Average Fracture Load Research Articles

Fracture resistance and failure mode of three esthetic CAD-CAM post and core restorations

BackgroundThe rising demand for improved aesthetics has driven the utilization of recently introduced aesthetic materials for creating custom post and core restorations. However, information regarding the fracture resistance of these materials remains unclear, which limits their practical use as custom post and core restorations in clinical applications.Aim of the studyThis study aimed to evaluate the fracture resistance of three non-metallic esthetic post and core restorations and their modes of failure.Materials and methodsThirty-nine single-rooted human maxillary central incisors were endodontically treated. A standardized post space preparation of 9mm length was performed to all teeth to receive custom-made post and core restorations. The prepared teeth were randomly allocated to receive a post and core restoration made of one of the following materials (n=13): glass fiber-reinforced composite (FRC), polyetheretherketone (PEEK) and polymer-infiltrated ceramic-network (PICN). An intraoral scanner was used to scan all teeth including the post spaces. Computer-aided design and computer-aided manufacturing (CAD-CAM) was used to fabricate post and core restorations. Post and core restorations were cemented using self-adhesive resin cement. All specimens were subjected to fracture resistance testing using a universal testing machine. Failure mode analysis was assessed using a stereomicroscope and SEM. The data was statistically analyzed using One-Way ANOVA test followed by multiple pairwise comparisons using Bonferroni adjusted significance level.ResultsCustom PEEK post and core restorations displayed the least fracture load values at 286.16 ± 67.09 N. In contrast, FRC exhibited the highest average fracture load at 452.60 ± 105.90 N, closely followed by PICN at 426.76 ± 77.99 N. In terms of failure modes, 46.2% of specimens with PICN were deemed non-restorable, while for PEEK and FRC, these percentages were 58.8% and 61.5%, respectively.ConclusionsWithin the limitation of this study, both FRC and PICN demonstrated good performance regarding fracture resistance, surpassing that of PEEK.

Dependence of Monocrystalline Sapphire Dicing on Crystal Orientation Using Picosecond Laser Bessel Beams

Dicing is a critical step in the manufacturing process for the application of sapphire. In this work, the dependence of sapphire dicing on crystal orientation using picosecond Bessel laser beam drilling combined with mechanical cleavage was studied. By using the above method, linear cleaving with on debris and zero tapers was realized for the A1, A2, C1, C2, and M1 orientations, except for the M2 orientation. The experimental results indicated that characteristics of Bessel beam-drilled microholes, fracture loads, and fracture sections of sapphire sheets were strongly dependent on crystal orientation. No cracks were generated around the micro holes when laser scanned along the A2 and M2 orientations, and the corresponding average fracture loads were large, 12.18 N and 13.57 N, respectively. While along the A1, C1, C2, and M1 orientations, laser-induced cracks extended along the laser scanning direction, resulting in a significant reduction in fracture load. Furthermore, the fracture surfaces were relatively uniform for A1, C1, and C2 orientations but uneven for A2 and M1 orientations, with a surface roughness of about 1120 nm. In addition, curvilinear dicing without debris or taper was achieved to demonstrate the feasibility of Bessel beams.

Influence of New Sleeve Composite on Fracture Behavior of Anterior Teeth with Flared Root Canals.

We evaluated the fracture strength and failure mode of non-ferrule teeth with flared root canals that were restored using new experimental sleeve composites. Fifty endodontically treated anterior teeth with flared root canals were restored with direct restorations utilizing different techniques. Group A had teeth (non-ferrule) restored using commercialized MI glass fiber post + Gradia Core as core build-up. Group B had teeth (non-ferrule) restored with commercialized i-TFC glass fiber post + sleeve system. In Group C, the teeth (non-ferrule) were restored with an experimental sleeve composite with commercialized MI glass fiber post and Gradia Core. Group D, teeth (non-ferrule), were restored using custom-made tapered E-glass filling post and Gradia Core. Group E, teeth (with ferrule), were restored with commercialized MI glass fiber post + Gradia Core. After core construction, all specimens underwent direct composite crown restoration and were loaded until fracture using a universal testing machine. Average fracture loads were compared, and the failure modes were observed. Group C exhibited significantly greater fracture strength than other groups (p < 0.05). Favorable fracture teeth ratio of group C was more than that of the other groups. Thus, the new experimental sleeve composite could be clinically useful for core construction of non-ferrule teeth.

Study on cleavage fracture probability-load curves of ferritic steel 20MnMoNi55 by using the new local approach model

In this paper, the new local approach model (NLAM) is first adopted to predict the cleavage fracture probability-load curves of notched round bar tensile specimens of ferritic steel 20MnMoNi55 at the temperatures of 123 K and 223 K. Then, based on the obtained curve, the probability density function, and the extreme value theorem of function, two significant cleavage fracture probabilities are proposed. Finally, the proposed probabilities are employed to predict the load which is most likely to cause cleavage fracture of 20MnMoNi55 steel. The results show that when the value of cleavage fracture probability is 45.29%, the corresponding predicted load is basically equal to the average fracture load of the specimens. Besides, the load corresponding to the cleavage fracture probability of 59.7% is the most likely one to induce cleavage fracture of the structure.

Finite element analysis of the femoral diaphysis of fresh-frozen cadavers with computed tomography and mechanical testing

BackgroundThe purpose of this study was to validate a diaphyseal femoral fracture model using a finite element analysis (FEA) with mechanical testing in fresh-frozen cadavers.MethodsWe used 18 intact femora (9 right and 9 left) from 9 fresh-frozen cadavers. Specimens were obtained from 5 males and 4 females with a mean age of 85.6 years. We compared a computed tomography (CT)-based FEA model to diaphyseal femoral fracture loads and stiffness obtained by three-point bending. Four material characteristic conversion equations (the Keyak, Carter, and Keller equations plus Keller’s equation for the vertebra) with different shell thicknesses (0.3, 0.4, and 0.5 mm) were compared with the mechanical testing.ResultsThe average fracture load was 4582.8 N and the mean stiffness was 942.0 N/mm from actual mechanical testing. FEA prediction using Keller’s equation for the vertebra with a 0.4-mm shell thickness showed the best correlations with the fracture load (R2 = 0.76) and stiffness (R2 = 0.54). Shell thicknesses of 0.3 and 0.5 mm in Keller’s equation for the vertebra also showed a strong correlation with fracture load (R2 = 0.66 for both) and stiffness (R2 = 0.50 and 0.52, respectively). There were no significant correlations with the other equations.ConclusionWe validated femoral diaphyseal fracture loads and stiffness using an FEA in a cadaveric study.

Diode Laser Welding/Brazing of Aluminum Alloy to Steel Using a Nickel Coating

Joining Al alloy to steel is of great interest for application in the automotive industry. Although a vast number of studies have been conducted to join Al to steel, the joining of Al to steel is still challenging due to the formation of brittle Fe–Al intermetallic compounds. In this work, the microstructure and mechanical properties of the dissimilar Al/steel joints with and without a nickel coating are comparatively investigated. A homogenous reaction layer composed of FeZn10 and Fe2Al5 is formed at the interface in the joints without Ni coating, and the joint facture load is only 743 N. To prevent the formation of brittle Fe2Al5, Ni electroplated coating is applied onto a steel surface. It has been shown that a nonhomogeneous reaction layer is observed at the interfacial region: Ni5Zn21 is formed at the direct irradiation zone, while Al3Ni is formed at the fusion zone root. The microhardness of the interfacial layer is reduced, which leads to the improvement of the joint mechanical properties. The average fracture load of the Al/Ni-coated steel joints reaches 930 N. In all of the cases, failure occurs at the Ni coating/fusion zone interface.

Fracture load of metal-ceramic, monolithic, and bi-layered zirconia-based posterior fixed dental prostheses after thermo-mechanical cycling

ObjectivesThis study aims to evaluate the fracture load of differently fabricated 3-unit posterior fixed dental prostheses (FDPs) with an intermediate pontic. MethodsFifty sets of two stainless-steel abutments were randomly assigned to five groups (n = 10 each) depending on the material and technique used for manufacturing the FDPs: (1) Metal-ceramic (MC, control); (2) Lava Zirconia (LZ, bi-layered); (3) Lava Plus (LM, monolithic); (4) VITA In-Ceram YZ (YZ, bi-layered); and (5) IPS e-max ZirCAD (ZZ, bi-layered). After being luted to the dies, all FDPs were submitted to thermo-mechanical cycling (120,000 masticatory cycles, 50 N; plus 774 thermal cycles of 5 °C/55 °C, dwell time: 30 s). Samples were then subjected to a three-point bending test until fracture in a universal testing machine (cross-head speed: 1 mm/min). Fracture load of the veneering ceramic (VF) and total fracture load (TF) were recorded. Microstructure and failure patterns were assessed. Data was analysed using one-way ANOVA and Tukey HSD post-hoc tests (α = 0.05). ResultsMC restorations recorded higher VF and TF values than did zirconia FDPs (p = 0.0001), which showed no between-group differences. Within the bi-layered groups, TF was significantly higher than VF. LM pieces registered lower average grain size than did LZ specimens (p = 0.001). Overall, the connector was the weakest part. ConclusionsAll of the groups tested could withstand clinical chewing forces in terms of average fracture load. Zirconia-based samples performed similarly to each other, but showed lower mean fracture load values than did metal-ceramic ones. Clinical significanceMonolithic zirconia may be recommendable for solving the chipping problem.

Effect of laser-GTAW hybrid welding heat input on the performance of Mg/Steel butt joint

The most common welding method for Mg/steel dissimilar metals is melt-brazing with lap joints; however, the butt fusion welding of Mg/steel joint remains a challenging task. 1.6mm AZ31B Mg alloy was joined with 1.0mm Q235 steel by using laser-gas tungsten arc welding (GTAW) hybrid butt welding with AZ61 filler metal. The influence of welding heat input on the morphology, interface bonding, mechanical properties and fracture modes of welded joints was systematically analyzed. Resultsshow that laser-GTAW hybrid welding with filling wire can be used to combine high-performance joints and Mg/steel dissimilar metals during butt assembly. The maximum average tensile fracture load of the joints can reach up to 3265N (the load equal to that of steel base metal). The tensile load of the joints decreases with the decrease in heat input. A reaction layer is formed on the welding interface enriched with Al and Mn elements. The morphology of Mg/steel joint and the thickness of interface transition layer are the key factors affecting the properties and fracture mechanism of the welded joint. With the decrease in heat input, the Mg/steel butt joints had three fracture modes, including the fracture on the base metal of steel, Mg weld seam, and Mg/steel interface.

The effect of glass fiber posts and ribbons on the fracture strength of teeth with flared root canals restored using composite resin post and cores

PurposeThis study evaluated the fracture strength and mode of failure of structurally compromised teeth with flared root canals restored using composite resin with four different systems. MethodsSixty endodontically treated bovine teeth were uniformly shaped to simulate human mandibular premolars with flared root canals. The roots were divided into four groups of 15 specimens each based on the type of restoration: composite resin core only (control), glass fiber post, cylindroid glass fiber ribbons, and glass fiber post and ribbons. All specimens were loaded until fracture occurred using a universal testing machine. Average fracture loads were compared with a one-way ANOVA and Tukey HSD test (α=.05). The modes of failure were observed and the Fisher exact test and Bonferroni correction were used for statistical analysis. ResultsThe fiber post and ribbon group (1035.70N) and the fiber ribbon group (881.77N) showed significantly higher fracture strength than the controls (567.97N) (p<.05). The fiber post and ribbon group also showed significantly higher fracture strength than the fiber post group (769.40N). Almost all specimens showed unrestorable root fractures (p<.008). The control group had a significantly higher ratio of core sectional fractures (p<.017). ConclusionsCylindroid glass fiber ribbons significantly increased the fracture strength of the composite resin post and cores in the case of the dentin within the thin root canal wall. Based on the results, this study recommends the combined use of glass fiber post and ribbons.

Self-riveting friction stir lap welding of aluminum alloy to steel

A novel self-riveting friction stir lap welding technique was developed by the prefabricated holes to realize sound and effective joining of 6082-T6aluminum alloy to QSTE340TM steel. Partial dynamic recrystallized aluminum flows downwards and fills the prefabricated holes forming self-riveting structure. Metallurgical bonding as diffusion layers formed at the interface of Al/Steel. The joint fracture process contains separation of self-riveting structure and unilateral hole wall, tearing of metallurgical bonding and final failure of self-riveting structure. An average fracture load of 3.21kN is obtained, which is stronger with high average elongation than that without prefabricated holes. The self-riveting structure is beneficial to prevent total failure of different materials joints with applicative perspectives.

Fatigue behavior of friction spot welds in lap shear specimens of AA5754 and Ti6Al4V alloys

The fatigue behavior of single friction spot welded AA5754 and Ti6Al4V dissimilar joints was investigated based on experimental observations. Fatigue tests were performed in a constant amplitude load control servo-hydraulic machine with a load ratio of R=0.1 at room temperature. Two-parameter Weibull distribution was used to analyze statistically the fatigue data for the joined overlapped sheets. Weibull graphics were plotted for each stress amplitude value. Subsequently, S–N curves were drawn for different reliability levels (10%, 50%, 90% and 99%) for the benefit of designers. A fatigue limit of 25MPa was determined for the AA5754/Ti6Al4V joints (at survival level of 50%), corresponding to 15% of the maximum average static fracture load. Two different types of fatigue failure modes were observed for the specimens. Under high cyclic loads, the failure occurred in the upper Al sheet because of the higher stress concentration at the notch tip leading to crack initiation followed by its propagation through the thickness toward the joint surface and, subsequently, along the specimen width direction. Under low cyclic loads, “through weld” failure was observed as the crack rapidly propagated along the interfacial surface until a critical point at which the remaining cross section could no longer sustain the shear overload and failed. A fatigue crack growth mechanism based on the paths of the dominant fatigue cracks was then proposed and used to explain the fatigue fracture evolution under different loading conditions.

Enhanced interfacial reaction and mechanical properties of laser welded-brazed Mg/Ti joints with Al element from filler

Dissimilar metal joining between magnesium and titanium alloys was performed using laser welding-brazing process. Metallurgical bonding of immiscible AZ31 magnesium and TC4 titanium was successfully achieved by adding Al element into Mg based filler, compared to mechanical bonding when direct joining. An ultra-thin serrated-shape reaction layer was evidently observed at the interface of AZ91 (9%Al) fusion zone/Ti and identified as Ti3Al phase by transmission electron microscopy (TEM), which could prevent crack propagation effectively and thus improved the joint strength. The average fracture load of joint with AZ91 filler could reach 1947±61N representing 47.3±1.5% joint efficiency relative to the magnesium base metal, which was 85% higher than that with AZ31filler.

Composite Resin Core Buildups With and Without Post for the Restoration of Endodontically Treated Molars Without Ferrule.

The aim of this study was to investigate the restoration of highly damaged, broken-down endodontically treated molars without the ferrule effect using glass ceramic crowns on different dual-cure composite resin core buildups. Thirty (N=30, n=15) decoronated, endodontically treated teeth (no ferrule) were restored without a ferrule with a direct buildup using the dual-curing composite Multicore HB (group MHB) or the dual-curing composite core buildup Multicore Flow in combination with glass-fiber-reinforced composite post (FRC post; group MFP). All teeth were prepared to receive bonded glass ceramic crowns (Empress CAD luted with Variolink II) and were subjected to accelerated fatigue testing. Cyclic isometric loading was applied to the palatal cusp at an angle of 30 degrees and a frequency of 5 Hz, beginning with a load of 200 N (×5000 cycles), followed by stages of 400, 600, 800, 1000, 1200, and 1400 N at a maximum of 30,000 cycles each. Specimens were loaded until failure or to a maximum of 185,000 cycles. Groups were compared using the life table survival analysis (log rank test at p=0.05). Average fracture loads and number of survived cycles were compared with one-way analysis of variance (Scheffé post hoc at p=0.05). Previously published data from the same authors about core buildups made of high-performance polymers (group HPP, n=15) and light-curing composite resin without FRC posts (group TEC, n=15) and with FRC posts (group TECP, n=15) using the same experimental setup were included for comparison. None of the tested specimen withstood all 185,000 load cycles. There was no significant difference in mean fracture load (p=0.376), survived cycles (p=0.422), and survival (p=0.613) between MHB (facture load 859.4 N±194.92) and MFP (796.13 N±156.34). Group HPP from a previous study appeared to have significantly higher performance than all other groups except MHB. All groups with posts were affected by an initial failure phenomenon (wide gap at the margin between the buildup/crown assembly and the root). HPP and MHB enhanced the performance of all-ceramic leucite-reinforced glass ceramic crowns, and insertion of a fiber-reinforced composite post was not influential when using other materials.

CAD/CAM Polymer vs Direct Composite Resin Core Buildups for Endodontically Treated Molars Without Ferrule.

The aim of this study was to investigate the restoration of broken-down endodontically treated molars without ferrule effect using glass ceramic crowns on different composite resin core buildups. Forty-five decoronated endodontically treated teeth (no ferrule) were restored with a semidirect buildup using an experimental computer-aided design/computer-aided manufacturing (CAD/CAM) high-performance polymer (HPP group) or with light-curing composite core buildups of Tetric EvoCeram with (TECP group) or without (TEC group) a glass-fiber-reinforced post. All teeth were prepared to receive bonded glass ceramic crowns (Empress CAD luted with Variolink II) and were subjected to accelerated fatigue testing. Cyclic isometric loading was applied to the palatal cusp at an angle of 30° and a frequency of 5 Hz, beginning with a load of 200 N (×5000 cycles) and followed by stages of 400, 600, 800, 1000, 1200, and 1400 N at a maximum of 30,000 cycles each. Specimens were loaded until failure or to a maximum of 185,000 cycles. Groups were compared using the life-table survival analysis (log rank test at p=0.05). Average fracture loads and number of survived cycles were compared with one-way analysis of variance (Scheffé post hoc at p=0.05). None of the tested specimen withstood all 185,000 load cycles. There was a significant difference in mean fracture load, survived cycles, and survival; the HPP group (fracture load 975.27N±182.74) was significantly higher than the TEC (716.87N±133.43; p=0.001) and TECP (745.67±156.34; p=0.001) groups, and the TEC and TECP groups showed no difference (p=0.884). Specimens in the TECP group were affected by an initial failure phenomenon (wide gap at the margin between the buildup/crown assembly and the root). Semidirect core buildup made from high-performance polymer enhanced the performance of all-ceramic leucite-reinforced glass ceramic crowns compared with direct light-curing composite resin buildups. The use of a fiber-reinforced post system did not influence the fatigue strength of all-ceramic crowns.

Load to failure of different zirconia abutments for an internal hexagon implant

Statement of problemVarious zirconia abutment designs are available to restore implant systems. Fracture resistance is one of the criteria involved in selecting among these options. PurposeThe purpose of this in vitro study was to measure and compare load to failure for 5 zirconia abutments for an internally hexagon implant. Material and methodsFive 4.1×11.5-mm Zimmer tapered screw-vent implants were individually secured in a loading apparatus, and 3 specimens of each of the 5 different abutments (Zimmer Contour with a Ti ring, anatomic-contour Atlantis-Zr, anatomic-contour Inclusive-Zr, anatomic-contour Astra Tech ZirDesign, Legacy Straight Contoured abutment with Ti core) (N=15) were loaded at a 30-degree angle until the implant abutment complex failed. Data for load to failure were compared with analysis of variance and a Tukey-Kramer post hoc test (α=.05). ResultsThe custom anatomic-contour abutment (Inclusive) showed the lowest load to fracture, and the stock anatomic-contour (AstraTech ZirDesign) the second lowest load to fracture. These were significantly lower than all other abutments (P<.05). The highest overall fracture strength was of a zirconia abutment with a titanium core-hexagon (Legacy Straight Contoured), which was significantly greater than all other abutments (P<.05). Anatomic-contour zirconia abutments fractured at an average of 275 N compared with the average fracture load of 842 N for zirconia abutments with titanium component (P<.05). ConclusionThe stock zirconia abutment with a titanium ring and the zirconia abutment with a titanium core-hexagon (Legacy Straight Contoured) had significantly greater fracture resistance than that of any of the 1-piece anatomic-contour zirconia abutments tested.

Reconstruction of Porcine Critical-Sized Mandibular Defects with Free Fibular Flaps: The Development of a Craniomaxillofacial Surgery Model

Vascularized bone flaps are the well-known standards of care for reconstruction of segmental mandibular defects ≥ 6 cm. We developed a large animal critical-sized mandibular defect model in which osseous free fibula flaps were used for reconstruction.In this study a total of eight 3-month-old Yorkshire pigs underwent 6 cm full-thickness resection of the left hemimandible. An osseous free fibula flap from the left leg was harvested and contoured to the mandibular defect. Bone placement and plate position was confirmed with fluoroscopy. Animals were followed with serial radiographs and clinical evaluations.Free fibulas were transferred successfully in all eight animals. The average operative time was 346 minutes, and the average flap ischemia time was 86 minutes. The average volume ratio of reconstructed hemimandibles to nonoperated control hemimandibles was 0.72 ± 0.33. The average maximum fracture load was 689 ± 262 N, and the average ratio of biomechanical fracture load for these samples compared with contralateral control hemimandibles was 0.88 ± 0.25.It is concluded that the porcine osseous free fibula flaps can be reliably harvested and viably transferred to critical-sized posterior mandibular defects with acceptable long-term results. The described microsurgical large animal model is acceptable for use in craniomaxillofacial experimentation.

The Effects of Locked and Unlocked Neutralization Plates on Load Bearing of Fractures Fixed With a Lag Screw

Because locked plates as a neutralization device for lag screw fracture fixation have different biomechanics than that of unlocked plates, we investigated how this would affect lag screw load at the fracture site. The purpose of this study was to assess the load at a fracture site compressed with a lag screw when both locked and unlocked plates are used as neutralization devices. Nine cadaver femurs had a mid-shaft oblique fracture created and were fixed with a lag screw, incorporating load transducers at the fracture site and lag screw. Three neutralization plate constructs (a standard plate, a locked plate, and an offset locked plate) were sequentially applied and loaded. Loads at the fracture site and the lag screw were measured after sequential application of axial loads. Plate application to the lag screw fixations did not significantly change (P > 0.26) the load at the fracture site or on the lag screw that were approximately 200 N. The unlocked, locked, and offset locked plates behaved similarly. The addition of a load to the specimens did not change the lag screw loads but increased the average fracture loads by approximately 20% of the applied load. Unlocked and locked neutralization plates do not affect the initial compressive load across a fracture fixed by a lag screw, and both behave similarly in transferring the load when the fracture was loaded. For a well-fixed stable fracture fixed with a lag screw, there is no advantage in using a more expensive locked plate over a standard plate for neutralization purposes if adequate screw purchase can be achieved.

Effects of cement thickness and bonding on the failure loads of CAD/CAM ceramic crowns: Multi-physics FEA modeling and monotonic testing

ObjectiveTo determine the influence of cement thickness and ceramic/cement bonding on stresses and failure of CAD/CAM crowns, using both multi-physics finite element analysis and monotonic testing. MethodsAxially symmetric FEA models were created for stress analysis of a stylized monolithic crown having resin cement thicknesses from 50 to 500μm under occlusal loading. Ceramic–cement interface was modeled as bonded or not-bonded (cement–dentin as bonded). Cement polymerization shrinkage was simulated as a thermal contraction. Loads necessary to reach stresses for radial cracking from the intaglio surface were calculated by FEA. Experimentally, feldspathic CAD/CAM crowns based on the FEA model were machined having different occlusal cementation spaces, etched and cemented to dentin analogs. Non-bonding of etched ceramic was achieved using a thin layer of poly(dimethylsiloxane). Crowns were loaded to failure at 5N/s, with radial cracks detected acoustically. ResultsFailure loads depended on the bonding condition and the cement thickness for both FEA and physical testing. Average fracture loads for bonded crowns were: 673.5N at 50μm cement and 300.6N at 500μm. FEA stresses due to polymerization shrinkage increased with the cement thickness overwhelming the protective effect of bonding, as was also seen experimentally. At 50μm cement thickness, bonded crowns withstood at least twice the load before failure than non-bonded crowns. SignificanceOcclusal “fit” can have structural implications for CAD/CAM crowns; pre-cementation spaces around 50–100μm being recommended from this study. Bonding benefits were lost at thickness approaching 450–500μm due to polymerization shrinkage stresses.

Improved prediction of proximal femoral fracture load using nonlinear finite element models

Hip fracture, which is often due to osteoporosis or other conditions affecting bone strength, can lead to permanent disability, pneumonia, pulmonary embolism, and/or death. Great effort has been directed toward developing noninvasive methods for evaluating proximal femoral strength (fracture load), with the goal of assessing fracture risk. Previously, computed tomographic scan-based, linear finite element (FE) models were used to estimate proximal femoral fracture loads ex vivo in two load configurations, one approximating joint loading during single-limb stance and the other simulating impact from a fall. Measured and computed fracture loads were correlated (stance, r=0.867; fall, r=0.949). However, precision for the stance configuration was insufficient to identify subjects with below average fracture loads reliably. The present study examined whether, for this configuration, nonlinear FE models could be used to identify these subjects. These models were found to predict fracture load within ±2.0 kN ( r=0.962). This level of precision is sufficient to identify 97.5% of femora with fracture loads 1.3 standard deviations below the mean as having below average fracture loads. Accordingly, 20% of subjects with below average fracture loads, i.e. those with the lowest fracture loads and likely to be at greatest risk of fracture, would be correctly identified with at least 97.5% reliability. This FE modeling method will be a powerful tool for studies of hip fracture.

The effect of cement thickness on the fracture strength of all-ceramic crowns.

The effect of increasing cement thickness on the fracture strength of aluminous porcelain jacket crowns was investigated in this study. Thirty-three aluminous porcelain jacket crowns were divided into three groups. In Group 1, only platinum foil was used to provide cement space. In Group 2 two layers and Group 3 four layers of die spacer were painted onto the metal die before impression taking. Each crown was cemented onto a metal die with zinc phosphate cement and loaded until fracture. The maximum force to break each crown was recorded. The average fracture load was 408 N for Group 1,365 N for Group 2 and 324 N for Group 3. ANOVA (SPSS) found a significant difference (p < 0.05) between groups. A Scheffe Test revealed a significant difference between Groups 1 and 3. It was concluded that increasing the cement thickness above 70 microns reduced the fracture strength of porcelain jacket crowns.