3-D Finite element analysis of all-ceramic posterior crowns

The purpose of this study was to evaluate the stress distribution under various loading conditions within posterior all-ceramic crowns. A three-dimensional finite element model representing a lower first molar was constructed. Variations of the model had two types of single layer all-ceramic crowns (Dicor and Empress) and two types of double layer all-ceramic crowns (In-Ceram and Empress2) cemented. A load of 600 N, simulating the maximum bite force, was applied vertically to the crowns. Loads of 225 N, simulating masticatory force, were applied from three directions (vertically, at a 45 degrees angle, and horizontally). In the test simulating maximum bite force, the maximum tensile stresses on all crowns (17.4-19.4 MPa) concentrated around the loading points. In the masticatory force simulation test, the specimens experienced maximum tensile stresses of 19.7-27.0 MPa under a horizontal load and 10.8-10.9 MPa under a vertical load. When the load was applied horizontally, the maximum tensile stress was observed around the loading points on the surface in the case of the single layer crowns, and of the cervical area of the inner core of the double layer crowns. Within the limitation of this study, it was found that the strength of occlusal contact points is important to the integrity of posterior all-ceramic crowns and that bite forces applied from the horizontal direction are a critical factor.     

Mechanical behavior of ProTaper universal F2 finishing file under various curvature conditions: a finite element analysis study

Introduction

The purpose of this study was to visualize the stresses and strain distribution patterns in ProTaper Universal F2 files (Dentsply Maillefer, Ballaigues, Switzerland) and to establish the stress- and strain-curvature relationship for this instrument under various conditions by using a dynamic, three-dimensional finite-element model.

Methods

An accurate geometric model of a ProTaper Universal F2 instrument was created. Two short, straight tubes were also modeled to represent the parts of root canal apical and coronal to the curvature. Then, the file was constrained to a curve of varying degree, curve length, and position. The maximum von Mises stress and strain on the tension side of the instrument was measured at 5-degree intervals in a numerical simulation package (LS-DYNA; Livermore Software Technology, Livermore, CA).

Results

The mechanical performance of the ProTaper F2 file under various conditions was simulated. A long curvature length produced lower values of stress and strain under the same angle of curvature. An increase in the curvature angle generally induces higher stress and strain. For the same degree and curve length, the stress and strain increased if the curved portion was situated further up the shaft of the instrument (ie, with a larger diameter).

Conclusions

The dynamic, numerical model may be used to evaluate and compare the effect of various root canal curvatures on the behavior of different designs of root canal instrument. The magnitude of stress and strain imposed on the instrument is influenced by the abruptness and degree of curvature as well as the location of the curved portion.     

Galvanic skin response of oral cancer patients during speech

Severe speech difficulty is often caused after surgery of an oral cancer. Prosthetic treatment with a removable obturator prosthesis is generally provided for such patients. Although some speech ability is recovered with prosthetic treatment, patients sometimes complain of continued dissatisfaction with their speech. However, it is difficult to evaluate the dissatisfaction. Therefore, a new method for evaluation is desirable. In this study, such a new method using the galvanic skin response as the index for the dissatisfaction of the patient was developed, and its objectivity was investigated. Eleven patients with maxillary bone defects were selected. Prior to the evaluation, improvement of speech with the removable prosthesis was confirmed using the speech intelligibility test and the visual analogue scale. The electrical resistant value at pronunciation was measured with the measuring system composed with the apparatus (galvanic skin response (GSR) measuring apparatus), the personal computer program. The changes for the electrical resistant value after pronunciation were evaluated by calculating the decrease ratio at pronunciation [(the mean electrical resistance before pronunciation - the mean electrical resistance after pronunciation)/the mean electrical resistance before pronunciation]. This decrease ratio at pronunciation was defined as the index of the speech dissatisfaction of the subject. The mean values for the decrease ratio with prosthesis were significantly smaller than the values without prosthesis (P < 0.05). From the results of this study, it is suggested that the measurement of the electrical resistance change of the skin during speech could be a new method for evaluating the speech dissatisfaction of the post-oral-cancer patient.     

Anticipating the Future and Change

As in all of medicine, there are basic assumptions underlying the present system of dental education and delivery. These basic assumptions are facing challenges due in part to the marked reduction in the incidence of caries and the development of more effective therapies. This current period of time is described as an era of true change, rather than the mere modification of existing technology. Changes are going to occur in a number of areas. The move will be from government to private, from small to large, from generalist to specialist. Since change is inevitable, one can learn about and prepare for the stress of change, the changes happening now, and how to adapt to change. Although many changes are beyond the individual's control, their occurrence may be beneficial.     

The relationship between life-events and periodontitis A case-control study

Abstract This case-control study (n=100 dental patients, matched for age and sex) investigated the rôle of life-events in periodontitis. Data collected included life events, tobacco use, oral health behaviours and socio-demographics. The results of conditional simple logistic regression analysis showed that periodontitis was associated with the negative impact of life-events (p<0.01), the number of negative life-events (p<0.05), high levels of dental plaque (p<0.01), tobacco smoking (p<0.01) and being unemployed (p<0.05). These associations remained statistically significant after adjusting for oral health behaviour and socio-demo-graphic variables, but not tobacco smoking (p>0.05). Marital status became statistically significant after adjusting for the other variables (p<0.05). A model is suggested to explain the pathways through which life events may affect periodontal health. It was concluded that psychosocial factors and oral health risk behaviours cluster together as important determinants of periodontitis.     

下颌骨骨折动力加压固定断面应力分析

实验采用反射光弹模拟应力分析法,通过受力模型等差线条纹图观测、分析、量化换算,对下颌骨骨折轴向和偏心动力加压固定作断面压应力比较研究.结果显示两种固定状态下骨折断面均表现为挤压应力,沿骨折线应力分布不均匀,板周区和牙槽区呈压应力高峰带,最大应力分量在轴向和偏心加压分别为1.8MPa和2.3MPa(板周区),以及0.5MPa和1.0MPa(牙槽区),中间区受力较弱.轴向动力加压总压力为100N,偏心动力加压总压力为140N,后者比前者断面应力分布稍趋合理.     

Psychosocial stressors in oral lichen planus

BACKGROUND: Psychosomatic factors and their association with dermatological disorders are well recognized, yet their importance in oral lichen planus (OLP) is still debated. This study was done to determine the importance of psychosocial stressors in patients with OLP. METHODS: An analytical age-sex matched double controlled study. Stratified serial random sampling was done for sample selection. The experimental group consisted of biopsy proven OLP subjects (n = 41), negative control consisted of apparently healthy general Out Patient Department (OPD) patients (n = 73) and positive controls were patients with burning mouth syndrome, atypical facial pain and myofacial pain dysfunction syndrome (n = 36). The General Health Questionnaire-version 28 (GHQ-28) and the Hospital Anxiety and Depression Scale (HADS) were used to evaluate psychosocial stressors in terms of stress, anxiety and depression respectively. RESULTS: The OLP patients had significantly higher stress (Z = 4.331; p < 0.05, significant), anxiety (Z = 4.260; p < 0.05, significant) and depression levels (Z = 4.942; p < 0.05, significant) than the negative controls when measured by GHQ and HADS respectively. Similar findings were noted between the two control groups, with higher stress (Z = 5.784; p < 0.05, significant), anxiety (Z = 6.416; p < 0.05, significant), and depression (Z = 4.841; p < 0.05, significant) levels in the positive control group. However, differences in stress (Z = 2.0416; p > 0.05, non-significant), anxiety (Z = 1.681; p > 0.05, non-significant) and depression levels (Z = 0.195; p > 0.05, non-significant) were found to be non-significant between OLP and positive control. CONCLUSION: Significantly higher stress, anxiety and depression levels were found in the OLP and positive control than the general population. No significant difference was noted between the OLP and the positive controls (in which psychosocial stressors are the sole etiology). These suggest that psychological stressors play an important role in the causation of OLP. It may be further hypothesized that these stressors form a starting point for the initiation of various autoimmune reactions, which have been shown to be contributory to the pathogenesis of OLP. Further longitudinal studies need to be done globally before definitive conclusions can be drawn.     

An experimental analysis of stresses in simulated flared root canals subjected to various post-core applications

Cervical root fracture is a major clinical problem in flared root canals treated with different post and core applications. This in vitro study evaluated the stress transfer of different post and core systems to the cervical part of the artificially created flared root canals, by using strain gauges. The post-core systems investigated were: (a) cast post-core system without resin reinforcement, (b) cast post-core system with resin reinforcement, (c) pre-fabricated post and resin core with resin reinforcement. The post-core systems which were cemented on simulated roots were subjected to a load applied at an angle of 45 degrees to the long axis of the simulated roots. The strain gauges which were cemented to the cervical part of simulated roots were connected to the data acquisition module to measure and record the changes in strain data. Specimens restored with resin reinforcement either with cast post-core or pre-fabricated post and resin core transferred the stress to the cervical part of the artificial roots at a rate lower than conventional cast post-core system (P = 0.001; anova and Tukey's post hoc test). It was concluded that the resin reinforcement of root canals before post-core applications reduces the stresses at the cervical part of the root surfaces.     

二单位式与四单位式种植牙周骨界面应力分布规律差异的三维有限元分析

为了探讨人工种植牙的数目、上部结构对种植牙周骨界面应力分布的影响，本实验应用三维有限元分析方法，对二单位和四单位式杆式覆盖种植义齿种植牙周骨界面的应力分布规律进行了探讨。结果表明：最大压应力、最大拉应力二单位式与四单位式均位于颈周密质骨，二单位式大于四单位式，两者有显著差异性，（Ｐ＜０．００１）。四单位式最大拉、压应力，远中种植牙要大于近中种植牙。最大位移运动二单位式小于四单位式，四单位式近中种植牙大于远中种植牙。二单位式与四单位式位、压应力主要集中于颈部，其它部位与颈部相比有非常显著的差异性，（Ｐ＜０．０００１）。结论：种植牙数目的增加，可以减小种植牙周颈部密质骨内的最大应力值。四单位式种植义齿颈周骨内应力要小于二单位式种植牙，从这点上看，四单位式种植义齿要优于二单位式种植义齿。多个种植牙种植时，杆的连接，改变了种植牙周骨内的应力分布规律，其应力主要由种植牙颈周密质骨来承担