Temporary Anchorage Devices: Biomechanical Opportunities and Challenges

Temporary anchorage devices (TADs) used to improve anchorage during routine orthodontic therapy have become popular in the past 5 years. Reasons for the increased interest in using such devices for anchorage include their commercial availability, the ease of placement, the lack of necessary patient cooperation, and the possibility of achieving better anchorage control during mechanotherapy. This chapter discusses the use of TADs in specific orthodontic situations, presents appliance designs in use, and analyzes the biomechanics of the force systems involved. Recommendations are also included to optimize TAD placement to better control the force system generated by these appliances, thus improving quality of treatment outcome.

ANCHORAGE IN ORTHODONTICS

During orthodontic therapy, the movement of teeth is achieved through the application of a force system on the teeth and the transduction of that mechanical signal into a biological response. As orthodontic forces are applied to move teeth, anchorage is required from teeth in the same or opposing arch to achieve differential tooth movement. Anchorage can be obtained from adjacent teeth or groups of teeth consolidated as a unit. Anchorage to achieve a variety of tooth movements is also obtained extraorally through headgear (occipital, cervical, and combination) and requires significant patient compliance. Teeth that serve as anchorage units should ideally remain stationary; they should not express any of the forces or moments resulting from application of the desired force system to the teeth that need to be moved. In reality, however, anchorage teeth are subjected to the often-undesirable side effects of the mechanics used.

The introduction of implants in dentistry by Branemark in 1969 led to the possibility of developing anchorage systems that could be used during orthodontics and remain stationary because of the implant’s osseointegration. Linkow was the first author to report the use of implants in conjunction with orthodontic therapy. He advocated use of endosseous blade implants as space maintainers to avoid drifting of the teeth and as posterior anchorage in patients with posterior edentulous areas. He also reported the first clinical application of mandibular implants to support class II mechanics through class II elastics. Creekmore and Eklund used bone screws as skeletal anchorage placed in the anterior nasal spine of patients who needed intrusion and torque control of the maxillary incisors. Kanomi described a mini-implant specifically designed to be used as direct anchorage for orthodontic purposes. Costa et al. introduced the first miniscrew that could be used as direct or indirect anchorage because it incorporated a bracket configuration in the design of its head.

Since these early reports on the use of skeletal anchorage to support orthodontic therapy, several authors have described the use of such anchorage devices in the hard palate, maxillary molar region, mandibular retromolar area, and maxillary tuberosity to achieve a variety of orthodontic tooth movements, including intrusion, distalization, uprighting, and space closure with torque control. The use of endosseous dental implants, miniplates, and miniscrews has been described since the early 1980s, with miniscrews gaining popularity because they are easily placed by the orthodontist with minimal tissue invasion. The use of miniplates has been limited, however, because placement requires the intervention of an oral surgeon and a more invasive placement procedure.

Currently, several terms are used to refer to “skeletal anchorage devices,” the most inclusive being temporary anchorage devices. Other names include implants, mini-implants, miniscrews, microscrews, screws, miniplates, and plates. Implants and mini-implants usually necessitate osseointegration for stability, whereas screws, miniscrews, and microscrews are generally loaded immediately after placement and receive their stability from mechanical retention in the bone. Plates are attached to the bone through a surgical procedure necessitating the elevation of a flap. A portion is left emerging in the oral cavity to serve as a point of application for the force system. According to Melsen, these devices need to be classified into an osseointegrated or a nonosseointegrated group. Other authors consider that all these devices provide anchorage to bone and that all considered anchorage devices.

Definition of Anchorage

The concept of anchorage in orthodontics corresponds to the resistance (or lack of) that a tooth or group of teeth may provide when subjected to the application of a force. Clinically, anchorage control is central to achieving an ideal buccal occlusion with ideal overjet and overbite. The lack of posterior teeth can seriously jeopardize the type of tooth movement that can be achieved through orthodontics unless clinicians have options of additional anchorage techniques, such as incorporation of TADs, in treatment planning. Some orthodontic tooth movements (e.g., molar intrusion, mandibular molar distalization) are unpredictable when achieved through conventional orthodontics because of the side effects of the mechanics. Such movement can be achieved with simple force systems and essentially no undesirable side effects on adjacent teeth when TADs are incorporated in the treatment plan.

Historically, the concept of anchorage was introduced by E.H. Angle, and his classification recognized several different types of anchorage, including simple, stationary, reciprocal, intermaxillary, and occipital anchorage. Angle distinguished among these different categories based on the treatment goals for tooth movement, as follows:

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Simple anchorage corresponded to a situation where an equivalent tooth or larger tooth was used as anchorage.
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Stationary anchorage was typically found where teeth were connected to each other in a rigid manner. In this clinical situation, teeth could not tip when subjected to orthodontic forces because of the rigidity of the attachment that connected them, but they translated slowly because of the resistance that the bone offered. The concept of stationary anchorage was primarily based on the rigidity of the anchorage unit.
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Reciprocal anchorage corresponded to an anchorage situation where two teeth were moved into their corrected position in the arch and acted as anchorage teeth to each other.
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Intermaxillary anchorage, first described by Angle in 1890, was defined as a situation where the anchorage necessary to move one tooth or a group of teeth was found in the opposite arch. A variation of the concept of intermaxillary anchorage included the “Baker anchorage” first introduced by H.A. Baker. In this specific situation, teeth of the entire arch were moved as a unit with respect to the teeth of the opposite arch in the sagittal plane to correct a Class II or III malocclusion, as described by Tweed (1966). The Baker anchorage corresponded to contemporary class II or III mechanics.
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Occipital anchorage was the last type of anchorage described by Angle and corresponded to occipital extraoral anchorage. The anchorage required in these situations was attained by placing forces on the top and back of the patient’s head using a headgear. When adequate patient compliance was achieved, this type of anchorage was particularly advantageous because the desired tooth movement was achieved without unfavorable movement of other teeth within the arch or on the opposing arch.

Optimum management of anchorage during orthodontic tooth movement was an important treatment step to ensure successful outcomes.

Burstone developed the segmented-arch technique in the early 1960s and introduced an anchorage classification based on the differential movement of the anterior and posterior segments of teeth when a force system was delivered to the dentition. The segmented-arch technique was developed to determine the optimal appliance design that delivered the desired force system to the teeth that needed to be moved. This technique introduced a systematic approach to analyze force systems applied to teeth to effectively control the undesirable side effects. This approach resulted in more predictable and reproducible tooth movement. The segmentation of the arch was a central feature in defining anterior and posterior segments of teeth between which a force system was delivered.

One of the advantages of such an approach to orthodontic mechanotherapy was the possibility of determining the force systems that act on the anterior and posterior segments of teeth, then identifying the desired forces and moments and the undesirable side effects generated by an appliance. This is more difficult to achieve with straight-wire mechanics because the force system generated by a continuous wire connecting multiple teeth is harder to determine than it is for two teeth. The force system delivered by a continuous arch is largely “indeterminate,” and the advantage of a segmented approach is the delivery of a “determinate” and reproducible force system. The segmented-arch technique also allows good control of the force and moment magnitudes on the different segments of teeth. It uses wires of different cross sections, lengths, and configurations and different materials to combine high and low deflection rate appliances within one arch, allowing better control of the anchorage unit. Finally, the segmented-arch technique introduced a novel way to manage anchorage by dividing the anchorage requirement in three groups: group A (or maximum) anchorage, group B (or reciprocal) anchorage, and group C anchorage (or en masse posterior protraction).

The utilization of the principles of the segmented-arch technique and a proper understanding of biomechanics have allowed for the delivery of the desired force systems, a better control of anchorage through the identification of the undesirable side effects, and an overall improved and more efficient appliance design.

The TAD introduced a new way to incorporate the principles of the segmented-arch technique into a straight-wire system. Because TADs essentially provide an immediate absolute or stationary anchorage, it becomes possible for the clinician to deliver the desired force system using a TAD as anchorage without observing the undesirable side effects on the anchor teeth. In these situations, anchorage can be set up directly through the use of a TAD (direct anchorage) or indirectly through connecting teeth to a TAD and making this segment of teeth virtually stationary (indirect anchorage).

Also, TADs have become attractive alternatives to traditional anchorage that is subjected to undesired side effects and that highly relies highly on patient compliance. In clinical situations where maximum anchorage is required to retract anterior teeth (Burstone’s group A anchorage), TADs can provide a simpler approach to anterior tooth retraction, applying the basic principles of biomechanics but simplifying the appliance design and activation. In this situation the clinician can use simple forces to retract the anterior segment of teeth, rather than elaborate retraction loops that need to incorporate appropriate moment of force ratios to control both anterior and posterior segments of teeth. The use of extraoral anchorage such as headgear provides good posterior anchorage but relies heavily on patient compliance and is thus unpopular. Nance appliances, which do not rely on patient cooperation, also provide posterior anchorage, but they can be uncomfortable and become embedded in the palatal mucosa. With headgear and Nance appliances, the anchorage unit of teeth is subjected to a force system and does not remain totally unaffected. TADs offer the possibility to manage anchorage differently because they replace the traditional use of teeth in their capacity as anchorage units. This allows the side effects from the mechanics to affect TADs, not teeth.

One of the keys to successful treatment is to understand and analyze the force system applied to the dentition, to predict and minimize the side effects. Planning to shift the expression of undesired side effects from teeth to TADs can increase the effectiveness of treatment and improve outcome. Again, a TAD in such clinical situations offers direct or indirect anchorage that does not affect tooth position (direct anchorage) or that can stabilize a group of teeth (indirect anchorage).

GENERAL CONSIDERATIONS FOR USE

The stability of TADs after implantation and during treatment and the location of their placement are critical in designing the proper appliance that delivers the appropriate force system. TADs are essentially used as stationary or absolute anchorage. TADs obtain their stability from mechanical retention and do not osseointegrate into bone. This stability is not absolute if the TADs fail, and this possibility necessitates attention. Studies in dogs have looked examined the pull-out strength of force necessary to dislodge the screw from its supporting bone and showed that screws placed in the anterior mandible required significantly less force to be pulled from bone than screws in the posterior mandibular regions. The correlation between the cortical bone thickness and the amount of force necessary was significant but weak.

The stability of TADs also has been studied when implanted in the zygomatic buttresses for direct anchorage. Miniscrews were stable but did not remain stationary when subjected to orthodontic forces. The screws showed about 0.4 mm of tipping forward at the level of the head of the screw and some signs of combined extrusion and tipping forward. The authors suggested that the movement of the screws was consistent with the loading pattern in some patients and recommended placement of screws in non–tooth-bearing areas or in tooth-bearing areas, allowing a minimum of 2 mm of safety clearance between the screws and the roots of the teeth.

Buchter et al. evaluated the transverse loading of miniscrews and concluded the failure of implants was directly related to the magnitude of the moment applied. The value of the moment needed to be smaller than 900 centinewtons per millimeter (cN/mm) for immediate loading of the screws.

The stability of miniscrews is also related to the location of their insertion into the bone. Ishii et al. measured the buccolingual and the mesiodistal lengths and areas of sectioned interalveolar septum from randomly selected maxillary bones using micro–computed tomography (micro-CT). They concluded that the safest position for implantation was the interalveolar septum between the maxillary first molar and the second premolar, 6 to 8 mm apical to the alveolar crest on the palatal side.

Poggio et al. repeated the study using 25 maxillae and 25 mandibles retrieved from patients’ records (data were collected with NewTom [DVT9000] Volume Scanner, QRsr1, Verona, Italy). They confirmed that the safest place in the maxilla to place a TAD was on the palatal side between the first permanent molars and the second premolars. The least amount of bone in the maxilla was found in the tuberosity area. The greatest thickness of bone measured buccopalatally was observed between the first and second molars. In the mandible, the least amount of bone was found between the first premolar and the canine, with the greatest bone thickness found between the first and second molars. The authors recommended a number of areas as safe for implant placement. They included the posterior aspect of the maxilla on the buccal and palatal aspects, and a generally more apical placement as the implantation site became more mesial. In the mandibular arch, the safest areas to place TADs were in the posterior areas, and implantation needed to be at least 11 mm from the alveolar crest as placement was performed in the premolar and canine areas.

Wilmes et al. reported on the potential factors affecting the primary stability of orthodontic mini-implants and concluded that the thickness of the compact bone, implant design, and implant site preparation were critical to the implant’s stability.

Deciding on the appropriate placement of TADs is critical for safety and for the appropriate design and delivery of the desired force system. During the treatment planning of orthodontic therapy, the clinician needs to determine the desired force system and its equilibrium diagram to identify the force system acting on the active unit of teeth (the unit of teeth that needs movement) and the side effects affecting the anchorage unit of teeth. This will dictate ideal placement of the TAD to ensure safety and optimal appliance design.

Clinicians should be fully aware of the anatomical limitations to implantation of TADs. These limitations drive important decisions in appliance design and the resulting force system (e.g., point of force application, direction of force and moment generated). Also, the force system applied to the TAD needs to be carefully evaluated to control the moments generated that tend to unscrew the TAD, to prevent the miniscrew from loosening. If the application of such undesirable moments to the screw cannot be avoided, indirect anchorage is recommended to optimize the anchorage system and minimize the undesirable side effects resulting from the force system.