Our objectives were to evaluate the long-term posttreatment changes of orthodontically corrected mandibular anterior malalignment and to determine the factors explaining these changes.
The sample consisted of 66 subjects (mean age, 15.4 ± 1.7 years) selected from 7 private practices. The teeth had been retained for approximately 3 years and followed for 15.6 ± 5.9 years posttreatment. Longitudinal study models and cephalograms were analyzed to quantify the malalignment and growth changes that occurred.
Crowding (1.2 ± 0.9 mm) and irregularity (1.5 ± 1.8 mm) showed only small average increases over the postretention period; only 26% of the sample had more than 3.5 mm of postretention irregularity. Variation in crowding explained 16% of the differences among subjects in irregularity. Growth variables (posterior facial height and mandibular rotation) and interarch variables (incisor-mandibular plane angle, interincisal angle, overbite, and overjet) were not significantly related to malalignment. Postretention malalignment changes were related to posttreatment anterior arch perimeter, intercanine width, and arch form, together indicating that narrower arch forms are likely to show greater posttreatment malalignment changes. Patients treated with extractions showed significantly greater malalignment than those treated without extractions; this was related to arch form. Patients who received interproximal restorations after treatment also showed significantly greater postretention malalignment than patients who did not.
Orthodontic treatment is not inherently unstable. Narrow arch forms and interproximal restorations are potential risk factors for the development of postretention malalignment.
Mandibular anterior malalignment is the most significant problem in patients having orthodontic treatment. Clinically significant incisor irregularity occurs in approximately 40% of the untreated population between 15 and 50 years of age, with approximately 17% exhibiting severe amounts (≥7 mm) of mandibular irregularity. Significant malalignment has also been regularly reported after orthodontic treatment. To further challenge orthodontists, treated patients are more aware of their malocclusion than are untreated subjects. Since patients want to maintain straight teeth, a primary objective of the orthodontist must be long-term stability.
When established guidelines of traditional orthodontic treatment have been followed (ie, no excessive flaring or canine expansion), treatment-related factors do not appear to explain postretention malalignment. There also appears to be no relationship between pretreatment and postretention dental malalignment. Although excessive incisor proclination during treatment appears to be related to posttreatment retroclination, limited amounts of incisor proclination during treatment show little or no relationship to posttreatment changes. Numerous studies have reported postretention decreases in intercanine width, but only one has shown a correlation between the amount of expansion during treatment and the amount of relapse after treatment. The available evidence suggests that when established treatment guidelines have not been violated, postretention malalignment appears to be primarily related to nontreatment factors.
Malalignment of teeth regularly occurs in untreated subjects, even in those with normal occlusions. Incisor irregularity of untreated subjects shows the greatest rate of increase during adolescence; the rates progressively decrease thereafter. Importantly, the irregularity and crowding increases reported for untreated subjects are similar to the malalignment increases reported in long-term follow-up studies of patients treated in private practices ( Table I ). This supports the finding of no long-term differences in irregularity and crowding between matched treated and untreated subjects. The similarities of treated and untreated subjects further supports the notion that the commonly reported postretention changes are due to factors not associated with treatment.
|Ext, nonext, both, or untx||Initial (posttreatment) age (y)||Final (postretention) age (y)||ΔII (mm)||ΔTSALD (mm)||ΔII (mm/y)||ΔTSALD (mm/y)|
|Vaden et al||Ext||15.3||21.6||+0.6||NA||+0.09||NA|
|Moussa et al||Nonext||15.7||22.0||+0.8||NA||+0.13||NA|
|Elms et al , ∗||Ext||14.5||23.1||+0.4||NA||+0.05||NA|
|Ferris et al||Nonext||13.7||24.3||+1.1||NA||+0.10||NA|
|Driscoll-Gilliland et al||Ext||14.8||26.1||+1.3||−1.0||+0.10||−0.09|
|Glenn et al||Nonext||14.9||26.7||+1.2||NA||+0.10||NA|
|Dugoni et al||Nonext||13.6||27.9||+1.6||NA||+0.11||NA|
|Park et al||Both||14.2||30.3||+1.5||NA||+0.09||NA|
|Park et al||Both||21.5||37.2||+0.9||NA||+0.06||NA|
|Vaden et al||Ext||21.6||30.5||+0.8||NA||+0.09||NA|
|Boley et al||Ext||15.5||31.6||+0.7||NA||+0.06||NA|
|Overall treated average||Both||15.8||27.8||+1.1||−1.1||+0.09||−0.08|
|Sinclair and Little||Untx||9.5||12.5||–0.2||NA||−0.07||NA|
|Sinclair and Little||Untx||12.5||19.5||+0.7||NA||+0.10||NA|
|Driscoll-Gilliland et al||Untx||14.3||23.2||+0.9||−1.1||+0.10||−0.12|
|Richardson and Gormley||Untx||21.0||28.0||NA||−0.2||NA||−0.03|
|Bishara et al||Untx||25.0||46.0||NA||−0.3||NA||−0.04|
There are age-related effects on the development of posttreatment malalignment that indicate an association with growth. Compared with sex, ethnic affiliation, and number of teeth present, age is by far the most important factor explaining incisor irregularity. Vertical facial growth and incisor eruption appear to be the morphologic components most closely related to postretention irregularity. Since mandibular rotation influences incisor inclination, and inclination changes might affect space relationships, rotation might affect the development of malalignment, but this relationship has not been previously explored. The contained-arch principle suggests that as overbite deepens after treatment, the lingual force imposed on the mandibular incisors from the maxillary incisors tends to squeeze and crowd the mandibular arch. There has been only 1 study substantiating this relationship ; other studies have evaluated, but have not been able to support, such a relationship.
Irregularity has been reported to worsen for teeth located farther from the mandibular midline, indicating that arch form can also be related to anterior malalignment. It has been suggested that the canines might be “slipping” into a position anterior to the lateral incisor in crowded dentitions, giving the arch a more square appearance. Previous studies evaluating the effects of mandibular arch form on malalignment have been based on linear depth, length, and transverse measurements, which provide only limited information about arch shape and no information about the positional (rotation and displacement) changes of the dentition. Positional changes could explain the pattern of irregularity observed if alignment is most susceptible at the portions of the arch with the greatest curvature. This is important because the anterior component of occlusal forces might be expected to have its greatest impact at the contacts located at the greatest curvature. Oversized restorations, which also provide an anterior component of force, must be considered as another potential contributing factor.
The aims of this study were to evaluate the long-term posttreatment positional changes of the mandibular teeth and to investigate whether and how growth and intra-arch dental relationships influence these changes. Whether long-term stability is related to novel measures of dental arch shape, including the contact angles between adjacent teeth, and to the angles between dental counterparts in the same jaw, has not been previously established. These relationships should give orthodontists evidence that will enhance their understanding of malalignment, guide them in the treatment of patients, and perhaps provide guidelines for the prevention of future dental malalignment.
Material and methods
A sample of 66 orthodontically treated patients was collected from 7 orthodontists who used various appliances and mechanics but shared a similar philosophy of maintaining the teeth over the basal bone (ie, they did not excessively procline the incisors or expand the canines). One orthodontist performed interproximal reduction of the mandibular anterior teeth when the retainers were removed. Many patients were treated in the 1960s and 1970s with full banded appliances. It was considered important to include a wide spectrum of treatment approaches and philosophies ( Table II ).
|Orthodontist||Experience (y)||Treatment approach||Treatment philosophy||Appliances used||Retention protocol|
The records collected included plaster study models, cephalograms, and panoramic radiographs or full-mouth radiographic series. For inclusion in this study, the patients’ final posttreatment and long-term postretention records needed to be of acceptable quality. At posttreatment, the patients had to be less than 21 years of age. Postretention records had to be taken a minimum of 5 years posttreatment and 3 years postretention. The patients reported compliance with the retainer protocol, which lasted for 3 years on average. Treatment outcome was not an inclusion criterion. Rejection criteria included pretreatment Class III malocclusions, craniofacial anomalies, orthognathic surgery, circumferential supracrestal fibrotomy, postretention spacing, or abnormal incisor anatomy such as restorations, interproximal reduction, or large lingual marginal ridges. The first 25 patients who met the selection criteria were included in the study.
All posttreatment records were taken within 3 months of debonding or debanding. They were collected at a mean age of 15.4 ± 1.7 years. Postretention records were collected at a mean age of 31.1 ± 6.3 years. The average posttreatment duration was 15.7 ± 6.0 years.
Overjet and overbite were directly measured on the study models with digital calipers, accurate to 0.01 mm.
Overjet, measured parallel to the occlusal plane, was the distance from the incisal edge of the most labial maxillary central incisor to the facial surface of the most labial mandibular central incisor.
Overbite, measured perpendicular to the occlusal plane, was the overlap of the maxillary to the mandibular central incisors.
Eight additional model measurements were collected from standardized digitized photographs, taken with a single-lens reflex camera and a 100-mm macro lens positioned 27 in from the study models, which were secured in a standardized position. The photos were imported into Dolphin software (version 11.0; Dolphin Imaging & Management Solutions, Chatsworth, Calif), and a customized protocol was used to calculate the following.
Intercanine width, measured between the cusp tips of the mandibular canines ( Fig 1 , A ).
Anterior arch perimeter, measured as the sum of 4 segments, 2 segments on each side. The 2 canine segments were measured from the mesial aspect of the first premolar to the contact of the canine and the lateral incisor. The 2 incisor segments were measured from the contact of the canine and the lateral incisor to the contact of the central incisors on the midline ( Fig 1 , A ).
Tooth size, measured as the sum of the mesiodistal diameter of the 6 anterior mandibular teeth.
Contact irregularity, measured between pairs of teeth as the distance between the contact points of the teeth anterior to the first premolars.
Contact angle, measured by lines through the mesial and distal contact points of adjacent teeth anterior to and including the first premolars ( Fig 1 , B ).
Interdental angle, measured as the angle formed by lines through the mesial and distal contact points of the contralateral teeth anterior to and including the first premolars ( Fig 1 , B ).
Incisor irregularity, calculated as the sum of the 5 contact irregularity measurements of the 6 anterior teeth.
Tooth size-arch length discrepancy, calculated as the difference between tooth size and anterior arch perimeter.
The posttreatment and postretention lateral cephalograms were scanned and imported into digitizing software. Sella, gonion, menton, the incisor apices, and the incisor cusp tips were digitized by the principal investigator (S.A.M.) using standard landmark definitions. Structures necessary to superimpose on the cranial base and the mandible were also traced. Based on the structures traced and the landmarks digitized, the following measurements were calculated with the Dolphin software.
Posterior facial height, measured as the linear distance from sella to gonion.
Incisor to mandibular plane angle, measured as the angle formed by the long axis of the mandibular incisor to the mandibular plane.
Interincisal angle, measured as the angle formed by the long axis of the mandibular incisor with that of the maxillary incisor.
Apparent rotation (as described by Solow and Houston ), measured as the angle formed by the postretention and posttreatment mandibular planes after cranial base superimposition.
Modeling rotation, measured as the angle formed by the postretention and posttreatment mandibular planes after mandibular superimposition.
True rotation, calculated as the difference between apparent rotation and modeling rotation.
The panoramic and full-mouth radiographic series were evaluated to determine whether interproximal dental restorations had been performed. Restorations were counted based on radiopacity.
Twenty random plaster models and radiographs were selected and measured twice to determine the intraexaminer reliability. There were no statistically significant systematic errors between replicate measurements. There were also no differences between replicates in the numbers of interproximal restorations. Random errors, measured by Dahlberg’s method error statistic, ranged from 0.03 to 0.48 mm for linear measurements and from 0.10° to 0.40° for angular measurements.
Descriptive statistics were calculated at each time point, as well as for changes over the posttreatment period. Because some distributions were significantly skewed or kurtotic, nonparametric statistical comparisons were used. Spearman rank order correlations were used to assess bivariate associations. Mann-Whitney tests were used to evaluate the effects of sex, interproximal restorations, and extraction treatment.
The subjects had small amounts of incisor irregularity (1.48 mm) and minor spacing (0.39 mm) at the end of treatment ( Table III ). Although irregularity and tooth size-arch length discrepancy increased significantly over time, the average increases in incisor irregularity (1.50 mm) and crowding (−1.22 mm) were minor. Posttreatment and postretention irregularity and tooth size-arch length discrepancy were not significantly correlated. Changes in irregularity were positively correlated with both posttreatment age ( r = 0.361, P = 0.003) and posttreatment duration ( r = 0.313, P = 0.010). Changes in tooth size-arch length discrepancy were negatively correlated ( r = −0.275, P = 0.025) with posttreatment duration. There was a significant negative correlation between postretention tooth size-arch length discrepancy and irregularity ( r = −0.603, P <0.001). Tooth size-arch length discrepancy and irregularity changes over time were also significantly intercorrelated ( r = −0.406, P = 0.001).
|Malalignment (mm)||Posttreatment (T1)||Postretention (T2)||Change T2-T1|
Posterior facial height increased by 2.9 mm, and the mandible underwent 1.34° ± 0.24° of true forward rotation during the posttreatment period ( Table IV ). One quarter of the true rotation was masked by backward modeling rotation (0.33° ± 1.18°), resulting in −1.01° ± 2.11° of apparent rotation. Although these growth measures were not related to posttreatment duration, posttreatment age was negatively correlated with changes in posterior facial height ( r = −.441, P <0.001) and positively correlated with true mandibular rotation ( r = 0.281, P = 0.022). Changes in posterior facial height and true rotation were also intercorrelated ( r = −0.402, P = 0.001). There were no other significant correlations between these growth variables and the interarch or intra-arch measurements evaluated.
|Variable||Posttreatment (T1)||Postretention (T2)||Change T2-T1|
|SGo (mm)||73.95||6.56||76.82||6.93||2.87 ∗||2.40|
|OB (mm)||2.30||0.70||2.83||1.05||0.53 ∗||0.93|
|OJ (mm)||2.38||0.60||2.80||0.77||0.42 ∗||0.78|
|U1L1 (°)||128.43||8.25||129.86||8.61||1.43 ∗||5.56|
Overbite and overjet increased slightly, but significantly, over time ( Table IV ). Changes in overbite were positively correlated with changes in overjet ( r = 0.408, P = 0.001). The interincisal angle increased by 1.4° at posttreatment. Although the incisor to mandibular plane angle showed no statistically significant change over time (ie, it both increased and decreased), the changes were negatively correlated with the changes in the interincisal angle ( r = −0.656, P <0.001). The interarch measures showed no other significant correlations.
Anterior arch perimeter and intercanine width decreased significantly over time ( Table V ). Changes in anterior arch perimeter and intercanine width were positively intercorrelated ( r = 0.426, P <0.001) and were weakly correlated with postretention tooth size-arch length discrepancy ( r = 0.275 P = 0.025; and r = 0.388; P = 0.001, respectively); intercanine width changes were negatively correlated with postretention incisor irregularity ( r = −0.371, P = 0.002). The changes in anterior arch perimeter and intercanine width were not significantly correlated with any other measures.
|Intra-arch measure||Posttreatment (T1)||Postretention (T2)||Change T2-T1|
|AP (mm)||37.63||1.83||35.66||1.72||−1.97 ∗||1.05|
|ICW (mm)||26.30||1.30||25.19||1.61||−1.10 ∗||1.00|
|Contact irregularity (mm), distance from anatomical contact points of adjacent teeth|
|CI L3-2||0.42||0.29||0.66||0.58||0.23 ∗||0.61|
|CI L2-1||0.19||0.19||0.52||0.50||0.33 ∗||0.53|
|CI 1-1||0.18||0.20||0.49||0.53||0.30 ∗||0.56|
|CI R2-1||0.23||0.23||0.55||0.52||0.32 ∗||0.53|
|CI R3-2||0.46||0.35||0.77||0.69||0.31 ∗||0.73|
|Contact angles (°), angles formed by lines through the mesial and distal contact points of adjacent teeth|
|CA 1-1||161.48||4.74||164.38||11.92||2.90 ∗||9.73|
|Interdental angles (°), angles formed by lines through the mesial and distal contact points of contralateral teeth|
|IDA 1-1||161.48||4.74||164.38||11.92||2.90 ∗||9.73|
|IDA 2-2||126.02||7.73||129.60||13.15||3.57 ∗||10.28|