06 March 2019

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Recent findings for better understanding fatigue and wear resistance of dental composites: What’s new on fatigue and wear resistance of dental composites?

Lorenzo Breschi

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Teeth are subjected to cyclic stresses that repeat more than 300.000 times per year for the average person. When dental composites restorations are performed, they are constantly subjected to a hostile ambient where forces coming from mastication, bruxism or clenching and the presence of saliva and acidic attacks may undermine the longevity of the reconstructions.
In this contest, it is fundamental to have a deep knowledge of how and why the restorative materials react to cyclic stress and fatigue, in order to make the correct clinical choice for the patient.
Fatigue and wear processes are closely related with subsurface cracks propagation and are considered driving key in the wear mechanisms of composites. Furthermore, mechanical cyclic loading can intensify roughness of the restorations, increasing the ability of bacteria to colonize the surface of the material and potentially create a favorable environment for secondary caries, which is the most common reason for the replacement of composite restorations. Moreover, water hydration is well known to influence the mechanical properties of dental composites by affecting the matrix, filler, or matrix-filler interfaces. 

Some potential effects of water that may accelerate fatigue cracks growth include:

• Plasticization and softening of the resin matrix
• Hydrolytic degradation of the silane coupling agent promoting interfacial debonding

A recent article published on the Journal of the Mechanical Behavior of Biomedical Materials by Kruzic et al. underlined the importance of considering the microstructure of the composites when analyzing the wear behavior of dental restorative materials. 

Increased resistance to wear of resin-based polymers is generally related to:

• Volume of the filler particles;
• Diameter of the filler particles;
• Degree and the depth of polymer matrix conversion rate;
• Quality of the bonding interface. 

The antagonist also plays an important role in how a composite restoration behaves under fatigue. Indeed, significantly higher wear was demonstrated to occur when the antagonist contains hard large fillers (e.g.,~3.5 m zirconia) compared to the resin-based reconstruction performed with a nano-sized and/or softer fillers material. Furthermore, also immediate tooth-brushing after exposure to an alkaline solution proved to be responsible of the increase in wear in a range between 60 – 1400% (Sarkar, 2000).
Recent findings in the field are in accordance to suggest that the oral environment of the patient substantially influences the amount of the material loss and the surface quality and indicates that hybrid and micro-hybrid composites suffered greater surface damage (increase in roughness/wear), while nano-composites have better resistance to the chemical and mechanical assault.