Monoblocks in restorative dentistry

Endodontically treated teeth are susceptible to fracture because of extensive restorations and reduced amounts of remaining tooth structure . The strength of an endodontically treated tooth is directly proportional to the amount of remaining sound tooth structure. As tooth structure is lost, the potential for tooth fracture increases . Although the manufacturer of Hydron did not infer that filling root canals with Hydron helps to strengthen roots and prevent root fractures, it is of interest that the moduli of elasticity of porous poly(HEMA) hydrogels such as Hydron ranges from 180–250 MPa . In order to reinforce roots, the modulus of elasticity of a root filling material would need to approximate that of dentin (i.e. 14,000 MPa) . Thus, with the advantages of hindsight , we know that one of the first monoblocks employed in root canals (Hydron) was not stiff enough to strengthen roots even if it could have bonded to root canal surfaces.

Orthograde obturation with mineral trioxide aggregate (MTA; ProRoot MTA, Dentsply Tulsa Dental, Tulsa, OK) as an apexification material represents a contemporary version of the primary monoblock in attempts to strengthen immature tooth roots . MTA is composed principally of Portland cement with the addition of bismuth trioxide to render it radiopaque (. Being an entirely inorganic material, Portland cement undergoes chemical shrinkage following hydration. This volume reduction is associated with the interaction between the cement and water, and is estimated to be 0.001 mL/g of cement (i.e. 0.1%). Thus, a certain amount of volumetric shrinkage should also occur during the setting of MTA. As MTA is not bonded to dentin (Tay and Pashley, unpublished results), this shrinkage does not result in the generation of shrinkage stresses along the cavity walls . It is prudent to point out that the so called “high bond strength” (ca. 38–40 MPa) reported on the MTA in root sections were produced using a push-out test design . This mechanical testing protocol can generate high interfacial strength values that represents the frictional resistance of a material to the cylindrical cavity walls  Although MTA does not bond to dentin, interaction of the released calcium and hydroxyl ions of MTA with a phosphate-containing synthetic body fluid results in the formation of apatite-like interfacial deposits. These deposits fill up any gaps induced during the material shrinkage phase and improves the frictional resistance of MTA to the root canal walls. The formation of these non-bonding, gap-filling apatite deposits probably also accounts for the seal of MTA in orthograde obturations and perforation repairAlthough the elastic modulus of MTA is not available, studies on Portland cements indicated that the compressive elastic modulus of the latter is approximately 1.7 GPa (i.e. 1,700 MPa) during the early setting stage. The compressive elastic moduli of Portland cement increase after 14 days to 15 GPa (i.e. 15,000 MPa) with a water/cement ratio of 0.6, and around 30 GPa (i.e. 30,000 MPa) with a water cement ratio of 0.33, with minor further increases on further aging . Unlike Hydron, MTA should theoretically be able to strengthen roots (elastic modulus 14,000 – 18,600 MPa, according to location and orientation of the dentinal tubules). Nevertheless, in a recent study that examined the fracture resistance of MTA applied to immature sheep roots, it was found that there was no difference in roots that were filled with saline, versus those that were filled with MTA . Although the sample size was small, this study illustrates that MTA does not confer any perceivable benefit in root strengthening, apart from its ability to stimulate cementogenesis in apexification and root end fillings . The inability of MTA to strengthen roots is probably a combination of its lack of bonding to dentin, and that although it has high stiffness in compression, it has little strength in tension (Tay and Pashley, unpublished results).

It can be seen from the discussion on primary monoblocks that two prerequisites are simultaneously required for a monoblock to function successfully as a mechanically homogenous unit. First, the materials that constitute a monoblock should have the ability to bond strongly and mutually to one another, as well as to the substrate with which the monoblock is intended to reinforce. Second, these materials should have moduli of elasticity that are similar to the substrate. The interaction of these two parameters is nicely illustrated in a recent finite element analysis study of different cements in combination with posts used to restore weakened roots . With the increase of the moduli of elasticity of the different cements , the Von Mises stress concentrations in the root dentin decreased from 24.5 MPa to 20.8 MPa. 

The results excerpted from the above study were derived from the use of a titanium post  that has a modulus of elasticity of 120 GPa (. It would be desirable for the study to be repeated using fiber posts that have moduli of elasticity similar to that of root dentin . Nevertheless, important conclusions could be derived with respect to the interaction between dentin adhesion and elastic moduli of the materials employed. For example, although Superbond C&B  has been recommended by Bouillaguet et al.  for bonding to post spaces due to its adhesive property and its low setting characteristics that permit shrinkage stress relief via resin flow, it would not be the ideal resin cement to restore weakened roots. The deformation of the resin cement was greater than that of the root dentin when occlusion force was passed to the root. As a result, the majority of the stresses were borne by the root. Although zinc phosphate cement demonstrated push out strengths comparable with other resin cements for the cementation of titanium  or fiber posts , (its elastic modulus was close to that of dentin and its stress concentrations in dentin were low), posts cemented with zinc phosphate cements often failed because of the cement’s relatively great elastic modulus, fragility and low bonding potential to the root dentin and the post surfaces  This explains why roots reinforced with posts that were cemented with dentin adhesives are more fracture resistant than those cemented with zinc phosphate cements . Likewise, the lower modulus of elasticity of glass ionomer cements  also explains why they are less efficient than dentin adhesives and composites in strengthening immature roots and roots bonded with quartz-coated carbon fiber posts .

With an understanding of these principles, it is appropriate to examine the ability of some classic secondary monoblocks described in the restorative and endodontic literature to function as mechanically homogeneous units. The first implied existence of a mechanically homogeneous monoblock in the root canal space was reported in 1996 with the bonding of epoxy resin-based, carbon fiber-reinforced posts (i.e. carbon fiber posts) to root dentin . The authors claimed, based on their clinical experience, that carbon-fiber posts, having a modulus of elasticity very similar to the modulus of elasticity of dentin, can achieve a tooth-post-core monoblock instead of an assembly of heterogeneous materials. This should help to distribute masticatory loads homogeneously and reduce stresses during function. Although such a concept is exceedingly appealing in terms of product marketing and advertising, it is too advanced to be realized by the materials available at that time (in the late eighties and early nineties). Although the strongest (ultrahigh modulus) carbon fibers have a tensile modulus (500–1000 GPa) that are 2.5–5 times as strong as that of steel (200 GPa) , and that carbon fibers can be made to bond to epoxy resins via a carefully controlled oxidative process in nitric acid, ozone or electrochemical oxidation ( they are no longer surface active once a fiber post is exposed by roughening or with a bur. Furthermore, the stiffness of carbon fiber posts is lowered by the presence of epoxy resin. Epoxy resin is not usually bondable to methacrylates, as ring opening of epoxide groups and chemical grafting of methacrylic groups cannot be achieved under physiological temperatures . The beneficial claims of the carbon fiber post-root dentin monoblock could not be validated in independent in vitro and retrospective in vivo studies   Over the years, the carbon fibers in this type of first generation fiber posts have been replaced by quartz-coated carbon fibers  and glass fibers that are amendable to silane coupling . The epoxy resin embedding matrix in older generations of fiber posts is also replaced with highly cross-linked, oxygen inhibition layer-free methacrylate resin matrices that, theoretically, have the potential to bond to methacrylate-based resin cements . Different modalities of surface treatments of posts are also available to render these newer generations of fiber posts more conducive to bonding to methacrylate-based resins. Although the use of these newer generations of fiber posts has not yet attained the scientific rigor of an ideal monoblock, they are reported to have performed well in vivo . This is probably due to the similarity in the moduli of elasticity between fiber posts and root dentin.

By definition, root canal obturations, being indirect fillings of the root canal space created by cleaning and shaping, may be regarded as secondary monoblock systems. However, as conventional root canal sealers do not bond strongly to dentin and gutta-percha , they do not behave as mechanically homogenous units with the root dentin. Although glass ionomer cements and resin-modified glass ionomer cements bond to root dentin and have been marketed as root canal sealers , they do not bond to gutta-percha. Even if they do, the modulus elasticity of gutta-percha points (ca. 80 MPa)  is 175–230 times lower than that of dentin (ca. 14,000–18,600 MPa) , making them too plastic (i.e. not stiff enough) to reinforce roots after endodontic therapy. Thus, it is dubious that a glass ionomer-based sealer can be used to prevent root fracture in gutta-percha filled root canals .

Interest in utilizing the classical monoblock concept for sealing and reinforcing the root canal space was rekindled in 2004 with the advent of bondable root filling materials that are advocated as alternatives to conventional gutta-percha. To-date, there are three bondable root filling materials available commercially. Of these, Resilon  is the only bondable root filling material that may be used for either lateral or warm vertical compaction techniques. As Resilon is applied using a methacrylate-based sealer to self-etching primer treated root dentin, it contains two interfaces, one between the sealer and primed dentin and the other between the sealer and Resilon, and hence may be classified as a type of secondary monoblock. Initial studies on Resilon-filled root canals were highly favorable. Resilon-filled root canals were found to be better than conventionally gutta-percha filled canals in resisting bacterial leakage and improving the fracture resistance of endodontically treated teeth . Based on these promising properties, Resilon, together with the Epiphany primer and sealer system (Pentron Clinical Technologies, Wallingford CT) was subsequently referred to as the Resilon Monoblock System (RMS)  that produces ideal root obturations in terms of coronal sealing and fracture resistance . Although Resilon-filled root canals do achieve good apical and coronal seals, it is equivocal from subsequent independent research studies whether such seals are better than those achieved using gutta-percha and conventional root canal sealers .

The two other types of bondable root filling materials previously mentioned also belong to this category, as an additional circumferential interface is introduced by coating the non-bondable gutta-percha points with materials that render them bondable to the root canal sealers. As the tertiary interface exists as an external coating on the surface of the gutta-percha, both systems are designed to be used with either a single-cone technique or a technique that involves the passive placement of accessory cones without lateral compaction, to avoid disruption of these external coatings.

In the EndoRez system  conventional gutta-percha cones are coated with a proprietary resin coating . This coating is created by first reacting one of the isocyanato groups of a diisocyanate with the hydroxyl group of a hydroxyl-terminated polybutadiene, as the latter is bondable to the hydrophobic polyisoprene component of the gutta-percha cones. This is followed by the grafting of a hydrophilic methacrylate functional group to the other isocyanato group of the diisocyanate, producing a gutta-percha resin coating that is bondable to a hydrophilic, methacrylate-based dual-cured resin sealer . In this system, no dentin adhesive is employed and the generation of an endodontic seal is dependent on the penetration of the hydrophilic sealer into the dentinal tubules and lateral canals following removal of the smear layer. To-date, leakage and morphologic studies showed that the seal of the EndoRez system is mediocre , although long resin tags could be identified within the dentinal tubules . This may be attributed to the polymerization shrinkage of the methacrylate-based sealer . Also, the sealer bonds weakly to the pre-polymerized proprietary coating, as the latter lacks free radicals for bonding due to the removal of the oxygen inhibition layer for packing purposes . Inconsistency of the external proprietary resin coating was also observed in the form of uneven circumferential thickness or partial detachment . Although both the tensile bond strength  and apical seal  of the EndoRez system to intraradicular dentin may be improved using a dual-cured self-etching primer/adhesive such as Clearfil Liner Bond 2V , there is a potential problem of rapid polymerization of the adhesive in an environment with reduced oxygen concentration. Moreover, even with the adjunctive use of an adhesive, it is unrealistic to expect the establishment of a mechanically homogenous unit with the root canal with the EndoRez system, as the bulk of the material inside the root canal still consists of thermoplastic gutta-percha, an elastomeric polymer that flows when stressed.