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Fully customized placement of orthodontic miniplates: a novel clinical technique
© Hourfar et al.; licensee BioMed Central Ltd. 2014
Received: 14 February 2014
Accepted: 22 April 2014
Published: 3 May 2014
The initial stability and survival rate of orthodontic mini-implants are highly dependent on the amount of cortical bone at their insertion site. In areas with limited bone availability, mini-plates are preferred to provide effective skeletal anchorage. The purpose of this paper was to present a new clinical technique for the insertion of mini-plates.
In order to apply this new technique, a cone-beam image of the insertion area is required. A software (Galaxy Sirona, Bensheim, Germany) is used to construct a three-dimensional image of the scanned area and to virtually determine the exact location of the mini-plate as well as the position of the fixation screws. A stereolithographic model (STL) is then created by means of a three-dimensional scanner.
Prior to its surgical insertion, the bone plate is adapted to the stereo-lithographic model. Finally, a custom transfer jig is fabricated in order to assist with accurate placement of the mini-plate intra-operatively.
The presented technique minimizes intra-operative decision making, because the final position of the bone plate is determined pre-surgically. This significantly reduces the duration of the surgical procedure and improves its outcome.
A novel method for surgical placement of orthodontic mini-plates is presented. The technique facilitates accurate adaptation of mini-plates and insertion of retaining surgical screws; thereby enabling clinicians to more confidently increase the use of bone plates, especially in anatomical areas where the success of non-osseointegrated mini-screws is less favorable.
Orthodontic mini-implants (MIs) have gained popularity among orthodontists mainly because they provide an effective tool in orthodontic cases with high anchorage demands. However, there are several factors affecting the survival rate of the implants that need to be taken into consideration prior to their insertion. Previous literature has emphasized the significance of cortical bone thickness for initial stability of orthodontic mini-implants[2–4].
Clinical investigations assessing quality and quantity of alveolar bone in the maxilla and the mandible revealed that there are not many areas of sufficient bone quality able to guarantee successful placement of MIs[5–9].
A particularly challenging area is the anterior mandible. For cases that require unilateral or bilateral protraction of mandibular posterior teeth, placement of a skeletal anchorage device around the canine area can provide suitable possibilities supporting treatment mechanics. However, the only inter-radicular spaces in the mandible presenting adequate bone quality and quantity are distal to the first premolars. Moreover, insertion of mini-implants in the mandibular canine region is not recommended in patients younger than 11 years of age, because of incomplete bone maturation and due to increased risk of interrupting normal eruption of the permanent canine[10, 11].
In order to overcome the above-mentioned limiting factors, orthodontic mini-plates can be recommended as anchorage devices. The introduction of mini-plates in orthodontics has further enhanced treatment possibilities for complex orthodontic and orthopedic problems. There are numerous reports in the literature proposing the use of mini-plates to address various orthodontic treatment needs, such as molar intrusion[13–15], correction of anterior open bite[16–18], maxillary[19, 20] and mandibular molar distalization, maxillary protraction[22–24] and maxillary impaction.
Mini-plates have a very high success rate (91.4% - 100%), low morbidity and are usually well accepted by patients. In addition, there are few restrictions regarding the insertion sites for mini-plates; their placement is less dependant on the anatomy of the mucogingival tissues. In contrast to mini-implants, the fixation screws of bone plates are mostly placed sub-apically, where bone quality is adequate. Thereby, tooth movements are also performed without obstruction and the risk of injuring the roots of adjacent teeth, during placement, is eliminated[11, 28, 29]. It has also been suggested that mini-plates may provide more reliable anchorage when higher forces, such as orthopedic forces, are needed[22, 23].
Despite the efficiency of utilizing mini-plates as skeletal anchorage, there is a major drawback compared to mini-implants, i.e. the need for a surgical procedure to secure the plates with titanium screws. For placement in the anterior mandible, it is essential to adapt the mini-plates to the bony contour prior to fixation. This process is not only technique sensitive, but can also be time consuming. Therefore, mini-plates are not as commonly used as mini-implants, although they present a high acceptance rate among both, orthodontists and patients[30, 31].
Taking into consideration the disadvantages associated with the placement of orthodontic mini-plates, a new technique is presented to help simplify the process, reduce the time of the surgery and possible decrease failure rates. The technique allows for pre-surgical adaption of the mini-plates to the bony contour of a printed model of the insertion area and utilizes a custom made transfer jig to accurately position the plates during surgery.
Materials and methods
The present article is not an experimental clinical study. Patient consent was requested and obtained prior to publication of clinical pictures. All clinical pictures are intraoral and do not include any identifying patient information.
After virtual planning is complete, the CBCT dataset is converted from the initial DICOM (Digital Imaging and Communication in Medicine) format to a Stereo-lithography (STL) format. For this purpose, a DICOM imaging software (OsiriX®, Version 2.0.1, 64 Bit, Pixmeo, Bernex, Switzerland) for MacOS (Apple, Cupertino, Ca, USA) was used in the case presented in Figure 3. This conversion was undertaken, to input the CBCT dataset into a 3-D printer (Rasteder KFO - Spezial Labor, Rastede, Germany) and create a stereo-lithographic working model of the mandible.
The drill guide is fabricated by an external service provider (SICAT, Bensheim, Germany), also based on the virtual data.
Adaptation of the bone plates on the stereo-lithographic model
Commercially available orthodontic anchorage plates (Promedia Medizintechnik, Siegen, Germany) are adapted to the stereo-lithographic model. The initial adaptation is simple, because their position is determined by the holes that were created with the help of the drill guide. Precise adaptation of the base and the connecting arm of the mini-plate to the contour of the anterior mandible are performed on the model with orthodontic pliers.
The plate is subsequently secured to the stereo-lithographic model using screws, and its adaptation is verified. The screws used in this stage are identical to the ones used during the actual surgical placement.
Finally, a transfer jig is fabricated with a light-cured tray material (Bredent, Senden, Germany), which is adapted to cover the incisal part of the mini-plate as well as the incisal edges of at least three teeth to key the position of the plate to those teeth when it is transferred to the patient’s mouth.
Surgical placement of the mini-plates
The surgical procedure is performed under local anesthesia. In most cases, a mental nerve block appears to be sufficient for this purpose.
Following the surgical incision, a full thickness flap is raised and the plate is transferred to the surgical site using the custom jig. The patient is asked to bite on the jig, in order to avoid minor movements or displacement. Once the correct position of the plate has been confirmed clinically, pilot holes are created and two 5 mm long and 2 mm in diameter titanium screws (Promedia Medizintechnik, Siegen, Germany) are inserted to secure the mini-plate on the mandibular bone.
A novel technique is presented to precisely determine the desired final position of orthodontic mini-plates prior to surgery, and therby simplify as well as significantly reduce the time for surgical placement. A pre-operative CBCT image allows the clinician to thoroughly evaluate the bone around the insertion site in all three dimensions. Three-dimensional diagnostic imaging has been previously suggested to improve the outcome of implant placement by eliminating distortion errors associated with two-dimensional images and by reducing the risk of injury of adjacent structures[35, 36].
Furthermore, virtual placement of the pilot holes and mini-plate on the CBCT image, using dedicated software, allows the clinician to precisely determine the final position of the plate during the surgery. Several studies have proposed virtual treatment planning as a means for achieving higher surgical success rates by reducing intra-operative decision-making[35–38]. The duration of surgery can be crucial for its final outcome, since longer surgeries are associated to larger edemas and more post-operative pain.
The final step of pre-surgical preparation for the technique described here is the adaptation of the mini-plate on a stereolithographic model and the construction of a custom transfer jig. The fabrication of surgical guides on stereolithographic models for the placement of mini-implants has been described in the past[35, 40], however, no similar technique has to our knowledge been reported for the placement of mini-plates. Pre-operative adaptation of the mini-plates on the model surface is the main advantage of the present method. It allows for maximum contact between the plate and the bony surface during the surgery and therefore significantly reduces the risk for infection and the possibility of mini-plate failure.
Despite its advantages, when our technique is compared to the use of mini-implants, it is indeed associated with more patient discomfort due to the need for a more extensive surgical procedure. However, in certain areas of the maxilla and the mandible, the use of mini-implants is limited by the anatomy of the oral tissues or the quality of the bone. In such cases, mini-plates can be used to provide effective skeletal anchorage.
Another potential concern of the present technique is the requirement for an initial CBCT. Radiation exposure for a sectional CBCT of the mandible ranges between 35 μSv – 113 μSv depending on the resolution[42, 43]. The total effective dose of absorbed radiation from a mandibular CBCT is approximately 3.5 times larger compared to a panoramic x-ray and 15 times larger compared to a periapical radiograph, depending on the area that is scanned. These significant differences in radiation dosages need to be taken into serious consideration prior to applying the proposed method, especially in younger patients.
A new technique is presented to improve the accuracy and potentially decrease the failure rate of surgical placement of orthodontic mini-plates. It is based on virtual treatment planning and accurate positioning of the plate with a custom made transfer jig. Despite the benefits of the technique, the need of an initial CBCT might be a limiting factor for using the method especially in younger patients due to increased radiation exposure.
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