Abstract
Aim
The trial aimed to compare the gingival health and clinical performance of prefabricated zirconia crown (PZC) versus resin-bonded composite strip crowns (RCSC) in the restoration of primary maxillary incisors over 36-month.
Materials and methods
A parallel randomized controlled trial included 200 maxillary primary incisors (100 teeth per group) restored either with PZCs or RSCS. The gingival health and plaque accumulation were assessed using gingival index (GI) and plaque index (PI) respectively. To evaluate the clinical performance, the modified United States Public Health Service (USPHS) criteria were used. Intergroup comparison was performed using a Chi-square test. Restoration longevity was assessed over the predetermined intervals using Kaplan-Meier analysis. Generalized Estimating Equation (GEE) with robust standard errors was used to test the effect of time over the changes of repeated measures. The significance level was set to 5%.
Results
The gingival health around PZCs is significantly better than that of RCSCs after 6 months. However, at the following follow-ups, GI scores of both restorations were comparable ( p > 0.05). All PZCs showed no plaque accumulation with a significant difference compared to the RCSCs over the follow-up intervals. The clinical performance of PZCs was significantly superior to that of RCSCs over the follow-ups in terms of retention and anatomical contour after 24 and 36 months. The survival rates of PZC and RCSC were 100% and 87% respectively.
Conclusions
After 36 months, the gingival health of PZC and RCSC was comparable. The retention and anatomical continuity of PZC were superior compared to RCSC.
Highlights
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Long-term evaluation of aesthetic restoration is mandatory during decision making to restore maxillary primary incisors
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There is a shortage in the evidence regarding the gingival health, clinical performance and longevity of PZCs compared to RCSCs.
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The present study showed comparable gingival health of both restorations with superior longevity of PZCs.
1
Introduction
Aesthetic restoration of primary teeth has become a trend in pediatric dentistry. With the growing interest of parents in the restoration of their children’s teeth with aesthetic restorative materials, there is an obvious shift from traditionally used non-aesthetic preformed stainless steel crowns (SSCs) to aesthetic restorations [ ]. Many aesthetic restorations are commonly used in pediatric practice. For instance, pre-veneered SSCs, resin-bonded composite strip crowns (RCSCs), polycarbonate crowns, and recently introduced preformed pediatric zirconia crowns (PZCs).
The PZC provides reasonable aesthetic properties with superior translucency [ ]. Zirconia presents in a metastable tetragonal phase which is stabilized by additives such as Yttrium Oxide (Y 2 O 3 ). Zirconia is milled from a highly dense, sintered single block substructure [ , ]. A laboratory study compared the fracture loads of three different brands of PZCs before and after aging. The findings showed the superiority of NuSmile PZC (NuSmile, Houston, TX, USA) over Kinder Krowns (St Louis Park, MN, USA) and EzCrowns (Sprig Oral Health Technologies, Loomis, CA, USA) before aging with saliva. Its average fracture load value of 1582 ± 638 N. However, after aging, Kinder Krowns showed the highest mean of fracture loads followed by NuSmile, and EzCrowns respectively [ ]. Clinically, the findings of two previous prospective randomized clinical trials confirmed excellent gingival health, less plaque accumulation, and the highest clinical performance of PZCs in the restoration of primary teeth over different follow-ups ranging from 12 to 24 months [ , ]. Nevertheless, the lack of crimping, contouring and their availability in very limited shades are the major disadvantages of ZCs use in primary dentition [ ]. Additionally, to allow a passive fit of PZCs, aggressive occlusal and circumferential tooth reduction are essential [ ].
The RCSC is a good aesthetic option to restore the coronal portion of anterior teeth. RCSCs have the advantage of replacing chipped or fractured parts of resin composite [ ]. However, RCSCs choice to restore anterior teeth and their longevity depend mainly on the amount of remaining tooth structure and available tooth surfaces. The retention of RCSCs is criticized with isolation precautionary measures [ ]. Retrospective analysis included 200 records of children aged from 22 to 48 months over two years of follow-up and reported a retention rate exceeding 80% of RCSCs [ ].
Few prospective clinical trials compared PZC to RCSC, a prospective randomized clinical trial showed better gingival health of PZC over RCSC for 3 and 6 months. While this significant superiority diminished after one year of follow-up [ ]. While significant dental plaque accumulation was found over RCSCs over 12-month follow-up intervals. Regarding crown retention, no significant difference between PZCs and RCSCs over the follow-ups with 100% and 98.3% retention rates of RCSCs and PZCs respectively [ ].
Pursuing a restoration with optimum standards is necessary to achieve the maximum benefit for the patients. Up to the best of the author’s knowledge, limited data concerned with comparing the PZC to RSCS over a long period of follow-up. The study aimed to compare clinical success rates of RCSC versus PZC in restoration of maxillary primary incisors. The primary outcome measure of this study was to compare the gingival health of RCSC versus PZC in the restoration of primary maxillary incisors. The secondary outcome was addressed through evaluation of the clinical performance of both restorations up to 36 months. The null hypotheses supposed no difference in the gingival health and clinical performance between both restorations over the follow-up period.
2
Materials and Methods
2.1
Ethical compliance
The parents of each child signed informed consent before launching the trial. The study procedures involving human participants followed the ethical standards of the Helsinki Declaration in 1964 and its later amendments or comparable ethical standards. The trial is a part of previously registered project at ClinicalTrials.gov record ID number NCT04973761.
2.2
Design, setting, and sample size estimation
From March 2018 to August 2023, a parallel randomized controlled trial with repeated measures over time design was conducted. To calculate the sample size, the following software ( http://glimmpse.samplesizesshop.org/ ) was used. A pilot study included 10 children to specify the mean GI of 0.07 and standard deviation (SD) values of 0.22 at each time point. The interaction between participants (i.e. treatment) and follow-up periods (i.e. time within-participants factors) using the Hotelling-Lawley Trace test was adopted. The study power and type-1 error were set to 0.80 and 0.05 respectively [ , ]. The finding of the multivariate test showed that 200 teeth (100 per group) were required after adding 10% to compensate for a withdrawal.
2.3
Eligibility criteria
2.3.1
Children’s-related measures
Inclusion criteria
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Only healthy children (i.e. categorized as class 1 or 2 according to the American Society of Anesthesiologists (ASA))
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Children’s ages were ranged from 36 to 48 months
Exclusion criteria
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Children with intellectual, severe emotional, or behavioral problems
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Presence of parafunctional habits such as clenching or bruxism
2.3.2
Teeth-related measures
Inclusion criteria
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Presence of at least two maxillary primary incisors with extensive caries involving one or two tooth surfaces [ , ]
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Teeth indicated for restorative dentistry or indirect pulp treatment
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Normal findings of preoperative periapical radiograph
Exclusion criteria
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Non-restorable teeth
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Clinical signs and/or symptoms of irreversible pulpitis or non-vital pulp (history of unprovoked tooth pain, sinus tract, gingival swelling related to the offending tooth, abnormal tooth mobility) [ ]
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Radicular pulp tissues with suppuration or excessive bleeding for a prolonged duration that can’t be controlled by a cotton pellet [ ]
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Radiographic evidence of furcation or periapical radiolucency or external or internal root resorption [ ].
2.4
Randomization, allocation, and blinding
The current trial was conducted based on the Consolidated Standards of Reporting Trials (CONSORT) statement. An independent clinician (A.M.A) who was masked to the treatment nature and procedures handled the randomization and allocation process. A randomization list was generated using the following software ‘ https://www.sealedenvelope.com/simple-randomiser/v1/listsGenerating ’. To achieve two balanced groups, block randomization technique of block size of four. With an allocation ratio of 1:1, a total of 200 identical opaque adequately sealed opaque envelopes contained standard-sized papers enclosed within foils. Each paper contained the restoration type code. Envelopes were shuffled and placed in two separate plastic containers according to the treatment codes. At the time of the procedure, four envelopes (two from each plastic pile were randomly selected and shuffled by an independent nurse (S.D.N). The child’s parent randomly selected an envelope that was opened by the assistant nurse. Children were assigned into two groups; group “1” teeth were restored with PZC and group “2” teeth were restored with RCSC.
2.5
Clinical procedures
Treatment was carried out under local anaesthesia for cooperative children rated 3 (i.e. accept treatment; cautious behaviour at times; willingness to comply with dentist, at times with reservation, but child follows dentist’s directions cooperatively) or 4 (i.e. good rapport with the dentist, interest in the dental procedures, laughter and enjoyment) according to Frankl Behaviour Rating Scale (FBRS). If the child’s behaviour during treatment procedures became negative, the findings of restored teeth were excluded from the final statistical analysis. While preoperatively children rated 1 or 2 according to FBRS were treated under general anaesthesia. In our institution, performing dental management under sedation is not available.
2.5.1
Restorative dentistry
For teeth decided to be treated under local anaesthesia, appropriate behaviour management approaches were applied. A small amount of topical anaesthesia was applied for 2 min. Labial infiltration injection was performed using lidocaine hydrochloride 2% and epinephrine 1:100,000 (Lignospan® standard, 1.7 mL, SEPTODONT Inc.). The teeth were isolated with a rubber dam. For carious teeth without pulp exposure, caries was excavated.
2.5.2
Coronal preparation for PZC group
Following the manufacturer’s guideline for crown preparation, the incisal edge was reduced by 1.5 mm using TR-13 and WR-13 diamond burs (Mani, Inc., Japan). Circumferential crown reduction of the axial walls by 20%–30% was performed. The preparation ended into a smooth feather-edged cervical margin and extended 1–2 mm subgingivally. To control bleeding a pellet of cotton placed in epinephrine 1 mg/mL (Primer Dental Products Company) was kept for 1 min over the gingiva under pressure with “2 × 2” cm gauze. The ruminant coagulum was removed by moistened gauze. The pink crown was tried to check the occlusion and passive fitting of the crown without loading a pressure. Finally, the corresponding PZC (NuSmile Ltd., Houston, Texas, United States) was cemented with dual-cured, self-adhesive resin cement (TotalCem, ITENA Clinical Products). After initial curing for 3 s, excess material was removed with a sharp scaler and dental floss. Curing was completed for additional 40 s. Curing was done using a light cure (1200 mW/cm 2 ) (Elipar™, 3 M ESPE).
2.5.3
For the RCSC group
Based on the mesiodistal width of each tooth, a suitable size of strip crown was chosen. To ensure proper gingival fitting, the crown was trimmed with a scissor. For resin composite, the shade was matched using 3 M Filtek Z350 XT shade guide. On the crown palatal side, a vent hole was cut. Etching of the prepared tooth with 37% phosphoric acid gel (3 M Scotchbond™ Etchant) for 15 s was applied, rinsed for 60 s, and dried for 30 s with moisture-free air. Over the etched enamel, a layer of Scotchbond light-cured bonding agent (3 M ESPE dental product) was brushed, thinned, and cured. The celluloid crown was filled with resin composite (Filtek Z350 XT WD (3 M ESPE Dental Products) It was packed in increments of maximum thickness of 1.5 mm. Resin composite was light-cured for 20 s by 3 M™ Elipar™ DeepCure-L LED curing light (with an output of 1000–2000 mW/cm2). After peeling the strip crown from the punched-in palatal vent, occlusal necessary check and required adjustments were performed. The restoration was finished with diamond burs (KG Sorensen), and SofLex discs (3 M). Finally, all restoration surfaces were polished except for the labial surface to maintain its glossiness with polishing strips.
2.6
Clinical outcomes and assessment
Children were recalled at 4 intervals [6 months (T 1 ), 12 months (T 2 ), 24 months (T 3 ), and 36 months (T 4 )].
2.6.1
Gingival status and plaque accumulation assessment
To assess the gingival health, Silness and Löe gingival index (GI) and plaque index (PI) were used [ ]. The GI uses the following scoring system: 0 = normal gingiva; 1 = mild inflammation: a slight change in colour, slight edema, no bleeding on probing; 2 = moderate inflammation: redness, edema, and glazing, or bleeding on probing; 3 = severe inflammation: marked redness and edema, a tendency toward spontaneous bleeding, ulceration. The PI scoring system: 0 = No plaque; 1 = a film of plaque adhering to the free gingival margin and adjacent area of the tooth the plaque may be seen in situ only after the application of disclosing solution or by using a probe on the tooth surface; 2 = Moderate accumulation of soft deposits within the gingival pocket, or the tooth and gingival margin which can be seen with the naked eye; 3 = Abundance of soft matter within the gingival pocket and/or on the tooth and gingival margin.
2.6.2
Clinical performance assessment
For the secondary outcome; clinical performance was evaluated in terms of crown retention, marginal adaptation, colour match, and anatomic form adopting the Public Health Service (USPHS) criteria [ , , ] ( Table 1 ).
| 1. Crown retention | |
| Retained | Complete retention of the restoration |
| Mobil/lost | Mobility present or lost |
| 2. Marginal adaptation | |
| Alpha | No visible evidence of crevice along margin; no catch or penetration of explorer |
| Bravo | Visible evidence of crevice and/or catch of explorer, no penetration of explorer |
| Charlie | Visible evidence; penetration of explorer |
| 3. Marginal discoloration | |
| Alpha | No marginal discoloration |
| Bravo | Marginal discoloration, not penetrated toward pulp |
| Charlie | Marginal discoloration, penetrated toward pulp |
| 4. Colour match | |
| Alpha | Crown shade and translucency are matched with adjacent tooth |
| Bravo | Crown shade and translucency are mismatched with adjacent tooth but within the normal range |
| Charlie | Crown shade and translucency are mismatched with adjacent tooth and out of the normal range |
| 5. Anatomical form | |
| Alpha | Crown is continuous with tooth anatomy |
| Bravo | Crown not continuous with tooth anatomy; slightly under- or over-contoured |
| Charlie | Crown not continuous with tooth anatomy; restoration material is missing; a surface concavity is ascertainable |
2.6.3
Survival rate of restoration over assessment over time
The restoration was considered clinically failed and needed replacement (i.e. survival rate) in case of an unacceptable marginal adaptation, marginal discoloration, anatomical form (i.e. classified as “Charlie” according to the USPHS criteria), restoration mobility, or complete debonding [ ].
2.7
Standardization and calibration
Before starting clinical procedures, scaling and polishing for all children’s teeth applied to provide neutral baseline scores of PI and GI. Additionally, tooth brushing steps and oral hygiene instructions were taught to the children’s guardians. Two dentists (T.G.A. and S.M.A.) with a Ph.D. in dentistry and an experience of 15 years in restorative dentistry in paediatric patients evaluated the two restorations independently at different follow-up periods. Cohen’s kappa coefficient (κ) was used to detect the inter-observer reliability.
2.8
Statistical analysis
Data were analyzed using IBM SPSS software package version 20.0. (Armonk, NY: IBM Corp). The GI scores were dichotomized as follows: (1) absence of gingival signs of inflammation or mild gingivitis (scores 0 and 1) were categorized as a “success” and (2) moderate or severe gingival inflammation (scores 2 and 3) were rated as “failure”. Similarly, the PI scores were dichotomized into success (scores 0 and 1) and failure (scores 2 and 3). To assess the clinical performance of both restorations, the USPHS criteria of the outcomes were dichotomized to success (Alpha and Bravo scores) and failure (Charlie score). A Chi-square test was used to test statistically significant difference between the dichotomized frequencies of GI, PI, and clinical performance items in the two independent restoration groups. To compare the survival rate of both restorations, Kaplan-Meier analysis was performed. To study the effect of time (i.e. follow-up periods) over the changes of repeated measures both restorations, Generalized Estimating Equations (GEE) with robust standard errors were used. The significance level was set to 5% at a 95% confidence interval (95% CI ).
3
Results
The inter-observer reliability was high at different follow-ups (Kappa = 0.92, 0.90, 0.87, and 0.88 at T 1 , T 2 , T 3 , and T 4 respectively).
3.1
Participant’s baselines
Of 122 children with 286 maxillary incisors examined for eligibility, 90 children with 200 maxillary primary incisors were selected. The details of enrollment that have been structured according to the teeth number were illustrated in Fig. 1 . In the PZC group, 44 children (26 boys and 18 girls) with an average age of 39.2 ± 0.54 months and in the RCSC group, 46 children (25 boys and 21 girls) of an average age of 40.3 ± 0.66 months were included. More data were elucidated in Table 2 .
