A simple guide to orthodontic miniscrews. Part 1: history and success rates

Background

Since their introduction, orthodontic miniscrews/temporary anchorage devices (TADs), have shifted the paradigm of anchorage in orthodontics. Tooth movements that were once challenging have become easier. Their placement is relatively straightforward and complications are uncommon if placed and utilised correctly.

In this series of blogs, we will look at the history, development, evidence and clinical considerations for TAD placement. Moreover, we will simplify the biomechanical principles associated with TAD assisted tooth movements.

Unlike osseointegrated implants, TADs derive their retention by mechanical means only. Osseointegration is not desired (to enable easy removal). TADs were introduced as absolute anchorage devices in orthodontic treatment because the procedures for placement and removal are less invasive than for other implants and require minimum patient cooperation.

Anchorage is defined as the prevention of unwanted tooth movement (Proffit 2000). Traditionally, anchorage has been provided in orthodontics by anchor sites within the mouth (intraoral anchorage) or outside the mouth (extraoral anchorage). Intraoral anchor sites include the teeth or other oral structures, such as the palatal vault. Extraoral anchorage is achieved by using headgear, neck straps or facemasks. A more recent method of reinforcing anchorage is the use of intraoral Temporary anchorage devices (TADs).

TADs are indicated when a considerable amount of tooth movement is required or when dental anchorage is insufficient, such as in hypodontia cases. These devices are also useful when asymmetric tooth movements are required, during intrusive mechanics, for intermaxillary fixation/traction and during orthopaedic traction. TADs are rapidly establishing their effectiveness and application in routine orthodontic practice.

History

1945: Gainsforth and Higley (1945) first suggested the use of metallic screws as anchors as long ago as 1945. They experimented the use of vitallium screws on five dogs. The vitallium screws were placed in the anterior border of the ramus of the mandible. Orthodontic traction was applied to the screw by means of an orthodontic elastic connected to a maxillary appliance. In two dogs, vitallium screws were placed in the rami and no traction applied. Each screw came out on the twenty-first day. It was concluded that “tooth movement was accomplished using basal bone anchorage, but an effective force could not be maintained for more than thirty-one days in any case”.

1969: Linkow described the use of an endosseous blade implant for retraction of anterior teeth in 1969.

1983: Following this, Creekmore and Eklund (1983) inserted a similar device below the nasal cavity in 1983. They used the titanium osteosynthesis screw to perform maxillary incisor intrusion.

1988: Turley and colleagues used endosseous implants to investigate the influence of absolute anchorage on tooth movement in dogs. “Not all experimental implants remained firm enough through the initial healing period to be used for anchorage”.

1997: In the late nineties, Kanomi (1997) first described a 1.2mm miniscrew, used specifically designed TADs for orthodontic use, namely for intrusion of mandibular anterior and buccal teeth.

1998: In the following year, Costa et al. (1998) described a screw with a special bracket-like head that could be used for either direct or indirect anchorage.

Since then, various types of TADs have been introduced in the market. In more recent years, there have been numerous publications describing novel methods of reinforcing anchorage using a variety of devices temporarily anchored in bone.

Evidence

survival and success rate

The success rates of TADs is very high. On average, we would expect our TADs to be successful in 85-90% of the time.

The nicely done retrospective study by Sarah Abu-Arqub found that the overall survival rate of palatal TADs was very high, about 92%. Buccal inter-radicular TADs on the other hand, had the highest survival rate of 75.5% after 12 months. Buccal shelf TADs had the lowest success and survival rates after 12 months, 31%.

A systematic review and meta-analysis by Alharbi et al. 2018 has found that TADs have high success rates generally, with overall failure rates of only 13.5%.

A meta-analysis of non-randomised clinical studies (Hong et al. 2016) showed that jaw of insertion, age, TAD length and diameter are critical risk factors to the success of TADs. The authors concluded that TADs with significantly higher success rates were those inserted in the maxilla, 8 mm or longer, 1.4 mm or wider and placed in older patients defined as >20 years old.

We now know that the success rate is also dependant on the length and diameter of the TAD. In a systematic review it was found that 8 mm TADs were associated with 22% higher success than 6 mm ones. In the same review, it was shown that TADs of minimum 8 mm length and 1.2 mm diameter achieve good stability and are associated with minimal root damage.

Consideration should be given to the soft tissue at the site of placement. Whenever possible, the TAD should be inserted through attached keratinised gingiva (Luzi et al. 2009) otherwise the screw can become buried beneath the soft tissues.

The primary stability depends on multiple factors, including length, diameter, thread geometry, insertion method and bone quality. The thread geometry is an important factor to consider before selecting a TAD. A recent study found that a wider thread pitch reduced microdamage, and decreased thread height resulted in increased bone compression, ultimately resulting in increased primary stability. On the other hand, a TAD with narrower thread pitch led to maximum bone compression and extensive bone microdamage.

For patients:

The success rates of bone screws in orthodontic treatment have been reported to be high, with studies showing a success rate ranging from 85% to 97%. Bone screws, also known as temporary anchorage devices, have revolutionised orthodontic treatment by providing stable anchorage, especially in cases where traditional methods may not be sufficient. Their small size, ease of placement, and versatility make them valuable tools in achieving successful outcomes in orthodontic treatment. Proper selection, placement, and monitoring of miniscrews contribute significantly to their success rates, making them a reliable option for orthodontic patients.