Pocket Dentistry

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Principles of Cavity Preparation

Cavity preparation, the procedure used to remove demineralized enamel and infected dentin consists of four steps:

1. Opening a cavity or removing a poorly fitting restoration
2. Removing infected dentin
3. Evaluating residual tooth tissue and removing unsupported or structurally compromised enamel
4. Finishing cavity margins

The extent of preparation always depends on clinical and radiographic analysis of the caries lesion (Table 5-1). The clinician will shape the cavity based on the extent of the lesion and in keeping with the principle of minimal invasiveness. This chapter discusses the various levels of tooth preparation according to the type of caries lesion and patient risk level.

TABLE 5-1 Treatment options based on the initial clinical situation

Clinical situation

Therapeutic options

Unpronounced occlusal anatomy, no active lesions, good oral hygiene

No treatment

Very pronounced occlusal anatomy, no active lesions, good oral hygiene

• None for adults
• Sealing if the patient is young2

Pronounced occlusal anatomy with suspect grooves in patients at risk (eg, pigmented, discolored grooves)

Ultraconservative opening of grooves for diagnostic purposes (preventive resin restoration [PRR])

• In the absence of caries: restoration/sealant
• If caries present: Class 1 restoration

Small, medium, and large occlusal caries lesions

Class 1 restoration

Areas of interproximal demineralization

• Remineralizing maneuvers and aids
• Infiltration method (only in strict accordance with guidelines)
• Class 2 restoration with direct access, preserving the marginal ridge
• Class 3 restoration with removal of marginal ridge

This chapter would have looked very different before the advent of dental bonding.1 The principles of tooth preparation for direct bonded restorations have evolved because of increased knowledge and application of caries prevention as well as dental bonding and the improved mechanical performance of contemporary bonding materials. Nowadays, the principles of cavity preparation are simpler than in the past. The requirements are as follows:

• Removing carious dental tissue
• Removing unsupported healthy tooth to prevent its mechanical breakdown during function
• Preparing space required for the restoration material, respecting the dental structural requirements and minimum thicknesses for the material in question

With the phasing out of amalgam and gold preparations, the following concepts have disappeared:

• Extending preparations into healthy adjacent grooves
• Accessory mechanical retention areas (swallowtails, dovetails, etc)
• Occlusal convergence
• Using retention pins

Dental bonding has changed the preparation and restoration of small, medium, and large caries-related cavities and assumed a fundamental role in treatments on the borderline between primary and secondary prevention: minimally invasive seals and tooth preparations.

The level of destructiveness involved in treatment depends on the initial clinical situation2 (see Table 5-1). A relatively unpronounced occlusal anatomy without any diagnostic problems does not require any type of treatment. Clinical case management becomes more complicated if grooves are more pronounced. No treatment will be performed in a patient without active lesions, while it will be prudent to perform minimally invasive treatment if the individual has a medium or high risk profile.3

Types of Grooves

The anatomy of molar and premolar occlusal surfaces is highly variable in terms of groove and pit depth. The enamel anatomy can be more or less pronounced and more or less complete, and this can generate grooves and fissures that are particularly subject to caries. Arhatari et al4 used microcomputed tomography (microCT) to show how deep and variable grooves and pit anatomy can be (Fig 5-1). Longitudinal sections through teeth show variable anatomies (open and closed V-shaped, drop-shaped, I-shaped; Fig 5-2).

Under anatomical conditions favoring plaque accumulation and where the enamel on the base of the groove is thin, any newly established caries activity would lead to sudden spread of the caries lesion. It is therefore imperative to be able to effectively evaluate groove types (Fig 5-3).

Groove evaluation

The essential premise for reliable groove evaluation is that the tooth surface should be clean and accurately observable. After careful cleaning using a high-pressure glycine spray, it is always advisable to observe the grooves using magnifying systems. The possibility of diagnostic error must be considered.5

Magnification systems are an aid for more detailed diagnostic evaluation of grooves. Galilean and prismatic lenses are most commonly used in routine dentistry, but microscopy can be used when the diagnosis is problematic. Once the occlusal surface has been thoroughly analyzed in relation to the patient’s risk rating, it will be possible to decide whether to do nothing, carry out sealing, perform a preventive resin restoration (PRR), or apply a direct bonded restoration (see Table 5-1).

Sealing

Sealing is a preventive procedure to prevent caries lesion formation in deep occlusal surfaces that are difficult to clean. It can reduce caries lesions by up to 51%.2 Filling difficult-to-clean surfaces improves oral hygiene procedures and prevents bacteria proliferating in their ideal habitat.

One of the biggest concerns raised about sealing is the possibility that active caries lesions might be covered. Although there are conflicting views about deliberate sealing of active caries lesions, it has been shown that it is difficult for sealed lesions to grow.6 These findings are reassuring when a practitioner decides to carry out sealing in accordance with the parameters set out in Table 5-1, because sealing could stop the development of an (albeit minimal) early caries lesion that was not identified during clinical evaluation.

The step-by-step procedure for sealing involves:

1. Thorough cleaning of grooves using glycine spray.
2. Removal of aprismatic enamel—which has been shown to be resistant to orthophosphoric acid, leading to an unreliable adhesive bond and loss of sealant.7-9 Preparation must not be aggressive and can be done with air abrasion, fissurotomy burs, or very small-diameter burs.10
3. A total-etch bonding procedure (37% orthophosphoric acid and bonding system).
4. Light-curing sealant.

Minimally Invasive Cavity Preparation

As shown in Table 5-1, a minimally invasive cavity will be made for exploratory purposes if the grooves are suspect. This minimally invasive procedure involves an exploratory preparation to dispel doubt about grooves that are difficult to diagnose. After cleaning the grooves, they are opened to no more than 1 mm to allow direct assessment of tooth tissue quality. If the cavity is negative, ie, without caries, the exploratory cavity can be sealed (type I PRR)11-13; otherwise, it may be decided to proceed beyond the DEJ and carry out a type III PRR (using restoration material and sealant) or a true Class 1 restoration.

A cavity is negative even when it is pigmented with a black line but cannot be probed by a dental probe. The clinical case described in Fig 5-4 shows the approach used for an occlusal surface characterized by grooves affected or not by initial demineralization processes with more or less pronounced cavitation.

Handpieces

There are essentially two types of handpieces used to prepare and finish cavities: a multiplier handpiece (Fig 5-5a) fitted with high-speed burs and a contra-angle handpiece (Fig 5-5b) fitted with low-speed burs. The high-speed turbine allows very high speeds but limited control, so is not recommended as a tool for preparation or for finishing and polishing. Two other useful handpieces are oscillating/reciprocating (Fig 5-5c) and sonic (Fig 5-5d). The nonrotary action of the latter allows certain movements that facilitate cavity preparation and finishing procedures.

Burs

Figure 5-6 shows a selection of preparation burs for conservative direct restoration of posterior teeth. Although sonic or oscillating tips are sometimes used, those described in Fig 5-6 comprise an essential set to manage almost all cavity preparations in posterior sectors. Note that FG indicates friction grip (high-speed bur, for multiplier handpiece) and CA indicates contra-angle (burs for a contra-angle handpiece).

Occlusal Cavities (Class 1)

Occlusal cavities (historically defined by Black as Class 1 in terms of topography and treatment type) are the only cavities related to grooves and fissures. All the other classes relate to caries activity on smooth surfaces. This also involves a different caries lesion configuration and propagation rate compared, for example, to interproximal caries lesions. The classic caries lesion topography is a triangle with its tip at the external surface and base at the DEJ. In interproximal caries lesions, however, the external surface is larger, and progression toward the DEJ is slower (ie, triangular topography with base on the external surface and tip pointing toward the DEJ). As mentioned at the beginning of the chapter, dental preparation aims to remove both caries lesions and undercuts in the cavity, evaluate residual structural factors, and provide restorative material in the necessary thickness.

Essentially two types of burs are used to prepare the cavity:

• Diamond cylindric burs with rounded head for a multiplier handpiece (see Figs 5-6a and 5-6b)
• Tungsten carbide multiblade round burs for a contra-angle handpiece (see Fig 5-6c)

“ THE EXTENT OF PREPARATION ALWAYS DEPENDS ON CLINICAL AND RADIOGRAPHIC ANALYSIS OF CARIES ACTIVITY AND MUST ALWAYS OBSERVE THE PRINCIPLE OF MINIMAL INVASIVENESS.”

The cavity margin is then finished and polished with fine-grained or multibladed burs, stones, or polishers (see Figs 5-6f and 5-6g).

Step-by-step Class 1 preparation

Class 1 preparation (Fig 5-7) follows very specific criteria whether used for treating established caries lesions, reconstruction, or a PRR. As already mentioned, even if the caries lesion is exclusively occlusal, it is advisable to isolate by quadrant (see Fig 5-7a) in order to:

• Increase the visibility of the operating field
• Avoid the need to remove and refit a dam if there is an interproximal extension (which may not be perceptible radiographically)
• Reveal the anatomy of adjacent teeth (to help in assessing the occlusal plane)

The operating field, particularly the occlusal surface, is cleaned with a high-pressure glycine or bicarbonate spray (see Fig 5-7b) or cleaned with a brush and a paste containing pumice and chlorhexidine. When the biofilm is removed from the occlusal surface, it is easier to see which grooves are affected by caries and thus plan a more selective preparation. If possible, the cavity should be opened using a small-diameter bur (size 006, 007, or 008; see Figs 5-7c and 5-7d) to a depth of 1 to 1.5 mm. Once a small cavity has been opened, the location of the caries lesion can be immediately evaluated (more or less buccal, palatal/lingual, mesial, or distal).

The next step is to increase the size of the cavity overlying the caries lesion (see Figs 5-7d and 5-7f), stepping up to a larger-diameter cylindric bur with a round head as soon as possible. As a rule of thumb, it is always advisable to use the largest-diameter bur that can freely enter the cavity. Large-diameter burs allow greater control as well as faster tissue removal. Once a cavity has been opened to allow access, the infected dentin is removed using low-speed tungsten carbide round burs (see Figs 5-7g and 5-7h). In this case too, it is advisable to use the largest bur possible. This allows effective removal of the infected dentin and excellent control compared with small-diameter burs.

Caries lesions extend horizontally through the DEJ. This very often means that the cavity in the dentin is larger than the access cavity in the enamel. The numerous resulting undercuts must be evaluated and ground down if necessary (see Figs 5-7i and 5-7j). A cylindric diamond bur with rounded head is reused to reduce or remove undercuts. Small undercuts are permissible, particularly if they are positioned deeply and if the enamel is partly supported by dentin. The cavity margin is then smoothed using fine-grained diamond burs (red/yellow ring), burs with a high number of blades, or stones before polishing (see Figs 5-7k and 5-7l). The clinical case illustrated in Fig 5-8 shows an exploratory approach to suspect grooves that prompt Class 1 preparations. The clinical case in Fig 5-9 shows the preparation and restoration of an established occlusal caries lesion.

Should the margin be beveled?

In occlusal cavities, beveling of the margins is not required. Because of the way the prisms are oriented at this level, the enamel prisms are already cut correctly by a bur placed at a right angle to the occlusal table.14,15

The cavity margin must be finished and polished. An unpolished margin contains unsupported prisms that can be lost by cohesive fracture during curing and over the lifetime of the restoration, causing marginal pigmentation and, potentially, secondary caries. A smooth margin also allows the bonding and restoration material to adapt better and prevent the incorporation of bubbles.16-19

Interproximal Cavities (Class 2)

Multisurface restorations in molars and premolars are the most common type of restoration.20 Interproximal cavities (historically defined by topography and treatment type as Black Class 2) are very common, and their restoration, performed using the techniques and materials indicated, is predictable and supported in the literature by reviews and meta-analyses.21 Heintze and Rousson22 analyzed 59 clinical studies over at least 2 years of observation, concluding that the most clinically effective procedure for treating Class 2 preparations is a bonding system including etching with 37% orthophosphoric acid, a hybrid composite, a cavity with no need for a bevel, and—when possible—rubber dam.

A Class 2 preparation procedure involves the following steps:

1. Removing the caries lesion with access through the marginal ridge, avoiding damage to the adjacent tooth
2. Defining the position of the cervical step
3. Defining the axial walls of the box-form preparation (sometimes referred to in this text as box for brevity) for reconstructive purposes

Accessing an interproximal lesion involves protecting the adjacent tooth. Although an interproximal cavity can be accessed while preserving the marginal ridge of the tooth involved, it is better to insert a wedge and interproximal protection or a system that includes both23,24 (Figs 5-10 and 5-11). After penetrating the lesion edges, it is advisable to stop and visualize the position of the lesion and decide how far to extend the preparation in a buccolingual direction (Fig 5-12). Some lesions develop in a more buccal or palatal direction and allow a good portion of the interproximal wall to be preserved.

The wedge plays a very important role in Class 2 preparations. The authors recommend positioning it before starting the preparation. It offers certain advantages:

• Protects the interproximal dam
• Apicalizes the interproximal tissue (papilla) to provide more space for rotary instruments
• Can be modified by grinding to make a custom wedge for use during reconstructive steps (Fig 5-13)
  

  

  

  

If the cavity is clearly large, an opening can sometimes be achieved correctly without the help of protection and wedges since a very large cavity can be opened at the occlusal level (Fig 5-14).

Step-by-step Class 2 preparation

The burs used to prepare a Class 2 restoration (Fig 5-15) are the same as those used for a Class 1 restoration (see Figs 5-6a to 5-6c, 5-6f, and 5-6g) with the addition of a flame bur (see Fig 5-6e) and an end-cutting bur (see Fig 5-6d). As with Class 1 restorations, it is preferable to isolate the entire sector (see Figs 5-15a and 5-15b) to manage sectional matrix system sizes and evaluate the anatomy of the other teeth. In a Class 2 preparation, it is always advisable to place the clamp on the tooth distal to the tooth to be treated. Even if the cavity is mesial, it is preferable to position the clamp more distally to allow accessibility, visibility, and space for reconstruction aids.

The wedge protects the deep interproximal area (see Figs 5-15c and 5-15d; see also Figs 5-13b and 5-13c) as well as the dam, which could become damaged during box preparation. The wedge can be inserted buccally or lingually. It is advisable to insert it where the embrasures are wider. Devices are available to protect the adjacent tooth (see Figs 5-15e and 5-15f; see also Fig 5-10). As discussed, Lussi et al showed that the adjacent tooth is always involved during a Class 2 preparation, even when magnification is used.23,24

A small round cylindric bur is used to prepare the inside of the marginal ridge (see Figs 5-15g and 5-15h), preserving the interproximal wall. The preparation is generally deepened by 2 to 3 mm to gain a direct view of the extent of the caries lesion (see Fig 5-12).

It often feels as though the bur is drilling into nothing as it enters the demineralized area, which offers less resistance. The preparation is carried out in a buccolingual direction, preserving the marginal ridge (see Fig 5-15i). Maintaining as much of the interproximal wall as possible, the cavity design is finalized in buccolingual and mesiodistal directions (see Figs 5-15j and 5-15k). The final design will be further modified after cleaning the dentin, given that enamel walls often turn out to be unsupported. The same bur (or a flame bur) is used to define the axial walls (see Fig 5-15l). If not already removed, the interproximal wall is gradually weakened to make it easier to remove (see Figs 5-15m and 5-15n).

A flat cervical step is prepared (see Fig 5-15o).The cervical step must be prepared accurately to achieve:

• A good fit for the sectional matrix
• Good bonding material wettability
• Good restoration material fit
• Correct emergence

VIDEO: CLASS 2 PREPARATION

The margin can be easily achieved by using end-cutting burs to avoid damaging the neighboring tooth (if protection has been removed) or the axial walls (Fig 5-16). The movement to be adopted when using this bur is gentle pressure but relatively fast buccolingual movements (see Fig 5-15p).

The axial walls of Class 2 box preparations must be defined and finished (Fig 5-17), and their surfaces must not be uneven. Various options are available to achieve this end. The first method is to use flame burs (see Figs 5-15q and 5-17a). If the walls are diverging and can be accessed buccally or palatally, reciprocating diamond files (diamond surface on only one side; see Figs 5-15r and 5-17b) or coarse-grained disks (see Figs 5-15s and 5-17c) can be used. This allows the axial wall of the box to be quickly defined and finished, but there must be enough space to allow the disk to work inside the wall, otherwise the outer wall of the box may be damaged. This stage can also be performed using manual tools (scalpels; see Figs 5-15t and 5-17d). Although this is an excellent method, manual tools should be used with caution because there is a risk of leaving steps and sharp angles in the transition between the axial wall and the step. The axial walls and cervical step also easily can be defined using sonic inserts (Figs 5-15u, 5-15v, and 5-17e). These tools are not essential, but they are convenient and allow significant time savings. The axial wall margin is smoothed using a medium-grained disk. This is not a bevel but simply a way of finishing the margin (see Figs 5-15w and 5-15x).

Sometimes it may be necessary to modify the emergence of the cervical step. This maneuver should be performed only when strictly necessary. This can happen when:

• Cervical anatomy is not linear, and gaps arise between the matrix and the cervical step when the sectional matrix is fitted (imperfectly fitting matrix).
• The cervical step is too close to the adjacent tooth, with consequent problems fitting the sectional matrix and accurately managing the emergence profile.

Changes can be made to the cervical step anatomy, provided these are contained within the enamel structure and are of minimal size. The ideal tool for this modification is a reciprocating file (see Figs 5-15y and 5-15z).

Defining the angle between the axial walls and the external surface is important for determining the strength of the restoration and the tooth tissue (Fig 5-18). The angle between the outer surface of the tooth and the box wall should be approximately 90 degrees (green angle, type 1 in Fig 5-18b). More acute angles (red angle, type 3 in Fig 5-18b) must be avoided because they indicate that the dental tissue is very thin and more likely to fracture during function. More obtuse angles (orange angle, type 2 in Fig 5-18b) are acceptable, but they result in areas that are more difficult to fill during reconstruction, making it necessary to apply only very thin layers of restorative material.

Sonic instruments and associated inserts are recommended as a user-friendly method. For the same tip size, rotary instruments need much more working space than a sonic instrument. If preparation is carried out using sonic instruments, cavity preparation on the most distal teeth is a simpler process for the clinician and less stressful for patients, who will not have to put so much effort into keeping their mouths open. Because sonic inserts come in many types with many angles, they can easily reach the most inaccessible areas of the tooth (Fig 5-19). If the most appropriate angle is selected, they comply with the principle of preserving healthy dental tissue where possible, which is not always feasible with the most common rotary instruments.

Interproximal cavities without ridge access: Tunnel or slot technique?

Although the marginal ridge plays an important structural role, it is often difficult to preserve in Class 2 preparations. Clinicians will ideally try to preserve it, relying on the fact that the caries lesion is far from the ridge and often below the contact point. One tried and tested method is the tunnel technique. This involves preparing the interproximal area from an initial occlusal access point. The literature reports highly variable results and a high failure rate (50% survival rate after 6 years in the longest-term study22,25). Failures arise due to marginal ridge fracture and secondary caries. Failure also has been attributed to residual caries that is impossible to remove with a blind access route. A caries lesion spreads significantly through the DEJ, and with tunnel access it is impossible to verify the DEJ coronal to the cavity. Until long-term studies are conducted with favorable results, the authors consider this technique to be risky.

Conversely, a review by McComb25 reports more promising results for slot preparations, known also as proximal slot or box-only preparations. The opportunity to view the cavity directly and therefore remove all the carious tissue and be able to predictably evaluate a compromised marginal ridge makes the restoration prognosis more favorable. In a slot preparation, marginal ridge height appears to be a critical factor for survival. A thin ridge could lead to mechanical failure. The authors suggest preserving a marginal ridge height of at least 1.5 mm. For lower values, it is advisable to form a traditional Class 2 preparation. Some slot preparations and their restorations are shown in Figs 5-20 to 5-22. Preparation of such cavities often depends on access and is carried out using round rotary instruments or angled sonic inserts (Fig 5-23). Cavities can be reconstructed using different composite material viscosities (flowable/paste) since these areas are not subject to mechanical stress. Sometimes it is not possible to perform a slot preparation if the marginal ridge is compromised. In this case, a conventional Class 2 preparation is performed (Fig 5-24).

Apicocoronal position of the cervical step

The apicocoronal position of the cervical step in a Class 2 preparation depends on many factors. Though the approach must always be extremely conservative, this does not rule out the possibility of extending the preparation (cervically or mesiodistally) in order to optimize:

• Restoration procedures (eg, matrix fitting)
• Marginal finishing and polishing procedures
• Marginal cleaning and checking
• Changing the emergence of the interproximal profile to optimize contact points

The diagram in Fig 5-25 shows that, although the cavity can be fully cleaned, the preparation must be extended cervically (seeking to stay within the enamel at all times) to fit a matrix and finish and check the restoration margin (note that the idea that the restoration can be purely self-cleaning is now outmoded). The cervical step does not require apicalization if enough space can be made for a matrix through wedging or by using a mechanical separator.

In Fig 5-26, a cavity completed without allowing for future emergence of the restoration may lead to a horizontal overcontour (see Fig 5-26c) or an inconsistent emergence (see Fig 5-26d) that is often accompanied by marginal ridge fragility. In this case too, preparing the box more apically makes for more consistent emergence (see Fig 5-26e). Given the predictability of bonding, it is in any case advisable to preserve the enamel cavity margins as much as possible.

Decision-Making Criteria for Direct Versus Indirect Restorations

Composite materials have become more popular mainly due to two factors:

1. Development of dentin-enamel bonding systems
2. Development of composite resins

Since 1962, when Bowen resin was used for the first time, the physical, mechanical, and esthetic properties of resins have continually developed and improved, allowing composite materials to be used for more indications. Composite resins advanced significantly due to:

• Introduction of fillers and particularly their silanization (allowing a strong bond with organic resin)
• Different types of fillers
• Different sizes of fillers
• Inclusion of a light-curing system

This led to materials that performed better from an esthetic and a mechanical viewpoint. Clinical procedures involving a composite combined with a bonding system have been tried and tested for years, displaying encouraging longevity.22-29 A meta-analysis published by Opdam et al in 2014 established the annual failure rate after 10 years to be 2.4%.29

With the passage of time, new material developments have prompted clinicians to aim for bolder treatments. While in 1998 American Dental Association guidelines for a Class 1 or 2 restoration recommended a moderate size, nowadays clinicians even aim for direct coverage of one or more cusps with satisfactory results.30,31 Direct treatments have been successfully used to treat clinical situations that are difficult to resolve, such as cracked tooth syndrome.32 We must consider that the more surfaces a restoration contains, the higher the risk of failure,30 and if the margin position after preparation is below the cementoenamel junction, the restoration is nearly 30% more likely to fail.33 These two reasons could make an indirect composite restoration favorable to direct composite treatment, because a prosthetic product confines shrinkage problems to the cement film.

For example, an in vitro study by Dietschi et al34 comparing direct and indirect Class 2 restorations shows that the seal of indirect composite bonded restorations is superior to that of direct restorations because there is less shrinkage due to curing (only the cement shrinks). This was also confirmed by Dejak and Młotkowski, who conducted a finite element study (with all its attendant limitations) and established that an inlay’s internal stresses are decidedly lower than those of a direct restoration and that indirect restorations provide a better seal than direct restorations.35

However, these findings do not seem to have any clinical consequences. In an 11-year randomized clinical trial on Class 2 restorations, Pallesen and Qvist established that there were no significant clinical differences between direct and indirect restorations and that indirect restorations did not show improved marginal integrity. In this type of configuration (inlay versus direct Class 2), there is therefore no justification for a more complex and costly treatment such as indirect restoration.36

Cusp Coverage and Analysis of Structural Factors

During cusp preparation, proximity of the cavity to a cusp must prompt a biochemical and structural analysis of the residual healthy tooth substance. Khers et al37 highlighted how cusps weakened by restorations or caries are more prone to fractures. Firstly, one must remember the principle of cusp independence stated by Sakaguchi et al in 1991: Cusps that undergo stress are subject to deformation that does not extend to adjacent cusps. They can therefore be kept intact, and only compromised cusps need be removed.38

A study by Hood39 emphasizes that the extent of cusp deformation strictly depends on missing tissue depth: Total deflection affecting a premolar cusp subject to an in vitro load is 11 μm for a healthy tooth, 16 μm for a tooth with a minimal Class 1 cavity, 20 μm for a mesio-occlusal cavity with a narrow isthmus, 24 μm for mesio-occlusodistal (MOD) cavities with a narrow isthmus, and 32.5 μm for extensive MOD cavities.

How does the practitioner decide whether to remove a cusp? When must it be removed? When is it acceptable to take the risk of not removing it? Schillingburg et al and Fichera et al identified deficiencies and structural factors to be considered when deciding whether to cover cusps adjacent to a cavity.40,41 In order of importance, the factors to be considered are as follows:

1. Interaxial dentin
2. Marginal ridge
3. Pulp chamber roof
4. Residual cusp dentinoenamel complex

The roof of the pulp chamber (whose absence indicates devitalization) is paradoxically much less important structurally than the dentin present between the cusps (interaxial dentin) and marginal ridges. Many authors41-44 have confirmed what initially seems a paradox: Teeth prepared for root canal access are much stronger than vital teeth with missing structural factors, such as the two marginal ridges and the interaxial dentin (MOD preparation). This concept is further confirmed by Howe and McKendry (1990), who state that “occlusal endodontic opening does not reduce resistance to fracture, which is significantly higher than that of a conservative MOD preparation.”45 These considerations are crucial to the endodontist, who is free to perform conventional access cavities without having to resort to pointless ultraconservative root canal openings that prevent proper cleaning and risk compromising the success of root canal treatment.

A 1992 study by Goel et al46 confirmed the importance of residual dentin, finding that the less dentin remains, the more stress forms immediately above or immediately below the cavity floor and that this can trigger a cusp fracture. In another study on the importance of interaxial dentin in relation to the marginal ridges, Larson et al claimed that extending an occlusal preparation with a relatively shallow Class 2 box preparation in the dentin does not significantly alter tooth strength.47

However, many of the studies and models referred to nowadays regarding fracture resistance following cavity preparation are somewhat dated and do not consider the use of bonding materials. Adhesive bonding can now add to a tooth’s structural integrity by reinforcing residual tooth structure.48,49 However, this alone cannot be relied on because adhesive bond quality decreases over time.50

Although a bonding material acts as a reinforcement for the intracoronal tooth structure, one study compared different types of reconstruction systems (ie, direct composite, amalgam, indirect glass-ceramic reinforced with leucite, CAD/CAM lithium disilicate, lithium silicate reinforced with zirconia CAD/CAM, ceramic-infiltrated resin CAD/CAM, and gold) for devitalized teeth with the loss of one or more marginal ridges. Researchers subjected them to simulated chewing and heat cycles and claimed that the best solution (in terms of fracture resistance) is still a full-coverage indirect cemented gold restoration.51

Returning to the subject of structural factors, Shahrbaf et al found that a marginal ridge of at least 2 mm in devitalized premolars restored in composite offered increased fracture resistance.52 The last structural factor to be considered is the residual cusp. This must be evaluated at the base of the cusp with the knowledge that at the cervical level, noncaries cervical lesions, caries lesions, or preexisting Class 5 restorations can reduce residual cusp thickness.53 When several structural factors are compromised, including the roof of the pulp chamber (ie, when dealing with a severely compromised devitalized tooth), the treatment of the residual structures must be carefully considered, with the practitioner often opting for total coverage of the occlusal surface (see Fig 5-27). A highly conservative approach in such cases can lead to radical consequences, as evidenced by the annual failure rate reported by many authors.51,54-57

As has been shown, when dealing with a devitalized tooth, one is faced with significant structural losses that often lead to the breakdown of compromised cusps. Multiple reconstructive strategies are available. Zarow et al58 classified restoration types based on the residual structure of a devitalized tooth. Direct restoration of a devitalized tooth based on this classification can be performed mainly in Class 0 (without the need for a fiber post) and in Classes I and II (fiber post needed, without and with surgical lengthening of the clinical crown). It is also important to consider which tooth is to be constructed, its position, and its relationship with adjacent or opposing teeth. The fracture risk of a tooth with both contact points intact is lower than that of a tooth that has only one adjacent element or—worse still—is isolated.59

According to Fichera et al,41 the cusps adjacent to a Class 2 box preparation can be maintained if they are at least 1.5 to 2.0 mm thick in a vital tooth (ie, if the pulp chamber roof is present). If the tooth is devitalized, cusps that are not at least 2.5 to 3.0 mm thick should be covered. Patient-centered factors must also be considered. Teeth must never be treated in isolation. They must be considered in the context of the patient as a whole. Parafunctions such as bruxism and grinding can cause premature wear of the restoration and fracture of the restored tooth. Residual structural factors must be evaluated by relating them to occlusal problems affecting the patient.

If dental substance is to be removed as a precaution, factors such as the material, the patient’s chewing pattern, and the nature of the opposing tooth will help determine the extent of removal. However, a certain amount of thickness is required to distribute masticatory stress uniformly.60

Choosing Between Direct and Indirect Composite Restorations

When we look at composite used for direct restoration or for indirect restoration, we can highlight some of the strengths of composite materials:

• Better mechanical properties. These are achieved using postpolymerization treatments that can be carried out only in the laboratory, where, the composite can be cured under a vacuum; in the presence of inert gases; and under light, heat, or a combination of all of these.61
• Greater wear resistance (for the reasons described).62-64
• Reduction in internal stress given increased polymerization (for the reasons described).65 In direct restorations, the material accumulates internal stress due to polymerization contraction. This is not the case for indirect restorations, in which such stresses are limited to the thin film of adhesive cement used for retention.
• Better repairability compared with other materials. Indirect composite restorations offer the indubitable advantage of easy repair. This is also possible in etched ceramic restorations but is a much more complex procedure.66

Clinical strategies

The question the clinician must ask is whether direct or indirect restorations work better. Various systematic literature reviews and meta-analyses have been conducted to establish whether indirect restorations last longer than direct restorations. At present, the literature reveals no significant differences in longevity, meaning that there are no particular indications for one over the other.67-69 No scientific evidence is available to suggest that composite differs significantly from other materials in this respect, although ceramic seems to fare slightly better.70

The above meta-analyses do not seem to show significant differences between direct and indirect restorations based on the tooth to be treated (molars or premolars). Clinicians therefore have a certain amount of decision-making freedom and can make their choices based on different criteria: knowledge, experience, patient type, patient’s chewing patterns, patient’s socioeconomic and employment status, and so on.

As shown in chapter 7, it is possible to restore an anatomy that is very faithful to the original if enough existing anatomical information can be interpolated with the practitioner’s knowledge of morphology. When this information is lost (minimal residual cusp, missing cusp, or cusp requiring coverage) nothing prevents a direct restoration, but the potential consequences are unpredictable restorations requiring numerous adjustments that detract from the anatomy and are very time-consuming.

If little of the occlusal perimeter is present, it will not be possible to interpret the gradient of the cusp slopes, and the occlusal surface will probably be over- or under-modeled as a result. If a cusp is missing or if it is decided to cover it following structural assessment, it will be impossible to determine:

• The mesiodistal, buccolingual, and apicocoronal position of the cusp tip to be restored
• The geometry (inclination, convexity, etc) of the occlusal and external cusp slopes constructed freehand

Indirect restoration provides clinicians with esthetic results, benefits in managing polymerization shrinkage, as well as an anatomy that considers all aspects of static and dynamic occlusion. In addition, it will generally require minimal adjustment (Fig 5-27). Under such conditions (eg, entire missing cusp) a very large amount of composite would be used for a direct restoration, with a consequent increase in internal residual stress and conversion that is not optimal and not comparable to that obtained in the laboratory. Based on all these points, the authors propose the following guidelines when deciding between a direct and indirect restoration.

Direct approach

• Minimally invasive cavities (Fig 5-28a)
  

  

• Class 1 cavities that retain peripheral anatomical information (Fig 5-28b)
• Multiple cavities that maintain structural requirements (Fig 5-28c)
• Small- to medium-sized Class 2 cavities (Figs 5-28d and 5-28e)
• Class 1or 2 cavities with palatal or buccal extensions (Fig 5-28f)
• MOD cavity with sufficient residual cusp thickness (Fig 5-28g)
• Large Class 2 cavities if there is sufficient residual cusp and if conditions are right for reliably reestablishing the contact area with the adjacent tooth (Figs 5-28h and 5-28i)

Direct or indirect approach (depending on circumstances)

• Medium-sized MOD: Often a lot of material must be inserted, and the residual cusps are not thick enough. Coverage of one or more cusps is indicated (Fig 5-28j).
• A Class 2 restoration with considerable buccal and lingual extension sometimes makes it difficult to achieve an effective contact surface. In such cases, an indirect restoration appears to be the best solution, even if the residual cusps adjacent to the box preparation have been assessed as structurally sound (Fig 5-28k).

Indirect approach

• A cusp is missing or is to be removed, and no information is available on how to reconstruct it (Fig 5-28l).
• The entire occlusal surface is missing, or it has been decided to remove it (Fig 5-28m).

When an indirect approach is advantageous

• The cavity size has minimized the residual anatomy (even though this reduction has left what seems to be a sufficiently thick cusp), and this prevents the practitioner from obtaining enough information to be able to interpolate the preexisting anatomy.
• A structural diagnosis led to a decision to remove one or more cusps with the same consequences as in the previous point.
• The Class 2 box is so big that it becomes difficult to reproduce the interproximal area (emergence and contact point) using a direct method.
• The cavity is so large that too much material must be placed even though residual anatomical information has been retained. With direct restorations, it is very difficult to effectively control shrinkage due to having to cure a large amount of composite within a time frame compatible with performing a direct restoration.
• For greater convenience: A tooth with a medium to large cavity that would normally be treated by means of direct restoration is located in a sector including other teeth being treated by means of indirect restoration.
• The restoration to be performed is medium to large and the operator has insufficient practical skill to model it effectively.

Conclusions

As already mentioned, the criteria used to decide between direct and indirect approaches set out in this chapter reflect the authors’ own opinions and experience. However, every clinical situation is different. In some instances, indirect restorations are performed even for simple occlusal cavities. In other cases, the patient’s socioeconomic circumstances make it difficult if not impossible to offer an indirect restoration to treat seriously compromised teeth.

Subgingival Margin Position

Contributed by Dr Roberto Kaitsas

Subgingival positioning of the caries lesion and its restoration may or may not involve supracrestal connective tissue or epithelial attachment (previously known as biologic width), a structure that is always present at natural teeth.71-73 The intrinsic relationship between the epithelial and connective tissue components of the restoration must always be considered, given the problems that may arise when this attachment is invaded.74-77 Two clinical situations can be distinguished based on the depth of the caries lesion, which determines the future margin: (1) the restoration can be performed without resective surgery, or (2) the situation requires resective bone surgery78,79 (Fig 5-29).

A restoration margin that does not invade the supracrestal connective tissue attachment can be performed without resective bone surgery. If the margin is supragingival or in the sulcus and can be easily isolated (nonsurgical margin exposure), a direct or indirect restoration will normally be performed. If isolation is precarious given the depth of the margin, especially for operational maneuvers that could move the rubber dam and cause contamination, the margin can be relocated for convenience (ie, deep margin elevation).80,81 This procedure involves applying restoration material to the margin to create another more coronal margin with the aid of circumferential matrices. After relocating the margin, a choice can be made between direct or indirect restoration.

If the restoration margin does not invade the supracrestal connective tissue attachment but cannot be isolated, the cervical margin can be temporarily exposed using surgical methods. After this, rubber dam is applied, the restoration (direct or indirect) is performed, and the flap is repositioned without any resective bone surgery (surgical margin exposure).82,83

The option of positioning the margin within the supracrestal epithelial attachment (junctional epithelium) must be carefully evaluated and limited to patients with exemplary maintenance habits. Any change in the balance of this portion can cause inflammation and loss of periodontal support. Sometimes, when surgically exposing the margin of an interproximal restoration, a decision is made to carry out simultaneous minor remodeling of the interproximal bone tissue. Although this may seem simple and convenient, it essentially amounts to surgical clinical crown lengthening and risks creating buccal and palatal/lingual bone overhangs if not combined with appropriate osteoplasty.84

Conversely, if the supracrestal connective tissue attachment is involved, inflammation will certainly result with consequent loss of periodontal support tissue; a clinical crown lengthening operation will therefore have to be performed with involvement of the underlying bone tissue (surgical crown lengthening).84-86 Another option outside the scope of this book is to use orthodontic extrusion to recover the tooth or extract the tooth if it cannot be restored.87

Direct Pulp Exposure and Direct Pulp Capping

Contributed by Dr Lucio Daniele

Maintaining pulp vitality is one of the main aims of conservative dentistry. Placing a dressing material on pulp that has been directly exposed to caries or trauma has always be considered a controversial procedure, and in the past, conventional endodontic therapy has often been recommended in such situations.88-92

Clinicians have used many materials and techniques for direct pulp capping, such as gold foil, zinc oxide-eugenol paste, hydrophilic resins, resin-modified glass-ionomer cement, lasers, and ozone technology to induce pulp defenses.93-98 Calcium hydroxide, once considered the gold standard for capping materials, is one option for the formation of reparative dentin, but long-term studies have shown variable and unpredictable results.99-102 The material does not adapt well to the surrounding dentin, does not promote odontoblastic differentiation, and has been found to be cytotoxic in cell cultures. More recently, clinicians have used mineral trioxide aggregate (MTA) and bioceramic materials.

A recent study of 70 patients with 3-year follow-up showed a success rate of 85% when MTA was used as a direct pulp capping material and 52% in cases where calcium hydroxide was used103; another study on 229 teeth with follow-up for up to 10 years showed a success rate of 80.5% in the group of teeth where MTA was used and 59% in the group where calcium hydroxide was used.104 According to Li et al,105 groups of teeth treated with MTA showed a significantly higher success rate than those treated with calcium hydroxide, with a lower inflammatory response by the pulp and formation of dentin bridges with more predictable durability. The advent of MTA changed everything. This material can withstand bacterial infiltration and provide effective protection for the pulp, allowing its repair and maintaining tooth vitality when used properly and in combination with appropriate crown restoration.106-111

As its name suggests, MTA is an aggregate of mineral trioxide. From a physical viewpoint, MTA is a powder made up of fine hydrophilic particles that harden in the presence of moisture. In chemical terms, MTA can mainly be divided into calcium oxide and calcium phosphate. Further physical and structural analysis demonstrated the coexistence of a crystalline phase (rich in calcium, silica, and oxygen) and an amorphous phase (rich in calcium and phosphates).108 The crystalline phase turns into calcium hydroxide when it comes into contact with the exudate, stimulating the formation of reparative hard tissue.111-113 The material functions successfully because of its small particle size, ability to create a marginal seal and fit, alkaline pH once set, and slow release of calcium ions.114

The researchers reported that MTA induces pulp cell proliferation and stimulates osteoblasts to release interleukins, with consequent formation of hard tissue and an interface with dentin that is very similar to hydroxyapatite in composition.109,114-117 The thickness and hardness of the resulting new dentin bridge are far superior to the results obtained using calcium hydroxide dressings.118 The material also sets much more quickly than calcium hydroxide. It is not resorbable, hardens in the presence of moisture, and has an alkaline pH that enhances its antibacterial properties, but it has low resistance to compression.108 Immediately after mixing, the pH is 10.2 and rises to 12.5 over the next 3 hours, stabilizing at this value over the first few days and then decreasing slightly with time.108,109 The setting time of MTA at a temperature of 37°C with relative humidity of 95% to 100% is 2 hours and 45 minutes; compression resistance after 24 hours is 40 MPa but increases to 67.3 MPa 21 days after mixing.109

An observational study conducted by Bogen et al in 2008 reported a success rate of 97.96% after 9 years on 49 teeth in a total of 40 patients aged between 7 and 45 who had undergone direct pulp capping using MTA.119 The clinical study conducted by Daniele in 2017 showed a 10-year success rate of 92.5% in 80 cases of direct capping with MTA.120 In this study on 77 patients aged between 14 and 68, direct pulp capping with MTA was carried out on 80 teeth affected by caries in which reversible pulpitis had been diagnosed by a cold thermal test and radiographic examination. Four patients experienced painful symptoms. In three teeth, a small area of bone rarefaction due to enlargement of the periodontal space was observed on the radiograph. The caries was removed using only rotary instruments, and a thin layer of MTA was applied to the exposed pulp and surrounding dentin. In some cases, a 5% sodium hypochlorite solution was used to achieve hemostasis of the operating field. The teeth were reconstructed during the second session with two-component bonding systems and new-generation composite resins after checking the material had set and evaluating pulp vitality by means of a cold thermal test. The patients were recalled at regular intervals for 10 years to evaluate pulp status, potential formation of a reparative dentin layer, presence or absence of pulp and canal calcifications or root resorption, presence or absence of pain on percussion, and presence or absence of an endodontic lesion visible on a radiograph. After a 10-year observation period, 6 teeth out of 80 had undergone endodontic treatment; no increase was observed in pulp calcifications, presence of root resorption, or endodontic lesions visible on a radiograph. All the remaining vital teeth responded positively to a cold thermal test. The three lesions present had disappeared by the last radiographic check. All four symptomatic teeth retained their pulp vitality.

More recently, numerous calcium silicate-based cements known as bioceramics have appeared on the market. These crown and root dentin substitutes are classified as a new class of Portland cement with high mechanical properties, excellent workability, radiopacity, and a much longer setting time.121,122 Studies by Gandolfi et al confirm that calcium silicate-based cement is bio-interactive (ion-releasing), bioactive (apatite-forming), and functional.123-127 The high calcium release rate and rapid apatite formation easily explains the formation of a new dentin bridge, which constitutes an effective scaffolding for clinical healing. In a recent study examining 716 articles and 83 patents, a significant increase was observed in patents for bioactive materials (containing bioactive proteins), MTA-derived materials (calcium silicate-, calcium phosphate-, and calcium aluminate-based cements), and MTA.128 The study confirmed that MTA and bioceramic materials could be successfully used to treat vital pulp and that their benefits outweigh the disadvantages of materials used in the past.

The great advantage of using calcium silicate-based bioceramics is their low curing time of approximately 12 minutes. When these materials are used for direct pulp capping, the treated tooth can be reconstructed in the same session. This is financially advantageous because everything can be completed in one appointment. It is also operationally advantageous because it prevents possible contamination of the treated tooth if the temporary filling falls out or fails to seal properly. If the exposure occurs on the axial wall of a cavity, it is more difficult to manage and apply the material. Great care must be taken, and it may be necessary to use special application syringes.129 If the pulp exposure is mechanical (as with a tooth fracture), bacterial contamination is absent, and the long-term success of direct capping approaches 100%.118

Significant bacterial contamination is sometimes present in the case of pulp fracture due to caries. Sodium hydroxide is a pulp disinfectant that can achieve satisfactory hemostasis if bleeding occurs during caries cleaning procedures.129,130 The exposed pulp often begins to bleed when touched by rotary or manual instruments. Bleeding can stop spontaneously after a few minutes, and once the necessary time has elapsed, the field will be ideal for direct pulp capping. If bleeding does not stop after a few minutes, satisfactory hemostasis must be obtained by placing a cotton pellet soaked in 5% sodium hypochlorite and maintaining it in contact with the pulp from a time period ranging from 30 seconds to 1 minute. This procedure can be repeated twice if necessary. If the exposed pulp continues to bleed after this procedure, endodontic therapy is required because the pulp hyperemia gives rise to internal inflammation, and irreversible pulpitis will presumably result.

Direct pulp capping is more successful in young patients,131 but the worst results occur only after the age of 60.132 The age factor therefore no longer affects the outcome of vital pulp therapy, and nowadays direct pulp capping can even be performed in elderly patients.

The step-by-step procedures for direct pulp capping are outlined in Box 5-1. This 10-point operating protocol must be rigorously implemented, including careful case selection, to ensure effective treatment.

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