Effect of orthodontic forces on root length of immature mandibular second premolars: a split-mouth randomized clinical trial

ABSTRACT Objective: To assess the effect of orthodontic forces on changes in root length of immature mandibular second premolars. Methods: Sixty-four mandibular second premolars (MSP) with immature apices (left and right sides) of 32 patients aged between 10 and 13 years were evaluated. Orthodontic treatment was started after obtaining periapical radiographs (T1) from the MSPs of each patient. Brackets were bonded, except the ones of MSPs (left or right by random as control MSP, and the other side as test MSP). After 9-12 months, a second periapical radiograph (T2) was obtained from the MSPs of each patient. Then, brackets were bonded to the control MSPs, which were not bonded before. After 18 ± 3 months, a third periapical radiograph (T3) was obtained. Changes in root length were evaluated by using a new formula. The test and control MSPs at T1, T2 and T3 were compared using repeated measures ANOVA and parametric tests. P-value smaller than 0.05 was statistically significant. Results: There was no significant difference between the test and control groups in the mean root length of MSP at T1 (p= 0.48) and T3 (p= 0.078). The root length at T2 (p= 0.001) was significantly different between test and control MSPs, and the test group showed longer root length than the control group. Conclusions: Orthodontic force applied for leveling and alignment of immature MSPs may not have destructive effects on the roots, and may accelerates root formation in short-term. Normal root length was achieved at the end of root development.


INTRODUCTION
Advanced apical root resorption is a possible consequence of orthodontic treatment, which is also related to some biological factors. 1 Shortening of root length as the result of external apical resorption is irreversible and unpredictable. 2 The mean magnitude of root resorption varies from 0.5 mm to 3 mm, as reported in the literature. 3,4 In the majority of orthodontic patients, the magnitude of root resorption is clinically irrelevant. In a limited number of patients, however, the teeth are severely affected. Resorption of more than half of the root length affects the function and compromises the survival of the teeth. A number of factors may play a role in this respect, including genetic susceptibility, 5,6 thumb sucking, 7 tooth root shape, history of trauma to the teeth prior to treatment onset, endodontic treatment, type of orthodontic appliance, the magnitude of load and tooth movement, duration of treatment and developmental stage of the root. 2 In general, it has been well accepted that the late mixed dentition period and early permanent dentition period (at the time of eruption of canines and premolars) are the most suitable times for initiation of orthodontic treatment. 8 At this time, the roots of the majority of permanent teeth have not yet fully developed.
The effect of orthodontic force on immature teeth with incomplete apex closure has not been adequately studied. Some authors 8,9 believe that movement of teeth with open apex may Dalaie K, Badiee M, Behnaz M, Kavousinejad S -Effect of orthodontic forces on root length of immature mandibular second premolars: a split-mouth randomized clinical trial 5 serve as a risk factor for root resorption, short root length or early apex closure. Malformation of the Hertwig's epithelial root sheath alters the trend of root apex calcification, and the root may not reach its normal length. 8,9 Consolaro et al. 10 reported that movement of open-apex teeth would decrease root length due to early apex closure, and not root resorption. On the other hand, some authors have claimed that teeth with undeveloped roots are more resistant to root resorption caused by orthodontic forces 7, 11,12 and tooth movement has no adverse effect on open-apex teeth. 13 It was reported that orthodontic treatment did not affect dentin formation in young permanent teeth, or even activated odontoblasts and accelerated dentin mineralization. 14 Mavragani  Periapical (PA) and panoramic radiographic images (OPG) can be used to assess root length during orthodontic treatment. Panoramic radiography has some limitations. For instance, the quality of a pantomograph (OPG) depends on the patient's position and the distance between the anatomical structures and the focal trough. 15 The magnification rate in different parts of the head and face is variable in OPG, ranging from 20 to 35%. 16 Dalaie K, Badiee M, Behnaz M, Kavousinejad S -Effect of orthodontic forces on root length of immature mandibular second premolars: a split-mouth randomized clinical trial 6 However, the magnification rate in PA radiography is usually <5%. 17 OPG is not suitable for assessment of root shape and other anomalies such as root dilaceration, since it can overestimate root resorption by 20%. 15 Considering the high demand for orthodontic treatment in adolescents, the need for early initiation of orthodontic treatment and the existing controversies on the effect of orthodontic treatment on root development of open-apex teeth, the present study aimed to assess the effect of orthodontic treatment on root length of immature mandibular second premolars. Other studies evaluated the effect of orthodontic movement in immature incisors. In this study, posterior immature teeth (second premolars) were evaluated, and PA was the radiographic method applied.

DESIGN
This randomized split-mouth clinical trial evaluated mandibular second premolars (MSPs). The mandibular second premolar of one side served as the test (test MSP), and the contralateral mandibular second premolar served as the control tooth (control MSP). Sixty-four mandibular second premolars of 32 orthodontic patients who required orthodontic treatment for mandibular anterior crowding were evaluated. The teeth had open apices, according to the primary panoramic radiograph. The patients were aged between 10 and 13 years, and were recruited from a dental clinic and two private offices in Tehran/Iran. All patients were treated by the same orthodontist. The inclusion criteria were: presence of MSP with immature roots (late F or early G stages, according to Demirjian 18 ) in both the right and left sides of the mandible, the need for fixed orthodontic treatment, mild crowding (1-3 mm on each side) in the posterior mandible, Class I malocclusion, no caries or restorations in premolars, and no evidence of developmental dental anomalies before treatment. The exclusion criteria were root dilaceration, developmental dental anomalies and severe posterior crowding of the mandible (>4 mm). All patients were matched in terms of dental developmental stage (late F or early G, according to Demirjian 18 ). The dental developmental stage of the MSPs on both sides of the mandible was the same. Figure 1 shows the CONSORT flow diagram of the study.   The root length of MSPs was considered as the primary outcome, which was measured at three time points: T 1 (baseline), T 2 (after 9 to 12 months) and T 3 (at the end of treatment, after 18 ± 3 months). Root length was measured on PA radiographs and compared between different time points. In other words, root length was compared between T 1 and T 2 , and also T 1 and T 3 , at the test and control sides. All PA radiographs were scanned digitally as an image and saved with the same reso- To reduce the digital image measurement error, two adjustment factors were applied. The pixel to millimeter conversion factor (PMCF) was applied for the T 1 , T 2 and T 3 , and the correction factor (CF) was applied for T 2 and T 3 to minimize the measurement errors. To convert pixels to millimeter, all PA films were scanned with a transparent ruler (Fig 3). PMCF was calculated by dividing the digitally measured length on the ruler (e.g., 1 mm) by the number of pixels on the same length (e.g., measuring 1 mm on the ruler in the scanned film, in pixels)   The cusp tip, mesial and distal CEJ, and root tip were identified as reference points. The reference point of the root tip is the center of the line connecting the mesial and distal edges of the immature root. The length of the crown is the perpendicular distance of the crown tip to the mesial and distal CEJ connecting line. T2) Reference points and distances were identified. The mesial and distal CEJ are exactly similar to the T1 stage. Due to the development of the root, the root length is higher than T1. The second premolar crown was masked for blinding the study, but after the correction factor was calculated for T1 and T2, while CEJs are visible for measuring root length. T3) The reference points and distances were identified, the mesial and distal CEJ is exactly similar to the T1 and T2 stages. The root is mature in this stage and the apex is closed.

T3
Dalaie K, Badiee M, Behnaz M, Kavousinejad S -Effect of orthodontic forces on root length of immature mandibular second premolars: a split-mouth randomized clinical trial 16 The CF eliminates the possible difference between the T 1 and T 2 , and also T 1 and T 3 radiographs. Since the crown length was constant, by dividing the crown length at T 1 by the crown length at T 2 , the CF was obtained and multiplied by the root length values obtained at T 2 and T 3 . The following formula was used to calculate CF: In this split-mouth clinical trial, the right and left MSPs were randomized into the test and control groups using a table of random numbers (simple randomization). Accordingly, at the onset of study, in 14 patients left MSP and in 18 patients right MSP were subjected to orthodontic force application (test group).

BLINDING
Patients were aware of the allocation of their teeth to the test or control group, but this knowledge had no significant effect on the intervention and was not a confounder. After obtaining the CF, the crown of all second premolars at T 2 (in both the test and control groups) was masked, for blind measurement of root length (Fig 5). Thus, for measurement of root length at T 2 , it was not clear for the observer whether the second premolar belonged to the test (with bracket) or the control group (no bracket), and only the right and left side were noticeable.
However, the mesial and distal CEJs were visible for root length measurement on all radiographs. Thus, this study had a single-blind design.

RESULTS
Of 32 patients evaluated in this study (Fig 1), 11 (34.4%) were males and 21 (65%), females. The mean age was 11.99 ± 0.75 years (ranging from 10.2 to 13.0 years). Table 1  treatment alone, the two groups had a significant difference at T 2 . Also, a significant correlation was noted between root length at T 1 , T 2 and T 3 time points. Root length increased from T 1 to T 2 and from T 2 to T 3 in both groups, irrespective of treatment. Regarding the interaction effect of time and treatment, the two groups were not significantly different at T 1 (mean difference of 0.032 ± 0.044, p = 0.48) but a significant difference was noted between them at T 2 , when the root length in the test group (13.64 mm) was significantly higher than that of the control group (13.2 mm) (mean difference of 0.46 ± 0.042, p = 0.001). At T 3 , the two groups were not significantly different in the mean root length (mean difference of 0.063 ± 0.035, p = 0.078) ( Table 1).
Multivariate ANOVA was applied to assess the effect of side of load application on root length, during application of orthodontic force, at the three time points. The results showed no significant association between side of load application and changes in root length (p = 0.94).  One-sample t-test was used to compare the mean root length in the two groups at T 3 and the normal root length of MSP, according to Wheeler's Dental Anatomy. 20 Table 2 showed no significant difference between the values of the test and control groups with the normal values (p = 0.535 for the test group, and p = 0.267 for the control group). To assess the intra-examiner reliability, the Cronbach's alpha was calculated to be 0.99, which was considered excellent. One-sample t-test was applied to assess the difference between MSP apex closure age and the normal value reported in the Standards of Human Tooth, 19 which revealed a significant difference in males (p = 0.007) but not in females (p = 0.267) (Table 3). However, no significant   Level of significance: p ≤ 0.05; *One-sample t-test. SD = standard deviation.

Root length (Mean ± SD in mm)
Difference with the normal root length of MSP according to Wheeler's Dental Anatomy (mm)* correlation was noted between sex and the age of MSP apex closure (p = 0.629). The measurement error between the first and the second (14 days later) measurements of 20 randomly patients was 0.33 ± 0.14 mm.

HARMS
Premolar teeth were not harmed in this study. Repeated PA radiographs may be the only harm in this trial, but this is less than OPG X-ray exposure. 21  treatment onset was significantly larger than that of developed roots. Accordingly, they suggested initiation of orthodontic treatment at a young age. 4  This finding may be due to developmental stage of the teeth, and small magnitude of movement because of mild crowding Since the dental pulp of immature teeth is less affected by changes in blood supply during application of orthodontic forces, they have higher tolerance threshold than teeth with closed apices. 29 It appears that the tissue surrounding an immature root can protect the mineralized tissue against resorption during orthodontic treatment, and allow continuation of root formation and maturation.
Considering all the aforementioned, as well as the findings of the current study, it may be stated that orthodontic force application does not cause shortening of the roots; instead, it enhances root formation, at least in short term.
The limitation of this study is that the accuracy of measurements on 2-D digital radiographs is lower than that of 3-D radiographs.
Another limitation was the difficulty in marking a point on the crown of some MSPs, due to restorations, rotations and etc. Writing the article:

Critical revision of the article:
KD, MBa, MBe, SK.

Final approval of the article:
KD, MBa, MBe, SK.

SK.
The authors report no commercial, proprietary or financial interest in the products or companies described in this article.