Anselm Wiskott

Low temperature degradation of a Y-TZP dental ceramic

Bars of Y-TZP ceramic for dental restorations were subjected to hydrothermal degradation via in vitro exposure to water steam at 140°C for 7 days. X-ray diffraction, atomic force microscopy and scanning electron microscopy techniques were applied to observe and quantify the tetragonal-monoclinic (t-m) phase transformation associated with the process. Nanoindentation was used to assess the ceramics mechanical properties before and after hydrothermal degradation. Texture associated with martensitic t-m transformation was observed at the grain surface. The t-m transformation followed nucleation-and-growth kinetics, with predominance of the nucleation process. The transformation occurred within a layer of 6 μm below the surface. Mechanical properties deteriorated with hydrothermal degradation, resulting in a 30% reduction of Youngs modulus and hardness. A strong correlation was found between the increasing monoclinic fraction and the decline in mechanical response. It was thus concluded that the emergence of the monoclinic phase and the associated microcracking were the most likely causes for the degradation of mechanical properties.

Monotonic flexure and fatigue strength of composites for provisional and definitive restorations.

STATEMENT OF PROBLEM Ordinarily, the mechanical strength of composites is characterized by their flexural strength. Information as to the materials fatigue strength is seldom provided. PURPOSE The purpose of this study was to compare the flexural strength and the resistance to fatigue loading of composites and an acrylic resin for provisional and definitive restorations. MATERIAL AND METHODS Artglass, Colombus, and Targis (composites) and Jet, Protemp II, Protemp Garant, and Provipont DC (provisional restorations) were subjected to mechanical tests. Fatigue tests (MPa) (n = 30 specimens/group) were conducted with the rotating-bending cantilever design. Monotonic flexural strength (MPa) (n = 10) was determined in 3-point bending tests. Fatigue resistance was analyzed via the staircase procedure, and flexural strength was examined by use of the 2-parameter Weibull distribution (confidence intervals at 95%). RESULTS The mean fatigue resistances (S(50)) in MPa +/- SD were: Targis, 62.1 +/- 7.0; Artglass, 58.5 +/- 3.7; Colombus, 54.6 +/- 6.2; Provipont DC, 29.5 +/- 3.2; Protemp II, 23.1 +/- 5.3; Jet, 22.8 +/- 8.3; Protemp Garant, 19.6 +/- 4.6. The flexure strengths (Weibulls S(0)) in MPa and their shape parameters (m) were: Colombus, 145.2 (13.1); Targis, 110.3 (7.8); Artglass, 5.9 (5.4); Jet, 150.9 (17.3); Provipont DC, 97.3 (23.8); Protemp II, 57.9 (6.4); Protemp Garant, 54.2 (12.8). The S(50) of Targis was significantly higher than that of Colombus but not different from Artglass. In flexion, the S(0) of Colombus was significantly higher than that of Artglass and Targis. The S(50) ranged between 40% and 60% of the S(0) for the composites and between 15% and 30% for the provisional restorative materials. CONCLUSIONS Correlations between monotonic flexure strength and resistance to fatigue loading were weak. Because fatigue tests are considered more pertinent than monotonic tests as to their predictive value, it is concluded that flexure strength data alone may not provide relevant information for long-term clinical performance. The materials resistance to fatigue loading should also be determined.

Hydrothermal degradation of a 3Y-TZP translucent dental ceramic: A comparison of numerical predictions with experimental data after 2 years of aging

OBJECTIVES The purpose of the study was to assess the hydrothermal resistance of a translucent zirconia with two clinical relevant surface textures by means of accelerated tests (LTD) and to compare predicted monoclinic fractions with experimental values measured after two years aging at 37°C. METHODS Polished (P) and ground (G) specimens were subjected to hydrothermal degradation by exposure to water steam at different temperatures and pressures. The t-m phase transformation was quantified by grazing incidence X-ray diffraction (GIXDR). The elastic modulus and hardness before- and after LTD were determined by nanoindentation. RESULTS G specimens presented a better resistance to hydrothermal degradation than P samples. Activation energies of 89 and 98kJ/mol and b coefficients of 2.0×10(-5) and 1.8×10(-6) were calculated for P and G samples respectively. The coefficients were subsequently used to predict transformed monoclinic fractions at 37°C. A good correlation was found between the predicted values and the experimental data obtained after aging at 37°C during 2 years. Hydrothermal degradation led to a significant decrease of the elastic moduli and hardness in both groups. SIGNIFICANCE The dependency of the t-m phase transformation rate on temperature must be determined to accurately predict the hydrothermal behavior of the zirconia ceramics at oral temperatures. The current prevailing assumption, that 5h aging at 134°C corresponds to 15-20 years at 37°C, will underestimate the transformed fraction of the translucent ceramic at 37°C. In this case, the mechanical surface treatment influences the ceramics transformability. While mild grinding could potentially retard the hydrothermal transformation, polishing after occlusal adjustment is recommended to prevent wear of the antagonist teeth and maintain structural strength.

Regional structural characteristics of bovine periodontal ligament samples and their suitability for biomechanical tests

Mechanical testing of the periodontal ligament requires a practical experimental model. Bovine teeth are advantageous in terms of size and availability, but information is lacking as to the anatomy and histology of their periodontium. The aim of this study, therefore, was to characterize the anatomy and histology of the attachment apparatus in fully erupted bovine mandibular first molars. A total of 13 teeth were processed for the production of undecalcified ground sections and decalcified semi‐thin sections, for NaOH maceration, and for polarized light microscopy. Histomorphometric measurements relevant to the mechanical behavior of the periodontal ligament included width, number, size and area fraction of blood vessels and fractal analysis of the two hard–soft tissue interfaces. The histological and histomorphometric analyses were performed at four different root depths and at six circumferential locations around the distal and mesial roots. The variety of techniques applied provided a comprehensive view of the tissue architecture of the bovine periodontal ligament. Marked regional variations were observed in width, surface geometry of the two bordering hard tissues (cementum and alveolar bone), structural organization of the principal periodontal ligament connective tissue fibers, size, number and numerical density of blood vessels in the periodontal ligament. No predictable pattern was observed, except for a statistically significant increase in the area fraction of blood vessels from apical to coronal. The periodontal ligament width was up to three times wider in bovine teeth than in human teeth. The fractal analyses were in agreement with the histological observations showing frequent signs of remodeling activity in the alveolar bone – a finding which may be related to the magnitude and direction of occlusal forces in ruminants. Although samples from the apical root portion are not suitable for biomechanical testing, all other levels in the buccal and lingual aspects of the mesial and distal roots may be considered. The bucco‐mesial aspect of the distal root appears to be the most suitable location.

Effect of different surface treatments on the hydrothermal degradation of a 3Y-TZP ceramic for dental implants

OBJECTIVES Implant surface modifications are intended to enhance bone integration. The objective of this study was to assess the effect of different surface treatments on the resistance to hydrothermal degradation, hardness and elastic modulus of a 3Y-TZP ceramic used for dental implants. METHODS Samples grouped according to their surface morphologies (AS, as-sintered; C, coated; P, dry-polished; R, roughened; PA, polished and annealed; RA, roughened and annealed) were subjected to accelerated hydrothermal degradation (LTD) by exposure to water steam (134°C, 2bars) for 100h. The t-m phase transformation was quantified by grazing incidence X-ray diffraction (GIXDR) and by combined focused ion beam and scanning electron microscopy (FIB-SEM). Elastic modulus and hardness before- and after prolonged aging (100h) were assessed by nanoindentation. RESULTS AS and C specimens presented a better resistance to hydrothermal degradation than P and R samples. After prolonged aging, the depth of the monoclinic transformed layer ranged from 11μm to 14μm. Hydrothermal degradation led to a significant decrease of elastic modulus and hardness. SIGNIFICANCE Surface treatments affected the resistance to hydrothermal degradation of the 3Y-TZP ceramic. Dry mechanical surface modifications should be avoided since a high t-m transformation rate associated to the initial monoclinic content was observed. Annealing was useful to reverse the initial t-m transformation, but did not improve the resistance to hydrothermal degradation.

TiNOx coatings on roughened titanium and CoCr alloy accelerate early osseointegration of dental implants in minipigs

INTRODUCTION Titanium nitride oxide (TiNOx) coatings are known for their biocompatibility, hardness and high resistance to corrosion and wear. Further, they can be applied by plasma vapor deposition onto a wide variety of metallic, mineral, or organic substrates. In cell cultures, TiNOx coatings applied onto SLA (sandblasted, large grit, acid etched)-roughened titanium surfaces increased human primary osteoblast proliferation by 1.5 times in the first 2 weeks after seeding, while maintaining a high degree of cell differentiation. Therefore, the objectives of the present study were (i) to determine whether these findings would translate into the enhanced osseointegration of TiNOx-coated implants in vivo and (ii) to compare the osseointegration of Ti-SLA (titanium-SLA) and CoCr-SLA (cobalt-chromium-SLA) implants coated with TiNOx. METHODS Forty-eight cylinders made of Ti-SLA, Ti-SLA-TiNOx (TiNOx-coated Ti-SLA) and CoCr-SLA-TiNOx (TiNOx-coated CoCr-SLA) were implanted into the lower jawbone of 8 minipigs. The bone-to-implant contact was determined after 1 week, 2 weeks, 1 month and 3 months. RESULTS Osseointegration proceeded normally on all 3 surfaces, with equal activity after the first week of healing. After 2 weeks, bone-to-implant contact was 1.8 times higher on TiNOx coatings, either deposited on Ti or on CoCr. These differences fell off after 1 and 3 months of healing. CONCLUSIONS When compared to standard SLA titanium, TiNOx coatings enhance implant osseointegration during the first month of healing. Furthermore, this stimulating effect is independent of the substrate, leading to similar results whether the coating is applied onto SLA-Ti or onto SLA-CoCr.

Grinding damage assessment for CAD-CAM restorative materials

OBJECTIVES To assess surface/subsurface damage after grinding with diamond discs on five CAD-CAM restorative materials and to estimate potential losses in strength based on crack size measurements of the generated damage. METHODS The materials tested were: Lithium disilicate (LIT) glass-ceramic (e.max CAD), leucite glass-ceramic (LEU) (Empress CAD), feldspar ceramic (VM2) (Vita Mark II), feldspar ceramic-resin infiltrated (EN) (Enamic) and a composite reinforced with nano ceramics (LU) (Lava Ultimate). Specimens were cut from CAD-CAM blocs and pair-wise mirror polished for the bonded interface technique. Top surfaces were ground with diamond discs of respectively 75, 54 and 18μm. Chip damage was measured on the bonded interface using SEM. Fracture mechanics relationships were used to estimate fracture stresses based on average and maximum chip depths assuming these to represent strength limiting flaws subjected to tension and to calculate potential losses in strength compared to manufacturers data. RESULTS Grinding with a 75μm diamond disc induced on a bonded interface critical chips averaging 100μm with a potential strength loss estimated between 33% and 54% for all three glass-ceramics (LIT, LEU, VM2). The softer materials EN and LU were little damage susceptible with chips averaging respectively 26μm and 17μm with no loss in strength. Grinding with 18μm diamond discs was still quite detrimental for LIT with average chip sizes of 43μm and a potential strength loss of 42%. SIGNIFICANCE It is essential to understand that when grinding glass-ceramics or feldspar ceramics with diamond discs surface and subsurface damage are induced which have the potential of lowering the strength of the ceramic. Careful polishing steps should be carried out after grinding especially when dealing with glass-ceramics.

Grinding damage assessment on four high-strength ceramics

OBJECTIVES The purpose of this study was to assess surface and subsurface damage on 4 CAD-CAM high-strength ceramics after grinding with diamond disks of 75 μm, 54 μm and 18 μm and to estimate strength losses based on damage crack sizes. METHODS The materials tested were: 3Y-TZP (Lava), dense Al2O3 (In-Ceram AL), alumina glass-infiltrated (In-Ceram ALUMINA) and alumina-zirconia glass-infiltrated (In-Ceram ZIRCONIA). Rectangular specimens with 2 mirror polished orthogonal sides were bonded pairwise together prior to degrading the top polished surface with diamond disks of either 75 μm, 54 μm or 18 μm. The induced chip damage was evaluated on the bonded interface using SEM for chip depth measurements. Fracture mechanics were used to estimate fracture stresses based on average and maximum chip depths considering these as critical flaws subjected to tension and to calculate possible losses in strength compared to manufacturers data. RESULTS 3Y-TZP was hardly affected by grinding chip damage viewed on the bonded interface. Average chip depths were of 12.7±5.2 μm when grinding with 75 μm diamond inducing an estimated loss of 12% in strength compared to manufacturers reported flexural strength values of 1100 MPa. Dense alumina showed elongated chip cracks and was suffering damage of an average chip depth of 48.2±16.3 μm after 75 μm grinding, representing an estimated loss in strength of 49%. Grinding with 54 μm was creating chips of 32.2±9.1 μm in average, representing a loss in strength of 23%. Alumina glass-infiltrated ceramic was exposed to chipping after 75 μm (mean chip size=62.4±19.3 μm) and 54 μm grinding (mean chip size=42.8±16.6 μm), with respectively 38% and 25% estimated loss in strength. Alumina-zirconia glass-infiltrated ceramic was mainly affected by 75 μm grinding damage with a chip average size of 56.8±15.1 μm, representing an estimated loss in strength of 34%. All four ceramics were not exposed to critical chipping at 18 μm diamond grinding. CONCLUSIONS Reshaping a ceramic framework post sintering should be avoided with final diamond grits of 75 μm as a general rule. For alumina and the glass-infiltrated alumina, using a 54 μm diamond still induces chip damage which may affect strength. Removal of such damage from a reshaped framework is mandatory by using sequentially finer diamonds prior to the application of veneering ceramics especially in critical areas such as margins, connectors and inner surfaces.

Consensus report ‐ reconstructions on implants. The Third EAO Consensus Conference 2012

INTRODUCTION This group was assigned the task to review the current knowledge in the areas of implant connections to abutments/reconstructions, fixation methods (cement vs. screw retained) for implant-supported reconstructions, as well as the optimal number of implants for fixed dental prosthesis and implant-supported overdentures. MATERIAL AND METHODS The literature was systematically searched and critically reviewed. Four manuscripts were produced in the four subject areas based on systematically search strategy and consensus statements, clinical recommendations and implication for future research were formulated. RESULTS The following four position papers were the basis for the consensus statements, the clinical recommendations and directions for future research: Internal vs. external connections for abutments/reconstructions: A systematic review. Cemented and screw-retained implant reconstructions: A systematic review of the survival and complication rates. What is the optimal number of implants for fixed reconstructions? A systematic review. What is the optimal number of implants for removable reconstructions? A systematic review on implant-supported overdentures. The remit of this working group was to update the existing knowledge in the areas of reconstructions on implants. The group acknowledged the results of previous workshops and systematic reviews were provided by the following position papers: Stefano Gracis, Konstantinos Michalakis, Paolo Vigolo, Per Vult von Steyern, Marcel Zwahlen, Irena Sailer. Internal versus external connections for abutments/reconstructions: a systematic review. Irena Sailer, S. Mühlemann, Marcel Zwahlen, Christoph Hämmerle, Daniel Schneider. Cemented and screw-retained implant reconstructions: A systematic review of the survival and complication rates. Guido Heydecke, Frank Renouard, Ailsa Nicol, Marcel Zwahlen, Tim Joda. What is the optimal number of implants for fixed reconstructions? A systematic review. Mario Roccuzzo, Francesca Bonino, Luigi Gaudioso, Henny Meijer. What is the optimal number of implants for overdentures? A systematic review.

Medium-Term Function of a 3D Printed TCP/HA Structure as a New Osteoconductive Scaffold for Vertical Bone Augmentation: A Simulation by BMP-2 Activation

Introduction: A 3D-printed construct made of orthogonally layered strands of tricalcium phosphate (TCP) and hydroxyapatite has recently become available. The material provides excellent osteoconductivity. We simulated a medium-term experiment in a sheep calvarial model by priming the blocks with BMP-2. Vertical bone growth/maturation and material resorption were evaluated. Materials and methods: Titanium hemispherical caps were filled with either bare- or BMP-2 primed constructs and placed onto the calvaria of adult sheep (n = 8). Histomorphometry was performed after 8 and 16 weeks. Results: After 8 weeks, relative to bare constructs, BMP-2 stimulation led to a two-fold increase in bone volume (Bare: 22% ± 2.1%; BMP-2 primed: 50% ± 3%) and a 3-fold decrease in substitute volume (Bare: 47% ± 5%; BMP-2 primed: 18% ± 2%). These rates were still observed at 16 weeks. The new bone grew and matured to a haversian-like structure while the substitute material resorbed via cell- and chemical-mediation. Conclusion: By priming the 3D construct with BMP-2, bone metabolism was physiologically accelerated, that is, enhancing vertical bone growth and maturation as well as material bioresorption. The scaffolding function of the block was maintained, leaving time for the bone to grow and mature to a haversian-like structure. In parallel, the material resorbed via cell-mediated and chemical processes. These promising results must be confirmed in clinical tests.