TAD Selection and Placement

Michael P. Chaffee, DDS, MS - Coeur d’Alene, ID

Introduction

Over the last several years, the use of temporary anchorage devices has grown exponentially. Today there is a myriad of choices of brands of microscrew implants. The purpose of this article is to give an overview of our TAD selection, placement techniques, and placement locations.

Temporary anchorage devices can include both miniplates and microscrew implants. The vast majority of skeletal anchorage patients that I have treated have been treated using microscrew implants (MSI). For the several patients that I have treated with surgical miniplates, the plates were chosen because of the severity of the malocclusion and required posterior intrusion, or because of bone density issues resulting in microscrew failure or anticipated failure (Fig 1). Our success rate with surgical miniplates has been 100% to date when unidirectional static intrusive or anterior-posterior forces have been used. Our only failures with miniplates have been in cases with Bollard plates used in early adolescent class III correction utilizing DeClerck mechanics¹. In those cases dynamic forces are involved with class III elastic wear, thus increasing the chances of failure. While our use of miniplates has been extremely successful, there are disadvantages. Miniplates are certainly more invasive, and are unable to be placed by the orthodontist. Their placement by an oral surgeon is relatively expensive compared to the placement of microscrew implants. Surgical flaps are required for placement and removal.

(click on images for larger view)

(Figure 1)

On the contrary, microscrew implants are much less expensive relative to miniplates. It’s becoming more common for the orthodontist to place microscrews as opposed to referring them out. Placement techniques have become simplified since the development of self-drilling screws.

Microscrew Design and Selection

Choosing a brand of microscrew implant is similar to choosing a brand of orthodontic bracket in that it is essentially a choice of personal preference, or “what works best in your hands”. Most microscrew implants today are very similar in most aspects. Most are self-drilling, not requiring a pilot hole. All self-drilling screws are tapered in at least the apical one-third and have sharp edges that create their own threads while being inserted. Most are highly polished with the intention of not allowing for osseointegration². There is a surface-treated microimplant system that encourages partial integration known as the C-Implant system³. However, this successful system does require a pilot hole. In polished microimplant systems, retention is purely mechanical, and stability is not related to screw length. Little stability comes from medullary bone⁴. The greatest variation in microscrews is in head design. Microscrews  rely on an undercut, a hole, and/or a slot in the head of the screw in order to accept elastic thread, a coil spring, elastomeric chain, or a segmental wire. While I have used a variety of screws over the last six years, my personal preference is the system I began with in 2003, the Dentos Absoanchor, now distributed in the U.S. by Great Lakes Orthodontics. Absoanchors have a wide variety of diameters and head design to choose from (Fig. 2). Diameters range from 1.2 to 2.0 mm. Head lengths and styles vary. Long heads can be used in areas where soft-tissue overgrowth may occur.  A slot head is available. While I have not yet personally used a bracket-headed MSI, Dentos even provides one in a left-handed thread in case anticipated moment forces will unscrew a conventional MSI. Lengths vary from 5 to 12 mm.

(Figure 2, courtesy of Dentos, Inc.)

Microscrew location will depend on desired mechanics. The most common locations in our practice are (in order) facial and mesial to the maxillary first molar, palatally between the maxillary first and second molars, facially between the mandibular first and second molars, and facially between maxillary central and lateral incisors. The average interroot space between maxillary first molars and second premolars is approximately 3.2 mm, based on a very informative study by Hyo-Sang Park⁵. This study looked at CT images taken at 5-7 mm from the alveolar crest, and measured cortical bone thickness, distance between surface bone and root, and distance between roots in various locations in the mouth (Fig. 3). This adequate interroot space between fist molars and second premolars allows for relatively easy and safe insertion, and allows for both vertical control and maxillary A-P anchorage. On the contrary, the distance between the facial roots of maxillary first and second molars is only 2 mm. With a 1.5 mm MSI, the room for error is extremely small. Because of the single palatal roots of maxillary first and second molars, average interroot space in this area is tremendous at 4 mm. Screws in this location are very effective for posterior intrusion to close anterior open bites. Microscrews placed facially between mandibular first and second molars in combination with a .032” X .032” Burstone lingual arch, allows for vertical control in both extraction and non-extraction cases (Figs. 4). Maxillary anterior microscrews provide anchorage for anterior intrusion. They can also prevent anterior extrusion in high angle anterior open bites that are not treated segmentally. Posterior intrusion at the maxillary first/second molar area can result in an extrusive moment force on anterior teeth, resulting in anterior extrusion⁶. There are a number of other locations we use less often for various other mechanics.

 

(Figure 3, courtesy of H.S.Park, KyoungPook University, Taegu, Korea)

(Figures 4)

MSI diameter and length will depend on location, but we have essentially narrowed our inventory down to two diameters and lengths. Facial screws are tapered and 1.5 mm in diameter and 6 mm in length. Palatal screws are 1.8 mm in diameter and 10 mm in length. Because MSI stability relies primarily on mechanical retention in cortical plate, the shortest MSI to completely penetrate cortical plate at the screw’s full diameter, yet long enough to traverse the soft tissue is ideal. Long palatal screws are required because of the soft tissue thickness in that area.

Microcrew Insertion

Our insertion protocol begins with the patient rinsing with chlorhexidine for one minute. A cheek retractor is then placed. Topical anesthetic gel (Profound Gel, Steven’s Pharmacy, Costa Mesa, CA) is then placed in the desired location. The gel is kept in place for three minutes before being removed. One quarter carpule of anesthesia is then administered. Following anesthesia, a periodontal probe is placed against the gingiva to accomplish three things; to verify anesthesia, to measure the desired distance from the archwire to the screw location, and to place a temporary indentation in the gingiva between the roots of the teeth to aid in correct insertion and angulation of the microscrew. The microscrew is initially engaged with the screw perpendicular to bone. After an initial “bite” into bone, the angulation is changed so that the MSI is between 30° and 60° to the long axis of the tooth. This angulation accomplishes two things. More microscrew surface area is engaged in cortical plate⁷. Also, as the screw is inserted at this angulation the apex of the screw is driven in a more apical direction where there is a greater interroot space (Figs 5).

(Figures 5)

Microscrew implants should be activated at the time of placement. Studies have shown that immediate static forces increase success rates². While I do use an occasional NiTi coil spring, the majority of our activation is by way of .018” X .018” elastic thread. Forces of less than 200 grams are very effective at A-P movement. Excessive forces can also increase screw failure⁸. Intrusion mechanics may require slightly heavier forces. Following microscrew insertion, the patient is given a supersoft implant toothbrush for cleaning around the MSI. The remaining four ounce chlorhexidine bottle is sent home with the patient and used daily until finished.

Conclusion

Skeletal anchorage is making a tremendous impact on the orthodontic profession. Their possibilities require the orthodontist to re-evaluate his or her treatment planning process. The ability of skeletal anchorage to control the vertical dimension and anterior-posterior anchorage requirements can now allow us to achieve acceptable non-surgical results in some cases.

References

1. www.hugodeclerck.net
2. Cope J. Orthodontic Products. Special Edition; 18-22.  May 2009
3. Chung KR, Kim SH, Kook YA. The C-orthodontic micro-implant. J Clin Orthod. 2004; 38:478-486.
4. Dalstra M, Cattaneo PM, Melsen B. Load transfer of of miniscrews for orthodontic anchorage. Orthodontics. 2004; 1:53-62.
5. Park HS. An Anatomical study using CT images for the implantation of micro-implants. Korean J Orthod. 2002; 32(6): 435-41.
6. DeVincenzo JP. A new non-surgical approach for treatment of extreme dolichofacial malocclusions. Part 1. Appliance design and mechanotherapy. J Clin Orthod 40(3): 161-170, March 2006.
7. Noble J, Karaiskos NE, Hassard TH, Hechter FJ, Wiltshire WA; Stress on bone from placement andremoval of orthodontic minscrews at different angulations. J Clin Orthod. 43:332-334, May 2009.
8. Gray JB, Steen ME, King GJ, Clark A.E. Studies on the efficacy of implants as orthodontic anchorage. Am J Orthod. 83:311-317, 1983.