M645: Temporary Anchorage Devices: Indications and Applications

Abstract

It should be self–evident that good diagnosis and treatment planning are vital in providing a favorable orthodontic outcome. Anchorage management is an integral part of that process. Anchorage, the resistance to unwanted tooth movement, determines the limits of desired tooth movement possible and accordingly the amount of correction possible. Traditionally, anchorage strategies for maximum tooth movement were largely dependent on patient compliance. Intraoral and extraoral auxiliaries were used in addition to grouping multiple teeth and pitting them against few. Over the last two decades, there has been increasing interest in establishing absolute anchorage with implantable devices which respond like ankylosed teeth. Although interest in this approach has grown dramatically in the last decade, the notion is not new. In their 1945 paper, Gainsforth and Higley reported the concept of basal bone anchorage via orthopedic bone screws placed in the mandibular ramus. Currently, although a multitude of vendors proffer a confounding variety of devices, there are only four basic approaches to implantable anchorage. These involve the use of osseointegrated implants, modified bone screws, modified bone plates and special purpose midpalatal implants. In the mutilated dentition, osseointegrated implants provide excellent anchorage and with the appropriate planning and placement, can subsequently be incorporated into the final prosthetic treatment. The remaining three categories are intended to be temporary and are removed at the conclusion of active use. The majority of temporary devices being used fall into the modified bone screw (mini–screw) category. The have been shown to successfully serve as stable anchors in research models, but the clinical results have been less promising. These devices range from systems using short screws of less than a millimeter in diameter to those which are transalveolar and over 2 mm in diameter. Not surprisingly, small diameter screws which require drilling seem to be more failure–prone than longer screws with a larger diameter. The mini–screws have the advantage of being minimally invasive and can be used both directly and indirectly in a wide variety of locations. They have the disadvantage of having a higher failure rate than the modified bone plates or palatal devices. Modified bone plates require an incision and suture closure for both placement and removal. The expense for materials is roughly 2-3 times that for mini–screws. However, they have the advantages of lower failure rate and more favorable force vectors. Although perhaps more limited than mini–screws in terms of application, they are ideal for placement when the sinus is pneumatised or for mandibular protraction of posterior teeth. Mid–palatal implants are perhaps the most expensive and least versatile. The are limited to indirect anchorage in the maxillary arch, largely for en–masse retraction of the anterior teeth. These devices are essentially short osseointegrated implants with special attachments to secure a molar–stabilising transpalatal bar. By virtue of this design, they are delayed load and require a considerable amount of paraphernalia for placement and the associated laboratory phase. They are placed as one–stage implants and removed by trephine following the completion of treatment.