A cusp supporting framework design can decrease critical stresses in veneered molar crowns.

OBJECTIVES: Veneered zirconia restorations predominately fail due to veneering fractures. It is hypothesized that a cusp-supporting framework design can prevent these catastrophic failures in all-ceramic restorations. Therefore, we investigated the influence of framework design and framework material on the stress distribution in a single tooth restoration using the numerical finite element method. METHODS: A three-dimensional model of a veneered lower molar (36) crown with constant outer shape was used. The framework design was either cusp supporting or with a constant framework thickness. Zirconia, alumina, and a gold alloy were used as framework material. A glass ceramic material was used as veneering material for both cases. Two different load cases were simulated: terminal occlusion with load distributed over the occlusal surface of the tooth and a fairly extreme load case with all force concentrated on one cusp. RESULTS: Maximum tensile stresses in the glass ceramic veneering material concentrated in the fissure region for all models. A cusp supporting framework design decreased the maximum tensile stresses significantly up to 30.5%. The distolingual load case resulted in an approximately fourfold higher stress level compared to the terminal occlusion load case. SIGNIFICANCE: A cusp supporting framework design can significantly decrease the maximum tensile stresses in the veneering material of single crowns. Based on the numerical results of this study it can be expected that such a design could decrease the risk of veneering failure in vivo.

Two-photon laser scanning microscopy as a useful tool for imaging and evaluating macrophage-, IL-4 activated macrophage- and osteoclast-based in vitro degradation of beta-tricalcium phosphate bone substitute material.

Two-photon microscopy is an innovative technology that has high potential to combine the examination of soft and hard tissues in vitro and in vivo. Calcium phosphates are widely used substitutes for bone tissue engineering, since they are degradable and consequently replaced by newly formed tissue. It is well known that osteoclasts are responsible for the resorption processes during bone remodelling. We hypothesize that also macrophages are actively involved in the resorption process of calcium phosphate scaffolds and addressed this question in in vitro culture systems by two-photon laser scanning microscopy. Beta-tricalcium phosphate specimens were incubated with (1) macrophages, (2) interleukin-4 activated macrophages, and (3) osteoclasts for up to 21 days. Interestingly, macrophages degraded beta-tricalcium phosphate specimens in an equivalent fashion compared to osteoclasts and significantly more than IL-4 activated macrophages. An average of ~32% of the macrophages was partially filled with ceramic material while this was 18% for osteoclasts and 9% for IL-4 activated macrophages. For the first time by applying two-photon microscopy, our studies show the previously unrecognized potential of macrophages to phagocytose ceramic material, which is expected to have implication on osteoconductive scaffold design.

The effect of crystallization of bioactive bioglass 45S5 on apatite formation and degradation.

OBJECTIVE: Amorphous bioglass 45S5 has been used for many years as bone substitute material. Bioactive glasses are also suitable as coating materials for implants in order to improve the bone ongrowth behavior. We hypothesize that both the apatite formation on the surface and the chemical stability can be improved by crystallization of the bioglass. METHODS: Synthesized amorphous bioglass 45S5 specimens as well as samples which were crystallized at 1000 °C were stored in simulated body fluid for 1, 7, and 14 days. The respective apatite formation was gravimetrically determined and characterized by SEM and XRD analysis. Moreover, the degradation behavior was studied after storage in distilled water. RESULTS: The weight of the crystallized samples decreased 6.3% less than that of the amorphous samples. Calcium silica and calcium carbonate layers were found on amorphous bioglass after 7 and 14 days. However, apatite formation was observed only on the crystallized 45S5 samples after storage. SIGNIFICANCE: We conclude that the chemical resistance can be improved and, in parallel, a pronounced apatite formation on the surface of 45S5 can be obtained by controlled crystallization of the material for the particular test setup. Therefore, crystallized bioactive glasses should be considered to be promising coating material for dental implants.

The clinical accuracy of single crowns exclusively fabricated by digital workflow--the comparison of two systems.

OBJECTIVES: The purpose of the study was to compare the accuracy of crowns exclusively fabricated by the digital workflow of two systems. The null hypothesis stated was: Both systems do not differ with respect to marginal and internal accuracy. MATERIALS AND METHODS: In 14 patients, 13 molars and 1 premolar were prepared. Each preparation was scanned intraorally with two different digital impression systems, Lava COS and Cerec AC. On the basis of these data, Lava DVS crowns [DVS] and Vita Rapid Layering Technique crowns [RLT] were fabricated, respectively. Both systems contained of a zirconia framework and a digitally fabricated silicate ceramic veneering. The marginal and internal fit of the crowns was documented by a replica technique. The replicas were examined under microscope with a magnification of ×200. The Wilcoxon signed rank test was applied in order to test if the values of the two systems showed significant differences at p ≤ 0.05. RESULTS: The results were as follows in micrometers (±standard deviation): at the marginal gap, 51 (±38) for [DVS] and 83 (±51) for [RLT]; mid-axial, 130 (±56) for [DVS] and 128 (±66) for [RLT]; axio-occlusal, 178 (±55) for [DVS] and 230 (±71) for [RLT]; and centro-occlusal, 181 (±41) for [DVS] and 297 (±76) for [RLT]. According to the Wilcoxon signed rank test, the results differed significantly at the marginal, axio-occlusal, and centro-occlusal gaps. CONCLUSIONS: The null hypothesis had to be rejected. CLINICAL RELEVANCE: The exclusively digital workflow on the basis of intraoral digital impressions delivered clinically satisfying results for single crowns with both systems.

Influence of tooth mobility on critical stresses in all-ceramic inlay-retained fixed dental prostheses: a finite element study.

OBJECTIVES: Inlay-retained fixed partial dentures are conservative prosthetic restorations. Their failure resistance is influenced by the stress distribution that depends on the material properties as well as the loading conditions. Finite element analysis provides the ability to estimate the loading capacity by simulating the stress distribution in all-ceramic dental restorations. The null-hypothesis of this study was that tooth mobility or tooth bearing condition significantly influences the stress distribution and therefore the failure resistance of all-ceramic inlay-retained fixed dental prostheses. Therefore, the stress distribution under different loading and bearing conditions of the teeth was analyzed using the finite element method. METHODS: Three different bearing conditions, one fixed and two flexible were chosen to simulate tooth mobility. The flexible models were constrained with spring elements to a virtual center of rotation. In addition, loading conditions were varied. RESULTS: The influence of tooth mobility on the stress distribution depended on the degree of modeled tooth mobility, as well as the loading conditions. The maximum first principal stresses differed significantly in magnitude and location depending on the modeled bearing condition and the simulated load case. The maximum difference between fixed and flexible model was more than 100%. SIGNIFICANCE: Tooth mobility and occlusal loading conditions have to be considered in finite element analyses as the simulated stress distribution is strongly influenced by these factors.

Influence of connector design and material composition and veneering on the stress distribution of all-ceramic fixed dental prostheses: a finite element study.

OBJECTIVES: Finite element analysis is a standard method to simulate the stress distribution in all-ceramic dental restorations in order to estimate the loading capacity of the brittle components. The hypothesis of this study was that stresses in the connector area of a veneered FDP are strongly influenced by the framework dimensions and the veneering material. METHODS: Finite element analyzes of bilayered fixed dental prostheses with three different framework-designs and three different veneering materials were conducted, applying the loads onto the veneering as well as directly onto the framework. The outer shape of the veneering ceramic remained constant for all cases. RESULTS: The maximum first principal stresses in the framework of the fixed dental prostheses (FDP) decreased with smaller framework dimensions when the load was applied on the veneering. By applying the load directly onto the framework of the FDP without veneering a converse tendency was found. The variation of the veneering material lead to the conclusion that stresses in the framework became higher with decreasing Young's modulus of the veneer, while the stresses in the veneer increased at the same time. SIGNIFICANCE: The veneering material plays a significant role for the failure of a FDP and cannot be neglected neither in testing nor in simulation. Thus the loading capacity of dental restorations can only be reasonably evaluated when the whole restoration is taken into account, including framework and veneering.

Synthesis of novel tricalcium phosphate-bioactive glass composite and functionalization with rhBMP-2.

A functionalization is required for calcium phosphate-based bone substitute materials to achieve an entire bone remodeling. In this study it was hypothesized that a tailored composite of tricalcium phosphate and a bioactive glass can be loaded sufficiently with rhBMP-2 for functionalization. A composite of 40 wt% tricalcium phosphate and 60 wt% bioactive glass resulted in two crystalline phases, wollastonite and rhenanite after sintering. SEM analysis of the composite's surface revealed a spongious bone-like morphology after treatment with different acids. RhBMP-2 was immobilized non-covalently by treating with chrome sulfuric acid (CSA) and 3-aminopropyltriethoxysilane (APS) and covalently by treating with CSA/APS, and additionally with 1,1'-carbonyldiimidazole. It was proved that samples containing non-covalently immobilized rhBMP-2 on the surface exhibit significant biological activity in contrast to the samples with covalently bound protein on the surface. We conclude that a tailored composite of tricalcium phosphate and bioactive glass can be loaded sufficiently with BMP-2.

Subcritical crack growth behavior of dispersion oxide ceramics.

Zirconia (Y-TZP) is used as material for components of implants and prostheses because of its high short-term strength. The mechanical long-term reliability, however, is limited for Y-TZP because of hydrothermal aging effects and a pronounced tendency for subcritical crack growth. The hypothesis of this study was that a substantial amount of alumina in a zirconia matrix can help to significantly suppress subcritical crack growth and thereby improve the mechanical long-term reliability. The Weibull parameters as well as the parameters of the subcritical crack growth were determined for Alumina, Y-TZP, and two dispersion ceramics, that is Alumina Toughened Zirconia (ATZ, 20% alumina/80% Y-TZP), and Zirconia Toughened Alumina (ZTA, 75% alumina/25% Y-TZP). The long-term failure probability as a function of service time was predicted for the four ceramics. The parameter n of the subcritical crack growth was approx. 80% higher for ATZ compared to Y-TZP. In consequence, the estimated lifetime revealed a significant better mechanical long-term reliability for ATZ. It can be concluded that tailored dispersion oxide ceramics can address the aging problem of monolithic zirconia. This makes ATZ very interesting for components of joint replacement as well as for dental prostheses and implants.