As already explained in detail in the previous chapters, hypomineralizations can differ enormously in their appearance and form. The spectrum ranges from small whitish opacities to severe enamel breakdowns.1 The need for restorative therapy increases with the occurence of enamel breakdowns. Small and medium-sized defects can be restored with a direct restoration. For larger enamel breakdowns with deep cervical cavities and eventually required cusp build-up, the spectrum of therapeutic approaches can be extended to indirect restorative options² (Fig 13-1).

Fig 13-1 Long-term restorative options within the therapy plan of the Würzburg concept (after Bekes et al²; courtesy of DGKiZ). a) Low caries risk. b) High caries risk.

Direct composite restorations enable a purely defect-oriented restorative therapy without having to remove the remaining sound tooth structure by preparation procedures. In addition, the restoration can usually be performed in one appointment and at a relatively low cost. However, direct restorations have the disadvantage of polymerization shrinkage and show lower abrasion resistance.³

Moreover, direct composite restorations have clinical limitations, particularly in the posterior region, especially in the case of multi-surface defects. Although direct multi-surface, occlusion-supporting restorations are also possible in principle, the probability of partial or complete loss and thus restoration failure becomes more likely with increasing size. In a recent meta-analysis, it was shown that each additional surface included in a restoration increases the risk of failure by 30% to 40%.⁴

In situations with an (almost) complete inclusion of the occlusal surface, indirect restorations can represent a suitable therapy alternative.⁵ The spectrum ranges from simple prefabricated steel crowns⁶ as an interim solution to analogue, laboratory-fabricated restorations made of various materials⁷ or CAD/CAM fabrication techniques.⁸ However, long-term studies on their effectiveness are lacking.

The option to be favored depends on other parameters besides the tooth-related factors: the patient’s ability to cooperate; the complexity of treatment; and the treatment modality (outpatient or ITN).

This chapter presents various indirect therapy options, highlights the indications for the different types of restoration, explains the procedure step-by-step, and discusses the advantages and disadvantages of the respective type of restoration in a comprehensive and differentiated manner.

13.1 Prefabricated crowns as an interim solution

13.1.1 Stainless steel crowns

In pediatric dentistry, the use of prefabricated steel crowns was first described in the 1950s.^9,10 These are factory-made preformed crowns, which in modern times are made of an alloy of iron, chromium, nickel, and a small percentage of magnesium, silicon, phosphorus, and sulfur. Steel crowns are available in various sizes. The main field of application of steel crowns is usually the primary dentition.

Indications

Prefabricated steel crowns are a useful medium-term and effective interim solution for molars with extensive defects, especially in the case of significant cusp involvement and subgingival preparation margins.^7,11,12 They protect the hypomineralized tooth from further posteruptive enamel breakdown and caries. Furthermore, they can stop any hypersensitivity that may occur.⁷ The repeated treatments frequently observed with direct restorations due to fractures on the margins are eliminated with this type of restoration.¹³

The term “interim” refers to a period of time ranging from a few months to a decade and longer.¹¹ The cost-effective solution provides all parties involved with the necessary time to protect the tooth until the ideal restoration time for a laboratory-fabricated permanent restoration. In addition, the restoration offers the option of preserving a molar with a poor prognosis until the ideal extraction time – which is usually at a later time point – is reached. A potential disadvantage is the need for preparation, which may result in the loss of healthy/sound tooth structure.

Special features

The special anatomy of primary teeth makes prefabricated crowns a widely used treatment option in this dentition, especially after endodontics, in the presence of extensive multi-surface defects, in cases of high caries activity (to protect against new caries), in patients with low compliance or in cases of mineralization disorders. Primary molars show pronounced buccal and oral enamel ridges, with the buccal ridge located further apically than the oral ridge. The natural crown converges strongly from the tooth equator toward the occlusal surface, which is itself very small.¹⁴ The proximal contacts are more flat. This particular crown shape of the dentes decidui in the molar region allows good retention and easy adaptation of the prefabricated steel crown. The crown has an elastic margins and can therefore “snap” over the enamel ridge, providing an acceptable marginal seal.

Although primary and permanent molars have a similar position and function within the dentition, they differ in terms of size and shape (Fig 13-2). A permanent molar is larger than a primary molar in all dimensions. It has a broad, multicusped occlusal surface. The cusps themselves are higher and more pronounced than in the primary dentition. An important difference with regard to the restoration with a fabricated crown is the absence of a cervical enamel ridge.

Fig 13-2 Comparison of a primary molar and a permanent molar.

Preparation

The restoration with a prefabricated steel crown is an interim solution. As in the treatment in the primary dentition, a simple preparation technique is used, which consists of the following steps: occlusal reduction, proximal separation, and rounding of the edges (Fig 13-3). These steps may result in the partially unavoidable removal of non-hypomineralized tooth tissue.

Fig 13-3 Preparation steps for a stainless steel crown in a permanent molar (source: Cornelia Jungwirth, Katrin Bekes). a) Step-by-step procedure: occlusal reduction, separation, and rounding of the edges. b) The proximal space is prepared without any steps (tangential preparation) to prevent the crown from sitting on top.

Step 1: Occlusal reduction

After caries excavation (if necessary), the occlusal surface is reduced by approx. 1.5 to 2 mm – if it is still present. This creates more favorable conditions for the subsequent proximal preparation.¹¹

Step 2: Proximal separation

A conical diamond is suitable for proximal separation. It is important to avoid excessive conicity of the preparation in order to preserve as much tooth structure as possible.

The preparation margin is just below the level of the free gingiva. No shoulder preparation is carried out, as otherwise there is a risk that the steel crown will sit on the proximal step.

Proximal space can also be created by placing orthodontic separators. These can be inserted 3 to 5 days before the appointment for preparation.^7,15 This loosens the proximal contact with the neigboring teeth.

If the second permanent molar has not yet erupted, it is important to ensure that there is sufficient space distally for its eruption, otherwise there is a risk of impaction.

Omitting a preparation in the buccal and lingual region offers the option of selecting not only a full crown but also an onlay as the subsequent definitive restoration.

Step 3: Rounding of the edges

Finally, all edges are rounded: proximal-buccal, proximal-lingual, and proximal-occlusal. If these areas have sharp edges, the fit of the crown may be impeded.

For crown selection, the mesiodistal dimension should be measured at the proximal contact points before preparation. Alternatively, if practicable, preoperative study models could be used to obtain details of crown width and height in advance.

Clinical case 1

Figure 13-4 shows the restoration of the right hypomineralized mandibular molar in a 9-year-old girl. The patient has congenitally missing mandibular second premolars, so preserving the first molars was the goal. The restoration was chosen to protect against further posteruptive enamel breakdown and to treat the hypersensitivity. Tooth 36 had already been treated with a stainless steel crown in the previous appointment.

Fig 13-4 A 9-year-old girl with MIH of teeth 36 and 46 and aplasia of the second premolars. a) Tooth 36 has already been restored with a prefabricated steel crown. Initial situation before restoration of tooth 46. b) Application of a steel crown on tooth 46 to protect against further enamel breakdown.

Special features

Various scenarios are likely when restoring an MIH affected molar with a stainless steel crown. For example, the application of a crown may be necessary at a relatively early eruption stage. In order to do justice to these special features (not only in MIH, but also in other structural anomalies in the permanent dentition), the stainless steel crowns fabricated for permanent teeth are not pre-contoured. This allows a variation of the crown length. By trimming with crown scissors, bending with pliers, and final finishing, the edge can be precisely adjusted (Fig 13-5).

Fig 13-5 Illustration of the possibility of shortening in order to place a stainless steel crown even in the case of a molar that has not yet completely erupted (source: Cornelia Jungwirth, Katrin Bekes).

Study situation

Clinical studies focusing on the use of stainless steel crowns in MIH-affected molars are scarce and currently limited to short-term studies.

Zagdwon et al12 compared the longevity of stainless steel crowns and nickel-chromium alloy cast crowns in the restoration of permanent first molars with amelogenesis imperfecta, severe enamel hypoplasia, or MIH. In 17 adolescents, 42 restorations (19 steel crowns, 23 NiCr crowns) were placed and followed up to 17 months. One of the stainless steel crowns placed was considered a failure after 2 years.

Kotsanos et al¹³ retrospectively investigated the management of MIH molars in a pediatric dental practice. In total, 136 interventions (sealants, fillings, SSCs) were placed in 133 molars. The authors reported that no replacement was needed for prefabricated crowns placed in 24 molars over a period of 3 to 5 years.

13.2 Tooth-colored zirconia crowns

For the primary dentition, prefabricated crowns made of zirconium oxide have been offered for many years by various companies for both the anterior and posterior regions. They are characterized by excellent esthetics and durability.^16–18 For some time now, they have also been manufactured for permanent molars. In contrast to stainless steel crowns, however, they require more extensive preparation, as they must fit passively on the tooth to be restored,¹⁶ otherwise there is a risk of fracture. It is not possible to adapt the crown using pliers. In addition, zirconium crowns per se have somewhat thicker walls than stainless steel crowns, which is also reflected in the preparation and substance removal.^19,20

The zirconia crown does not hold via the snap-on effect like the stainless steel crown, but via the adhesiveness of the selected cement.¹⁶ The cement layer, in turn, is susceptible to moisture and possible sulcus bleeding. However, such bleeding often cannot be prevented due to the more invasive preparation required.

Esthetic crowns currently available on the market are more cost-intensive than steel crowns.

Clinical case 2

Figure 13-6 shows the treatment of a hypomineralized mandibular first permanent molar in a 10-year-old girl with a prefabricated zirconia crown. The patient reported that the tooth previously showed a metal-colored restoration. It can be assumed that this was a stainless steel crown.

Fig 13-6

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