Monoblocks in root canals: a hypothetical or a tangible goal

Abstract

The term monoblock has become familiar in the endodontic literature with recent interest in the application of dentin adhesive technology to endodontics. Endodontic monoblocks have generated controversial discussions among academicians and clinicians as to whether they are able to improve the quality of seal in root fillings and to strengthen roots. This review attempts to provide a broader meaning to the term monoblock and to see how this definition may be applied to the materials that have been used in the past and present for rehabilitation of the root canal space. The potential of currently available bondable materials to achieve mechanically homogeneous units with root dentin is then discussed in relation to the classical concept in which the term monoblock was first employed in restorative dentistry and subsequently in endodontics.

Fig. 1

A schematic depicting the classification of endodontic monoblocks. In a primary monoblock, there is only one interface between the root filling material and the root dentin. In a secondary monoblock, there are two interfaces, one between the fiber post/root filling material and the cement/root canal sealer, and the other between the cement/root canal sealer and the root dentin. In a tertiary monoblock, a third interface is created when a bondable coating is present on the surface of the fiber post/root filling material.

Fig. 2

Orthograde filling of root canals with mineral trioxide aggregate (MTA) as an apexification material represents a contemporary version of the primary monoblock in attempts to strengthen immature tooth roots. A. Pre-operative radiograph of a central incisor (tooth 8) with incompletely formed root and open root apex that was necrotic and exhibiting radiographic signs of chronic apical periodontitis. B. Post-operative radiograph of an apexification procedure that was performed with orthograde filling of the entire root with white MTA. C. A 14 month post-treatment radiograph showing gradual bone regeneration along the periapex.

Fig. 3

Bonding of a pre-fabricated fiber post and a resin cement to a post space represents the classic secondary monoblock depicted in the restorative and endodontic literature. Although there is ample evidence in the literature to support the use of fiber posts for improving the fracture resistance of endodontically treated teeth, a case is illustrated showing that fiber posts do not necessarily strengthen roots and that crown fracture may still occur after the use of fiber posts. A. A radiograph of a retained root (tooth 21; pointer) with an incomplete root filling, a fractured fiber post containing radiopaque fibers, and horizontal root fracture along the gingival margin. B. A photograph of the fractured crown with a ceramometal crown and the coronal portion of the fractured fiber post (pointer). In this particular case, the presence of eccentric masticatory forces caused by the absence of molar tooth support and the possible pre-existence of a cervical abfraction lesion (see tooth 20) probably accounted for the failure of the fiber post-supported tooth.

Fig. 4

Surface coating of conventional gutta-percha cones with glass ionomer fillers (ActiV GP, Brasseler USA, Savannah, GA) represents an example of a part of the components of a tertiary endodontic monoblock, in which these filler-coated gutta-percha cones are bonded to intraradicular dentin with the use of a glass ionomer root canal sealer. A. A low magnification scanning electron micrograph of a cryofractured ActiV GP gutta-percha cone depicting the representative locations from which the higher magnification micrographs were derived. B. A high magnification interfacial view showing the surface of the fractured gutta-percha cone (between asterisks) with the glass ionomer fillers (arrow) on top of the surface and the filler-dense gutta-percha cone below. C. A high magnification surface view showing a region that is heavily-coated with glass ionomer fillers. The dimensions of these angular fillers ranged from submicrometer to 2 μm in diameter. D. Incomplete or uneven coating of the gutta-percha cone surface could often be observed along different regions of the same coated gutta-percha cone. In this micrograph, the glass ionomer fillers were sparse (open arrowhead) and numerous dimpled, filler-free areas (pointer) could be identified.