The use of fiber posts, i.e. fiber-reinforced composite (FRC) root canal posts, has increased considerably since their introduction in the late 1980s. It is estimated that prefabricated solid fiber posts are used in tens of millions of teeth annually. There are two ways to use fiber posts. The conventional way follows the design of metal posts, which are used for providing friction-based retention against vertically directed forces, and the resistance for bending forces is received optimally from the metal ferrule of a crown . Another way to use FRC as a post material is based on the concept of the individually formed post, which utilizes resin-based adhesion and uses a larger amount of FRC close to the dentin walls of tooth-restoration systems .
Recently, there has been critical discussion and published information about risks, which relate to the use of solid prefabricated fiber posts in the way conventional metal posts have been used . For instance, this was critically discussed in the meeting of the European Society of Endodontology (ESE) in Barcelona in September 2015. Indeed, it seems to be the case that there are many unsuccessful fiber post-core-crown treatments, which fail clinically a few years after treatment. Failures are often serious, showing severe secondary caries at the time point of failure observation. Problems relate most often to the use of all ceramic and composite restorations, which cannot utilize the supporting effect of the ferrule. Damage can be due to fiber post material, the post itself, improperly performed fabrication or technique in terms of biomechanical considerations.
The stages of damage of conventionally used prefabricated solid fiber posts are: (1) marginal breakdown, due to the lack of a ferrule and deteriorated or insufficient dentine bonding, (2) interfacial fracture propagation between dentine and core-built-up resin composite, (3) delamination of core-built-up resin composite and resin composite cement from the fiber post and (4) delamination of the fiber post itself by splitting. Thus, damage is associated with the stress caused by occlusion and articulation contact of teeth, and resistance of the restored tooth to withstand dynamic stresses for years. It seems that the adhesive interface of core-built-up resin composite on the flat surface of dentine is not durable enough unless the post system is able to carry the load and consequently diminish the tensile stress at the crown margin. A simple way to diminish tensile stress at the crown margin is to increase stiffness of the post, which – without the ferrule – is known to predispose the restoration to another type of severe failure, namely root fracture. From the material perspective, presently used glass fiber posts mechanically match to the properties of dentine. However they are used typically as small diameter posts, which are located in the neutral axis of stress of a tubular root. This, in combination with poor adhesion of highly cross-linked polymer matrix posts in core-built-up resin composites and resin composite luting cements, does not provide a mechanically stable tooth-restoration system.
Clinically, dentists should make all attempts to minimize damaging tensile stress at the ceramic and composite crown margin. With ceramic and resin composite materials metal ferrules cannot be used. The answer to lowering the magnitude of damaging tensile stress is to be found inside the root, at the canal opening: by placing a larger amount of FRC at the root canal opening and locating the FRC close to the dentine walls, where the highest stresses are located. Additionally important are the use of adhesive systems which provide the highest possible adhesion to dentine and FRC post material, in doing so, damaging tensile stresses can be diminished . Thin prefabricated fiber posts should be used only in very limited clinical cases because they do not provide mechanical support and load-bearing capacity, although the material itself may demonstrate adequate strength in mega-Pascals .