Degradation of composite materials

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The oral cavity is a complex and aggressive environment that affects hard dental tissues and restorative materials. It contains different substances, such as saliva (water, ions, enzymes, proteins), plaque (bacteria, bacterial metabolic products – acids which lower the pH to 4.5), food and chemical substances (carbonated drinks which can lower the pH to 4.5). less than 2, fruit acids, toothpastes, mouthwashes, fluoride, chlorhexidine, bleaching gels containing peroxides, drugs, vitamins), all of which can contribute to the aging of materials.

In addition, mechanical stresses caused by biting (400 N when chewing), forces resulting from bruxism and other oral habits, and forces acting when brushing teeth can contribute to material aging. In addition, chemical or electrochemical factors and long-term mechanical loads act synergistically in the oral cavity, accelerating the aging of materials.1

Aging is the deterioration of a material as a result of a reaction with its environment which increases over time. If the changes in the properties of the material are negative, it can be considered degradation. The aging of materials is caused by internal (chemical, mechanical and physical) and external factors (light, temperature, humidity, stresses during operation, mechanical stresses, interactions with other materials).

The consequences of material aging are loss of anatomical shape (abrasion or fracture) and marginal fracture, decrease in aesthetic properties, decrease in mechanical strength and durability, and release of material components and their breakdown products in the human body. The aging of biomaterials is an important concern. It is often manifested by 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 alteration of functionality.

Composite resins 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 ionizing radiation). Solvolysis, or more specifically hydrolysis when the solvent is water, is the most studied and relevant biochemical degradation process, as it acts on unprotected ester bonds in methacrylate-based resin monomers, polymers and coupling agents. By definition, hydrolysis is a chemical reaction in which water splits a molecule into two parts.

Resin composites have high failure rates.4 The main causes of failure are secondary caries and fractures of the restoration.5 Concerns about their reduced durability and the prevalence of secondary caries have arisen, along with concerns about the safety of resin-based materials that could release chemicals like BPA, TEGDMA, bis-GMA, and HEMA.6, 7 The main problems with composite resins relate to shrinkage (sealing and adhesion problems), degradation of function (abrasion, dissolution and fatigue) and bonding to hard tissues (efficiency and durability problems).

Shrinkage can be decreased by masking the effects of contraction when using the lamination technique when applying the composite resin and/or by modifying the behavior of the composite by its composition, thus decreasing its polymerization shrinkage . The latter has the advantage of reducing the shrinkage stress, thus clearly improving the clinical behavior of the composite.

One possibility regarding the composite composition is to keep linear dimethacrylates as the base and to change the nature and proportions of the other components of the resin matrix – this is the option used by products like els (Saremco Dental). By very precisely controlling the nature and proportions of the components of the resin matrix, it is possible to optimally reduce polymerization shrinkage.

Editorial note:

A list of references can be obtained from the publisher.

During the 2022 UAE International Dental Conference and Arab Dental Expo, Dr. Maha H. Daou will hold a lecture titled “Resin Composite Restorative Materials: Properties, Characteristics and Clinical Implications” on February 1 from 2-3 PM GST .


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