Fatigue behavior of three thin CAD/CAM all-ceramic crown materials.

The purpose of this study was to study the effect of crown thickness on the fatigue life of CAD/CAM ceramic materials. CAD/CAM ceramic materials for the crown were virtually designed with three thickness designs of (a) ultra-thin occlusal crown average 0.7 mm thick (group A), (b) thin occlusal crown 1.1 mm average thick (group B), (c) thick occlusal crown 1.5 mm thick. The materials are: zirconia Cercon ZC and IPS e.max CAD (LD). Finite Element Analysis (FEA) simulations were carried out to estimate the fatigue lives of restorative materials. The lives for groups B and C under fatigue load were not significantly different from each other for Zirconia. The predicted lives for group A zirconia crowns, under fatigue load 50 N, 100 N, 120 N is 24 years, 4.3 years, 1.9 years, respectively. Results for crowns made of LD can be summarized as follows: under load 50 N, all groups have survived longer than 5 respectively, while under the load of 100 N, only group C survived longer than 5 years. 0.7 mm thick full contour Zirconia crowns possessed adequate endurance strength to survive under physiologic conditions. On the other hand, the crown made of LD should have at least 1.5 mm thickness to survive longer than 5 years.

Fatigue Resistance of 3-Unit CAD-CAM Ceramic Fixed Partial Dentures: An FEA Study.

PURPOSE: To estimate the fatigue life of 3-unit molar fixed partial dentures (FPDs) made from two different monolithic ceramic systems, zirconia cercon (ZC) and lithium disilicate (LD). The effect of the connector size on the fatigue resistance of the monolithic FPD was also investigated. METHODS: Two models for the FPDs were built, a 3-unit all-ceramic and a porcelain-fused-to-metal. The porcelain-fused-to-metal FPD model was used as the control. Actual 3-unit FPDs (replacing the second lower premolar) were constructed using a computer aided design and computer-aided manufacturing (CAD-CAM) system. Finite element analysis (FEA) was executed. A hemispherical indenter was used to simulate occlusal load. The occlusal load phase of the chewing cycle was applied at the premolar pontic. RESULTS: The failure location for the monolithic FPD was always located at the distal connector. Connector size played a key role in determining the long-term survival of the prosthesis. The fatigue resistance was predicted to be 670 N for the ZC with a connector size 4 × 3 mm, while it was only 226 N for LD. As for porcelain-fused-to-metal (PFM), FEA predicts that fatigue resistance can reach up to 770 N. Under the cyclic load of 670 N, the fatigue life for the zirconia FPD with connector size 4 × 3 mm was 2.23 × 106 cycles while it survived only 3.1 × 105 cycles when the connector was reduced to 3.5 × 2.5 mm. The angle of the oblique load has a significant effect on the stress distribution. CONCLUSION: 3-unit monolithic FPDs made of ZC have superior fatigue performance compared to those made of LD. The fatigue life of the zirconia FPD was about three times longer than that made of LD with a connector size of 4 mm × 3 mm. The survival rates of ZC FPDs are comparable to porcelain-fused-to-metal. A significant reduction in fatigue strength is predicted for reduced connector size. Therefore, it is necessary to establish general guidelines for the minimum connector size.

Fatigue Failure Load of Molars with Thin-Walled Prosthetic Crowns Made of Various Materials: A 3D-FEA Theoretical Study.

PURPOSE: The aim of the study was to compare the fatigue lifetime of thin-walled molar crowns made of all-ceramic CAD/CAM materials under three different cyclic load conditions. METHODS: The crowns were fatigued using a range of forces similar to which crowns in the molar region might be subjected. Crowns of two thin-walled thicknesses (0.7 mm and 1.1 mm) were prepared from Zirconia and lithium disilicate. Numerical methodologies to simulate the behavior of a restored tooth were applied to evaluate the fatigue lifetimes under multiple cyclic loading; 50 N, 100 N, 150 N. An 8 mm hemispherical indenter was used to simulate the mechanical stress of opposing teeth during mastication, and applied the fatigue load at the center of the crowns. RESULTS: The results show that the predicted survival rates for 0.7 mm and 1.1 mm Zirconia crowns were not significantly different. The number of life cycles predicted for Zirconia under all fatigue loads indicates that these crowns can live longer than five clinical years (when crowns are in service). However, crowns made from lithium disilicate also can be predicted to survive longer than five clinical years (under load up to 100 N). Crowns made of lithium disilicate should have 1.1 mm thickness to survive longer than five clinical years (when crowns are in service). CONCLUSION: Zirconia crowns exhibit significantly higher fracture resistance compared with lithium disilicate crowns, making them better suited to handle higher masticatory loads encountered in the posterior region of the mouth. Lithium disilicate can survive more than five clinical years (when their thickness is 1.1 mm).

Algebraic tangles and Jones polynomial.

We develop a method for computing Kauffman bracket and Jones polynomial for algebraic tangles and their numerator closures. We also introduce the notion of connectivity type of pretzel tangles and give a way of computing it. Several examples are given.

Effect of different biocompatible implant materials on the mechanical stability of dental implants under excessive oblique load.

BACKGROUND: Metallic implants such as titanium are much harder than the neighboring bone. This high difference may generate larger stress at the bone-implant interface during load transfer which leads to implant failure. The use of biocompatible polymer such as CRF-polyether ether ketone (PEEK) with strength comparable with the bone might lead to lower and better stress distribution to the supporting peri-implant bone. PURPOSE: The aim of this study is to investigate the effect of using carbon reinforced PEEK composite material for fixture/abutment on stress distribution in peri-implant bone. MATERIALS AND METHODS: Three-dimensional (3D) model of dental implant placed in the first mandibular molar is constructed from computed tomography scan. Five distinct models using a combination of titanium, CRF-PEEK, lithium disilicate for implant/abutment materials are studied. 3D finite element analysis (FEA) is used to evaluate the stress distribution at implant-bone interface under excessive oblique load. The physical interaction between several contacting bodies is numerically investigated. The effect of friction coefficients between the indenter and occlusal surface and between the implant and peri-implant bone is determined. RESULTS: FEA results show that there is no significant difference in the distribution pattern of stress at implant-bone interface among the different material models studied. The highest maximum and lowest minimum principal stresses were always located in the cortical bone and never in the cancellous bone which is consistent with the existing literature. Off-axis loading can result in unfavorable forces on the implant, jeopardizing the long-term success because of excessive lateral loads. Current FEA results agree with previously published work. CONCLUSION: Substitution of titanium implant by PEEK implant does not provide any advantages in regards to better stress distribution to the peri-implant bone. The strain thresholds of Frost's mechanostat theory that are suitable for long bone could not be applied for alveolar bone.

Developing of an environmental cell TEM holder for dynamic in situ observation.

This paper deals with the subject of "in situ" development of environmental-transmission electron microscope (E-TEM) holder assemblies. In E-TEM, the sample is continuously subjected to gases as opposed to conventional TEM where the sample is under high vacuum. E-TEM offers the possibility of achieving a new level of material analysis. E-TEM allows obtaining information about chemical information during the reaction at atomic level. Rarefied gas dynamics analysis is used to assess the proposed design. The analysis is focused on determining the molecular distribution inside the vacuum chamber and calculating the impingement rate on the target surface of the specimen. Simulations are performed to predict the molecular interaction with the specimen at given pressures to determine the proper position of a specimen within a vacuum chamber to optimize and predict reaction characteristics. Results of direct simulation Monte Carlo show that the two sides of the sample operate at different temperatures due to the gas flow and experience different molecular distributions.