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Year : 2021  |  Volume : 32  |  Issue : 2  |  Page : 174-180
A prospective randomized controlled trial on the comparative clinical efficiency and hygiene of a ceramic inter locking retainer and a flexible spiral wire bonded retainer

Department of Orthodontics and Dentofacial Orthopedics, Institute of Dental Studies and Technologies (IDST), Ghaziabad, Uttar Pradesh, India

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Date of Submission26-Nov-2020
Date of Decision02-Dec-2020
Date of Acceptance15-Jun-2021
Date of Web Publication22-Nov-2021


Background: The lower canine to canine retainer is one of the most commonly used retention appliance. Objective: Compare and assess the clinical efficiency and oral hygiene maintenance with 2 retainers: flexible spiral wire and ceramic interlocking bonded. Setting and Sample Population: Orthodontic department of a dental college. Participants, Study Design and Methods: Prospective randomized controlled trial that involved 40 patients who had completed orthodontic treatment and were randomly allotted to either group: flexible spiral wire (FSW) or ceramic interlocking retainers. Allocation ratio was 1:1. Eligibility Criteria: Completed fixed orthodontic treatment with an acceptable treatment result. Main Outcomes: Oral hygiene was assessed at 3 (T1) and 6 (T2) months intervals after debonding using Gingival and Plaque index. The contact point displacement was assessed using 3D model superimposition and bonded retainer failure was noted. Randomization and Blinding: Computer-generated random allocation, only data analyzer was blinded. Statistics: Data normality was tested using the Shapiro-Wilk and Q-Q plot. Mixed model analysis was used to assess differences between the two groups for the above-mentioned parameters. The failure rate of the retainer was calculated by the Chi-Square test. Results: Only the plaque index score was significantly greater in the FSW group and showed a statistically significant difference (P < 0.05), rest of the parameters showed no statistically significant difference over the specified time points, although a mild increase in relapse was noted in both groups No serious harms were reported. Conclusion: Both retainers seem to be effective in maintaining the treatment results however the FSW seems to accumulate more plaque over a given period of time thereby mandating proper hygiene maintenance and follow-up.

Keywords: Ceramic interlocking retainer, flexible spiral wire, orthodontic relapse, retention.

How to cite this article:
Sinha A, Sonar S, Batra P, Raghavan S. A prospective randomized controlled trial on the comparative clinical efficiency and hygiene of a ceramic inter locking retainer and a flexible spiral wire bonded retainer. Indian J Dent Res 2021;32:174-80

How to cite this URL:
Sinha A, Sonar S, Batra P, Raghavan S. A prospective randomized controlled trial on the comparative clinical efficiency and hygiene of a ceramic inter locking retainer and a flexible spiral wire bonded retainer. Indian J Dent Res [serial online] 2021 [cited 2021 Dec 8];32:174-80. Available from:

   Introduction Top

Orthodontic relapse is well recognized and documented reality in orthodontic literature.[1],[2] After active treatment is completed, long-term preservation of the corrected tooth positions is desirable, both for the clinician and for the patient.[3],[4],[5] Contemporary retaining strategies basically include removable and flexible retainers. The canine-to-canine banded retainer along with a rigid 0.036-inch stainless steel wire was one of the most commonly used methods of rigid retention.[6] Zachrisson in 1983 introduced a multi-stranded wire (0.0215 inch) bonded lingual retainer; bonded on the lingual surface of all the teeth in the mandibular anterior segment. In different clinical situations, bonded retainers using multistranded flexible wire of dimension from 0.015” to 0.020” can be used.[7] Later on, many dimensions (0.015 inch, 0.0175 inch, 0.0195 inch, 0.020 inch, 0.021 inch, 0.0215 inch, 0.032 inch) of flexible spiral wire (FSW) have been used to overcome the rigidity and permitting the physiological movement of the tooth within the periodontal ligament premises.[8],[9],[10],[11],[12] These bonded lingual retainers are intended to serve for long periods of time in the mouth and various attempts were made to increase the success rate of these devices with the maintenance of oral hygiene.[8],[13],[14],[15]

The clinical trials which were retrospective[3],[5],[16],[17] as well as prospective[14],[18],[19],[20],[21] that have been conducted to date concluded an increase in failure rates and decreasing oral hygiene status with flexible spiral wires and glass fiber reinforced retainers.[19],[22] The detachment of the wire in bonded lingual retainers occurs from cracks within the composite which arises from deformation of the inter-dental wire.[22],[23] To counter this, an alternative rigid flossable ceramic interlocking retainer (Ling Lock™ 3 M Unitek, Monrovia, USA) was introduced by Amundsen and Wisth[24] in 2006 with the main objective for long-term retention of the lower anterior teeth without increasing the risk to the patients of developing periodontal disease or caries in the actual retention area. Another advantage of using the ceramic interlocking retainer in place of a flexible spiral wire bonded retainer is with prevention of adverse effects from long term use of stainless steel in mouth there are chances of oral mucosa contamination with the nickel released from stainless steel.[24]

Unfortunately, till date no literature has been published comparing this ceramic interlocking retainers with other fixed lingual retainers. The aim of the present study was to compare and evaluate the clinical efficiency in terms of retention, and oral hygiene status between the two retainer systems.

Specific objectives and hypothesis

The null hypothesis for the trial was that there is no difference in the clinical efficiency in terms of retention, and oral hygiene maintenance status between the two retainer systems.

Trial design and any changes after trial commencement

This prospective randomized controlled trial included 40 patients visiting the Department of Orthodontics and Dentofacial Orthopedics with an allocation ratio of 1:1. All the patients were informed about the study and a written consent was taken. The ethical committee of the institute had approved the trial - (Ethical Clearance Number: IDST/IERBC/2015-18/09. The trial followed the Declaration of Helsinki Guidelines and the reporting of the trial has been done according to the CONSORT guidelines. No changes were made after trial commencement.

Participants, eligibility criteria, and settings

Patients who participated in the present trial were screened from October 2015 till May 18. The inclusion criteria for case selection include healthy gingival status with probing depth less than 3 mm with no radiographic evidence of bone loss, Little's Irregularity Index ≤2 mm and patients treated with optimum occlusion with the all-conventional orthodontic treatment objectives satisfied. The patients selected had undergone orthodontic treatment without extractions. The excluded patients were those with impacted teeth and patients with anomalies in the lower anterior teeth.


The study sample was divided randomly into two groups. Group I bonded with the ceramic interlocking retainer canine to canine [Figure 1] or Group II with the flexible spiral wire retainer canine to canine. [Figure 2] After achieving optimal occlusion and treatment goals, the patients were de-bonded and retainers (flexible spiral wire and ceramic interlocking retainer) were placed. The retainers were placed by a single orthodontist.
Figure 1: Ceramic interlocking retainer

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Figure 2: Flexible spiral wire retainer

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The ceramic interlocking retainer

The ceramic interlocking retainer (Ling Lock™ 3 M Unitek, Monrovia, USA) was developed in a private practice and the research and developmental program was continued and conducted in collaboration with the Department of Orthodontics, University of Bergen, Norway and currently is not in commercial production. The retainer was made up of pairs of separate, but co-working retention elements constructed in ceramic aluminum oxide (Al2O3) [Figure 3]. The base of the retainer was typically adhesive pre-coated (APC) which aid in the adhesion of the retainer on to the tooth surface. An application strip was present in between the two parts of ceramic interlocking retainer. This application strip helps in guiding the retainer in the interproximal surfaces of the neighboring teeth. The retainer was brought into tooth contact and the guide strip is used to establish the position in superior/inferior direction. Pairs of elements were bonded to the lingual aspect of the lower anterior teeth from canine to canine. Retention was created by intimate contact of the complimentary shaped and outlined contact surfaces of the retention elements. These are co-working interlocking male and female parts.[25] Pair of retention elements had similar radio-opacity to a ceramic bracket.[24]
Figure 3: Parts of Ceramic interlocking retainer

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Flexible spiral wire retainers

Flexible spiral wire retainers used in the study were made of round coaxial wire of dimension 0.016 inch. The retainer was bonded by direct method using ligature wire for stabilizing on the lingual surface of mandibular anterior teeth in the patient's mouth and the light cure adhesive (Transbond XT, 3 M Unitek, Monrovia, USA) was used.

After placement of flexible spiral retainers or ceramic interlocking retainers, oral hygiene instructions and inter-dental cleaning using dental floss were given to the patients [Figure 4]. Plaque and gingival indices were evaluated at T0 (time of retainer placement), T1 (3 months after retainer placement) and T2 (6 months after retainer placement).[26]
Figure 4: Flossing in Ceramic interlocking retainer and Flexible spiral wire retainer

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Primary and secondary outcomes

The clinical efficiency in terms of retention (using Little's irregularity index), retainer failure and oral hygiene status (using the plaque and gingival index) between the two retainer systems were evaluated after 3 (T1) and 6 (T2) months of debonding. A baseline reading was noted at the time of debonding as well. The study models of the 40 patients at T1 and T2 were scanned using the white light scanner (Comet5, 100-200-400, Steinbichler Optotechnik, Germany) [Figure 5] and 3D reverse modeling software program. The 3D models obtained were sterolitho-graphic models in STL format. Measurements were done by superimposing the 3D models over each other and comparing the discrepancies in all the planes using Polyworks Software Version 12.0 (InnovMetric Software Inc, Canada) to the nearest 0.001 mm (resolution = ±0.000001 mm) [Figure 6]. The precision of the superimposition of the 3D models over each other helped us in evaluating the discrepancies of the mandibular anterior teeth malalignment in all the three planes. A sample of 16 randomly selected patients (8 from each group) was re-measured by the same assessor after 2 weeks for repeatability and re-test reliability. Intra class co-efficient was found to be 0.93 and standard error of mean was calculated using the paired t-test, and was found to be statistically not significant.
Figure 5: White light scanner (Comet5, 100-200-400, Steinbichler Optotechnik, Germany)

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Randomization was done using a computer-generated random allocation sequence amongst the 2 groups. The sequences were concealed and were chosen by the patient. The primary investigator had no part in the randomization sequencing.

Sample size calculation

The sample size estimation for this study yielded a sample size of 20 participants per group (minimum power of 80% and an α error of 0.05 and Cohen's effect size of 0.4) to detect mean difference in irregularity of 5 (.25 SD).


Blinding of the participants, primary investigator, data analyzer was not possible due to trial nature.

Statistical analysis

Baseline parameters (at the time of debonding) like age, sex and Little's Irregularity index were compared between the 2 groups using the independent 't' test to ensure a homogeneous baseline and the results were statistically insignificant, thereby ensuring a comparative pre-treatment baseline. Normality of the data was evaluated using the Shapiro Wilk test and a Q-Q Plot, the data was found to be normally distributed, therefore parametric inferential tests were used.

The means of Plaque index, Gingival index, Irregularity index and 3-D model deviations were tabulated and the data was analyzed using the SPSS (Statistical Package for Social Service) version 20.0. As baselines within randomized groups are unreliable,[27] a mixed model analysis factoring the time-retainer interaction was done to assess the influence of time in the two retention groups for all the parameters. The failure rate of the retainer was calculated by the Chi-Square test done for each group. Level of significance was set at 0.05.

   Results Top

A total of 68 patients were screened for the trial out of which 61 patients fulfilled the inclusion criteria. 21 patients did not give their consent for the participation so a total of 40 patients participated in the trial (20 females and 20 males). Mean age for the FSW group was 15.61 ± 1.13 years and for the ceramic interlocking retainers was 15.83 ± 1.07 years respectively.

Gingival index

The mean and standard deviation of gingival index score of the both groups at T1 and T2 were tabulated [Table 1]. No statistically significant difference was noted when analyzed using the mixed model analysis between the groups. A mild decrease was noted in scores of both groups between T1 and T2.
Table 1: Intergroup comparison of gingival index score in Group I and Group II at T1 and T2

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Plaque index

The mean and standard deviation of plaque index score of the both groups at T1 and T2 were tabulated [Table 2]. There was a statistically significant difference noted between the two groups over the two time points (P < 0.05) with greater plaque noted in the FSW group.
Table 2: Intergroup comparison of plaque index score in Group I and Group II

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Irregularity index and 3D model deviations

The mean and standard deviation of Little's irregularity index score was tabulated along with the respective deviation in the X, Y and Z axis i.e., the displacement in the mesiodistal, labiolingual and vertical direction respectively for the both groups at T1 and T2 were tabulated [Table 3] and [Table 4]. There was no statistically significant difference noted in any of the above-mentioned parameters when analyzed using the mixed model method, although mild increases in the irregularity and 3-D deviations were noted in both groups.
Table 3: Intergroup comparison of little's irregularity index score in Group I and Group II

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Table 4: Intergroup comparison of 3-D superimposition in the respective T1 and T2 in Group I and Group II

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Bond failure

Both appliances had 2 bond failures (10%) reported at T1 which increased to 4 (20%) at T2 for the ceramic interlocking group [Figure 7] while it remained stable at 2 failures for the FSW group [Figure 8]. The differences between both the groups at both time points was statistically not significant when assessed using Chi-Square test [Table 5].
Table 5: Intergroup comparison of failure rates in the respective time intervals between the two groups

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Figure 6: 3-D model superimposition

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Figure 7: Breakage in Ceramic interlocking retainer

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Figure 8: Breakage in Flexible spiral wire retainer

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Harms evaluated for the trial were wire breakage, subsequent impingement in the gingival tissue and accidental swallowing or aspirations. Adequate instructions were given to the participants and guidance in case of an emergency. No harms were reported during the trial.

   Discussion Top

Main findings in the context of existing evidence

Mandibular alignment instability is one of the major pitfalls of orthodontic treatment[4],[6],[28] Many studies have demonstrated a high relapse rate of the alignment of the mandibular anterior teeth after orthodontic treatment and during the retention period as well. As a result, orthodontists tend to recommend long-term use of retainers for enhanced stability, which may require years or even decades of retainer wear.[4],[29],[30]

The flexible wire retainer is indicated in those situations where individual tooth movements like rotations are prone to relapse.[4],[6],[7] The disadvantages of these retainers are attributed to the demanding technique of placing the retainer and the potential for tooth movement due to distortion or lack of passivity of the wire. Bond failures may also constitute a problem, estimated to range between 6 to 35 percent, depending on the technique used and follow-up observation period.[14],[31],[32] These bonded flexible retainers have been shown to increase plaque and calculus accumulation compared with removable retainers. This, however, was not found to have detrimental effects on the integrity of the dental hard tissues adjacent to the wire.[4],[8]

A few years ago, a flossable ceramic interlocking rigid type of retainer, was introduced with the main objective of long-term retention of the lower anterior teeth; without increasing the risk to the patients of developing periodontal disease or caries in the actual retention area.[15],[24] The rigidity of this appliance is due to the intimate contact of the two parts of a unit bonded at interdental area on the lingual surfaces of the lower anterior teeth. Due to the rigidity of the appliance, 3-dimensional control can be better achieved in ceramic interlocking retainer whereas flexible spiral wire retainer permits physiological movement of the tooth within the periodontal ligament premises.

The unique design of the ceramic interlocking retainer enables the patient to floss the teeth in the actual retention area, while maintaining the incisal alignment. This additional objective helps in the long-term retention protocols in the mandibular anterior teeth by maintaining adequate periodontal health and also preventing orthodontic relapse[24] whereas in case of flexible spiral wire retainer dental flossing is possible only up to the level of the bonded wire.

In the present study, subjects given the ceramic interlocking retainer (Group I) showed statistically significant lower levels of plaque accumulation when compared with patients with flexible spiral wire retainers (Group II); even though oral hygiene was reinforced at each assessment to the subjects of both groups. However, subjects with ceramic interlocking retainers (Group I) and flexible spiral wire retainers (Group II) showed no statistically significant difference in gingival health scores. Though flexible spiral wire retainers (Group II) showed statistically significant increase of plaque accummulation, there was no evidence of increased periodontal disease or enamel decalcification in relation to lingual bonded retainers and there was no evidence of greater plaque deposits on multistranded wire which is in agreement with the long term studies conducted in this regard[3],[12],[15] with the flexible bonded retainer. However, a long term follow up study by Pandis et al.,[22] in 2007 revealed the deepening of periodontal pockets and increase of calculus deposits and gingival recession in patients with bonded lingual retainers over 10 years. The result of this study suggests that the placement of retainer for a long time has a direct effect on the plaque and gingival index score. Since our study had a lesser follow-up period, this would be best probable explanation for the difference in the periodontal health scores.

In the present study, both Group I and Group II had matching baseline irregularity index scores at the time of debonding. The findings of the present study suggested comparable irregularity index scores in both the groups suggested similar retentive control by both appliances. Similarly, deviations in 3-planes were also similar between the 2 appliances although an trend of mild increase was noted in both the irregularity index scores and the 3-D deviations which was neither statistically nor clinically significant.

The immediate tendency of the teeth to rebound towards their original positions, and also the mobility associated with these teeth immediately after the appliance removal along with the fact in case of flexible spiral wire, the designs and flexibility of the spiral wire permits physiological movement of the tooth within the periodontal ligament premises could explain some of the relapse observed. Additionally, the two-part assembly of the ceramic interlocking retainer allows some degree of play between the two adjoining teeth as opposed to the claims made by the manufacturer.[24]

Another important factor that might account for the increase of the irregularity index over the two timepoints T2 was bonding failure. In the present study, failure in the retainer was noted in wire - composite interface and composite - tooth interface. In case of flexible spiral wire retainer, 10% failure rate was observed. According to Lie Sam Foek et al.,[16] and Nagani et al.,[14] in flexible spiral wire retainer maximum failure rate occurs in the first six months due the increased mobility of the teeth in the initial post treatment period that favors detachments. Failures can be inherent, as a result of poor chair-side technique, or acquired, from wear or direct trauma to the retainer. In case of ceramic interlocking retainer, a 20% failure rate was observed mostly in the canine-lateral incisor interface. This can be due to alteration or relapse in the arch forms from pretreatment to debonding or post debonding, poor chair-side technique, or acquired, from wear or direct trauma to the retainer as well.

Limitations of the study

A smaller sample size and the limited followed up period could be considered as limitations for the study.


The results may be considered practical in a conventional clinical scenario as the trial was conducted in an accredited and recognized dental college with an outpatient which can potentially mimic a typical orthodontic case load.

   Conclusions Top

In the present study, both retainers were efficient in controlling the post-treatment relapse however the ceramic interlocking retainer seemed to permit the maintenance of better oral hygiene and lesser plaque retention.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

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Correspondence Address:
Dr. Sreevatsan Raghavan
Department of Orthodontics and Dentofacial Orthopedics, Institute of Dental Studies and Technologies (IDST), Delhi-Meerut Road, Kadrabad, Modinagar, Ghaziabad, Uttar Pradesh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijdr.IJDR_1050_20

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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