Anchorage can be reinforced in many ways. Due to the variety of anchorage concepts, only a few general conclusions can be drawn. Therefore, more research is needed to investigate specific concepts with specific indications. The objective of this trial was to compare the anchorage capacities of miniscrews and molar blocks.
This randomized controlled trial was conducted on 2 parallel arms. The trial was conducted at the Public Dental Service Orthodontic Clinic in Gävle, Sweden. Participants were adolescents who needed orthodontic treatment with a fixed appliance, extraction of the maxillary first premolars, and anchorage reinforcement. In group A, miniscrews were used as direct anchorage during space closure. In group B, molar blocks were used as anchorage reinforcement during leveling and alignment and space closure. The primary outcome was loss of anchorage assessed as maxillary first molar movement. Random allocation was maintained with a simple randomization stratified by sex. The observer was blinded to the allocations during the measurements.
Forty participants each were randomized to groups A and B. Results were analyzed on an intention-to-treat basis, meaning that all participants, successful or not, were included in the analysis. Group A showed a mean anchorage loss of 1.2 mm during leveling and alignment. During space closure with miniscrews, no significant anchorage loss was found. Group B showed mean anchorage losses of 1.4 mm during leveling and alignment and 2.4 mm during space closure. No serious harms were detected. The first molar rotation, torque, and tipping showed different characteristics during the treatment phases.
Miniscrews can be recommended for anchorage reinforcement. Depending on the need for anchorage reinforcement, miniscrews can be inserted at the beginning of treatment or when space closure starts. Molar blocks cannot be recommended for anchorage reinforcement.
The protocol was published after trial commencement.
This trial received funding from the Center for Research and Development, Uppsala University/Region Gävleborg; Thuréus Foundation for the Promotion of Dental Science; and the Swedish Dental Associations Scientific Funds.
Miniscrews as direct anchorage provided increased anchorage capacity.∖.
Molar blocks did not provide sufficient anchorage reinforcement.
Miniscrews can be recommended for anchorage reinforcement.
In orthodontics, teeth are moved using active elements such as tie-backs, elastomeric chains, and coil springs. Active elements always deliver the same but opposite force on the teeth being moved and on the anchor teeth. This opposite force can cause undesired movement of the anchor teeth. Anchorage, the ability to resist undesired tooth movements, needs to be reinforced in many patients. Anchorage reinforcement has traditionally been provided by adding resistant units, such as headgear or intermaxillary elastics. The basic principle of anchorage reinforcements is to distribute the reaction forces and reduce the pressure on the anchor units.
These traditional methods of anchorage reinforcement have their drawbacks. Headgear can deliver outstanding anchorage reinforcement when used 10 to 12 hours every day. However, this implies that patients must actually wear the appliance for the suggested period. Clinical trials have shown that, in real life, approximately one third of patients are not accurate in reporting their headgear usage. Furthermore, up to 50% of patients treated with headgear had unacceptable anchorage loss.
The same applies to Class II elastics. Used full time, they are as effective as functional appliances in correcting Class II malocclusions, but even here compliance is an issue.
Anchorage, however, can be reinforced in ways that do not rely on compliance. The basic assumption is that every tooth has a certain anchorage value that is correlated to its root surface. The resistance of moving teeth can be overcome by uniting several teeth to an anchorage block. The more osseous tissue that needs to be remodeled, the less likely it is that these teeth will move. Molars from the left and right sides can be united with a transpalatal arch. This construction produced by a dental technician can theoretically reinforce anchorage. However, a systematic review showed that this concept was not sufficient in patients who needed space closure after premolar extractions.
A convenient way to reinforce anchorage is to undertie adjacent teeth with a stainless steel ligature. When the second molar is added to the appliance and tightly connected to the first molar, this so-called molar block has twice as much root surface as 1 molar. This technique is especially suitable when the first premolars are extracted because then also the second premolars can be added to the anchorage block, further increasing the anchorage value. In that way, the root surface ratio between the anchor blocks and the front teeth is changed. Theoretically, this results in less mesial movement of the anchor teeth. Molar blocks do not involve a dental technician, are not based on cooperation, and can be inserted within minutes. The anchorage capacity of the molar block has not been investigated in clinical studies.
Due to the limited anchorage capacity of dental noncooperation-based techniques, skeletal anchorage with miniscrews has been claimed to be the ideal anchorage reinforcement. However, the literature includes only moderate evidence that miniscrews can provide good anchorage. General conclusions must be drawn with caution due to the heterogeneity of the published data. To fill these knowledge gaps about miniscrews, more research is needed about their use for specific indications in specific insertion sites.
Anchorage capacity has traditionally been discussed mainly in the sagittal and vertical dimensions. Changes in the transverse dimension are rarely reported.
The main objectives of this trial were to evaluate anchorage capacity in its three dimensions at different timepoints: during leveling and alignment anchorage loss with and without molar blocks was evaluated (T1-T2); when molar blocks and buccal miniscrews were used during space closure for en masse retraction (T2-T3). It was hypothesized that miniscrews deliver better anchorage capacity than molar blocks and that the molar block is capable of certain anchorage reinforcement.
The sample was collected at the Public Dental Service Orthodontic Clinic in Gävle, Region Gävleborg, Sweden. The sample consisted of adolescents, 11 to 19 years of age, who needed orthodontic treatment with a fixed appliance, extraction of the maxillary first premolars, and anchorage reinforcement. Anchorage need was assessed according to the dental visual treatment objective. Moderate anchorage need corresponded to approximately 75% retraction of anterior teeth during space closure. All subjects had permanent dentition, including erupted maxillary second molars, and had received regular dental care since the age of 3 years. Adolescents who previously had orthodontic treatment or needed maximum anchorage or orthognathic surgery were excluded from the trial.
All patients were treated with extraction of the maxillary or maxillary and mandibular first premolars and fixed appliances in both jaws. Treatment with fixed appliance (Victory Series stainless steel brackets, 0.022-in slot size, MBT prescription; 3M Unitek, Monrovia, Calif) followed a straight-wire concept. The recommended archwire sequence was 0.016-in heat-activated nickel-titanium alloy, 0.019 × 0.025-in heat-activated nickel-titanium alloy, and 0.019 × 0.025-in stainless steel with posted hooks (3M Unitek). Space closure was accomplished as en-masse retraction of the 6 anterior teeth. The treatment was conducted by the first 2 authors, and the staff had several years of experience in various systems for skeletal anchorage.
The 2 treatment groups were different in their anchorage strategy depending on the treatment phase. During leveling and alignment (T1-T2), the molars in group A had no anchorage reinforcement. In group B, anchorage was reinforced with molar blocks. During space closure (T2-T3), anchorage reinforcement was provided by miniscrews in group A, whereas group B continued with molar blocks.
Leveling and alignment phase (T1-T2)
In the maxilla, the appliances were bonded on all teeth from the right first to the left first molars. Lacebacks were used in both groups to control canine proclination with the 0.016-in heat-activated nickel-titanium alloy archwire. In contrast to group A ( Fig 1 , A), anchorage in group B was reinforced by bonding the maxillary second molars. The second molars were then united with the first molars and second premolars using a stainless steel ligature ( Fig 1 , B). Leveling and alignment were considered completed when the 0.019 × 0.025-in stainless steel archwire was in place and space closure was started.
Space closure (T2-T3)
In group A, all patients received 1 miniscrew on each side in the maxilla (Spider Screw K1 SCR-1510 or SCR-1508, diameter 1.5 mm, length 8-10 mm; Health Development, Sarcedo, Italy). The miniscrews were placed from the buccal side between the maxillary second premolar and first molar under local anesthesia according to the protocol published in the clinical trials register. The miniscrews were immediately loaded with 150-g closed-coil springs (Ortho Technology, Tampa, Fla) ( Fig 2 , A ) In group B, the molar block was loaded with 150-g active tie-backs ( Fig 2 , B). To reduce friction, all archwires were cut distal of the first molars during space closure.
Space closure was considered completed when the canines reached a Class I relationship or all spaces were closed.
Alginate impressions for plaster casts were taken at the start of treatment (T1), after leveling and alignment (T2), and after space closure (T3). The plaster casts were produced at the clinic’s laboratory within 24 hours, and then the casts were digitized with a desktop scanner (R700; 3Shape, Copenhagen, Denmark).
The primary outcome measures were loss of anchorage during leveling and alignment (T1-T2), defined as changes in tooth position of the maxillary right and left first molars, and loss of anchorage during space closure (T2-T3), defined as changes in tooth position of the maxillary right and left first molars.
Superimposition was performed on digital 3-dimensional models with a computer program (Final Surface; GFaI, Berlin, Germany) and according to the raw, fine matching, and deformation superimposition technique. This technique calculates individual reference points for every subject. Reference points were detected with an algorithm-based deformation analysis that identified unchanged areas in the palate. Tooth movement was assessed in millimeters. Rotations, tipping, and torque were assessed in degrees.
Sample size calculation
The sample size calculation was based on values for anchorage loss and standard deviation in previous studies in our research group (headgear, anchorage loss 1.2 mm, SD 1.96 mm). Additionally, we assumed that the loss of anchorage would be half as much when miniscrews were used. The smallest clinical difference for the margin of superiority was set at 1 mm. The significance level was set at 5%.
Thus, under these circumstances, a sample of 26 subjects in each group would give 90% power. In addition, dropouts due to discontinued treatment or patients moving from the area and a 15% to 20% failure rate for the miniscrews were expected. Consequently, a sample size of 40 patients was established for both groups A and B.
This trial was conducted as a randomized controlled trial with 2 parallel arms. The protocol was approved by the regional ethical review board of Uppsala University, Uppsala, Sweden (number 2009/188).
All participants were randomly allocated to either group A or group B. The allocation was conducted by an independent person not involved in the trial. Each participant was given a sealed opaque envelope that contained a note with either “Group A” or “Group B,” and all envelopes were assigned using simple randomization, stratified on sex, with SPSS statistical software (version 18; IBM, Chicago, Ill). After informed consent, the allocation was revealed when the participant opened the envelope.
All measurements were performed by 1 examiner (N.G.), who was blinded during the assessments of the outcomes. All details revealing the groups, such as the maxillary buccal part of the second molar and the buccal portion of the attached gingiva from the maxillary first premolar to the second molar, were removed from the plaster casts before the scanning.
Statistical analysis was performed using the programming language R (version 3.42). Arithmetic means and standard deviations were calculated for numeric variables. For every patient, the maxillary right and left first molars were included in the analysis, giving 2 dependent observations for every treatment phase. The maxillary first molar movements were analyzed with adjustments for the left and right sides using linear mixed effect models. Linear mixed effect models are statistical models containing fixed and random factors, which are particularly suitable for analysis of repeated measurements and dependent data. The statistical model was built with the following fixed factors: treatment group and maxillary molar position on the left and right sides. Each subject was assigned as a random factor. Data were analyzed separately for the treatment phases (T1-T2 and T2-T3) and for the total observation period (T1-T3). Differences with probabilities of less than 5% ( P < 0.05) were considered statistically significant.
Data on all patients who were randomly assigned to the 2 groups were analyzed on an intention-to-treat basis. This implies that all subjects irrespective of success were included in the final analysis. In addition, if there were any dropouts during the trial, they were considered unsuccessful. Unsuccessful anchorage was defined as reciprocal space closure: ie, mesial movement corresponding to 50% of a premolar width (3.75 mm). All other variables such as transverse and vertical movements, rotation, tipping, and torque were set to the mean value of the variables calculated from the per-protocol subsample.
Method error analysis
Repeated superimpositions and measurements were performed on 15 randomly selected subjects after at least 2 weeks. No significant mean differences between the 2 series of records were found with the linear mixed effect models. The arithmetic mean error was −0.01 mm and the absolute mean error was 0.14 mm (95% confidence interval [CI], 0.10-0.18), for distance measurements and 0.02°,with an absolute mean error of 0.34° (95 % CI, −0.14-0.12), for rotational measurements.
Participant flow and baseline data
Ninety-eight patients matched the inclusion criteria and were invited to participate in this trial; 18 patients declined to participate. Thus, 80 patients were enrolled in the trial. Informed consent was collected from all patients and their parents. There were 7 dropouts in group A and 2 in group B. The details are given in the CONSORT flow diagram ( Fig 3 ). The baseline demographic characteristics are presented in Table I .