From Volume 42, Issue 10, December 2015 | Pages 933-938
This article details the clinical techniques for conventional preformed metal crown placement. It aims to increase the readers' awareness of the clinical advantages of preformed metal crowns and the indications for their use. The second part will discuss the Hall Technique.
This two-part article will be looking at preformed metal crowns (PMCs), also known as stainless steel crowns (SSCs), which have been in use since the 1950s. They are prefabricated crown forms that can be adapted to individual primary molars as a definitive restoration. In this first part, the indications, evidence and the conventional technique will be described. The second paper will discuss the new and innovative technique for placing these crowns known as the ‘Hall Technique’, together with aesthetic crowns.
Primary teeth in young children are vital to their development as they:1
However, the most important reason to restore the primary dentition is the management of caries in children, which can otherwise progress to pain, infection/sepsis and hospital admissions.2
PMCs are widely accepted by paediatric dentists, including the American Association of Paediatric Dentistry (AAPD) and the British Society of Paediatric Dentistry (BSPD) as the restoration of choice for primary molars.3,4
The indications for PMCs are:4
They are contra-indicated in situations where:5
Although PMC is the preferred term, these crowns were traditionally called stainless steel crowns. This is a misnomer, as they are actually nickel chromium. The constituents of 3M ESPE (Seefeld, Germany) preformed metal crowns, the market leader, are as follows:6,7
Nickel is one of the constituents and therefore nickel allergy is one of the contra-indications to the use of PMCs. It is suggested that 11% of young female adults and 2% of young male adults are nickel sensitive.8 From the orthodontic literature, it is estimated that only 0.1–0.2% of patients show a harmful response to nickel. This is because it is thought that much greater concentrations of nickel are required intra-orally than on the skin to elicit a reaction. Further, it has been shown that only small amounts of the metal constituents leach from PMCs.6
A questionnaire study of a random sample of general dental practitioners (GDPs) drawn from the Midlands of England and Scotland reported 6% potentially using PMCs to restore primary teeth.9 An interview study the same year of GDPs in the North West of England reported again only 6% placing PMCs.10 In this study, the commonest reasons cited for the low uptake included cost, time consuming to fit and patient co-operation issues. Appearance and parental acceptance were also cited as a concern.
The longevity of preformed metal crowns is their key advantage over multi-surface restorations. One retrospective study conducted in NHS general practice found the 5-year survival rate of conventional technique preformed metal crowns to be 100%; over twice that of the multi-surface amalgams, though the small number of only 18 PMCs sampled is a valid criticism of this study.11 This issue was rectified in a five-year retrospective study, conducted in a private specialist paediatric practice, that found only 2.1% of preformed metal crowns required replacement, compared to 14.7% of Black's Class II amalgams over a 10-year period. A second prospective study conducted in the same specialist practice found very similar survival rates between PMCs and intra-coronal restorations, but the mean follow-up was only just over two years.12
These observational studies have a number of drawbacks and confounders, including a lack of standardization on use of local anaesthetic, variable use of rubber dam and, most importantly, the PMC tended to be selected for larger carious lesions.
This longevity and the consequent reduction in the need for replacement can have significant implications in terms of time- and cost-effectiveness, along with patient and parent satisfaction.
None of these studies is a randomized controlled trial. A Cochrane review on PMCs found that there were no studies that met the inclusion criteria and therefore no relevant data were available for analysis.13 Therefore, no conclusions could be made as to whether PMCs were more successful than alternatives for restoring primary molar teeth. However, ‘it is important that the absence of evidence for PMCs is not misinterpreted as evidence for their lack of efficacy’, especially as the ‘evidence[s] that have been produced … have strength in that the clinical outcomes are consistently in favour of PMCs’.13
The conventional crown technique requires a good degree of patient co-operation. This is necessary firstly to allow the administration of local anaesthetic.
Show the crown to the child and his/her carer and describe the procedure (Figure 1).
Local anaesthetic is best provided following application of topical anaesthetic.
Figure 2 shows a tooth prior to preparation. The tooth is reduced occlusally by 1.5 mm, using a diamond fissure bur. This reduction should take into account the extent of tooth surface loss already present, as further reduction in a tooth already displaying signs of significant erosion or attrition could compromise the pulp further.
Approximal reduction is the most technically-challenging aspect of the conventional technique. A visible space should be created with a fine-pointed diamond bur, ensuring that only the enamel of the tooth to be crowned is reduced (Figure 3). The adjacent tooth should not be contacted by the bur, unless, of course, it is to be extracted or crowned in the same treatment session. Even the slightest iatrogenic damage approximally has been shown to increase risk of development of a carious lesion at that site.14 This is clearly highly likely in an existing cariogenic environment.
Buccal and lingual reduction is not normally required. However, it is frequently necessary to remove the buccal prominence on first primary molars, particularly in the lower arch.
Any remaining caries is then removed (Figures 4a and b). The final preparation should be free of ledges, steps or shoulders, with smooth and rounded edges to facilitate placement of the crown.
There are two main techniques employed by dentists to select the appropriate choice of crown. The first involves measuring the mesio-distal space with either dividers or a periodontal probe. The second utilizes trial and error, a process which becomes easier with experience.
The crown which adequately restores the space with a snug fit to the tooth should be selected. Occasionally, adjustment may be required in the form of crimping or trimming the crown. The former tends to be required when the tooth requires a crown between two sizes. Crimping of the larger crown to engage interproximally and beneath any bulbosities can often solve the problem (Figures 5a and b). Alternatively, manual adjustment of the crown form can be undertaken simply by squeezing the crown slightly between finger and thumb (Figure 6). Trimming followed by polishing with a stone may be necessary in cases where there is extensive tooth surface loss occlusally, which has reduced the crown height. Trimming should be avoided whenever possible, as this reduces retention and, even with stoning, it can be hard to remove rough sharp edges. Moreover, trimmers should be used as opposed to scissors, as the latter can produce distortion.
The prepared tooth should then be washed and dried. The crown chosen is generously filled with the luting material and fully seated (Figure 7). Whilst resin-modified glass ionomer cement is reported to have superior bonding properties as a luting agent for PMCs, conventional glass ionomer cements are widely used.7,15 Excess cement should be removed with damp gauze and an instrument; knotted floss passed interproximally may be helpful (Figure 8a and b).
Consideration must be given to airway protection during both try-in and cementation. This can be done by sitting the patient upright, using a sheet of gauze as a physical barrier or using tape to hold the crown.
Durability, plasticity and malleability are some of the key properties of preformed metal crowns that contribute to their clinical success and ease of use. These properties hail from the metallic components that in turn produce the silver-coloured appearance. Clearly, one would expect aesthetics to be a potential issue for parents and children alike. This issue was noted by Page and colleagues, who sought opinions from both children and their parents on the appearance of their PMC(s).16 Whilst the children interviewed gave generally positive or neutral feedback, a few parents were concerned that their child's PMC would be viewed by adult peers as an indicator of dental neglect. Bell et al's questionnaire study looked at the views of children and their parents, on conventional PMCs and those placed by the Hall Technique in more depth.17 Only 8% of children did not find having the crown placed ‘okay’, and approximately 16% seemed to have concerns about its appearance, particularly other people commenting on it. A small number of parent's (5%) reported strong concerns about the appearance.
For this reason, it is important that parents and children can view a crown, or a photo of a crown, in situ prior to placement and are aware of on which tooth it is planned to be placed.
Rarely, instances may arise where PMC removal is indicated. Reasons for this include:
Removal is relatively simple and does not require anaesthesia because the luting cement insulates and protects the tooth.
The technique for crown removal involves:
As discussed, aesthetics is an issue with PMCs. In the past it has been suggested that, following placement, a window could be cut in the buccal surface of the crown to improve its aesthetics. This compromises the PMC integrity and has a limited improvement on aesthetics. Therefore, anterior and posterior tooth-coloured crowns have been developed. These are usually veneered, although all-ceramic crowns such as zirconium are also available. Owing to the thickness of these aesthetic crowns, a more extensive tooth preparation is required. In primary teeth this is significant due to the enamel/dentine/pulp ratio. There is also an issue with the loss of the veneer over time, although these crowns are improving in quality with advancements in manufacturing techniques.
The use of PMCs has been shown to be a successful restorative option, accepted by patients and their carers. The technique of conventional PMC placement is, in the authors' opinion, relatively simple and certainly much simpler to carry out than intra-coronal restorations in primary teeth.
The second of these papers will discuss the Hall Technique, which does not involve tooth preparation. Some practitioners will have a preference for either the conventional or Hall technique. For those who adopt the latter, it is essential to understand the conventional technique for instances when the Hall Technique is not possible. These occasions include, amongst others, the presence of a ledge or significant space loss, preventing a crown from seating without preparation.
