Stability, survival and costs of vacuum-formed retainers versus bonded lingual braided rectangular and multistranded round wire retainers in the mandible of orthodontic patients over a two-year period

Abstract

Background

To assess the efficacy in relation to costs of three retention techniques: vacuum-formed retainers (VFR) with lingual-bonded braided and multistranded wire retainers in the mandible.

Methods

A total of 93 participants, aged 12–21 years at the start of the retention phase (median age: 16 years; 66% females) were assessed. Analysis included retainer survival rate, costs associated with retention procedures, degree of mandibular incisor irregularity, and patient satisfaction. The cost-minimization analysis accounted for direct costs (material and production expenses, utility costs, and chair time based on personnel wages).

Results

The survival rate of retainers was 74% for VFR, 71% for rectangular wire retainers, and 62% for round wire retainers, with no significant differences among groups. Incisor alignment relapse was minimal overall but was highest in the VFR group compared to both wire types (p < 0.001). No differences in satisfaction were observed between groups. However, total retention costs varied significantly (p = 0.001), with the VFR group showing the lowest median cost (34.39 €) and rectangular wire group the highest (44.61 €).

Conclusion

The retainers demonstrated similar survival rates, provided equal patient satisfaction, and maintained alignment effectively, with differences in stability remaining below evidence-based thresholds. However, VFR incurred 21–23% lower costs than fixed retainers, a finding with practical relevance for public or resource-limited settings.

Trial registration

ClinicalTrials.gov, NCT05121220. Registered 02 October 2021.

Introduction

Proper retention plans and protocols are essential in orthodontic treatment planning. Following the active movement of teeth during orthodontic treatment, teeth often occupy an unstable position, making them prone to relapse towards their original position [1]. To address that challenge, both fixed or removable retainers are used to stabilize outcomes and enhance patient satisfaction [2–4].

Recently, orthodontists have increasing preferred fixed bonded retainers (FBR) for their aesthetic benefits and because they do not rely on patient compliance [5]. However, these retainers restrict cleaning to the approximal surfaces, potentially increasing the risk of plaque build-up, gingivitis, and caries development [6–8]. Conversely, long term use of removable retainers can suffer from inconsistent patient compliance, raising the likelihood of relapse [9, 10]. It is still unclear which retainer type best reduces complications while providing safe retention, comfort, and cost-effectiveness [11].

A recent meta-analysis revealed that evidence regarding different retention methods is generally of low quality [12]. FBR and vacuum-formed removable retainers (VFR) show similar efficacy in maintaining mandibular incisor alignment. Although the difference between them is statistically significant, it is not clinically significant, and the evidence supporting this finding is of low quality [12]. This is in line with the Cochrane systematic review, which pointed out that while removable retainers may lead to more discomfort, they also exhibit lower failure rates and better periodontal health than FBR [13].

Given limited public dental care resources, it is crucial to understand the costs and effects associated with the treatment when making decisions. There are limited studies available regarding the costs of various orthodontic retention methods [14–16]. Different types of analyses, such as cost-minimization, cost-effectiveness, cost-utility, and cost–benefit analyses, can be conducted to evaluate various aspects of costs and benefits [17–19].

Due to the lack of combined clinical and cost-effectiveness data in the current literature this study aimed to compare the stability of the orthodontic treatment outcome (primary outcome), survival rate of retainers, patient satisfaction, and costs (secondary outcomes) of the three retainer types during a 2-year period. The null hypothesis proposed that all retention methods would be equally effective with respect to stability, patient satisfaction, and cost.

Materials and methods

Trial design and ethics

This study was designed as a three-arm, randomized controlled trial. Block randomization was used (11 blocks of size 12 and 9 blocks of size 3) and sequentially numbered, opaque, sealed envelopes were prepared in advance of the trial by a person independent of the study. The recruitment timeframe was 1 year (November 2019 – November 2020) and the study ended in November 2022. One person enrolled patients (VZB) and another (MM) assigned them to the interventions. Orthodontic treatment outcome was not evaluated in relation to the initial status. Researcher assessing outcomes (SKU) was blinded to the incisor irregularities before the start of orthodontic treatment, treatment-induced changes, costs, and patient satisfaction. Because a FBR is visible on the teeth, blinding was not possible for incisor irregularity in retention and retainer failure.

All participants signed an informed consent form. In individuals younger than the age of 16, written informed consent to participate was obtained from their parents or legal guardians.

The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committees of the Faculty of Dental Medicine, University of Rijeka (No. 2170-57-006-01-22-1) and the School of Dental Medicine, University of Zagreb (No. 05-PA-30-III-12/2021).

Participant eligibility criteria

The inclusion criteria were permanent dentition, absence of hypodontia, no history of tooth loss and age < 18 years prior to orthodontic intervention, a healthy periodontium in the lower anterior region, and Angle dental class I, or mild classes II and III. Exclusion criteria were cases involving tooth extraction or orthognathic surgery.

Interventions

Orthodontic treatment was performed using a multibracket fixed appliance (MBT 0.022 inches). Following the removal of the orthodontic appliances, the participants were assigned to one of three groups. Each group received thermoplastic VFR for the maxilla made from a 1 mm thick thermoplastic foil (Erkodur, Erkodent Erich Kopp, Pfalzgrafenweiler, Germany). Retention protocols varied for the mandible.

The first group had a rectangular passive steel wire measuring 0.673 × 0.268 mm (0.027 × 0.011 inches) placed in the mandible from canine to canine, composed of eight flattened and braided wires (Bond-A-Braid Wire, Reliance Orthodontic Products, Itasca, USA). All retainers were customized to fit the plaster models and affixed lingually using an adhesive technique to the lower canines and incisors of each tooth individually. A flowable composite and adhesive (Filtek and Single Bond, 3 M, St. Paul, USA) were utilized.

The second group was fitted with a round steel wire in the mandible (0.406 mm; 0.016 inches) and comprised of six thinner twisted wires (Forestaflex, Forestadent, Pforzheim, Germany). The same fitting procedure and adhesive technique as in the first group were used.

The third group, which served as the control, did not incorporate wires, and the participants in this group received VFR for the mandible. They were instructed to wear it full-time for the first 3 months and then only during the night. All wire adaptations, bonding procedures, and follow-ups were performed by the same researcher at the Orthonova clinic, Zagreb, Croatia.

All patients received consistent instructions on oral hygiene, including tooth brushing and interdental flossing.

Regular check-ups were scheduled at 3, 6, 12, and 24 months following retainer bonding/delivery. However, the patients were advised to report to the office immediately if they noticed any retainer failure outside of these appointments. The trial ended after 2 years owing to a high dropout rate. Participants who experienced retainer debonding were not excluded from further examination. Instead, the retainers were reattached. Additionally, individuals who broke or lost their removable retainers were provided with replacements.

Outcomes

The irregularity of the position of the mandibular incisors (primary outcome) was quantified as the sum of distances between the contact points (using the Little index) [20] on the plaster casts, using a digital caliper (Digital Caliper, Burg-WächterG, Wetter, Germany). A value of 1–3 is considered minimal irregularity, 4–6 moderate, 7–9 severe while 10 very severe irregularity.

The reproducibility was assessed by the intraclass correlation coefficient using repeated measurements of 30 dental casts at one-week interval. The reliability was excellent ranging 0.993–0.999.

The frequency of wire detachment, breakage and fractures of the VRF or loss of the retainer was documented for each participant, and these were considered failures.

Participants reported satisfaction with the outcome after 2 years of retention using one question (“Are you satisfied with the stability of the tooth position after orthodontic treatment?”) and a Likert-type scale ranging from 1 = not at all to 5 = very much.

A cost-minimization analysis focused on direct costs: material and production costs, utility costs and chair time based on personnel wages in Croatia. Material costs were based on supplier price list for used material and divided by amount needed: rectangular retention wire for 6 teeth 1.199 €, round wire 0.376 €, retention foil 2.142 €, alginate for mandibular arch impression 0.365 €, plaster for mandibular cast 0.138 €, etching gel 0.03 € per tooth, adhesive 0.006 € per tooth, flowable composite per tooth 0.139 €, adhesive brush 0.063 €, plastic glass 0.025 €, pair of glows 0.125 €, mask 0.20 €, dental rolls 0.041 € per piece and suction tube 0.035 €. The time necessary to produce or bond retention appliances included chair time together with the patient entering and exiting the office, impression taking, bonding, time needed for laboratory technicians to pour plaster casts and manufacture appliances, and delivery of appliances by orthodontists (with entering and exiting the office). Time was converted into average salary costs determined by the state for public services with supplements for 15 years and a tax allowance for one child. The gross hourly rate for an orthodontist is 22.67 €, while for a technician and assistant 7.51 €. The hourly utility costs of the office and laboratory (when using one dental unit and one working place for technician) were 0.11 € for electricity, 0.05 € for gas, and 0.04 € for water. Some modifications to the cost calculation methods were made after the trial commenced.

Sample size calculation

The sample size was based on a significance level of 0.05 and an 80% power to detect difference of 2 mm in incisor irregularity between the two retention methods, with a standard deviation of 3 in each group [20, 21]. The calculation was conducted using the Statulator online tool and indicated that 36 participants per group would be required [22]. For the standard deviation of 2, 16 participants per group would be enough. To accommodate for dropouts, the number was increased, resulting in the randomizing 159 patients into three groups of equal size (N = 53).

Statistical analysis

Shapiro–Wilk test was used to check the normality of the data distribution. Because the data did not have a normal distribution, non-parametric statistics were used for comparisons of mandibular incisor irregularity, satisfaction and retention costs between the experimental groups (Kruskal–Wallis and Mann–Whitney tests with Bonferroni correction for multiple comparisons). Spearman’s correlation coefficient was used to explore the relationship between continuous variables (satisfaction, changes in the irregularity index, retainer failure, time of retainer breakage, or number of breakages). A comparison of frequencies (drop out rate and incidence of relapse) between groups was made by χ2 and Fischer’s exact test. The effect size for χ2 and Fisher test was quantified by Cramer’s V, for Mann–Whitney test by the formula r = Z/√N while for Kruskal–Wallis test by the formula ε²=(H-k + 1)/n-k. For interpretation of effect size and correlation, Cohen’s criteria were employed (for r and V): 0.1–0.3 = small effect size/low correlation, 0.3–0.5 = moderate, 0.5–0.7 = large/high and > 0.7 = very large effect size/very high correlation. For the interpretation of ε² in the Kruskal–Wallis test, the squared values of Cohen’s criteria were used. Kaplan–Meier analysis with a log-rank test was used to analyse the survival rate. All analyses were conducted using a commercial software (IBM SPSS version 22, IBM, Armonk, USA).

Results

Baseline data

Participant flow

Figure 1 present participants’ flow. The pretherapeutic age of enrolled participants (N = 152) ranged from 11 to 18 years, with a median age of 15 years (interquartile range (IQR) 13–16 years) and comprised 63% females. No significant differences in age or sex between groups was observed. Therapy lasted 13–45 months (median 21; IQR 17–25 months). Age at the start of retention ranged 12–21 years (median 16 years; IQR 15–18 years). No significant difference was recorded between groups in age, sex, or treatment duration. Over the 2-year retention period, 39% of patients failed to attend the final check-up, despite reminders and invitations, or insisted on removal of the retention wire (N = 3). Two patients required retreatment owing to relapse, both occurring after 2 years of retention with a round wire, attributed to wire breakage. They were not excluded from analysis as retreatment was done after assessing Little’s irregularity index. Dropout rates were not associated with age or sex. Discontinuation differed between the retainer groups, with a moderate effect size (p < 0.001; V = 0.327), being least frequent in the rectangular retainer wire group (18%) compared to the round wire (41%) and VFR group (57%).

Fig. 1.

Participant flow

Ninety-three patients were analysed (41 rectangular, 29 round, and 23 VFR group). All those who missed planned check-ups but came to the final stage were included in the analysis (N = 93). The pre-therapeutic age of the patients ranged 11–18 years (median: 14 years; IQR: 14–16 years; 66% females). No significant differences in age or sex were observed between groups.

Primary outcome - assessment of mandibular incisor irregularity

The Little’s incisor irregularity range at the beginning of therapy in recruited samples (N = 152) was 0.2–11.2 mm (median: 4.2; IQR: 2–6.2) while at the end of treatment it ranged 0–1.8 (median 0.3; IQR 0–0.7.7).

The sample that was available for a 2-year follow up (N = 93) had Little’s irregularity index at the start 0.2–11.2 mm (median: 4.2; IQR: 2–5.7), while at the start of retention Little index ranged 0–1.4 mm (median: 0.3; IQR: 0–0.7) and after 2 years of retention 0–5.2 mm (median: 0.5; IQR: 0.2–1.0). A comparison between the groups in the irregularity index is presented in Table 1. The amount of relapse in incisor alignment was small, but it differed between groups with moderate effect size (p < 0.001; ε² = 0.160; Fig. 1). VFR showed a higher median change in incisor alignment than round (p = 0.027) and rectangular (p < 0.001), with no differences between the last two. Incidence of relapse differed between groups, with moderate effect size (p < 0.001; V = 0.437) being significantly higher in VFR (70%) than both round wire (35%) and rectangular wire groups (17%), with no significant differences between the last two. No harmful or unintended side effects were observed.

Table 1.

Comparison of mandibular incisor irregularity (Little index) and retention costs across groups (median, interquartile range (IQR), mean, standard deviation (SD), minimum (min), maximum value (max), level of significance (p), effect size (ε²))

Appliance	Median (IQR) †	Mean ± SD	Min-Max	p ‡	ε2
Little before treatment (N = 152; with drop-outs) Rectangular wire	4.5 (1.0–5.9.0.9)	5.2 ± 3.0	0.2–10.2
Round wire	4.2 (1.9–6.1)	4.2 ± 2.4	0.2–9.2
VFR	4.4 (2.1–6.7)	4.7 ± 2.9	0.3–11.2	0.704	−0.020
Little before treatment (N = 93; without drop-outs) Rectangular wire	5.2 (3.3–6.5)a	5.0 ± 2.6	0.4–10.2
Round wire	3.3 (1.6–5.4)ab	3.6 ± 2.2	0.2–9.0.2.0
VFR	2.3 (1.4–4.9)b	3.4 ± 2.7	0.4–11.2	0.009	0.060
Little after treatment (N = 152; with drop-outs) Rectangular wire	0.2 (0–0.5.5)	0.3 ± 0.4	0–1.1.1
Round wire	0.3 (0–0.8.8)	0.4 ± 0.5	0–1.8.8
VFR	0.3 (0–0.7.7)	0.4 ± 0.4	0–1.5.5	0.601	−0.020
Little after treatment (N = 93; without drop-outs) Rectangular wire	0.3 (0–0.7.7)	0.4 ± 0.4	0–1.1.1
Round wire	0.3 (0–0.7.7)	0.4 ± 0.4	0–1.0
VFR	0.3 (0–0.7.7)	0.4 ± 0.4	0–1.4.4	0.894	−0.042
Treatment-induced change in incisors irregularity (N = 93) Rectangular wire	4.6 (2.8–6.5)a	4.7 ± 4.6	0–9.9.9
Round wire	2.9 (1.0–5.1.0.1)ab	3.2 ± 2.3	0.8–8.8
VFR	2.0 (1.3–4.4)b	3.0 ± 2.7	−0.1-11.2	0.015	0.049
Little 2y retention (N = 93) Rectangular wire	0.4 (0–0.8.8)a	0.4 ± 0.4	0–1.2.2
Round wire	0.6 (0.3–1.3)ab	0.9 ± 1.2	0–5.2.2
VFR	0.9 (0.3–1.6)b	1.1 ± 1.1	0–4	0.028	0.035
Irregularity change in retention (N = 93) Rectangular wire	0 (0–0)a	0.1 ± 0.2	0–0.8.8
Round wire	0 (0–0.3.3)a	0.5 ± 1.2	0–4.2.2
VFR	0.5 (0–0.8.8)b	0.7 ± 1.0	0–3.7.7	< 0.001	0.160
Total costs (eur) Rectangular wire	44.61 (44.30–48.41)a	47.02 ± 5.46	42.76–69.07
Round wire	43.27 (43.27–47.69)ab	46.57 ± 6.49	42.04–68.69
VFR	34.39 (33.16–88.72)b	48.56 ± 25.34	32.54–91.19	0.001	0.117

† groups that do not share the same superscript letter differ significantly, ‡ Kruskal-Wallis test

Secondary outcomes

Assessment of survival rate and time

Over the 2-year follow-up period, there were 17/23 retainers that were not lost, broken or deformed in any way (survival rate: 74%) in the VFR group, 29/41 (71%) in the rectangular, and 18/29 (62%) in the round wire retainer group, with no significant differences between the groups. Additionally, there were no substantial differences found in the mean survival time (first breakage time point), which was 20.6 months in the VFR and round wire group, and 22.4 months in the rectangular retainer group (Fig. 2). The number of teeth involved in breakage/detachment ranged from 0 to 6 (median: 0), with no differences between the wire retainer groups. No correlation was observed between changes in the irregularity index, retainer failure, time of retainer breakage, or number of breakages.

Fig. 2.

Severity of incisor irregularity changes (left) and retention appliances survival rate without breakage (right)

Assessment of satisfaction

No differences in satisfaction were observed between the groups. It ranged 2–5 with a median of 5 in each group. Satisfaction was linearly related to a slight change in irregularity, with a low correlation (r = 0.252; p = 0.020). This was not related to the number of check-ups, breakages, or the total cost of retention.

Retention costs

The median time required for the impression of the mandibular arch was 1.7 min, while 15.3 min was needed to adjust the FBR on the plaster casts and bond the mandibular teeth. Total mandibular retainer production time was significantly shorter for the VFR group (48.3 min) than for the two fixed retainer groups (54.4 min for rectangular and 53.9 for round) with a very large effect size (p < 0.001; ε² = 0.569; Fig. 3). Additionally, less chair time (including entering and exiting the office) was required for VFR (8.0 min) than for FBR (median: 21.4, rectangular and 20.9, round; p < 0.001; ε² = 0.569). The time needed for check-up in retention without repairing retainer was 1.2 min (median). An additional 5.4 min per tooth was required to bond the detached retainer. Total time spent on retention (retainer fabrication, check-ups, bonding of detached wire and fabrication of new VFR) was shorter for the VFR group (median: 61.1 min) than both rectangular and round wire group with moderate effect size (80.6 and 79.6 min, respectively; p = 0.001; ε² = 0.106; Fig. 3).

Fig. 3.

Chair time for initial retainer fabrication (left), total fabrication time including laboratory processing (middle), and cumulative retention time over 2-year follow-up (right)

The number of regular check-ups ranged from 1 to 4 (median: 4), the number of additional visits ranged from 0 to 2 (median: 0), and the total number of visits ranged from 1 to 6 (median: 4), with no differences between the groups.

The number of teeth to which detached retainers were rebonded ranged from 0 to 8 (median: 0). New mandibular retainers were made 0 to 1 time (median: 0), once in the rectangular group (2%) and six times in the VFR group (26%).

Gross salary costs for initial fabrication of retainers were lower for VFR (9.31 €) than rectangular and round (15.22 and 14.93 €, respectively; p < 0.001; ε² = 0.569). The total gross salary costs during 2-year were also lower for VFR (31.16 €) than rectangular and round (41.11 and 40.60 €, respectively; p = 0.002; ε² = 0.090).

The total costs of retention differed between groups with moderate effect size (p = 0.001; ε² = 0.117) with the VFR group showing the lowest median cost (34.39 €) and rectangular wire group the highest (44.61 €; Fig. 4; Table 1).

Fig. 4.

Personnel gross salary costs related to retention in 2-year follow-up (left), and total costs related to retention (right)

Discussion

The present study revealed that VFR and bonded retainers effectively maintained the alignment of the lower front teeth over a period of 2 years, with minor differences in stability and equal patient satisfaction. The total expenses were lower for VFR than for bonded retainers, primarily because of the chair time/gross salary costs.

The incidence of relapse was higher in the VFR than in the wire retainer groups, and the change in incisor alignment reached statistical significance. However, the amount of change was minimal, below evidence-based thresholds. Some studies contrast with our findings. A recent meta-analysis reported higher long-term stability with fixed retainers than with VFR [23]. Similarly, Kucera and Marek found significantly greater irregularity in VFR users compared to bonded retainers [24]. Higher irregularities in VFR and a high dropout rate could be interrelated and influenced by a lack of compliance. Irregularity in the VFR group may have been underestimated, as dropouts could have avoided follow-up due to relapse and feelings of guilt or embarrassment about non-compliance with retainer wear. The low dropout rate in the rectangular retainer group could be related to the bulkiness of the wire, which was a reminder of the necessity to check the situation. The survival time was similar, while other studies have indicated that fixed retention in the mandible has a longer survival time and offers better sustainability of alignment in the long term (>4 years) than VFR [10, 25]. Some occlusal habits and accidental breakages contribute to the failure of removable retainers.

When comparing stability and survival between FBR, differences in our study did not reach a level of statistical significance. In line with our findings, no major differences in mandibular incisor alignment between round and rectangular wire retainers were reported in the 1-year period [26]. According to recent meta-analysis, the failure rate of mandibular FBR was 25% in the short (up to 1 year), 46% in the medium (1–4 years), and 54% in the long term (5–6 years) [27].

Concerns have been raised about the breakage of FBR and whether patients promptly report broken or detached retainers for replacement. This issue was also reported in a 20-year observational study on mandibular FBR [28]. While compliance with VFR usage is a common concern, FBR also demands a high level of patient diligence. Patients need to report breakage or debonding as soon as it is observed and maintain good oral hygiene.

The total expenses in this study were around 20% lower for VFR than FBR. The wide range in total cost values, particularly in the VFR group, suggests heterogeneity. Chair time for VFR was shorter than for bonded retainers, as the wire for bonded retainers required adaptation to plaster casts, transfer to the mouth, and bonding. Many orthodontists prefer VFR owing to reduced chair time and fewer retention-related issues. The minimal irregularities and high patient satisfaction with results 2 years following orthodontic treatment justify the use of this appliance. Few studies have compared chair time and costs between removable and fixed retainers. A recent analysis examined stability and chair time cost over a 2-year period, but only in the maxilla, finding no significant differences in incisor alignment stability or chair time costs (costs per hour for the orthodontist) between FBR and VFR, although material and emergency costs were higher for VFR [29]. Because failure rates are similar in both jaws, these findings may apply to the mandible as well [27]. Long-term costs may vary, as VFRs wear out and require replacement, while bonded retainers are prone to material fatigue and debonding because of repeated masticatory movements [14]. Therefore, long term follow-up with a cost assessment is required.

Indirect costs, such as travel expenses, school absence for adolescents, work absenteeism for parents, and reduced productivity, were not included. Therefore, from a broader economic perspective, this may have changed the overall societal costs. Because the number of regular and emergency visits did not differ between the groups, the indirect costs were similar.

When the treatment outcomes were similar, the costs were comparable. In this study, the differences between the retention protocols were minimal. Higher costs were associated with fixed retention. Results of this study have clinical applicability in selection of retainer, especially in public or resource-limited settings. Retainer cost might vary in different places, which limits the generalizability of these findings because the price of labour and energy varies, and the price of materials depends on the quantity ordered and discounts.

Limitations of the study

A key limitation of this study was the high dropout rate, particularly in the VFR group, which reduced the final sample size and may have contributed to the nonsignificant differences observed in several outcomes. Follow-up challenges were common among adolescents who relocated for study or work. Patient compliance, a known factor in VFR effectiveness, was not assessed, limiting interpretation of relapse. Satisfaction was measured using a single-item question at the 2-year follow-up; the use of validated, multi-item tools and repeated assessments would enhance reliability. While FBRs were placed by a single operator, reducing inter-operator variability, this may limit generalizability. The study benefited from the integration of both clinical and economic assessments conducted in a non-academic setting by an average practitioner. Future studies should include digital retainer production methods and a longer follow-up period. Additionally, by focusing on patient compliance and considering overall societal costs, direct and indirect expenses can provide a more comprehensive understanding of the efficacy and efficiency of retention strategies.

Conclusion

VFR and FBR had similar survival rates, induced an equal level of patient satisfaction, and effectively maintained lower front tooth alignment over 2 years, with differences in stability remaining below evidence-based thresholds. However, VFR incurred 21–23% lower costs than FBR, a finding with practical relevance for public or resource-limited settings.

Abbreviations

VFR

Vacuumformed retainers

FBR

Fixed bonded retainers

IQR

Interquartile range

SD

Standard deviation

€

Euro currency

MBT

McLaughlin-Bennett-Trevisi

UBT

University for Buissness and Technology

VZB

Vojka Zgombic Popovic

MM

Marija Miholjevic

SKU

Silvija Kanizaj Ugrin

Authors’ contributions

Formal analysis: SS. Funding acquisition: SS, Investigation: SKU, Methodology: SS, SKU, JK, EC, PS. Project administration: SS. Supervision: SS. Writing–original draft: SKU, JK. Writing–review & editing: EC, PS, SS.

Funding

Funded by University of Rijeka grant (uniri-iz-25-57), principal investigator Stjepan Spalj.

Data availability

The datasets generated and analyzed during this study can be accessed upon reasonable request from the corresponding author.

Declarations

Ethics approval and consent to participate

All participants signed an informed consent form. In individuals younger than the age of 16 consent to participate was obtained from their parents or legal guardians. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committees of the Faculty of Dental Medicine, University of Rijeka (No. 2170-57-006-01-22-1) and the School of Dental Medicine, University of Zagreb (No. 05-PA-30-III-12/2021).

Consent for publication

Written informed consent was obtained from each patient, parent or legal guardian.

Competing interests

The authors declare no competing interests.