Principles of operative dentistry 1st Ed

2

Principles of direct intervention

PRESERVATIVE MANAGEMENT

Over recent years the dental profession has shifted towards practising preventive dentistry and adopting more conservative and tooth-preserving procedures. Such progression is considered to be a response to the decline in the level of dental caries and increased consumer demands with regards to comfort of treatment and advances in materials science. This shift in caries management, based on rational clinical and scientific principles, will no doubt continue over the coming decades1.

PRINCIPLES OF OPERATIVE INTERVENTION

Modern cavity preparation and design and the evolution thereof cannot, or perhaps should not, be considered without reference to G.V. Black. Black’s text A Work on Operative Dentistry in 19082 was the first to prescribe a systematic method of cavity preparation and the

‘ideal’ cavity form. These features relate to the instruments available at the time (slowly rotating burs with poor cutting efficiency and chisels), caries incidence and pattern, as well as restorative materials available. Although modifications to the classical cavity forms and principles to achieve these were suggested in the early 1900s, these principles remained appropriate and largely unchallenged for a period of over 50 years. The basic shape, and some of the ideals, of Black’s cavities have been popular until recent times and indeed to a degree are still prevalent.

The last 35 years have seen tremendous advances in dentistry, in particular related to tooth-coloured restorative materials and in the 27

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Table 2.1 Black’s classification of carious lesions versus current terminology.

Black’s classification

Current terminology

Class I

Affecting pits and/or fissures also termed

occlusal lesions

Class II

Affecting the proximal surfaces of posterior teeth Class III

Affecting the proximal surfaces of anterior teeth Class IV

Affecting the proximal surfaces of anterior teeth and involving the incisal angle

Class V

Affecting the cervical surfaces

bonding of restorative materials to tooth tissue. Such developments have brought about a re-evaluation of Black’s principles and, furthermore, a move away from Black’s classification of carious lesions and prescribed preparation form. Carious lesions are best described by the site in which they occur and the size of lesion, an approach taken by Mount and Hume3 in their proposal for a new classification of cavities. Many of the modifications have been made on an empirical basis, with scientific evaluation and suggestions more prevalent in the latter part of the last century (Table 2.1).

In contrast to Black’s principles of cavity preparation, which included the establishment of outline form including extension for prevention, the development of resistance and retention form, creation of convenience form, the treatment of residual caries, the finishing of cavity margins and cavity toilet, now the general principles of tooth preparation are determined by:

• The nature and extent of the lesion.

• The quantity and quality of the tooth tissue remaining following preparation.

• Functional load.

• The nature and properties of the restorative system to be used.

In general the minimum amount of tooth substance should be removed to ensure appropriate access and the placement of the required restoration. With developments in the range and properties of the materials available for the restoration of teeth, it is now possible to consider the preparation of teeth as an exercise in damage limitation, with due consideration of both the macroscopic and microscopic features of the biophysical environment into which it is intended to introduce a restoration. This concept was neatly described by

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Anusavice1 as a preservative approach to the operative management of dental caries and associated lesions.

To be able to prepare teeth efficiently and effectively, it is essential to understand the processes of the diseases of teeth, have a detailed working knowledge of tooth anatomy4, the structure and properties of the tooth tissues and pulp biology, and have a clear understanding of the basic principles of occlusion. In addition, one must understand the mode of action, functions and limitations of the instrumentation used to shape and fashion enamel and dentine in the oral environment.

The process of preparing teeth may be considered to comprise the following stages.

Gaining access

In order to remove caries, create the required form of preparation, and enable restorative materials to be placed, adapted and contoured to restore form and function, it is generally necessary initially to cut through and then cut away part of the enamel of the tooth to be treated. Even when the tooth contains a large lesion, it is generally necessary to gain access using a friction-retained, water-cooled, diamond bur held in an air turbine handpiece. If the lesion to be treated is associated with an existing restoration, the whole restoration may need to be removed using the air-turbine handpiece; however, increasingly the benefits of repairing rather than replacing existing restorations are being acknowledged.

Removal of caries

With access established, caries is removed, first from around the amelodentinal junction and then, working apically, towards the areas overlying the pulp. When caries extends down to a vital pulp, one should aim to remove all soft, stained, infected dentine leaving either some stained but firm dentine or possibly some slightly softened, unstained dentine protecting the pulp from exposure. The rationale for this is that affected dentine (rather than infected dentine) may be retained and remineralised with the use of a therapeutic liner. It is common to experience difficulties in distinguishing between dentine that should be removed, and that which should be left. Fluorescence-aided caries excavation5 or a caries detector dye have been suggested as aids in such situations, but may actually lead to over-preparation6.

The area of the amelodentinal junction must always be made completely

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caries-free, although again the necessity for this has recently been questioned.

Development of final form

Once the caries has been removed, before proceeding to create the final cavity form, it is necessary to consider the biological, functional and mechanical demands that will be placed on the final tooth-restorative ‘system’. In particular, the following should be considered.

Minimisation of the effect of preparation on

tooth strength

Any preparation will weaken a tooth and predispose it to fracture. To minimise this effect, all internal line angles should be rounded.

Choice of restorative material

The material to be used is dictated largely by the size of the cavity/preparation and an assessment of the functional demands that will be placed on the tooth-restorative system. If the tooth is non-functional then mechanical properties of the material will not be a large consideration, but for a large preparation in a functional tooth a material that is strong (e.g. amalgam) and able to withstand the stresses encountered during function will be required. The choice of material will influence the final form of the preparation, particularly the cavo-surface angle (more critical with amalgam restorations) and presence of retentive features (more required with non-adhesive restorations).

Integrity of the remaining tooth structure

The preparation should be planned to maximise the preservation and protection of remaining tooth structure. Increasing cavity depth and width increases the potential for outward flexion of buccal and lingual walls7. Preparations with a curved floor show less cuspal movement than those with a flat floor and a flat floor with its sharp angles and stress concentrations may lead to fracture. This flexure may also have effects on subsequent buccal restorations8. If caries has undermined the remaining tooth structure to a significant degree, the tooth may fracture during function. The planned removal of such healthy tissue may, in fact, preserve tooth structure in the long term

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by minimising the subsequent risk of fracture, which may otherwise lead to loss of a large quantity of strategic tooth structure. Also, it has long been established that there is increased fracture incidence in teeth with restorations of a wide isthmus and having three or more surfaces. The provision of cuspal protection should be considered in such cases.

Placement of margins

Black originally proposed that margins should be placed well into the embrasures in cleansable areas, but the degree to which this has been adopted has slowly reduced over the years with the acceptance that good oral hygiene is sufficient. Cervically, Black recommended that margins should be placed in a caries-free zone subgingivally, but this zone is the area of gingival attachment! It is now accepted that margins should be kept free of the gingivae to avoid periodontal problems and that incidence of overhangs and marginal gaps must be avoided. It may be necessary to extend the preparation if the margin (i.e. interface between tooth structure and restorative material) is close to a contact with an opposing tooth as there is the potential for early breakdown at this weak interface. This emphasises the need to mark the occlusal contacts before preparation is commenced, especially if rubber dam is being used. Similarly, for cavities involving the proximal surface it may be necessary to extend the gingival margin in an apical direction to allow placement of a matrix band. This differs significantly from Black’s ‘ideal’ preparation with predefined placement of margins (Fig. 2.1).

Elderton9,10 has argued that many amalgam restoration failures are due to marginal breakdown owing to a low amalgam marginal angle (AMA) and high cavo-surface angle (CSA). He has suggested that preparations with AMA of at least 70° (ideally 90°) will yield longer lasting restorations (Fig. 2.2). In a 2-year clinical study of amalgam restorations in preparations with such margins, Stratis and Bryant11

commented on the difficulty of consistently achieving these angles.

They showed that utilising these angles (together with finishing procedures) resulted in fewer marginal fractures although the short-term nature of the study was noted.

Planning of the retentive form

If a non-adhesive restoration is to be placed, some mechanical retention must be included in the preparation. The nature of caries lesions

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Fig. 2.1 Rounded cavity form and position of occlusal contacts relative to cavity margins.

is such that removal of the caries should result in a preparation that is undercut in cross-section. This general form may provide sufficient retention for the restoration. However, in a large preparation, additional retentive features may need to be provided – these include undercuts, grooves, boxes etc. and these aspects are addressed later in this chapter.

Integrity of the restoration

Internal preparation features affect the stresses that occur within the restoration. This is particularly important in the case of proximal lesions in which the placement of a groove in the floor of the ‘box’ will minimise the lateral forces to which the restoration is subjected and thus reduce failure due to fracture of the restorative material.

A wide range of rotary and hand instruments may be used to remove unsupported tooth structure and form features within the remaining tooth structure. Such procedures should always be performed in such a way as to minimise pulpal damage. Completed preparations must always be meticulously cleaned, dried and then carefully inspected for evidence of residual caries or small exposures, which may have been overlooked.

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Fig. 2.2 Cavo-surface angle of proximal preparations.

ALTERNATIVE PREPARATION METHODS

Chemomechanical caries removal

Chemomechanical techniques are one of the recently documented alternatives to traditional mechanical rotary techniques and mechanical non-rotary techniques.

Chemomechanical caries removal involves the application of a gel to tooth tissue. This selectively softens the carious dentine, thus facilitating its removal. Removal of sound tooth structure, the cutting of open dentinal tubules, pulpal irritation and pain are all reduced when compared with conventional mechanical methods12.

The first commercially launched product for chemomechanical caries removal was Caridex (National Patent Medical Products Inc.), initially introduced on the US market in 1985. This system involved

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the intermittent application of preheated N-monochloro-DL-2-aminobutyric acid (GK-101E) to the carious lesion. The solution was claimed to cause disruption of collagen in the carious dentine, thus facilitating its removal. Caridex was not widely adopted, possibly because of the expense, additional clinical time and the bulky delivery system, which consisted of a reservoir, a heater, a pump and a handpiece with an applicator tip.

Carisolv

During the 1990s a more efficient and effective chemomechanical caries removal system was developed called Carisolv™ (Medi Team).

The formulation of Carisolv is isotonic in nature and consists of the following:

• Sodium hypochlorite (0.5%)

• Three amino acids (glutamic acid, leusine, lysine)

• Gel substance (carboxymethylcellulose)

• Sodium chloride/sodium hydroxide

• Saline solution

• Colouring indicator (red)

Carisolv can be used in the management of the majority of caries lesions, either in isolation or in conjunction with a handpiece, which may be required to gain access or remove existing restorations. The clinical situations in which Carisolv could be considered the preferred method of caries removal include:

• When the preservation of tooth structure is important (this should be every case).

• The removal of root/cervical caries, where access and visibility are good.

• The management of coronal caries with cavitation, thus avoiding the use of dental handpieces.

• The removal of caries at the margins of crowns and bridge abutments, thus decreasing the likelihood of replacing the entire crown/bridge.

• The completion of tunnel preparations (where access to approximal caries is gained via the occlusal surface, leaving the marginal ridge intact).

• Ensuring complete caries removal.

• Where local anaesthesia is contraindicated.

• The care of caries in dentally anxious patients (needle phobics).

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• Management of primary carious lesions in deciduous teeth.

• Atraumatic restorative technique (ART) procedures.

• Caries management in special needs patients.

The last five situations should result in the avoidance of local anaesthetic administration.

The clinical technique employed can be quickly and easily mastered.

However, careful case selection is initially required. For the first few cases, it is advisable to select fully visible and easily accessible lesions such as buccal root caries or occlusal caries with 1–2 mm entry opening, thus allowing the procedure to be observed. Early cavitation usually helps to provide easy access for gel application and instrumentation, and does not necessitate the use of a handpiece to gain access. From a patient perspective the response to the technique has been almost universally positive, with patients reporting less pain, discomfort and shorter perceived treatment times when compared with traditional drilling13. The avoidance of both slow-speed cutting and, in many cases, the use of a high-speed handpiece, makes the experience relatively pleasant for the patient. However in some instances, it is still necessary to use the high-speed handpiece with water coolant to gain access.

A number of theories have been postulated as to why there may be reduced pain and need for local anaesthesia. These include the lack of cutting into caries-free dentine, relatively few dentine tubules are exposed, no vibrations from drilling, no great temperature variations and the dentine is constantly covered with an isotonic gel at body temperature. The possible psychological input of a quiet and less traumatic experience may also play an important role. In certain cases it is necessary to administer a local anaesthetic to complete deep cavity preparation or where existing restorations, crown and bridgework require removal before cavity preparation.

Sonic preparation

Sonic instruments have been used within the field of dentistry for many decades, principally for scaling and root surface debridement.

Their use for cavity preparation has been revisited recently. The system was initially marketed for proximal lesions with matching size preparation tips and ceramic inserts. This type of approach proved to be destructive to the tooth tissue. The newer sonic handpieces allow for interchangeable tips and multiple applications, such as minimally invasive caries therapy, cavity preparations, endodontics, periodontics, luting procedures and prophylaxis.

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Air abrasion

Air abrasion has also been revisited in recent years in light of developments in restorative materials and changes in cavity preparation design. Most units work by delivery of a jet of aluminium oxide particles at a pressure of 40–149 psi (276–1028 kPa) through a fine nozzle. It is these spray particles that effectively cut the tooth tissue and restorative materials. Air abrasion is best suited to the treatment of small lesions in pits and fissures, cervical caries and recurrent caries around existing restorations. The advantages of such a system include: a local anaesthetic is usually not required; several lesions in different quadrants can be completed at one visit; saucer-shaped preparations can be produced and these are ideal for resin-bonded restorations; and there is less noise and vibration compared with the slow handpiece. However, over-spray can contaminate the surgery, clog the handpiece bearings, block the suction units and damage unprotected adjacent teeth. It is claimed that newer air abrasion units eliminate these problems with high volume suction and water to reduce the over-spray. This method is not very effective for removal of soft caries, therefore manual excavation or slow handpiece removal is required. Some practitioners use chemomechanical caries removal in conjunction with air abrasion. Air abrasion cannot be used for precise cavity preparations, such as inlays or crowns.

Lasers

The field of laser technology has developed considerably over recent years, and many types of lasers are available for cutting of dental hard tissue. The combined CO /erbium substituted: yttrium 2

aluminium garnet (Er:YAG) dental laser is designed for cutting both hard and soft tissue. It has been reported to be as fast as a turbine handpiece, silent and does not require the use of a local anaesthetic for the preparation of enamel and dentine. Their use for soft-tissue surgery has been well documented; however there is limited literature available on dental hard tissue and some concern has been raised about heat generation.

PULP PROTECTION

In discussing aspects of pulp protection, it is useful to consider definitions of a sealer, liner and base14:

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• Sealer – a cavity sealer is a material which seals the dentinal tubules and provides a protective coating for the freshly cut tooth structure of the prepared cavity.

• Liner – a cavity liner is an aqueous or volatile organic suspension or dispersion of zinc oxide or calcium hydroxide that can be applied to a cavity surface in a relatively thin film. Glass-ionomer cement and resin-modified glass-ionomer cements are also suitable for use as lining materials.

• Base – a cavity base is a material, usually a type of cement, used to base a prepared cavity before the insertion of a permanent restoration, to protect the pulp and act as a dentine replacement.

Historical concepts

Microleakage is the term used for the passing of fluids, micro-organisms or ions between the restoration and the adjacent preparation walls. Microleakage occurs around all restorations currently used in restorative dentistry, including those that are adhesively bonded to enamel and dentine. Such leakage provides a path for the ingress of bacteria and their products around restorations and has been implicated in a variety of clinical conditions, including marginal discoloration, pulpal irritation and subsequent necrosis, postoperative sensitivity, recurrent caries and eventual failure of restorations15,16.

The methods used to treat the preparation walls before restoration placement have changed over the years. This is thought to be a response to better understanding of the cause of pulpal damage, Brannstrom’s hydrodynamic theory of pulpal pain17,18 and the development of new dental materials.

Traditional dental teaching advocated the generous use of bases and liners under restorations (especially amalgam) to limit postoperative sensitivity and to act as a thermal insulator. It was originally thought that the primary cause of pulpal inflammation was related to the direct cytotoxic effect of the dental restorative material. However, this has been shown to be a mild and transitory effect18.

Current concepts

Most authorities now recognise that the presence of bacteria is the most important determinant factor of pulp inflammation and ultimately pulp death. Bacterial contamination may be derived from the initial carious lesion, cavity preparation and restoration placement, the smear layer or microleakage. Hilton14 stated that ‘an understanding

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of the properties of the currently available materials, and how they interact with the pulpal tissue, can help the practitioner decide when to use bases and liners and which products to choose.’ The routine placement of a preparation liner or base is now not advocated. All preparations should have some form of preparation sealer and some preparations (usually deep) will require a liner and/or base.

The pulp may be damaged during the restorative procedure by inadequate water cooling of the burs, use of worn burs or by accidental entry into the pulp chamber (pulpal exposure) by hand or rotary instruments. Accurate knowledge of the anatomy of each tooth is therefore essential to ensure that tooth preparation is completed with the minimum of iatrogenic damage. An important consideration here is the age of the patient, in that younger patients have larger pulp chambers than older patients.

To prevent further noxious stimuli reaching the pulp it has been usual practice to protect further the pulp by applying therapeutic materials to the floor and/or the pulpo-axial wall of the preparation.

These materials were commonly placed under amalgams and resin composites to prevent thermal stimulation of the pulp and acid contamination of dentine respectively. It has now been demonstrated that thermal stimulation of dentine is not a problem clinically and that routine basing of amalgams, to prevent thermal stimulation, inherently weakens the restoration. It is also now accepted that dentine can be etched and therefore routine lining for resin composites is now contraindicated.

Sealer

Traditionally, cavity varnishes have been routinely used to provide a protective coating for freshly cut tooth structure. A cavity varnish is a natural gum, such as copal or rosin, or a synthetic resin dissolved in an organic solvent such as acetone, chloroform or ether, which evaporates and leaves a protective film behind. Many studies support the view that the application of a cavity varnish under amalgam restorations provides a temporary seal, decreasing microleakage until corrosion products are deposited. Doubts have been expressed as to the effectiveness of Copalite varnish to seal teeth restored with high copper amalgam long enough for corrosion products to be deposited at the interface19. The increased microleakage seen with some high copper amalgam restorations may be due to the fact that the varnish dissolves before the corrosion products are fully formed. Recent advances in dentine bonding agents have led to recommendations for

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their use under amalgam restorations to seal the dentinal tubules, eliminate dentinal fluid movement, decrease microleakage and postoperative temperature sensitivity.

In recent years various desensitising agents have been used in the management of tooth hypersensitivity. These agents are reported to be effective by reducing the diameter of the dentinal tubule and limiting fluid movement20. It has been postulated that the application of the same mechanism allows desensitising agents to be equally effective in preventing postoperative sensitivity when amalgam restorations are placed. These materials may be of value in the treatment of cavity surfaces before amalgam placement.

Liner

Cavity liners are placed to a thickness of typically less than 0.5 mm.

They act as cavity sealers and may have the additional therapeutic benefits of fluoride release, adhesion to tooth structure and antibacterial properties. Liners may not have sufficient thickness or strength to be used alone in deep preparations; therefore they are frequently overlaid by a base material. The most popular currently used cavity liners are calcium hydroxide and glass-ionomer cements. Resin-modified light-activated glass-ionomer liner materials have gained increased popularity and have the added advantage of ease of placement, command set and early resistance to moisture contamination.

Eugenol based materials are contraindicated as liners for resin composite restorations, because the eugenol may be absorbed into the resin composite, act as a plasticiser and decrease bond strength. This view has, however, been disputed over recent years21,22.

Base

The ideal base material is a thermal insulator, non-toxic, cariostatic, has persistent antibacterial properties, is able to stimulate reparative dentine formation, and is strong enough to withstand the forces of amalgam condensation and masticatory forces. Bases are traditionally dentine replacement materials, and may also be used to block out undercuts for indirect restorations. All cement bases dissolve slowly and disintegrate with time in the oral environment. They act as a mechanical barrier between the restorative material and the underlying pulp. The remaining dentine thickness overlying the pulp is the single most important factor when deciding whether or not to place a base. In vitro studies have shown that a remaining dentine thickness of

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between 0.5 and 1 mm reduces the toxicity levels of materials by 75%

and 90% respectively23. Dentine is said to be the most effective base and should not be removed to accommodate a proprietary material.

The most commonly used bases have been zinc polycarboxylate, glass-ionomer cements, zinc oxide eugenol and zinc phosphate cements.

On the basis of research, the philosophy of basing a preparation to an ideal form has fallen into disrepute. Bases have few benefits and make the restoration more prone to fracture. The main question today has to be whether cement bases under amalgam restorations are necessary and have any value in current operative dentistry techniques.

Materials that are used for bases can sometimes be used for temporary dressings. If a patient has lost a restoration or when tooth preparation is not completed, it is usual to insert a temporary restorative material (temporary dressing). This is designed to seal the preparation and prevent pain from exposed tooth substance and to preclude further carious activity until a permanent restoration can be placed.

Indications for use

Dentine itself is an excellent insulator; therefore, in preparations estimated to have more than 2 mm of remaining dentine thickness, there is generally no requirement for any pulp protective material beneath the restorative material. However a preparation sealer should be placed to seal dentinal tubules and thus prevent postoperative sensitivity and bacterial contamination of dentinal tubules.

In the case of simple amalgam restorations two coats of a dentine desensitiser (such as a HEMA/glutardialdehyde combination, available as a proprietary product) may be used. Current research would seem to suggest that cavity varnish will be replaced by dentine bonding agents in the near future. However, there is insufficient evidence, at present, to support the routine use of dentine bonding agents under amalgam restorations. There is growing evidence that compound or complex amalgams would benefit from the application of a dentine bonding agent. Dentine bonding agents should be used routinely used under all resin composite restorations.

For deeper cavities in which there is less than 2 mm of remaining dentine a preparation liner should be placed in the deepest aspects of the preparation. Traditionally, calcium hydroxide was used routinely as a liner but is now reserved for use in deeper cavities or for ‘capping’

procedures (Fig. 2.3).

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Fig. 2.3 Features of a restored deep lesion (RmGIC = resin-modified glass-ionomer cement).

Table 2.2 Amalgam and resin composite restoration preparation.

Minimum

Moderate

Deep

(just into dentine)

(more than 2 mm of (less than 2 mm of dentine remaining)

dentine remaining)

Amalgam

(S) with dentine

(L) with glass-ionomer

(L) with glass-ionomer

restoration

desensitiser

cement

cement always

and

and

(S) with dentine

(S) with dentine desensitiser.

desensitiser

Hard setting calcium

hydroxide may be placed

in deeper areas if indicated

Resin composite

(S) with dentine

(S) with dentine

(L) with calcium hydroxide

restoration

bonding agent

bonding agent

and glass-ionomer cement

in deeper areas if indicated

and

(S) with dentine bonding

agent

(S) = Sealer; (L) = Liner; (B) = Base.

Table 2.2 provides a guide to the use of pulp protection materials, indicating suitable combinations of sealers, liners and bases to be used in resin composite and amalgam preparations.

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Stepwise caries removal

Where gross caries is present in a tooth, an assessment should be made of the likelihood of creating a carious exposure should all caries be removed. In the situation in which risk of an exposure is high, it is prudent to remove only the peripheral caries and the majority of caries on the pulpal floor. A calcium hydroxide dressing (to encourage formation of tertiary dentine and kill any remaining bacteria) and a well-sealed temporary restoration is then placed. Approximately 3 months later, re-exploration of the cavity is performed and remaining caries removed. Such an approach reduces the incidence of pulpal exposures and subsequent loss of vitality24,25.

Pulp capping

Direct pulp capping

A direct pulp cap is the term for the procedure in which a dressing/lining (or restorative material) is placed into direct contact with exposed pulpal tissue. This is usually carried out following a carious or traumatic exposure. Calcium hydroxide is most commonly used; however some workers have directly bonded resin composite over exposures and mineral trioxide aggregate may show promise as an alternative (although currently it is relatively expensive).

Indirect pulp capping

An indirect pulp cap is essentially where not all carious affected dentine has been removed and involves placement of a dressing on the deepest dentine. There is some confusion in the literature (even in recent publications) with regards to definition of ‘indirect pulp cap’. This term is often used to describe the situation where stained, demineralised dentine is not removed and a calcium hydroxide lining placed to encourage formation of tertiary dentine and kill any remaining bacteria. An alternative definition is where calcium hydroxide is used in a similar manner over soft, carious dentine.

Greater understanding of the caries process has led to the distinction between infected and affected dentine26. Stained dentine may be affected by caries (may be slightly demineralised or conversely may be sclerosed) but may not necessarily be infected and thus removal of such dentine would, in fact, be over preparation with unnecessary loss of tooth structure. Thus it could be argued that the first definition

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of an indirect pulp cap (where stained, demineralised dentine is not removed and a calcium hydroxide lining placed) reflects nothing more than routine practice for pulp protection.

Although several studies have been completed with regard to progression of caries and prognosis of teeth in which permanent restorations are placed over caries, there is at present insufficient evidence to support this approach. Thus the second approach to indirect pulp capping (where soft, carious dentine is left) describes a procedure that, with current evidence, should not be performed.

Given the above arguments and with confusion over the term indirect pulp cap, it is best to avoid this term and consider the use of calcium hydroxide for three lining purposes:

• For deeper cavities where it is estimated that less than 2 mm of dentine remains, a preparation liner should be placed in the deepest parts of the preparation (to encourage formation of tertiary dentine and minimise risk of future exposure).

• For direct pulp capping procedures (to stimulate formation of calcific repair).

• For stepwise caries removal (to encourage formation of tertiary dentine, kill any remaining bacteria and reduce risk of exposure).

SUPPLEMENTARY RETENTION FOR DIRECT RESTORATIONS

Retention is the ability of a restoration to resist forces that would dislodge it in the long axis of the tooth. Resistance of a restoration is the ability of a restoration to resist forces that would dislodge it in a lateral or rotational direction. In general terms, features of a preparation that provide resistance form will also provide some degree of retention, and the two terms are often interchanged.

Retention of a restoration within the tooth relies primarily on there being sufficient coronal tooth tissue which can provide:

• Adequate bulk of dentine to form an undercut preparation or allow for placement of undercuts without resulting in weakened tooth structure.

• Sufficient coronal tooth tissue to provide ‘bracing’ to lateral forces and hence provide some resistance to displacement of the restoration.

Often with extensively broken down teeth it is impossible to develop appropriate retention and resistance form with the remaining tooth

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tissues and alternative methods to retain the restoration must be considered. A variety of techniques may be employed (perhaps in combination) to provide the extra retention.

Bonding

The principle of acid etching of enamel and the use of resin-based adhesives with resin composite materials is well established and the continuing development of multi-purpose bonding systems has allowed such materials to be bonded to tooth structure in a broad range of situations without the need to sacrifice healthy tooth structure in order to increase retention and resistance. A more difficult situation arises when the material being used does not bond to tooth structure. Retention is normally provided by undercuts and preparation features; however at times it may not be possible to create these features. This is classically the case with amalgam restorations in large preparations. With any operative procedure, there is a fundamental need to preserve tooth structure wherever possible and this equally applies to situations in which additional retention is required. In this respect, the ability to achieve additional retention through bonding restorative materials to tooth structure (and rely less on mechanical means of resistance) offers obvious advantages.

A variety of propriety adhesives that are specifically for use in bonding amalgam to tooth structure are now available27. These are principally bi-functional polymeric resins, for example phosphonated esters of bis-GMA, 4-META and HEMA. Most of these adhesives bond to enamel and dentine in a similar way to resin composite bonding systems, though the bond between amalgam and adhesive is thought to be purely micromechanical28.

Numerous in vitro studies related to bonded amalgam restorations have been reported in the literature. Despite there being few long-term clinical studies, there are definite short-term advantages, including: preservation of tooth structure, decreased immediate postoperative sensitivity, increased retention and increased fracture resistance of remaining tooth structure29–31. The long-term benefits are, however, less certain, reflecting uncertainty regarding the durability of the resin bond. In addition it should not be forgotten that bonding an amalgam restoration requires a well-controlled operating field and may be more time consuming.

When the short-term benefits of bonding amalgam restorations are of use, for example to reinforce weakened cusps before providing a cuspal coverage restoration (large cores, or endodontically treated

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teeth), there seems little excuse for not adopting such a procedure.

The long-term benefit of bonding amalgam restorations is uncertain.

Therefore, given the lack of long-term clinical data, increased cost and technique sensitivity, the use of adhesive liners under amalgam restorations cannot yet be advocated as a routine procedure.

Preparation design features

In general, once carious dentine has been removed from a cavity, the resulting shape will be undercut; however this general shape may not be sufficient to retain a restoration when the preparation/restoration is particularly large. In order to increase the retention and resistance form of a preparation, the placement of well-defined preparation features such as undercuts, slots and grooves will often suffice (Fig. 2.4). These include parallelism or relative undercuts of all preparation walls, proximal box form, retention grooves in the proximal line angles and box form in buccal and lingual groove areas of molars.

Although healthy tooth structure should be retained whenever possible, the careful and judicious removal of dentine to create retentive features will result in enhanced service of the restoration and ultimately greater longevity of the tooth itself due to fewer interventions over the lifespan of the tooth. Defined preparation features can be easily and safely cut into remaining dentine with a variety of small burs. In order for these features to have maximum benefit, they should be placed in opposing dentine walls.

Fig. 2.4 Supplementary features for retention of direct restorations.

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In a large preparation with one or more missing cusps, the resulting preparation floor is often fairly flat in its entire profile. If this preparation were filled with no additional preparation features, even when there are significant undercuts in the remaining dentine walls, the ability of the restoration to withstand lateral forces will be limited.

The placement of a circumferential groove or shelf will provide a significant increase in the resistance form of the preparation.

The preparation of circular chambers cut vertically into dentine of about 1–1.5 mm diameter and 2 mm deep, into which restorative material is placed, can provide resistance and retention. These features have also been termed amalgam inserts or amalgapins.

A disadvantage of slot-retained amalgam restorations is that they are particularly sensitive to displacement during matrix removal, and great care must be taken not to dislodge the restoration when removing the matrix.

Dentine pins

The use of dentine pins is well established as a method of providing additional retention (Fig. 2.5). Three types of pin (cemented, friction-Fig. 2.5 Typical position of a dentine pin.

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locked and self-threading) have been used to retain dental restorations. Although cemented and friction-locked pins have certain advantages, all but a few of the pins presently used in clinical practice are of the self-threading type. These are relatively simple to use and are the most retentive. A wide variety of self-threading pins are available, a typical dentine pin system comprises a twist drill with a matching pin, which is usually threaded and self-tapping (i.e. cuts its own thread on insertion). It is usual that both drill and pin have a latch grip enabling them to be used in a conventional handpiece. Some pins consist of a simple length of threaded pin, while others have features such as a shoulder stop (to control depth of insertion) and altered shapes for more mechanical retention of the restorative material.

Although the use of dentine pins has an advantage over preparation features in that less removal of sound tooth structure is required, it has long been recognised that their use also has considerable problems. The stress produced within the dentine during placement of a dentine pin causes cracking and subsequent weakening. Errors during placement are not uncommon and may result in perforations into the pulpal space or the periodontal ligament with subsequent problems. An additional problem may occur due to the mismatch of the modulus of elasticity of the pin and the restorative material. This, combined with the lack of homogeneity of the restorative material due to pin placement, may cause localised stress concentrations during load and a subsequent predisposition to failure (i.e. pins weaken rather than reinforce or strengthen restorations). This can be a problem with amalgam and more so with polymeric restorative materials where such a mismatch will be even larger. Obviously, the more pins that are placed, the greater the risks.

Given the routine use of bonding agents with polymeric restorative materials, the additional use of dentine pins with these materials is questionable as the disadvantages would seem to outweigh any advantages. In addition if an adhesively retained restoration is supplemented with dentine pin placement, catastrophic bond failure may go unnoticed and rapidly progressing caries is then a risk.

There is a lack of clinical data on survival of large amalgam restorations placed with or without pin retention32,33. There are however, some studies that suggest that large amalgam restorations placed without the use of dentine pins, but using preparation features as described above have equal strength, resistance and longevity to those restorations placed with pin retention. It is becoming apparent that supplementary retention/resistance is probably not as essential as was once thought, and when necessary can be achieved without the

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use of dentine pins and their potential problems. For very large restorations, the placement of a dentine pin can aid in stabilising the amalgam during removal of the matrix band and during finishing. In this way dentine pins may be useful for a particularly large amalgam restoration that is otherwise retained by preparation features as described above.

Amalgam dowel core

An amalgam dowel core, also termed a Nayyar core (after the clinician who first reported the technique), is a method described for restoration of endodontically treated molars before provision of an indirect restoration such as a crown34. This direct core utilises the coronal pulp space and a few millimetres of the radicular canal for retention. This corono-radicular core (Nayyar core) is described fully in Chapter 4.

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