Mandibular molar uprighting using orthodontic miniscrew implants: a systematic review

Conventional methods for molar uprighting

Several orthodontic approaches are suggested for mandibular molar uprighting, such as Australian uprighting spring, cantilever spring, prefabricated Sander spring, helical uprighting spring, NiTi coil spring, push spring appliance, and traction from removable appliances are few of the currently available options [6, 8, 9]. Molar uprighting requires good anchorage control, and subsequently, a full-arch fixed appliance is necessary. Furthermore, ankylosed teeth, dental implants, and extraoral appliances could also be effective, enhancing anchorage, and protecting from undesirable tooth movements [8, 10].

Among others, the Uprighter Jet developed by Carano provides a complete control of molar uprighting, minimizing extrusion, requiring no brackets, and no special patient cooperation [1]. In another uprighting case of an impacted molar, in combination with rapid maxillary expansion (RME), vertical elastic forces were directed from a hook on the RME device to an orthodontic attachment bonded on the tooth to be uprighted. This method saves time, requires no additional anchorage preparation, and appears more physiologic as the force vector is in the direction of normal eruption path [9]. According to Pogrel [11], surgical uprighting of lower second molars is a quick procedure with minimal morbidity and long-term prognosis. Most of the uprighted teeth remained firm with excellent bone formation and periodontal status after 18 months of follow up [11].

However, conventional treatment methods for molar uprighting have some disadvantages, including extrusion of the target molar, unwanted reciprocal movement of the anchorage units, need for bulky appliances, and longer treatment time [1, 2, 12–15]. In order to minimize the first two side-effects, intra-arch stabilization is usually needed [2, 12, 13], which is undertaken through the use of osseointegrated implants. Yet, an osseointegrated dental implant is costly, needs sufficient bone space, limiting our choices, and is very difficult to be removed after the treatment. It also requires osseointegration before orthodontic force application, increasing the treatment time [5, 10, 12].

In addition, surgical uprighting should not be considered as a routine method due to the possible pulp necrosis, ankylosis, external root resorption, or even rupture during the procedure. After treatment, occlusal equilibration may be needed, and the post-surgical stability of the tooth may be questionable [4]. Further, the possibility of pulpal calcification and vitality loss is high [11].

Use of orthodontic miniscrew implants for molar uprighting

The development of orthodontic miniscrew implants (MIs) provided solutions to most of the aforementioned problems. MIs are fabricated from pure titanium or titanium alloy with a diameter of 1–2 mm and length of 8–20 mm [16]. They remain stable during orthodontic treatment with minimum anchorage loss and are more effective than conventional anchorage means [17–19]. Their success rate ranges from 59 to 100% with an average of 86.5% [18–20].

Their attachment to the bone is mechanical with no intent to establish any form of osseointegration [21, 22]. Therefore, after the treatment, when they are no longer needed, they can be removed through a simple procedure, with negligible risks for the patient [23].

This new type of skeletal anchorage is simpler, smaller, less-invasive, and more economical than conventional osseointegrated implants [16]. Moreover, MIs do not require a long interval between placement and force application since loading can occur immediately after placement [10, 16, 24].

Their main advantage though is their ability to move specific teeth or even the entire quadrants directly, without involving other teeth or using inter-arch mechanics. Thus, they eliminate the reaction forces usually applied on the anchor teeth, leading to unwanted tooth movement and anchorage loss [5, 14, 15]. Patients are also more satisfied with the more invisible treatment compared to conventional methods [10].

MI anchorage is preferable to conventional mechanics when a third molar is in direct contact with the second molar root [4]. In some cases, it is better from a biomechanical perspective not to extract the third molar bud, since its extraction can change the center of resistance of the second molar and uprighting can be realized with distal crown tipping. This is undesirable when the second molar is planned to be uprighted mostly with mesial root tipping [24].

Molar uprighting using MIs with direct anchorage

Molar uprighting using MIs with direct anchorage was the sole or partial subject of the 15 included papers. One of these 15 papers [2] describes two cases treated with direct use of MIs and one case with indirect use, and this is why it will be discussed again in the following paragraph.

Regarding the use of direct application of forces on the MIs for the correction of molars on the sagittal plane, cases of uprighting all three mandibular molars were found, which were initially either mesially tilted or impacted. In order to treat these cases, several options regarding the insertion sites of the MIs were used: (a) in the retromolar area [2–5, 12, 15, 23, 25] (Figs. 2 and 3), (b) vertically in the alveolar ridge of a mesial edentulous molar site [2, 26] (Fig 4), or (c) mesial to the mandibular molar and between the roots of the adjacent teeth [4, 15, 27] (Fig 5).

The origin of force differed also, as in several cases uprighting was achieved (a) by using an open or closed coil spring [3, 4] (Fig 6) or (b) by using buttons and elastomeric chains [2, 4, 5, 25, 27]. In a case of a mesially tilted second molar, buttons were placed on its labial, lingual, and mesial surface, and uprighting force was applied through three elastic chains, which were connected to a MI in the retromolar area [25]. A distalizing and uprighting movement was produced, which avoided undesirable rotation of the molar [25] (Fig 7). Other treatment options included uprighting springs [26], a small cantilever [2], a sequence of archwires and a running loop [15], and a modified version of the Uprighter Jet called Uprighter Screw [1] (Fig 8).

As regards the direct force application on the MIs for the correction of molars on the transverse plane, two cases were reported [10, 28], where there was a lingually tilted second molar and a lingual crossbite of a second mandibular molar, respectively. In the first case, described by Sohn, the MI was inserted in the buccal alveolar bone and the force was applied through an elastomeric chain to a buccal button for 3 months [10]. In the second case, two MIs were placed in both the mandible and the maxilla in order to treat the crossbite with elastomeric threads attached to buttons, by producing a buccal and intrusive force on the lower molar and a palatal and intrusive force on the upper one, respectively [28] (Fig 9).

Direct force application was also used in three cases, where there was a correction of molars in more than one planes of space. In the first case report [12], a second molar was mesially and lingually tipped and was simultaneously in crossbite with the maxillary molar. In this specific case, the MI was placed distobuccally in the retromolar region, and for the following 3 months, an uprighting force was applied to a lingual button on the mesiolingual surface of the second mandibular molar, through an elastomeric thread. In the second case [29], after the distal uprighting of a mesially tipped and obliquely impacted molar via a removable appliance and an uprighting spring, a MI was inserted buccally between the roots of the maxillary molars. Force was applied via an elastic chain placed between the buccal MI and a wire on the occlusal surface of the mandibular molar in order to upright the tooth buccally and vertically (Fig 10). The third case [30] describes the simultaneous uprighting and intrusion of a lingually tipped and supraerupted lower first molar. The clinicians used elastomeric chains connecting a lingual attachment on the molar and a MI placed in the interradicular region. Buccolingual uprighting of 2.3 mm and intrusion of 1.8 mm was achieved in 45 days.

Molar uprighting using MIs with indirect anchorage

Two out of the 17 included papers, which reported only two cases, assessed molar uprighting using MIs with indirect anchorage. As mentioned above, the paper of Musilli et al. [2] presents cases treated with both types of anchorage and will be described again in the next paragraph.

The first case of a second mandibular molar tilted on the sagittal plane was presented by Yun et al. [13]. A MI placed between the second premolar and the first molar was connected to the tooth by a rigid stainless steel wire serving as indirect anchorage. In addition, a peerless single tube was bonded on the first molar, and a metal button was attached on the second molar, in order to achieve appropriate traction with an uprighting spring. In the second case presented by Musilli [2], two mesially tilted molars (teeth #37 and #46) were simultaneously uprighted in 9 weeks using a long cantilever on each side. Two MIs were used indirectly, locking the molars with steel ligature in order to prevent molar’s extrusion and distal tipping. Thus, the uprighting is mainly achieved by mesial root tipping (Fig 11).

Molar uprighting using MIs both with direct and indirect anchorage

Finally, molar uprighting using MIs both with direct and indirect anchorage was the subject of the one remaining paper.

There was one clinical trial included in the current review in which 181 MIs were inserted in 102 patients and were used both as a direct and indirect anchorage for lower molar uprighting [31]. With regard to the direct anchorage, 65 MIs were inserted in 31 patients using two different options. The first option included two vertically inserted MIs in the edentulous alveolar crest area and a bracket that was bonded on a resin uniting the two MIs. A segment of wire from this bracket to the molar was activated to realize the uprighting. The second option included the use of a cantilever directly activated on a MI that was inserted perpendicular to the buccal surface of the alveolar bone. With regard to the indirect anchorage, 116 MIs were inserted in the buccal surface of the alveolar bone of 71 patients and were connected via a segment of stainless steel wire with the canine and premolars. Then, a single or a double cantilever system was used to upright the molar. According to the authors, MIs showed high success rates in both anchorage methods with a slight superiority of the direct anchorage. Only 18 MIs failed, including 15 that were used as indirect anchorage and 3 that were used as direct anchorage.

Direct vs. indirect anchorage

According to Lee et al. [4], direct MI for molar uprighting is simpler, as it requires one MI and a single bracket or button attachment, minimizing patient’s discomfort and also reducing chair time compared to more complex indirect anchorage. Furthermore, direct MI connection with the target tooth eliminates the possibility of unwanted movement of the anchorage unit, which can occur even with indirect MI anchorage as a result of technical errors. However, direct anchorage has some limitations and especially in cases of lingually tipped or rotated molars because a single force may be insufficient to upright the tooth. Usually, such cases require a sequential application of different force systems and repeated changes of appliances. Lee et al. also report that direct MI application is not indicated in cases of extruded molars since the force system lacks an intrusive component [4].