Introduction
The objectives of this research were to evaluate changes in occlusal components in 3 subperiods during a 10-year posttreatment time span and to examine the long-term effects of fixed retention on maxillary and mandibular anterior alignment.
Methods
Ninety-six patients were examined; the Peer Assessment Rating Index and Little’s Irregularity Index were measured at pretreatment, posttreatment, and 3 (T3), 5 (T5), and 10 (T10) years posttreatment. Unweighted Peer Assessment Rating component scores were analyzed for differences between all subperiods. The effect of fixed retention on posttreatment changes in Little’s Irregularity Index was analyzed for both jaws with regression analysis. For the maxilla, 2 groups were compared: MX0, removable retainer until T3 (n = 52) and MX10, removable retainer until T3 combined with a fixed retainer until T10 (n = 23). For the mandible, 3 groups were compared: MD3, fixed retainer until T3 (n = 19); MD5, fixed retainer until T5 (n = 19); and MD10 fixed retainer until T10 (n = 48).
Results
The Peer Assessment Rating Index percentage of improvement was 79% at T10. A gradual deterioration of occlusal components was seen, with small insignificant changes in each subperiod. Corrected for pretreatment irregularity, MX10 showed 0.6 mm lower LII than MX0. MD10 had significantly better alignment than MD3 (1.1 mm) and MD5 (0.7 mm).
Conclusions
Gradual occlusal changes of limited clinical importance were seen during a 10-year posttreatment period. Long-term fixed retention in the maxilla was of minor importance in patients also wearing removable retainers. In the mandible, a 10-year fixed retention protocol gave moderately lower alignment scores compared to a 3-year protocol and slightly better alignment compared to a 5-year protocol.
Highlights
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Components of the occlusion underwent gradual changes throughout the period.
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In maxilla, the effect of dual retention over removable retention was minor.
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In the mandible, 10 years of fixed retention gave better alignment than 3 and 5 years.
Total stability of the occlusion after orthodontic treatment seems to be unlikely to achieve. With time, changes in tooth alignment are inevitable, and both relapse and growth are contributing factors. To counteract unwanted changes, orthodontists prescribe different types of retainers, sometimes intended to be used indefinitely.
Both removable and fixed retainers have been found to be equally effective in controlling relapse up to 2 years posttreatment. Part-time wear of thermoplastic and Hawley retainers has proved to be as efficient as full-time wear. Methods for controlling the anterior alignment even without retainers such as interproximal reduction, sometimes used in combination with overcorrection, have been shown to be viable in the short term.
It is unclear at what point after debonding the greatest posttreatment changes occur. Some authors have found that most changes take place during the first 2 years after treatment, a period corresponding well with relapse and settling. In contrast, early stability has been reported for all components of occlusion 2 years after treatment. Others have reported that most occlusal changes take place during the first 4 years. Moreover, long-term studies have concluded that significant occlusal changes take place even between 19 and 31 years of age. With more information about posttreatment changes, one could possibly improve retention strategies.
Prolonged retention can interfere with the natural reduction in dental arch parameters. Since long-term compliance with a removable retainer is expected to be limited and less practical, a semipermanent or permanent retainer is often the bonded type. It has been stated that lifetime permanent retention is the only way to prevent relapse. Al Yami et al found a positive effect of fixed retainers on the Peer Assessment Raing (PAR) score 11 years posttreatment. Furthermore, presence of fixed retainers gave better occlusal results 17 years posttreatment in a long-term follow-up. In contrast, some studies have concluded that fixed retention is not of major importance to the treatment outcome. Rather than being a protective measure against long-term changes on an occlusal level, fixed retainers were found to primarily inhibit changes in anterior alignment. The efficacy of fixed retainers on mandibular anterior relapse has been reported. Nonetheless, satisfactory alignment has also been found at 10 years postretention, even after a short retention protocol. In the maxilla, fixed retainers appear to have less influence on the stability of alignment compared with the mandible. There seems to be uncertainty about how much a fixed retainer will improve the alignment in the long term.
For the time being, the preferred type and duration of retention have not been established. Use of retention appliances varies between countries and largely depends on personal preferences. It is therefore important to improve our knowledge about the effect of different retention protocols, types, and durations on long-term treatment outcomes.
The aims of the study were to evaluate changes in occlusal components in 3 stages during a 10-year posttreatment period and to examine the effect of type and duration of retention on maxillary and mandibular anterior alignment.
Material and methods
The Department of Orthodontics at the University of Oslo in Norway routinely summons patients for checkups at 3, 5, and 10 years posttreatment. Included in this retention archive are nonsurgical patients aged 20 years or younger at the beginning of treatment, without agenesis, trauma, or autotransplantations to the anterior regions. To detect a minimum difference of 10 PAR score points between pretreatment and 10 years posttreatment with a standard deviation of 11, a sample size of 12 patients was required to provide 80% statistical power with an alpha of 0.05. Attendance at the 10-year follow-up appointment per March 22, 2017 served as inclusion criteria for this longitudinal analytical study. Approval was granted by the Regional Committee for Medical and Health Research Ethics and the Norwegian Data Protection Official for Research. One hundred twenty-five patients met the inclusion criteria. Exclusions were made according to the following criteria: missing or damaged dental cast (pretreatment, posttreatment, or 10-year follow-up) (n = 18), retreatment (n = 5), single-arch treatment (n = 4), and extractions of incisors (n = 2). The final study sample included 96 patients (43 male, 53 female) treated with full fixed appliances. Study casts were available for all patients at pretreatment (T0), posttreatment (T1), and 10 years posttreatment (T10), as well as for 70 patients at 3 years posttreatment (T3) and 86 patients at 5 years posttreatment (T5). Fifty-six patients were treated without extractions, 28 patients were treated with extraction of 4 premolars, 8 patients had extraction of 2 maxillary premolars, and 4 patients had extraction of 2 mandibular premolars. Mean pretreatment age was 12.2 years (±1.4). Mean treatment duration was 2.6 years (±0.9). Average follow-up periods were 3.1 years (±0.4) at T3, 5.3 years (±0.6) at T5, and 10.1 years (±0.8) at T10.
The PAR index was used to assess the occlusion at all time stages. Anterior tooth alignment was scored using Little’s Irregularity Index (LII). In addition, several dental cast measurements were registered ( Table I ). All variables were measured by the same examiner (R.B.) to the closest 0.1 mm using a digital caliper (Digital 6; Mauser, Oberndorf, Germany), except for overjet and overbite, which were measured to the nearest 0.5 mm using a ruler. All patients had received a removable retainer in the maxilla with instructions for initial full-time wear followed by a gradual reduction until cessation at around the 3-year checkup (T3). Duration of fixed retainer wear varied for both arches. To examine the effect of duration of retention on anterior tooth alignment, retention subgroups were created based on the applied retention protocol ( Table II ). For the maxilla, 2 groups of patients were compared: MX0, removable retainer until T3 (n = 52); and MX10, removable retainer until T3 combined with fixed retainer until T10 (n = 23). Only patients with a retainer bonded to the four incisors (n = 12) or all six anterior teeth (n = 11) were included in the MX10 group. For the mandible, 3 groups were compared: MD3, fixed retainer until T3 (n = 19); MD5, fixed retainer until T5 (n = 19); and MD10, fixed retainer until T10 (n = 48).
| Measurement | Definition |
|---|---|
| PAR index | Measured according to the conventions described by Richmond et al |
| LII | The sum of the linear displacements of the anatomic contact points from canine to canine, according to Little |
| Overjet | Distance parallel to the occlusal plane from the buccal surface of the most protruding maxillary incisor to the buccal surface of the corresponding mandibular incisor |
| Overbite | Maximum distance of the mandibular incisors overlapped by the maxillary central incisors |
| Canine relationship ∗ | Distance from the cusp tip of the maxillary canine to the distal contact point of the mandibular canine |
| Molar relationship ∗ | Deviation from a neutral occlusion, defined as occlusion of the mesiobuccal cusp of the maxillary first molar within the buccal groove of the mandibular first molar |
| Intercanine distance | Distance between the cusp tips of fully erupted teeth |
∗ Distal occlusions were recorded as positive values; mesial occlusions as negative values.
| Maxilla | Mandible | |
|---|---|---|
| Total patient sample | ||
| No fixed retention (removable retainer in maxilla) | 52 | 10 |
| Fixed retainer until T3 | 6 | 19 |
| Fixed retainer until T5 | 7 | 19 |
| Fixed retainer until T10 | 31 | 48 |
| Subgroups used in retention analysis | ||
| MX0 Removable retainer until T3 | 52 | |
| MX10 Removable retainer until T3 combined with fixed retainer until T10 | 23 | |
| MD3 Fixed retainer until T3 | 19 | |
| MD5 Fixed retainer until T5 | 19 | |
| MD10 Fixed retainer until T10 | 48 | |
Intraexaminer reliability for the dental cast measurements was determined by scoring 30 randomly selected sets of models twice, 4 weeks apart; intraclass correlation coefficients were between 0.94 and 0.99. Skeletal characteristics and incisor positions were measured before and after treatment with cephalometric analysis of lateral radiographs ( Fig 1 ; Table III ). Reliability for the cephalometric analyses was tested by retracing 30 cephalograms after 3 weeks; intraclass correlation coefficient values were between 0.93 and 0.99.
| T0 | T1 | |||
|---|---|---|---|---|
| Mean | SD | Mean | SD | |
| SNA (°) | 81.1 | 3.4 | 80.3 | 3.8 |
| SNB (°) | 77.0 | 3.4 | 77.3 | 3.7 |
| ANB (°) | 4.1 | 2.6 | 3.1 | 2.2 |
| NL/NSL | 7.5 | 3.5 | 7.8 | 3.7 |
| ML/NSL | 32.9 | 5.3 | 32.6 | 5.7 |
| ML/NL | 25.4 | 5.2 | 24.8 | 5.3 |
| Isb-NA | 5.3 | 2.2 | 4.3 | 1.6 |
| lib-NB | 4.9 | 2.6 | 5.1 | 2.0 |
| ILs/NA | 22.6 | 9.3 | 23.3 | 5.7 |
| ILi/NB | 25.0 | 7.1 | 27.7 | 6.4 |
| ILi/ML | 95.1 | 7.1 | 97.8 | 7.9 |
| ILs/lLi | 128.3 | 13.3 | 126.0 | 8.7 |
Statistical analysis
For the time periods T0-T1, T1-T3, T3-T5, T5-T10, T0-T10, and T1-T10, continuous variables were analyzed for differences using a paired t test. The preventive effect of different retention protocols on posttreatment alignment deterioration was compared using a forced-entry linear regression analysis: with the retention subgroups coded as dummy variables 1 regression model was needed to compare the 2 maxilla groups, and 2 models were needed to compare the 3 mandible groups. A number of other variables, including dental cast measurements, pretreatment skeletal characteristics, and posttreatment dental protrusion and inclination, were tested for confounding and included in the regression models if required ( Table IV ). When performing the regression analysis for the maxilla, treatment changes in LII were found to be a confounding variable; the variable was consequently included as a covariate in the adjusted regression model. No confounding variables were detected related to the regression analysis for the mandible. There was no violation of the assumptions of normality, linearity, and homoscedasticity. A post hoc power analysis was conducted for each regression model using the software package G*Power (version 3.1.9.2; Franz Faul, Universität Kiel, Kiel, Germany). The statistical analyses were performed with SPSS software (version 24; IBM, Armonk, NY), using a significance level of 0.05.
| Obtained from | Variable | Description | MX0 | MX10 | MD3 | MD5 | MD10 |
|---|---|---|---|---|---|---|---|
| Dental casts | Treatment change in LII | –3.9 | –6.0 | –3.0 | –3.0 | –4.4 | |
| Treatment change in overjet | –2.8 | –3.3 | –3.1 | –3.1 | –2.6 | ||
| Treatment change in overbite | –2.0 | –1.5 | –1.2 | –2.0 | –1.8 | ||
| Treatment change in intercanine distance | 0.2 | 0.5 | –0.6 | 0.2 | 0.2 | ||
| Treatment change in canine relation | –2.3 | –1.5 | –2.0 | –2.9 | –1.5 | ||
| Lateral cephalogram | Mesial basal jaw relationship at T0 | ANB <1 | 6 | 1 | 3 | 3 | 4 |
| Distal basal jaw relationship at T0 | ANB >4 | 26 | 13 | 10 | 9 | 27 | |
| Low mandibular plane angle at T0 | ML/NSL <29° | 11 | 7 | 4 | 7 | 9 | |
| High mandibular plane angle at T0 | ML/NSL >37° | 7 | 7 | 4 | 0 | 12 | |
| Maxillary incisor protrusion at T1 | Isb-NA >2 mm above norm | 16 | 9 | 9 | 4 | 16 | |
| Maxillary incisor retrusion at T1 | Isb-NA >2 mm below norm | 2 | 0 | 0 | 1 | 1 | |
| Maxillary incisor proclination at T1 | ILs/NA >3° above norm | 23 | 12 | 11 | 9 | 22 | |
| Maxillary incisor retroclination at T1 | ILs/NA >3° below norm | 6 | 3 | 3 | 3 | 5 | |
| Mandibular incisor protrusion at T1 | Iib-NB >2 mm above norm | 8 | 7 | 4 | 5 | 11 | |
| Mandibular incisor retrusion at T1 | Iib-NB >2 mm below norm | 3 | 2 | 1 | 1 | 4 | |
| Mandibular incisor proclination at T1 | ILi/NB >3° above norm | 14 | 8 | 8 | 4 | 14 | |
| Mandibular incisor retroclination at T1 | ILi/NB >3° below norm | 3 | 3 | 1 | 2 | 6 | |
| Written records | Age at T1 (y) | 14.5 | 14.8 | 14.7 | 15.1 | 14.7 | |
| Follow-up time at T10 (y) | 10.1 | 10.0 | 10.1 | 10.3 | 10.2 | ||
| Sex (M/F) | 26/26 | 7/16 | 9/10 | 12/7 | 18/30 | ||
| Extraction/nonextraction | 13/39 | 13/10 | 6/13 | 7/12 | 18/30 |
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