Temporary anchorage devices in orthodontics: A review

According to shape and size

a. Conical (cylindrical)

- Miniscrew implants

- Palatal implants

- Prosthodontic implants

b. Miniplate implants

c. Disk implants (onplants)

According to implant bone contact

a. Osseointegrated

b. Non-osseointegrated

According to the application

a. Orthodontic implants

b. Prosthodontic implants

Endosseous implants ^[13]

These are osseointegrated implants that can withstand more mechanical force compared to the mechanically retaining implants. The drawbacks of this type is that there is a waiting period before loading, extensive surgical procedure is required for placement and the removal is problematic. These were used earlier in the palate for molar distalization.

Surgical miniplates- ^[3], ^[13]

The modified or conventional surgical titanium miniplates with intraoral extension can be used for orthodontic anchorage. Regions with thick cortical bone such as the zygomatic region and the buccal shelf area of the mandible are suitable for such implants. Applications of these implants include en masse molar distalization and intrusion of buccal segment in open bite cases. The downside of these implants was the need for extensive surgical procedure and post-operative discomfort for the patient.

Miniscrew implants (MSIs)

These implants have mechanical retention and provide short duration anchorage in orthodontics. The small diameter of these screws provides high versatility for placement site. Inter-radicular bone is the most common site for placement. These implants are absolute anchorage units.

Head

It is the orally exposed portion of the screw which provides attachment for the springs and elastics. It has a screw driver slot or a particular design to engage the miniscrew driver for implant placement. Numerous kinds of head design are available for different types of anchorage and for prevention of soft tissue irritation. Most common type is the button like design with a sphere or double sphere-like shape or a hexagonal shape. A 0.8mm diameter hole in the head or neck of the screw is mostly used for direct anchorage. Furthermore, bracket-like designs are also available that can be used for both types of anchorage. ^[14] The driving head can only be used on the miniscrew given by that specific manufacturer. The head of the screw encompasses a hole and a collar for various attachments.^[6] [Figure 1] shows the parts of a miniscrew.^[15]

Neck

Screw neck or the trans-mucosal part passes through the mucosa and connects the screw with head. Variable lengths of neck are available for different mucosal thickness. The surface of the neck should be smooth and well-polished to diminish plaque accumulation around the neck. The junction of TAD with the mucosa is crucial as most of the implant failure due to peri-implantitis usually begins from this site.

Screw

This part gets embedded in the cortical or medullary bone to provide retention. The thread of the screw around shank or main body of the TAD has the cutting edge that facilitates insertion. The depth of the cutting edge and its angle determines the stresses generated during insertion and the amount of insertion torque required for insertion.

Thread design can be conical as in miniscrews or parallel tapering only at the end as in Orthodontic Mini Implant. ^[4] [Figure 2] shows different types of miniscrews. The length of TAD is defined as the length of the threaded body. It can range from 5-12mm for various clinical procedure according to anatomical considerations. ^[16] Costa et al evaluated the intraoral hard and soft tissue depths and established that miniscrew implants of 4-6mm length are safe in most regions, but subject to variations according to the bone depth of individual patients. ^[17] Total screw length id determined by the screw, neck and head length. The major diameter of TAD is the maximum diameter determined by the outer diameter of the threads and can range from 1.2 -2 mm. Diameters greater than 2mm should not be used in the inter-radicular region and those less than 1.2mm are weak and prone to breakage. ^[18] The minor diameter refers to the inner core diameter, which usually ranges from 0.2 to 1.6mm. The pitch of the screw is the distance between the two threads. High pitch denotes threads placed far apart and vice versa. A screw with a larger pitch gets inserted at a faster rate. [Figure 3] shows the design features of a miniscrew. [Table 1] shows the currently available orthodontic miniscrew systems.

Biocompatibility

Medical type IV or type V titanium alloy (Ti6Al4V) which is an alloy of titanium, aluminium and vanadium is the material of choice for the production of majority of the implants except for Orthodontic Mini Implants, which is made from stainless steel. The medical grade titanium offers biocompatibility and strength which is higher as compared with commercially pure (CP) titanium.

Osseointegration

TADs are designed to be mechanically retained in the bone. For easier removal of TADs, extensive osseointegration is detrimental. For this reason, these devices are made with smooth surfaces in order to reduce the bone ingrowth and to promote soft tissue attachment and without any special surface treatment regimen.

Types of anchorage

Two discrete types of anchorage can be provided by TADs. Direct anchorage implies that the implant receives the reactive forces directly by acting as an anchor unit. In indirect anchorage, bars or wires are used to attach the implant to the reactive unit. Although orthodontic mechanism requires not more than 300g force, TADs are designed to withstand up to 500g force. FEM studies indicate that direct loading may overload the TADs and the peri-implant bone leading to failure of TAD. ^[19], ^[20] Hence, indirectly loaded TADs is a healthy option in clinical situations where direct loading is not preferable. ^[21] [Figure 4], [Figure 5] shows direct anchorage and indirect anchorage respectively.

Indications for use of TADs

Absolute anchorage in maximum retraction requirements.

For patients not compliant with the use of headgear, TADs are viable option for anchorage.

In case of missing first molars, TADs can provide anchorage as well as help manage the space judiciously.

For difficult tooth movements such as anterior /posterior intrusion, en masse distalization of upper/lower arches, molar up righting and molar distalization.

In adult orthodontics for complex tooth movements.

TADs can also be used for the attachment of orthopaedic forces to jaws when there is a lack of anchorage units.

Correction of midline asymmetry and cant of occlusion.

Case selection, informed consent and records

Patient’s medical history and assessment of oral cavity for absence of gingival inflammation and periodontal disease are to be done initially. Informed consent should always be obtained from the patient or the parent. Apart from the usual orthodontic records, intraoral radiographs of the proposed miniscrew site have to be taken to assess the bone morphology and roots of adjacent teeth. ^[22] In case further assessment of bone quality is required; cone beam computed tomography (CBCT) may be taken for bone density values. It has been suggested that D4 and D5 bones types are not suitable for implants. ^[23]

Sites of Insertion

In the maxilla, TADs can be placed in the incisive fossa, canine fossa, infra-zygomatic ridge, pre-maxillary region or mid-palatal region. While in the mandible, TADs can be placed in symphysis, canine fossa, anterior external oblique ridge, retro-molar area or sub-maxillary fossa. ^[17]

Studies have shown that the safest site for placement of TADs in the maxilla was in the anterior and apical portion and the tuberosity region was most unsuited for implant placement due to reduced bone thickness in this region. In the mandible, safest insertion site was between the first and second molars and between first and second premolars. ^[24] In the palate, the strongest bone support for implant insertion was found 6 to 9mm posterior to the incisive foramen and 3 to 6 mm paramedian. ^[25]

Placement of TADs in the extra-alveolar bone will diminish tooth root contact and allows the force vector closer to the centre of resistance of the tooth. But such implants will lay in mobile alveolar mucosa which can be overcome by using trans-mucosal attachments. ^[17]

Direction of Insertion

The miniscrew implants are inserted obliquely in an apical direction in the maxilla and parallel to the roots in the mandible. ^[26] In the maxilla, insertion angulation is 30° to 40° to the long axes of the teeth while it is 10° to 20° in the mandible. ^[27] In the region of maxillary sinus, a more perpendicular direction of insertion of mini implant is suggested to minimize the chances of perforation of maxillary sinus. ^[28]

Technique for Placement

The miniscrew implants can be self-drilling or self-tapping. An adjustable acrylic template or surgical guide can be used before implant placement. Recently, clinicians have used 3D CBCT, and customized surgical guide fabricated using stereolithographic techniques. This method helps for placement of self-drilling miniscrews with precision adjacent to dental roots and maxillary sinuses. ^[29], ^[30]

In self-tapping screws, predrilling is done under a small amount of local anaesthesia, preferably by an oral surgeon. At the site of placement, soft tissue is removed using a soft tissue punch and the pilot hole is drilled using a drill bit and a drill rotating at a speed not exceeding 1000rpm. The pilot hole should be maximum 2 to 3mm deep and should be 0.3mm smaller than the screw diameter. The implant is then inserted using an appropriate screw driver.

The self-drilling screws have specially formed tips and cutting flutes that can be inserted into the bone without predrilling, thereby decreasing the likelihood of damage to the tooth root, tooth germ or nerves, thermal necrosis of the bones and the fracture of the drill bit. ^[4] However, self-drilling screw requires pilot hole drilling if the thickness of bone cortex is more than 2mm which may cause the bending of the fine tip of the screw. ^[14] Figure 6 shows self-drilling and self-tapping screws.

Loading and anchorage consideration

Unlike dental implants immediate loading of orthodontic miniscrews can be done using light forces. ^[28] It is advisable to provide at least 2 mm clearance between the implant and the adjacent tooth root as the implant might move during orthodontic loading in some patients. ^[31]

Implant removal

The miniscrew can be removed using the same screw driver with or without local anaesthesia. Normally, the wound after implant removal does not require any special treatment and heals uneventfully. In case the screw cannot be retrieved during the removal appointment, it is advisable to wait for 3 to 4 days. The micro fractures or bone remodelling caused due to the initial attempt will loosen the screw. In case of implant fracture during removal, a small surgical procedure may be required for removal. ^[14]