The oral cavity is a complex and aggressive environment that affects the hard dental tissue and restorative materials. It contains different substances, such as saliva (water, ions, enzymes, proteins), plaque (bacteria, bacterial metabolic products—acids which decrease the pH value to 4.5), food and chemical substances (soft drinks which can decrease the pH value to below 2, acids from fruits, toothpastes, mouthrinses, fluoride, chlorhexidine, bleaching gels containing peroxides, drugs, vitamins), all of which can contribute to ageing of materials.
Also, mechanical stress caused by biting (400 N during chewing), forces arising from bruxism and other oral habits, and forces acting during toothbrushing can contribute to material ageing. In addition, chemical or electrochemical factors and long-term mechanical loads act in the oral cavity synergistically, accelerating material ageing.1
Ageing is the deterioration of a material as a result of a reaction with its environment that becomes more pronounced over time. If the changes in material properties are negative, this can be considered degradation. Ageing of materials is caused by internal (chemical, mechanical and physical) and external (light, temperature, humidity, stress during function, mechanical solicitations, interactions with other materials) factors.
The consequences of material ageing are loss of anatomical shape (abrasion or fracture) and marginal fracture, decrease in aesthetic properties, decrease in mechanical resistance and durability, and release of the material components and their degradation products in the human body. Ageing of biomaterials is an important concern. It often manifests as a decrease in the normal performance of the biomaterial, physical degradation and subsequent release of substances, alteration of properties (aesthetics, hardening or softening, alteration of permeability, etc.) and impairment of functionality.
Resin composites are commonly used in restorative dentistry. However, these composites degrade in the oral cavity.2, 3 The processes leading to the degradation of composites are thermolysis (decomposition by temperature), oxidation (loss of electrons), solvolysis (decomposition by a solvent), photolysis (decomposition by light) and radiolysis (decomposition by ionising radiation). Solvolysis, or more specifically hydrolysis when the solvent is water, is the most investigated and most relevant biochemical degradation process, as it acts on the unprotected ester linkages in methacrylate-based resin monomers, polymers and coupling agents. By definition, hydrolysis is a chemical reaction during which water divides a molecule into two parts.
Resin composites have high failure rates.4 The main reasons for failure are secondary caries and fractures of the restoration.5 Concerns over their reduced durability and the prevalence of secondary caries have come to the fore, as well as concerns with respect to the safety of resin-based materials that might release chemicals like BPA, TEGDMA, bis-GMA and HEMA.6, 7 The main problems with composite resins relate to shrinkage (hermeticity and adhesion problems), degradation in function (abrasion, dissolution and fatigue) and bonding to hard tissue (efficiency and durability problems).
Shrinkage can be decreased by masking the effects of contraction when using the layering technique in applying the composite resin and/or by modifying the behaviour of the composite through its composition, thus decreasing its polymerisation shrinkage. The latter has the advantage of reducing shrinkage stress, thus improving the clinical behaviour of the composite markedly.
One possibility regarding composite composition is to keep the linear dimethacrylates as the basis and change the nature and proportions of the other components of the resin matrix—this is the option employed by products like els (Saremco Dental). By controlling the nature and proportions of the components of the resin matrix very precisely, it is possible to decrease polymerisation shrinkage optimally.
Editorial note:
A list of references can be obtained from the publisher.
At the 2022 UAE International Dental Conference and Arab Dental Exhibition, Dr Maha H. Daou will be holding a lecture titled “Resin composite restorative materials: Properties, characteristics and clinical implications” on 1 February from 2 p.m. to 3 p.m. GST.
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