Focus Group -
dental composite wear behaviour through in-vitro analysis -
For more than a century, dental amalgam (an alloy of up to 50% mercury mixed with silver, tin and copper) has been the choice restorative material for posterior teeth. However, its usage is declining due to the fear of mercury toxicity, the potential hazardous environmental effects, and increased aesthetic demands by patients. Dental composites are gaining popularity as a "directly placed" alternative for amalgam restorations. An optimal formulation for any dental composite material must possess two key elements of marginal adaptation and wear behaviour. The wear process in the mouth can be categorized into Occlusal Contact Area (OCA) and Contact Free Area (CFA) wear. OCA wear is a result of sliding wear caused by direct tooth contact during involuntary grinding of teeth and indirect tooth contact during eating. CFA wear is caused by the suspension of food and water during eating and tooth brushing. OCA wear of composites may be three to five times greater than CFA wear.
The wear of commercial dental composites was studied by a team comprising staff from the Department of Restorative Dentistry and Centre for Biomedical Materials Applications and Technology (BIOMAT), Faculty of Engineering. By employing an integrated biological and engineering approach, the scientists have designed a CFA wear apparatus and an OCA wear instrument (Figure 1) that controls contact stress, wear environment and the number of contact cycles. The researchers defined and evaluated the different variables that could influence wear, and looked at their effects on several commercial composite restoratives, using a dental amalgam for comparison. They found that increased contact stress and cyclic loading resulted in greater OCA wear. At low contact stresses the composites generally showed abrasive wear and filler dislodgement due to preferential loss of the resin matrix (Figure 2). At higher contact stresses, possible cohesive failure (i.e., failure within the body of the composite) of the resin matrix occurred subsequent to micro-crack formation as fillers transmitted forces to the surrounding matrix. Conditioning and wear testing in water demonstrated the greatest wear. For all materials, conditioning and wear testing in heptane (which simulates butter, fatty meats and vegetable oils) resulted in the least wear. The amalgam alloy and one mini-filled composite (average particle size from 0.1 to 1.0 micron) exhibited fatigue wear mechanisms (Figure 3) with extended wear testing. Although fatigue wear did not occur with the micro-filled composite (average particle size from 0.01 to 0.1 micron), extended wear testing resulted in deep and wide micro-cracks (Figure 4) that may precipitate catastrophic failure. The investigators did not observe a significant relationship between the change in composite hardness and OCA wear. Results of OCA wear were markedly different from those arising from CFA wear testing.
The laboratory findings from this study on composite wear gave an insight into the high data variance observed in many previous clinical wear studies. This team of researchers believed that the standardization of variables for in-vivo wear assessment will enable better data discrimination and avoid misinterpretation.
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