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Factors affecting the stability of maxillary extraction site closure

ABSTRACT Introduction: A side effect observed in cases treated with extractions is the instability of orthodontic space closure. Objective: The aim of this study was to investigate the influence of gingival invagination, presence of third molars and facial pattern, on the stability of orthodontic space-closure in the maxillary arch. Methods: Ninety-nine subjects (41 male and 58 female) with Class I malocclusion treated with four premolars extraction were evaluated. Extraction sites reopening and gingival invaginations were evaluated in scanned dental models in the posttreatment and 1-year posttreatment stages (mean age 16.1 years). Third molars presence was evaluated at 1-year posttreatment panoramic radiographs, and the facial pattern (SN.GoGn) was evaluated in the initial lateral headfilms. Multiple logistic regression analysis was used to estimate the influence of the aforementioned independent variables on the frequency of extraction space reopening. Results: Space reopening was observed in 20.20% of the subjects 1-year post-debonding. Gingival invaginations were present in 25.73% of quadrants after debonding and in 22.80% 1-year posttreatment. The mean pre-treatment SN.GoGn was 35.64 degrees (SD=5.26). No significant influence was observed of the three independent variables on the instability of extraction site closure. Conclusions: The presence of gingival invaginations, third molars and facial growth pattern do not seem to influence maxillary extraction sites reopening.


INTRODUCTION
Maintaining extraction spaces fully closed in the long-term remains a challenge for clinical Orthodontics. [1][2][3][4] Extraction space reopening determine both esthetic and functional problems, such as interproximal food impaction. 1 Approximately 30% of Class I patients presented extraction space reopening 1-year posttreatment. 5 The group with space relapse presented smaller initial dental crowding and greater amount of incisors retraction during orthodontic treatment.
Some factors such as inadequate dental interdigitation, imbalance between intraoral and extraoral forces, deficient occlusal results after orthodontic treatment, lack of proper retention protocol, distortion of the periodontal fibers, growth pattern and root parallelism have been considered to influence the stability of closed-spaces. 1,3 Nevertheless, reevaluation of closed-spaces stability has shown no correlation with some of these factors. 3,4 No previous study has evaluated the influence of gingival invagination, presence of third molar and facial growth on opening of extraction space using regression analysis.
After closure of an extraction site, excess of gingival tissue appears in a papillary form between the approximated teeth. 1 This gingival deformation, denominated gingival invagination, is not rapidly reorganized by the oral physiologic process and Bressane L, Janson G, Naveda R, Freitas MR, Garib D Factors affecting the stability of maxillary extraction site closure 5 appears to be associated with orthodontic space relapse in extraction areas. 1 However, the association between extraction space reopening and gingival invaginations has not been demonstrated so far. 1,3 It has been suggested that the presence of third molars may influence the long term stability of mandibular alignment. 6 Although there is no scientific evidence of the third molars role in orthodontic retention, 7 some studies sustain that third molars may move teeth mesially in the long term. 8 Considering that there is physiologic mesial movement during third molars development, these mesial forces may possibly influence the long-term stability of extraction-site closure, maintaining the spaces closed.
One essential factor for orthodontic diagnosis and prognosis is the facial growth pattern. Several studies have demonstrated greater instability of anterior dental alignment in hyperdivergent patients. 9-11 As a dental compensation of the growth pattern, the incisors tend to develop more vertically, increasing their retroclination. 9 Considering this long-term behavior Bressane L, Janson G, Naveda R, Freitas MR, Garib D Factors affecting the stability of maxillary extraction site closure 6 Considering the elevated prevalence of extraction space reopening in the first-year posttreatment, 5 the present study aims to assess whether gingival invagination, presence of maxillary third molars and facial growth pattern are associated with extraction space relapse in the maxillary arch. The information regarding initial, final and 1-year posttreatment ages, as well as treatment time is described in Table 1.

This study was approved by the Ethics in Research Committee of
All patients were treated by graduate students with 0.022x0.028-in fixed Edgewise appliances. Patients with severe anterior crowding required initial canine retraction. The archwire sequence for leveling and alignment was 0.015-in twist-flex or 0.016-in NiTi archwires, followed by 0.016, 0.018, 0.020, and 0.019x0.025-in stainless steel archwires. The extraction spaces were closed with en-masse retraction of the anterior teeth, with elastic chains on a rectangular stainless steel archwire. After the end of treatment, a modified Hawley retainer was used in the maxillary arch, and a fixed canine-to-canine archwire was bonded in the mandibular arch, as retention (Fig 1). The Hawley retainer was recommended to be used full-time for six months, followed by nights-only use for additional six months. The mandibular canine-to-canine bonded fixed retainer was recommended to be used for 3 years.

ERROR STUDY
The quantitative variable SN.GoGn angle was re-measured in thirty randomly selected patients after a 30-day interval.
Intraobserver random and systematic errors were calculated with Dahlberg's formula 13 and dependent t-test, respectively, at a significance level of 5%. This variable was also tested for normality using Shapiro-Wilk test.

STATISTICAL ANALYSIS
A multiple logistic regression analysis was used to estimate the influence of each factor - gingival invagination, presence of third molars and facial growth pattern -on the occurrence of extraction space reopening. The significance level was 5%.

RESULTS
The random error for the variable SN.GoGn was within acceptable limits (2.74) 14  The prevalence of gingival invaginations at the end of treatment in the sample subjects was 34.34% (34 out of 99 - Table 3).   In relation to the growth pattern, the mean value for the SN.GoGn angle was 35.64° (Table 4).

Considering the number of quadrants with fully-closed
Results of the multiple logistic regression analysis showed no influence of the independent variables (gingival invagination, presence of third molars and growth pattern) on the frequency of extraction space reopening (Table 5).  fixed appliance removal. Given this high incidence, the first and second years of retention after appliance removal would be essential for space closure stability. 5,15 According to Thilander et al, 16 the orthodontist must distinguish the rapid relapse, occurring during the period of remodeling of periodontal structures, from the slow relapse, which responds to late changes occurring during the postretention period. Thus, this study evaluated the "rapid relapse" of extraction-sites reopening.   14 Gingival invaginations were observed in 34.34% of the subjects and in 25.73% of the quadrants, at the end of treatment (Table 3) follow-up, while 11.37% disappeared (  3 have not confirmed such assumption. No significant correlation was observed between gingival invagination and space reopening (Table 5). Thus, periodontal surgery for solving the invaginations in order to avoid space relapse is not substantiated.
The presence or absence of third molars were not related to space relapse/stability ( Table 5). The possible explanation is that the irruption of third molars do not have enough force to produce mesial posterior teeth movement. 24,25 The mean value for the SN.GoGn angle was 35.64°, indicating that the sample had a slight vertical growth tendency (Table 4). 26 The inclusion criteria may explain this result, once patients incisors at the end of treatment. 29 The hypothesis that hyperdivergent patients would present greater stability of extraction space closure was rejected. No significant correlation between SN.GoGn angle, which is usually used to determine growth pattern, and extraction space reopening was found ( Table 5).
Stability of extraction space closure remains uncertain, considering that most investigations that searched for associated factors did not detect significant results. 3 However, a greater amount of initial crowding and smaller anterior retraction seem to positively influence the stability of extraction space closure. 5 Therefore, treatment of biprotrusion performed with extractions, needing accentuated retractions, would demand longer retention time.
Despite the limitations of having evaluated gingival invaginations on dental casts, the results of this study should be considered when closed-spaces reopening is evaluated. Future studies should investigate closed-space reopening and the predisposing factors in Class II and Class III compensatory treatment.