Grey areas in restorative dentistry: part 6. Direct or Indirect Restoration

Restoration is defined as ‘an act of restoring or the condition of being restored, such as bringing back to a former position or condition’¹ and is a broad term applied to any material or prosthesis that restores or replaces lost tooth structure, teeth or oral tissues.² This is especially applicable to restorative dentistry, whose aims, by and large, are to restore damaged, ill-shaped, malpositioned or malfunctioning teeth so that they look good and function well.

In this context there are three words beginning with ‘F’ that are important to know and apply (Figure 1).

It is imperative that in achieving these aspects when restoring a tooth, the patient is left with a situation that provides for the maintenance of good oral health and wellbeing. Practically, this means providing good access for plaque control, good occlusal harmony, and patient satisfaction with the aesthetics. It seems counterintuitive to satisfy aesthetic demands while leaving the patient with situations that do not allow for adequate prevention of disease processes and the prospect of early failure. Each of these parameters demand equal consideration (Figure 2).

Quality of indirect dental restorations

In deciding whether to provide an indirect restoration, as opposed to a direct restoration, there may be an implicit assumption that the indirect restoration will be of an optimal standard. Clinical experience shows that this optimism may be misplaced. How well a restoration is made, and the accuracy with which it will fit the mouth, are materially affected by the cast, which is dependent on good impressions and good laboratory technique. At present, most indirect restorations are based on impressions, and the evidence is that the quality of impressions sent to laboratories is suboptimal in many cases. Rau et al⁷ reported that 86% of examined impressions had at least one detectable error, and 55% of the noted errors were critical errors relating to the finishing line. There was an increase in errors at the finish line with dual-arch impression techniques and in the presence of blood. Others, have also reported on the suboptimal impressions received at commercial dental laboratories for the construction of crowns and bridges, and on the quality of dental casts used in crown and bridgework.^8,9,10 The potential, therefore, to provide a suboptimal restoration should be at the forefront of the clinician's thinking.

Scanning techniques obviate the potential errors associated with impression taking and there is good evidence of the accuracy of this technique, although it does require good communication between the practitioner and the laboratory, and a laboratory that can meet the demands of this technique.¹¹

Indirect restoration preparation

When preparing an indirect restoration, the following should be borne in mind:

• Retention form: the feature of a tooth preparation that resists dislodgment of a crown in a vertical direction or along the path of placement;¹²
• Resistance form: the features of a tooth preparation that enhance the stability of a restoration and resist dislodgement along an axis other than the path of placement;¹³
• Ferrule: a band or ring used to encompass the root or crown of a tooth;¹⁴
• Amount of supported remaining tooth tissue after the preparation;
• Status of the pulp;
• Status of any root filling;
• Status of peri-apical tissues;
• Presence of a post(s) in the root canal(s);
• Potential to provide posts;
• Ability to create a good preparation and accurate margins;
• Ability to obtain an accurate impression of the preparation;
• Ability to create a well-fitting temporary crown that is in occlusion;
• Status of the periodontium;
• Bone levels;
• Status of the occlusion;
• Ease of repair or replacement in event of failure;

It will be apparent to the reader that some of the above factors are also applicable to direct restorations (Figures 3 and 4, Table 1).

Several restorations in the same tooth, even if they are small, will usually present a longer restoration–tooth interface when compared with the length of the margins of a full crown, so that it might be deemed preferable to provide one restoration to reduce the potential for marginal failure. When deciding which to do, it is important to bear in mind the potential cost–benefit analysis in terms of the health of the tooth (Figure 5). A full crown will inevitably be placed on a tooth that already has a large amount of natural tooth tissue missing and the potentially harmful effects of preparing a tooth for a crown, with the loss of more tooth tissue, should be borne in mind.

Alternatively, with proven bonding techniques, extensive tooth tissue loss can be replaced with directly placed restorations, as shown in Figure 6. The operator must be confident that the criteria for a successful restoration can be met.

Criteria for a successful restoration

Form: the shape allows for adequate plaque control by the patient. Emergence profile allows food to be shed away from the gingival margins. The contact point(s) prevent food packing between adjacent teeth and the embrasures allow for adequate approximal cleaning.

Function: the occlusal contours allow the tooth to return to a functional relationship with opposing teeth in both static and dynamic positions.

Features: restoration or improvement of the appearance of the tooth in the context of the adjacent teeth.

Repairability: the ability to make good if/when the restoration fails, partially or completely.

Figure 7 demonstrates the dilemma of choosing between a direct or indirect restoration, while Figure 8 shows similar situations resolved with direct and indirect restorations. In Figure 7, the upper right first molar and second premolar have indirect restorations showing good contact points, embrasures that encourage good interstitial plaque control, allowing for ease of maintenance of the periodontal tissues, and occlusal contours allowing for good chewing function. The upper right second molar had a three-quarter crown, in gold, in place for over 10 years. The core of the tooth tissue that contributed to the retention of the restoration, came away inside the crown, leaving just one small area of unsupported enamel distally. The stained dentine was hard and not carious. There is now a major problem regarding this second molar as to how, if possible, to make this tooth a functional member of the arch. This begs a question, what if the upper first and second molars had been restored with a direct restoration? The chances are high that more tooth tissue would have remained, and that repair would have been easier.

The diagnosis and management of the tooth in Figure 9 is complicated by the presence of the crown so that the true extent of the carious process cannot be fully assessed without its removal. The extensive cavity in the lower right second premolar is noted in the context of the caries activity.

Tooth wear

Historically, tooth wear has been treated with indirect restorations and/or removable prostheses, often involving full mouth rehabilitation, on the basis that the materials were stronger than those available for direct restorations and that the occlusion could be restored to its original or to an assumed improved function. This involved highly skilled and complex procedures both intra-orally in the preparation of severely distressed teeth and extra-orally, in the laboratory processes to achieve form, features and function.

The modern trend has moved away from this to minimally invasive direct or indirect restorative procedures as the evidence for rehabilitation with crowns has been equivocal. This trend was boosted by the work of Loomans and Opdam in their Radboud Tooth Wear Project conducted in the Netherlands.¹⁷ Their philosophy is one of additive restorations to compensate for tooth substance loss, where the materials used may be direct composite resin or indirect restorations such as ceramic, resin ceramics, or composite. The restorations are designed as an additive ‘uplay or tabletop’ and can be produced by the technician in the laboratory or digitally designed and manufactured. A number of studies have shown the effectiveness of directly placed composites for the management of tooth wear. Milosevic and Burnside concluded that ’the worn dentition presents a restorative challenge, but composite is an appropriate restorative material, and that posterior occlusal support is necessary to optimize survival’.¹⁸ Others support the effectiveness of composite resin restorations for managing tooth wear but point out that outcomes vary, and that maintenance will be needed over time.^19,20,21 Molar restorations may require higher maintenance.

A comprehensive overview of tooth wear and its management is featured in a 2023 special issue of Dental Update.²²

Clinical conundrums

Conclusion

The role of a dental practitioner undertaking restorative treatment is to provide teeth that have the appropriate form, function, and features for the particular clinical situation and for the particular patient. The choice to achieve these parameters is between direct or indirect restorations. It is good to have a choice between these two restorative options, but for the dental practitioner the decision-making process is challenging in the absence of any clear evidence of the benefit of one method over the other. Perhaps less is more! Several factors must be considered before embarking on the treatment of a distressed tooth; the aetiology of the problem; the extent of tooth tissue loss; the skill set of the practitioner, and laboratory support where appropriate. There must be a full and frank discussion with the patient about the risks and/or benefits of any proposed treatment, together with the need for any ongoing maintenance. The financial implications of each option, for both the patient and the practitioner, should be considered. The higher initial cost for the provision of an indirect restoration should be weighed against the cost of the care and repair of a direct restoration over time.