L.H. Mair

Wear: mechanisms, manifestations and measurement. Report of a workshop

OBJECTIVE The purpose of this workshop was to bring together a tribologist (T.A.S.), a clinician (L.H.M) and a dental materials scientist (R.W.V) to discuss the fundamental mechanisms of wear and how these relate to the manifestations and measurement of wear in dentistry. Eighty delegates contributed to a valuable discussion led by the workshop chairman (C.H.L.). KEY POINTS Wear is the net result of a number of fundamental processes: abrasion, adhesive effects, fatigue and corrosive effects which act in different combinations on the various classes of materials. In ceramics, sliding compression and surface corrosion greatly increase wear, whereas the ability to deform either plastically or elastically influences the wear of polymers. The filler particle distribution is a major variable influencing the wear of composites. In the mouth, wear can be considered in terms of its site, timing and mechanism. The latter may involve direct contact between surfaces or result from the action of slurries; both of which are affected by surface corrosion (erosion). Although wear can be categorized at the chairside, its precise measurement involves the use of replica models and surface contouring. Laboratory simulation is useful to study fundamental wear mechanisms; but it is not able to predict clinical wear. CONCLUSIONS The management of clinical wear requires a proper understanding of the underlying mechanisms. This can only be achieved through close co-operation between all the disciplines which seek to understand and manage wear. The attendance of so many delegates at this workshop indicates the willingness to participate in this process.

Wear in dentistry—current terminology

There is some confusion in the dental literature concerning the phenomenon of wear which is usually considered in terms of its clinical manifestations rather than its underlying processes. The parent discipline for the study of these processes is tribology. The terms attrition, abrasion and erosion are common to both dentistry and tribology; however their meanings are different. Even within the various disciplines of dentistry there are differences in the understanding of the same terms. This article relates the dental appreciation of wear to the underlying processes.

The recharge of esthetic dental restorative materials with fluoride in vitro—two years' results

OBJECTIVES A range of esthetic restorative dental materials are now available to dental clinicians. The aim of this study was to evaluate the relative fluoride recharge potential of a number of these materials in vitro over two years. METHODS Disc samples (12 mm x 2 mm) of each of the materials were placed into artificial saliva at neutral pH. The materials tested were: two glass ionomers (Chemfil; Ketac-Fil), two resin-modified glass ionomers (Photac-Fil Aplicap; Vitremer), two compomers (Dyract; Compoglass) and two composites (Heliomolar; Concise). At 13 intervals over a two years period the discs were subjected to a 2 min fluoride recharge by exposure to sodium fluoride (500 ppm F). The pre- and post-recharge fluoride release values were determined using an ion sensitive electrode. RESULTS At all time intervals the post-recharge fluoride release of the materials was significantly different (MANOVA p<0.0001). The mean values for post-recharge fluoride release after two years were: Chemfil 10.3, Ketac 3.0, Vitremer 9.0, Photac-Fil Aplicap 12.1, Compoglass 5.0, Dyract 3.6, Concise 0.3, Heliomolar 0.2 (units = microgF/cm(2)/h). SIGNIFICANCE Esthetic fillings vary significantly in their capacity to absorb and re-release fluoride. The glass ionomer-based materials displayed a far greater potential for fluoride recharge than the composites, in which the recharge was virtually negligible. Whilst it cannot be assumed that fluoride release is directly proportional to cariostatic potential, these results suggest that, where possible, glass ionomer-based materials should be used in patients who have a high caries rate.

Surface permeability and degradation of dental composites resulting from oral temperature changes

The surface changes in dental composites caused by thermal cycling and different temperatures were evaluated by use of silver nitrate to stain the surface layers. Rapid temperature change resulted in the formation of layers within the surface which may have resulted from microcracking. Slow rates of change increased the depth and rate of diffusion of silver nitrate. Determination of the temperature changes at the surface of a restoration in the mouth indicated that in vivo temperature change is more likely to increase the depth of diffusion of oral fluids than to cause microcracking of the surface.

An investigation into the permeability of composite materials using silver nitrate

A new effect of silver-nitrate staining on dental composites and unfilled polymers has been observed which may be useful in the study of polymer permeability. After extended storage in silver nitrate, the materials developed a layer of brown stain in the subsurface. The color was thought to be caused by the presence of finely divided silver precipitated in microcavities within the composite. Energy Dispersive Analysis by x-ray confirmed the presence of silver in the stained zone. The width and shade of staining varied in different materials. After 90 days, layers of different shades could be seen within the stained zone. The exact shade of the stain is thought to depend upon the size of the precipitated silver particles, which is directly related to the sizes of the microcavities.

The effect of thermal cycling on the fracture toughness of seven composite restorative materials

Short-red chevron-notch samples of seven composite materials were thermal-cycled between 0 degrees C and 60 degrees C. The stress intensification factor, KIC, was found for four sample groups subjected to 10(1)-10(4) cycles, respectively. One group of uncycled samples was tested at the commencement of the investigation and another group after exposure to water for the equivalent time taken by 10(4) cycles (42 days). The KIC decreased for all materials, but the decrease after 10(4) cycles was not significantly different from that after storage in water for an equivalent period of time. These results indicate that, in the determination of KIC, the filler/matrix bond is of secondary importance to the presence of fillers. The effects of cyclic temperature changes likely to be encountered in the mouth are not a significant factor in reduction of the fracture strength of composite filling materials.

Effect of surface conditioning on the abrasion rate of dental composites

Seven dental composites were conditioned prior to wear testing in a three body abrasion machine. The physical conditioning comprised thermal cycling or storage in water for equivalent times. Chemical conditioning was with 75 per cent ethanol or n-heptane. The results indicated that specimens conditioned by 50,000 thermal cycles had an increased surface abrasion rate. With chemical conditioning, 75 per cent ethanol increased the abrasion rate in a number of materials. Both of the latter treatments are clinically unrealistic. As the specimens in other groups were not statistically separable, the results do not support the concept that surface degradation in the mouth is a major variable for three body abrasion.

The silver sorption layer in dental composites: three year results

OBJECTIVES The purpose of this investigation was to observe the behaviour of a recently discovered silver sorption layer in seven dental composites for three years. METHODS Rectangular block testpieces (3.0 x 2.5 x 2.5 mm) of seven resin composites were fabricated and the resin rich layer removed from one surface by grinding on silica carbide paper. The testpieces were immersed in aqueous AgNO3 (3 mol/l). After 26, 42, 90, 180, 360, 540, 720 and 1085 days, respectively, nine specimens of each material were removed to measure the depth of silver stain in the different composites. RESULTS The depth of silver stain continued to increase at a rate proportional to (time)0.5. After three years, five homogeneous subsets [HS] were distinguished for the resin rich surface [HS1] Occlusin (stain depth = 45.6 microns); [HS2] Clearfil (117.8 microns), [HS3] Heliomolar (145.6 microns), Concise (148.8 microns), P-30 (168.9 microns); [HS4] Silux (243.3 microns); [HS5] Profile-TLC (446.7 microns). For the ground surface, the materials were in similar subsets but the depth of stain was less. Different coloured layers were seen within the sorption layer in some materials. SIGNIFICANCE The linear relationship between the depth of stain and (time)0.5 indicate that the mechanism controlling the sorption is Case 1 (Fickian) diffusion. The different depths in the individual materials may indicate differences in the segmental mobility of the polymer chains and free space within the resin phase of the composites. If the silver sorption layer marks the extent of water penetration, then the results show different depth distributions for individual composites.

Subsurface compression fatigue in seven dental composites

OBJECTIVES The purpose of this study was to evaluate subsurface fatigue in seven dental composites. METHODS Cylindrical test pieces were subjected to 2000 compression cycles with a load of 120 N. The area of stained subsurface was measured, and subsequently, the specimens were examined by scanning electron microscopy. RESULTS The greatest staining occurred in the composites with large quartz fillers, whereas there was little staining in the microfilled materials. Electron microscopy demonstrated very little evidence of cracks, suggesting that silver nitrate stained a network of crazes and microcracks in the subsurface. The pattern of staining indicated that the microcracks formed within the depth of the subsurface rather than by direct nucleation from the indenter. SIGNIFICANCE The different patterns of subsurface damage occurring as a result of compression should be considered when studying the wear and degradation of these materials.

The colors of silver with silver nitrate staining in dental materials.

Silver nitrate has been used as a stain to study the permeability, degradation, and marginal integrity of dental composite restorations. The reagent results in various colors in the composite or marginal interface. So that the cause of these colors could be investigated, this study compared the appearances of the stained layers by light and electron microscopy. The results indicated that the colors could be explained by the distribution of silver particles ranging from minute colloidal particles (20 nm) in the composite or dentin, to clusters of large particles (0.1-1 micron) in marginal interfaces. The colors can be explained by the absorption and scattering of light by these particles. At elevated temperatures, fused concretions of silver formed in the composite subsurface, resulting in direct reflection of incident light. The colors which appear with silver nitrate staining may indicate the size and distribution of microporosity in the material.