Richard J. Mitchell

Fracture resistance of teeth restored with class II composite restorations

Teeth with large mesio-occlusal-distal cavity preparations fracture more easily than intact teeth. An intracoronal restoration capable of increasing tooth fracture resistance is desired. This in vitro study compared the fracture resistance of maxillary premolars restored with enamel bonding and dentin bonding. The effects of the type of curing system (chemical or photochemical) and marginal preparation (beveled or not) were also evaluated. Premolars restored with large MOD composite resin restorations were approximately twice as strong as the unrestored prepared tooth. However, all restored teeth were no more than half as strong as the intact tooth. Dentin bonded restorations were not significantly stronger than enamel bonded restorations.

Effect of cementation method on the retention of anatomic cast post and cores

This study determined the influence of the cementation method on the retention of anatomic posts. Three cementation methods were used for each post/tooth combination: (1) cement on post only, (2) cement in canal only, and (3) cement on post and in canal. The mean retentive force for method 2 was significantly higher than for method 1. The mean force for method 3 was not significantly different from either method 1 or method 2.

Effect of a hydraulic jet prophylaxis system on composites

Because air-powered devices that remove surface stain from teeth are commercially available, recent literature concerning the effect of the hydraulic jet prophylaxis system on oral tissues and restorative materials was reviewed. A study designed to compare the volume loss by different types of composites abraded by this system revealed a relatively high rate of abrasion for the small-particle composite compared with composites containing larger filler particles.

Resistance to mercury embrittlement of some alloys for amalgam

Abstract Dental amalgam is a composite material in which dispersed unconsumed alloy particles reinforce the system. The strength of amalgam is governed by the fracture resistance of the particles and the bond between the particles and the matrix. The purpose of the present experiment is to examine the resistance to mercury embrittlement of some alloys used to make powder for dental amalgam. Tensile specimens of pure silver, pure tin, and six different alloys were prepared: Eut (71.9 w/o Ag-28.9 w/o Cu), ACS (Eut+10 w/o Sn), AgCu (49 w/o Ag-51 w/o Cu), Op (70 w/o Ag-26 w/o Sn-4 w/o Cu), Ty (60 w/o Ag-27 w/o Sn-13 w/o Cu) and Sy (40 w/o Ag-30 w/o Sn-30 w/o Cu). Mercury was electroplated onto the middle portion of the specimens. Four specimens of each pure metal and alloy were tested at 5 min, 30 min, 1 hour and 24 h after the plating with a loading rate of 0.25 mm/min. As-cast specimens were also tested. The present study revealed that: 1) 5 min after plating, all the alloys, except Op, Sy, and Eut, are significantly weaker than the unplated alloys; 2) the alloys with the lowest percentage of their unplated strength 5 min after plating are ACS (41.4 w/o) and Ty (41.8 w/o); 3) after 30 min, the effect of plating diminishes so that significant decreases in strength are consistently found only in ACS and Sn; 4) the silver-copper alloys were the least affected by plating. The results suggested that mercury released during aging or corrosion of amalgams in vivo might embrittle unconsumed alloy particles, weakening the amalgam restoration.

Surface coating and leakage of dentin-bonded resin composite restorations

OBJECTIVES To compare the marginal leakage of dentin-bonded resin composite restorations in tooth sections coated with nail varnish and similarly restored sections coated with cyanoacrylate cement. METHODS MO and DO cavities were prepared with the gingival floor below the CEJ in 11 non-carious extracted human molars. Each cavity was restored with a dentin adhesive and resin composite. Sectioning yielded four specimens per tooth (N=44). One specimen from each restoration was coated with varnish. The other specimen was coated with cyanoacrylate cement. The coatings were applied to all surfaces except that a 1mm window on either side of the interproximal gingival margin was left uncoated. Specimens were thermocycled and stained with silver nitrate. Silver penetration into the gingival margin of each section was measured with a measuring microscope. The predominant leakage path for each coating type was determined by scanning electron microscopy. RESULTS There was no significant difference between the leakage of the varnish-coated and cyanoacrylate-coated specimens. No marginal gaps were observed either by optical or by electron microscopy. However, the both optical and electron microscopy revealed leakage in nearly all specimens. This leakage was confined to either the dentin/hybrid layer interface or the adhesive resin/hybrid layer interface. CONCLUSIONS The results suggest that these coating materials are not confounding factors in laboratory investigations of marginal leakage along dentin-bonded interfaces of resin composite restorations. Although marginal gaps were undetectable even at high magnification, leakage was observed along the gingival margin of almost all of these Class II resin composite restorations.

A comparison of abrasion resistance of dental stones.

Abstract The objective of this study was to evaluate the abrasion resistance of gypsum dental die stones. A review of literature of previous evaluations of gypsum products is presented, and the relevancy of the various tests is discussed. Testing was done at 24 h and 7 days after mixing. There are significant differences between die stones, and aging from 24 h to 7 days makes a substantial difference with some stones.

Non-carious cervical lesions imaged by focus variation microscopy

OBJECTIVE The aim of this study is to characterize the morphological and histological features of NCCLs in a group of extracted teeth using a focus variation optical microscopic technique that is capable of detecting minute variation in surface topography. METHODS Twenty three extracted teeth containing NCCLs were collected. Histological features of the surface and longitudinal cross sections were examined using a focus variation microscope (FVM). RESULTS The sample included 8 teeth with wedge-shaped lesions; the remaining 15 contained saucer-shaped lesions. Wedge-shaped lesions showed obliterated dentinal tubules, parallel furrows and micro-fractures in the surface; longitudinal cross sections revealed subsurface micro-fractures extending toward the pulp chamber. The surfaces of typical saucer-shaped lesions were smooth and relatively featureless. CONCLUSIONS FVM imaging shows microstructures that are consistent with simultaneous erosion and toothbrush abrasion. Saucer shaped lesions have a smooth featureless surface as well as craters and dimples that could be attributed to uneven acid attack. In wedge-shaped lesions, the presence of scratches and furrows could be attributed to mechanical forces such as tooth brush abrasion. The furrows and in-surface micro-factures of wedge shaped lesions suggest a possible role of tensile stresses but require further investigation. CLINICAL SIGNIFICANCE NCCLs present in two morphologies, either saucer-shaped or wedge-shaped. Erosion and toothbrush abrasion are important in both types of lesions. Tensile stresses due to occlusal loading may be important in some wedge-shaped lesions. Knowing the etiology of cervical lesions is the key for their prevention.

Measurement of the Shear Modulus of Tissue-Like Materials Using Magnetic Resonance Elastography

We developed a technology to measure the shear modulus of tissue-like materials using Magnetic Resonance (MR) imaging. In this technique, called MR elastography or MRE, the sample is vibrated at 100–300 Hz while MR images are made. The shear wave generated by the vibration will propagate into the material at a speed proportional to the square root of the shear modulus. We compared the shear modulus of gel samples measured in this way with the modulus derived from a static compression test and assuming the material is linearly elastic. The results show good agreement of the two methods provided the shear modulus of the material is below approximately 10 kPa. Differences in the two measurements are explainable by differences in the compression rate of the two techniques.Copyright