The aims of this study were to compare the changes in posterior dental inclination and angulation, and the posterior tooth crown sizes and alveolar ridge thicknesses consequent to the orthodontic procedures of closing and opening of mandibular first molar edentulous spaces.
The sample comprised 16 patients (4 men, 12 women) with an initial mean age of 34.17 years and unilateral or bilateral absence of mandibular permanent first molars. The space closure group (SCG) comprised 12 hemiarches with a mandibular first molar edentulous space varying from 2 to 7 mm, orthodontically treated with space closure. The space opening group (SOG) comprised 14 quadrants with a mandibular first molar edentulous space varying from 7.1 to 12 mm, orthodontically treated with space reopening for prosthetic replacement. Digital dental models were obtained before treatment and after space closure or opening, and posterior tooth angulation and inclination, cervico-occlusal crown height, and alveolar ridge thickness were evaluated. Interphase and intergroup comparisons were performed with dependent t tests and t tests, respectively ( P <0.05).
Mandibular second molar uprighting and changes in buccolingual inclination of the posterior teeth were similar in the groups. The second molar cervico-occlusal crown height increased in the SCG and decreased in the SOG. The alveolar ridge thickness increased in the SCG and remained stable in the SOG.
The only significant intergroup differences were that the second molar cervico-occlusal crown height and the alveolar ridge thickness increased in the SCG, and decreased and remained stable in the SOG.
We evaluated dental changes after treatment in adults with mandibular first molar loss.
Space was closed or opened for prosthetic rehabilitation.
Both opening and closure demonstrated appropriate results.
Treatment planning should consider the cost-benefit ratio and treatment time.
Dimensions of edentulous spaces, third molars, and periodontal conditions are also important.
The mandibular permanent first molars have a higher prevalence of loss from caries. The high frequency of first molar loss can be explained by the early eruption time and the complex anatomy of its occlusal surface. The neighboring teeth move as a result of first molar loss, with changes varying in magnitude and movement type. Usually, the mandibular second and third molars show marked mesial and lingual tip, whereas the premolars distally tip. The edentulous alveolar ridge remodels, showing thickness and height decreases.
Treatment planning in patients with loss of mandibular first molars may involve space reopening, followed by prosthetic replacement, or orthodontic closure of the edentulous space. However, mesial movement of the second molars toward the remodeled alveolar ridge area has been scarcely studied in the literature.
Stepovich evaluated changes in edentulous alveolar ridges before and after space closure with orthodontic tooth movement, concluding that orthodontists can close spaces of 10 mm or more in adults. The author emphasized the high frequency of relapse and recommended fixed retainers. Space closure caused crestal bone loss of 1.3 mm at the mesial aspect of the mandibular second molars. Hom and Turley evaluated orthodontic treatment of adults with loss of mandibular first molars and reported that second molar root resorption was minimal.
Digital dental models have been proven to be an appropriate and accurate reproduction of dental arch morphology, and digital measurements have been shown to be as reliable as manual measurements with a caliper. The accuracy and reproducibility of linear measurements performed directly on dental casts and in digital models with the 3Shape Scanner was confirmed in a previous study.
Comparisons of closing and opening procedures of lost mandibular first molar spaces have not been previously evaluated with digital models. Therefore, the aims of this study were to compare the changes in posterior dental inclination and angulation, and the posterior tooth crown sizes and alveolar ridge thicknesses consequent to the orthodontic procedures of closing and opening of mandibular first molar edentulous spaces. The hypothesis was that the neighboring tooth positions and the cervico-occlusal crown heights show no differences in both treatment modalities.
Material and methods
This clinical study was approved by the ethical committee in human research of the Bauru Dental School, University of São Paulo, Bauru, São Paulo, Brazil, and all patients signed informed consent.
Adults with missing mandibular permanent first molars were orthodontically treated with either with space closure through mesial movement of the mandibular second molars or space reopening and second molar uprighting.
The sample included patients who attended the orthodontic clinic at Bauru Dental School, University of São Paulo, Bauru, São Paulo, Brazil from July 2011 to March 2012. The inclusion criteria were (1) initial age between 18 and 45 years, (2) Class I malocclusion, (3) no craniofacial anomalies, (4) absence of at least 1 mandibular permanent first molar with a history of extraction exceeding 2 years, (5) edentulous space size from 2 to 12 mm, (7) presence of the mandibular permanent second molar adjacent to the edentulous space, and (8) no active periodontal disease.
Sixteen patients were selected: 6 with unilateral mandibular permanent first molar loss and 10 with bilateral loss. Two of the 10 patients with bilateral mandibular first molar loss were treated with space opening on 1 side and space closure on the opposite side. The final sample comprised 26 edentulous hemiarches that were divided into 2 groups ( Figs 1 and 2 ).
The space closure group comprised 12 hemiarches with first molar edentulous spaces from 2 to 7 mm. These hemiarches were orthodontically treated with space closure. Comprehensive orthodontic treatment was performed with preadjusted brackets and continuous nickel-titanium and stainless steel archwires. A cantilever spring of 0.017 × 0.025-in beta-titanium alloy wire was used for uprighting the mandibular second molars. After uprighting, the second molars were included in the continuous wire. Mesial movement of the second molars was performed on a 0.019 × 0.025-in stainless steel archwire using 150-g tension produced by an elastic chain or a nickel-titanium coil spring. Mini-implants (6 × 1.5 mm; Morelli, Sorocaba, São Paulo, Brazil) were used as anchorage in 4 of the 12 hemiarches. The mean treatment time until space closure was 1.54 years (18 months 14 days). Figure 1 shows the initial and final digital models and panoramic radiographs of a patient who had space closure on both sides.
The space opening group comprised 14 hemiarches with first molar edentulous spaces from 7.1 to 12 mm. Comprehensive orthodontic treatment was performed using preadjusted brackets and continuous nickel-titanium and stainless steel archwires. A cantilever spring of 0.017 × 0.025-in beta-titanium alloy wire was used for uprighting the mandibular second molars. After molar uprighting, leveling and alignment were conducted with space reopening using open-coil springs. The mechanics ended when the edentulous space had the same mesiodistal size as a mandibular second molar crown. The mean treatment time until space reopening was 1.44 years (17 months 8 days). Figure 2 shows the initial and final digital models and panoramic radiographs of a patient whose spaces were opened on both sides.
All patients were treated by 3 orthodontists (F.S.H.S., PB.D.S., M.C.F.) in the same clinic. The mandibular digital dental models were obtained before and after closing or opening the spaces, using the 3Shape scanner (3Shape; Copenhagen, Denmark). At both times, the following variables were measured on the digital models using the OrthoAnalyser software (3Shape).
For posterior tooth angulation, the occlusal plane was used as the reference to measure the angulation of the canine, premolars, and second molar and was defined by 3 points: buccal cusp tips of the second premolars and the contact point between the mandibular central incisors at the occlusal level ( Fig 3 ). The long axis of the clinical crown was constructed according to the method of Andrews, using the buccal groove as the reference for the mandibular molars. The digital model was rotated to position the occlusal plane parallel to the horizontal plane. The angulation measurements were performed on the perspective of a buccal segment plane ( Fig 4 , A ). Values greater than 90° indicated that the crown was mesially angulated, and values smaller than 90° indicated that the crown was distally angulated.
For posterior tooth buccolingual inclination, the long axis of the posterior buccal surface was measured relative to the same occlusal plane as previously explained. The long axis of the buccal surface was constructed according to the method of Andrews on the frontal section of the models (with the dental arch tool) as shown in Figure 4 , B . The digital model was rotated to position the occlusal plane parallel to the horizontal plane during all steps.
To measure the posterior tooth cervico-occlusal heights, the digital model was positioned as described for the angulation measurement. The distance between the occlusal and cervical limits of the buccal surface long axis (along the buccal groove) was measured ( Fig 4 , C ).
The buccolingual thickness of the alveolar ridge was measured on the frontal section of the dental models passing through a point 1.5 mm distal to the second premolar. The buccolingual thickness was obtained 2 mm above the apical limit of the distal gingival papilla of the second premolars ( Fig 4 , D ). The digital model was rotated to position the occlusal plane parallel to the horizontal plane during all steps.
The test power was 90% considering the alveolar ridge thickness variable with a standard deviation of 1.15 (space opening group) and an interphase difference of 0.95 mm.
Normal distribution of the variables was confirmed with Shapiro-Wilk tests.
Comparability of the study groups regarding sex was evaluated using the Fisher exact test. Interphase and intergroup comparisons were performed using dependent and independent t tests, respectively. All statistical analyses were performed with Statistica software (Statistica for Windows, release 7.0; StatSoft, Tulsa, Okla). Results were considered significant at P <0.05.
The same operator (F.S.H.S.) measured all digital models a second time within a month after the first measurements. The 2 measurements were compared with the intraclass correlation coefficient.
Intraexaminer reproducibility coefficient (intraclass correlation coefficient) was high and varied from 0.97 to 0.99.
The results showed intergroup comparability for sex distribution, initial age, facial pattern, and treatment time ( Table I ). The initial dimension of the edentulous space was significantly greater for the space opening group compared with the space closure group. There was also intergroup comparability of all pretreatment variables ( Table II ).
|Variable||Space opening group||Space closure group||P|
|Male, n||2||3||0.635 †|
|Initial age (y), Mean (SD)||34.69 (7.06)||33.66 (8.15)||0.733 ‡|
|Facial pattern (FMA) (°), Mean (SD)||20.36 (3.45)||23.08 (4.34)||0.568 ‡|
|Initial dimension of the edentulous space (mm), Mean (SD)||8.45 (1.38)||3.01 (1.62)||0.038 ‡ , ∗|
|Treatment time (mo), Mean (SD)||17.28 (3.48)||18.48 (3.84)||0.470 ‡|
|Space closure group
(n = 12)
|Space opening group
(n = 14)
|Cervico-occlusal crown dimension (mm)|
|Alveolar ridge thickness (mm)|
|Second premolar-second molar||4.90||1.35||3.94||1.15||0.060|
The treatment changes in the space closure group consisted of significant reduction of the mandibular second molar mesial angulation of 15.16°, increase of its cervico-occlusal crown dimension, and increase in the alveolar ridge thickness of 2.60 mm ( Table III ).
|Pretreatment (T1)||After closure (T2)||Difference
|Second molar||121.47||10.81||106.31||8.19||−15.16||0.000 ∗|
|Cervico-occlusal crown dimension (mm)|
|Second molar||5.71||1.16||5.96||1.20||0.25||0.040 ∗|
|Alveolar ridge thickness (mm)|
|Second premolar-second molar||4.90||1.35||7.50||1.03||2.60||0.000 ∗|