A core is the term used to describe the restoration that is placed in order to build up a broken down tooth before receiving an indirect restoration. In some cases it may not be necessary to place a separate core, but an indirect restoration may be constructed to replace all of the missing tooth structure. Typical examples of this include root-filled teeth in which an integral corono-radicular restoration and core may be placed, or for teeth that have suffered cusp fracture where placement of a traditional restoration would leave very little tooth structure (e.g. premolars with a previous mesio-occlusodistal restoration and one lost cusp, a ‘one-piece’ onlay may be the treatment of choice). However, for most teeth requiring an indirect restoration, a core restoration will need to be provided.
The exact nature of any particular core will depend on the degree to which the tooth in question is broken down and how much coronal dentine remains. When attempting to understand the rationale for choice of core restoration, it is helpful to consider the concept of extremes, from a simple space-filling core to a structural core (Fig. 5.3) and relate the different functions to the materials available.
Core Build up Video
When much coronal dentine remains, the role of a core is simply to fill out any undercuts and give an appropriate shape that will provide adequate retentive and resistance form (described later). The restorative material simply acts to prevent or ‘block out’ any undercuts to the path of insertion of the intended indirect restoration. This situation commonly arises when an intra-coronal restoration (i.e. an inlay) is planned to replace a previous direct restoration. The preparation will have to be modified to eliminate the undercuts; extending the preparation would be unnecessarily destructive compared with placement of a core material to block out the undercuts. Similarly, the same approach can be taken to give smooth axial walls when an extra-coronal restoration such as a full coverage crown is planned.
In the above example, the functional demands and stresses encountered by the core material will be minimal. The mechanical properties of a core material in this situation are not critical, and the material choice is largely determined by secondary factors such as ability to bond to tooth structure, cariostatic properties and ease of handling (e.g. command set).
Types of core restorations, (a) structural and (b) space-filling.
When a large amount of coronal dentine has been lost, it is more likely that an extra-coronal full-coverage restoration will be planned. The core material will replace a substantial part of the clinical crown and will form the bulk of the final preparation. In this case, the core material will be subjected to significant functional demands and stresses, particularly in molar teeth, and must therefore have adequate mechanical properties to resist these. Although a full-coverage crown may afford some protection to the core if the margins are extended gingivally beyond the core6, this protection is limited. The strongest materials available at present are amalgam (for a direct core) or a cast metal such as gold (for an indirect core in an endodontically treated tooth).
Essentially, there are four types of direct core material available; amalgam, resin composite, glass ionomer or hybrid materials such as light-cured resin-modified glass-ionomer cement (RmGIC).
Amalgam has perhaps the best track record when used for substantial posterior core build-ups. Amalgam has good contrast with tooth substance and is easy to prepare. The long time to full set may predispose to early fracture, which is unfortunate, as the preparation cannot usually be prepared at the same visit, although newer high-copper amalgam alloys have high strength within a short time and may be prepared at the same visit after a short delay. The thermal expansion of amalgam is quite dissimilar to dentine, and this factor may predispose to failure after a period of time. In addition, amalgam cannot be bonded to tooth substance without resorting to proprietary products for amalgam bonding.
The use of resin composite as a core material has advantages and disadvantages. The composite does not require a two-visit crown preparation technique and, when necessary, the crown preparation can be commenced immediately. However, against this, the resin composite is difficult to prepare to the correct form because it may be difficult to differentiate between tooth tissue and core substance, though resin composites of contrasting colour are available. Lightcured resin composites should be used with caution, as full depth of cure may not be achieved in substantial core build-ups. Chemicalcure or dual-cure resin composites (the latter having the advantage of ‘command set’) have an advantage in that those portions of the material not exposed to the curing light will still undergo polymerization due to the chemical cure. However, they may undergo increased discoloration (due to the tertiary amine activator7) compared with light-cured resin composites and thus should be used with caution in anterior teeth in which non-opaque aesthetic restorations are planned. Some resin composites are marketed specifically for core build-up, are coloured and have advantages over tooth-coloured composites. It has been suggested that because of water sorption and expansion, additional die relief should be provided during construction, or impression taking should be delayed after preparation to prevent discrepancy between the working die and the prepared tooth.
Glass ionomers and resin-modified glass-ionomer cement
Traditional glass-ionomer cements are only suitable for use as a space-filling core, where they will not be subjected to any stresses, as they are inherently weak materials. Several glass-ionomer materials are marketed specifically for use as a core build-up material such as RmGIC. They bond to dentine, release fluoride, have comparable thermal behaviour to dentine, can be made a contrasting colour to tooth (e.g. blue) and are easy to prepare, although the long-term behaviour of these materials is not well documented. Water sorption and expansion are higher with these materials than with resin composites and, for this reason, after preparation there should be a delay before impression taking. At present, their use as a structural core may be questionable. However they may eventually become the materials of choice with further developments.
Choice of core material
The choice of core material depends on several clinical variables. The role of the core material with regard to a space-filling or functional role is critical and the degree to which the core will be subjected to stress and the amount of bracing provided by remaining coronal dentine should be considered when selecting the material. Amalgam alloy should not be used beneath anterior full-veneer crown restorations as corrosion products from the amalgam core may stain the dentine peripheral to the restoration and result in poor aesthetics.
Similarly, an amalgam core underneath a three-quarter crown may shine through the remaining tooth and be unaesthetic. Restoration of the endodontically treated tooth is covered in detail in later posts, though points of particular relevance are repeated here. In most situations the general principles above apply. When little tooth structure remains it is usual to place a post-retained core, although molar teeth may successfully be restored with an amalgam dowel core (Nayyar core). If a direct intra-radicular post has been placed in order to retain a core, then care should be taken to ensure that the properties of the core material are not mismatched to those of the post (e.g. avoid glass-ionomer cement or resin-composite cores with metal intra-radicular posts), although some studies suggest that fibre posts (with a relatively low modulus of elasticity) perform better with a rigid metal core.
In general terms, when there is sufficient coronal dentine remaining to provide some support to the core material, then resin-based restorative materials are the core materials of choice. However, for a tooth that has lost much coronal tooth structure then a stronger core material (amalgam or cast metal if root treated) should be placed.