurately described by a second polynomial equation based on two independent variables (amounts of binder and binder solution). This regression equation also gave a good correlation for three physical properties of tablets (distintegration time, compactibility, compression force variance), but the correlation for tablet hardness and weight variation was poor. These results imply that not only the above physical properties of granules but also the rheological behavior and porous structure of granules are closely related to tablet properties. Using an optimization of five tablet properties using the generalized distance function, the predicted values of the physical properties of both granules and tablets agreed well with experimental values. This agreement indicates that the computer optimization technique is useful for optimizing the granulation process for designing tablets.

Computers↗
PubMedDate not supplied

Physical properties of proprietary light-cured lining materials.

Physical parameters of four photopolymerized lining materials were evaluated for comparative purposes and to contribute to the understanding of these novel materials. Properties investigated included compressive, diametral and flexural strength, modulus of elasticity, fluoride and calcium release for four weeks, and pH values for 24 hours. In general Cavalite and TimeLine were significantly stronger than Vitrabond and XR-lonomer and were essentially neutral in pH. All the materials exhibited a yield upon compressive strength testing. The fluoride release was intermediate for TimeLine and negligible with Cavalite. Vitrabond and XR-lonomer released significant amounts of fluoride at all time periods. XR-lonomer was the only material investigated to have significant calcium release, and this may have been related to its visible dissolution in water. This material also showed a low initial pH of 2.2. It, along with Vitrabond, demonstrated a gradual increase in pH over time. The behavior of TimeLine and Cavalite is therefore more consistent with that of a modified composite resin, whereas Vitrabond and XR-lonomer are similar in nature to glass-ionomer liners.

Acrylic Resins↗
PubMed2005

Preparation and characterization of ibuprofen pellets based on Eudragit RS PO and RL PO or their combination.

The aim of this study was to assess the application of Eudragit RL PO and RS PO and their combination for production of ibuprofen pellets by extrusion-spheronization. The pellets were prepared based on full factorial design. Independent variables were % ibuprofen (40, 60, 80), the ratio of Eudragit RS to Eudragit RL (1:0, 1:1, 0:1) and % PVP (1, 3, 5). The evaluated responses were mean dissolution time (MDT), crushing strength and elastic modulus of pellets. Surface characteristics, sphericity and aspect ratio of pellets were also evaluated. Linear regression and response surface modeling was used for analyzing results. It was shown that the amount of water required to prepare a proper wet mass was affected by composition of formulations. The required amount of water decreased with increasing drug load and percent of PVP. It was also shown that formulations containing Eudragit RL need more water. Increasing percent of PVP slightly decreased MDT and elastic modulus but had negligible effect on crushing strength. Increasing the percent of ibuprofen up to 60% decreased MDT but beyond that increased MDT. Increasing the percent of ibuprofen also decreased elastic modulus of pellets. Eudragit RL PO compare with Eudragit RS PO resulted in pellets with high crushing strength; however, Eudragit type did not have a significant effect on elastic modulus.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed2004

Mechanical failure of human fetal membrane tissues.

Mechanical integrity of the chorioamnion membrane, and the component chorion and amnion layers, was assessed with biaxial puncture testing. Fetal membranes were obtained from term placentas following labored natural delivery or scheduled cesarean section. Preterm specimens were obtained from deliveries prior to 37 weeks gestation. Dividing and peripheral membranes were obtained from multiple gestation pregnancies. Specimens were gripped between parallel plates with circular openings and loaded with an instrumented, hand-held blunt probe until rupture occurred. Peak force was recorded and rupture sites were examined. Defects in multi-layered membranes differed in both size and shape in the individual layers. Compared with chorion and whole chorioamnion, amnion was more mechanically sensitive to different obstetrical conditions. Amnion varied in response at different physical locations within the same patient. Membrane and component puncture force data were used to calculate biaxial failure strength. Membrane stresses arising from amniotic fluid pressure were computed as a function of gestational age, and compared to membrane strength to examine the criterion for membrane failure in vivo. Possible mechanical conditions for preterm membrane rupture were examined.

Amnion↗
PubMed1997

Normalizing strength for body size differences in older adults.

The purpose of this study was to compare the normalization methods of ratio standards, allometry, and ANCOVA with knee extensor strength of older adults. The apparently healthy older volunteers were 71 men (mean +/- SD; age, 71 +/- 4 yr; body mass, 81 +/- 10 kg; height, 174 +/- 7 cm) and 77 women (71 +/- 4 yr, 65 +/- 8 kg, 160 +/- 5 cm. respectively). Strength was defined as peak torque (N.m-1) and measured with a Cybex II isokinetic dynamometer. Body composition was estimated with dual energy x-ray absorptiometry. With allometry, the body mass exponent (0.74) was not statistically different from theory (0.67). Body mass adjusted strengths were 34.7% (allometry), 32.0% (ANCOVA), and 29.4% (ratio standards) greater in older men than women. Allometry revealed that the bone-free lean tissue mass exponent was not different from ratio standard exponent of 1.0. After adjustment by bone-free lean tissue mass, strength in men remained 16.0% (allometry and ratio standards) higher than in women, but, strength differences between genders were eliminated with ANCOVA. The methods used to normalize strength yielded similar results with body mass but conflicting results with bone-free lean tissue mass.

Aged↗
PubMed1999

Do dentin bond strength tests serve a useful purpose?

PURPOSE: The publication of dentin bond strength data is prolific in the dental literature and will remain so as more and more new dentin bonding agents are marketed. It is the purpose of this presentation to assess the usefulness of this information. METHODS: The limitations of dentin bond strength tests are examined in the context of current knowledge about the methods used. RESULTS: The evidence provided shows that dentin bond strength measurement techniques provide data that is not able to act as a consistent predictor of clinical performance. Fundamental flaws in the most popular methods adopted have been identified and alternative approaches need to be explored. Unfortunately, we do not as yet have in place methodologies that allow us to establish a reliable link between laboratory bond strength data and clinical performance. CONCLUSION: For the present, clinically based evidence remains the only reliable means for the selection of dentin bonding agents.

Adhesiveness↗
PubMed2004

Spheroidization of glass powders for glass ionomer cements.

Commercial angular glass powders were spheroidized using both the flame spraying and inductively coupled radio frequency plasma spraying techniques. Spherical powders with different particle size distributions were obtained after spheroidization. The effects of spherical glass powders on the mechanical properties of glass ionomer cements (GICs) were investigated. Results showed that the particle size distribution of the glass powders had a significant influence on the mechanical properties of GICs. Powders with a bimodal particle size distribution ensured a high packing density of glass ionomer cements, giving relatively high mechanical properties of GICs. GICs prepared by flame-spheroidized powders showed low strength values due to the loss of fine particles during flame spraying, leading to a low packing density and few metal ions reacting with polyacrylic acid to form cross-linking. GICs prepared by the nano-sized powders showed low strength because of the low bulk density of the nano-sized powders and hence low powder/liquid ratio of GICs.

Compressive Strength↗
PubMed1998

Evaluation of micro-tensile, shear and tensile tests determining the bond strength of three adhesive systems.

OBJECTIVES: The aim of this study was to determine bond strength between dentin and three adhesive systems, by means of microtensile, shear and tensile tests. METHODS: Extracted human molars were embedded in acrylic resin and had the dentin exposed on three of their smooth surfaces. On each surface a specimen was prepared to be submitted to either micro-tensile, shear or tensile bond strength testing. For shear and tensile tests, after adhesive application, a cone 3 mm high and diameter of 3 mm in the small surface was built with composite resin. The shear test was performed with a chisel. Tensile testing was made by pulling the resin cone via a metallic clamp. For micro-tensile testing, composite approximately 5 mm high was placed over the entire exposed dentin. Then, using a diamond disk perpendicular to the bonding interface, 'sticks' with 0.25 mm2 rectangular cross-sectional area were obtained and subjected to tensile force. RESULTS: All tests ranked the adhesives in the same order. Mean values obtained by the micro-tensile test were not statistically different. For shear and tensile tests, Single Bond gave higher bond strength than Etch & Prime 3.0 (p < 0.05). Scotchbond Multi-Purpose Plus originated bondings that were statistically similar to both Single Bond and Etch & Prime 3.0. Comparing the three tests, a higher mean (p < 0.05) and a smaller coefficient of variation were found with the micro-tensile test. SIGNIFICANCE: The one-bottle adhesive system obtained higher bond strength values than the self-etching adhesive upon shear and tensile strength tests. Depending on the test applied, differences among materials may not be disclosed.

Analysis of Variance↗
PubMed2004

Evaluation of modifying the bonding protocol of a new acid-etch primer on the shear bond strength of orthodontic brackets.

The purpose of the study was to evaluate the shear bond strength of orthodontic brackets when light curing both the self-etch primer and the adhesive in one step. Fourty eight teeth were bonded with self-etch primer Angel I (3M/ESPE, St Paul, Minn) and divided into three groups. In group I (control), 16 teeth were stored in deionized water for 24 hours before debonding. In group II, 16 teeth were debonded within half-an-hour to simulate when the initial archwires were ligated. In group III, 16 additional teeth were bonded using exactly the same procedure as in groups I and II, but the light cure used for 10 seconds after applying the acid-etch primer was eliminated, and the light cure used for 20 seconds after the precoated bracket was placed over the tooth. This saved at least two minutes of the total time of the bonding procedure. The teeth in this group were also debonded within half-an-hour from the time of initial bonding. The teeth debonded after 24 hours of water storage at 37 degrees C had a mean shear bond strength of 6.0 +/- 3.5 MPa, the group that was debonded within half-an-hour of two light exposures had a mean shear bond strength of 5.9 +/- 2.7 MPa, and the mean for the group with only one light cure exposure was 4.3 +/- 2.6 MPa. Light curing the acid-etch primer together with the adhesive after placing the orthodontic bracket did not significantly diminish the shear bond strength as compared with light curing the acid-etch primer and the adhesive separately.

Acid Etching, Dental↗