A successful clinical outcome of endodontically treated teeth depends on adequate root canal instrumentation (1, 2) and three-dimensional obtuartion (3, 4) as well as on adequate restorative treatment performed afterwards (5). The loss of more than a half of the coronal tooth structure, caused by caries, fracture or extensive cavity preparation, mandates the use of posts (6) as they provide restorations with enhanced retention and stability (7). Traditionally used metal posts do not meet the requirements of modern dental medicine, due to disadvantages such as color, corrosion potential, non-adhesive bonding and high modulus of elasticity which can lead to root fracture. In order to enhance the esthetic aspects, physical properties and biocompatibility, a wide range of esthetic posts have been developed and become commercially available. In addition to esthetic and health benefits, they hold the capacity of adhesive bonding to tooth tissue and core buildup and that potentiates the creating of the monoblock, a gap-free single unit in which the loading stresses are evenly distributed and borne by all the monoblock components (8).
CERAMIC ZIRCONIA POSTS
All-ceramic restorations have gained popularity due to their excellent esthetic properties and biocompatible nature (9), consequently increasing the use of white and translucent posts made of zirconia and other ceramic materials for the same reasons. Partially stabilized zirconium dioxide (Zr20)?? mislim da je to (LŠ) (Zr02), a ceramic material, formed by adding yttrium oxide (Y2O3), was introduced by the end of 1980's (10, 11) and recommended for the fabrication of posts because of high fracture and bending strength by which the posts can withstand functional loads.
Apart from its favorable chemical stability, good esthetic and physical properties (12), zirconia also yields excellent radiographic opacity, superior light transmission properties (13) and Young's modulus similar to that of the stainless steel alloy (14). However, the high elastic modulus of zirconia posts at 200 Gpa causes stress to be transferred to the less rigid dentin, thereby resulting in root fractures (15). Furthermore, zirconia posts are stiff, but at the same time very brittle, without any ductility (9). Therefore, it is necessary to make a deep post preparation which, on the other hand, prevents the use of a minimally invasive approach when it comes to removing the dentin tissue. Furthermore, if a failure occurs and there is a need for endodontic retreatment, the reported strength becomes a significant disadvantage because it is nearly impossible to remove a zirconia post from the root canal (16).
Few techniques for zirconia post and core reconstruction are available: direct composite build-ups, adhesively luted ceramic core, and ceramic core heat-pressing and one-piece ceramic post-and-core restorations. Although in vitro zirconia posts exhibit lower fracture strength when used with composite cores than with ceramic cores (14), clinical long-term success appears to be excellent for a combination with composite core system (17). Within the group with ceramic cores, zirconia posts achieve higher fracture strength when used with bonded prefabricated ceramic core than with bonded custom-made ceramic core (18). This lower value of heat-pressed custom-made ceramic core over zirconia post is a result of changes in the inner structure of the zirconia material during the heating process. Conversely, the retentive strength of zirconia post with direct heat-pressed ceramic core is significantly higher than with adhesively bonded ceramic core (19).
Zirconia posts are indicated in grossly destructed tooth, areas with heavy forces and in high lip line and thin gingival tissue for achieving better esthetics (20). To improve the bond strength of zirconia post to core and root dentin, it is recommended to use resin cements (21) and do surface pretreatment of which the most effective is airborne particle abrasion using silicated Al2O3 particles in a combination with silanization (22).
FIBER REINFORCED POSTS
In 1990, a new nonmetallic material for the fabrication of posts based on the carbon fiber reinforcement principle was introduced. (23). Although these post are easy to manipulate and have good mechanical properties, low toxicity and low modulus of elasticity, more similar to that of dentin (24), their color and difficulty of concealment under composite or all-ceramic restoration prevents them to be classified as esthetic posts. In order to meet the cosmetic requirements, posts reinforced with glass and quartz fibers were manufactured. Apart from good esthetics, their advantageous properties are also high impact resistance, attenuation and softening of vibration, shock absorption and increased fatigue resistance (25).
PREFABRICATED GLASS- AND QUARTZ-FIBER POSTS
Translucent and tooth colored silica-based posts were introduced in 1992. They are made of high volume percentage prestretched silanizated glass- or quartz- fibers bounded by methacrylate- or epoxy-polymer matrix with high degree of conversion and highly cross-linked structure that binds the fibers (26). The fibers offer strength and stiffness, while the polymer matrix transfers loads to the fibers and also protects them from the moisture of the oral environment (27).
BisGMA and epoxy resins are commonly used as a resin based material for dental fiber posts which are therefore compatible with adhesive restoration techniques. This allows chemical and micromechanical bonding of fiber post to the root dentin that leads to a uniform stress distribution (28). Due to this fact and the similarity in elastic modulus with dentine (18-42 GPa) (29), biomechanical performance is better and fracture resistance increases. Additionally, stress distribution and fracture resistance of a silica-based post are not significantly influenced by post length and diameter, hence the restoration technique is less sensitive to post dimension in this case than when stainless steel posts are included (30).
The silica-based fibers can be made of glass or quartz. Quartz is a crystalline form of silica, whereas glass is monocrystalline. It has been found that that the quartz fiber posts are more radiopaque (31), have a higher flexural strength than the glass fiber posts (32) and that the teeth restored with quartz fiber posts have higher fracture strength than those with glass fiber posts (33).
However, in dentistry, E- and S-glass fibers have become the most commonly used reinforcing fibers. Glass fibers stretch uniformly under stress to their breaking point, and on removal of the tensile load short of breaking point, the fiber returns to its original length. E (electrical application)-glass has good tensile and compressive strength, as well as electrical insulation and rather low cost, but relatively poor fatigue resistance, while S (stiff, strong)-glass has a different chemical composition, giving higher tensile strength and better wet strength retention, but is rather expensive (34).
The most frequent failure of fiber post restoration is post debonding (35) which can happen on the post/cement or cement/dentine interface. The bond between glass fiber post and composite substrates is difficult to achieve by means of free radical polymerization bonding because the organic component of fiber post is a polymer matrix that is highly cross-linked with a high degree of conversion and a small number of carbon-carbon double bonds on the surface (36). Because of this inert and nonreactive polymer matrix, recent research suggest to treat the post with H2O2 (24%) for 1 minute to selectively dissolve the polymer matrix and expose the glass fibers, allowing micromechanical interlocking of adhesive/cement with the post (37). H2O2 is frequently used in dental practice, mainly for dental bleaching- It is easy and safe to use and doesn't affect the integrity of the fiber posts. Furthermore, after the H2O2 treatment, the exposed fibers become available to chemically bond to the adhesive/cement through the silane agent (38). For glass fiber post cementation light-irradiated dual-cure materials provide the most reliable option (39).
Although clinical studies have not showed significant difference for root fracture incidence between metal and glass fiber post (40), the latter are often first clinician’s choice, not only because of their sufficient esthetics, biocompatibility, flexural and fatigue strength, favorable elastic modulus and bond strength of fibers to composite substrates, but also because they are cheap, easy to handle allowing one-visit therapy. Besides, they are easily removed if necessary. However, when restoring teeth with prefabricated glass fiber post, it should be considered that indication applies only to teeth with well conserved radicular structure (20) because they require preparation of the root canal to fit the shape of the post, which causes the loss of dentin and makes the root more vulnerable to root fracture (41). Also, by using the prefabricated FRC posts, the free space of a larger coronal root canal opening remains filled only with cement and due to changes during polymerization shrinkage. This may cause detachment of the luting resin from the dentin, consequently leading to microleakage along the post space and, also, to the failure of the post.
INDIVIDUAL GLASS FIBER POSTS
In the early 2000s, individually formed glass fiber reinforced posts were introduced (29) to eliminate the shortcomings and improve the advantages of the prefabricated glass reinforced posts. They are made of unidirectional, silanated E-glass fibers impregnated with a combination of two non-polymerized polymers, PMMA as a linear phase and poly Bis-GMA as the cross-linked phase that form semi-interpenetrating polymer network (semi-IPN). PMMA chains, with molecular weight of 220 KD, plasticize the cross-linked Bis-GMA based matrix and thus reduce the stress formation in the fiber-matrix interface during deflection (29). Because of the non-polymerized semi-IPN, the monomers of adhesive resins and cements can diffuse into the linear polymer phase and by polymerization, form interdiffusion bonding and so-called secondary semi-IPN structure (42) that contributes to the better load transfer from the core to the root. Resin substrates suitable for dissolving the IPN matrix, i.e. linear polymer, are the ones containing monomers such as Bis-GMA, TEGDMA or HEMA (36) and belong to those that are most commonly used in restorative dentistry.
Since they are not polymerized, the IPN posts can be easily adapted to the shape of the root canal, thereby possibly reducing the number of voids and potentially reducing the number of post decementations (43). The concept of using individually formed FRC posts is based on minimizing the preparation need to the deeper parts of the root canal thus allowing addition of higher quantity of FRC material to the coronal root canal opening of the tooth. In this way, the concept saves the dentin, minimizes stress at the apical parts of the post and enables stiff and fracture resistant post with larger diameter to the core that forms strong support for the core (41). Additionally, the coronal part of the IPN-post can be bent to the desired angulation and adapted to meet the requirements of the crown restoration. Individually glass fiber posts can also be used in curved and oval root canals as well as in very large canals, where several posts of different lengths and diameters can be placed in the same canal by means of lateral condensation.
Although individually glass fiber posts, in a comparison with prefabricated glass fiber posts, show higher flexural strength (29), higher fracture resistance (44), higher bonding strength without adhesive (post-cement interface) failure (43, 45), they are more complicated to use for an inexperienced clinician due to a highly sticky nature of a non-polymerized matrix and fibers tendency to separation.
POLYETHYLENE FIBER POSTS
Several years before the IPN-post was introduced, a fiber composite laminate endodontic post and core system based on a woven polyester bondable ribbon had been (46). This reinforcement material, that has been commercially available since 1992, is composed of plasma treated ultra-high molecular weight polyethylene fibers woven into three dimensional structure, leno wave or triaxial braid. Due to special patterns of cross-linked threads, a higher mechanical interlocking is provided. Also, the fiber’s superficial tension is reduced due to cold gas plasma pretreatment in order to ensure good chemical bond to resin materials (47).
According to the manufacturer, apart from having excellent properties in translucency, polyethylene fibers exceed the breaking point of fiberglass and are so tough that special scissors are required to cut them. In addition, unlike loosely braided or bundles of unidirectional fibers, these fibers do not spread or fall apart when manipulated because the dense network of locked-stitched threads prevents the fibers from shifting during manipulation and adaptation before polymerization (48). Because of these characteristics, polyethylene fibers can be bent to sharp angles and woven to make tight mechanical inter-locking from one thread to another. Also, these ultra-high molecular weight polyethylene fibers have a high elasticity coefficient and a high resistance to stretch, distortion and traction (49) that allows them to adapt closely to the contours of the root canal and to properly condense, increasing the content of individually made endodontic post thus decreasing the luting-agent thickness and consequently its polymerization shrinkage. Since the fibers adapt to the root canal, the canal enlargement is not required, hence the natural strength of the tooth is preserved and a possibility of the root perforation is eliminated. In a comparison with other fiber reinforced posts (including the individually glass fiber reinforced post), an individually shaped polyethylene post cemented with dual-cure resin cement shows the least amount of microleakage in overflared root canals (50).
Unlike the continuous unidirectional fibers that give the highest strength and stiffness for the composite only in the direction of the fiber orientation, polyethylene fibers reinforce the polymer in all direction so that the mechanical properties are isotropic (51). Woven fiber architecture enhances fracture resistance which is also evident in the case of cracks induced by load. The crack stops at the node of the leno-lock-stitch weave, thereby preventing the crack propagation from the restoration to the tooth and helping maintain the integrity of the fiber reinforcement (52).
It has been shown that polyethylene-reinforced resin provides sufficient retention required for clinical success of a post and core system (53) and adequate fracture resistance with increased incidence of repairable fractures in structurally compromised canals (54). However, high price of polyethylene fibers limits their use in daily practice despite the abovementioned excellent characteristics.
The primary function of endodontic post is to provide retention for the core and enable full sealing of coronal portion of the root canal. Therefore, it should bond firmly to the root dentin and buildup core. Additionally, an endodontic post should have sufficient fracture strength to withstand the loads, but elastic modulus similar to that of dentin to enable more uniform stress distribution and prevent root fractures.
Since much attention is given to esthetic aspects, more esthetically pleasing posts with composite/ceramic cores have become very common in restorative dentistry. In fact, they are becoming a standard because they are esthetically pleasing, biocompatible and have good physical properties.
The successful restoration of an endodontically treated tooth is an ongoing challenge for a restorative dentist. Before inserting a post, factors such as remaining amount of coronal tissue, root canal size and configuration, tooth position, functional requirements and occlusion need to be analyzed.
The clinician should be aware of the differences between the endodontic posts in order to select and use the most appropriate post system in each specific situation.