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Replicas of a dentate human mandible were prepared in photoelastic resin material. They were positioned in a supporting frame by means of struts, representing the principal muscles of mastication. Occlusal loading was simulated, and the stresses generated within the models were examined by three-dimensional photoelastic stress analysis.
The wear resistance of three different types of restorative materials for posterior teeth was examined in a 2-year clinical study. Reduction in vertical dimension was measured with the aid of profilometric grafical curves of the restoration occlusal relief. The following results were obtained: (formula: see text). The results show that, relative to wear resistance, the composite restorative material, strengthened with a new type of anorganic microfiller, is clearly superior to the conventional composite, however, is not equal to amalgam. Besides resistance to wear, there are numerous other demands of a posterior tooth composite system which have yet to be met before these microfiller composites can be recommended for the restoration of conventionally prepared Class I and II cavities.
STATEMENT OF PROBLEM: Coronoradicular reconstruction techniques of pulpless teeth include prefabricated post systems that retain a core material such as silver amalgam, composite, glass ionomer, or modified glass ionomer cement. Mechanical properties of these materials are critical to sustain masticatory forces. PURPOSE: This in vitro study compared the mechanical resistance of 3 core materials (silver amalgam, composite, and silver-reinforced glass ionomer) under masticatory conditions. MATERIAL AND METHODS: Industrially fabricated teeth were used and a total of 75 specimens were divided into 3 groups of 25 specimens. Titanium canal posts were placed, followed by core buildups in amalgam, composite, or glass ionomer. Teeth were prepared for full cast crowns and the crowns were fabricated and cemented with glass ionomer cement. Twenty specimens from each group were placed in a mastication simulator cyclically loading the teeth with a 400 N force for 1.5 million cycles. The 5 remaining specimens were used as controls. Teeth were sectioned and observed macroscopically and microscopically to determine the rate of defects for each material. Observed defects were verified with the Kruskal-Wallis test. The 3 core materials were ranked with the Tukey multiple comparison test. RESULTS: Significant differences of mechanical behavior were found for the 3 materials. At P <.01, silver amalgam was significantly superior to composite and glass ionomer. Composite was significantly superior to glass ionomer. CONCLUSION: Cores fabricated with amalgam had the lowest rate of defects when tested under artificial crowns. Glass ionomer, when used as a core material under artificial crowns, showed the highest rate of defects after an instantaneous load of 400 N for a 1,500,000 repetition cycle.
Magnetically retained sectional obturator prostheses, consisting of a hollow obturator portion made of thin silicone layer over a resin frame and a denture portion, were fabricated to alleviate some of the limitations of silicone materials, and applied to two edentulous patients with maxillectomy defects. The silicone obturator allowed profound engagement of undercuts within the defect, resulting in adequate retention, support and stability of the prostheses for over 3 years. The patients have achieved improvement in speech and mastication without complications, by use of the obturator prostheses.
Several methods have been developed to evaluate attritional wear of restorative materials. A distinction was made between attritional wear in occlusal contact areas (OCA) and wear in contact-free occlusal areas (CFOA). Only limited data dealing with approximal wear in vitro are available. Using a new configuration of the computer-controlled masticator, approximal wear of two composites (P-50, 3M and Tetric Ceram, Ivoclar-Vivadent) was assessed in a two-body wear test after thermo-mechanical loading. Wear was quantified using a profilometer. In addition, micromorphology was assessed by SEM. Wear showed a non-linear pattern, which was comparable to occlusal abrasion. After the first loading cycle, wear increased significantly (p < or = 0.05), and subsequently decreased. After a 5-year-equivalent, the mean substance loss for composite specimens was 20.3 +/- 15.6 microns for P-50 und 17.5 +/- 3.1 microns for Tetric Ceram. Approximal wear between enamel surfaces was 3.9 +/- 4.3 microns. In another experiment, the influence of the approximal wear on an enamel bonding agent was assessed. After thermo-mechanical loading, specimens were immersed in a demineralisation gel. The protective potential of the sealant was assessed under a polarized light microscope. The results suggested that partial sealant loss could lead to caries-like demineralisation in the affected areas.
Acid-etched prostheses are exposed in the oral cavity to pressure of mastication, to corrosion due to oral fluids, to changes in temperature due to ingestion, and to iatrogenic influences. The changes in temperature which were measured in the adhesive layer were up to +/- 40 degrees C within a range from -2 degrees C up to about 80 degrees C. The bonding agents, which had a high softening temperature and little thermal expansion showed the least influence of temperature after the in-vitro tests. The fact that the tested bonding agents are different in chemistry influenced significantly the adhesive and cohesive characteristics. The thermal tests lead to the conclusion that for clinical use the following adhesives are to be preferred: Bis-GMA modified by monomeric phosphate (e.g. Panavia) and Bis-GMA (e.g. Microfill pontic).
A parotid cup has been widely used as an intraoral device for the collection of parotid saliva. However, in long duration sampling, the cup tends to disconnect under circumstances such as mastication, sleep or physical exercise. We have designed a modified device of parotid cup for saliva collection to overcome these unfavorable conditions. The parotid cup was modified by fixing it to the bite-plate appliance which was adjusted individually to the occlusal plane in order to avoid disconnection of the cup under varying conditions. Using this device, we could measure salivary flow rate during mastication, sleep or physical exercise. Additional advantages of this device are that it is easy to make with acrylic resin and is inexpensive. The preliminary study using this device showed that flow rates of unstimulated parotid saliva varied from 0.06 +/- 0.04 ml/min. to 0.16 +/- 0.10 ml/min. (mean +/- s.d) from 7 a.m. to 12 p.m. in 7 subjects. In five subjects, the flow rate of parotid saliva during meal (lunch) was much higher than that of resting on a chair, playing table tennis, or sleeping at night for 6-8 hours. This might be considered due to the stimuli of the meal content. Flow rates of saliva were also examined with various stimuli. The results showed that salty pickled plum > sour lemon juice > 10% ascorbic acid > sweet milky candy > 1% ascorbic acid. These data suggest that our modified device is useful for various experiments in salivary research.
Eight intraoral porcelain repair systems, ie, Oral Ceram-Etch (Gresco), Scotchprime (3M), Rocatec (ESPE), Command Ultrafine (Kerr), Silistor (Kulzer), Clearfil Porcelain Bond (J Morita), All-Bond (Bisco), and Monobond S (Vivadent) were used in this study. The control specimens consisted of unetched porcelain surfaces onto which a resin composite was polymerized without the use of an adhesive. Load fatigue was used as the testing method to simulate the repetitive action of mastication. The peak stress applied to each test specimen was 1500 psi (10.34 MPa), and an upper limit on the number of load cycles applied to any specimen was set at 2,000,000 cycles. Statistical analysis revealed two significant subsets. Only Clearfil Porcelain Bond and All-Bond did not fail before reaching the 2,000,000-cycle upper limit.
A technique has been described for fabrication of a removable prosthesis that restores proper cheek contour. This prosthesis is a modified removable partial denture with a retentive surface to hold an acrylic resin bolus to recontour the cheek from within. The advantages of such a prosthesis for the patient with facial nerve paralysis include restoration of fullness and proper contour to the cheek (Figs. 7 and 8), prevention of trauma to the buccal mucosa during mastication, and stretching of the cheek, which indirectly improves oral hygiene in the buccal sulcus. The disadvantages of this technique include limited improvement in esthetic and functional contour of the cheek, and food accumulation beneath the acrylic resin bolus, which requires more meticulous hygiene.
The purpose of the study was to explore the fundamental wear behaviour of a dental composite with different filler loadings under two-body wear conditions. The parent resin and filler components were mixed according to different weight ratios to produce experimental composites with filler loadings ranging from 20 to 87.5% by weight. A two-body wear test was conducted on the experimental composites using a wear-testing machine. The machine was designed to simulate the impact of the direct cyclic masticatory loading that occurs in the occlusal contact area in vivo. The results showed that there was little increase in the rate of wear with filler loadings below 60 wt%, but a sharp increase between 80 and 87.5 wt% in filler loading. Wide striations and bulk loss of material were apparent on the wear surfaces at higher filler loadings. Coefficients of friction increased with filler loading and followed the increase in rate of wear loss closely. It was concluded that, under two-body wear conditions, addition of high levels of filler particles into the resin matrix could reduce the wear resistance of dental composites. This finding may help when designing future dental composites for use in particular clinical settings.
Prosthetic treatment failures related to the biomechanical deficiencies of post and cores still represent a problem of clinical significance. To overcome the difficulties of clinical studies, numerous in vitro methods were developed to address specific properties of post-and-core restorations. Most of them, however, were based on an oversimplified mechanical testing of the restored tooth. Experience proved that the fatigue of restorative materials was a primordial factor in clinical failures. Therefore, special devices were built that stimulate the physiological masticatory cycle. Tests performed with adhesive post-and-core systems seem to indicate that materials placed in the tooth should have physical properties as close as possible to those of natural tissues. New carbon-epoxy posts appear to offer a promising solution for restoring the endodontically treated tooth.
The potential correlation of the force versus time behavior of four resilient lining materials and their acceptance by patients was investigated. Two modified acrylic resins and two silicone denture lining materials were tested. The response of the soft lining materials to a programmed indentation test was used to define a parameter for each denture liner product. A parameter defined as the "Rebound Index," using force-time data obtained from the indentation measurements, was significantly different for the modified acrylic resins compared to the silicone materials. Improved patient acceptance was correlated with higher values of this index.
Twenty-two commercially available composites, a new resin without inorganic filler and two amalgams have been investigated to compare their abrasive behaviour. In laboratory tests, an abrasion model of tooth against filling has been simulated. The results indicate a dependence of the abrasive behaviour on the type of filler in the different composites. Composites with quartz as filler have the greatest resistance to abrasion, whereas those with lithium-aluminium-silicate show the strongest abrasion. Mean abrasion values are obtained for the composites with barium-glass or a mixture of barium-glass and quartz as filler. The resin and the two amalgams have abrasion values comparable to the abrasive resistant composites.
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The influence of several experimental parameters on the sliding-wear behavior of a composite restorative has been examined. The results demonstrate that: 1) Changes in surface finish and sliding velocity have little effect on the moderate wear-rate observed at nominal levels of stress, and 2) increased contact stress can profoundly alter wear mechanisms and produce marked surface failure at levels well within the range associated with human mastication.
The soft denture liner was applied to dentures for the edentulous patients with poor residual ridge and thin mucosa. It is useful for minimizing the soreness caused by the denture during mastication, however, its effect on masticatory functions has been rarely investigated. The purpose of this study was to determine the effect of a soft liner applied to complete dentures on masticatory functions. Five edentulous volunteers who were satisfied with their new complete dentures took part in this study. The lower dentures were lined with a soft liner. Comparison of occlusal force, masticatory performance, masseter muscular activity, and mandibular movement were made between new dentures and relined dentures. The results of applying soft materials to lower dentures obtained by comparison with complete dentures using hard acrylic resin bases were follows: 1. Occlusal forces were significantly larger (p < 0.01). 2. Masticatory performance in chewing peanuts increased slightly. The number of strokes and time for masticating a peanut decreased significantly (p < 0.01). 3. Masticatory muscles functioned more rhythmically and mandibular movements became smoother. 4. Integrated EMGs per stroke of all patients was similar.
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