Armin Kirsten

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

ObjectivesThe 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 methodsIn 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.ResultsThe 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.ConclusionsThe null hypothesis had to be rejected.Clinical relevanceThe exclusively digital workflow on the basis of intraoral digital impressions delivered clinically satisfying results for single crowns with both systems.

Fracture load of different crown systems on zirconia implant abutments.

OBJECTIVES The purpose of this study was to evaluate the fracture load of single zirconia abutment restorations using different veneering techniques and materials. MATERIALS AND METHODS The abutment restorations were divided into 6 groups with 20 samples each: test abutments (control group A), lithium disilicate ceramic crowns bonded on incisor abutments (group B), leucite ceramic crowns bonded on incisor abutments (group C), premolar abutments directly veneered with a fluor apatite ceramic (group D (layered) and group E (pressed)) and premolar abutments bonded with lithium disilicate ceramic crowns (group F). The fracture load of the restorations was evaluated using a universal testing machine. Half of each group was artificially aged (chewing simulation and thermocycling) before evaluating the fracture load with the exception of the test abutments. RESULTS The fracture load of the test abutments was 705 ± 43N. Incisor abutments bonded with lithium disilicate or leucite ceramic crowns (groups B and C) showed fracture loads of about 580N. Premolar restorations directly veneered with fluor apatite ceramic (groups D and E) showed fracture loads of about 850N. Premolar restorations bonded with lithium disilicate ceramic crowns (group F) showed fracture loads of about 1850N. The artificial ageing showed no significant influence on the strength of the examined restorations. SIGNIFICANCE All ceramic crowns made of lithium disilicate glass-ceramic, adhesively bonded to premolar abutments showed the highest fracture loads in this study. However, all tested groups can withstand physiological bite forces.

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.

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 Youngs 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.

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.

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.

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.

Bioactive and Thermally Compatible Glass Coating on Zirconia Dental Implants

The healing time of zirconia implants may be reduced by the use of bioactive glass coatings. Unfortunately, existing glasses are either bioactive like Bioglass 45S5 but thermally incompatible with the zirconia substrate, or they are thermally compatible but exhibit only a very low level of bioactivity. In this study, we hypothesized that a tailored substitution of alkaline earth metals and alkaline metals in 45S5 can lead to a glass composition that is both bioactive and thermally compatible with zirconia implants. A novel glass composition was analyzed using x-ray fluorescence spectroscopy, dilatometry, differential scanning calorimetry, and heating microscopy to investigate its chemical, physical, and thermal properties. Bioactivity was tested in vitro using simulated body fluid (SBF). Smooth and microstructured glass coatings were applied using a tailored spray technique with subsequent thermal treatment. Coating adhesion was tested on implants that were inserted in bovine ribs. The cytocompatibility of the coating was analyzed using L929 mouse fibroblasts. The coefficient of thermal expansion of the novel glass was shown to be slightly lower (11.58·10–6 K–1) than that of the zirconia (11.67·10–6 K–1). After storage in SBF, the glass showed reaction layers almost identical to the bioactive glass gold standard, 45S5. A process window between 800 °C and 910 °C was found to result in densely sintered and amorphous coatings. Microstructured glass coatings on zirconia implants survived a minimum insertion torque of 60 Ncm in the in vitro experiment on bovine ribs. Proliferation and cytotoxicity of the glass coatings was comparable with the controls. The novel glass composition showed a strong adhesion to the zirconia substrate and a significant bioactive behavior in the SBF in vitro experiments. Therefore, it holds great potential to significantly reduce the healing time of zirconia dental implants.

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. Microsc. Res. Tech. 77:143–152, 2014.