Introduction
Our objectives were to compare the stability of treatment and periodontal health with fixed vs removable orthodontic retainers over a 4-year period.
Methods
A 4-year follow-up of participants randomly assigned to either mandibular fixed retainers from canine to canine or removable vacuum-formed retainers was undertaken. Irregularity of the mandibular anterior segment, mandibular intercanine and intermolar widths, arch length, and extraction space opening were recorded. Gingival inflammation, calculus and plaque levels, clinical attachment level, and bleeding on probing were assessed. The outcome assessor was blinded when possible.
Results
Forty-two participants were included in the analysis, 21 per group. Some relapse occurred in both treatment groups at the 4-year follow-up; however, after adjusting for confounders, the median between-groups difference was 1.64 mm higher in participants wearing vacuum-formed retainers ( P = 0.02; 95% confidence interval [CI], 0.30, 2.98 mm). No statistical difference was found between the treatment groups in terms of intercanine ( P = 0.52; 95% CI, −1.07, 0.55) and intermolar ( P = 0.55; 95% CI, −1.72, 0.93) widths, arch length ( P = 0.99; 95% CI, −1.15, 1.14), and extraction space opening ( P = 0.84; 95% CI, −1.54, 1.86). There was also no statistical difference in relation to periodontal outcomes between the treatment groups, with significant gingival inflammation and plaque levels common findings.
Conclusions
This prolonged study is the first to suggest that fixed retention offers the potential benefit of improved preservation of alignment of the mandibular labial segment in the long term. However, both types of retainers were associated with gingival inflammation and elevated plaque scores.
Highlights
- •
Prolonged retention with fixed retainers was more effective than with vacuum-formed retainers.
- •
Periodontal outcomes were similar in the treatment groups 4 years after treatment.
Prolonged and indeed indefinite retention is routinely prescribed following orthodontic treatment to mitigate against posttreatment change related to unstable positioning of teeth, physiological recovery and age-related changes Notwithstanding this, there is a lack of high-quality evidence concerning the relative effectiveness of fixed and removable variants. Moreover, the long-term impact of fixed or removable retention on the periodontium has been the subject of little prospective analysis and compliance levels with prolonged removable retention is unclear
Relatively few randomized controlled trials (RCTs) have involved comparisons of the effectiveness of fixed and vacuum-formed retainers (VFRs). Neither of these studies involved follow-ups in excess of 2 years. Thus, they reported little difference in terms of stability, with mean mandibular anterior irregularity scores less than 2.0 mm in both trials, indicating acceptable levels of stability in the short term. It is intuitive to expect that irregularity would increase over time, with important differences between these interventions conceivably only emerging over a more prolonged period. In particular, compliance with removable retainer wear may wane, leading to the development of posttreatment changes primarily due to unchecked maturational changes in the medium term. Failure of fixed retainers may also promote deterioration of the posttreatment outcome. Notwithstanding this, in view of the dearth of prolonged, prospective evaluation, the relative impact of these eventualities can only be speculated.
In terms of periodontal health, fixed retainers may hinder scrupulous oral hygiene measures; however, it is not known whether this necessarily leads to worsening of periodontal outcomes, particularly in the long term. A number of observational studies have involved assessment of periodontal integrity during the retention phase. The retrospective nature of these studies risks selection bias, and those with poorer hygiene may not be considered suitable for fixed retainers. Consequently, prospective analysis with random allocation to retainer types is preferable. It is important, therefore, to undertake a more holistic assessment of benefits and harms with prolonged use of orthodontic retainers.
Aims
The primary aim of this study was to compare the stability of orthodontic outcomes with fixed and removable retainers over a period of at least 4 years. The secondary aim was to investigate periodontal outcomes with fixed vs removable retainers over this period.
Material and methods
Follow-up was undertaken in an RCT conducted at the Institute of Dentistry, Queen Mary University of London, which had involved assessment of stability at up to 18 months posttreatment. Ethical approval was obtained (10/H0713/57, Bloomsbury Research Ethics Committee), and all participants in the previous clinical trial were contacted for possible inclusion at least 48 months after withdrawal of active appliances with an appointment arranged at their convenience. In the previous RCT, 82 participants were randomly allocated by computer-generated random allocation with the allocations concealed from the treating clinician using an opaque, sealed-envelope system. Participants received either a mandibular VFR (Essix Ace Plastic, 120 mm in diameter; DENTSPLY, Islandia, NY) or a fixed retainer (0.0175-in coaxial archwire; Ortho-Care, Shipley, United Kingdom) bonded with Transbond LR composite material (3M Unitek, London, United Kingdom). Those in the removable retainer group were instructed to wear the mandibular VFR on a full-time basis for the first 6 months, nights only for the second 6 months, and alternate nights from 12 to 18 months after removal of active appliances. Thereafter, intermittent nights-only wear (1 to 2 nights weekly) was recommended. Of the 82 participants in the previous trial, data were obtained from 48 at the 18-month follow-up.
An information sheet was given to patients willing to participate at a minimum of a 48-month follow-up after removal of active appliances, and oral and written consent was obtained. They were advised not to visit their dentist for scaling for 1 month before their appointment, and those taking medications known to have an effect on gingival health were excluded from the periodontal assessment.
Orthodontic stability was based chiefly on the irregularity of the mandibular incisors using Little’s irregularity index to assign a cumulative score for the contact point displacement in the mandibular intercanine region. Allied measurements including intercanine and intermolar widths, arch length, and extraction space opening were also recorded. Five clinical measures of periodontal health were scored: gingival inflammation, calculus and plaque levels, clinical attachment level, and bleeding on probing ( Appendix ).
An impression of the mandibular arch was taken for all participants using hydrophilic vinyl polysiloxane (Virtual; Ivoclar Vivadent, Schaan, Lichtenstein). The impression was then cast in hard (type III gypsum) stone. Orthodontic stability was measured from the study models, adopting the same technique used in the previous study. The lingual surfaces of the mandibular labial segment were obscured on the study models using prosthetic dental wax (Ribbon Wax; Metrodent, Huddersfield, United Kingdom) to ensure assessor blindness. Measurements were performed by 1 researcher (D.A.-M.) using a digital caliper (150 mm DIN 862, ABSOLUTE Digimatic caliper, model 500-191U; Mitutoyo, Andover, Hampshire, United Kingdom) with a resolution of ±0.01 mm. Periodontal measurements were recorded for the labial and lingual surfaces of the mandibular canines, and central and lateral incisors. Each tooth surface was divided into thirds using vertical lines based on the morphology and position of the dental papilla to demarcate mesial, middle, and distal surfaces. The periodontal measures were scored clinically by the same researcher ( Appendix ).
All participants were asked about frequency, duration, type of toothbrushing, and the time since the last visit to the dentist. Patients wearing mandibular VFRs were also asked to complete a retainer wear chart. The self-reported compliance levels were categorized as follows: compliant, reported wear of retainers was as advised; partially compliant, retainer wear instructions were not followed precisely; and noncompliant, not wearing retainers.
The status of the fixed retainer and the history of retainer repair and previous breakage were recorded in the fixed retainer group.
Interexaminer and intraexaminer reliabilities of clinical and study model measurements were tested by assessing agreement between repeated measurements. For stability outcomes, intraexaminer reliability was determined on 10 randomly selected study models 4 weeks after the initial measurements. Interexaminer reliability (D.A.-M., N.O.) was determined on 10 randomly selected study models. There was excellent agreement for intraexaminer (0.97) and interexaminer (0.92) reliabilities. Because the examiner (D.A.-M.) was an orthodontist, familiarization with the measurement of periodontal outcomes was required and was facilitated by completion of an online course with oversight from a specialist in periodontology (C.G.-M.) before recruitment. Intraexaminer reliability for scoring the modified gingival index and plaque index was assessed by repeating the measurements on 10 intraoral photographs at a 4-week interval. Repeated measurements were performed on 10 healthy volunteers 30 minutes apart to assess the repeatability of measurements of calculus scores and clinical attachment levels. Excellent agreement was observed (0.94 to 0.97) for interexaminer reliability.
Sample size calculation
The initial sample size was calculated based on previous research, although a higher level of attrition was to be expected after the more prolonged follow-up. A total of 72 participants (36 in each group) were required for power of 90% to detect a difference of 0.5 mm at the 0.05 level of statistical significance. To compensate for a dropout rate of at least 15%, the final number enrolled in the trial was 82 participants at the outset.
Statistical analysis
Since the data were not normally distributed, median regression was used to compare the effectiveness of the 2 types of retainers on orthodontic stability accounting for baseline differences between the groups. Similarly, the median differences between fixed and removable retainers in terms of gingival inflammation, calculus and plaque levels, clinical attachment levels, and bleeding on probing were assessed using the Mann-Whitney U test. A subgroup analysis was performed to compare the median differences in periodontal outcomes between the fixed and removable groups on the labial and lingual surfaces independently. If significant differences were identified in relation to gingival inflammation, plaque, or calculus scores, probing depth, or bleeding on probing, median regression analysis was used to assess the influence of age, sex, brushing frequency and duration, and type of retainer on the outcome. A similar model was used to evaluate the effect of retainer type on the clinical attachment level. The level of statistical significance was set at 0.05 with all analyses undertaken using the Stata statistical software package (version 14.1; StataCorp, College Station, Tex).
Material and methods
Follow-up was undertaken in an RCT conducted at the Institute of Dentistry, Queen Mary University of London, which had involved assessment of stability at up to 18 months posttreatment. Ethical approval was obtained (10/H0713/57, Bloomsbury Research Ethics Committee), and all participants in the previous clinical trial were contacted for possible inclusion at least 48 months after withdrawal of active appliances with an appointment arranged at their convenience. In the previous RCT, 82 participants were randomly allocated by computer-generated random allocation with the allocations concealed from the treating clinician using an opaque, sealed-envelope system. Participants received either a mandibular VFR (Essix Ace Plastic, 120 mm in diameter; DENTSPLY, Islandia, NY) or a fixed retainer (0.0175-in coaxial archwire; Ortho-Care, Shipley, United Kingdom) bonded with Transbond LR composite material (3M Unitek, London, United Kingdom). Those in the removable retainer group were instructed to wear the mandibular VFR on a full-time basis for the first 6 months, nights only for the second 6 months, and alternate nights from 12 to 18 months after removal of active appliances. Thereafter, intermittent nights-only wear (1 to 2 nights weekly) was recommended. Of the 82 participants in the previous trial, data were obtained from 48 at the 18-month follow-up.
An information sheet was given to patients willing to participate at a minimum of a 48-month follow-up after removal of active appliances, and oral and written consent was obtained. They were advised not to visit their dentist for scaling for 1 month before their appointment, and those taking medications known to have an effect on gingival health were excluded from the periodontal assessment.
Orthodontic stability was based chiefly on the irregularity of the mandibular incisors using Little’s irregularity index to assign a cumulative score for the contact point displacement in the mandibular intercanine region. Allied measurements including intercanine and intermolar widths, arch length, and extraction space opening were also recorded. Five clinical measures of periodontal health were scored: gingival inflammation, calculus and plaque levels, clinical attachment level, and bleeding on probing ( Appendix ).
An impression of the mandibular arch was taken for all participants using hydrophilic vinyl polysiloxane (Virtual; Ivoclar Vivadent, Schaan, Lichtenstein). The impression was then cast in hard (type III gypsum) stone. Orthodontic stability was measured from the study models, adopting the same technique used in the previous study. The lingual surfaces of the mandibular labial segment were obscured on the study models using prosthetic dental wax (Ribbon Wax; Metrodent, Huddersfield, United Kingdom) to ensure assessor blindness. Measurements were performed by 1 researcher (D.A.-M.) using a digital caliper (150 mm DIN 862, ABSOLUTE Digimatic caliper, model 500-191U; Mitutoyo, Andover, Hampshire, United Kingdom) with a resolution of ±0.01 mm. Periodontal measurements were recorded for the labial and lingual surfaces of the mandibular canines, and central and lateral incisors. Each tooth surface was divided into thirds using vertical lines based on the morphology and position of the dental papilla to demarcate mesial, middle, and distal surfaces. The periodontal measures were scored clinically by the same researcher ( Appendix ).
All participants were asked about frequency, duration, type of toothbrushing, and the time since the last visit to the dentist. Patients wearing mandibular VFRs were also asked to complete a retainer wear chart. The self-reported compliance levels were categorized as follows: compliant, reported wear of retainers was as advised; partially compliant, retainer wear instructions were not followed precisely; and noncompliant, not wearing retainers.
The status of the fixed retainer and the history of retainer repair and previous breakage were recorded in the fixed retainer group.
Interexaminer and intraexaminer reliabilities of clinical and study model measurements were tested by assessing agreement between repeated measurements. For stability outcomes, intraexaminer reliability was determined on 10 randomly selected study models 4 weeks after the initial measurements. Interexaminer reliability (D.A.-M., N.O.) was determined on 10 randomly selected study models. There was excellent agreement for intraexaminer (0.97) and interexaminer (0.92) reliabilities. Because the examiner (D.A.-M.) was an orthodontist, familiarization with the measurement of periodontal outcomes was required and was facilitated by completion of an online course with oversight from a specialist in periodontology (C.G.-M.) before recruitment. Intraexaminer reliability for scoring the modified gingival index and plaque index was assessed by repeating the measurements on 10 intraoral photographs at a 4-week interval. Repeated measurements were performed on 10 healthy volunteers 30 minutes apart to assess the repeatability of measurements of calculus scores and clinical attachment levels. Excellent agreement was observed (0.94 to 0.97) for interexaminer reliability.
Sample size calculation
The initial sample size was calculated based on previous research, although a higher level of attrition was to be expected after the more prolonged follow-up. A total of 72 participants (36 in each group) were required for power of 90% to detect a difference of 0.5 mm at the 0.05 level of statistical significance. To compensate for a dropout rate of at least 15%, the final number enrolled in the trial was 82 participants at the outset.
Statistical analysis
Since the data were not normally distributed, median regression was used to compare the effectiveness of the 2 types of retainers on orthodontic stability accounting for baseline differences between the groups. Similarly, the median differences between fixed and removable retainers in terms of gingival inflammation, calculus and plaque levels, clinical attachment levels, and bleeding on probing were assessed using the Mann-Whitney U test. A subgroup analysis was performed to compare the median differences in periodontal outcomes between the fixed and removable groups on the labial and lingual surfaces independently. If significant differences were identified in relation to gingival inflammation, plaque, or calculus scores, probing depth, or bleeding on probing, median regression analysis was used to assess the influence of age, sex, brushing frequency and duration, and type of retainer on the outcome. A similar model was used to evaluate the effect of retainer type on the clinical attachment level. The level of statistical significance was set at 0.05 with all analyses undertaken using the Stata statistical software package (version 14.1; StataCorp, College Station, Tex).
Results
Eighty-two participants were enrolled in the original RCT. Of these, 48 attended the 18-month follow-up. At the 4-year follow-up, 42 participants returned: 21 per group ( Fig ). The groups were well matched in terms of age, sex, and treatment protocol; most were female, and 43% and 48% had extraction-based treatment in the fixed and removable groups, respectively ( Table I ). In terms of fixed retainer integrity, all (100%) were in place at the recall, although 3 (14%) were partially detached, and 2 (10%) had a history of repairs. In the removable retainer group, the reported noncompliance levels increased from 0% over the initial 6 months to 19% at 6 to 12 months, 52% in the second year, and 67% thereafter.
| Overall sample n = 42 |
FR group n = 21 |
VFR group n = 21 |
|
|---|---|---|---|
| Mean age in years (SD) | 21.15 (2.41) | 21.54 (3.06) | 20.77 (1.49) |
| Sex | |||
| Men | n = 10 | n = 3 | n = 7 |
| Women | n = 32 | n = 18 | n = 14 |
| Mean years in retention (SD) | 4.16 (0.35) | 4.09 (0.25) | 4.23 (0.42) |
| Treatment protocol | |||
| Extraction | n = 19 | n = 9 | n = 10 |
| Nonextraction | n = 23 | n = 12 | n = 11 |
| Type of toothbrush | |||
| Manual | n = 37 | n = 18 | n = 19 |
| Electric | n = 5 | n = 3 | n = 2 |
| Daily toothbrushing frequency | |||
| 1 time | n = 7 | n = 6 | n = 1 |
| 2 times | n = 35 | n = 15 | n = 20 |
| Time spent in toothbrushing | |||
| <1 minute | n = 1 | n = 0 | n = 1 |
| 1-2 minutes | n = 29 | n = 14 | n = 15 |
| >2 minutes | n = 12 | n = 7 | n = 5 |
| Use of other oral hygiene measures | |||
| None | n = 23 | n = 13 | n = 10 |
| Dental floss | n = 10 | n = 4 | n = 6 |
| Interdental brush | n = 3 | n = 2 | n = 1 |
| Toothpick | n = 9 | n = 4 | n = 5 |
| Last visit to the dentist | |||
| <6 months | n = 10 | n = 5 | n = 5 |
| 6 months to <1 year | n = 5 | n = 3 | n = 2 |
| 1-2 years | n = 12 | n = 9 | n = 3 |
| >2 years | n = 15 | n = 4 | n = 11 |
| Smokers | n = 4 | n = 3 | n = 1 |
| Gingival biotype | |||
| Thick | n = 17 | n = 7 | n = 10 |
| Thin | n = 24 | n = 14 | n = 10 |
| Frenal attachment | |||
| Low | n = 41 | n = 21 | n = 20 |
| High | n = 1 | n = 0 | n = 1 |
Stay updated, free dental videos. Join our Telegram channel
VIDEdental - Online dental courses
