PubMed · 2284546
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G W Braze. Esthetic modifications using intrinsic techniques. 4.. https://pubmed.ncbi.nlm.nih.gov/2284546/
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Hertzian cone cracks visible at the loading site of 20 all-ceramic fixed partial dentures (FPDs), tested in vitro, led to the hypotheses that failure was due to the propagation of localized contact damage crack systems (Hertzian stress state) and that such damage was an unlikely clinical failure mode. Fractographic analysis of the 20 laboratory-failed and nine clinically-failed all-ceramic FPDs allowed for definitive testing of these hypotheses and a comparison between in vitro and in vivo failure behavior. In all cases, failure occurred in the FPD connectors (none from contact damage), with approximately 70 to 78% originating from the interface between the core and veneer ceramics. The coincidence between failure origins provides strong evidence that the in vitro test modeled aspects of structural behavior having clinical importance. The fractographic observations, coupled with the in vitro failure load data, furnished very specific boundary conditions which were applied to constrain mathematical models of FPD connector failure. Finite element analysis (FEA) of the laboratory FPDs found that maximum principal tensile stresses would occur at locations consistent with the fractographic observations only if: (1) there were appropriate elastic moduli differences between the ceramics; and (2) a small amount of abutment rotation was allowed. Weibull failure probability (Pf) calculations, incorporating FEA stress profiles, very closely replicated the laboratory failure distribution only when: (1) the veneer ceramic was much weaker than the core ceramic; and (2) the Weibull modulus of the core-veneer interface was much lower than that for the free veneer surface (i.e., the interface is of lower quality with regard to defects).(ABSTRACT TRUNCATED AT 250 WORDS)
In medical ultrasound imaging, two-dimensional (2-D) array transducers are desirable to implement dynamic focusing and phase aberration correction in two dimensions as well as volumetric imaging. Unfortunately, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance near the resonance frequency. This results in poor signal-to-noise ratio (SNR) of the array elements. It has previously been demonstrated that transducers made from multilayer PZT ceramics have lower electrical impedance and greater SNR than comparable single layer elements. A simplified circuit model has been developed to optimize the SNR for multilayer ceramic (MLC) transducers. In this model, an electronic transmitter excites the array element and in the receive mode, the element drives a coaxial cable load terminated by a high impedance preamplifier. The transducer impedance is Zt/N2, where N is the number of piezoelectric layers. Maximum transmit signal is obtained when N = Ntx such that the transducer impedance, Zt/Ntx2, is matched to the source impedance. Maximum receive signal is obtained when N = Nrx such that the transducer impedance, Zt/Nrx2, is matched to the coaxial cable reactance. For maximum pulse-echo signal, the transducer should be designed with N = square root of Ntx Nrx, the geometric mean of Ntx and Nrx. Using this optimization technique, a 1.5-D array was designed with 3 layers for maximum pulse-echo SNR. Results of simulations from the simplified circuit analysis were consistent with those of the KLM model. The 3 layer array was fabricated as well as a single layer control array. The measured transmit signal and receive signal agreed with the simulation results.
The Cerec-2 unit offers three different options to generate the occlusion: anatomically extrapolated ("Extrapolation"), correlated to a wax-up or functionally generated path ("Correlation") and bucco-orally flat ("Linear") as known from Cerec-1. These three computer-generated occlusions were evaluated by measuring the vertical discrepancy and the specific time needed for manufacturing (design, machining, contouring and adjusting the occlusal relation) of inlays and overlays. The occlusion of inlays generated with "Extrapolation" (0.42 +/- 0.16 mm) and "Correlation" (0.80 +/- 0.17 mm) showed significantly (p < 0.001) less vertical discrepancy than "Linear" (2.46 +/- 0.50 mm). The specific production time of inlays differed significantly (p < 0.001) between "Extrapolation" (16.05 +/- 0.80 min), "Linear" (19.25 +/- 1.14 min) and "Correlation" (27.33 +/- 0.75 min). The vertical discrepancies of overlays generated with "Correlation" (1.26 +/- 0.43 mm) were significantly (p < 0.001) lower than with "Extrapolation" (2.46 +/- 0.50 mm) and "Linear" (2.75 +/- 0.57). The specific overlay production time was significantly (p < 0.001) longer with "Correlation" (39.44 +/- 1.36) than with "Linear" (23.9 +/- 1.26 min) and "Extrapolation" (23.71 +/- 1.01 min). Related to the technical status of 10/1994 and as a result of the present evaluation, "Extrapolation" was chosen to be the most practical of the computer-generated occlusions evaluated in the present study.