When moving teeth orthodontically, it is important to identify which specific teeth need to be moved in order to reach the treatment goal. For example, premolar extraction spaces can be closed by moving the posterior teeth forward or the anterior teeth distal. The same can be said about correcting a deep bite, which can be corrected with posterior extrusion or anterior intrusion. Similar considerations can be made for open bites as well. This concept is termed the anchorage requirement. Certain movements require anchorage that cannot be achieved without pre-planning and the use of auxiliaries. Additional anchorage can come in the form of headgear, elastics, use of adjacent teeth, and appliances, many of which rely on patient compliance. The use of implants in orthodontics has evolved through publications by Gainesforth and Higley in 1945;60 Kanomi who introduced the use of mini-implants in 1997;61 and Costa et al, who described a 2mm titanium implant that could be used for both direct and indirect anchorage in 1998.62 Experimental data on Temporary Anchorage Devices (TADs), also known as mini-implants or miniscrews, began being published in 2000. The initial implants used in orthodontics were conventional implants placed in edentulous, palatal, or retromolar spaces.63 Skeletal anchorage has made possible the ability to correct malocclusions without having to rely on patient compliance and with minimal effect of Newton’s third law of physics regarding opposite and equal forces.64 This type of anchorage is considered absolute anchorage, with no movement of the anchorage unit.65 TADs are typically no more than 3mm in diameter and 11mm in length, and their placement is usually facilitated via the use of local or topical anesthetic.64
One of the great advantages of using a TAD over a traditional implant is the ability to load it with force shortly after placement. While traditional implants require two months of healing time, TADs are usually loaded immediately or after a short 2-week healing time. This primary stability in the shorter waiting period is a product of the mechanical retention formed between the screw and the bone. Research has demonstrated this stability in forces up 250g.66 It is important to maintain 1-2mm between the screw and any vital anatomic structure to prevent damage.66 The close proximity of TADs to roots has also led to the concern of potential root damage should a TAD make contact with the root, although several studies have suggested that repair after contact of a TAD and root may be possible after TAD removal.64,67 Despite this, root damage may be associated with higher failure rates. The risk of damaging a root is enhanced by improper placement, migration after loading, axial deviation during insertion, anatomic root variation, and tooth contact with TAD in orthodontic treatment.67 Proper precaution should be taken to avoid this if possible.
Compared to conventional implants, TADs are typically not expected to osseointegrate more than 25% to enhance the practitioner’s ability to remove them. The stability of the screw is related to its diameter as opposed to its length63 and is based on mechanical gripping to the cortical bone.68 Additionally, TADs may be self-drilling, requiring no pilot hole, or self-tapping which do require a pilot hole.68 Overall, lower failures have been seen with the use of self-drilling screws and placement in keratinized mucosa.63 A study conducted in 2009 surveyed orthodontic residents and found that 92.03% of them planned on using TADs in their office and 72.26% planned to place them themselves.69
Since their development, a number of studies have been published discussing the ability of TADs to assist in difficult orthodontic movements such as uprighting, intruding, and distalizing molars; retracting the anterior segment (Figure 4); closing first molar sites; and correcting midlines without affecting the remaining occlusion or dentition.70 A transpalatal arch (TPA) placed 3-4mm away from the palate can be used in conjunction with TADs for molar intrusion, using tongue pressure to aid in the intrusion.68 Scheffler et al. successfully demonstrated intrusion of the maxillary molars of 2.3mm, with relapse of 0.5mm occurring mainly in the first year after treatment. When attempting this, it is important to apply a passive force, such as an essix or lower lingual holding arch, to the mandibular molars to prevent their eruption, and thus negating the intrusive effects.71
Another good use for TADs is retraction of the maxillary anterior dentition when first premolars are extracted in Class II molar patients (Figure 4). With a miniscrew located further apically in the edentulous space, a force can be placed to an attachment also reaching apically to move the teeth in a straight distal vector. A force to a short attachment will provide an intrusive force as well if needed. Improper force vectors can cause unwanted movements, such as extrusion or rotation of the occlusal plane.72 TADs can also be used to distalize full maxillary arches, with or without appliances, such as the pendulum appliance. Distalization of the maxillary arch should also involve some intrusion to prevent changes to the vertical dimension of the face. One complication of this procedure is the formation of a pseudopocket distal to the second molar which could potentially require future periodontal surgery to correct.73
The success rate of TADs in published literature has ranged from 61 to 100% depending on the source.65 The most common complications are mucosal inflammation, soft tissue hypertrophy and infection. The main cause of failure is placement in non-keratinized tissue, leading to peri-implant inflammation and infection.65 Providing proper oral hygiene and care information to patients at the initial placement appointment can reduce the amount of plaque and inflammation of the surrounding tissues.74 Potential risk factors for TAD placement include osteoporosis, uncontrolled diabetes, periodontal disease, smoking and bisphosphonates. Factors that may potentially increase their clinical success include greater cortical thickness for improved primary stability, placement in keratinized tissue, and sufficient distance from nearby roots.75