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A simplified approach to true molar intrusion
© Flieger et al.; licensee BioMed Central Ltd. 2012
Received: 24 September 2012
Accepted: 30 October 2012
Published: 7 November 2012
Orthodontic management of anterior open bites is a demanding task for orthodontists. Molar intrusion as a primary means of open bite correction entails the need for appropriate anchorage. Orthodontic mini implants can provide the required mechanical support. The suggested procedure aims to reduce the risk of complications such as root damage or soft tissue irritations while minimizing overall complexity.
Three female patients aged 14, 18 and 19 years who decided against a surgical correction were treated with a device consisting of mini implants in the palatal slope, a palatal bar and intrusion cantilevers.
In all three patients, an open bite reduction of more than a millimeter occurred within four months. An anterior overbite of 2 mm or more could be established within 6 to 9 months.
The method presented in this article enables the practitioner to use mini implants in an easily accessible insertion site. A lab-side procedure is optional but not required.
The management of anterior open bites is considered one of the most difficult tasks in orthodontics.
Orthodontic solutions may involve an unfavorable extrusion of the incisors and may result in an increased display of gingiva. In many cases, posterior intrusion is favorable but it requires appropriate anchorage. A high-pull headgear can provide for an adequately directed force but depends on patient cooperation. Furthermore, it is usually not an option in adults. Skeletal anchorage using mini plates has been proven efficient in the management of open bites. However, this approach requires incision and suture.
Various mini implant insertion sites are available for the purpose of molar intrusion. Interradicular insertion bears the risk of root damage. Even after correct implant insertion, root contact may still occur later due to the progress of the intrusion, which may impair implant stability. Insertion beyond the attached gingiva may also lead to an increased failure rate. The palate as a skeletal anchorage site has been described by several authors and for different treatment tasks[7, 8]. It offers good bone support[9–11].
The aim of this work is to present an approach that does not require incision, interradicular implant placement or placement in the movable alveolar mucosa while keeping the complexity of the orthodontic mechanics at a minimum.
Implant and insertion
The mini implants were inserted according to the following protocol:
Informed consent regarding potential risks, complications and behavior via standardized documents
surface anesthesia with 1% lidocaine spray applied using a cotton ball
infiltration using 4% articaine solution
mouth rinse with 0.2% chlorhexidine digluconate
assessment of gingival thickness using a probe
choice of JS neck length: 3 mm neck for gingival thicknesses not exceeding 3.5 mm, otherwise 5 mm
insertion using a surgical handpiece or a hand screw driver
A palatal bar served to avoid palatal tipping of the molars during intrusion. Also, it helped in preventing the cantilevers from interfering with the gingiva. Notably, the palatal bar was adjusted to leave at least 3 mm of space to the roof of the palate in order to obviate excessive soft tissue contact as the intrusion progresses.
Once attached to the mini implant, the cantilevers rest on the palatal bar. To establish the desired intrusive force level, they were then tied to the molar bands with steel ligatures. An initial force level of 60 cN per side was chosen for the intrusion of the first molars and verified using a spring balance. A force of 200 cN per side was applied when second bicuspids, first molars and second molars were being intruded simultaneously. During subsequent appointments the ligatures were gradually tightened to maintain the intrusive force.
At the time of the most extensive over-correction, the anterior open bite was remediated and an iatrogenic lateral open bite of 3.5 mm on the right side and 4 mm on the left had occurred. In the following four months, the lateral open bite closed incompletely. A lateral open bite of 1.5 mm on each side remained. The anterior overbite persisted. It amounted to 1 mm at the right lateral incisors and to 4 mm in the left central incisor area.
The x-ray images (Figure6) showed no discernable sign of root resorption although a change of the first molars’ position in relation to the maxillary sinus floor became visible.
In Figure7, the digitized models are shown in conjunction with a reference plane.
In addition to the vertical correction, an improvement of the molar relationship was observed. On the right side, a class I molar relationship was established. On the left side, a quarter of a unit class II remained.
The transversal distance between the upper first molars increased by 2.5 mm which may be attributed to a slight over-activation of the palatal bar.
Within five months, the anterior open bite was closed and a lateral open bite of 1 mm was established. Occlusal contacts remained between the second molars and first bicuspids. On both sides, a super class I occlusion resulted.
Five months of intrusion resulted in an anterior overbite and a vertical distance of 2 mm between upper and lower first molars.
The aspired treatment objective was reached in all cases. The clinically visible open bite reduction can be attributed to the intrusion mechanics since no intermaxillary elastics were used. Still, an incisor extrusion was observed in case 2. It may result from the fact that the intrusive force was applied distal to the center of resistance of the maxillary dentition entailing a clockwise rotation of the entire dental arch.
In terms of implant placement, the suggested procedure benefits from an insertion site which is accessible with a surgical hand piece as well as with a straight hand screw driver unless pathological limitations of mouth opening are present. Slight deviations regarding the implants’ position as well as their angulation can be compensated for by adapting the cantilevers accordingly.
Maxillary molar intrusion may also be conducted by using miniplates instead of mini implants. However, this approach involves an incision and a more complex surgical procedure in both insertion and removal of the anchorage unit.
Mini implant insertion in the buccal alveolus may bear the risk of root damage. Additionally, in order to achieve a favorable force vector, a high insertion location in the area of the movable mucosa would be necessary, which is suboptimal in terms of implant survival. The palatal alveolus is another known insertion site for molar intrusion, too. It eliminates the issues linked to the movable mucosa but still requires interradicular implant placement. While successful molar intrusion has been described using single screws in the posterior palate, this insertion location is less easily accessible and eliminates the option of using a hand screw driver. Also, only the midline of the posterior palate offers sufficient bony support, raising the requirements on surgical accuracy.
The suggested procedure can be performed chair side. The clip connectors can be pre-fabricated. A lab-side fabrication of the palatal bar is optional.
Although the reliability of the presented method requires further investigation, the results appear propitious. The first case is especially insightful since the observed effect can be fully attributed to the intrusion mechanics. No brackets, elastics or other appliances were being used. It can, however, be argued that treatment might have been more efficient if molar intrusion had been performed simultaneously with leveling and aligning.
The intrusion cantilevers were fabricated out of .016”×.022” stainless steel wire. The decision for this material was made because TMA wires are more prone to breakage whereas NiTi wires cannot as easily be bent. A hybrid construction with a steel clip on one end and a NiTi lever is conceivable but was not deemed necessary. The sheer length of the levers provided for a sufficient level of elasticity. Finally, with regard to the possibility of root resorption, dissipating forces may even be favorable.
While additional research is required, present results indicate that the proposed method is suitable for treatment of anterior open bites. It is advantageous in terms of surgical difficulty and mechanical complexity.
Written informed consent was obtained from all patients for publication of this report and the accompanying images. In all cases, a medical indication for the respective treatment was present. The surgical procedure constitutes a routine treatment. The authors declare that no ethical approval was necessary.
We thank Dr. Jens Reimann for reviewing the English version of this article.
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