Core restorations-Restorative Dentistry Lecture note

A core is the term used to describe therestoration that is placed in order to build up a broken down tooth beforereceiving an indirect restoration. In some cases it may not be necessary toplace a separate core, but an indirect restoration may be constructed toreplace all of the missing tooth structure. Typical examples of this include root-filledteeth in which an integral corono-radicular restoration and core may be placed,or for teeth that have suffered cusp fracture where placement of a traditionalrestoration would leave very little tooth structure (e.g. premolars with aprevious mesio-occlusodistal restoration and one lost cusp, a ‘one-piece’ onlaymay 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 corewill depend on the degree to which the tooth in question is broken down and howmuch coronal dentine remains. When attempting to understand the rationale for choiceof core restoration, it is helpful to consider the concept of extremes, from asimple space-filling core to a structural core (Fig. 5.3) and relate thedifferent functions to the materials available.

Core Build up Video

Space-filling core

When much coronal dentineremains, the role of a core is simply to fill out any undercuts and give anappropriate shape that will provide adequate retentive and resistance form(described later). The restorative material simply acts to prevent or ‘blockout’ any undercuts to the path of insertion of the intended indirectrestoration. This situation commonly arises when an intra-coronal restoration (i.e.an inlay) is planned to replace a previous direct restoration. The preparationwill have to be modified to eliminate the undercuts; extending the preparationwould be unnecessarily destructive compared with placement of a core materialto block out the undercuts. Similarly, the same approach can be taken to givesmooth axial walls when an extra-coronal restoration such as a full coveragecrown is planned.

In the above example, thefunctional 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 asability to bond to tooth structure, cariostatic properties and ease of handling(e.g. command set).

 

Types of corerestorations, (a) structural and (b) space-filling.

Structural core

When a large amount of coronaldentine has been lost, it is more likely that an extra-coronal full-coverage restorationwill be planned. The core material will replace a substantial part of theclinical crown and will form the bulk of the final preparation. In this case,the core material will be subjected to significant functional demands andstresses, particularly in molar teeth, and must therefore have adequate mechanicalproperties to resist these. Although a full-coverage crown may afford someprotection to the core if the margins are extended gingivally beyond the core6,this protection is limited. The strongest materials available at present areamalgam (for a direct core) or a cast metal such as gold (for an indirect corein an endodontically treated tooth).

Essentially, there are fourtypes of direct core material available; amalgam, resin composite, glass ionomeror hybrid materials such as light-cured resin-modified glass-ionomer cement(RmGIC).

Amalgam

Amalgam has perhaps the besttrack record when used for substantial posterior core build-ups. Amalgam hasgood contrast with tooth substance and is easy to prepare. The long time tofull set may predispose to early fracture, which is unfortunate, as thepreparation cannot usually be prepared at the same visit, although newerhigh-copper amalgam alloys have high strength within a short time and may be preparedat the same visit after a short delay. The thermal expansion of amalgam isquite dissimilar to dentine, and this factor may predispose to failure after aperiod of time. In addition, amalgam cannot be bonded to tooth substancewithout resorting to proprietary products for amalgam bonding.

Resin composite

The use of resin composite asa core material has advantages and disadvantages. The composite does notrequire a two-visit crown preparation technique and, when necessary, the crownpreparation can be commenced immediately. However, against this, the resin compositeis difficult to prepare to the correct form because it may be difficult todifferentiate between tooth tissue and core substance, though resin compositesof contrasting colour are available. Lightcured resin composites should be usedwith caution, as full depth of cure may not be achieved in substantial corebuild-ups. Chemicalcure or dual-cure resin composites (the latter having theadvantage of ‘command set’) have an advantage in that those portions of the materialnot exposed to the curing light will still undergo polymerization due to thechemical cure. However, they may undergo increased discoloration (due to thetertiary amine activator7) compared with light-cured resin composites and thusshould be used with caution in anterior teeth in which non-opaque aesthetic restorationsare planned. Some resin composites are marketed specifically for core build-up,are coloured and have advantages over tooth-coloured composites. It has beensuggested that because of water sorption and expansion, additional die reliefshould be provided during construction, or impression taking should be delayedafter preparation to prevent discrepancy between the working die and the preparedtooth.

Glass ionomers andresin-modified glass-ionomer cement

Traditional glass-ionomercements are only suitable for use as a space-filling core, where they will notbe subjected to any stresses, as they are inherently weak materials. Severalglass-ionomer materials are marketed specifically for use as a core build-upmaterial such as RmGIC. They bond to dentine, release fluoride, have comparablethermal 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 thesematerials is not well documented. Water sorption and expansion are higher with thesematerials than with resin composites and, for this reason, after preparationthere should be a delay before impression taking. At present, their use as astructural core may be questionable. However they may eventually become the materialsof choice with further developments.

Choice of core material

The choice of core materialdepends on several clinical variables. The role of the core material withregard to a space-filling or functional role is critical and the degree to whichthe core will be subjected to stress and the amount of bracing provided byremaining coronal dentine should be considered when selecting the material.Amalgam alloy should not be used beneath anterior full-veneer crownrestorations as corrosion products from the amalgam core may stain the dentineperipheral to the restoration and result in poor aesthetics.

Similarly, an amalgam coreunderneath a three-quarter crown may shine through the remaining tooth and beunaesthetic. Restoration of the endodontically treated tooth is covered indetail in later posts, though points of particular relevance are repeated here.In most situations the general principles above apply. When little toothstructure remains it is usual to place a post-retained core, although molarteeth may successfully be restored with an amalgam dowel core (Nayyar core). Ifa direct intra-radicular post has been placed in order to retain a core, thencare should be taken to ensure that the properties of the core material are notmismatched to those of the post (e.g. avoid glass-ionomer cement orresin-composite cores with metal intra-radicular posts), although some studiessuggest that fibre posts (with a relatively low modulus of elasticity) performbetter with a rigid metal core.

In general terms, when thereis sufficient coronal dentine remaining to provide some support to the corematerial, then resin-based restorative materials are the core materials ofchoice. However, for a tooth that has lost much coronal tooth structure then astronger core material (amalgam or cast metal if root treated) should beplaced.