Faculty Bibliography

The complex and patient-unique geometry of posterior all-ceramic dental crowns represents a particularly interesting set of challenges to understanding stress concentration and fracture evolution in response to loading. A series of numerical and physical experiments, with both single cycle and fatigue loading, show that geometry profoundly influences the stress concentration and fracture initiation and propagation. In stylized crowns with uniform axial wall height, stresses concentrate beneath the indenter. As the height of the axial wall increases, loads to cause failure increase linearly. In crowns with variation in axial wall height around the periphery, stresses concentrate both beneath the indenter and at the margin of the core ceramic. The magnitude of the stress concentration at the margin is directly related to the amount of variation in axial wall height around the periphery of the crown. Anatomically correct veneered zirconia core crowns subjected to single-cycle loads, fracture in areas of greatest stress concentration identified by finite element models. Fractures and stress concentrations that occur in response to single-cycle loading are important indicators of initiation sites for fatigue failure. (c) 2008 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2008

OBJECTIVE: To evaluate the mouth-motion step-stress fatigue behavior of two porcelain-zirconia all-ceramic crown systems. METHODS: The average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modeled by reducing proximal walls by 1.5 mm and occlusal surface by 2.0 mm. The CAD-based tooth preparation was made by rapid prototyping and used as a master die to fabricate all-ceramic crowns with 1.0 mm porcelain veneered on 0.5 mm Y-TZP cores (LAVA veneer+LAVA frame, 3M/ESPE, and Vita veneer+CERCON frame, Dentsply). Crowns were cemented on aged (60 days in water) composite (Z100, 3M/ESPE) reproductions of the die. Three crowns from the LAVA group were subjected to single cycle load to failure for stress profile design; remainder subjected to step-stress mouth-motion fatigue (three step-stress profiles). All mechanical testing was performed by sliding a WC indenter of 6.25 mm diameter 0.7 mm lingually down the mesio-distal cusp. Master Weibull curves and reliability for missions of 50,000 cycles at 200 N load were calculated (Alta Pro 7, Reliasoft). RESULTS: Single load to failure showed fractures through the zirconia core. Reliability for a 200 N x 50K cycle mission was not significantly different between systems. In fatigue, failure occurred by formation of large chips within the veneer originating from the contact area without core exposure. CONCLUSIONS: LAVA and CERCON ceramic systems present similar fatigue behavior; fatigue loading of both systems reproduces clinically observed failure modes

Clinically, zirconia-supported all-ceramic restorations are failing by veneer-chipping without exposing the zirconia interface. We hypothesized that mouth motion step-stress-accelerated fatigue testing of standardized dental crowns would permit this previously unrecognized failure mode to be investigated. Using CAD software, we imported the average dimensions of a mandibular first molar crown and modeled tooth preparation. The CAD-based tooth preparation was rapid-prototyped as a die for fabrication of zirconia core porcelain-veneered crowns. Crowns were bonded to aged composite reproductions of the preparation and aged 14 days in water. Crowns were single-cycle-loaded to failure or mouth-motion step-stress- fatigue-tested. Finite element analysis indicated high stress levels below the load and at margins, in agreement with only single-cycle fracture origins. As hypothesized, the mouth motion sliding contact fatigue resulted in veneer chipping, reproducing clinical findings allowing for investigations into the underlying causes of such failures

The objective of this study was to physico/chemically characterize and determine the corrosion resistance of a Calcium-Phosphate (Ca-P) based bioceramic thin coating processed by a sputtering process on titanium alloy (Ti-6Al-4V). The samples utilized in this study were uncoated and coated disks of 10 mm diameter by 3 mm thickness. The coating was characterized by SEM, XPS + ion beam milling (IBM), thin-film mode XRD, and atomic force microscope (AFM) (n = 3). Coated and uncoated Ti-6Al-4V disk surfaces were tested in Phosphate Buffered Saline (PBS) at 25 degrees C through an area of 0.79 cm(2). A three-electrode cell set-up was used with a saturated calomel electrode (SCE) and a platinum wire as reference and counter electrodes. After 3, 17, and 25 days of immersion, electrochemical impedance spectroscopy (EIS) experiments were performed (n = 3). The EIS tests were carried out in potentiostatic mode at the open circuit potential (OCP). The frequency range considered was from 100 kHz to 10 mHz, using 10 mV root mean square as the amplitude of the perturbation signal. A potentiodynamic polarization scan using a frequency response analyzer potentiostat, was acquired following 3 days of immersion in PBS. The potentiodynamic polarization scans (n = 3) were carried out with a scan rate of 1 mV/s ranging from -0.8V(SCE) to 3.0V(SCE). Results: The physico/chemical characterization showed an amorphous Ca- and P-based coating of ~400-700 nm thickness with Ca-P nanometer size particles embedded in a Ca-P matrix. The Bode phase angle diagrams showed highly capacitive results at low and medium frequencies for both surfaces tested. The polarization curves showed low current densities at the corrosion potential (E (corr)), in the order of 10(-8)A/cm(2), typical of passive materials with protective surface films. Coated sample current densities were comparable to the uncoated samples. Conclusion: Coated and uncoated samples were stable in the test solution with a protective film maintained throughout the 25 day immersion test period

The fundamental aspects of damage initiation and accumulation in one-piece zirconium oxide endosseous dental implants remain to be investigated. Aims: This study tested the null hypothesis that there is no influence on mouth-motion fatigue reliability and failure modes between as-received and after full crown preparation on one-piece ceramic implants. Methods: Forty-eight one-piece Y-TZP ceramic implants (Nobel Biocare, Goteborg, Sweden) were utilized. All specimens were embedded in acrylic resin exposing the first two threads at 30 degrees angulation with respect to the vertical axis (as per ISO specification 14801). Full crown preparations were performed following prosthodontic guidelines for half of the specimens. As-received and prepared specimens were distributed among three step-stress profiles based on the specimens ultimate fracture strength. Specimens were step-stress fatigued until failure or survival. A master Weibull curve was generated from the data and the reliability for completion of a mission of 50,000 cycles at 600 N load calculated. Results: No differences between the groups' reliability was observed. Failure mode for both groups was similar, where cracks initiated mainly at the tensile bending side of the second thread's internal diameter. The low Weibull modulus (<1) indicates that fatigue (<150,000 cycles) did not influence failure. Failure depended upon the applied load. Conclusion: Crown preparation did not influence the reliability of the one-piece ceramic implant. The null hypothesis was accepted. Fatigue did not influence the life time of ceramic implants at loads under 600 N. (c) 2008 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2008

PURPOSE: Mechanical analyses of idealized crown-cement-tooth systems through finite element analysis (FEA) has provided valuable insight concerning design parameters and materials that favor lower stress patterns. However, little information regarding variation of basic preparation guidelines in stress distribution has been available. The primary objective of this study was to evaluate maximum principal stresses on a molar crown veneer plus core system natural tooth configuration preparation with variations in the ratio of proximal axial length (PAL) to buccal axial length (BAL) as well as loading condition and position. MATERIALS AND METHODS: Three-dimensional models comprising a crown veneer (porcelain), crown core (zirconia), cement layer, and tooth preparation (4.2 mm BAL with PAL reductions of 0.8 mm, 1.0 mm, and 1.2 mm) yielding BAL:PAL ratios of 1.23, 1.31, and 1.4 were designed by computer software (Pro/Engineering). The models were imported into an FEA software (Pro/Mechanica), with all degrees of freedom constrained at the root surface of the tooth preparation. Each tooth preparation crown configuration was evaluated under a vertical (axial) 200 N load, and under a combined vertical 200 N and horizontal (buccally) 100 N load applied at different positions from the central fossa to the cusp tip. Maximum principal stress (MPS) was determined for the crown core for each crown BAL:PAL ratio, loading condition, and position. RESULTS: Under both vertical and combined loading conditions, the highest MPSs were located at the occlusal region and in the occlusogingival region of the ceramic core. MPS values increased in the proximal region as the BAL:PAL ratio increased. Combined loading resulted in a general increase in MPS compared to vertical loading. CONCLUSION: Increasing the BAL:PAL ratio (reducing the proximal axial length of the preparation) acted as a stress concentrator at regions near the crown margins, suggesting this area may be vulnerable to damage from fit adjustment as well as during function. Such increases in stress concentration should be considered in clinical scenarios, especially when inherent flaws are present in the material, since extensive high-magnitude tensile stress fields have been noted under all loading conditions

This study evaluated the effect of core-design modification on the characteristic strength and failure modes of glass-infiltrated alumina (In-Ceram) (ICA) compared with porcelain fused to metal (PFM). Premolar crowns of a standard design (PFMs and ICAs) or with a modified framework design (PFMm and ICAm) were fabricated, cemented on dies, and loaded until failure. The crowns were loaded at 0.5 mm min(-1) using a 6.25 mm tungsten-carbide ball at the central fossa. Fracture load values were recorded and fracture analysis of representative samples were evaluated using scanning electron microscopy. Probability Weibull curves with two-sided 90% confidence limits were calculated for each group and a contour plot of the characteristic strength was obtained. Design modification showed an increase in the characteristic strength of the PFMm and ICAm groups, with PFM groups showing higher characteristic strength than ICA groups. The PFMm group showed the highest characteristic strength among all groups. Fracture modes of PFMs and of PFMm frequently reached the core interface at the lingual cusp, whereas ICA exhibited bulk fracture through the alumina core. Core-design modification significantly improved the characteristic strength for PFM and for ICA. The PFM groups demonstrated higher characteristic strength than both ICA groups combined

OBJECTIVE: The purpose of this study was to determine the microtensile bond strength of various resin composite/adhesive systems to alumina particle abraded Ti-6Al-4V substrate after aging for 24h, 10 days, and 30 days in distilled water at 37 degrees C. METHODS: Four laboratory resin composite veneering systems (Gradia, GR; Solidex, SOL; Ceramage, CER; and Sinfony, SF) were bonded to 25 mm diameter machined disks of Ti-6Al-4V with their respective adhesive and methodology, according to the manufacturer's instructions. Microtensile bars of approximate dimensions 1 mm x 1 mm x 6 mm were prepared for each resin composite/adhesive system. After cutting, groups (n=12) from each adhesive system were separated and either stored in water at 37 degrees C for 24h (baseline) or aged for 10 or 30 days prior to loading to failure under tension at a cross head speed of 1.0mm/min. Failure modes were determined by means of scanning electron microscopy (SEM). Statistical analysis was performed through one-way ANOVA and Tukey's test at 95% level of significance. RESULTS: Significant variation in microtensile bond strength was observed for the different systems and aging times. SOL and GR showed the highest mean bond strength values followed by SF and CER at baseline. Aging specimens in water had an adverse effect on bond strength for SOL and CER but not for the SF and GR groups. SIGNIFICANCE: In vitro bond strength of laboratory resin composites to Ti-6Al-4V suggests that strong bonds can be achieved and are stable for certain systems, making them useful as an alternative for esthetic fixed prosthetic restorations

Among dental implant design alterations, surface modifications have been by far the most investigated topic. Regarding implant surface research, the lack of hierarchical approaches relating in vitro, in vivo, clinical trials, and ex vivo analyses has hindered biomaterials scientists with clear informed rationale guidelines for implant surface design. This manuscript provides a critical hierarchical overview of the in vitro, laboratory in vivo, clinical, and ex vivo methodologies used to investigate the performance of novel biomaterials aiming to allow dental professionals to better evaluate the past, present, and future dental implant surface research. This manuscript also contains an overview of the commercially available surface texture and chemistry modifications including novel nanotechnology-based fabrication processes. Over the last decade, surface texturing has been the most utilized parameter for increasing the host-to-implant response. Recently, dental implant surfaces utilizing reduced length scale physico/chemical features (atomic and nanometric) have shown the potential to synergistically use both texture and the inclusion of bioactive ceramic components on the surface. Although surface modifications have been shown to enhance osseointegration at early implantation times, information concerning its long-term benefit to peri-implant tissues is lacking due to the reduced number of controlled clinical trials. Given the various implants/surfaces under study, the clinician should ask, founded on the basic hierarchical approach described for the in vitro, laboratory in vivo data, as well as the results of clinical studies to effectiveness before use of any dental implant