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Our reference: YMOD 6084 P-authorquery-v9

AUTHOR QUERY FORM

Journal: YMOD

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Q1 It is not clear what “22/3-17” meants. Can you state this in words?

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Q3 Please confirm that all tables are as you meant. They had to bew converted to the format for this journal.

Q4 Please give the city in Germany for this company.

Q5 Should “MX0” be “MX3”?

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Highlights

� Components of the occlusion underwent gradual changes throughout the period.

� In the maxilla, the effect of dual retention over removable retention was minor.

� In the mandible, 10 years of fixed retention gave better alignment than 3 and 5 years.

Q13

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ORIGINAL ARTICLE

64656667686970717273

Occlusal changes during a 10-yearposttreatment period and the effectof fixed retention on anterior toothalignment

74757677787980

Ragnar Bjering and Vaska Vandevska-RadunovicOslo, Norway

FromOslo,All auPotenAddreulty oe-maiSubm0889-� 201https:

81828384858687888990919293949596979899

100

Introduction: The objectives of this research were to evaluate changes in occlusal components in 3 subperiodsduring a 10-year posttreatment time span and to examine the long-term effects of fixed retention onmaxillary andmandibular anterior alignment. Methods: Ninety-six patients were examined; the Peer Assessment Rating in-dex 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 differ-ences between all subperiods. The effect of fixed retention on posttreatment changes in Little's Irregularity Indexwas analyzed for both jaws with regression analysis. For the maxilla, 2 groups were compared: removableretainer until T3 (n 5 52) and removable retainer until T3 combined with a fixed retainer until T10 (n 5 23).For the mandible, 3 groups were compared: fixed retainer until T3 (n 5 19), fixed retainer until T5 (n 5 19),and fixed retainer until T10 (n 5 48). Results: The Peer Assessment Rating index percentage of improvementwas 79% at T10. A gradual deterioration of occlusal components was seen, with small insignificant changes ineach subperiod. Corrected for pretreatment irregularity, the group with the maxillary removable retainer until T3combined with a fixed retainer until T10 showed a 0.6-mm lower Little's Irregularity Index score than atpretreament. The group with the mandibular fixed retainer until T10 had significantly better alignment than atT3 (1.1 mm) and T5 (0.7 mm). Conclusions: Gradual occlusal changes of limited clinical importance wereseen during a 10-year posttreatment period. Long-term fixed retention in the maxilla was of minor importancein patients also wearing removable retainers. In the mandible, a 10-year fixed retention protocol gavemoderately lower alignment scores compared with a 3-year or 5-year protocol. (Am J Orthod DentofacialOrthop 2018;-:---)

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Total stability of the occlusion after orthodontictreatment seems to be unlikely to achieve. Withtime, changes in tooth alignment are inevitable,

and both relapse and growth are contributing factors.1

To counteract unwanted changes, orthodontists pre-scribe different types of retainers, sometimes intendedto be used indefinitely.

Both removable and fixed retainers have been foundto be equally effective in controlling relapse up to 2 years

the Department of Orthodontics, Faculty of Dentistry, University of Oslo,Norway.thors have completed and submitted the ICMJE Form for Disclosure oftial Conflicts of Interest, and none were reported.ss correspondence to: Ragnar Bjering, Department of Orthodontics, Fac-f Dentistry, University of Oslo, PO Box 1109, Blindern, 0317 Oslo, Norway;l, ragnar.bjering@odont.uio.no.itted, August 2017; revised and accepted, December 2017.5406/$36.008 by the American Association of Orthodontists. All rights reserved.//doi.org/10.1016/j.ajodo.2017.12.015

FLA 5.5.0 DTD � YMOD6084_pro

116117118119120121122123124125126

posttreatment.2,3 Part-time wear of thermoplastic andHawley retainers has proved to be as efficient as full-time wear.4,5 Methods for controlling the anterioralignment even without retainers such as interproximalreduction, sometimes used in combination withovercorrection, have been shown to be viable in theshort term.3,6

It is unclear at what point after debonding the great-est posttreatment changes occur. Some authors havefound that most changes take place during the first2 years after treatment, a period corresponding wellwith relapse and settling.7 In contrast, early stability hasbeen reported for all components of occlusion 2 years af-ter treatment.8 Others have reported that most occlusalchanges take place during the first 4 years.9 Moreover,long-term studies have concluded that significantocclusal changes take place even between 19 and 31 yearsof age.10 With more information about posttreatmentchanges, one could possibly improve retention strategies.

of � 4 June 2018 � 11:03 pm

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Table I. Dental cast measurements with definitions

Measurement DefinitionPAR index Measured according to the conventions

described by Richmond et al20

LII The sum of the linear displacements of theanatomic contact points from canine tocanine, according to Little21

Overjet Distance parallel to the occlusal planefrom the buccal surface of the mostprotruding maxillary incisor to thebuccal surface of the correspondingmandibular incisor

Overbite Maximum distance of the mandibularincisors overlapped by the maxillarycentral incisors

Canine relationship* Distance from the cusp tip of the maxillarycanine to the distal contact point of themandibular canine

Molar relationship* Deviation from a neutral occlusion,defined as occlusion of the mesiobuccalcusp of the maxillary first molar withinthe buccal groove of the mandibularfirst molar

Intercanine distance Distance between the cusp tips of fullyerupted teeth

*Distal occlusions were recorded as positive values; mesial occlusionsas negative values.

2 Bjering and Vandevska-Radunovic

127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189

190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252

Prolonged retention can interfere with the naturalreduction in dental arch parameters. Since long-termcompliance with a removable retainer is expected to belimited and less practical, a semipermanent or perma-nent retainer is often the bonded type. It has been statedthat lifetime permanent retention is the only way to pre-vent relapse.11 Al Yami et al7 found a positive effect offixed retainers on the Peer Assessment Raing (PAR) score11 years posttreatment. Furthermore, fixed retainersgave better occlusal results 17 years posttreatment in along-term follow-up.10 In contrast, some studies haveconcluded that fixed retention is not of major impor-tance to the treatment outcome. Rather than being aprotective measure against long-term changes on anocclusal level, fixed retainers were found to primarilyinhibit changes in anterior alignment.12 The efficacy offixed retainers on mandibular anterior relapse has beenreported.13 Nonetheless, satisfactory alignment hasalso been found at 10 years postretention, even after ashort retention protocol.14 In the maxilla, fixed retainersappear to have less influence on the stability of align-ment compared with the mandible.13,15 There seems tobe uncertainty about how much a fixed retainer willimprove the alignment in the long term.

For the time being, the preferred type and duration ofretention have not been established.16,17 Use ofretention appliances varies between countries andlargely depends on personal preferences.18,19 It istherefore important to improve our knowledge aboutthe effect of different retention protocols, types, anddurations on long-term treatment outcomes.

The aims of the study were to evaluate changes inocclusal components in 3 stages during a 10-year post-treatment period and to examine the effect of type andduration of retention on maxillary and mandibular ante-rior alignment.

MATERIAL AND METHODS

The Department of Orthodontics at the University ofOslo in Norway routinely summons patients forcheckups at 10 years posttreatment. Included in thisretention archive are nonsurgical patients aged 20 yearsor younger at the beginning of treatment, without agen-esis, trauma, or autotransplantations to the anterior re-gions. To detect a minimum difference of 10 points onthe PAR between pretreatment and 10 years posttreat-ment with a standard deviation of 11, a sample size of12 patients was required to provide 80% statistical po-wer with an alpha of 0.05. Attendance at the 10-yearfollow-up appointment per 22/3-17 served as inclusioncriteria for this longitudinal analytical study. Approvalwas granted by the Regional Committee for Medical

- 2018 � Vol - � Issue - American

and Health Research Ethics and the Data Protection Offi-cial for Research of Q���. One hundred twenty-five pa-tients met the inclusion criteria. Exclusions were madeaccording to the following criteria: missing or damageddental cast (pretreatment, posttreatment, or 10-yearfollow-up) (n 5 18), retreatment (n 5 5), single-archtreatment (n 5 4), and extractions of incisors (n 5 2).The final study sample included 96 patients (43 male,53 female) treated with full fixed appliances. Study castswere availale for all patients at pretreatment (T0), post-treatment (T1), and 10 years posttreatment (T10), as wellas for 70 patients at 3 years posttreatment (T3) and 86patients at 5 years posttreatment (T5). Fifty-six patientswere treated without extractions, 28 patients weretreated with extraction of 4 premolars, 8 patients hadextraction of 2 maxillary premolars, and 4 patients hadextraction of 2 mandibular premolars. Mean pretreat-ment age was 12.2 years (61.4). Mean treatment dura-tion was 2.6 years (60.9). Average follow-up periodswere 3.1 years (60.4) at T3, 5.3 years (60.6) at T5,and 10.1 years (60.8) at T10.

The PAR index20_ENREF_20 was used to assess theocclusions at all times. Anterior tooth alignment wasscored using Little's Irregularity Index (LII).21 In addi-tion, several dental cast measurements were registered Q

(Table I). All variables were measured by the same

Journal of Orthodontics and Dentofacial Orthopedics

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Table II. Number of patients and distribution amongthe retention protocols for the total sample and sub-groups in the retention analysis

Maxilla MandibleTotal patient sampleNo fixed retention (removableretainer in maxilla)

52 10

Fixed retainer until T3 6 19Fixed retainer until T5 7 19Fixed retainer until T10 31 48

Subgroups used in retention analysisMX0

Removable retainer until T3 52MX10

Removable retainer until T3combined with fixed retaineruntil T10

23

MD3Fixed retainer until T3 19

MD5Fixed retainer until T5 19

MD10Fixed retainer until T10 48

A subgroup was created only if a sufficient number of patients couldbe assigned to the group. Time points used are 3 (T3), 5 (T5), and10 years posttreatment (T10).

Fig 1. Landmarks used in the pretreatment andposttreatment Q12cephalometric analyses of skeletal charac-teristics and incisor protrusion and inclination. The trac-ings were oriented horizontally 7� down from the sella-nasion line. S, Sella; N, nasion; A, point A; B, point B;Isa, ���; Isb, ���; Ii, ���; Is, ���; Iib, ���; Iia, ���; iGo,���; Me, menton.

Bjering and Vandevska-Radunovic 3

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examiner (R.B.) to the closest 0.1 mm using a digitalcaliper (Digital 6; Mauser, ���, Germany), except foroverjet and overbite, which were measured to the near-est 0.5 mm using a ruler. All patients had received aremovable retainer in the maxilla with instructions forinitial full-time wear followed by a gradual reductionuntil 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 ante-rior tooth alignment, retention subgroups were createdbased on the applied retention protocol (Table II). Forthe maxilla, 2 groups of patients were compared:MX0, removable retainer until T3 (n 5 52); andMX10, removable retainer until T3 combined with fixedretainer until T10 (n 5 23). Only patients with a 12-22(n5 12) or 13-23 (n5 11) retainer were included in theMX10 group. For the mandible, 3 groups werecompared: MD3, fixed retainer until T3 (n 5 19);MD5, fixed retainer until T5 (n 5 19); and MD10, fixedretainer until T10 (n 5 48).

Intraexaminer reliability for the dental cast measure-ments was determined by scoring 30 randomly selectedsets of models twice, 4 weeks apart; intraclass correla-tion coefficients were between 0.94 and 0.99. Skeletalcharacteristics and incisor positions were measuredbefore and after treatment with cephalometric analysisof lateral radiographs (Fig 1; Table III). Reliability forthe cephalometric analyses was tested by retracing 30

American Journal of Orthodontics and Dentofacial Orthoped

cephalograms after 3 weeks; intraclass correlation coef-ficient values were between 0.93 and 0.99.

Statistical analysis

For the time periods T0-T1, T1-T3, T3-T5, T5-T10,T0-T10, and T1-T10, continuous variables wereanalyzed for differences using a paired t test. The pre-ventive effect of different retention protocols on post-treatment alignment deterioration was compared usinga forced-entry linear regression analysis: with the reten-tion subgroups coded as dummy variables; 1 regressionmodel was needed to compare the 2 maxilla groups, and2 models were needed to compare the 3 mandiblegroups. A number of other variables, including dentalcast measurements, pretreatment skeletal characteris-tics, and posttreatment dental protrusion and inclina-tion, were tested for confounding and included in theregression models if Qrequired (Table IV). When perform-ing the regression analysis for the maxilla, treatmentchanges in LII were found to be a confounding variable;the variable was consequently included as a covariate inthe adjusted regression model. No confounding vari-ables were detected related to the regression analysisfor the mandible. There was no violation of the assump-tions of normality, linearity, and homoscedasticity. A

ics - 2018 � Vol - � Issue -

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Table III. Mean cephalometric values for the patientsample at T0 and T1

T0 T1

Mean SD Mean SDSNA (�) 81.1 3.4 80.3 3.8SNB (�) 77.0 3.4 77.3 3.7ANB (�) 4.1 2.6 3.1 22NL/NSL 7.5 3.5 7.8 3.7ML/NSL 32.9 5.3 32.6 5.7ML/NL 25.4 5.2 24.8 5.3Isb-NA 5.3 2.2 4.3 1.6lib-NB 4.9 2.6 5.1 2.0ILslNA 22.6 9.3 23.3 5.7ILi/NB 25.0 7.1 27.7 6.4ILi/ML 95.1 7.1 97.8 7.9ILs/lLi 128.3 13.3 126.0 8.7

NL/NSL, ���; ML/NSL, ���; ML/NL, ���; Isb-NA, ���; Iib-NB,���; ILslNA, ���; ILiNB, ���; ILiML, ���; ILs/ILi, ���.

4 Bjering and Vandevska-Radunovic

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442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504

post hoc power analysis was conducted for each regres-sion model using the software package G*Power (version3.1.9.2; Franz Faul, Universit€at Kiel, Kiel, Germany). Thestatistical analyses were performed with SPSS software(version 24; IBM, Armonk, NY), using a significance levelof 0.05.

RESULTS

The mean weighted PAR score at pretreatment was24.0 (69.8). The posttreatment score was 2.6 (63.4),which increased to 3.6 (63.7) at T3, 4.5 (64.8) at T5,and 5.1 (64.9) at T10. The PAR index percentage im-provements were 89.2% at T1, 85.0% at T3, 81.3% atT5, and 78.8% at T10. Total weighted PAR scoresshowed statistically significant changes between allstudied time periods (Table V). The unweighted PARcomponent scores showed a significant reduction fromT0 to T1 and, with the exception of the centerlinecomponent, increased some from T1 to T10. At 10 yearsposttreatment, all scores were still significantly lowercompared with pretreatment. For the stages T1-T3,T3-T5, and T5-T10, the main trend was small insignifi-cant changes in the different parts of the occlusion(Fig 2). Only the anterior components showed signifi-cant, albeit minor, changes from T3 to T5 and T5 to T10.

The measurements for LII, overjet, overbite, canine/molar relationship, and maxillary/mandibular caninedistances showed the same pattern as the PAR compo-nent scores. Apart from significant but small changesin anterior alignment in all posttreatment stages, therewere essentially no changes in these variables.

Comparison of the 2 retention subgroups in themaxilla, adjusted for treatment correction in LII, showed

- 2018 � Vol - � Issue - American

that a fixed retainer for 10 years posttreatment with aremovable retainer lowered the posttreatment increasein LII by 0.6 mm more than in patients who used onlya removable retainer (Table V). Statistical power for themaxillary regression analysis with 2 independent vari-ables was above 98% at a large effect size (R2 5 0.26)and above 79% at a moderate effect size (R2 5 0.13),with a sample size of 75 and an alpha level of 0.05.

The increase in the mandibular LII from posttreat-ment to 10 years posttreatment was 1.1 mm lower forpatients wearing a fixed retainer for 10 years comparedwith 3 years, and 0.7 mm lower compared with 5 years.There were no significant differences in posttreatmentchanges in LII between the groups with fixed retentionfor 3 and 5 years (Table VI). Statistical power for themandibular analyses with 2 independent variables wasabove 99% at a large effect size (R2 5 0.26) and above85% at a moderate effect size (R25 0.13) , with a samplesize of 86 and an alpha level of 0.05.

DISCUSSION

This study shows that changes in occlusal parameterssuch as overjet, overbite, incisor alignment, and poste-rior occlusion underwent gradual deterioration duringa 10-year period after orthodontic treatment. In eachsubperiod, the changes were insignificant. When viewedover a 10-year perspective, the changes were statisticallysignificant, although of limited clinical importance. Forpatients wearing a removable maxillary retainer, theadditional benefit of having a fixed retainer for 10 yearswas significant, but of minor clinical importance. In themandible, fixed retention for 10 years gave moderatelybetter tooth alignment compared with fixed retentionfor 3 or 5 years.

The results indicate that dual retention in the maxillais unnecessary for the average patient. When adjustedfor corrections made during treatment, the effect oflong-term wear of maxillary fixed retention in additionto a removable retainer was small in terms of improvedstability. A removable retainer will probably be sufficientfor patients with low to moderate pretreatment irregu-larity. However, since posttreatment deterioration inthe maxilla correlated with Qthe degree of alignmentcorrection, a fixed retainer could be considered for pa-tients with greater pretreatment irregularity, and certainpatients might need the combination with a removableretainer. Previous long-term studies found a marked as-sociation between treatment and posttreatment align-ment changes in the maxilla.14 Nonetheless, maxillaryalignment tends to be quite stable in the long term,more so than mandibular alignment. These suggestedretention strategies do not apply to special clinical

Journal of Orthodontics and Dentofacial Orthopedics

Table IV. Variables investigated for possible confounding when performing linear regression analyses to predict theinfluence of fixed retention on the outcome of maxillary and mandibular posttreatment change in LII at T10

Obtained from Variable Description MX0 MX10 MD3 MD5 MD10Dental 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 –2.6Treatment change in overbite –2.0 –1.5 –1.2 –2.0 –1.8Treatment change in intercanine distance 0.2 0.5 –0.6 0.2 0.2Treatment 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 4Distal basal jaw relationship at T0 ANB .4 26 13 10 9 27Low mandibular plane angle at T0 ML/NSL\29� 11 7 4 7 9High mandibular plane angle at T0 ML/NSL .37� 7 7 4 0 12Maxillary incisor protrusion at T1 Isb-NA .2 mm above norm 16 9 9 4 16Maxillary incisor retrusion at T1 Isb-NA .2 mm below norm 2 0 0 1 1Maxillary incisor proclination at T1 ILs/NA .3� above norm 23 12 11 9 22Maxillary incisor retroclination at T1 ILs/NA .3� below norm 6 3 3 3 5Mandibular incisor protrusion at T1 Iib-NB .2 mm above norm 8 7 4 5 11Mandibular incisor retrusion at T1 Iib-NB .2 mm below norm 3 2 1 1 4Mandibular incisor proclination at T1 ILi/NB .3� above norm 14 8 8 4 14Mandibular 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 14.7Follow-up time at T10 (y) 10.1 10.0 10.1 10.3 10.2Sex (M/F) 26/26 7/16 9/10 12/7 18/30Extraction/nonextraction 13/39 13/10 6/13 7/12 18/30

ML/NSL, ���; Isb-NA, ���; Iib-NB, ���; ILiNB, ���.For each retention subgroup (MX0, MX10, MD3, MD5 and MD10) dental cast mean values in millimeters and frequencies of cephalometric valuesare shown. Treatment change was defined as T1 value minus T0 value. Normal values for maxillary and mandibular incisor protrusion and incli-nation were corrected for Steiner's acceptable compromises.

Table V. Paired sample t test of mean changes in unweighted PAR component scores, total weighted PAR scores, andother dental cast variables from T0 to T1, T1 to T3, T3 to T5, T5 to T10, T0 to T10, and T1 to T10

T0-T1 T1-T3 T3-T5 T5-T10 T0-T10 T1-T10

(96) (70) (66) (86) (96) (96)Anterior component- maxilla –4.1* 0.0 0.1y 0.2y –3.8* 0.3z

Anterior component-mandible –2.0* 0.0 0.1y 0.2z –1.7* 0.3*Posterior component –2.1* 0.1 0.1 0.1 –1.8* 0.3y

Overjet component –1.7* 0.1 0.1 0.1 –1.5* 0.2*Overbite component –1.0* 0.1 0.1 0.1 –0.7* 0.3*Centerline component –0.3* 0.0 0.1 0.0 –0.3* 0.0Total weighted PAR score –21.4* 0.7y 1.1y 0.9§ –18.8* 2.5*LII-maxilla –4.6* 0.4* 0.2§ 0.3* –3.8* 0.8*LII-mandible –3.6* 0.3* 0.3§ 0.4* –2.7* 0.9*Overjet –2.8* 0.2 0.1 0.1 –2.5* 0.3z

Overbite –1.7* 0.1 0.2 0.1 –1.3* 0.4z

Canine relationship –1.9* 0.0 0.0 0.0 –1.9* 0.0Molar relationship –1.9* –0.1 0.0 0.0 –1.9* 0.0Maxillary intercanine distance 0.3 0.2 –0.1 –0.2§ 0.2 –0.1Mandibular intercanine distance 0.0 0.1 –0.1 –0.1 0.0 0.1

LII, Molar/canine relationships, intercanine distances measured in millimeters.Total patient sample (n 5 96) was tested. Values in parentheses are number of subjects, for some periods reduced by limited availability of studycasts at T3 and T5.*P #0.0001; yP #0.05; zP #0.001; §P #0.01.

Bjering and Vandevska-Radunovic 5

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circumstances such trauma to the anterior teeth, largediastemas, autotransplants, anterior tongue thrust, peri-odontitis, extensive anterior space closures, and so on.

American Journal of Orthodontics and Dentofacial Orthoped

For the mandible, a fixed retainer for 10 years gavesignificant but small to moderate improvements inalignment compared with 3-year and 5-year retainer

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Fig 2. Overview ofmean unweighted PAR component scores at pretreatment (T0), posttreatment (T1),3 years posttreatment (T3), 5 years posttreatment (T5), and 10 years posttreatment (T10) for the totalpatient sample (n 5 96). U, ���; L, ���.

Table VI. Overview of linear regression models showing the effect of different retention protocols on changes in LIIfrom T1 to T10 (Little_T1T10)

Regression model Coefficients b value 95% CI P valueMaxilla*Model P value: 0.009y (constant) 0.367 (–0.118, 0.852) 0.135R2: 0.12 MX10* –0.643 (–1.205, –0.080) 0.026z

Reg equation: Little_T1T10 5 0.37 - 0.64*MX10 - 0.14*Little_T0T1 Little_T0T1 –0.143 (–0.243, –0.044) 0.005y

Mandible§

Model P value: \0.001k (constant) 1.611 (1.194, 2.027) \0.001k

R2: 0.21 MD5§ –0.468 (–1.058, 0.121) 0.118Reg equation: Little_T1T10 5 1.61 - 0.47*MD5 - 1.12*MD10 MD10§ –1.115 (–1.607, –0.623) \0.001k

Mandible{

Model P value: \0.001k (constant) 1.142 (0.726, 1.559) \0.001k

R2: 0.21 MD3{ 0.468 (–0.121, 1.058) 0.118Reg equation: Little_T1T10 5 1.14 1 0.47*MD3 - 0.65*MD10 MD10{ –0.646 (–1.138, –0.154) 0.011z

Reg, ���.Durations of retention until T3, T5, and T10 were examined. For the maxilla, 2 groups were compared: MX0 and MX10; the model was adjusted fortreatment change in LII (Little_T0T1). For the mandible, 3 groups were compared: MD3, MD5, and MD10. The retention subgroups were coded asdummy variables (0 as absence and 1 as presence of the retention protocol) and compared with the reference group, requiring 1 regressionmodel forthe maxilla and 2 for the mandible.*MX0 as reference variable; yP #0.01; zP #0.05; §MD3 as reference variable; kP #0.0001; {MD5 as reference variable.

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wear. The small difference in posttreatment irregularitybetween patients with a 3-year and a 5-year retentionprotocol was not statistically significant. Consequently,our results are inconclusive. Nevertheless, the overall re-sults indicate that a time factor is present regardingduration of mandibular retention. With larger samplesizes in subgroups MD3 and MD5, the differencesobserved between the 2 groups would have been, most

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probably, statistically significant. Previous researchfound that mandibular alignment is best preservedwith a fixed retainer, and the longer the duration, thebetter the alignment.13,22 But adequate long-term sta-bility of mandibular alignment has also been reportedafter only 1.8 years of fixed retention.14 Our findingsfor the mandible show that the longest retention timegave the best alignment scores, but the advantage over

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a shorter retention time was small to moderate in a 10-year perspective. This situation may of course change astime progresses. Little et al23 regarded an irregularityscore of less than 3.5 mm to be clinically acceptable. Au-thors of other studies comparing different retentionmethods have considered a mean difference of 1.4 mmin mandibular alignment irregularity to be of no clinicalimportance.24 The discrepancies in LII that we found byseparating the retention subgroups in this study arewithin these values. However, to what extent a deviationwill be tolerated will vary from person to person, and theorthodontist should strive to determine when minoralignment changes could become disturbing to the pa-tient.

The PAR score improvement of 79% at 10 years post-treatment demonstrates that overall orthodontic treat-ment can be stable to a great extent, even in the longterm. Previous reports on treatment stability 10 to17 years posttreatment have shown improvements inthe PAR index ranging from 49% to 73%.7,8,10,14,25

Compared with these, our study showed favorablelong-term outcomes. We found a gradual change inthe total weighted PAR index and most of the un-weighted occlusion parameters during the 10-yearfollow-up. This is in contrast to the findings of AlYami et al7 and Otuyemi and Jones,8 who found thatthe largest changes in occlusion took place in the first2 and 4 years posttreatment, respectively. On a compo-nent level, the centerline did not change, complement-ing previous findings of good long-term midlinestability.7,8 The remaining components showed minoradjustments, and the anterior alignment stood out asthe component most prone to changes. Altogether, theobserved changes resembled growth changes seen inuntreated persons.26

There are some limitations to this study. As a retro-spective study, it does not have the scientific impact ofa randomized controlled trial. However, a randomizedcontrolled trial with a follow-up period of 10 years canbe difficult to complete because of dropouts. Also, theregression models for the maxilla and the mandiblediffer in terms of patient samples and the number ofgroups. They were based on slightly different patientsamples according to the disparity in duration of fixedretainer wear. Therefore, the results from them are notdirectly comparable with each other.

Furthermore, the low R2 values imply that the signif-icant regression variables lack some precision in their es-timates. The confidence intervals for these variables wereapproximately 1 mm wide, demonstrating some uncer-tainty in the expected differences in retentive effects be-tween the retention protocols. This shows that thevariability in posttreatment alignment deterioration

American Journal of Orthodontics and Dentofacial Orthoped

was not broadly explained by differences in retainerwear alone, indicating that other factors also contributeto the process. It seems reasonable to expect that com-plex processes such as growth and maturation accountfor some variations in the patient sample. Since cephalo-metric measurements were not available for T10, we donot know how growth and incisor positions changedduring the posttreatment period. However, decisionsregarding retention in clinical practice are made at thetime of debonding at the latest. Any information tosignificantly influence the choice of retention strategyshould be present at that time. Growth prediction hasproved to be difficult.27 For our patients, neither pre-treatment sagittal/vertical jaw relationship nor post-treatment incisor position outside the normal rangewere significantly associated with the dependent and in-dependent variables. This contrasts with the findings ofFranklin et al,28 who reported cephalometric values suchas vertical dimension and mandibular incisor to the A-Pgplane to be important predictive factors for long-termalignment stability. However, in our study, many pa-tients still wore retainers at T10, whereas the study ofFranklin et al was done at least 5 years postretention.From a clinical standpoint, it seems reasonable thatthe advantages of prolonged retention must outweighthe potential disadvantages. Therefore, all aspects oflong-term effects of retention appliances must be stud-ied to establish a preferred duration of retention.

CONCLUSIONS

1. Stability of occlusion measured by the PAR indexpercentage improvement method was 79% at10 years posttreatment.

2. Components of the occlusion underwent gradualand minor deterioration dispersed evenlythroughout the 10-year posttreatment period.

3. Posttreatment changes in maxillary alignment weresignificantly negatively correlated with the inducedtreatment changes.

4. In patients wearing a removable maxillary retainerfor 3 years, the additional benefit of wearing a fixedretainer for 10 years was minor regarding stability ofmaxillary alignment.

5. In the mandible, fixed retention for 10 years gavemoderately better alignment compared with fixedretention for 3 or 5 years.

ACKNOWLEDGMENTS

We thank Andrej Grijbovski at the Norwegian Insti-tute of Public Health for statistical guidance and TanyaFranzen for editing.

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