Treatment management of extensive defects of vital first permanent molars in children

2.1 Composite resin restorations

The composite resin restoration is currently the commonly used method for tooth defects [18]. It has excellent aesthetic and mechanical properties, ease of handling, and it preserves more of the dental tissue [19]. With the development of dental materials, the indications for resin restoration have become increasingly popular. Guidelines published by the Academy of Operative Dentistry Europe Section (AODES) in 2014 stated that composite resin is the preferred material for posterior tooth restoration, despite the fact that resin restoration should be carefully selected in cases of poor development of dentin and enamel [20]. For small-scale dental defects, direct resin restoration is always the preferred treatment option.

The ideal composite resin direct restoration method requires a standardized operation, which poses high requirements for both dentists and patients [21]. Currently, most resin fillers require layered solidification due to polymerization shrinkage and the influence of light depth [22]. When there is a large area of tooth defects, the appearance and shape of the restoration is more time-consuming, which can implicate in the final result because young patients have limited cooperation. Insufficient eruption of the FPMs in pediatric patients often results in the gingival marginal and extensive defects being located beneath the gingiva, making it difficult or even impossible to install a rubber dam required for the restoration process. In addition, for extensive defects FPMs it is difficult to remodel good occlusal morphology and interproximal contact with resin restoration [21].

Several studies have shown that restoration with composite resin is a predictable option [23, 24, 25]. However when dealing with teeth affected by developmental defects like MIH, structure, chemical and mechanical properties of enamel should be considered [6]. Due to poor adhesion of remaining enamel and reduced bond strength, it is suggested that all of the hypomineralised enamel is removed for long-term success [6]. Generally, the success rate of direct restoration of posterior teeth is high. However, attention should be paid to potential failures mainly caused by secondary caries and fractures within the restoration area [26, 27]. The polymerization shrinkage of composite resins during the curing process is an inherent problem of the composite material. Long-term shrinkage leads to complications such as postoperative sensitivity, secondary caries and delamination of the bonding interface [18]. To improve the composition of composite resins, addition of inorganic particles could control the polymerization shrinkage [28]. Furthermore, some study has attempted to introduce bioactive particles into inorganic fillers to provide exogenous mineralization ion sources. These approach helps to compensate for early demineralization and caries development, and it induces remineralization at the bonding interface [29].

The more dental structure remaining, the better the long-term prognosis of teeth Study has shown that as the amount of missing tooth tissue walls increases, the 5-year failure rate of resin restorations gradually rise [30]. The enlargement of the defects frequently leads to problems such as thin tooth tissue walls and fragile tooth cusps. During the resin restoration process, polymerization shrinkage may cause deformation of the weak tooth tissue wall, and the lateral occlusal forces intensify the deformation and leads to tooth fracture [31]. Currently, there is still insufficient evidence regarding what constitutes a safe minimum thickness for cavity walls [31]. A current study shows a minimum thickness of more than 2 mm at the bottom of the remaining tooth cusp enables a more effective direct restoration [32]. When the wall of the cavity is less than 1 mm, it is recommended to restore indirectly with a cusp coverage approach [31]. Generally, researchers recommend that when the width of the defects is more than 2/3 of the distance between the buccolingual cusps, or when the remaining tooth tissue is less than 1/2 of its original size, direct restoration is difficult to provide sufficient coronal protection, and indirect restoration is a more appropriate choice [33, 34]

2.2 Glass ionomer cement

Glass ionomer cement (GIC) is another type of direct restoration material, that produces direct chemical bonding and is easy to use. The unique advantage of GIC lies in its sustained fluoride release ability and its ability to act as a fluoride reservoir [35]. Fluoride can not only disturb the formation of plaque biofilm but also promote the remineralization when the tissue is demineralized [36]. Studies have shown that GIC can reduce mineral loss by at least 80% within 0.22 mm of adjacent dental tissue [37]. In addition, as a water-based cement, the wet tooth surface at the initial stage of curing is conducive to its bonding [38]. Therefore, the requirements for moisture isolation are not as strict as those for resin restoration. An important consideration in pediatric dentistry is the cooperation of children during dental procedures [7]. Therefore, the above characteristics of GIC also make it widely used for pediatric patients [39].

Some studies have indicated that in teeth affected byMIH, dentin is highly sensitive, the crowns often exhibit extensive demineralization, and the bonding with resin during restoration is unsatisfactory. In such cases, GIC has shown better results [40, 41]. Furthermore, due to the poor cooperation and inadequate moisture control in the case of children, the effect of resin restoration is unsatisfactory. Therefore, GIC could be used as an interim restoration to be replaced by resin-based restoration when conditions are suitable.

Considering the insufficient pressure and abrasion resistance, many improvements have been made in the composition of traditional GIC. In a new generation of GICs, one approach involves incorporating silver alloy particles into ionomer glass portions and fusing silver powder particles with glass to create a metal-modified glass ionomer cement [42]. Furthermore, incorporating zirconia ceramic particles achieves greater compressive and flexure strengths and provides reduced occlusal wear along with fast setting reaction times, which make this approach a good option for enamel restoration [43]. Resin-modified glass-ionomers (RMGIs) have also been introduced. These contain hydrophilic monomers and polymers such as hydroxyethyl methacrylate (HEMA) [44]. A recent study suggests that RMGI typically has better mechanical properties compared to traditional GIC [44]. Although significant improvements have been achieved in GIC, evidence is still lacking on whether it can maintain long-term stability in the restoration of extensive PFMs.

3.1 Onlay restorations

Onlays are restorations that cover at least one cusp of a teeth. They can restore the shape of tooth and protect the remaining tooth tissue [50]. The edge line of the onlays is more complex than that of crowns. Therefore, obtaining accurate impressions in children with silastic or polyether rubber is difficult due to the gag reflex associated with this procedure [51, 52]. Therefore, the use of onlays in pediatric patients is not very common. In recent years, due to the increasing popularity of computer-aided design/computer-aided manufacturing (CAD/CAM) and other technologies including portable intraoral scanning equipment, dentition models in children can be obtained more quickly and accurately. Furthermore, owing to the development of adhesives and resin-based ceramics as well as other materials, the preparation of onlays that are accurately adjusted to the cavity shape, and the edge line are also gradually being facilitated. The emergence of new materials including the resin-based ceramics that closely mimic the elastic modulus of dental tissue, partially offsets previous concerns about the material’s mechanical properties. At present, onlays are widely used in the restoration of large-area defects molars. In addition, medical indications that can be addressed with onlays are even more frequently seen in dentistry than indications for full crowns onlay [53, 54].

Onlay restoration in dentistry has the following advantages: (1) Covering the tooth cusps to restore the tooth morphology, changing the tensile stress to compressive stress, and reducing the risk of tooth fracture; (2) The tooth preparation for onlays involves about 34%–39% of the tooth, while that for crowns is about 67.5%–75.6% [55]; (3) The integrity of the tooth cervical can be preserved without grinding the tooth tissue on the axial surface, destroying the adjacent surface of the non-defect side and the contact point; (4) The marginal line is generally located above the gingiva, which is conducive to the health of periodontal tissue; (5)The fracture mode of onlays, in cases where this occurs, is more favorable for future repair in the specific area than the fracture mode of crowns. Most fractures do not involve the root of the tooth. If the tooth were fractured, full crowns, post-core crowns, and other restorations can be performed according to the situation. Two studies have also confirmed the excellent restoration effect of onlays in children [13, 56].

In recent years, advancements in bonding technology and materials have facilitated the increasingly widespread application of onlays in dental restoration. Because current onlay restorations mainly rely on bonding and retention, it is critical that there is enough intact tooth enamel bonding area to ensure a stable and functional long-term outcome. Research has shown that reliable bonding effect could be achieved only when the remaining amount of enamel exceeds 50%, the enamel bonding surface accounts for more than 50% of the total bonding area, and the enamel bonding edge line accounts for more than 70% of the total bonding edge line, reliable bonding effect could be achieved [57, 58]. Therefore, for molars with defects exceeding 50% of the tooth, or when the enamel is extensively damaged and the bonding area is insufficient for durable onlay integrity, restoration with crowns is the choice of treatment to ensure improved long-term outcomes.

Currently, there is a lack of relevant scientific evidence regarding the minimum thickness of tooth walls. This creates some uncertainty in the preservation of tooth tissue and the removal of cavities. Additionally, various parameters can influence the decision-making process for the dentist. It is generally agreed that for large-area defects in thin-walled cavities, indirect restoration should be preferred over direct restoration [31].

3.2 Conventional preparation crown restoration

Crowns are the most commonly used method to repair large area tooth defects, as crowns have a good protective effect on weak tooth tissue. Numerous studies have confirmed that full crowns provide lasting protective effects on posterior teeth with large area defects [59]. Some scholars have performed crown restorations on the first molars of children aged 6 to 8 years. In cases involving enamel developmental defects, the results after 2 to 5 years of follow-up have been very satisfactory [60]. For vital pulp molars with large area defects, the study also confirmed that the overall success rate of full crown restorations over a period of 5 years was much higher than that of direct restorations when less than two complete tooth tissue walls were retained [30].

It is generally believed that the occluso-cervical height of abutment teeth is the most important factor that affects the retention of full crowns. When occluso-cervical height is less than 3 mm [61], it cannot provide an effective retention form, and the restoration is more prone to fall off. Therefore, the gingival attachment level of the first molar directly affects the repair space [62]. However, in clinical dental practice, the crown of the FPMs in children is relatively short, and the pulp cavity cannot be used for retention. These facts limit the application of full crown restorations in pediatric dentistry. Nowadays, the concept of restoration increasingly emphasizes the preservation of natural tooth tissue. The necessity of more extensive abutment preparations and the incurred pericervical dentin damage in crown restoration procedures, are inconsistent with current development trends [63]. Therefore, for pediatric patients, conventional preparation crown restorations should be chosen cautiously.

3.3 Indirect restoration materials

At present, the commonly used ceramics in dental restoration are divided into three categories: polycrystalline ceramics, such as alumina ceramics and zirconia ceramics; Glass-matrix ceramics, such as feldspar ceramics, lithium disilicate ceramics and resin-matrix ceramics. Nowadays, the most widely used polycrystalline ceramics in dental practice are zirconium-dioxide ceramics while alumina-based restorations are gradually phased out [64]. The high resistance to fracturing of zirconium dioxide allows for the deployment of crowns with a minimal thicknesses of approximately 0.5 mm, thereby contributing to the biological benefit arising from the preservation of dental matter [65]. However, despite the high popularity in restorative dentistry, sensitive procedures of bonding and concerns about wear in antagonist teeth limit the use of zirconium dioxide crowns in pediatric dentistry [66, 67].

For indirect restoration materials, glass-matrix ceramics are usually preferred, as they were thought to be stronger and more reliable than their composite counterparts [31]. The elastic modulus of glass-ceramic is close to that of natural tooth enamel, with good aesthetic performance, stable long-term functionality and good bonding properties with tooth enamel. Therefore, glass-matrix ceramics are an ideal choice when the enamel bonding area is sufficient and children tolerate the procedures involved well. It should be noted that because the elastic modulus of glass-matrix ceramics is 3–5 times that of natural dentin, the inner part of the restoration in the coronal segment may apply an adverse wedge forces on the tooth tissue and can cause fracture of the tooth tissue [68].

However, with the development of resin-matrix ceramics in recent years, the physical and chemical properties of these materials have been increasingly improved. Resin-matrix ceramics [69] are ceramic materials with inorganic ceramic materials as the main component and resin matrix as the auxiliary component. Study has shown that the bending strength of resin-matrix ceramics can reach 100–200 MPa, and its elastic modulus is close to that of natural dentin [70]. Experiments have shown that 80% of failure modes of resin-matrix ceramic onlays are restorable, while most of the failure modes of glass-matrix ceramic onlays are accompanied by fracture of abutment teeth, and the failure mode is unrepairable [71, 72]. This is related to the fact that the elastic modulus of resin-matrix ceramics is close to that of natural dentin, which provides greater resilience and enables more stress absorption by deformation and produces well-balanced stress distribution [73]. Contrary to the mechanical failure of ceramic restorations, the main failure of composite restorations is related to biological complications. These include secondary caries formation and pulp complications [53]. Therefore, in our opinion, for children, resin-matrix ceramic restoration can well protect the thin walls and weak cusps of developing teeth, repair and maintain good oral and maxillofacial system function, and create favorable conditions for subsequent restoration if that becomes necessary. Thus, resin-matrix ceramic restoration constitutes a priority in indirect tooth restorations for pediatric patients.

4.1 Preformed metal crowns

The restoration of teeth with prefabricated metal crowns (PMCs) uses biocompatible bonding materials to bond a single prefabricated metal crowns to the teeth through certain shape adjustment [74, 75]. Humphrey first reported the application of PMCs in pediatric dentistry in the 1950s, and subsequently, Mink promoted their use for repairing large area defects in primary and permanent molars [76]. Because of their excellent performance, PMCs are considered to be the best method to repair decayed primary molars in caries-prone children [77, 78] (whether by using the Hall technique or conventional preparation techniques). Meanwhile, in cases of severely damaged FPMs in children, PMCs are a successful interim restorative option until a permanent restoration, usually of the full coverage type, can be placed later.

For molars with large area defects, the protection of residual tooth tissue should be considered in the design of the restoration scheme. PMC restorations can not only meet the requirements of tooth protection, but are also relatively easy to apply, and the costs are significantly lower than onlays or full crown restorations, making PMCs a widely accepted option. Together, PMCs can restore tooth shape and function, and compared to traditional crown restoration, PMCs enable better preservation of original tooth tissue, which conforms to the principle of tooth preservation. PMC restorations are recommended in the following cases [14, 75]: (1) Extensive caries or multi-surfaces caries where direct restoration cannot ensure long-term stability; (2) Developmental defects (e.g., MIH, DI, AI), and or when failure of other available restorative materials is likely; (3) When transitional treatment such as apexification and revascularization is necessary; and (4). For intermediate restoration of fractured teeth.

However, there are also some deficiencies associated with the restoration of permanent molars with PMCs [25, 77, 79]. (1) The PMCs may become worn and perforated after a service life of about 2–5 years, and the require replacement; (2) Periodontal damage may occur when the crowns and cervical edges are poorly trimmed and polished; (3) Poor aesthetic effects, and allergic responses in some patients who do not tolerate Ni Cr alloy. Therefore, after the complete eruption of permanent teeth and stable occlusion, PMCs should be replaced with other more appropriate restoration methods.

4.2 Hall techniques (HT)

Although PMCs have shown predictive positive results in repairing FPMs, it is still impossible to avoid the use of dental burs and injections, which will cause discomfort in children [75]. The HT is a minimally invasive dental procedure that utilizes PMCs and glass ionomer cement to restore damaged primary molars [80, 81]. Moreover, HT not only significantly reduces the time spent in the dental chair, but also minimizes the level of anxiety associated with dental procedures [82]. Considering the above advantages, a trial has attempted to use HT to repair the first molars in children with extensive defects, and the outcome remains to be observed [83].

4.3 Preformed zirconia crowns

Considering the shortcomings of the PMCs mentioned above, two studies investigated to repair the FPMs by applying preformed zirconia crowns (PZCs) [84, 85]. Recently developed PZCs are a new treatment option that enables practitioners to provide excellent aesthetic effects, but mainly in primary dentition at first [86]. It has been reported that primary teeth restored with zirconia crowns may have a higher success rate and maintain better gingival health than PMCs [87].

Compared to a general restoration procedure which requires two treatment sessions, the main advantage of preformed crowns is to complete the restoration in one treatment process, without the need for impressions and manufacturing of prostheses. These are very important considerations for some patients with special needs, including the FPMs restoration under general anesthesia. Recently, a study compared the clinical and radiographic performance of PMCs and PZCs in restored FPMs. The result showed that PZCs could achieve the same or even better results than PMCs in relation to plaque accumulation and marginal integrity [85]. However, the time required for preparation and placement of the PZCs is longer than that of PMCs, which imposes higher requirements for dentists’ operations [84].