Miniscrew-assisted rapid palatal expansion (MARPE): how to achieve greater stability. In vitro study

ABSTRACT Objective: Assess the influence of mono- and bicortical anchorage and diameter of mini-implants (MIs) on the primary stability of these devices. Methods: 60 self-drilling MIs were distributed in six groups according to diameter (1.5mm, 1.8mm or 2.0mm) and type of anchorage (monocortical and bicortical) in bovine rib. The primary stability was evaluated by insertion torque, micromobility and pull-out strength tests. ANOVA and/or Tukey analysis were used to conduct intergroup comparisons (p< 0.05). Non-parametric statistics (Kruskal-Wallis and Mann-Whitney) were performed when normality was not found (p< 0.05). Results: MIs with larger diameters and bicortical anchorage showed greater primary stability regarding insertion torque (p< 0.05) and micromobility (p< 0.05). Only MI diameter had an effect on the pull-out strength test. Larger diameter MIs presented better retention in pull-out strength tests (p< 0.001), regardless of mono- or bicortical anchorage. Conclusions: MI primary stability is dependent on its diameter and type of anchorage. Bicortical anchorage showed greater stability when compared with monocortical anchorage, independently of other variables.


INTRODUCTION
Orthodontic mini-implants (MIs) have greatly impacted orthodontic biomechanics and anchorage, since their advent.
Movements that were very limited before, such as molar intrusion, became possible, and other routinely performed movements, such as molar distalization, were optimized. 1 It is known that 20% of mixed dentition patients have maxillary constriction, 2 and the most popular treatment is rapid maxillary expansion (RPE). When RPE with a tooth-borne appliance is used to treat adolescents and young adults, it produces 35% skeletal orthopedic expansion and 65% dentoalveolar tipping. 3 RPE skeletal effects diminish with patient aging, because of the progressive calcification and interdigitation of circummaxillary sutures, and the decreased elasticity of bone in adults. 4 In adult patients, where there is no potential for mid-palatal suture opening using conventional techniques, the treatment option is surgically-assisted rapid palatal expansion (SARPE). 5 However, this is a more invasive technique with considerable side effects, such as injury to the periodontium, root resorption, 6 sinus infection, 7 and injury to the branches of the maxillary nerve. 8 In addition, relapse of the transverse maxillary dimension has been demonstrated in the short term. 8 In 2010, MIs were associated with rapid palatal expanders for the first Copello FM, Brunetto DP, Elias CN, Pithon MM, Coqueiro RS, Castro ACR, Sant'Anna EF -Miniscrew-assisted rapid palatal expansion (MARPE): how to achieve greater stability. In vitro study time 9 and are still yielding promising results. This expansion technique, known as miniscrew-assisted rapid palatal expansion (MARPE), can make the expansion more efficient in adolescents and young adults, and more feasible in elderly adults. 10 When well indicated, this technique can become a potential alternative to SARPE. 9,11 From a clinical point of view, bicortical anchorage should be used in cases where heavy anchorage is desired. 12 The use of MIs allows tooth-bone-borne palatal expanders to apply forces directly into the basal bone, thus bringing horizontal expansion forces close to the midpalatal suture and right into the maxillary center of resistance. 9 Thus, MI stability is essential to resist the magnitude of the applied mechanical Few laboratorial studies have demonstrated that the MI diameter has a direct influence on its primary stability, and others have suggested that bicortical anchorage might impact it as well. 13,14 However, to our knowledge, no study has assessed the influence of these two factors simultaneously on MI primary stability. Our hypothesis is based on the possibility that larger diameters MI could positively influence the stability of these devices, as well as the bicortical anchorage.
The aim of this study was to compare the effects of monocortical and bicortical anchorage of MIs with different diameters on their primary stability, through mechanical in vitro tests.

MATERIAL AND METHODS
The project was approved by the Animal Ethics Committee of the Center for Health Sciences of the Federal University of Rio de Janeiro before the study began, under number 01200.001588/2013-87.

Insertion torque (IT)
The MI sites were predrilled with a lance (Orthodontic Kit, INP system, São Paulo, Brazil) to a depth of 1 mm, following the protocol of a previous study. 15 The insertion was conducted by a single operator by using a manual key connected to a digital torque meter (Lutron TQ-8800, Taipei, Taiwan). Each MI was inserted until all the threads were fully contained in the block. A mechanical device was used to align the torque meter, the MI and the bone blocks, maintaining the system in a perpendicular relationship. The peak insertion torque values were recorded in Newton centimeters (Ncm).

Mini-implant mobility
MI mobility was evaluated with the Periotest ® instrument (Medizintechnik Gulden, Modautal, Germany). A special acrylic device was used to fix both the sample and Periotest ® handpiece, and to standardize the distance between the sleeve and the MI. 16 The handpiece was calibrated before each screw was measured. Two recordings were collected for each MI, and the average value was designated as the Periotest value (PTV), ranging on a scale from -8 to +50, where the smaller the PTV value, the smaller the micromobility and the higher the primary stability.

Pull-out strength (PS)
This test was conducted in a universal testing machine (EMIC DL

RESULTS
The insertion torque results are displayed in Figure 3. Mechanical performance was clearly influenced by MI diameter and type of anchorage, given that higher insertion torque values were found in devices with greater diameter and bicortical insertion (p < 0.001). In addition, the insertion torque values for all the diameters evaluated were higher in the MIs with bicortical insertion  Higher IT values were found in mini-implants with greater diameter and bicortical insertion. * ANOVA two-way: a, b, c distinct letters indicate statistical difference (p ≤ 0.05).

Only MI diameter influenced pull-out strength values; MIs
with larger diameter were more resistant to traction (Fig 5), regardless of mono-or bicortical insertion.     In the present study, the MIs were selected with the same length of active threads to standardize both the insertion, with all the active threads of the MI inserted into the bone, and the same transmucosal portion leading out of the specimen, in order to reduce the moment of force variable.
We used bovine rib because it has been validated as a bone model in other biomechanical studies. 21,22 In addition, the thickness of the bovine rib in selected areas allows the simulation of monocortical and bicortical anchorage.
Since the MARPE technique is relatively recent, the primary stability and mechanical performance of the MI must be evaluated when it is correlated with the type of anchorage (mono-and bicortical). In this study, primary stability parameters such as insertion torque, Periotest ® and pull-out values were used as stability predictors. 23,24 Studies with finite element methods (FEM) were used to simulate the effectiveness of the midpalatal opening, the expansion resistance and the MI stability when using a tooth-bone-borne palatal expander. 12 Because of the inherent limitations of in vitro studies and mechanical tests, future studies using conventional clinical model analysis are needed to confirm our results. We also suggest that a mechanical in vitro analysis of the MI be conducted using the MARPE expander, bearing in mind that a microstructural assessment of the bone should also be made when this type of device is used.

CONCLUSIONS
The following conclusions can be drawn: • Mini-implant primary stability is dependent on the diameter and the type of anchorage (mono-or bicortical) of the device.