Temporary Anchorage Devices: The Bone Anchorage Appliance

Although TADs have been a relatively recent addition to the orthodontist’s arsenal, in reality there is a long history behind them. There are plenty of references in bibliography of clinicians using some sort of implant to move teeth many years before TADs were introduced. Roberts was probably one of the first researchers to realize the potential of titanium implants as orthodontic anchorage and conduct systematic research on the topic. His “first generation” of TADS featured a regular dental implant in the retromolar area that was used to protract second molars and close the space of the frequently extracted first molars.

However, it was Kanomi that established the term mini-implant and created the TAD in the way we use it today [4]. Nowadays there are hundreds of different types of this appliance and new field of orthodontic research. Anchorage value Newton's Third Law states that “for every action, there is an equal and opposite reaction”. When trying to move teeth orthodontists must acknowledge this law and realize that every time they try to move teeth there is a possibility to simultaneously create an undesired tooth movement. Orthodontic anchorage has been defined since 1923 as “the base against which orthodontic force or reaction of orthodontic force is applied” and essentially means the resistance to undesired tooth movement. Any structure that is covered by periodontal ligament (PDL) will more or less move under force application since PDL is effectively the apparatus that makes orthodontic movement possible. The philosophy behind using TADS as skeletal anchorage is that since they have no PDL, the reactive forces will be absorbed by osseous structures and only the desired therapeutic movements will be allowed. There are many different ways to achieve anchorage. One simple classification could be as follows:

1. Anchorage with the use of extra-oral support (headgear or face mask etc. )
2. Anchorage with the use intra-oral appliances (Nance, lingual arch etc. ).
3. Intermaxillary anchorage using support from teeth in the opposite dental arch (Class II or III elastics)
4. Anchorage by modification of fixed appliances (tip-back or gable bends, buccal root torque etc. ).
5. Skeletal anchorage (ankylosed teeth and all forms of implants or plates).

It’s a well-known fact that absolute anchorage or stabilization of teeth can be only reliably achieved using ankylosed teeth or some type of implant or plate. Every other type of anchorage either creates some sort of reciprocal force that needs to be manipulated or relies on patient compliance which has a certain. degree of unpredictability [6]. Biology One of the major advantages of TADS is the versatility of placement. TADS can be placed in the close vicinity of the anchorage requirement within the alveolar process typically in an inter-radicular location. This way the need for complex biomechanics is minimized while anchorage remains maximum.

There are numerous case reports and papers in the last 20 years emphasizing the clinical application and potential of TADS [7] However, both clinicians and researchers very often assume that TADS function in an identical manner to endosseous dental implants. It has been demonstrated that regular endosseous dental implants after a period of time are rigid and capable of withstanding high orthodontic forces and prolonged loads . On the other hand, TAD research has shown that larger forces (e. g. , 10 N) cannot be routinely supported over a prolonged duration (1– 2 years) and mini implants are typically used for movement of few teeth over a period of 6–8 months. Persistently high failure rates appear to be a major problem of TADS. The most significant difference between regular dental implants and many of these TADs is the lack of osseointegration of the mini implants. While it was desired that mini screws would not fully osseointegrate and could be removed upon completion of their use, the high failure rate (10–30%) and displacement can make that use challenging. For this reason, orthodontists explored other skeletal anchorage options such as miniplates [12] and other extra-alveolar sites such as the palate for a more favorable placement of TADs.

Osseointegration

Generally the definition and mechanism of a successful device implantation has been described by the term osseointegration. Osseointegration is the presence of vital load-bearing bone directly in contact with the implant. Most of the implant studies examine bone sections and quantify histological parameters at the bone-implant interface.

Some of the variables that can be measured are percent bone to implant contact (%BIC), percent bone volume fraction (% BV/TV) within the threads of an implant and bone remodeling (% bone formation rate/year, %BFR/ year). Nevertheless, the definition of a “successful implant “on a histological section is not easy and not readily measureable. Primary and secondary stability cannot be evaluated on a histologic section and the same occurs for almost all mechanical factors. The exact opposite occurs with a failed or failing implant. The presence of fibrous tissue and woven bone at the implant interface on histological sections indicates overload and predicts future failure. In general, there are many challenges in conducting endosseous implant research. Selection of an appropriate animal model, interpretation and extrapolation of results to humans, ability to mimic clinical by conducting long duration studies (>9– 12 months) and analysis of the in vitro cellular and molecular responses for clinical situation are just a few of the problems that need to be solved. Histological variables There are a lot of studies published on different animal models however, there are limitations and advantages to each animal model and direct extrapolation of the results to humans should be avoided. Some of the most important histomorphometric variables that provide information about mini implants are the following:

• Bone implant contact (BIC) is measured in most histological studies. Although bone contact as measured in studies is a static measurement, it actually describes a dynamic process. There is bone remodeling at the implant interface which makes that measurement dynamic. That means that different areas of the implant may contact bone at different times as bone is increased or decreased by remodeling. It has been shown that across species remodeling rate is elevated in close proximity to the implant and high at implant interface, which makes almost certain that bone contact changes. Literature indicates that the shape of the implant and the design of the implant threads can have an impact the amount of bone contact. While bone contact measurement is important is not a direct predictor for implant success.
• Βone volume (BV) adjacent to the implant and contained within the threads This specific bone is either generated by contact osteogenesis or distance osteogenesis[20]. The biologic basis here is that bone ingrowth occurs toward an osteogenic surface, in areas where bone did not exist before.
• Bone remodeling. Manifestation of viable bone at an implant interface is a key to success. One method to measure the metabolic activity at an implant interface is by estimation of bone remodeling in supporting cortical and trabecular compartments. Measurement of bone remodeling involves the use of intravital bone label. The rate of cortical and trabecular bone turnover in humans are estimated to be at 2–10%/year and 25–30%/year, respectively.

After implant placement there is elevated bone remodeling during initial phases of healing which is usually described with the term regional acceleratory phenomena (RAP)[22]To evaluate bone remodeling histomorphometric variables such as mineral apposition rate, mineralizing surface/bone surface (MS/BS) or bone formation rate (BFR) are measured in mineralized sections. These variables reveal the dynamic nature of the metabolic activity in the bone and certainly reveal more information than static variables such as BIC. It is interesting. Analysis of retrieval specimens from various animal species have demonstrated that that even after accounting for periods of time for typical bone healing, a persistent elevated remodeling rate is observed in implant adjacent bone in the long term (2 years out after implantation). It is unclear though if this is important for the long-term success of implants Microcomputed tomography Microcomputed tomography (μCT) is the latest innovation in the study of bone healing and adaptation. μCT images provide 3D reconstructions of the region of interest and help overcome one of the major limitations of standard histology. That is that only a select number of 2D sections can be examined and the true 3D nature of the implant interface cannot be visualized. It appears that μCT will revolutionize static histological measurements but at the moment cannot replace dynamic histomorphometry. This new technology has also some new type of problems to overcome like scatter and beam hardening. In comparison with traditional histology μCT can collect the same information only in static measurements. Materials for TADS Titanium is an ideal biocompatible material that allows for direct bone contact (osseointegration) between endosseous dental implants and the host bone. Mini screw implants are generally made of titanium alloys. Contrary to dental implants, a high degree of osseointegration is not a requirement for orthodontic mini implants to be functional as anchorage devices. Stainless steel bone screws have also been widely used in orthognathic surgery for fracture fixation. Unlike titanium alloy, stainless steel screws tend to develop a fibrous tissue interface between the screw and bone. This fact allows easier retrieval as it reduces removal torque.

Stainless steel TADs have been used for en-masse space closure showing promising results. Τhere are 2 major issues regarding steel anchorage devices primary stability and bone-healing responses. Primary stability is defined as the mechanical retention at insertion and is quantified by insertion torque. Ιt has been reported that a wide range of insertion torque values can achieve high TAD success rates. In contrast, excessive insertion torque might cause negative effects such as bone necrosis and increased microdamage. When microdamage accumulates, it can contribute to MSI failure. The reason is that bone mechanical properties are reduced as a consequence of microdamage initiating rapid bone remodeling and healing. Most clinicians choose to immediate load the mini implants and that can alter the microdamage healing process and the bone-to-implant contact. Once implant is placed, bone healing starts and new bone forms and remodels surrounding the implant. The bone-to- implant contact ratio (implant surface in contact with bone divided by total implant surface) is frequently used to designate the degree of bone-implant adaptation. It has been shown that even as little as 5% of bone-to-implant contact successfully resists orthodontic forces.

One of the best indicators of anchorage capability of TADs and the quality of bone-to-implant contact is removal torque. Clinically, removal torque of titanium alloy MSIs varies from 4 to 16 N-cm, depending on surface treatments. It is speculated that fibrous tissues develop around stainless steel screw threads, leading to reduced bone-to-implant contact and removal torque with increased potential for failure. Conclusions It is important to understand the physiology of bone implant interface in order to explain how a mini-implant will perform under orthodontic load. Osseointegration of TADS is not as important as in endosseous implants, nevertheless some new methods need to evolve for the scientific examination of newer designs and the importance in the retention of TADs. So far the biggest problem for this device appears to be the relatively high rate of failure. Lack of rigidity of the mini implant with resulting displacements within the bone needs to be investigated.