Introduction
Early childhood caries is a health epidemic that disproportionately affects the developing world and low socioeconomic communities (Çolak et al.
2013). Oral disease (primarily dental caries) in Australia accounts for 8.1% of non-fatal diseases for children aged 5–9, with numbers higher in indigenous children (Australian Institute of Health and Welfare
2021). A population-based study of dental caries noted children aged 5–10 had 1.5 decayed, missing, and filled deciduous teeth. Moreover, the rate of potentially preventable hospitalisations of children aged 0–14 years in 2017–2018 due to dental conditions was 17 per 1000 population. Due to high caries rate in children, the use of the full coverage preformed crowns (PCs) to restore the primary dentition has become widely adopted.
The full coronal coverage afforded by PCs makes them indispensable to the paediatric armamentarium, irrespective of the material composition. The preformed metal crown (PMC) was first introduced in 1950 to help restore the primary dentition affected by multi-surface lesions of dental caries, developmental defects, fractures, or extensive wear. PMC are also used after pulp therapy, or as abutments for space maintainers (Engel
1950; Humphrey
1950). The crowns are available in multiple sizes and can be modified chairside by means of crimping, trimming, and contouring, for improving adaptation to the tooth (Seale and Randall
2015). PMCs are considered the gold standard in paediatric restorative materials.
During clinical procedures, contamination of PMCs occurs for several reasons. When the crown of the tooth is prepared, subgingival margin preparation results in minor gingival bleeding. When the dental practitioner estimates the crown size, often several crowns of differing sizes are tried-in before one is chosen. Hence, PMCs will be exposed to the patient’s saliva and blood. Both bodily fluids are potentially infectious. The reprocessing and reuse of dental instruments and devices (e.g., rotary endodontic files or molar orthodontic bands) is acceptable only when industry standards and infection control requirements are met (Chan et al.
2016; Fulford et al.
2003). Adding to this, children are more likely than adults to contract infectious diseases from poor infection control practices due to their immunological naivety (Ygberg and Nilsson
2012). For this reason, if reuse is contemplated, there must be clear guidance around suitable decontamination processes, including the aspects of cleaning, sterilisation, and storage (British Dental Association
2003). The trial-and-error nature of sizing PMCs will predictably result in these being sterilised multiple times. Indeed, the high cost of PMCs is a disincentive for discarding crowns after try-in attempts (Abukabbos et al.
2018). Rates of PMC reuse after sterilisation are estimated to be from 86.4 to 98.92% (El Shehaby
2008; Farhin et al.
2013). This emphasises the need for clear guidance around reprocessing of PMCs that were found to be the wrong size, particularly since these are defined as single use devices.
Rising demands for more aesthetic restorations has led to the emergence of stainless steel preformed crowns that are pre-veneered with a tooth coloured resin material, as well as preformed fabricated from zirconia ceramics (Aiem et al.
2017). Limited data exist on the colour stability or structural integrity of aesthetic PCs. Pre-veneered PMCs have a fracture resistance beyond the average occlusal forces of young children, (Baker et al.
1996; Waggoner and Drummond
2006) although microstructural changes caused by steam sterilisation may alter such properties (Kiran et al.
2015; Yilmaz and Guler
2008). While steam sterilisation remains the most widely used sterilisation technique in dental practice, the high cycle temperatures may disturb the bond between the resin and the stainless steel at the interface of the two materials. In fact, some manufacturers advise chemical disinfection rather than steam sterilisation for aesthetic PCs, to avoid such degradation.
A range of detrimental effects on crown materials can occur during reprocessing. For example, resin materials can be discoloured, absorb water or lose leachable components (Villalta et al.
2006). Steam sterilisation can accelerate the ageing process for zirconia ceramics, with detrimental effects on their colour (Volpato et al.
2016). Processes that lead to crown discolouration are an aesthetic concern and a common reason for complaints (Kupietzky et al.
2003). Practitioners need to be aware of material limitations for PCs and their impact on reprocessing practices. In light of this, a systematic review of the literature on reprocessing of PCs used in paediatric dentistry was undertaken.
Discussion
Dentists have a responsibility to maintain high standards of infection control for patient care. The constant progression of industry guidelines to address infection control concerns has led to better outcomes for patients and staff alike. For example, the decontamination protocols used for orthodontic appliances and dental prostheses have received considerable scrutiny (Benson and Douglas
2007; Benyahia et al.
2012; Chassot et al.
2006). Whether practitioners can safety recirculate sterilised PCs is significantly underreported.
The current review revealed that PCs are susceptible to commonly utilised infection control practices. Indeed, several studies identified vestibular crazing and colour instability in PCs following steam sterilisation and/or chemical disinfection (Hogerheyde et al.
2021; Kiran et al.
2015; Padmanabh and Patel
2021; Wickersham et al.
1998; Yilmaz and Guler
2008). However, the most stable PC material was zirconia ceramics (Pate et al.
2021).
PCs are routinely placed by dentists to protect a compromised dentition (Kindelan et al.
2008). Most manufacturers define PCs as single use devices, however, discarding contaminated crowns is financially impractical for dental surgeries. Practitioners need clarification on whether contamination with bodily fluids constitutes treatment. Even the sterility of unopened PCs cannot be guaranteed. Studies have reported nonsterile endodontic files and dental burs supplied by the manufacturer (Kumar et al.
2015). Thus, the risk of environmental contaminants such as bacterial spores is highly plausible.
Inconsistent guidelines on PC reuse are problematic for practitioners with potential medicolegal issues. Researchers surveyed a cohort of paediatric dentists to determine the extent of material recirculation and infection control protocols. The overwhelming majority of respondents (98.92%) reuse PCs, with 57.65% using steam sterilisation as part of the decontamination procedure (Farhin et al.
2013). PCs are classified as a critical item, which requires sterilisation parameters to be met. The Centers for Disease Control and Prevention (CDC) recommends steam autoclaving for critical items as the gold standard (Kohn et al.
2003). Meanwhile heat sensitive items can be sterilised using ethylene oxide or hydrogen peroxide gas plasma (Kanemitsu et al.
2005; Mendes et al.
2007). The survey results reveal a significant portion of practitioners reusing a critical item that has not met international standards for medical treatment. As such, the basic requirements of infection control and patient care are not being met. The inconsistent methods of decontamination as well as wide-ranging infection control learning resources among respondents led the authors to recommend the need for clarity on PC sterilisation guidelines.
The benefits of reducing bacterial populations by steam autoclaving cannot be overstated. An in vivo study comparing decontamination methods on tried-in PMCs found autoclaving at 121 °C, 15 psi for 15 min significantly reduced the colony counts, compared with 5% sodium hypochlorite (5 min), 5% glutaraldehyde (5 min), 70% isopropyl (1 min), or glass bead groups (Darshan et al.
2019). The insufficient reduction in bacterial counts for rapid chairside sterilisation (i.e., sodium hypochlorite, glutaraldehyde, etc.) led authors to conclude that autoclaving was paramount for effective sterility. Similarly, the efficacy of infection control practices on contaminated pre-veneered PMCs using 70% ethanol, 10% sodium hypochlorite, or steam autoclaving has been studied (Shelburne et al.
2003). The adherent
Streptococcus mutans bacteria was then cultured using agar growth plates to assess the degree of bacterial reduction. All protocols reduced levels of bacteria by at least six logs. However, a significant number of variables can affect the outcome. Incomplete sterilisation can occur when any disparities in temperature, contact time, or concentration of liquid agents exists. For protocols that omit steam autoclaving the degree and consistency of microbial elimination is quite variable (Wickersham et al.
1998). Whilst some manufacturers recommend cold sterilisation for heat sensitive PC materials, the techniques need to be carefully calibrated (Benyahia et al.
2012).
Various PC materials may respond differently to infection control practices. The need for customised decontamination protocols may be essential for some crown materials. For example, whether PCs are vulnerable to steam autoclaving is debatable. Studies have reported conflicting evidence with PMCs showing evidence of crazing and microstructural changes (Kiran et al.
2015; Yilmaz and Guler
2008). A comparative study evaluating decontamination methods on PCs observed crazing and contour alterations following steam autoclaving (Yilmaz and Guler
2008). However, changes were not observable using stereomicroscope imaging. SEM analysis was necessary to discern notable microstructural changes. As such, practitioners may unknowingly recirculate PCs with compromised structural properties. To date the effects on clinical performance are unknown. The authors of another study recommended using an aldehyde-free high grade disinfectant to decontaminate PCs that have exposed to bodily fluids (Yilmaz and Guler
2008).
Additional studies noted deleterious effects for autoclaving at 121 °C, 15 psi, 20 min and 132 °C, 30 psi, 8 min with crazing in one-third to one-half of PMCs (Kiran et al.
2015). Such topographical irregularities may yield a more retentive surface for dental plaque as well as promoting unwanted corrosion (Marentes
2018; Wickersham et al.
1998). Others identified no microstructural changes to PMCs after five autoclave cycles (Marentes
2018). The authors postulated that material variations between manufacturers may account for corrosion sites in the trough regions. In addition, a recent study investigating the effects of infection control practices (i.e., fast/slow steam autoclaving, ultrasonic bath) on the physical–mechanical properties of preformed crowns found no evidence of fractures or colour changes (Padmanabh and Patel
2021). However, maximum crazing was seen in PMCs. The materials tested included PMCs, pre-veneered PMCs, and zirconia crowns.
A greater awareness of childhood psychology as well as demand from parents has led practitioners to offer aesthetic PCs. It has been shown that disinfectants can modify the colour stability of denture base resins (Bensel et al.
2018). Similarly, persistent thermocycling of monolithic zirconia can produce significant colour and translucency changes (Koseoglu et al.
2019). However, limited data have been published on the impact of infection control practices on aesthetics of crowns. For example, neither autoclaving nor glutaraldehyde sterilisation had minimal effect on colour parameters for pre-veneered PMCs (Wickersham et al.
1998). However, the same PCs underwent negative colour changes when subject to chemiclave sterilisation. The present authors reported that pre-veneered PMCs showed colour variations with loss of luminosity after reprocessing (Hogerheyde et al.
2021). Thus, clinical awareness of material compatibility is paramount to limit unwanted aesthetic variations. A recent study evaluated the aesthetic characteristics of four brands of zirconia crowns after autoclave and cold sterilisation (Pate et al.
2021). The authors concluded no significant differences in colour stability, gloss, or translucency between groups. Similar results were described by the current authors with zirconia crowns unaffected by reprocessing protocols.
The systematic review was limited by experimental disparities such as sterilisation parameters or manufactures used. Although considerable research has been conducted into the decontamination practices on restorative dental materials, the effect on paediatric materials is substantially unreported. The review herein was constrained by a lack of robust in vivo studies, indicating the need for widening the analysis to include non-paediatric dental materials. However, the assembled literature offers a blueprint for future studies by identifying consistent material deficiencies such as colour instability and microstructural variations. Future studies need to focus on identifying the specific conditions (i.e., temperature, cycle number, etc.) responsible for such material degradation. The maintenance and operational parameters of sterilisation equipment is paramount for effective sterility. For instance, challenge tests such as biological indicators require specific conditions such as temperature, pressure, and time to be met (Palenik et al.
1999). If practitioners chose to recirculate decontaminated materials, then utmost confidence in the sterilisation process is vital.
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