Introduction
The orthodontic intrusion of maxillary incisors is a routine treatment modality for increased visibility of maxillary incisors or gummy smile 1 There are several factors deciding the choice of treatment which includes incisor display, smile line and length of upper lip. 2 As per the routine orthodontic treatment, the ability of a fixed orthodontic appliance to intrude maxillary teeth is limited when compared to moving teeth in the sagittal plane.
Various conventional methods for incisor intrusion such as Reverse curve arch, Segmental Intrusive arch, Utility arch or Connecticut Intrusion arch have been used since long time but they are known to cause labial tipping of anterior teeth3or molar extrusion.3, 4
With the invent of temporary anchorage device in orthodontics, some tooth movements considered difficult previously can now be easily carried out which has led to a new envelope of discrepancy with greater range of tooth movement in all dimensions. Hence, in borderline surgical cases which have a gummy smile or deep bite, use of mini-implants for incisor intrusion is a promising option.5
As of now there were several studies conducted on maxillary incisors intrusion by using mini-implants,6, 7, 8, 9 where some studies have used one mini-implant placed between the roots of central incisors,6, 10, 11 while some have used two mini-implants placed between lateral incisors and canines bilaterally.7, 8 Although both methods have reported clinically acceptable success, but orthodontic literature lacks randomized controlled trials on comparison of these two methods for intrusion. Hence, the purpose of this trial is to evaluate and compare the effectiveness of one versus two mini- implant assisted intrusion mechanics for maxillary incisors. .
Specific objectives or hypothesis
The purpose of this trial was to evaluate and compare and amount of intrusion and changes in axial inclination of maxillary incisors with one or two anterior mini-implants. The null hypothesis is that there is no significant difference in the amount of maxillary incisor intrusion achieved with one mini-implant versus two mini-implants.
Materials and Methods
Trial design, ethical approval and registry
The trial was a two arm parallel, randomized clinical trial with an allocation ratio of 1:1. The trial was registered at www.ctri.nic.in (CTRI No- CTRI/2022/03/0XXXXX). The study follows the declaration of Helsinki guidelines (World Medical Association 2013) and the presentation of the report is according to the CONSORT (Moher et al. 2010) guidelines. The study was approved by ethical committee of the University and prior consent of the participating subjects was taken. The method was not changed after trial initiation.
Participants, eligibility criteria and settings
Total 150 subjects were screened in the OPD of department of orthodontics between January 2022 to October 2022, out of which 68 subjects met the inclusion and exclusion criteria; however 8 of the subjects refused to take part in the study. 60 subjects were equally divided into 2 groups, 30 were in group-1(one mini-implant group) and 30 were in group-2(Two mini-implant group).
Inclusion Criteria were as follows:
Subjects within the age group of 20-30 year.
Excessive gingival display on smiling (≥3mm), Maxillary incisor display at repose ≥ 4mm.
Deep overbite ( ≥ 4mm),
Skeletal class 1(ANB(≤40)
Patients treated with non-extraction treatment and maxillary incisor inclination less than 1100(U1-SN)
Healthy periodontium, healthy individuals devoid of any systemic diseases.
Intervention
All Subjects were treated with fixed pre adjusted therapy using .018 x.025 M.B.T prescriptions (ORMCO Pvt. Ltd). The maxillary arch was levelled and aligned with .014 and .016 superelastic nickel- titanium archwires and followed by .016x.022 NiTi wire. Finally 0.16x.022 stainless steel arch wires with no compensating curve were placed, in order to remove any bias.
Mini-implant placement
One mini-implant was placed between central incisors below the ANS in Group-1 and two mini-implants were placed between the roots of lateral incisors and canines (one mini-implant on each side) in Group-2. The template for mini-implant placement was fabricated with a .016X.022 stainless steel wire and three loops of 3mm diameter were made parallel to long axis of the tooth in the inter radicular space. This template was attached on to the main arch wire with the help of a ligature wire to demarcate the site of placement.
The patient’s mouth was rinsed with a Betadine mouth wash for two minutes as a disinfectant before the procedure began. Topical anesthestic agent was applied for 5 to 7 minutes followed by local infiltration anaesthesia at the site of mini-implant placement. A gingival bleeding point was marked using a probe in one of the loops of the template guide to mark a point of insertion and gain an access through the soft tissue to the cortical bone where the mini-implant was placed.
Self drilling mini-implants (length-8mm; diameter 1.3mm, SK Surgical, India) were inserted under strict sterilization protocol in the attached gingival just below the mucogingival junction, perpendicular to teeth. Periapical intraoral radiographs were taken before and after mini-implant placement to ensure its correct positioning.
Force application and activation
Force of 80 grams was applied in the single mini-implant group and 40 grams per side in the two mini-implants group with elastomeric chain. The elasomeric chain was placed on the arch wire in the same line as that of the mini-implant and stretched to the mini-implant. Force was measured with the help of a Dontrix gauge (Panama, New Delhi, India).The loading protocol involved immediate application of orthodontic forces after mini-implant insertion.The elastomeric chain was replaced every month for a total period of 4 months.
Primary and secondary outcomes and any changes after trial commencement outcomes
To assess the outcomes, lateral Cephalograms of these subjects were taken before intrusion (T1) and after 4 months (T2) of intrusive treatment to compare and evaluate the skeletal and dental changes produced by the two modalities of treatment. All cephalograms were digitally traced and measurements were taken using Nemoceph 11.3.1 software by the same investigator. The analysis included a combination of the variables described by Steiner, Rakosi, Burstone and Arnett. (Figure-1)
Primary outcome of this trial was to compare the amount of incisor intrusion achieved by using one mini-implant versus two mini-implants. To evaluate the primary outcome following linear parameters were analyzed on digital lateral cephalograms at T1 and T2 time intervals-
CR-SN (mm) - Vertical distance between centre of resistance (CR) of maxillary incisor to sella-nasion plane(SN) .
Overbite (mm) - Vertical overlap of maxillary central incisors over mandibular central incisors.
Secondary outcome of this trial was to evaluate changes in the axial inclination of upper incisors following the application of intrusive forces using one mini-implant versus two mini-implants. To evaluate the secondary outcome following angular and linear parameters were analyzed on digital lateral cephalograms-
Angular Parameters
U1.SN (O) - Angular relation between long axis of the maxillary central incisor and Sella Nasion plane.
U1.PP (O) - Angular relation between long axis of maxillary central incisor and palatal plane.
U1.NA (O) - Angle between long axis of upper incisor and line joining Nasion and Point A. Linear Parameters
U1_NA (mm) - Distance of most labial surface of incisor to NA line.
Sample size calculation
The sample was calculated to minimum power of 95% and α error of 0.05considering the standard deviation of a previous study8using G Power 3.0.10 software. Although calculations yielded a required sample of 14 per group; however, a sample of 30 per group was taken to increase the power of the study and to consider possible loss of subjects during the trial.
Randomization
In order to remove selection bias, at the time of intervention a random allocation was done by block randomization method with a block size of 6 in 1:1 ratio of allocation. The concealment of treatment modality which would be performed was done in sequentially numbered, sealed, opaque envelopes which were shuffled by the independent investigator. Thereafter, the envelope allotted to each subject was opened at chair side and the placement of mini-implants was undertaken accordingly.
Blinding
Blinding was done at the analytical level where all the cephalograms were coded and measurement and analysis was done by other independent investigator who was not a part of the trial and was blinded as to what treatment modality was used.
Error of method
A revaluation was done by the same blinded investigator after 1 week by measuring all the parameters in order to assess any method error and was calculated using the Intra-class correlation test, which in our study was seen ranging from 0.926- 0.997, thus indicating a high similarity between values from both the assessments and signify a near complete agreement between the observations.
Interim analyses and stopping Guidelines
Not applicable
Statistical analysis
A mean value of all observations was taken for final evaluation. The difference in pre and post values for each group were analysed and inter-group variation was also observed. Data was analyzed using Statistically Package for Social Sciences ( SPSS ver 21.0,IBM Corporation,USA) for MS Windows. Level of significance was kept at 5%. The data was subjected to normality testing using Shapiro Wilk test which showed that data deviated from normal. Hence changes in angular and linear measurements within Group 1 and 2 were compared using Wilcoxon signed rank test. Differences in angular and linear measurements between Group I and II were compared using Mann whitney test.
Results
Participant flow
A CONSORT chart showing participant flow during this trial is shown (Figure-2). All the 60 participants recruited between January 2022 and October 2022 and trial was completed in February 2023. All 60 subjects were randomly divided into Group 1 and Group 2 with 1:1 allocation ratio. However, due to TAD failure and patient non-compliance, 2 subjects from each group were lost in follow up. Finally 28 subjects in each group were analysed.
Baseline data
Baseline data regarding age, sex, and other variables of the subjects are listed in Table-1. Baseline variables of both groups were compared by independent t test and no statically significant difference found between them.
Numbers analyzed for each outcome, estimation and precision, subgroup analyses
For one mini-implant group (Table 2), there was a statistically significant difference in each angular and linear measurements of one mini-implant group. Upper incisors inclination (U1-NA, U1-SN and U1-PP) was statistically significantly (p ≤ 0.001) increased after intrusion. That indicates proclination of incisors occurred after intrusion with one mini- implants. Maxillary incisors were significantly intruded (p≤0.001) after 4 months (U1-PP 30.993mm±1.33 pre and 28.375mm±1.54 post) and overbite was also significantly reduced from5.804mm±1.45 to 3.286mm±1.22. Thus the amount of mean intrusion using one mini-implant was 2.62 mm with a proclination of 0.90
Two mini-implant group (Table 2) also showed that there was a statistically significant difference in all angular and linear measurements after 4 months of intrusion. Upper incisors inclination (U1-NA, U1-SN and U1-PP) showed statistically significant proclination of maxillary incisors after intrusion and sufficient intrusion of incisors take place (U1-PP reduced from 30.136mm±1.64 to 26.321mm±1.70) and overbite was also significantly decreased from 5.579mm±0.93 to 3.418mm±0.88. .However, U1-NA linear distance showed no statistically significant difference (p = 0.184). Thus the amount of mean intrusion using two mini-implant was 3.81mm with a proclination of 0.70.
Table-3&Figure-3 showed that amount of intrusion was significantly more in group-2 in comparison to group-1(3.81± versus 2.61) so overbite reduction was also greater in group -2(2.51 versus 2.16) as compare to group-1.Unlike intrusion, incisors were proclined more in group-1 as compare to group-2 (0.90 versus0 .70). Incisor proclination was depicted by the increased value of angular measurements (U1-NA, U1-SN and U1-PP) and the difference was statistically significant (p≤.05).
Discussion
Recently, Mini implant assisted intrusion of maxillary incisors become the main treatment modality for the correction of deep bite and gummy smile. This is the topic of debate whether one mini-implant is sufficient or two mini-implants are required for the intrusion of four maxillary incisors. Till date, very limited research was done to evaluate and compare the effect of one mini-implant with two mini- implant on maxillary incisors intrusion. This Randomized clinical trial was conducted on large number of subjects to produce the concrete scientific evidences about the effectiveness of one and two mini-implants assisted maxillary incisor intrusion. The present trial analysed the incisor intrusion, overbite reduction and changes in axial inclination of incisors. Previous studies of maxillary incisor intrusion stated that proclination of maxillary incisors is a known consequence of intrusion mechanics 12, 5, 13, 14 According to biomechanical principles, whenever point of force application is away and labial to center of resistance of four incisors during intrusion, flaring of incisors take place. So to minimize the proclination of incisors during intrusion, force should be applied through the center of resistance of four incisors. Practically it is not possible to apply the force exactly through the CRes but whenever possible we have to apply the force as much as close to CRes. There are many confounding factors that alter the position of CRes such as root morphology and height of surrounded alveolar bone. The other most critical factor is the axial inclination of maxillary incisors if the axial inclination of incisors are different, the relationship between point of force application and CRes will alter. To overcome all these confounding factors, subjects with healthly periodontium and incisor inclination less than 1100(U1-SN) were selected for this trial. To rule out the effect of growth, only adult subjects (above 20 years) were selected for this trial. In group-1, one mini-implant was placed between the roots of two central incisors at 12 mm from archwire and perpendicular to the occlusal plane as recommended by Namburi et al. 15 Many previous published studies stated that the CRes of four maxillary incisors lies 8-10 mm apical and 5-7mm distal to lateral incisors 16, 17, 18 so in group-2, two mini-implants were placed distal to lateral incisors root and 10 mm height from archwire for more balanced stress distribution. Consolidation of all anterior teeth was done in both the groups to minimize the undesirable labial flaring as suggested by Namburi et al13. An intrusive force of 80 gm in group-1 and 40 gm per side in group-2 was applied from .019x.025 stainless steel arch wire to mini-implant, since various previous studies recommended a very light force of 15-20 gm per tooth.19
Intrusion of maxillary incisors was measured on digital lateral cephalogram by measuring the vertical distance between CR of incisor and SN plane. The CRes was located at 40% of the distance from alveolar crest to root apex. According to Tilekar et al.20 and Vela- Hernandez et al21, incisal edge or root apex are not reliable landmarks since they dependent on inclination changes. CR of tooth act as a more accurate reference point for measurement of intrusion because it is not altered by incisor inclination. Polat- Qzsoy suggested that palatal plane can be altered by intrusion mechanics so in the present trial SN plane used as a reference plane for measurement of intrusion.
Polat-Ozsoy et al 9 and Namrawy et al.22 were conducted studies to compare the effectiveness of conventional intrusion arches with mini-implants. Results of their studies stated that mini-implant assisted intrusion of maxillary incisors was more effective and stable compare to conventional intrusion mechanics. There are two different approaches for mini-implant assisted anterior intrusion, first is one mini-implant assisted intrusion and second is two mini-implant assisted intrusion. Till date, only two comparative clinical studies were published in the literature where one mini-implant was compared with two mini-implants for intrusion. The results of both studies are contradictory with each other.
Results of present clinical trial showed that amount of maxillary incisor intrusion and overbite reduction were greater in group 2 than in group 1 , which was highly significant. Similar result was also observed by Vela-Hernandez et al.21 in their retrospective comparative study. However in their study, subjects were allocated in the groups on the basis of position of roots and frenum that could produce the selection bias and another pitfall of their study is that there was variation in the number of subjects among both groups( Group-1=16,Group-2=28). To overcome these confounding factors, in the present trials subjects were randomly allocated to both groups in 1:1 ratio. On the other hand, contradictory results was observed by Tileker at al, according to their study, the rate of intrusion and overbite reduction was similar with one or two mini-implants assisted intrusion
Proclination of maxillary incisor secondary to mini-implant assisted incisor intrusion is very common effect so the secondary outcome of our study was to evaluate and compare the amount of maxillary incisor proclination between both the groups.
The results of present study showed that the mean amount of proclination of incisors was greater in group 1 (one mini-implant) as compared to group 2 (two mini-implants). These results are similar with other two studies conducted by Vela-Hernandez et al and Tilekar et al .
Al-Falahi et al6 in their case series observed that on placing one mini-implant between the roots of the central incisor an increase in proclination of incisors by 3.4 ± 5.08 degree (U1.SN angle) was seen, which was similar to one mini-implant group in our study. In other case series by Polat-Ozsoyet al9 and Virnag et al23 two mini-implants were placed between the roots of the lateral incisor and canine and a minimal increase in incisor proclination (U1.PP=1.41±3.84) and (U1.SN= 0.7±4.0) was observed respectively, which was similar to two mini-implant group in our study.
Along with the results seen in our study, labial frenum may cause interference with the placement of single mini-implant at exact location which may result in unwanted gingival growth over the mini-implant. Whereas usually no such problem is encountered in two mini-implant cases.
The null hypothesis was rejected as the research indicated a significant difference in the amount of maxillary incisor intrusion achieved with one mini-implant versus two mini-implants.
Hence, it can be said that the position of mini-implants plays a very important role as placing mini-implants between lateral incisors and canines directs vector of force more towards the centre of resistance of the 4 incisors than when placed mesial to lateral incisors or between the roots of central incisors as Group I in our study.
Limitations of the study
Individual subject requirements were also considered and thus the intrusive period was limited to 4 months to avoid any bias in the study, although intrusion should have carried on for more duration to assess the extent of intrusion that can be achieved by using mini-implants and its effect on the envelope of discrepancy.
The effect of root resorption at the apex of maxillary incisor if any was not studied because it was difficult to evaluate precisely on the lateral cephalogram and for any additional investigations the subjects would have to undergo unwanted radiation exposure along with an overall increase in the research expenses.
Generalisability
The study was carried out in a national dental council accredited dental college with post-graduation residency programme in orthodontics. Subjects who were included in the trial represent a typical orthodontic case load requiring fixed mechanotherapy for intrusion of maxillary incisors. We could therefore assume that the results of present trial could be applicable in most clinical settings where true incisor intrusion without reciprocal tooth movement is desirable.
Trial Registration- Trial registered at Indian Council of Medical Research, CTRI/2022/01/XXXX
Conclusion
The mean amount of intrusion achieved by using two mini-implants was more as compared to one mini-implant.
The mean amount of proclination observed was greater in one mini-implant group as compared to two mini-implants group.
Therefore it can be concluded from our study that the use of two mini-implants as compared to one mini-implant is more appropriate since it causes more amount of incisor intrusion with a minimal amount of proclination thus helping in achieving a relatively true incisor intrusion without causing unwanted soft tissue problems which are more common while using one mini-implant.
Article DOI
