The purpose of this study was to investigate the variations of orthodontically induced tooth movement in the maxillary and mandibular arches between patients and the factors such as age, sex, and presence of an interference that might influence the amount of tooth displacement.
By using a standardized experimental orthodontic tooth movement in 30 subjects, 57 premolars were moved buccally during 8 weeks with the application of a 1-N force. Forty-four contralateral premolars not subjected to orthodontic tooth movement served as the controls. Plaster models from before and after the experimental tooth movement were digitized and superimposed to evaluate the amounts of tooth movement. Differences in tooth movement between the experimental and control groups were tested by an unpaired t test. For the experimental teeth, subject-related factors (age and sex) and tooth-related factors (location in the maxillary or mandibular dental arch, and the presence or absence of an intra-arch or interarch obstacle such as neighboring touching teeth or teeth interfering with the occlusion) were examined with analysis of variance. Multiple linear regression analysis was performed to determine correlations between tooth displacement, age, sex, tooth location, and presence of an interference.
Each subject contributed at least 2 experimental premolars and 1 control premolar. The displacement of the orthodontically moved teeth was 2.42 mm (range, 0.3-5.8 mm). Younger subjects (<16 years; n = 19; number of teeth, 36) had significantly greater amounts of tooth displacement compared with older subjects (≥16 years; n = 11; number of teeth, 21): 2.6 ± 1.3 mm vs 1.8 ± 0.8 mm; P <0.01. When an interarch or intra-arch obstacle was present, the amount of tooth movement was significantly less (2.6 ± 1.3 mm vs 1.8 ± 0.8 mm) ( P <0.05). Neither sex nor the location of the experimental teeth in the mandible or the maxilla had any effect.
Younger patients showed greater tooth movement velocity than did older ones. An interarch or intra-arch obstacle decreased the amount of tooth displacement.
Orthodontic tooth movement has been defined as “the result of a biologic response to interference in the physiologic equilibrium of the dentofacial complex by an externally applied force.” Only small amounts of force might be required to effect this outcome, which is accompanied by remodeling changes in the periodontal ligament and alveolar bone. The sequence of cellular, molecular, and tissue-reaction events during orthodontic tooth movement has been extensively studied. Several factors, alone or in combination, might influence remodeling activities and ultimately tooth displacement. Among these, the concept of “an optimal orthodontic force” has been the subject of investigation for several years. However, animal research has shown that even with standardized, constant, and equal forces, the rate of orthodontic tooth movement can vary substantially among and even within subjects. It was concluded that a wide range of forces can be applied to induce orthodontic tooth movement, and the rate is based mainly on patient characteristics. Several factors, such as age, drug consumption, diet, several systemic conditions, and other intrinsic genetic factors, have been shown to influence the rate of tooth movement.
Clinically, differences in the rate of tooth movement even in the same patient can be observed. In certain cases, the role of neighboring touching teeth or occlusal interferences by antagonist teeth seem to influence the amount of the tooth displacement. However, no study has investigated the role of such interferences on the rate of tooth displacement.
Owman-Moll et al and Owman-Moll introduced an experimental clinical model mainly to evaluate root resorption in previously moved and finally extracted premolars. Using the same model, our aims were (1) to study the variations of orthodontically induced tooth movement between subjects; (2) to identify factors such as age, sex, and location of the tooth in the mandible or the maxilla that could influence the amount of tooth displacement; and (3) to elucidate the importance of intra-arch (neighboring touching teeth) or interarch (occlusion-interfering antagonist) obstacles in the interference with the amount of tooth displacement.
Material and methods
Thirty patients (19 female, 11 male) were consecutively recruited from patients starting orthodontic treatment at the University of Geneva in Switzerland. Their mean age was 17.7 years, with a range of 11.3 to 43.0 years. The patients had to meet following criteria: (1) good general, dental, and periodontal health; (2) no previous orthodontic treatment; (3) severe crowding in both jaws; and (4) scheduled to begin orthodontic treatment with at least 2 or 4 first or second premolar extractions. Informed consent in written form was obtained from the patients before the beginning of the study. The protocol was approved by the medical ethics committee of our university.
In all the patients, standardized experimental tooth movement was carried out. Each patient contributed at least 1 experimental and 1 control premolar. Fifty-seven premolars, randomly assigned to the experimental group, were tipped buccally for 8 weeks. For this movement, a sectional archwire (0.019 × 0.025 beta-titanium alloy) was activated buccally and attached with a ligature to the bracket of the experimental tooth (1-point contact without the wire in the bracket slot engagement) to exert an initial force of 1 N (statically determinate force system) ( Fig 1 ). In the middle of the experimental movement (after 4 weeks), the amount of force was controlled and adjusted. A transpalatal arch and a lingual arch were placed as anchorage. Forty-four contralateral premolars bonded with brackets but not subjected to orthodontic tooth movement served as the controls.
Dental casts from before and after the experimental period were scanned at 600 dpi, 24 gray scale, and saved in TIFF format. The scanned models were superimposed on stable dental structures: teeth that were not moved during the relatively short experimental period. We made the superimpositions and measured the casts directly on the computer screen using Adobe Photoshop software (Elements 6, version 6.0; Adobe Systems, San Jose, Calif). The actual tooth movement was measured as the distance between the pretreatment tooth position compared with the postexperimental tooth position at the respective centroids on the occlusal surface. The centroid point was defined as the geometric center of the tooth in the occlusal plane. On the superimposed cast images, the distance on the line connecting the 2 centroid points represented the estimated tooth movement ( Fig 2 ).
For the experimental teeth, subject-related factors, such as age (<16 years/≥16 years) and sex, and tooth-related factors, such as location in the mandible or the maxilla and presence or absence of an intra-arch or interarch obstacle, were examined. An intra-arch interference was defined as a neighbor-touching tooth situation, and interarch interference meant an obstacle such as an occlusion-interfering antagonist. The evaluation was done on the dental casts at the molar level by moving them apart 1 mm and checking whether antagonists were interfering at this position.
Differences in tooth movement between the experimental and control groups were tested by an unpaired t test. For the experimental teeth, analysis of variance (ANOVA) was used to test the influence of the factors of age, sex, tooth location, and intra-arch or interarch obstacle on the amount of tooth displacement. The mean age of the patients was 17.7 years, and the median was 15.1 years. We used the cutoff of 16 years to have a reasonable distribution between the “young” and “old” groups. Multiple linear regression analysis was performed to determine correlations between tooth displacement, age, sex, tooth location, and the presence of an interference. The statistical analysis was processed with IBM SPSS software (release 19.0.0; IBM SPSS, Chicago, Ill).
To evaluate the error of our method, we repeated the superimpositions and measurements of the casts of 40 teeth 2 weeks later. A paired t test was used to estimate the differences in the measurements from these 2 superimpositions and to evaluate the systematic error. No differences were found at a significance level of 0.05.
Dahlberg’s formula (Se 2 = ∑d 2 /2n) ( d is the difference between measurements from the 2 superimpositions) was used to calculate the coefficient of reliability (CR = 1 − Se 2 /S t 2 ) ( S t is the standard deviation of measurements from superimposition 1). The result (CR = 0.997) showed excellent reliability of this method, and the error of the method was SE = 0.13 mm.
Significant differences were observed in the amounts of tooth movement between the orthodontically moved teeth and the controls: the former showed a mean displacement of 2.4 mm (±1.2 mm), and the latter showed only a slight mean displacement of 0.2 mm (±0.2 mm) ( P <0.001) ( Fig 3 ).
Concerning the influence of age, it was found that the amount of tooth displacement in younger subjects (<16 years; n = 19; 36 teeth) was significantly greater than the amount of tooth displacement of the 21 teeth of the older subjects (≥16 years; n = 11) (2.6 ± 1.3 mm vs 1.8 ± 0.8 mm; P = 0.007) ( Table I ). No difference was found in the amount of tooth displacement between the sexes or between teeth displaced in the maxilla and the mandible.
|Amount of tooth movement (mm)|
|df||Mean square||F||Significance||n||Mean||SE||95% CI|
Teeth without an obstacle moved significantly more than those with an obstacle (interarch or intra-arch). Thirty-three of 57 experimental teeth met no obstacle during their movement and showed a mean displacement of 2.6 mm (±1.3 mm), whereas the 24 experimental teeth meeting an obstacle during movement showed a mean displacement of 1.8 mm (±0.8 mm); the difference was statistically significant ( P = 0.017). More specifically, teeth with an interarch obstacle (n = 17) moved significantly less, with a mean displacement of 2.0 ± 1.3 mm ( P = 0.041). Less tooth displacement was found in the intra-arch obstacle group (n = 7), with a mean displacement of 1.6 ± 0.3 mm ( P = 0.044).
The multiple regression analysis showed that the amount of tooth displacement was associated with age and the presence of an obstacle (adjusted R 2 = 0.20; P = 0.003). No associations were found between tooth movement and sex and tooth location ( Table II ).