Mini-implants in contemporary orthodontics part 1: recent evidence on factors affecting clinical success

A revolution in orthodontic anchorage began at the start of the 21st century with the introduction of skeletal anchorage fixtures customized for orthodontic purposes. These are commonly known as orthodontic mini-implants (OMIs), mini-screw implants (MSIs) and temporary anchorage devices (TADs). The first two terms are widely regarded as the most accurate ones,¹ and are used interchangeably in most peer-reviewed journals. The author prefers the term OMI and this will be used throughout this series of two papers. These papers aim to provide a contemporary update for the orthodontic team, firstly on the principles of stable mini-implant anchorage and secondly on the wide-ranging clinical applications.

Maxillofacial surgical (fracture and osteotomy) fixation screws were used as the original OMIs in the late 1990s. Subsequently, during the first decade of the new millennium, this technology was customized for orthodontic purposes, especially in terms of the head and neck screw design, plus the associated insertion instrumentation. The head portion is used to connect to orthodontic appliances and the neck traverses the mucosa (in contrast to fixation screws which are placed submucosally). Typical OMI endosseous (body) dimensions are 1.3-2 mm in diameter and 5-10 mm in length. The majority of commercially available OMIs have polished and smooth endosseous surfaces, in contrast to dental (tooth) implants. Hence, they rely on mechanical retention within the bone. This is observed at the histological level in the form of bone-implant contact (BIC), but crucially this does not equate to osseointegration, where the latter is defined as an ankylotic union between a metal fixture and the adjacent bone. This is an important distinction since solely physical retention means that OMIs may be immediately loaded and are easily unscrewed during their removal (without the need for bone trephination).