European Journal of Orthodontics, 2015, 345–353
doi:10.1093/ejo/cju063
Advance Access publication 1 December 2014
Randomized controlled trial
Five-year postretention outcomes of three
retention methods—a randomized controlled trial
Gudrun Edman Tynelius, Sofia Petrén, Lars Bondemark and Eva Lilja-Karlander
Department of Orthodontics, Faculty of Odontology, Malmö University, Sweden
Correspondence to: Gudrun Edman Tynelius, Harvigslund Stubbarp 112, S-274 91 Skurup, Sweden.
E mail: [email protected]
Summary
Objective: Comparison of three different retention strategies 5 years or more postretention.
Design, Setting, and Participants: Randomized, prospective, single-centre controlled trial. Fortynine patients (33 girls and 16 boys) were randomly assigned to one of three retention methods
during 2 years by picking a ballot shortly before start of retention treatment. Inclusion criteria were
no previous orthodontics, permanent dentition, normal skeletal sagittal, vertical, and transversal
relationships, Class I dental relationship, space deficiencies, treatment plan with extractions of four
premolars followed by fixed straight-wire appliance. Maxillary and mandibular Little's irregularity
index (LII), intercanine and intermolar width, arch length, and overbite/overjet were recorded in
a blinded manner, altogether 10 measurements on each patient. Significant differences in means
within groups assessed by t-test and between groups by one-way analysis of variance.
Interventions: Retention methods: removable vacuum-formed retainer (VFR) covering the palate
and the maxillary anterior teeth from canine-to-canine and bonded canine-to-canine retainer in the
lower arch (group V-CTC); maxillary VFR combined with stripping of the lower anterior teeth (group
V-S); and prefabricated positioner (group P).
Results: Maxillary mean LII ranged from 1.8 to 2.6 mm, mean intercanine width 33.6–35.3 mm
with a significant difference between groups V-S and P, mean intermolar width 46.8–47.4 mm and
mean arch length 21.8–22.8 mm. Mandibular mean LII ranged from 2.0 to 3.4 mm with a significant
difference between groups V-S and P, mean intercanine width from 25.4 to 26.6 mm, mean
intermolar width from 40.8 to 40.9 mm and mean arch length from 16.9 to 17.3 mm. Mean overbite
ranged from 1.8 to 2.7 mm and mean overjet from 3.7 to 4.1 mm.
Limitations: A single centre study could be less generalizable.
Conclusions: The three retention methods disclosed equally favourable clinical results.
Trial registration: This trial was not registered.
Protocol: The protocol was not published before trial commencement.
Introduction
A major challenge in orthodontic treatment is to inhibit relapse and
ensure long-term stability of outcome. Growth, initial crowding, and
patient compliance have traditionally been regarded as the main determinants of orthodontic treatment stability (1). As long as the maxilla
and mandible are still growing, the position of the teeth and thus the
result of retention treatment may be affected. Most current knowledge about long-term stability is based on retrospective studies (2–5).
A recent review by the Cochrane group concluded that to date there
is insufficient evidence to single out any particular retention strategy
as the preferred method (6): it was recommended that future studies
should include true randomization, reporting of dropouts, adequate
sample size calculation, and a minimum follow-up period of 3 months.
There are a number of recent studies of the short-term effects
of different retention strategies. Two randomized controlled trials
(RCTs) comparing two appliances have shown equal short-term
© The Author 2014. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved.
For permissions, please email: [email protected]
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European Journal of Orthodontics, 2015, Vol. 37, No. 4
effects in retaining the outcome of orthodontic treatment of the
maxilla (7,8). Edman Tynelius et al. (9,10) compared the effect of
three different retention strategies on maintaining the outcome of
orthodontic treatment of both jaws and concluded that all three
were equally successful after 1 and 2 years of retention. However,
to our knowledge there is to date no RCT of the capacity of various
retention methods to counteract relapse in the long term.
The aim of this study, in the form of an RCT, was to evaluate and
compare the effects of three different retention strategies on counteracting orthodontic treatment relapse at least 5 years postretention.
The null hypothesis tested was that the three retention procedures
achieve equivalent long-term results.
Subjects and methods
Trial design
The study was an RCT performed at a single centre by one experienced orthodontic team. The Ethics Committee of Lund/Malmö
University, Sweden, approved the protocol and the informed consent
form (LU515-01).
Participants, eligibility, and setting
The study subjects comprised patients referred to an orthodontic
clinic in the National Health Service (NHS), Ystad, Sweden. The NHS
clinic was responsible for treatment of malocclusions of patients in
the southeast County Council of Scania. The NHS in Sweden offers
free dental care up to the age of 20 years, including orthodontic
treatment for patients with severe malocclusion. Between 2001 and
2007, all the study participants had undergone orthodontic treatment by one experienced orthodontist (GET). Each patient and
parent received oral as well as written information. Signed, written
consent was required before inclusion in the trial.
The following inclusion criteria were met:
•
•
•
•
No previous experience of orthodontic treatment
Permanent dentition
Normal skeletal sagittal, vertical, and transverse relationships
Class I molar relationship or a maximum of 3 mm anterior or
posterior deviation
• Space deficiencies in both jaws
• Treatment plan involving extraction of four premolars followed
by fixed straight-wire appliances in both jaws (11)
Interventions
The three retention methods investigated were:
• A removable VFR covering the palate and the maxillary anterior teeth from canine-to-canine and a bonded canine-to-canine
retainer in the lower arch (group V-CTC; Figure 1)
• An identical maxillary VFR as in group V-CTC, combined with
stripping of the mandibular anterior teeth (group V-S; Figure 1)
(2,12)
• A prefabricated positioner covering all erupted teeth in the maxilla and the mandible (group P; Figure 1) (13)
The VFRs were made of 2 mm Biolon (Dreve Dentamid GmbH,
Unna, Germany) in a Scheu Ministar press (Scheu-Dental GmbH,
Iserlohn, Germany).
The canine retainers consisted of 0.7 mm spring hard wire
(Dentaurum noninium, Dentaurum, Ispringen, Germany) bonded to
the mandibular canines with Transbond LC (3M Unitek Orthodontic
Products, Monrovia, California, USA).
Figure 1. Vacuum-formed retainer (VFR) in the maxilla and a canine-to-canine
retainer in the mandible, VFR in the maxilla and stripping of the mandibular
incisors and canines and prefabricated positioner.
Mechanical stripping of the mandibular incisors and canines was
performed either by hand with single sided medium and fine metal
blades (TP Orthodontics, La Porte, Indiana, USA) or with a backand-forth reciprocating handpiece with Ortho-Strips System (GAC
International, New York, USA). The method of stripping depended on
tooth form, non-triangular or triangular respectively, and was carried
347
G. Edman Tynelius et al.
out either at the appointment 5–6 weeks prior to debonding, or at
debonding. The reason for this was to avoid inflicting damage to the
papilla. When incisors were non-triangular, there was a greater risk
of damage to the papilla with the reciprocating handpiece and, thus,
single sided blades manually was the option. The aim of stripping was
to achieve small but distinct enamel flattening of the contact surfaces.
The reduction of any contact point between two teeth was approximately the thickness of the coarse blade of either system, i.e. 0.22 mm
for hand stripping or 0.34 mm for EVA-stripping.
The preformed positioner (Ortho-Tain Positioner, Ortho-Tain
Inc., Toa Alta, Puerto Rico) was a soft plastic appliance covering all
erupted teeth.
All retention appliances were distributed within 1 hour after debonding. The patients in groups V-CTC and V-S were instructed to wear the
VFR 22–24 hours per day for 2 days and nights and then at night for
12 months. In group P, the positioner was to be worn for 30 minutes during the daytime and during sleep for 12 months. During day-time wear
the patients were instructed to actively chew into their positioners. The
second year of retention patients wore their retainers every other night.
Outcomes
Dental casts were measured on four occasions: before orthodontic
treatment (T0), when the fixed orthodontic appliance was removed and
the retention appliance inserted (T1), when the retention appliance was
removed after 2 years (T2), and 5 years or more out of retention (T3).
The same examiner (GET) made the following linear measurements on dental casts with an electronic digital caliper (Mauser
Digital 6, Winterthur, Switzerland) to a precision of 0.01 mm:
• LII in the maxilla and the mandible—the summed displacement
of the anatomic contact points of the upper and lower incisors
and canines not including the distal contact points of the canines
to premolars (14)
• Intercanine width in the maxilla and the mandible—the distance
between the canine cusp tips
• Intermolar width in the maxilla and the mandible—the distance
between the mesiobuccal cusp tips of the first molars
• Arch length in the maxilla and the mandible—the perpendicular
distance from the midpoint of the incisal edges of the central
incisors to a line joining the mesial anatomic contact points of
the first molars
• Overbite—the overlap of upper to lower central incisors
• Overjet—the distance parallel to the occlusal plane from the
incisal edge of the most labial maxillary incisor to the opposing
mandibular central incisor
Data on patient ages and treatment times were retrieved from the
treatment records.
Randomization
The generation of randomization sequence was performed in blocks of
15 to ensure that equal numbers of patients were allocated to each of the
three retention groups. A ballot system was used for allocation, in lots of
15: five ballot sheets labelled ‘maxillary VFR and bonded mandibular
canine-to-canine retainer’, five labelled ‘maxillary VFR and mandibular
interproximal enamel reduction (stripping)’, and five with ‘positioner’
were placed in a basket by an independent person. The patient was then
allocated to one of the three groups by picking a ballot from the basket
shortly before debonding. When the first basket was empty, the second
basket was prepared, and another 15 ballots were extracted successively
as patients were recruited to the study. This procedure was altogether
repeated five times and concluded in 75 randomized patients.
Blinding
To achieve blinding when measuring dental casts only one dental
cast at a time was picked out of its box in a cross-sectional manner by an assistant, had its own protocol not showing any previous
measurements and then a new dental cast from a different patient
box was picked. Two dental casts from the same patients were never
measured in connection with each other. As the study was prospective with continuous patient visits it was inconvenient to anonymize
dental casts from the start of the study.
Statistical analysis
Sample size calculation
The calculated sample size for each group was based on a significance
level of 0.05 and 80 per cent power to detect a clinically meaningful
difference of 2.0 mm (SD = 2.0 mm) of the LII. The power analysis
showed that at least 16 patients would be required in each group.
To compensate for dropouts in the follow-up study, 25 patients were
enrolled in each group. Arithmetic means and standard deviations
(SD) on group level at times corresponding to pre-treatment (T0),
end of active treatment and start of retention (T1), end of 2 years of
retention treatment (T2), and end of 5 years or more out of retention
(T3) were calculated for each variable.
Primary and secondary outcomes
Significant differences in means between groups were assessed by
one-way analysis of variance and within groups by t-test to the previous time point using the Statistical Package for Social Sciences (version 22.0.0.0, IBM SPSS Inc., Chicago, Illinois, USA). P-values less
than 5 per cent (P < 0.05) were considered statistically significant.
Error of method
Twenty randomly selected dental casts were measured by the same
examiner (GET) on two separate occasions at an interval of 4 weeks.
The error of the method did not exceed 0.45 mm for any of the 10
measurements (15). No significant mean differences between the two
series of records were found using paired t-test.
Results
Participant flow
A total of 82 patients fulfilled the inclusion criteria. Seven patients (6
girls, 1 boy) declined to participate, and thus 75 patients were randomized at the start of the trial. Out of these 75 patients, 26 failed to
complete the trial: three declined to participate and 23 left the district
or were unable to be contacted. Consequently, 49 patients (33 girls, 16
boys) attended their 5-year or more appointment (Figure 2). Out of
these 49 patients, 9 were out of retention more than 5 years (Table 1).
Baseline data
There were no significant intergroup differences with respect to age
or active treatment time. The mean active orthodontic treatment
time was 1.7 years (SD = 0.4) and the mean age at start of retention
(T1) was 14.3 years (SD = 1.5). As there were no significant differences between boys and girls in any of the study variables, the data
for the genders were pooled for analysis (Tables 2–4).
Harms
No harms were reported. In two cases, patients choose to keep their
CTC in situ after 2 years of retention and consequently they were
excluded from the long-term study.
European Journal of Orthodontics, 2015, Vol. 37, No. 4
348
Eligible patients with appliance in both jaws n=82
51 girls and 31 boys
Declined to participate n=7
6 girls and 1 boy
Randomized patients n=75
45 girls and 30 boys
T1 Group V-CTC
n=25
18 girls and 7 boys
T1 Group V-S
n=25
14 girls and 11 boys
Unable to
reach
1 girl
Unable to
reach
2 boys
T2 Completed twoyear retention
n=24
17 girls and 7 boys
T2 Completed two-year
retention
n=23
14 girls and 9 boys
Refused to
participate
1 girl and 1 boy
Left the
district/unable to
reach
5 girls and 2 boys
T3 Completed >5
years without
retention
n=16
12 girls and 4 boys
Refused to
participate
2 boys
Left the
district/unable to
reach
4 girls and 2 boys
T3 Completed >5
years without
retention
n=17
10 girls and 7 boys
T1 Group P
n=25
13 girls and 12 boys
Unable to
reach
3 boys
T2 Completed twoyear retention
n=22
13 girls and 9 boys
Refused to
participate
3 boys
Left the
district/unable to
reach
2 girls and 4 boys
T3 Completed >5
years without
retention
n=16
11 girls and 5 boys
Figure 2. The flow diagram of the trial. Vacuum-formed retainer (VFR) in the maxilla and bonded canine-to-canine retainer in the mandible (group V-CTC), VFR
in the maxilla and stripping of the mandibular incisors and canines (group V-S), and positioner covering erupted teeth (group P) at start of retention (T1), after
2 years of retention (T2) and after 2 years or more without retention (T3).
Table 1. Postretention.
V-CTC
V-S
P
Totals
5 years
6 years
7 years
8 years
13
12
15
40
1
4
1
6
1
1
9 years
1
1
1
1
Table showing for how many years the 49 patients had been out of retention at T3.
Maxilla T0–T3
At start of orthodontic treatment (T0), the mean LII was 8.6 mm in the
V-CTC group, 11.7 mm in the V-S group, and 8.3 mm in the P group.
No significant differences were found between the groups. After
treatment (T1), the level of mean LII was below 0.5 mm in all three
groups, with no significant intergroup differences. Small changes in
mean LII occurred during the retention period (T1–T2). After 5 years
or more out of retention (T3), the mean LII was in group V-CTC
1.8 mm (SD = 1.4), in group V-S 2.6 mm (SD = 1.5) and in group P
2.3 mm (SD = 1.9) with no significant intergroup differences (Table 2).
In all three groups, there was an average increase in mean intercanine width of 1 and 2 mm during treatment (T0–T1), with a return
to pre-treatment levels in all groups at T3 with 34.4 mm (SD = 1.7)
in group V-CTC, 35.3 mm (SD = 2.1) in group V-S and 33.6 mm
(SD = 1.4) in group P. Small, albeit significant intergroup differences
were found between the V-S and P groups after 2 years of retention
(T2) and this difference remained at T3 (Table 2).
The mean intermolar width and the mean arch length decreased in
all groups because of extraction of premolars. The mean intermolar
8.6
0.4***
0.9*
1.8**
34.5
36.3*
35.1***
34.4*
48.7
48.7
47.6**
46.8**
28.8
23.1***
23.2
22.8*
LII T0
LII T1
LII T2
LII T3
Intercanine width T0
Intercanine width T1
Intercanine width T2
Intercanine width T3
Intermolar width T0
Intermolar width T1
Intermolar width T2
Intermolar width T3
Arch length T0
Arch length T1
Arch length T2
Arch length T3
4.9
0.4
0.9
1.4
2.3
1.6
1.6
1.7
2.9
1.5
2.0
1.9
3.2
1.4
1.7
1.7
Standard
deviation
5.5
0.2
0.5
1.1
33.0
35.4
34.2
33.5
47.2
47.9
46.5
45.7
27.1
22.4
22.3
21.9
Lower bound
11.6
0.7
1.4
2.5
36.0
37.1
35.9
35.3
50.3
49.5
48.6
47.8
30.5
23.9
24.1
23.7
Upper bound
95% confidence interval for
mean
11.7
0.5***
1.4**
2.6**
34.8
36.6*
35.9**
35.3*
49.8
48.9
47.7**
47.2**
29.1
22.6***
22.5
22.3
Mean
5.1
0.5
1.0
1.5
3.8
2.7
2.4
2.1
4.6
2.9
3.5
3.5
3.1
2.1
1.8
2.2
Standard
deviation
V-S (n = 17)
Within groups the t-test shows the significance corresponding to the previous time point. NS, not significant.
*P < 0.05, **P < 0.01, ***P < 0.001.
Mean
Maxilla
V-CTC (n = 16)
8.7
0.3
0.9
1.8
32.6
35.2
34.6
34.2
47.3
47.4
46.0
45.4
27.4
21.5
21.6
21.1
Lower bound
14.8
0.8
1.9
3.3
36.9
38.0
37.1
36.4
52.3
50.4
49.5
48.9
30.7
23.6
23.4
23.4
Upper bound
95% confidence interval for
mean
8.3
0.5***
1.7**
2.3*
35.0
36.4*
34.1***
33.6*
49.8
49.7
48.1***
47.4**
29.1
22.5***
22.5
21.8**
Mean
P (n = 16)
4.1
0.3
1.5
1.9
2.7
1.7
1.5
1.4
3.2
2.2
2.0
1.8
3.1
3.2
1.7
1.7
Standard
deviation
5.7
0.3
0.9
1.4
33.3
35.5
33.3
32.8
48.0
48.5
47.0
46.4
27.4
20.8
21.6
20.9
Lower bound
11.0
0.6
2.5
3.3
36.7
37.3
34.9
34.3
51.6
50.8
49.1
48.3
31.1
24.2
23.5
22.7
Upper bound
95% confidence interval for
mean
NS
NS
NS
NS
NS
NS
0.036* V-S/P
0.025* V-S/P
NS
NS
NS
NS
NS
NS
NS
NS
Analysis of
variance
Table 2. Mean pre-treatment measurements (mm) of the maxilla (T0), at the end of treatment/start of the retention treatment (T1), at the end of 2 years of retention treatment (T2), and after
5 or more years postretention (T3) in the three retention groups: removable vacuum-formed retainer (VFR) and bonded mandibular canine-to-canine retainer (V-CTC), VFR and mandibular
anterior stripping (V-S), and positioner (P). LII, Little’s Irregularity Index.
G. Edman Tynelius et al.
349
2.1***
26.2
27.3
27.5
26.6**
44.2
41.8***
41.9
40.9**
23.7
17.9***
18.2
17.1***
6.2
0.4***
0.9*
Mean
0.9
2.5
1.6
1.6
1.4
2.1
1.9
1.7
1.7
2.3
1.3
1.9
1.7
4.0
0.3
0.7
Standard
deviation
1.6
24.9
26.4
26.6
25.9
43.1
40.7
41.0
40.0
22.5
17.2
17.2
16.2
4.1
0.2
0.5
Lower bound
2.5
27.6
28.1
28.3
27.4
45.3
42.9
42.8
41.8
25.0
18.6
19.2
18.0
8.4
0.6
1.2
Upper bound
95% confidence interval for
mean
2.0**
26.3
27.7**
26.8**
26.5*
44.8
42.2**
41.6
40.8***
23.5
17.9***
17.7
17.3**
7.0
0.3***
1.2**
Mean
1.9
2.4
1.9
2.2
2.2
3.4
2.6
2.7
3.1
2.5
1.7
1.6
1.7
4.3
0.5
1.1
Standard
deviation
V-S (n = 17)
Within groups the t-test shows the significance corresponding to the previous time point. NS, not significant.
*P < 0.05, **P < 0.01, ***P < 0.001.
LII T3
Intercanine width T0
Intercanine width T1
Intercanine width T2
Intercanine width T3
Intermolar width T0
Intermolar width T1
Intermolar width T2
Intermolar width T3
Arch length T0
Arch length T1
Arch length T2
Arch length T3
LII T0
LII T1
LII T2
Mandible
V-CTC (n = 16)
1.1
25.1
26.7
25.7
25.3
43.1
40.9
40.2
39.3
22.3
17.0
16.9
16.4
4.7
0.1
0.7
Lower bound
3.0
27.6
28.7
28.0
27.6
46.6
43.5
43.0
42.4
24.8
18.8
18.5
18.2
9.3
0.5
1.8
Upper bound
95% confidence interval for
mean
3.4**
26.3
27.3*
26.0***
25.4**
44.7
43.0*
41.5*
40.9**
23.4
17.4***
17.6
16.9***
5.6
0.3***
2.1***
Mean
P (n = 16)
2.1
1.8
1.0
1.2
1.5
2.6
2.1
2.1
2.7
2.5
1.3
1.4
1.2
3.4
0.4
1.7
Standard
deviation
2.3
25.4
26.8
25.3
24.6
43.4
41.9
40.4
39.4
22.0
16.7
16.9
16.3
3.8
0.1
1.3
Lower bound
4.5
27.3
27.8
26.6
26.1
46.1
44.1
42.6
42.3
24.7
18.1
18.4
17.5
7.4
0.6
3.0
Upper bound
95% confidence interval for
mean
NS
NS
0.015* VCTC/P
0.037* V-S/P
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
Analysis of
variance
Table 3. Mean pre-treatment measurements (mm) for the mandible (T0), at the end of treatment/start of the retention treatment (T1), at the end of 2 years of retention treatment (T2), and after
5 or more years postretention (T3) in the three retention groups: removable vacuum-formed retainer (VFR) and bonded mandibular canine-to-canine retainer (V-CTC), VFR and mandibular
anterior stripping (V-S), and positioner (P). LII, Little’s Irregularity Index.
350
European Journal of Orthodontics, 2015, Vol. 37, No. 4
351
Within groups the t-test shows the significance corresponding to the previous time point. NS, not significant.
*P < 0.05, **P < 0.01, ***P < 0.001.
1.3
3.7**
T3
3.0
4.4
4.1
1.2
3.5
4.7
3.7
1.2
3.1
4.3
NS
NS
0.040* VCTC/V-S
NS
7.0
3.3
4.1
4.4
2.6
3.0
2.6
0.7
1.0
5.7
2.9***
3.5*
6.9
3.8
4.4
4.3
2.6
3.3
2.6
1.1
1.1
5.6
3.2**
3.8*
3.4
2.6
2.3
2.0
0.6
1.1
5.6
3.2
3.4
NS
NS
NS
NS
4.2
2.8
3.0
3.1
2.4
1.9
1.8
2.3
1.7
0.8
1.2
0.9
3.3
2.3*
2.4
2.7
3.7
2.2
3.0
3.3
1.6
1.5
1.9
1.9
2.0
0.7
1.0
1.3
2.6
1.8
2.5**
2.6
Analysis of
variance
Upper bound
Lower bound
Standard deviation
Mean
Upper bound
95% confidence interval for
mean
P (n = 16)
95% confidence interval for
mean
Lower bound
Standard deviation
Mean
Upper bound
2.9
2.6
2.2
2.7
1.2
1.6
1.0
1.0
1.6
0.9
1.1
1.6
Overbite
T0 2.0
T1 2.1
T2 1.6
T3 1.8
Overjet
T0 4.5
T1 2.9*
T2 2.9
After 5 years or more out of retention, all three retention methods
provided good long-term clinical stability. Thus, the null hypothesis
could not be rejected for the patient groups corresponding to the
inclusion criteria. Consequently, any of the three retention methods
Lower bound
Discussion
Standard deviation
During the observation period, including treatment, the mean overbite remained fairly unaltered and was in group V-CTC 1.8 mm
(SD = 1.6), in group V-S 2.6 mm (SD = 1.3), and in group P 2.7 mm
(SD = 0.9). After 5 years or more, no intergroup differences were
found (Table 4).
During treatment (T0–T1), the mean overjet decreased significantly in all three groups and without any group differences. At the
end of the retention period (T2), the mean overjet was significantly
higher in the V-S than in the V-CTC group (3.8 versus 2.9 mm). After
5 years or more postretention (T3), there were no significant intergroup differences in mean overjet with 3.7 mm (SD = 1.3) in group
V-CTC, with 4.1 mm (SD = 1.2) in group V-S, and with 3.7 mm
(SD = 1.2) in group P (Table 4).
Mean
Overbite and overjet T0–T3
V-S (n = 17)
At start of orthodontic treatment (T0) the mean LII was 6.2 mm in the
V-CTC group, 7.0 mm in the V-S group, and 5.6 mm in the P group:
the intergroup differences were not significant. After treatment (T1),
the mean LII was less than 0.4 mm in all three groups, with no significant intergroup differences. Small changes in mean LII occurred
during the retention period (T1–T2). In the P-group a small but significantly higher mean LII was apparent compared to the V-CTC,
mean 2.1 versus 0.9 mm. After 5 years or more out of retention (T3),
the mean LII in group V-CTC was 2.1 mm (SD = 0.9), in group V-S
2.0 mm (SD = 1.9), and in group P 3.4 mm (2.1). The average value
was significantly higher in the P group than in the V-S group (Table 3).
In all three groups, mean intercanine width increased on average
1 mm during treatment (T0–T1) and had returned to pre-treatment
levels at T3 with 26.6 mm (SD = 1.4) in group V-CTC, 26.5 mm
(SD = 2.2) in group V-S and 25.4 mm (SD = 1.5) in group P. No
significant intergroup differences were found after 5 years or more
(Table 3).
As was found in the maxilla, mandibular mean intermolar width
and mean arch length decreased during treatment (T0–T1) in all
groups because of extraction of premolars. The mean intermolar
width was at T3 for group V-CTC 40.9 mm (SD = 1.7), for group
V-S 40.8 mm (SD = 3.1), and for group P 40.9 mm (SD = 2.7). No
significant intergroup differences were found after 5 years or more
(Table 3).
The mean arch length was at T3 for group V-CTC 17.1 mm
(SD = 1.7), for group V-S 17.3 mm (SD = 1.7), and for group P
16.9 mm (SD = 1.2). No significant intergroup differences were
found after 5 years or more (Table 3).
95% confidence interval for
mean
Mandible T0–T3
V-CTC (n = 16)
width remained stable during the retention period as well during the
5-year post retention period. It was at T3 in group V-CTC 46.8 mm
(SD = 1.9), in group V-S 47.2 mm (SD = 3.5), and in group P 47.4 mm
(SD = 1.8). No significant intergroup differences were found at T3
(Table 2).
The mean arch length remained stable during the retention
period as well during the 5-year post retention period and was at T3
for group V-CTC 22.8 mm (SD = 1.7), for group V-S 22.3 (SD = 2.2),
and for group P 21.8 mm (1.7) (Table 2).
Table 4. Mean overbite and overjet (mm) pre-treatment (T0), at the end of treatment/start of the retention period (T1), at the end of the 2 year retention period (T2), and 5 or more years postretention (T3) in the three retention groups: removable vacuum-formed retainer (VFR) and bonded mandibular canine-to-canine retainer (V-CTC), VFR and mandibular anterior stripping (V-S),
and positioner (P).
G. Edman Tynelius et al.
352
can be recommended in patients with space deficiency in both jaws,
when treatment involves extraction of four premolars followed by
fixed straight-wire appliances.
Main findings
There are a number of studies evaluating stability after orthodontic
treatment (3,6,16–19). However, to our knowledge, this is the first
RCT specifically designed to evaluate and compare three different
retention strategies for counteracting relapse at least 5 years out of
retention. Thus, no comparison can be made with previous studies.
Whilst not directly comparable, a few RCTs of short-term stability of orthodontic treatment results have previously been published
(7,8,20,21). These also demonstrated and confirmed our results that
small but not clinically significant movements of teeth occurred.
From the second day after debonding, the patients in the V-CTC
and V-S groups wore their VFRs only at night. This regimen was
based on a report of equal stability of treatment result following
full- or part-time wear of Essix retainers (8). Vacuum-formed Essix
retainers have been shown to be superior to Hawley retainers in
retaining the maxillary anterior teeth and were thus the option of
choice (7). LII increased in patients with canine-to-canine retainers
in the present study: this is in accordance with other studies (19).
In group V-CTC, it was obvious that the canine-to-canine retainers
held the intercanine width well at T2, but the extraction sites opened
up during retention treatment, i.e. the arch length increased and the
overjet decreased compared to group V-S. This is probably due to
a proclination of the lower front teeth but it cannot be confirmed
cephalometrically as we did not take cephalograms at T2. At T3,
teeth were free to move without any influence of retention devices
and, thus, the groups grew similar.
The finding that stripping of the mandibular anterior teeth without any adjunctive methods was sufficient to retain the treatment
result on a short-term basis has not previously been demonstrated
in an RCT. A retrospective study reported that stripping of lower
anterior teeth in combination with overcorrection of rotated teeth
was sufficient to prevent relapse over a 3-year period (18). Stripping
of lower anterior teeth after debonding instead of wearing fixed or
removable retention appliances would probably have advantages,
not only for the patient but also for the clinician, in terms of cost,
chair-time, and the issue of lost appliances.
The positioner is an eruption guidance appliance used in the
early mixed dentition (13). No research has been presented about
the effectiveness of prefabricated positioners for retention of orthodontic treatment results. This study showed that the appliance could
be used in the permanent dentition as a retention device but in the
long-term perspective the positioner held the maxillary intercanine
distance less well than the VFR and likewise the mandibular LII
compared to the V-S group. Because the positioner is prefabricated,
it may be a less costly alternative to appliances made by dental technicians. However, a drawback is that the fitting cannot be as precise
as retention appliances made on individual dental casts. A further
disadvantage is that compliance is essential, but as the subjects were
randomly allocated, this uncontrolled factor was evenly distributed
among the groups.
Generalizability
An RCT was selected in order to reduce the risk of error from such
factors as selection bias, the clinician's preferred treatment method
and patient compliance. Furthermore, random allocation of subjects reduces bias and confounding variables by ensuring that both
European Journal of Orthodontics, 2015, Vol. 37, No. 4
known and unknown determinants of outcome are evenly distributed among the subjects. The prospective design also ensures that the
baseline characteristics, treatment progression, and side-effects can
be strictly controlled and accurately observed.
The standardized treatment of all patients by the same orthodontic
team makes it a valid comparison of the actual treatment methods.
In most cases there was substantial pre-treatment crowding (T0). In
all groups, the mean LII after treatment (T1) was below or equal to
0.5 mm in both jaws, indicating successfully treated cases (Tables 2
and 3). However, the mean intercanine width was generally somewhat
expanded after treatment and relapse could be expected: the mean
intercanine widths had returned to pre-treatment values in all groups.
Limitations
In a long-term study, the effect of attrition on outcomes must be considered and some attrition is inevitable. Hence, in the present study the
attrition rate was acceptable and according to the sample size calculation, the number of subjects remaining in each group was adequate to
ensure that the outcomes were not biased by loss of data.
The standardized treatment of all patients by the same orthodontic team could be a drawback as the same possible mistakes may
have been made on all patients.
The use of LII for measuring relapse of tooth positions may
have some limitations: it tends to exaggerate cases with considerable irregularity but little length shortage, i.e. a rotated tooth
without crowding (14). Another drawback with the LII is that
it does not include the distal contact point of the canine. This
means that canines can be in a very crowded position without
LII showing it.
In any retention treatment with removable appliances, the
responsibility and result lies with the patient and is out of control
to the orthodontist.
At the final registration, all patients in this trial had reached
adulthood. This means that overall growth had ceased but this does
not imply that changes in occlusion, arch dimensions, and tooth
position cannot occur. Thus, the increase in Little's index and diminished intercanine widths found in all groups during the postretention
period could coincide with the normal physiological changes during
the same period of time (22,23).
New studies are needed to confirm the results of this study.
Conclusions
After 5 years or more out of retention, the three retention methods had achieved equally favourable clinical results. Thus a maxillary VFR combined with a bonded canine-to-canine retainer in the
mandible; a maxillary VFR combined with stripping of the mandibular anterior teeth and a prefabricated positioner can all be recommended. Hence, the clinician is not limited to routine use of a
bonded mandibular canine-to-canine retainer: choice of retention
method can be individualized, taking into account such variables as
orthodontic diagnosis, the expected level of patient compliance and
the patient’s wishes and financial considerations.
Funding
Scania County Council, Sweden; the Swedish Dental Society and the
Faculty of Odontology, Malmö University, Sweden.
Conflict of interest statement. None declared.
G. Edman Tynelius et al.
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