The role of micro-implant-assisted rapid palatal expansion (MARPE) in clinical orthodontics — a literature review

Winsauer et al.21 introduced a strict bone-borne device, a MICRO (‘mini-implant collar-retained orthodontic’) expander, with four or six miniscrews (2–2.5 mm in diameter, 10–14 mm long) inserted exactly perpendicular to the anterior palate in the paramedian area. The MICRO-4 Hyrax, with four palatal miniscrews, was mainly used in adolescents, and the MICRO-6 Hyrax, with six miniscrews, was recommended for use in adults. After expansion, it was advocated that the MICRO-4 and the MICRO-6 Hyrax expanders remain in situ for 6 and 12 months, respectively, for retention.

Kim et al.16 demonstrated two kinds of bone-borne appliances, with and without palatal acrylic resin coverage. Four miniscrews (1.6 mm × 10 mm in the anterior and 1.6 mm × 8 mm in the posterior regions) with stainless-steel arms extending to the lingual surfaces of the upper premolars and first molars were inserted para-medially in the premolar areas. Acrylic resin was occasionally added over the expander and secured to the palatal miniscrews. It was suggested that, in patients with extremely narrow or high-vaulted palates, the use of an acrylic RPE was beneficial because it produced less gingival impingement and inflammation compared with the wire type.

Yoon et al.22 analysed the effects of the number, position, and length of miniscrews in a three-dimensional finite element analysis of bone-borne RPE. It was concluded that placing four minscrews antero-posteriorly was more advantageous and aided stress distribution and transverse expansion. It was further found that the length of the miniscrew or the antero-posterior position of the expander showed little effect on maxillary expansion. However, the results of Yoon et al. indicated that an anterior position of the expander seemed to encourage more extrusive displacement of the premaxilla than a posteriorly-positioned expander.

Lee et al.2,15 introduced a combined Hyrax RPE with four extension arms comprising helical hooks soldered under the body of a jackscrew to accommodate the miniscrews. Lateral arms from the body of the Hyrax expander were soldered to bands on the first premolars and first molars, respectively. Two anterior hooks were positioned in the rugae area and the other two were placed posteriorly in the parasagittal area. It was recommended that orthodontic miniscrews (1.8 mm collar diameter, 7-mm length) be placed in the centre of the helical hooks. Patients were advised to activate the expansion mechanism once a day following placement. The role of the miniscrews in the MARPE was intended to secure sufficient expansion in the dentoalveolar area with less dental tipping, and to reduce the excessive pressure on the buccal cortical plates. It was considered that the incorporation of four miniscrews provided better dissipation of the pressure along the sutures and a reduction of pressure on the buccal cortical plates.2

Montigny23 proposed a Hyrax expansion device supported by two molar bands and two anterior palatal mini-implants. The mini-implants were 1.8–2.2 mm in diameter, 7–9 mm in length and inserted into the paramedian area aligned with the premolar region. According to a cartographic study of the palate, this insertion position was reported to be the most favourable because it could produce 5–6 mm of intraosseous anchorage for primary stability.24 This was also supported by studies which located the centre of resistance of the nasomaxillary complex in the premolar region.25–27 The anterior arms of the appliance delivered a force via the mini-implants to the anterior palate, whereas the posterior arms delivered a force via the molars to the posterior segments. It was believed that placing the anchorage directly on the palate, i.e. closer to hemi-maxillary resistance centre, would allow better skeletal maxillary expansion.

In addition, Garib et al.17 suggested using a Hyrax expander supported by the permanent first molars and palatal implants placed bilaterally between the first and second premolars. The advice was to place the implants obliquely and lingually in order to avoid contact with important anatomical structures (nasal cavity, maxillary sinuses, dental roots), and to resist transverse expansion forces.28,29 Palatal expansion was recommended to begin one month after the implant surgery.

Nojima et al.30 advocated the utilisation of a CBCT scan to guide mini-implant insertion during the MARPE surgical procedure. The expansion device was similar to the design of Lee et al.2,15 and bone thickness at the expected miniscrew insertion sites was evaluated from the coronal plane of the CBCT image. Bicortical engagement of the miniscrews was recommended for the internal cortices of the hard palate and nasal fossa, and it was believed that this was fundamental to support anchorage during expansion and to overcome the resistance of the maxillary bone. By utilising CBCT scans, safe bicortical engagements of the miniscrews were guaranteed in a consideration of palatal bone height, palatal soft tissue thickness, fixation band height, and the clearance between band and palate.

Moon et al.31 developed a MARPE device named the Maxillary Skeletal Expander (MSE), which was deemed to simplify complicated procedures. The main body of the appliance contained an expansion screw with four parallel holes sited at the corners of the expander. Each hole was sized 1.8 mm in internal diameter and 2 mm in thickness and acted as a guide for the placement of the micro-implants. In addition, the holes prevented tipping of the implants during insertion and activation. Four lateral arms extended from the main body and were soldered to the molar bands. Moon et al. urged that force application should be delivered more posteriorly and so the expander was placed at the level of the first molar to overcome initial resistance. It was believed that, in this position, a lateral force could be exerted directly against the pterygomaxillary buttress and promote a parallel opening on the midpalatal suture.14,20 After cementation of the MSE, micro-implants were placed through the parallel holes with a manual driver.31 The inserted micro-implant was 1.8 mm in diameter, and either of 9 mm, 11 mm, or 13 mm in length according to the thickness of palate measured from the CBCT image. The length of the screw should provide at least 5–6 mm of bone engagement and ensure bicortical penetration.32 In a finite element analysis, it has been shown that bicortical micro-implant anchorage could improve screw stability and decrease screw deformation and fracture during expansion.33 The activation protocol varied according to the patient’s age as recommended by Moon et al. in Table I.20 The frequency of activation could be reduced to once a day after an inter-incisal gap appeared in the adult.

The use of a MARPE may be categorised by:

A constricted maxilla along with a unilateral or bilateral crossbite in skeletally mature patients.

In adolescents or adults presenting with a constricted maxilla along with a unilateral or bilateral crossbite, the palatal suture is often rigid and complexly interdigitated, which may need extra consideration to separate.14 A common method to diagnose and quantify a constricted maxilla based on Andrews’ six keys of occlusion was recommended for clinical use.34 Maxillary width was measured on a stone cast between the most concave points in the vestibule at the level of the mesiobuccal cusp of the first molars. Mandibular width was measured at the level of the mesiobuccal grooves of the first molars on the WALA ridge (the most prominent portion of the buccal alveolar bone). These two widths should be equal in a normally developing maxilla and mandible. Betts et al.35 developed a postero-anterior cephalometric analysis to calculate the variation in maxillomandibular widths using the Ricketts Rocky Mountain analysis.36 If the transverse discrepancy was greater than the Ricketts’ norm (19.6 mm), maxillary skeletal expansion might be considered.

Lee et al.2 introduced the Yonsei transverse index for the diagnosis of a transverse skeletal discrepancy using computed tomography (CT). On a CT scan, an estimated centre of resistance was used and located at the furcation level of the upper or lower first molar to represent transverse reference points. The average difference between the maxillary and mandibular transverse widths (Yonsei transverse index) at the estimated centres of resistance was reported to be −0.39 ± 1.87 mm in the Class I subjects, but −3.17 ± 3.17 mm in a Class III surgical group. The difference was statistically significant. It was believed CT could provide better insight into the transverse discrepancy than the Ricketts Rocky Mountain analysis in which landmarks were sited far from the alveolar basal bone.

Toklu et al.41 compared the periodontal, dentoalveolar and skeletal effects of tooth-borne and hybrid (tooth-bone-borne) expansion methods by CBCT scans. It was reported that the buccal bone thickness decreased over the premolar area in the purely tooth-borne group, whereas the buccal bone was maintained in the hybrid group. It was concluded that tooth-bone-borne expansion could be considered for patients who had a risk of periodontal destruction over the buccal areas during expansion. Wilmes37,42 and Ludwig et al.43 also suggested the utilisation of MARPE rather than a RPE if a patient lacked anterior dental anchorage for the expander in circumstances of missing deciduous teeth, or underdeveloped premolar roots.

Kim et al.54 used CBCT scans to measure the changes before MARPE (T0) to immediately after expansion (T1) and subsequently at a one-year follow-up (T2). It was found that nasal cavity volume increased 9.9% from T0 to T1, and 5.5% from T1 to T2. The total expansion increment from the beginning to the one-year follow-up was 15.4%. Correspondingly, the nasopharynx increased by 6.4, 4.1, and 10.5%, from T0 to T1, T1 to T2, and T0 to T2.

SARPE studies have indicated that the nasal cavity volume increases by 17.9–23.3% when measured by acoustic rhinometry (AR).55,56 However, only 5.1% of the increment was observed from CT studies.57,58 The results were less than Kim’s MARPE findings (15.4%). However, the increase in width of the nasopharynx after SARPE was not significant.59,60

Li et al.61 used an outpatient surgical procedure termed endoscopically-assisted surgical expansion (EASE) to expand the maxilla and reported an improvement in nasal breathing and OSA by widening the nasal floor in adolescents and adults. The apnoea-hypopnoea index (AHI) and the oxygen desaturation index (ODI) improved dramatically after treatment, and the Epworth Sleepiness Scale (ESS) and Nasal Obstruction Septoplasty Effectiveness (NOSE) scale were reduced significantly. The CBCT images showed 4.9 ± 1.2 mm and 5.6 ± 1.2 mm of expansion over the anterior and posterior nasal floors, with a 2.3 ± 0.8 mm diastema created between the central incisors. Interestingly, Chang et al.62 reported a conventional RPE can also produce a 25.9% increase in the pharyngeal volume in young adolescents, which might be due to growth and variation in the patient’s age.54

A recent study evaluated the respiratory airflow and muscle strength before and after MARPE.39 A significant 30.45% increase in nasal inspiratory peak flow was observed immediately after expansion, and the increment was maintained after five months of retention at a 30.28% level. Airway volume increased significantly by 26% after expansion. Respiratory muscle strength was measured by maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP). MIP showed a significant improvement of 20% after 5 months of expansion. MEP showed a significant increase of 10% immediately after expansion (T1) but no change was observed after 5 months. The authors concluded that MARPE could not only gain an increase in airway volume but also produce a significant positive improvement in respiratory function.

Hur et al.63 used computational fluid-structure interaction analysis to investigate the influence of MARPE on airflow in the upper airway of an adult patient who suffered from obstructive sleep apnoea syndrome. Seven and nine cross-sectional planes (of 10 mm interplane distance) were set along the nasal cavity and pharynx, respectively. The change in area increments after MARPE at the set planes was significantly larger in the anterior part of the nasal cavity and the upper part of the pharynx. The increment was larger in the pharynx than for the nasal cavity despite the pharynx being positioned farther from the MARPE appliance. However, the nasal cavity exhibited a greater decrease in pressure and velocity of airflow than the pharynx. It was concluded that MARPE could expand the upper airway and decrease the total resistance significantly during the respiratory cycle.

Li et al.64 conducted a similar study to compare the dimensions and volumes of each segment of the upper airway before and after MARPE in young adults. Twenty-two patients with the mean age of 22.6 ± 4.5 years were evaluated and it was found that the volumes of the nasal cavity and nasopharynx both increased. This finding supported the results of previous studies.39,54,63

In summary, airway studies demonstrated that MARPE could offer assistance in respiratory function by enlarging volume and decreasing the total resistance in the upper airway. However, the follow-up period was short in most of the studies. Further investigations assessing the long-term effects may still be needed.

The maintenance of a sutural gap during the consolidation phase would exert a favourable influence on the resulting stability following expansion.2 Choi et al.4 reported the stability of the maxillary expansion using hybrid-typed MARPE in young adults was good with a 10% relapse of the intermolar width during an average 30 months retention period. The gingival recession was not noted to be significant during the observation period.2,4 Also reported was a post-expansion change in middle alveolar width was correlated with age. The change in the inter-premolar width was positively correlated with the amount of expansion in this area, but not correlated with the amount of inter-molar expansion.

An alternative study evaluated the stability of dental, alveolar, and skeletal changes after the same hybrid-typed MARPE following a one-year review period.48 It was shown that the expansion amounts immediately after MARPE were due to a 39.1% skeletal effect, a 7.1% alveolar effect, and a 53.8% dental effect, respectively, but the proportions changed to 43.2, 15.0, and 41.8% one year after MARPE. These changes were explained by a greater tendency for dental relapse. The skeletal measurements of nasal cavity and nasal floor decreased 0.37 ± 0.36 mm and 0.64 ± 0.73 mm after one year of follow-up, and this accounted for a 23.0 and 29.1% loss of the original expansion although the authors considered that this was clinically insignificant. Dental measurements showed a range of change, which may be a result of orthodontic treatment administered after expansion. Orthodontic treatment might have masked the dental changes produced by the MARPE.48

These findings suggested that the MARPE procedure could provide a clinically acceptable and stable expansion result in young adults. The skeletal effects could be maintained better than the dental effects. However, more investigations, especially in older patients, may still be needed to address the issue of long-term stability.

The main disadvantages of MARPE are the difficulty in cleaning the expander and the invasiveness of the micro-implants.65 No severe complication of MARPE has been reported to date. The most frequent complication is inflammation and hyperplasia of the gingiva around the micro-implants, which is usually associated with inadequate local hygiene. Brunetto et al.20 suggested that a micro-implant should be removed if inflammation occurs. If inflammation only affects one micro-implant, treatment may progress normally after the offending implant is removed. Hyperplasia may occur when there is not enough clearance from the expander or its wires from the palatal gingiva. Loosening of the miniscrews could also complicate MARPE therapy; however, Kim and Helmkamp16 reported only 5.0% of the miniscrews dislodged during expansion but 13.0% showed clinical mobility. The rest remained stable during a retention period.

Asymmetric transverse expansion of the midpalatal suture has been described.34,66 Cantarella et al. reported one half of ANS moved more than its contralateral half by 1.1 ± 1.0 mm on average. The displacement of the anterior part of the maxilla after expansion could affect soft tissue expression over the midface and lead to an aesthetic problem. Therefore, the soft tissue changes after MARPE should be managed with care.

A narrow and high palatal vault, which may hinder the vertical position of the device, was an additional limitation which reduced the success rate of treatment. Some clinicians proposed the replacement of the expander and the use of the same screw holes for additional expansion if the original objective had not been met.16,18

Investigations directed at how reduced midfacial bone elasticity, especially in the zygomatic arch, could affect the lateral movement of zygomaticomaxillary complex in older patients, are still few.50 This could be a potential limitation when using MARPE, and so more information is needed which provides an impetus for future research.