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Postretention stability after orthodontic closure of maxillary interincisor diastemas

Anterior spaces may interfere with smile attractiveness and compromise dentofacial harmony. They are among the most frequent reasons why patients seek orthodontic treatment. However, midline diastema is commonly cited as a malocclusion with high relapse incidence by orthodontists. Objectives This study aimed to evaluate the stability of maxillary interincisor diastemas closure and the association of their relapse and interincisor width, overjet, overbite and root parallelism. Material and Methods Sample comprised 30 patients with at least a pretreatment midline diastema of 0.5 mm or greater after eruption of the maxillary permanent canines. Dental casts and panoramic radiographs were taken at pretreatment, posttreatment and postretention. Results Before treatment, midline diastema width was 1.52 mm (SD=0.88) and right and left lateral diastema widths were 0.55 mm (SD=0.56) and 0.57 mm (SD=0.53), respectively. According to repeated measures analysis of variance, only midline diastema demonstrated significant relapse. In the overall sample the average relapse of midline diastema was 0.49 mm (SD=0.66), whilst the unstable patients showed a mean space reopening of 0.78 mm (SD=0.66). Diastema closure in the area between central and lateral incisors showed great stability. Multivariate correlation tests showed that only initial diastema width (β=0.60) and relapse of overjet (β=0.39) presented association with relapse of midline diastema. Conclusions Midline diastema relapse was statistically significant and occurred in 60% of the sample, while lateral diastemas closure remained stable after treatment. Only initial diastema width and overjet relapse showed association with relapse of midline diastema. There was no association between relapse of interincisor diastema and root parallelism.


INTRODUCTION
Anterior diastemas may interfere with smile attractiveness, compromise dentofacial harmony 7,8,24 , and provoke dyslalias 5 . Since they are easily visible, anterior spaces are one of the most important reasons why patients look for long-lasting stable treatment outcomes 1 .
In the primary and mixed dentitions, anterior spaces are common and considered as normal. In the permanent dentition, reported incidence ranges from 1.7% to 38% 11,13,17,19,23 in different populations. This incidence is higher in black individuals than among white or yellow racial groups 11,13,17 .
Midline diastema is frequently cited as a malocclusion with high relapse incidence by orthodontists 2,4,15 . Some have suggested that its recurrence is associated with initial diastema width 18 , inadequate root parallelism at the end of treatment 4,15 , sucking habits or imbalanced musculature 4,15 , abnormal labial frenum 4 , and intermaxillary osseous cleft 18,21 . An increase in overjet and overbite as the diastema reopens was also mentioned 21 .
However, information on stability following maxillary diastema closure is limited and the majority of reports consists of case reports and authors' opinion. The rare follow-up evaluations on maxillary diastema treatment stability have divergent results. Edwards 4 (1977) found diastema relapse on 84% of the sample (recurrent diastema width larger than 0.5 mm), and showed a strong correlation between labial frenum and diastema relapse. Contrarily, other studies 18,21 demonstrated that relapse occurred at about one third of the subjects, but recurrent diastema widths were near 0.6 mm.
To date, no study has evaluated maxillary diastema relapse between central and lateral incisors, or the association between root parallelism and diastema closure stability.
The objectives of this study were: (1) to describe the frequency and severity of midline and lateral interincisor diastema relapses in patients with diastema before treatment; and (2) to identify treatment and posttreatment factors (interincisor widths, overjet, overbite and root parallelism) which could be associated to space reopening.

MATERIAL AND METHODS
A minimum sample size of 20 individuals was SURSRVHG IRU SRZHU DW D VLJQL¿FDQFH OHYHO RI 0.05 to demonstrate a true difference of 0.4 mm in the diastema width, with 0.6 mm of estimated standard deviation, according to Sullivan, et al. 21 (1996).
Ethical Committee approval was obtained from the Research Ethics Committee of Bauru School of Dentistry, University of São Paulo, to perform this retrospective study.
Thirty subjects were selected (17 female; 13 male) with Class I (18 subjects) and Class II (12 VXEMHFWV PDORFFOXVLRQV REWDLQHG IURP WKH ¿OHV of 4,331 patients treated in the Orthodontics Department at Bauru School of Dentistry, University of São Paulo. The subjects were treated nonextraction with standard edgewise appliances (0.022x0.028-inch) by graduate students. Inclusion criteria were patients presenting with at least a maxillary midline diastema equal to or greater than 0.5 mm after maxillary canines full eruption. Subjects with missing anterior teeth, periodontal disease with bone loss, generalized microdontia, maxillary pathologies, mesiodens, diastema closure by a non-orthodontic method, post-orthodontic restoration of maxillary anterior teeth resulting in increase in mesiodistal width, and patients with deteriorated or missing dental casts or radiographs were excluded. After treatment, patients wore a Hawley plate in the maxillary arch. Patients with maxillary fixed retention were excluded. Pretreatment (T1), posttreatment (T2) and postretention (T3) mean ages were 12.94 years (SD 1.27), 15.32 years (SD 1.61), and 22.38 (SD 3.90), respectively. Dental casts and panoramic radiographs taken at T1, T2 and T3 were used. Periapical radiographs taken at T1 and T2 were also evaluated. Treatment changes were calculated as T2-T1, and postretention changes were calculated as T3-T2. Diastema relapse occurred when diastema width at T3-T2 was greater than zero.

Dental cast analysis
All dental cast measurements were made with a 0.01 mm precision digital caliper (Mitutoyo, Kawasaki, Kanagawa, Japan) at T1, T2 and T3. The assessed variables were diastema width, overjet and overbite. To measure these variables, the following concepts were considered: Diastema widths (A, B, C): The smallest distance between adjacent teeth at the level of gum papilla ( Figure 1).
Overjet (OJ): Distance from the labial aspect of the medium point, on mesiodistal direction, of the central maxillary incisor incisal edge to the labial surface of the central mandibular incisor.
Overbite (OB): Distance between the incisal edges of maxillary and mandibular central incisors.
Values of overbite and overjet were obtained from right and left sides and averaged.

Radiographic analysis
Panoramic radiographs were taken at T1, T2, and T3 under standard conditions, with the Frankfurt KRUL]RQWDO SODQH SDUDOOHO WR ÀRRU DQG WKH IDFLDO midline plane in a vertical position, by means of the Rotograph Plus (Villa Sistemi Medicali, Buccinasco, 0LODQ ,WDO\ ZKLFK SURGXFHV D PDJQL¿FDWLRQ IDFWRU close to 25%. Panoramic radiographs were traced using a 0.5 mm pencil on a sheet of acetate paper (14x21 FP SODFHG RYHU WKH UDGLRJUDSKLF ¿OP 7KH LQIHULRU outlines of the zygomatic processes of the maxilla and the contours of the maxillary incisors were traced. A horizontal reference line (IZP line) was used, passing through the most inferior point of the right and left zygomatic processes of maxilla ( Figure  2) 10 . Maxillary incisor angulations were measured, using the following variables: RLI (angle between the long axis of the right maxillary lateral incisor and the IZP line), RCI (angle between the long axis of the right maxillary central incisor and the IZP line), LCI (angle between the long axis of the left maxillary central incisor and the IZP line), LLI (angle between the long axis of the left maxillary lateral incisor and the IZP line). Interincisor angles were assessed by measuring the angles (A°, B°, C°) between adjacent maxillary incisors. Figure  2 describes the anatomical structures, lines and angles used in the panoramic radiographic analysis.
Intermaxillary osseous cleft was evaluated by comparing periapical radiographs taken at T1 and T2, and was considered present when a v-shaped radiolucency in crestal bone between the central LQFLVRUV ZDV IRXQG LQ WKH LQLWLDO DQG ¿QDO WUHDWPHQW radiographs 18,21 .

Error study
A f t e r a m o n t h i n t e r va l f r o m t h e f i r s t measurement, 30 randomly selected dental casts and 30 panoramic radiographs were reevaluated by the same examiner (JFM). The casual error was calculated according to Dahlberg's formula (S 2 ƶG 2 /2n) 3 , where S2 is the error variance and d is the difference between two determinations of the same variable. The systematic errors were evaluated with dependent t tests at P<0.05.

Statistical analyses
Means and standard deviations of dental cast and panoramic radiograph variables were calculated DW WKH LQLWLDO ¿QDO DQG SRVWUHWHQWLRQ VWDJHV DQG were compared by repeated measures ANOVA, followed by Tukey tests.
Independent t tests were used to compare variables related with midline diastema relapse (B) in v-shaped crestal bone subjects and in subjects with normal intermaxillary crestal bone. Results ZHUH FRQVLGHUHG VLJQL¿FDQW DW 3 Variables significantly correlated at P<0.05 XVLQJ 3HDUVRQ FRUUHODWLRQ FRHI¿FLHQW WHVWV DQG WKH YDULDEOHV ZKLFK ZHUH VLJQL¿FDQWO\ GLIIHUHQW EHWZHHQ subjects with normal or v-shaped crestal bone were selected to be included in a backward multiple linear regression analysis.

RESULTS
From the 30 subjects, 8 presented with only a midline diastema. The retention time was 1.44 years (SD 0.48), and the mean period out of retention was 5.62 years (SD 3.89). The mean treatment time was 2.38 (SD 1.06).
There were no systematic errors. Casual errors ranged from 0.04 to 0.15 mm for linear (A, B, C, OJ, OB) and from 0.17° to 1.45° for angular variables  Considering only recurrent diastemas, midline diastema (B) relapse occurred in 18 patients (60%) with a mean increase of 0.78 mm (SD 0.66): six presented recurrent midline diastema of up to 0.5 mm, eight presented 0.5 to 1.0 mm widths, two had diastemas between 1.0 and 1.5 mm, and two diastemas larger than 1.5 mm. Only 5 patients (16.6%) showed right lateral diastema (A) relapse (mean increase of 0.37 mm, SD 0.12) and also only 5 patients (16.6%) presented left lateral diastema (C) reopening (mean increase of 0.47 mm, SD 0.10).    (Table 2). Thus, these variables were selected to be included in a multiple regression analysis to assess the level of relationship between midline diastema relapse and the selected variables.
Eight subjects showed intermaxillary osseous cleft. Initial midline diastema width and its relapse were larger in these patients than in subjects with normal crestal bone (Table 3). These findings suggest that intermaxillary osseous cleft could be a predisposing factor for midline diastema relapse. Therefore, this variable was also selected to be included in the multiple regression analysis.

DISCUSSION
Only midline diastema showed statistically VLJQL¿FDQW UHODSVH 7DEOH DQG RFFXUUHG LQ of patients. However, the diastema width at T3 was VLJQL¿FDQWO\ VPDOOHU WKDQ LWV GLVWDQFH DW 7 7KLV result was also observed by most authors 4,18,21,22 , which means that there is only partial relapse. Contrarily to our results, a previous study 18 showed stability of diastema closure in 75% of the patients. One reason for this divergence may be the difference between sample inclusion criteria, since WKH\ LQFOXGHG SDWLHQWV ZKR ZHUH VWLOO XVLQJ ¿[HG RU removable retention and the mean pretreatment space width was 1.22 mm. In our study, the mean pretreatment midline diastema width was slightly larger (1.52 mm) and the patients were at least 1.8 years out of retention, with an average postretention period of 5.6 years. Sullivan, et al. 21 (1996) found space closure stability in 66% of the patients, and the sample selection criteria were similar to ours, except for the minimum postretention time that was 1 year. Analyzing the relapse amount, most studies 18,21,22 also showed small space reopening. On the other hand, Edwards 4 (1977) demonstrated greater relapse, between 2.4 and 2.7 mm in 84% of the patients, in a sample consisting of patients with an average pretreatment midline diastema of 3.2 mm (minimum 2 mm). Therefore, the initial diastema width might explain the discrepancy EHWZHHQ RXU ¿QGLQJV DQG (GZDUGV 4 . Unlike midline diastema area, lateral space closure appeared to be stable (Table 1). Although lateral space relapse had not been investigated yet, it was previously observed that, when relapse occurs, even in patients presenting generalized anterior spacing before treatment, it is usually located at the midline 21 . Whereas only 22 patients presented lateral diastemas before treatment, post-hoc power analysis calculation showed that a sample composed of 18 subjects provides 91.4% power DW D VLJQL¿FDQFH OHYHO RI WR GHWHFW D PLQLPXP difference of 0.1 mm, with standard deviation 0.12.
Regarding the contributing factors for midline diastema relapse, spaces A, B, and C at T1, space A at T2, and posttreatment changes (T3-T2) in 2-VKRZHG VLJQL¿FDQW DVVRFLDWLRQ ZKHQ WHVWHG E\ univariate correlation test (Table 2). On the other hand, no association between midline diastema relapse and root angulations or parallelism were found.
,W LV VXJJHVWHG WKDW URRW SDUDOOHOLVP LQÀXHQFHV the stability after orthodontic closure of anterior diastema 4,15 . However, the present results did not FRQ¿UP DQ\ LQÀXHQFH RI URRW WLS RQ VSDFH UHODSVH 7DEOH DQG VR GLG WKH ¿QGLQJV SUHVHQWHG LQ D previous study 14 . Additionally, it was found that the slight mesial crown tip of the maxillary incisors occurred during the space closure remained stable posttreatment 14 .
An intermaxillary osseous cleft, which is also a contributing factor for diastema relapse 4,20 was present in eight patients. These patients showed larger initial midline diastema and greater relapse than those with normal intermaxillary crestal bone (Table 3). Nevertheless, multivariate analysis demonstrated that there was no association between intermaxillary osseous cleft and diastema reopening (Table 4). Therefore, it seems that the actual contributing factor for the greater midline diastema relapse in the osseous cleft group was its larger width at the pretreatment stage.
Among the treatment and posttreatment factors, RQO\ 2-77 SUHVHQWHG VLJQL¿FDQW DVVRFLDWLRQ with diastema relapse ( Table 4). As the overjet increased, so did the midline diastema. Muscle function and relapse of Class II malocclusion may explain this association. Forward tongue posture induces incisor proclination, increase in arch length and anterior space opening. If tongue pressure is stronger on the maxillary incisors, the overjet increases. Camouflage orthodontic treatment of Class II division 1 malocclusion usually is reached by maxillary incisor retroclination and great overjet decrease. Therefore, patients with this malocclusion might be more prone to relapse of overjet and, consequently, diastema relapse. Association between diastema reopening and incisor proclination was previously determined 21 .
Some investigators have suggested that labial frenum may contribute to diastema development 2,4,8 . Due to the retrospective follow-up study design, information about frenum anatomy would be available on clinical charts, photographies, or study models. These methods could be considered questionable 21 , and a previous study found no association between frequency of relapse and abnormal frenum 18 . Therefore, assessment of the LQÀXHQFH RI ODELDO IUHQXP RQ GLDVWHPD UHODSVH ZDV not performed.
Image magnification and distortion limit dimensional accuracy in panoramic radiography 9 . This is more critical for linear measurements.
Contrarily, angular measurements in panoramic radiography showed less distortion 6,12 , especially in the anterior region 16 . Panoramic radiography was used because it is a low-dose radiographic technique which provides a comprehensive view of the entire maxillomandibular complex in a single ¿OP $GGLWLRQDOO\ LW LV URXWLQHO\ UHTXLUHG LQ WKH orthodontic practice and its use for this research avoided extra radiation exposure of the patients who comprised this sample.
This study showed that midline diastema closure is highly unstable. The amount of relapse is proportional to its pretreatment width and occurs associated with increase in the overjet. It is suggested lifetime wear of a well-adapted PD[LOODU\ ¿[HG UHWDLQHU RQ DQ\ SDWLHQWV ZLWK PLGOLQH diastema, especially in cases with initial large spaces and/or muscle unbalance.
Only initial diastema severity width and overjet relapse showed association with relapse of midline diastema.
There was no association between relapse of interincisor diastemas and root parallelism.