Zirconia Prefabricated Crowns for Pediatric Patients With Primary Dentition: Technique and Cementation for Esthetic Outcomes

Frequently, when reviewing treatment options for restoration of carious anterior primary teeth, the recommendation is extraction. Undoubtedly, given an anterior tooth that is close to the dental age of exfoliation, this may rightly be the treatment of choice. However, for a child who has significant time until exfoliation of a primary tooth and subsequent eruption of its permanent successor, extraction may be a functional insult for speech and mastication, as well as an esthetic affront.^2,3 In addition, an extraction may make a young child feel self-conscious, which can be detrimental to overall well-being.

Esthetics is perhaps a result of today’s societal expectations. Nonetheless, the demand for esthetic restorations is evident. A study by Peretz and Ram⁴ described the increasing desire by both parents and children for tooth-colored restorations. Another study by Zimmerman et al⁵ documented parental attitudes on restorative materials, with esthetics ranking high. Clearly, the demand for esthetic dental restorations exists, and reliable techniques and biomaterials are available to practitioners to meet this need.

Crowns

The use of preformed esthetic primary restorations began in earnest in the early 1990s with the advent of pre-veneered stainless-steel crowns. Such a restoration is essentially a stainless-steel crown coated with an esthetic, tooth-colored facing. This type of restoration has served children and the dental profession well in restoring carious teeth both functionally and esthetically. However, challenges have accompanied the use of these crowns. One such issue includes restrictions on autoclaving, as autoclaving a pre-veneered stainless-steel crown may lead to discoloration and weakening of the crown–veneer interface. Additionally and significantly, these crowns have the risk for possible fracture and sometimes the loss of the esthetic facing,⁶ especially for posterior pre-veneered crowns on which biting forces are greater and occlusal wear is more common.

In recent years, dentistry has seen the emergence of esthetic full-coverage restorative alternatives for the primary dentition. Cost-effective prefabricated zirconia crowns have become available for restoring not only function but also form^7,8 for all primary teeth, molars, cuspids, and incisors. These crowns are available in North America through manufacturers such as Cheng Crowns (www.chengcrowns.com), EZ Pedo (www.ezpedo.com), Kinder Krowns® (www.kinderkrowns.com), and NuSmile® (www.nusmilecrowns.com). Pediatric prefabricated zirconia crowns are made of yttrium-stabilized zirconium and are either milled or injection molded. Zirconia offers many benefits, including far greater flexural strength than that of a natural tooth,⁹ wear at a similar rate to a natural tooth (in the case of NuSmile), full coverage, autoclavability, excellent fracture resistance,¹⁰ and a superior esthetics.

In vitro fracture load studies by Townsend et al¹⁰ showed variances in crown thickness and fracturability between crowns from 3 of the aforementioned manufacturers. Although the differences were found to be statistically significant, all required between 5 and 10 times the amount of force to cause fracture than the mean maximum biting force in the molar areas of children aged 10 to 12 years.

With the arrival of zirconia crowns to pediatric dentistry, clinicians must understand preparation technique and cementation protocols. Three key factors in pediatric dentistry make cementing crowns a unique challenge. First, a dry field is extremely difficult to maintain. Second, zirconia crowns must fit passively, as opposed to stainless-steel crowns, which have a more retentive fit; thus, the cement of choice must be as retentive as possible. Third, when preparing teeth to fit these prefabricated crowns, clinicians may encounter varying degrees of thickness; therefore, the cement’s properties must be able to handle this variance.

The following section briefly discusses available cements¹¹ and specifically reviews their properties for use with zirconia.¹²

Cements

Pure glass-ionomer (GI) cements—These cements bond ionically to tooth structure but provide no bond to zirconia. As a result, unless significant internal retention has developed on the interior of the crown, these cements are not recommended universally for use with zirconia. GI cement manufacturers note that, after test fitting and prior to cementation, saliva contamination must be removed from the intaglio surface of the crown. In addition, pure GI cements will “wash out” where an open margin is present, which will always be the case with a preformed zirconia crown.

Resin-modified glass-ionomer (RMGI) cements—RMGI cements provide ionic bond to tooth structure with the advantage of resin added, which provides a 2-fold benefit. The resin not only helps to prevent the dissolution of the glass ionomer from the interior of the crown, but also bonds to the receptors in the interior of the zirconia crowns. As with GI cements, saliva contamination must be removed before cementing with resin or RMGI cements.

Calcium aluminate cements—This unique category of cements provides an ionic bond to tooth structure and a mechanical bond to the zirconia via the deposition of katoite crystals and a gibbsite gel matrix. The cement is hydrophilic, biocompatible, and moisture tolerant and shows evidence of forming hydroxyapatite.^13,14 With excess calcium in the formulation, it also releases calcium during and after cementation. Calcium aluminate cement sets to a basic end pH level, which is, of course, a strong antibacterial. It is supplied in capsules, which must be activated and triturated. The only calcium aluminate cement marketed today is available exclusively as Ceramir® (Doxa Dental, www.ceramirus.com).¹⁵

Bioresponsive ionic resin cements—These are the newest in the unique category of resin-modified glass-ionomer cements and offer multiple advantageous properties. Bioresponsive ionic resin cement is biocompatible and hydrophilic. After bonding to the tooth, it remains flexible because of a rubberized urethane component and is able to absorb shock, thus reducing the risk for failure. Hydroxyapatite forms at the cement–tooth interface via a reaction between calcium and phosphates in the cement’s formulation. Bioresponsive ionic resin cement will allow for ion exchange of its calcium and phosphate as well as fluoride exchange. It is unique in its ability to offer 3 methods of cure: a self-cure and photocure resin component, and a self-cure bioresponsive ionomer component. Exclusively available as BioCem® (NuSmile), it is supplied in a double-barreled syringe with automix tips.

Cementation Process

Cementation of zirconia requires the clinician to have some background knowledge regarding how cements adhere to this material. The mechanism is via phosphate receptors on the zirconia that bond to the phosphates in cement. Blood and saliva also contain phosphates, and if the phosphates in blood and saliva come into contact with the zirconia prior to cementation, then the phosphate receptors on the zirconia are “taken up” by the blood and saliva. In the case of contamination of the zirconia crown, a cleaning paste product such as Ivoclean (Ivoclar Vivadent, www.ivoclarvivadent.com) must be used, which is a zirconia-rich compound that pulls away the contaminating phosphates and allows for proper cementation. Thus, when utilizing any resin and/or GI cement, zirconia must not be exposed to blood or saliva; if it is, the zirconia must be decontaminated.

To address this concern, a pink zirconia try-in crown (NuSmile) has been developed. Its use prevents contamination by helping the clinician avoid exposure of the final crown to any blood or saliva. The pink try-in crown is the same size as its tooth-colored counterpart and is used to accurately determine correct fit and occlusion prior to cementation. Autoclavable and reusable, try-in crowns allow the practitioner to be confident that a clean, uncontaminated crown is being cemented with optimal bond strengths for the resin and/or glass-ionomer–containing cement being used.

The following case studies illustrate both anterior and posterior placement of zirconia crowns.

Case 1: Anterior Zirconia Prefabricated Crowns

A 3-year-old boy presented with severe early childhood caries. In addition to affected posterior teeth, the maxillary central and lateral incisors were decayed. It was decided to restore these teeth with NuSmile ZR crowns in the following manner:

First, the incisors were isolated with rubber dam, bilateral ivory dam clamps 2A, and a slot-style rubber dam placement, ensuring that the distals of the lateral incisors were visible and easily accessible for reduction (Figure 1 and Figure 2). Then, using a coarse long-tapered diamond bur, an incisal reduction of 1.5 mm to 2 mm was achieved followed by a circumferential reduction of approximately 15% to 20% using the same bur. At this stage, it is easiest to complete a full circumferential reduction supragingivally to visualize the completeness and evenness of the preparation, and it may be desirable to reduce the palatal of the incisor with a football-shaped diamond bur (Figure 3).

Next, a subgingival preparation was achieved using a more tapered and finer diamond bur. A full subgingival reduction to approximately 1.5-mm depth was attained, ensuring a smooth, ledge-free feather-edged margin (Figure 4). Pink try-in crowns were then used to check preparation, size, fit, and occlusion. Any necessary refinements to the preparations would be done at this point and occlusion again checked (Figure 5). Also, pulpal therapy would be completed at this point if needed, and the try-in crowns would be checked for fit after such treatment.

As with all cases in which multiple crowns are placed simultaneously, it was important that all crowns could be seated passively at the same time. Once fit and occlusion were determined to be satisfactory, each prepared tooth was washed and dried, but not desiccated. The crown was filled with BioCem universal bioactive cement; work time was approximately 60 seconds. The crown was set into the correct position, and excess cement was wiped from visible surfaces.

The cement was photocured with a tack cure of 10 seconds each to the facial and palatal aspects. The cement was then cleaned and flossed interproximally, and, once all cement was satisfactorily removed, a final photocure of 10 seconds was done for the facial and palatal aspects.

Figure 6 shows the restorations immediately after the procedure, and Figure 7 depicts the patient at a 2-week recall appointment. The patient’s family was delighted with the restoration, including the esthetics and function of their child’s dentition.

Case 2: Posterior Zirconia Prefabricated Crown

A 6-year-old boy presented with extensive caries of his primary dentition. This case study will focus on the boy’s maxillary left first primary molar (Figure 8 and Figure 9). The molar required full coverage due to the extent of the decay. Because the parents were concerned about esthetics, the author chose a prefabricated zirconia primary crown (NuSmile ZR) as the restorative material. The restorative procedure was as follows:

The quadrant was isolated with a rubber dam, ivory dam clamp W7, and a slot-style rubber dam placement. Using a coarse long-tapered diamond bur, an occlusal reduction of 1.5 mm to 2 mm was achieved, followed by a circumferential reduction of approximately 15% to 20% utilizing the same bur. At this stage, it is easiest to complete a full circumferential reduction supragingivally to visualize the completeness and evenness of the preparation.

Next, a subgingival preparation was achieved using a more tapered and finer diamond bur. A full subgingival reduction to approximately 1.5-mm depth was achieved, ensuring no ledges and a smooth feather-edged margin. This was imperative because the crown must fit passively and be able to be seated completely unencumbered (Figure 10).

A pink try-in crown was used at this point to check size, fit, and occlusion. Any necessary refinements to the preparation would be done at this point and occlusion checked. An inappropriately highly seated crown will cause difficulties. If pulpal therapy is required, it would be completed at this point and the try-in crown checked for fit after such treatment.

Once fit and occlusion were satisfactory, the prepared tooth was washed and dried, but not desiccated. The crown was filled with BioCem universal bioactive cement; work time was approximately 60 seconds. The crown was set into the correct position, and excess cement was wiped from visible surfaces.

The cement was photocured with a tack cure of 10 seconds each to the facial and palatal aspects. The cement was then cleaned and flossed interproximally, and, once all cement was satisfactorily removed, a final photocure of 10 seconds was done for the facial and palatal aspects.

Two-year recall photographs (Figure 11 and Figure 12) show an excellent outcome, with a healthy, highly esthetic dentition restored.

Conclusion

For practitioners, being able to provide children with esthetics and function is truly satisfying. Using the advanced biomaterials available today, clinicians can achieve healthy and happy outcomes for their pediatric patients, giving both them and their parents not only what they desire, but also what they need.

About the Author

Carla Cohn, DMD
Private Practice Limited to Pediatric Dentistry
Winnipeg, Manitoba, Canada