State of the Art Contemporary Prefabricated Fiber-Reinforced Posts

6.1. Matrices used in Fiber-reinforced Posts

The functions of the matrix in fiber-reinforced posts are to hold the fibers together in the post, as well as interact with functional monomers contained in the adhesive cements for successful bonding of post to root dentine and to composite core materials [31]. Furthermore, the matrix transfers stresses between fibers and protects fibers from the outside environment such as chemicals, moisture, and mechanical shocks [42]. Thus, the matrix may influence the compressive strength of the post, as well as interlaminar shear properties between the matrix and the fiber [42].

During the manufacturing of fiber-reinforced posts, glass or quartz fibers are pre-stretched and treated with silane coupling agent before they are impregnated in the resin matrix [43]. The resin-impregnated fibers are then heat cured to form blocks of different shapes and diameters. Finally, the blocks are shaped into posts with different geometries and diameters through a milling process [44]. As a result of the milling process, some of the fibers are exposed onto the surface of the prefabricated posts (Fig. 4).

Two major types of matrices are used in prefabricated fiber-reinforced posts. The first type consists of a highly cross-linked polymer matrix polymerized by the manufacturers, while the second type consists of unpolymerized, the so-called interpenetrating polymer matrix where the dentist can polymerize it during the fabrication of the post-core restoration [11].

The most common types of matrices used in the polymerized cross-linked fiber-reinforced posts are epoxy-based or diamethacrylate-based cross-linked matrix. Less commonly, some manufacturers use polymethylmethacrylate-based resin matrix for their posts. Additionally, some investigators have suggested the use of aromatic polyimides as a matrix for fiber-reinforced posts [45].

Epoxy resin are thermosetting polymers, also known as polyepoxide, that are formed by the reaction of the base epoxide with the reactor polyamine [46]. On the other hand, the aromatic monomer Bis-GMA (bisphenol A glycidyl methacrylate) used widely as a matrix in dental composite resin materials have also been used as a matrix in fiber-reinforced posts. Bis-GMA matrix is known to be stiffer than the epoxy matrix [47]. As a result, flexural strength tests have shown that Bis-GMA-based matrix to experience greater stresses than epoxy-based matrix in fiber-reinforced posts [47]. Only a few manufacturers use polymethylmethacrylate of high molecular weight (> 220 KDa) as a matrix for fiber-reinforced posts [48].

The wide use of carbon-reinforced polyimide composite materials in aerospace and automobile industries has triggered researchers to investigate the possible use of these materials in fiber-reinforced posts [49]. Fiber-reinforced aromatic polyimides have been shown to have high strength and stiffness, lower density, high fatigue endurance, low thermal coefficient, and the ability to withstand extreme temperature changes [49, 50].

Gao and colleagues demonstrated that polyimide-based resin reinforced by high strength carbon fibers have good mechanical and biological properties and suggested its use in clinical situations [45]. Recently, Yang and Xu [51] found that blending polyimide and epoxy polymers have favorable mechanical properties and suggested its use as a matrix for fiber-reinforced posts. However, no fiber-reinforced post with polyimide matrix has been marketed.

6.2. Fibers used in Fiber-reinforced Posts

As it has been already alluded to, original fibers used in fiber-reinforced posts were made of carbon [22, 28]. As these carbon fibers did not fulfill aesthetic requirements under all-ceramic restorations, manufacturers developed more aesthetic fibers made of silica. Silica-based fibers can be either glass or quartz [30, 31]. The incorporated glass or quartz fibers imparted similar biomechanical properties, as carbon-fiber-reinforced posts, including elasticity, high tensile strength, low electrical conductivity, resistance to solubility and biochemical degradation [31,39,52]. On the other hand, some manufacturers used zirconia enriched glass fibers [15].

Due to their favorable mechanical properties and transparent appearance, glass fibers had become the most commonly used fibers in fiber-reinforced posts [46]. Based on their chemical composition there are several types of glass fibers available including A-glass (Alkali glass), C-glass (chemically resistant glass), D-glass (dielectric glass), R-glass (resistant glass), S-glass (high strength glass), and E-glass (electric glass) among others [42]. However, the most common types used in fiber-reinforced posts are the S-glass and E-glass fibers [46]. S-glass is known to have higher tensile strength and is rather expensive to produce, whereas E-glass has good tensile and compressive strength, as well as electrical properties and lower production cost [42]. However, E-glass has lower tensile modulus and lower fatigue resistance resulting in relatively poor impact resistance as compared to S-glass [42]. On the other hand, quartz fibers made out of pure silica in crystallized form, which is an inert material with a low coefficient of thermal expansion, has been used in several commercial fiber-reinforced posts [29] as seen in Table 1.

In addition to the types and properties of individual fibers, other fiber-related factors can affect the mechanical properties that may affect the clinical success of fiber-reinforced posts [42]. These include fiber orientation, fiber density, impregnation of fibers with the matrix polymer, and adequate adhesion of fibers to the matrix polymer [53, 42].

In fiber-reinforced posts, continuous unidirectional fibers are used [53]. Fibers direction influences the mechanical properties of fiber-reinforced posts [54]. The fibers are continuous and oriented parallel to the post longitudinal axis Fig. (5) with different diameters ranging from 6.0 to 21.0 microns [42, 48]. Fibers density (i.e. the number of fibers per mm² of the post-cross-sectional surface) usually provided by the manufacturers’ and is expressed by weight or volume and varies from one brand to the other. Increased fiber density improves the strength and load-bearing capacity of fiber-reinforced posts [53]. In a transverse section of the post 30-50% of the area is occupied by fibers [30, 55] with fibers of smaller diameter allowing higher packing density of up to 70%.

The fibers should be well impregnated, meaning that resin should come in contact with the surface of every fiber, in order to achieve adequate adhesion of the fibers to the polymer matrix [53]. With good impregnation, optimal reinforcement and transfer of stresses from the polymer matrix to the reinforcing fibers are achieved [42]. During the fabrication of posts, fibers are pre-stressed, and resin injected under pressure to fill the spaces between the fibers, giving them solid cohesion [56]. The smaller the diameter of the fiber filaments the better the matrix ability to spread between the fibers leading to an increase in interlaminar tightness [48]. In addition, fibers are pre-coated with silane in order to improve the adhesion at the fiber-resin matrix interface [42, 56]. This also protects the fibers from damage during handling, modifying the catalytic and wettability properties of fiber surfaces so that their chemical resistance increased [57].

A durable adhesion between fibers and matrix of posts ensures that the load is transferred to the stronger fibers, thus optimizing the function of fibers as the reinforcing component of fiber-reinforced posts. On the other hand, if adhesion is not so durable and if any voids appear between the fiber and the matrix, these voids may act as initial fracture sites that encourage the breakdown of the material [42]. Differences in the coefficient of thermal expansion between fibers and resin matrix may affect the structural integrity of the post following thermal fluctuations in the mouth. There are large variations in the coefficient of thermal expansion between the polymer matrix (40-80 X10^-6/°C) and that of E-glass (8 X10^-6/°C), quartz (0.2 X10^-6/°C), and carbon (0.4 X10^-6/°C) fibers [29]. It has been shown that thermocycling decreased the flexural modulus of different fiber posts by approximately 10% [29]. This suggests that the mismatch in the coefficient of thermal expansion between fibers and matrix polymers might affect the long-term integrity of fiber posts.

Studies reported that post debonding is the most common type of failure seen in teeth restored with prefabricated fiber-reinforced posts [58]. In vitro studies have shown that the bond strength of luted fiber-reinforced posts was significantly lower in the apical third of canals followed by the middle third as compared to a coronal third of root canals [28, 59]. In fact, various fiber-reinforced posts have been shown to reduce the transmission of polymerization light intensity differently as light travels from the coronal toward the apical third of root canals [15].

The difference in light transmission between different fiber-reinforced posts can be attributed to differences in type and number of fibers, as well as the diameter and orientation of fibers [15]. In addition, the refractive index of the matrix used which can be influenced by factors such as the type of monomer, pigments, and fillers used can also affect absorption and scattering of light transferred to various depths of posts in the root canals [60]. Recently a novel fiber-reinforced post (iLumi fiber optic post; iLumi Sciences), was introduced into the market with the premise that it is able to transmit light to the most apical parts of the canal, thus improving the retention of the post and reducing the debonding rate [61]. In the iLumi fiber optic post, each fiber is thermally coated with optical glass cladding made of a non-corrosive, biocompatible material, the exact nature of which is kept confidential by the manufacturer (Fig. 6). This monolayer around each fiber is believed to force the light to be internally reflected and transmits to the apical end of the post. Studies have shown that this results in complete polymerization of resin cement as compared to other types of translucent fiber-reinforced posts [61]. While manufacturers claim that this resulted in increased bond strength of the apical third of luted iLumi fiber optic post, this has not been independently demonstrated.

It is highly desirable that posts cemented in root canals be visible on radiographs for evaluation and follow up. Therefore, prefabricated posts should have radiopacity similar to or close to that of root dentine. Studies have shown that different types of prefabricated fiber-reinforced posts exhibited different radiopacity levels. The composition of the post appears to be the most significant factor affecting the radiopacity of prefabricated fiber-reinforced posts [17]. Elements that appear to affect the radiopacity of the prefabricated fiber-reinforced post include silicon (Si), which may present in the form of silica, as well as zirconia (Zr), barium (Ba), and aluminum (Al) [17]. Differences in percentages of the various radiopaque elements, their atomic number, and crystallization forms seem to affect the radiopacity from one type of fiber-reinforced to the other [17]. These elements can be present in the fibers, matrix and/or added as fillers in some commercially available prefabricated fiber-reinforced posts.