Indirect restorations: an update

In the UK, 85% of adults have restored teeth,¹ 37% have crowned teeth, with an average of three per person in this group.¹ It is estimated that there are 47.6 million crowned teeth in the UK, but unfortunately, surveys report that over one-quarter of these exhibit signs of failure, with secondary caries being the most common diagnosis.^1,2 For over 50 years, direct adhesive restorative techniques, particularly using resin composite, have progressively replaced the indications for traditional indirect restorations because they offer considerable advantages and are associated with fewer serious complications.

In clinical situations where indirect restorations are necessary, by using current concepts in adhesion and occlusion, multiple authors have described minimally invasive preparation designs that preserve significantly more natural tooth tissue than traditional full coverage crowns. The advent of implants and resin-bonded bridgework has also virtually eliminated the need for conventional fixed bridges, which commonly result in biological, functional and aesthetic problems, and may precipitate further tooth loss (Figure 1).

Fortunately, in the UK, younger patient cohorts have fewer crowns compared with older groups and are, therefore, better placed in the advancing restorative cycle process:^1,2,3

• 5% of age group 16–24 years have crowned teeth;
• 57% of age group 45–74 years have crowns.

Disadvantages of indirect restorations

Indirect restorations are associated with a range of widely documented disadvantages, dating back to some of the earliest professional publications, for example: crowns are ‘a mausoleum of gold over a mass of sepsis’ (Dental Briefs 1911).

Common complications include poor longevity, with the average indirect restoration lasting just 10 years before total replacement or extraction of the supporting tooth becomes necessary (Table 1).^1,3

Indirect versus direct restorations

Minimally invasive adhesive direct techniques have transformed restorative dentistry. They focus on biologically conservative interventions designed to address aetiological factors and maximize preservation of natural tooth tissue. They promote less extensive restorations of increased longevity that are easier to maintain and renovate, and seldom compromise future restorative options.

Direct composite is a proven, minimally invasive, cost-effective, aesthetic alternative to traditional indirect restorations in many clinical situations (Figure 2).

The complications listed in Table 1, coupled with the advantages of direct resin composite,¹² have extended the indications for resin composites to include the restoration of endodontically treated teeth and extensive rehabilitations of the worn dentition, where the use of full coverage crowns now has little to commend it (Table 2).¹³

In the authors’ opinion, the proven success of composite resins alongside the use of tooth bleaching, direct/indirect veneers and onlays, means that traditional full coverage crowns are rarely indicated, other than for the replacement of failed existing crowns.

Furthermore, the commonly catastrophic mode of failure of conventional fixed/fixed bridgework means that cantilever (ideally resin-bonded) bridges are the treatment of choice for replacing missing teeth in clinical situations where no treatment, implant-retained restorations, or removable prosthodontics are not an option (Figure 3).

Dento-legal considerations

Crown and bridgework is responsible for the majority of negligence claims against dentists, accounting for the largest proportion of all damages and legal costs.⁴

Complaints and allegations of negligence can result from any stage of case management and are notoriously difficult to defend.⁴ The main factors precipitating complaints and litigation are listed in Table 3.

Indirect restorations are often provided for teeth that have previously endured multiple restorative treatments, increasing the risk of complications, such as tooth fracture or pulp necrosis. Other factors that increase the risk of dento-legal problems include:⁴

• Irreversibly preparing previously healthy teeth for purely cosmetic restorations;
• Providing multiple/complex restorations;
• Using indirect restorations to reorganize the patients’ occlusal scheme.

Case selection

Careful case selection is critical in reducing the risk of unsatisfactory outcomes. A range of factors should be considered (Table 4), and the key questions, listed in Table 5, may be used to aid clinical decision-making.

As very few patients exhibit excellent oral health (10%)¹ or have very healthy periodontal tissues (17%),¹ advanced treatment, such as indirect procedures, should only be considered when they will clearly enhance the patient’s long-term oral health and welfare.¹¹

The replacement of failed crowns, bridges and implant-retained restorations should also be conditional on the diagnosis and complexity of the failure and strategies to mitigate these aetiological factors in the future.¹³

Consent

Obtaining valid consent is critical for indirect procedures, where over-prescription is a frequent source of complaint.⁴ For example, patients commonly complain that:

• They have been talked into treatment that they did not need, or treatment that is of little or no benefit to them.
• They had inadequate understanding of the proposed treatment, and would not have agreed to it if they had been fully informed.

Clinicians should be fully aware of the patient’s treatment expectations at the outset, and patients must have complete understanding of:

• The irreversible nature of the treatment;
• Potential complications (and how they will be managed);
• Prognosis/longevity/likely mode of failure;
• Financial considerations.

Considering the inherent challenges described above, the following step-by-step guide is designed to enhance the success of indirect restorative procedures by providing evidence-based practical tips for each clinical stage.

Clinical stages for indirect procedures

Assessment and diagnosis

As with any restorative procedure, diagnosis is based on information from a comprehensive patient assessment and interpretation of the results of special investigations (Table 6). Pulp testing and high-quality peri-apical radiography are mandatory when assessing patients for indirect procedures. Teamwork and interdisciplinary communication are vital for optimising patient assessments. Diagnostic findings can then be easily communicated to patients, enabling them to actively participate in decisions regarding their care.

In the authors’ opinion, clinical photography is essential for all stages of indirect procedures. High-quality clinical images optimise communication between clinicians, patients and technicians, and enable insightful reflection that optimises clinical outcomes.

Aesthetic diagnosis

Indirect restorations in the aesthetic zone must integrate with the optical properties of the residual dentition. Visual assessment supplemented by high-quality clinical photographs and/or videos will inform patient discussions, aid co-diagnosis and optimise laboratory communication (Table 7).

Shade selection

Shade selection and communication are key skills for indirect restorative procedures. The authors recommend the use of the VitaPan 3D Master shade guide (Vita Zahnfabrik. Germany). Compared to the 16-shade Vita Classical shade guide, shade selection is faster and more objective, and there is increased relevance to the range and distribution of tooth colours. Each shade is denoted by three digits, for example Shade 3M1 (where 3 = value, M = hue, 1 = chroma) (Figure 4).

As light quality can vary dramatically throughout the day (1000–10,000 degrees Kelvin colour temperature (K)), and dental unit and fluorescent lights can have large variations in the wavelengths of emitted light, the use of specialized, colour-corrected light sources is recommended. The standard for colour matching is 5500K, which encompasses the full spectrum of visible light wavelengths. Repeating colour assessments under different light sources avoids the effects of metamerism.

While bright illumination is essential for perceiving subtle variations in hue and chroma, lower-intensity light may be used for studying value. This also allows enhanced perception of surface texture detail.

Significant differences in shade and surface texture may be present in a single tooth, therefore an annotated ‘map’ optimises patient discussions and laboratory communication (Figure 5).

As digital scanning devices have limitations, meticulous training in the visual diagnosis and communication of tooth form, surface texture and shade is recommended for clinical teams.¹³ Practical tips are listed in Table 8.

Occlusal registration

The potential for irreversible damage to the functional occlusal relationship between a patient’s teeth is high when providing indirect restorations, and is significantly increased in proportion to the number of teeth being restored. The patient’s pre-operative occlusal scheme must be assessed, registered and documented accurately.

Provided that the presenting occlusion is stable, conforming precisely to the patient’s existing occlusal relationships is recommended.

The importance of accurate occlusal records cannot be overstated. Thin articulating paper, eg Hanel Occlusion foil 12 μm (Coltène, Switzerland), held in Miller forceps, is recommended to mark centric occlusal contacts. A second colour may be used to identify excursive contacts. It is good practice to photograph the pre-operative occlusal marks, and to document firm holding contact points with thin-gauge metal foil (eg 8-µm Shimstock).

Mounted pre-operative study casts enable the clinician and technician to orientate the occlusal plane in the same perspective as facing the patient. This reduces the risk of designing restorations with an inaccurate incisal plane, and avoids maxillary incisal edge canting.¹⁴

In many single-unit cases, the occlusal relationship can be determined with hand-articulated casts. Marked study casts, photographs or videos of the patient’s occlusion will enhance communication quality.

A formal registration may not be required if only a few teeth are being restored, and sufficient contacts remain between the unprepared teeth to allow the technician to establish the centric occlusion (CO) position. When there is any doubt, a bite registration material should be used, with the following properties:¹³

• Ability to easily and accurately record opposing occlusal surfaces;
• Exhibit limited flow to prevent mandibular deviation;
• Have suitable working time to allow mandibular positioning;
• Set with an abrupt transition to the solid state;
• Be dimensionally stable
• Capable of adjustment without distortion;
• Be suitable for decontamination procedures.

In situations where patients have lost posterior occlusal support, an occlusal registration using wax record rims may be required. However, the limitations of wax records for fixed prosthodontic rehabilitations should be recognized.¹³

Where functional occlusal relationships need to be recorded, registration procedures should also use a facebow transfer and a semi-adjustable articulator, which may be supplemented with excursive/protrusive occlusal records. Clinical situations where the use of a facebow transfer is recommended are listed in Table 9.

Treatment planning

When discussing treatment options and treatment planning, reference should be made to clinical notes, photographs, radiographs and study casts. The use of reversible intra- and extra-oral techniques, for example diagnostic wax-ups, digital smile design (DSD), demonstration models, diagnostic transfers and clinical videography, may also be used as valuable planning and communication resources (Figure 6).

All options (including no treatment) should be presented to the patient, before finally agreeing on a particular treatment strategy. Patients should be made aware of the implications, possible complications and anticipated life-expectancy of the restorations. Patients must be willing to accept that the success of the treatment will be highly dependent on their subsequent commitment to oral health maintenance protocols.

All treatment plans should be kept under continual review, at every stage, and contingency treatment options should form part of the overall strategy for patient care. The following fundamental principles may be useful when treatment planning for indirect restorations:¹³

• Use only techniques that are supported by a strong evidence-base, offering the best prognosis;
• Use minimally invasive restoration designs that preserve the maximum amount of tooth tissue, eg labial veneers, onlays, resin-bonded bridges;
• Avoid multiple restorations where possible;
• Never link crowns on adjacent teeth;
• Avoid fixed/fixed bridges (use cantilever, ideally resin-bonded, designs);
• Design conformative occlusal relationships that minimize loading;
• Design restorations that will maintain periodontal health and optimise oral hygiene procedures;
• Select materials that will satisfy biological, functional and aesthetic requirements;
• Consider financial implications for patients in the short and long term;
• Involve the laboratory technician in the planning stages;
• Use evidence-based materials, equipment and techniques, eg reputable, versatile implant systems;
• Consider implants first when replacement of missing teeth is necessary;
• Obtain specialist opinion where necessary.

Consent

Written consent should be obtained for all forms of fixed prosthodontics, following a comprehensive discussion of the proposed treatment with the patient. As consent is an ongoing process, each stage should be documented completely, legibly, unambiguously and contemporaneously.

Pre-preparation

As emphasized earlier, full coverage restorations should only be provided where more conservative treatment options are not possible. When crowns are indicated, the abutment tooth should be fully assessed for restorability and structural durability and an appropriate core constructed. Commonly used core restorative materials are compared in Table 10.

Silicone putty supported in a sectional tray (eg Kwik-Tray, Kerr, Uxbridge) may be used to record teeth (or wax-ups) prior to preparation, to allow construction of accurate provisional restorations.

A range of other procedures may need to be completed and reviewed, including:

• Pain relief;
• Extraction of unsavable teeth;
• Caries management;
• Periodontal treatment (stability essential);
• Definitive endodontic treatment;
• Preliminary occlusal adjustment;
• Implant osseo-integration and soft tissue response;
• Orthodontic treatment.

Tooth preparation

Tooth preparation design is an extensively researched subject (Table 10; Figures 7, 8).¹⁵

When preparing teeth with limited resistance and retention form, or when modifying existing preparations following removal of failed indirect restorations, it may be necessary to incorporate resistance grooves, slots or boxes to significantly increase resistance to dislodgment by lateral forces. As mandibular molar preparations are reported to commonly be over-tapered, the routine incorporation of mesial and distal resistance grooves is recommended to reduce the risk of decementation.¹⁵ Equally, the use of parallel resistance grooves in metal–ceramic resin-bonded bridge preparations will reduce stress on adhesive bonds and reduce the risk of ‘peel’ failure (Figure 9). Onlay preparations, that maximize enamel preservation, are recommended for posterior indirect restorations (Figure 10).

Margin design

Various forms of shoulder, chamfer and knife-edge margin designs have been proposed in the literature, but there is no universal agreement regarding superiority.¹⁵ General principles include:¹⁵

• Preserve enamel for adhesively bonded restorations;
• Maximize residual dentine thickness;
• Design margins that are appropriate for selected restorative material;
• Ensure margins are obvious to the technician/scanning device;
• Keep margins supragingival where possible;
• Follow gingival contour (especially interproximally where bone levels are higher);
• Prepare rounded internal line angles to reduce stress concentration;
• Flat shoulders will resist occlusal forces;
• Appropriate depth to allow ceramic shade match, eg 1.0-mm shoulder.

Chamfer margins are popular as they are easy to prepare, for example with a 1.0-mm diameter torpedo bur used to prepare a 0.5-mm chamfer. Chamfer margins are more minimally invasive than shoulder margins and are distinct/readable on preparations, impressions, dies and scans. They also create adequate space for bulk and rigidity of materials to allow development of physiological emergence profiles.

Supragingival margins are preferred, wherever possible, because the gingivae are sensitive to mechanical trauma from preparation, retraction, impressions and temporization. Biological width violation is a common problem associated with indirect preparations,¹⁶ and will result in chronic inflammation and rapid, significant recession (average 1.0 mm in 2 weeks).⁹ Visibility of a periodontal probe through the marginal gingiva indicates a thin gingival biotype, and in such cases, subgingival margins are contraindicated.¹⁴

Subgingival margins may become necessary for aesthetic reasons, to increase axial length for retention, to extend beyond restorations/cervical defects, or to create a ferrule for post-retained restorations. Where crown lengthening is necessary, preparations should be restored with wellfitting provisional restorations. The tissues should be allowed to heal sufficiently, for example 3–6 months, before impressions are taken for the definitive restorations.⁴

Impressions

Digital impressions are transforming indirect restorative techniques. They are expected to replace conventional techniques because they offer a range of advantages (Table 13, Figure 11).¹⁷

Moisture control

Moisture control is critical for both conventional and digital impressions. Retraction cord, soaked in an astringedent (eg placed with a cord packer), is a quick, efficient method of simultaneously achieving moisture control and soft tissue retraction. The authors recommend the Ultrapak retraction system (Ultradent, UT, USA) (Figure 8a).

Silicone impression materials are favoured for conventional impressions, using materials based on operator preference regarding:

• Handling properties;
• Setting properties, eg time/rigidity;
• Adequate tear resistance;
• Colour contrast;
• Decontamination protocol;

The most popular, convenient and reliable evidence-based technique for silicone impressions is full arch, single stage, putty/wash using a rigid non-perforated tray (eg Rimlock, Prestige Dental, Bradford, UK) or a custom-made special tray (Figure 12).^18,19 Material rigidity also influences impression tray selection. In comparison, two stage silicone impressions are associated with several disadvantages, notably:

• Seating pressure may deform impression materials and non-rigid impression trays.
• Incomplete seating of stage two may create steps in the wash material.
• Stage one may remove the retraction cord.

Although less accurate than silicone impression materials, alginate is commonly used for impressions of opposing arches. High-quality alginate and automatic alginate mixers are recommended for consistency, along with metal non-perforated Rimlock impression trays, which eliminate the need for tray adhesive.

Impressions should be washed, trimmed, dried and inspected (ideally with magnification) and carefully handled/stored, before transit to the laboratory.

In some cases, it may be useful to have casts returned for inspection, margin marking (with a sharp pencil), socketing (eg immediate replacement resin-bonded bridges) and occlusal checks (Figures 13 and 14).

Provisional restorations

High-quality provisional restorations have important functions (Table 14) and may also be used for longer durations to:

• Trial planned tooth shapes/
• aesthetic changes;
• Test occlusal changes, eg new anterior guidance;
• Test patient tolerance, eg to planned increases in occlusal vertical dimension;
• Overcome phonetic problems;
• Delay impression taking, eg for periodontal healing;
• Allow healing/stability after crown lengthening, eg up to 6 months.⁴

Injection moulding techniques are recommended, using an accurate pre-operative impression and bis-acryl composite or acrylic materials. Covering preparations with petroleum jelly may assist removal and may offer some protection against the exothermic setting reaction. Slight excess is preferred to test for the optimal removal time before complete setting.

Provisional restorations should be cemented with weaker temporary cements (eg TempBond NE, Kerr, Uxbridge, UK). Care should be taken to avoid overfilling restorations with temporary cements so that they can be easily removed and re-used if the definitive restoration requires adjustments following the try-in stage. Eugenol-free temporary cements may be used to prevent potential negative effects on resin luting materials.

When provisionally restoring multiple preparations, it is more convenient to let the material set completely and trim marginal excess in situ, eliminating the need to remove and re-cement the provisional restorations (Figure 15). Extra emphasis must be given to interappointment plaque control when using this method, and provisionals for multiple, adjacent, individual preparations should not be linked in cases where the occlusal prescription and the retention and resistance forms are being confirmed.

Laboratory communication

High-quality laboratory communication is a vital determinant of success for indirect procedures. Written or digital prescriptions should contain all the information necessary for the technician to construct restorations with the ideal fit, function and aesthetics (Table 15).

Annotated diagrams/photographs and/or digital smile design information can be sent along with relevant study casts, wax-ups or impressions of provisional restorations (Figure 16). Regular verbal dialogue between dentist and technician is recommended. Laboratory visits and/or video conferencing will also enhance communication, professional relationships and teamwork.

Try-in

Try-in of indirect restorations is usually performed at the fit appointment; however for complex or challenging aesthetic cases, it may be necessary to schedule a separate appointment to trial restorations to confirm:

• Marginal fit;
• Occlusal relationships;
• Contact points;
• Aesthetics/patient acceptance.

Restorations should be carefully assessed, in advance, on the articulated casts to ensure that they conform precisely to the prescription. Magnification and reference to study casts and impressions/scans is recommended. If minor adjustments/further laboratory instructions are necessary, it is recommended to complete these at the chairside while the patient is in attendance. Metal restorations have the significant advantage that they may be repolished at chairside to a laboratory-quality finish. Where ceramic restorations require adjustment, then they will need to be returned to the laboratory for repolishing/reglazing. This is because ceramic adjustments significantly increase their abrasivity on opposing teeth,¹⁰ and surface roughness increases the risk of staining. Where major adjustments are necessary, the cause should be identified to reduce the risk of similar errors in future indirect procedures.

Following local anaesthesia, provisional crowns should be carefully removed (ideally in one piece in case the definitive restorations require laboratory adjustment or remake). Water soluble try-in gel (eg NX3, Kerr, Uxbridge, UK) is recommended to allow accurate patient/operator assessment of function and aesthetics (Figure 17).

Try-in gels are also available in different shades, which correspond to those of luting resins. This facilitates testing of slight shade adjustments for translucent restorations, such as ceramic veneers.

In some situations, it may be necessary to cement full veneer restorations temporarily, so that patients can trial function and aesthetics at home over a longer period before returning for removal and definitive cementation.

When patients are satisfied with the restorations, it is recommended to ask them to confirm this (ideally in writing), and to document their understanding that the cost of replacements will be borne by them in the event that they subsequently change their mind, and that the process may result in further trauma to teeth and associated soft tissues.

Cementation

Following successful try-in, the restoration(s) should be cemented with luting materials and techniques appropriate to the tooth and fit surface substrates. Manufacturer’s instructions should be followed precisely. Isolation from saliva, blood and gingival crevicular fluid must be maintained throughout the entire cementation procedure. Use of retraction cord soaked in an astringedent is recommended, which may be retained in the gingival sulcus until after complete set. Rubber dam isolation is recommended for adhesively bonded restorations (eg resin-bonded bridges). General cementation stages include (Figure 18):

• Isolation, eg with a rubber dam;
• Pumice slurry may be used to clean preparations;
• Preparations washed and dried;
• Tooth surfaces should be conditioned, eg etching, washing, drying, priming, adhesive application;
• Conditioning of fit surfaces, eg silanation of ceramic restorations;
• Luting material should be dispensed and mixed in strict accordance with the manufacturer’s instructions;
• The volume of luting material applied to restorations (and/or subject tooth) should be sufficient to allow a slight excess of material along the entire restoration margin. NB: overfilling may impede seating;
• Fully seat restorations within the working time, using appropriate pressure to overcome hydraulic forces;
• Restorations should be held in place during the initial setting phase, or until ‘tack-cure’ or full polymerization have been completed;
• Excess luting material should be completely removed using magnification and appropriate instruments directed away from the margins;
• Approximal contacts may be cleaned/tested using dental floss, but with care to use only apical pressure and lateral removal to eliminate dislodging forces on newly bonded restorations.

Review and monitoring

The common complications related to indirect restorations mean that continuous monitoring is required at appropriately timed review appointments.

Detailed post-operative instructions (ideally written/digital) may be used to remind patients of their ongoing oral hygiene/maintenance requirements and to remind them to return immediately if complications present (Figure 19).

An initial review appointment should be scheduled to:

• Assess the patient’s biological and functional response to the restorations;
• Assess the quality of oral hygiene measures;
• Assess periodontal health;
• Confirm satisfaction with comfort and aesthetics;
• Double-check for excess luting material (drying thoroughly enhances identification).

Long-term reviews should include peri-apical radiographic monitoring at justified, patientspecific intervals,²⁰ as marginal failure may lead to rapidly progressing secondary caries (the most common cause of failure).^1,2 Oneyear post-operative peri-apical radiographic examinations are recommended for implant-retained restorations.²⁰

Observations from long-term reviews should be documented (and photographed) to inform similar future clinical cases. With (written) patient permission, clinical photographs may also be used to educate other patients (or professional colleagues) regarding restorative treatment options.

Summary

While direct techniques are progressively replacing the indications for indirect restorations, indirect procedures will remain an integral component of clinical dentistry, largely due to the high failure rate among millions of existing indirect restorations that will require replacement.

Careful case selection and provision of high-quality, minimally invasive fixed prosthodontics, accompanied by excellent maintenance, can produce long-term success, which is rewarding for both the patient and the dental team and will reduce the risk of complications.