Force-Distance Relation for Dental Magnets - Fitted Equation.

OBJECTIVE: The force-distance power law for dental magnets had been unresolved until a theoretical study found that only even inverse powers were allowed; for simple magnets inverse fourth power was the only possibility. It remained to demonstrate that this indeed did apply to real magnets, the present purpose. METHODS: The force exerted by a series of real dental magnets to a large steel plate, and in a few cases to dental magnet keepers, as a function of distance was recorded. Curve-fitting of that data was explored. using the equation previously used for long dipoles, but allowing the power to be a free parameter. An index of 4 was the only feasible value. Corresponding fitted parameter values were then examined in relation to magnet design and each other. RESULTS: The theoretical power law index was confirmed to be 4. For a satisfactory fit, a 'polar offset' and a 'stretch power' were again required to better approximate the experimental results. Polar offset appears to be a function of apparent pole strength; stretch power less clearly so. SIGNIFICANCE: The motivating question is settled.

Effect of heat treatment on the tensile strength of 'Elgiloy' orthodontic wire.

OBJECTIVE: Elgiloy is the trade name of a cobalt-chromium-nickel superalloy that is offered for orthodontic use as wire. Despite some years of use, there is very little information in the dental literature on its mechanical properties, and especially on the effect of the hardening heat treatment (HT), that may be used after forming, on the tensile strength (TS) in relation to the four 'tempers' that are available. METHODS: Straight lengths of round wire of the four available tempers, Blue, Yellow, Green and Red, were tested in direct tension at 5mm/min in air at 23°C to fracture, both as-supplied (AS) and with HT at 500°C for 5h, in air. HT was done in a high-uniformity, three-zone tube furnace in an alumina boat. The wires were then allowed to cool to room temperature in the boat, outside the furnace. The nominal (original cross-sectional area) peak stress was calculated. RESULTS: TS varied from 1.4 to 2.1GPa, AS, and 1.6 to 2.8GPa HT, according to temper, but with appreciable variation within tempers. Even so, the TS plot of HT vs. AS was very straight and of narrow distribution (intercept: -0.638±0.064GPa, slope: 1.575±0.036, r(2): 0.994918, n=12, F=1957.7, p∼8×10(-13)). SIGNIFICANCE: The strengthening due to HT was highly regular and TS can be reliably predicted on the basis of the AS value, but this of course cannot be known without specific batch testing. However, the unexpectedly large variation in the AS values within tempers renders such a prediction of lower reliability and usefulness in practice. Indeed, the distinction between tempers can be negligible, making selection according to application demands problematic, and differential property expectation less than certain. No such product data are provided commercially. Quality control is not as tight as might be expected. The implications for treatment need to be explored.

Alkaline biodegradable implants for osteoporotic bone defects--importance of microenvironment pH.

UNLABELLED: Change of microenvironment pH by biodegradable implants may ameliorate unbalanced osteoporotic bone remodeling. The present work demonstrated that a weak alkaline condition stimulated osteoblasts differentiation while suppressed osteoclast generation. In vivo, implants with an alkaline microenvironment pH (monitored by a pH microelectrode) exhibited a promising healing effect for the repair of osteoporotic bone defects. INTRODUCTION: Under osteoporotic conditions, the response of the bone microenvironment to an endosseous implant is significantly impaired, and this substantially increases the risk of fracture, non-union and aseptic implant loosening. Acid-base equilibrium is an important factor influencing bone cell behaviour. The present purpose was to study the effect of a series of alkaline biodegradable implant materials on regeneration of osteoporotic bone defect, monitoring the microenvironment pH (µe-pH) over time. METHODS: The proliferation and differentiation potential of osteoporotic rat bone marrow stromal cells and RAW 264.7 cells were examined under various pH conditions. Ovariectomized rat bone defects were filled with specific biodegradable materials, and µe-pH was measured by pH microelectrode. New osteoid and tartrate-resistant acid phosphatase-positive osteoclast-like cells were examined by Goldner's trichrome and TRAP staining, respectively. The intermediate layer between implants and new bone were studied using energy-dispersive X-ray spectroscopy (EDX) linear scanning. RESULTS: In vitro, weak alkaline conditions stimulated osteoporotic rat bone marrow stromal cells (oBMSC) differentiation, while inhibiting the formation of osteoclasts. In vivo, µe-pH differs from that of the homogeneous peripheral blood and exhibits variations over time particular to each material. Higher initial µe-pH was associated with more new bone formation, late response of TRAP-positive osteoclast-like cells and the development of an intermediate 'apatitic' layer in vivo. EDX suggested that residual material may influence µe-pH even 9 weeks post-surgery. CONCLUSION: The pH microelectrode is suitable for in vivo µe-pH detection. Alkaline biodegradable materials generate an in vivo microenvironmental pH which is higher than the normal physiological value and show promising healing effects in the context of osteoporotic bone defects.

Influence of surface treatment on the resin-bonding of zirconia.

OBJECTIVE: To compare the effect of various surface treatments on the bonding of luting resin cements to zirconia under four-point bending. METHODS: Bar specimens (n = 200) (2 mm × 5 mm × 25 mm) were prepared from zirconia blocks (VITA In-Ceram YZ, Vita Zahnfabrik) with the cementation surface (2 mm × 5 mm) of groups of 40 treated in one of five ways: airborne particle abrasion with 50µm Al2O3 (GB), zirconia primer (Z-Prime Plus, Bisco) (Z), glaze ceramic (Crystall.Glaze spray, Ivoclar Vivadent) + hydrofluoric acid (GHF), fusion glass-ceramic (Crystall.Connect, Ivoclar Vivadent) (CC), or left untreated as control (C). Within each treatment, bars were cleaned ultrasonically for 15 min in ethanol and then deionized water before bonding in pairs with one of two luting resins: Panavia F 2.0, (Kuraray) (P); RelyX U-200 (3M/Espe) (R), to form 10 test specimens for each treatment and lute combination. Mechanical tests were performed and bond strengths (MPa) were subject, after log transformation, to analysis of variance, Shapiro-Wilk and Holm-Sidak tests; also log-linear contingency analysis of failure mode distribution; all with α = 0.05. Fracture surfaces were examined under light and scanning electron microscopy. RESULTS: While the effect of surface treatment was significant (p = 1.27 × 10(-9)), there was no detected effect due to resin (p = 0.829). All treatments except CC (30.1 MPa ×/÷ 1.44)* were significantly better than the untreated control (24.8 MPa ×/÷ 1.35) (p = 3.28 × 10(-9)). While the effect of GB - which gave the highest mean strength (50.5 MPa ×/÷ 1.29) - was not distinguishable from that of GHF (39.9 MPa ×/÷ 1.29) (p = 0.082), it was significantly better than treatment with either CC or Z (33.1 MPa ×/÷ 1.48) (p < 0.05). (* After log transformation for analysis and back; asymmetric error bounds as s.d. in log values.) SIGNIFICANCE: The novel test method design, which has good discriminatory power, confirmed the value of gritblasting as a simple and effective treatment with low operator hazard. It gave the highest bond strengths regardless of the cement type. Glaze layer application followed by hydrofluoric acid-etching on zirconia before cementation might be viable for adhesive zirconia cementation, but represents a much greater hazard as well as having problems with thickness control.

Failure analysis of the ball bearings of dental air turbine handpieces.

BACKGROUND: The aim of this study was to identify the nature and causes of deterioration and failure in dental handpiece ball bearings and thus provide guidance for clinical handling for service longevity. METHODS: The bearings of 36 turbine assemblies were dismantled for visual inspection, documented using a digital camera, and examined using scanning electron microscopy, as appropriate. RESULTS: For the metal parts of the ball bearing assembly, defects observed were mainly wear arising from the running load and corrosion. This was in the form of scratches and discoloured circumferential bands on the balls, and dull or worn surfaces extending around the circumference of the raceways. Cage damage including cracking, fracture, surface rubbing and distortion occurred, in varying degrees, in every failed turbine. CONCLUSIONS: Dental ball bearing failure modes have been identified. Cumulative effects of damage from corrosion and mechanical factors lead to handpiece deterioration. The cage was found to be very vulnerable to damage, and this may be the key limitation on bearing lifetime. Autoclaving may contribute to that, as it does to corrosion in the absence of adequate lubrication, but this seems to be minor in comparison to the effects of abuse. There is no justification for failing to observe usage and sterilization instructions.

Effect of corrosion on the strength of dental silver amalgam.

OBJECTIVES: To characterize the effect of crevice corrosion on the strength of dental silver amalgam as determined by the Hertzian 'ball on disc' method, with a view to providing a test method for use in standards compliance testing. MATERIALS & METHODS: Sixteen dental silver amalgam products were tested using the 'ball on disc' protocol at 30 d after setting at 37°C in air or immersed in artificial saliva at pH 6.2. The mixed materials were packed into a tapered steel disc mold (10 mm diameter, 3 mm thick) resting on a glass surface, slightly overfilled and carved level with a sharp edge, then ejected at ∼10 min and placed immediately into an incubator at 37°C. For corrosion specimens, the disc was laid on a flat polystyrene surface, immersed in artificial saliva, to create a spontaneous crevice corrosion cell. Testing was in Hertzian mode, using a 20 mm steel ball, with the specimen resting on a disc of glass-filled polyamide (E=10 GPa) at a cross-head speed of 0.2 mm/min on a universal testing machine (E3000, Instron). The load at first crack was recorded, as was the number of radial cracks produced. RESULTS: Radial cracking into 2-4 pieces, in a clinically relevant (non-explosive) mode was observed in all cases. On average, corrosion caused a decrease in load at failure of ∼10%, although the interaction with alloy (analysis of variance) was significant (P∼0.03) indicating variation between products. Comparison of the 30 d dry (uncorroded) results with those at 24h obtained earlier showed that there was highly significant increase on average (P∼5×10(-12)), but again a significant variation between products (P∼5×10(-6)), the maximum effect being +22%. SIGNIFICANCE: The ball-on-disc test provides a facile means of ascertaining the sensitivity of dental silver amalgam to corrosion under clinically relevant conditions, and is viable as a standards compliance test.

Development of strength in dental silver amalgam.

OBJECTIVES: To characterize the development of strength during the setting process of dental silver amalgam in the context of 'early strength' measurements for standards compliance testing in relation to patient instructions, and demonstrate the applicability of the Hertzian 'ball on disc' method. MATERIALS AND METHODS: Sixteen dental silver amalgam products were tested using the 'ball on disc' protocol at 1, 2, 3, 4 and 24h after setting at 37°C in air. The mixed materials were packed into a tapered steel disc mold (10mm diameter, 3mm thick) resting on a glass surface, slightly overfilled and carved level with a sharp edge, then ejected at ∼10min and placed immediately into an incubator at 37°C. Testing was in Hertzian mode, using a 20mm steel ball, with the specimen resting on a disc of glass-filled polyamide (E=10GPa) at a cross-head speed of 0.2mm/min on a universal testing machine (E3000, Instron). The load at first crack was recorded, as was the number of radial cracks produced. RESULTS: Radial cracking into 2-5 pieces, in a clinically-relevant (non-explosive) mode was observed in all cases. Considerable variation in setting rate between products, as indicated by the development of load at failure with time, was found. The distribution of normalized failure load values overall was lognormal (Weibull was excluded). The RMS coefficient of variation overall was 12.4%. SIGNIFICANCE: The ball-on-disc test provides a facile, relevant measure of the strength of dental silver amalgam, and is viable as a standards compliance test. Early strength testing at a minimum of 2h is suggested.

Effect of elastic modulus mismatch on failure behaviour of glass ionomer cement under Hertzian indentation.

OBJECTIVE: To investigate the effect of the substrate: coating elastic modulus ratio on the failure behaviour of glass ionomer cement (GIC) under Hertzian indentation. METHODS: GIC (Amalgomer CR, Advanced Healthcare, Tonbridge, Kent, UK) discs, 2 mm thick and 10 mm diameter, were randomly divided into 11 groups, 10-22 specimens for each group, and stored at 37 °C in artificial saliva for 7 days. Discs were tested resting freely on 11 substrate materials at 23 °C in air by loading centrally through a 20 mm diameter hard steel ball at a crosshead speed of 0.2 mm/min. The elastic moduli of the substrates (E(s)) and the coating GIC (E(c)) were first measured with a method similar to that of the Hertzian indentation, except for the three harder materials: amalgam, Al6061 and stainless steel 304. The failure load at the first crack was recorded with the aid of acoustic emission detection. Fracture surfaces were observed by SEM. RESULTS: E(s) values ranged from 0.09 to 210 GPa; E(c) was 7.7 GPa. Failure load generally increased with E(s) at first and then became relatively stable; the turning point occurred for E(s)/E(c)=1. Bottom surface-initiated radial cracking was the principal failure mode on softer substrates (E(s)/E(c)<1), while top surface conical cracking usually occurred on harder substrates (E(s)/E(c)≥1). CONCLUSION: The elastic modulus ratio, substrate: coating, is a controlling factor for both the failure mode and load for the coating material under Hertzian indentation.

Reaction of silver diamine [corrected] fluoride with hydroxyapatite and protein.

UNLABELLED: Silver diammine fluoride (SDF) is used as an anticaries agent; however, its mode of action is uncertain, whether chemical, physical, mechanical or antibacterial. As a preliminary study, the effect of SDF on hydroxyapatite (HAp) and gelatin (as a chemically-representative protein) was examined. METHODS: 2.5mg HAp powder specimens and 0.5mL 10% gelatin (Riedel-de Haën) (initially as a sol at ∼37°C), were mixed with 0.5mL of 38% SDF (J. Morita), 4% NaF (Sigma) or 40% AgNO(3) (Sigma) and tumbled in 1.5mL polypropylene tubes (Sarstedt) for 48h at ∼23°C, in two series: exposed to laboratory lighting, and kept dark at all times. The HAp specimens were separated by centrifugation and decanting, then these and one set of gelatin specimens were dried at 60°C in situ; a second parallel set of gelatin specimens were dried at ∼23°C. Each was washed with 1mL deionized water for 1min, 3 times. Treated materials were observed, before and after washing, with scanning and transmission electron microscopy (SEM, TEM); energy-dispersive X-ray analysis (EDX), and electron diffraction (ED). RESULTS: SDF appeared to produce globular particles of CaF(2) on the surface of the HAp, but these disappeared on washing, whilst with AgNO(3) yellow cubic crystals of Ag(3)PO(4) formed which were not dissolved on washing, but which darkened, converting gradually to metallic silver, on exposure to light. NaF had no effect on gelatin, whilst with SDF and AgNO(3), particles of silver were produced which were resistant to washing. CONCLUSIONS: Both principal components of tooth tissue react with SDF; the solubility of the putative CaF(2) formed weakens the case for it exerting a caries-protective effect. The importance of the persistent silver needs further study.

"MTA"-an Hydraulic Silicate Cement: review update and setting reaction.

OBJECTIVES: To review the current status and understanding of Portland cement-like endodontic materials commonly referred to by the trade designation "MTA" (alias "Mineral Trioxide Aggregate"), and to present an outline setting reaction scheme, hitherto unattempted. METHOD: The literature was searched using on-line tools, overlapping an earlier substantial review to pick up any omissions, including that in respect of ordinary Portland cement (OPC), with which MTA shares much. The search was conducted for the period January 2005 to December 2009 using 'MTA', 'GMTA', 'WMTA', and 'mineral AND trioxide AND aggregate' as keywords, with various on-line search engines including ScienceDirect (http://www.sciencedirect.com), SAGE Journals Online (http://online.sagepub.com), Wiley Online Library (http://onlinelibrary.wiley.com), SciELO Scientific electronic library online (http://www.scielo.br/scielo.php), JSTOR (http://www.jstor.org), and Scopus (http://www.scopus.com). References of articles found were cross-checked where appropriate for missed publications. Manufacturers' and related websites were searched with Google Search (http://www.google.com.hk). RESULTS: A generic name for this class of materials, Hydraulic Silicate Cement (HSC), is proposed, and an outline reaction scheme has been deduced. HSC has distinct advantages apparent, including sealing, sterilizing, mineralizing, dentinogenic and osteogenic capacities, which research continues to demonstrate. However, ad hoc modifications have little supporting justification. SIGNIFICANCE: While HSC has a definite place in dentistry, with few of the drawbacks associated with other materials, some improvements in handling and other properties are highly desirable, as are studies of the mechanisms of the several beneficial physiological effects. Reference to the extensive, but complex, literature on OPC may provide the necessary insight.

Interactive effect of indenter size and specimen thickness in Hertzian indentation test.

OBJECTIVE: To investigate the effect of indenter size on the failure mode of amalgam and its interaction with thickness. METHODS: Dental silver amalgam (Lojic+, SDI) discs, 10 mm diameter, thickness h=0.4-8.0 mm, were stored in air at 37 degrees C for 7 days before testing in 'Hertzian contact', resting freely on a 30% glass fibre-reinforced nylon 6,6 substrate (E: 10GPa) at 23 degrees C in air at a crosshead speed of 0.2 mm/min. The load at the first crack, detected acoustically, was recorded. Two series of tests were conducted. In the first, a 3 or 20 mm diameter hard steel ball was used to apply the load to discs with h=0.4-8.0 mm. The second was performed on discs with h=2.0 and 5.0 mm with indenter diameters D=2-40 mm. RESULTS: The failure loads for these two groups were substantially different, with most values for D=3 mm being lower than those for D=20 mm. The transitions of the main failure mode with increasing thickness were similar, i.e., from RC-dominant for h small to CC-dominant for h large, except that this occurred over a greater range for the larger indenter. For h=2 mm, failure load was constant for D=2-40 mm; all specimens consistently failed from bottom RC. For h=5 mm, failure load increased with D with a quadratic relationship up to about 10 mm (r(2)=0.85), after that, values were near constant; the failure mode gradually changed with D from PD- or CC-prevailing for D small (< or =5 mm) to RC-prevailing for D very large (30 and 40 mm). SIGNIFICANCE: The failure mode of amalgam is dependent on the indenter size; indenter size and coating thickness have reciprocal effects.

Influence of LED irradiance on flexural properties and Vickers hardness of resin-based composite materials.

OBJECTIVE: To investigate the effect of irradiance through choice of LED light-curing unit (LCU) and fiber-optic tip on the mechanical properties of methacrylate resin-based composites (RBCs). METHODS: Rectangular bar-shaped specimens (25 mm x 2 mm x 2 mm) of four RBCs, irradiated from one side for the manufacturer's recommended times with one of two LED LCUs, and with either 8 or 13 mm tip diameter, were tested in three-point bend for flexural strength and modulus of elasticity; Vickers hardness was measured on top and bottom surfaces, on overlapped and non-overlapped irradiation sites. The effects and interactions of all factors were examined by analysis of variance. RESULTS: While the materials tested differed significantly for all properties (P < 1 x 10(-6)), there was no significant effect for LCU, tip, or irradiance for flexural strength or modulus of elasticity. For hardness, the only significant effect overall was due to irradiance on the bottom surface. CONCLUSIONS: Both LCUs and both tips could be used to give adequate top surface hardness, but the recommended maximum increment thickness is too great for adequate cure at the bottom with the tested LCUs at the recommended times. An extended experimental design would be valuable to test behavior at much elevated irradiances.

Solid titration of octacalcium phosphate.

Octacalcium phosphate (OCP) is of considerable importance as a precursor in the formation of dental enamel and an intermediate phase in the precipitation of hydroxyapatite (HAp) in bone. However, agreement is poor on the solubility product (pK(sp)), possibly due to the formation of the more stable phase HAp. The system was investigated using solid titration, which has shown reliability in work on HAp and related fluoride minerals, with OCP in 100 mM KCl at 37.0 +/- 0.1 degrees C. The constitution of the end point precipitate was determined by X-ray diffraction and selected-electron area diffraction; the particle morphology and elements present were examined by high-resolution field emission scanning, transmission electron microscopy and energy-dispersive X-ray analysis. The titration curve for OCP was found for pH approximately 3.4-7.4. The precipitate was HAp at pH 3.6 and 4.5; no residual OCP or other phase was detected. Dicalcium phosphate dihydrate (DCPD) was then found to form at pH 3.6 on further addition of OCP titrant after equilibrium had been achieved, possibly due to easier nucleation at lower pH. However, markedly crystalline HAp was formed in equilibrium for OCP titration with HAp seeding, verifying HAp as the more stable phase. A solubility isotherm for OCP was not obtained as HAp appears to be less soluble in the pH range studied. This adds weight to the view that HAp may be the most stable phase of all calcium phosphates, with further doubt being cast on DCPD being the most stable phase below pH 4.2. However, metastable DCPD may form in an Ostwald succession, depending on supersaturation and nucleation conditions.

Solubility of TTCP and beta-TCP by solid titration.

OBJECTIVE: Using solid titration with hydroxyapatite (HAp) and octacalcium phosphate, HAp has been found to be more stable than dicalcium phosphate dihydrate (DCPD) even at lower pH, inconsistent with the widely reported view that DCPD is less soluble than other calcium phosphates below pH 4.2. A check of the behaviour of other calcium phosphates (TTCP; Ca/P: 2.00 and beta-TCP; Ca/P: 1.33) is necessary. METHODS: Solid titration was used to determine the effective solubility of TTCP and beta-TCP in 100 mM KCl solution at 37.0+/-0.1 degrees C for pH approximately 2.9-9.2 and approximately 3-7.4 respectively. The constitution of the precipitate was determined by XRD, particle morphology was observed by SEM and TEM, and the precipitate Ca/P ratio was calculated by EDX. RESULTS: The only identified solid phase at equilibrium was HAp at both pH 3.60 and 4.50; no residual titrant or other phases were detected. A marked change of slope in the curve occurred at pH approximately 3.9 for TTCP. CONCLUSION: HAp was verified to be more stable than other calcium phosphates, especially at lower pH. That DCPD is more stable below pH 4.2 is contradicted.

Solubility of dicalcium phosphate dihydrate by solid titration.

Solid-titration results for hydroxyapatite (HAp), octacalcium phosphate, beta-tricalcium phosphate and tetracalcium phosphate have shown that the only stable phase in 100 mmol x l(-1) KCl at 37 degrees C is HAp. In particular, dicalcium phosphate dihydrate (DCPD) did not form at pH <4.2 (where it is otherwise believed to be stable) except as a metastable phase under conditions of slight supersaturation. The behaviour of DCPD itself under the same conditions requires checking. Solid titration was used to determine the apparent solubility of DCPD in a 100-mmol x l(-1) KCl solution at 37.0 +/- 0.1 degrees C over the pH range 3.2-11.6. The constitution of the precipitate was determined by X-ray diffraction, particle morphology was observed by scanning and transmission electron microscopy, and the precipitate Ca/P ratio was calculated by energy-dispersive X-ray analysis. The titration curve for DCPD was substantially lower than the position reported elsewhere. DCPD was the only identified phase at equilibrium at pH 3.60 and 4.50, but HAp was formed after seeding with 1 mg HAp at DCPD equilibrium at pH 4.47, 3.60 and 3.30. It is concluded that the titration curve observed for DCPD corresponds to the solubility isotherm for the phase, but that this represents a metastable equilibrium. HAp is more stable than DCPD, particularly below pH 4.2. The implications for calcium phosphate studies are profound as the reverse is generally believed to be true. Thus, solubility results and the nature of the carious lesion need reconsideration.

Hertzian load-bearing capacity of a ceramic-reinforced glass ionomer cement stored wet and dry.

OBJECTIVE: To investigate the development of the load-bearing capacity of a ceramic-reinforced glass ionomer cement (GIC), stored dry or wet, using Hertzian indentation. METHODS: GIC discs 2mm thick and 10mm in diameter were made (Amalgomer CR, Advanced Healthcare, Tonbridge, Kent, UK), randomly divided into 8 groups and stored at 37 degrees C in air or artificial saliva (AS) for 1, 7, 30 and 90 days. Discs were tested by loading centrally using a 20mm diameter hard steel ball while resting freely on a filled-nylon substrate (E: 10GPa), at 23 degrees C in air. The failure load at the first bottom-initiated radial crack was detected acoustically. RESULTS: All the failure loads of air-stored specimens were higher than those of AS-stored specimens. The failure load was relatively stable for air-stored specimens but showed a significant decreasing tendency for AS-stored specimens. SIGNIFICANCE: Zirconia-reinforced GIC is sensitive to moisture. Artificial saliva storage has a detrimental effect on the failure load of ceramic-reinforced GIC that may indicate long-term deterioration in service.

Solubility of strontium-substituted apatite by solid titration.

Solid titration was used to explore the solubility isotherms of partially (Srx-HAp, x=1, 5, 10, 40, 60 mol.%) and fully substituted strontium hydroxyapatite (Sr-HAp). Solubility increased with increasing strontium content. No phase other than strontium-substituted HAp, corresponding to the original titrant, was detected in the solid present at equilibrium; in particular, dicalcium hydrogen phosphate was not detected at low pH. The increase in solubility with strontium content is interpreted as a destabilization of the crystal structure by the larger strontium ion. Carbonated HAp was formed in simulated body fluid containing carbonate on seeding with Sr10-HAp, but the precipitate was strontium-substituted on seeding with Sr-HAp. Strontium-substituted HAp might be usable as a template for the growth of new bone, since nucleation appears to be facilitated.

Failure behavior of glass ionomer cement under Hertzian indentation.

OBJECTIVE: To investigate the load-bearing capacity and failure mode of various types of glass ionomer cement (GIC) under Hertzian indentation, exploring the relationship between the failure behavior and formulation, and examining claims of filler-reinforcement of GIC. METHODS: Discs 2mm thick, 10mm diameter, 8-18 replicates, were fabricated for two filler-reinforced GICs, four unmodified and unreinforced GICs, and four resin-modified GICs, with a dental silver amalgam and a filled-resin restorative material for comparison. Testing was at 23 degrees C, wet, after 7d storage at 37 degrees C in artificial saliva at pH 6, using a 20mm diameter hard steel ball and filled-nylon substrate (E: 10GPa). First failure was detected acoustically; mode was determined visually. At least 1/3 of specimens in each case were examined under scanning electronic microscope for corroboration. RESULTS: Reinforced and unmodified-unreinforced GICs were indistinguishable by failure load (one-way analysis of variance, P=0.425, overall 260+/-70N) and mode. Failure loads for resin-modified GICs were 360-1150N, amalgam approximately 680N, and filled resin approximately 1200N. Resin-modified GICs tended to be tougher (incomplete fracture), all others gave complete fracture (radial cracking). The stronger materials (two resin-modified GICs and filled resin) showed some cone cracking. SIGNIFICANCE: While resin-modified GICs showed various extents of increase of failure load over that of the plain GICs, consistent with the hybrid chemistry, filler-reinforcement was not evident for the two claimed products, consistent with structural and theoretical expectations.