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Biomechanics in the orthodontic treatment of complex multidisciplinary problems

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Flavio Uribe, USA

By Flavio Uribe, USA

Thu. 19. February 2009

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In recent years, the purview of orthodontics has grown to include an increasing number of adult patients, many of whom present with complex malocclusions that demand complicated treatment plans. Often, these more mature patients are referred by the restorative dentist because it is impossible to fabricate an adequate prosthesis for the missing teeth. These malocclusions are difficult to restore esthetically and functionally. This prosthetic challenge might include missing teeth, canted occlusal planes, overeruption of posterior teeth into opposing edentulous areas, impinging deep bites, and inappropriate intra arch space distribution.

Since these patients present with myriad problems, it is important to enlist other disciplines. Although the approach to the treatment plan in complicated cases is similar to that of an adult patient with the full dentition, in the edentulous case more disciplines may be required to contribute their expertise. Nevertheless, in all situations, a problem-based approach to analyzing the malocclusion is employed and leads to the objectives of treatment, and thereafter to the mechanics plan. Obtaining the treatment objectives are most efficiently achieved if the orthodontist is fully versed in biomechanics. Indeed, a lack of understanding of biomechanics limits the options the orthodontist can provide, and therefore the objectives of treatment with orthodontic appliances may be limited. For example, a significant amount of intrusion cannot be accomplished with a straight wire appliance. An orthodontist not mindful of biomechanical options might include a step bend to intrude a tooth or group of teeth. This approach will result in a minimal amount of movement and would in the end likely require a surgeon to fully implement the treatment objective.

With the advent of titanium as a biomaterial in orthodontics, skeletal anchorage has emerged as an alternative treatment tool in solving many complex orthodontic problems. In the past, complex cases would be limited to a solution by means of a surgical procedure. Today, titanium usage benefits patients with numerous missing teeth ultimately as an esthetic finish prosthesis as well as serving as an anchor unit necessary for predictable orthodontic movement.

Titanium anchorage devices can be divided into temporary or permanent. Although the temporary anchorage devices, TADs, are currently a very popular treatment modality, adult patients who are missing numerous teeth would benefit more by utilizing a conventional endosseous dental implant. The benefits are twofold: achieving the complex orthodontic movements and serving as prosthetic dental restoration at the end of the orthodontic treatment.

In order to maximize the cost-effectiveness of these skeletal anchorage devices, the orthodontist has to properly understand the basic biomechanical principles, which are the same fundamentals that apply to conventional orthodontic treatment. The only variation is that emphasis shifts to analyzing and understanding of the force system and prediction of tooth movement primarily in the active unit. The titanium anchorage device remains essentially stable or anchored. As such, the biomechanical analysis should focus on the active unit and assume stability from the skeletal anchorage device.

Endosseous dental implants can be placed before the initiation of orthodontic treatment. This treatment approach has very little room for error in the three dimensions of space. It is important to note that there is an approximate 1 mm margin of error in the mesiodistal, occlusogingival, and buccolingual final position of the implant. Any error above this margin will more than likely compromise the outcome. Compensation of this error will then fall to the shoulders of the restorative dentist to redress. Thus, in order to achieve this ideal placement, the interdisciplinary team has to have a 3-D model of the final result depicting the objectives of treatment. All the members of the team should be in agreement and understand how the objectives are going to be met.

At the University of Connecticut, this 3-D model starts on paper by means of an occlusogram.1 The occlusogram is a diagnostic tool that enables the orthodontist to visualize the changes that will be obtained with treatment (Fig. 1). The benefit achieved by using this tool is that the original relationship can be used as a reference for the desired movements. To complement the 3-D analysis, the vertical and anteroposterior movements are sketched in the conventional visualized treatment objectives (VTO) popularized by Ricketts.2 Based on the occlusogram and the VTO, which are produced on paper, a 3-D diagnostic wax-up reflecting these objectives of treatment is fabricated.

The diagnostic wax-up of the final endossous implant position is transferred to the original model in order to fabricate a surgical stent. The surgical stent is then evaluated clinically and radiographically to ensure that adequate alveolar bone levels are present in the implant site. If bone is lacking, a grafting procedure should be considered and the endosseous implant placed approximately three months later.

Once the implant is placed and osseointergration has occurred, it can be loaded by placing an abutment and a temporary crown of composite resin or acrylic. From the endosseous implants, the necessary anchorage can be generated to achieve complicated orthodontic movements such as: significant molar intrusion in the same and opposing arch, incisor intrusion, and retraction and protraction of adjacent teeth.

The mechanics for incisor intrusion from endosseous dental implants placed in any of the buccal segments consists of a cantilevered system. From this an intrusive force is generated and is capable of achieving more than 4 mm of incisors intrusion. As an osseointegrated stable unit, no side effects such as extrusion and tip back are seen on the reactive unit (Fig. 2).

Retraction and protraction of the teeth is usually difficult, but adjacent to an implant, this movement is rendered uncomplicated because of the anchorage available from the nearby implant. It is clear that a significant amount of movement can be accomplished without side effects by using a skeletal anchorage unit. The only caveat is that molar protraction is still slow, especially when the desired tooth movement is traslation.3

Finally, intrusion of molar teeth can be accomplished from a molar in the same arch. To effectively accomplish this an arm should be extended in order to allow an intrusive force to be delivered (Fig. 3). Intrusion of an overerupted segment can also be accomplished by placing an implant on the opposing arch and delivering an intrusive force from it through repelling magnets (Fig. 4).4,5

In summary, complex orthodontic problems in adult patients can be treated in multiple ways by exploiting the leverage of implants. Endosseous implants in patients with multiple missing teeth are a very cost-effective option. A proper understanding of biomechanics enhances the possibility of using skeletal anchorage to achieve orthodontic movements in different sites around the arch. Complex orthodontic movements such as intrusion of the posterior teeth and significant intrusion of the incisors can be accomplished using endosseous implants. Furthermore, the implants can be later restored prosthetically serving as a permanent solution to the missing teeth in the partially edentulous adult patient.

Editorial note: A complete list of references is available from the publisher.

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