Forces In Orthodontic Treatment

Newton’s third law of motion states that for every action there is an equal and opposite reaction. There are many situations in orthodontics, however, where this equal and opposite reaction is not desired. Some common orthodontic treatment goals where these reciprocal forces are of particular concern include:

1. Retraction of the anterior (front) teeth without protraction of the posterior (back) teeth.

2. Protraction of the posterior teeth without retraction of the anterior teeth.

3. Preventing protraction of the anterior teeth while using intraoral appliances to move molars distally e.g., Pendulum, Distal Jet.

4. Preventing undesired intrusion or extrusion of teeth.

5. Decreasing the amount of dental change and increasing the amount of skeletal change produced by orthopedic maxillary expansion.

In order to best meet the objectives of a given case, orthodontists have developed multiple methods of mechanical orthodontic anchorage, each with advantages and disadvantages. These anchorage methods include: 1) extraoral appliances, 2) dental anchorage, 3) intraoral appliances, and most recently 4) osseointegrated and dental implant-associated appliances.

Extraoral Appliances
Traditionally, the primary form of increasing anchorage was the use of extraoral traction (Figure 1) (Kingsley, 1880; Angle, 1897). By placing the reciprocal force on the back of the skull or neck, appliances such as facebows (Kloehn, 1947) and J-hook headgears (Merrifield and Cross, 1970; Vaden et al., 2000) allowed for distal movement of teeth without a reciprocal anterior force on the teeth. Extraoral traction remains an excellent means of providing maximum anchorage, but variable patient compliance is a significant disadvantage.

Dental Anchorage
The second category of orthodontic anchorage is dental anchorage. When teeth are pulled toward each other, teeth with greater root surface area tend to move less than teeth with less root surface area (Freeman, 1965; Gianelly and Goldman, 1971). With this principle in mind, consolidating a group of teeth into a single dental unit will result in the reciprocal tooth movement being reduced. The most common example is canine retraction following extraction of the first premolar. By lacing the second premolar, first molar, and second molar together, they form one dental unit with much greater total root surface area to pull against. While this dental unit will move mesially to some degree during canine retraction, the canine will move distally a far greater amount. In cases requiring minimum or moderate anchorage, dental anchorage may be acceptable.

Intraoral Appliances
The next category of orthodontic anchorage is the use of intraoral appliances such as the transpalatal arch and the Nance button (Figure 2). These appliances are thought to primarily work by consolidating teeth into dental units, but have the added advantage of crossing the palate to connect the right and left posterior teeth together into a single dental unit. The Nance button further attempts to increase anchorage by placing an acrylic button on the anterior palate. While intraoral appliances can serve multiple functions, the scientific evidence shows that they have little efficacy as anchorage devices (Bondemark and Thornéus, 2005; Zablocki, 2005). In the end, the reciprocal force still is placed on teeth, resulting in undesirable tooth movement in a case requiring maximum anchorage.

Osseointegrated and Dental Implant-Associated Appliances The final category of anchorage appliances are osseointegrated and dental implant-associated appliances such as implants (Figure 3) and mini-screws. Similar to extraoral traction, osseointegrated and dental implant-associated appliances allow for maximum anchorage by placing the reciprocal force on something other than teeth. The advantage of this approach over extraoral traction is that the need for patient compliance is minimal. For this benefit, however, there is the monetary cost of the implant placement as well as any surgical risks.
The use of implants for orthodontic anchorage first was reported in dogs (Gainsforth and Higley, 1945). Unfortunately, all of the implants failed and the idea was discarded for nearly 35 years. It was not until the Brånemark group’s (Adell et al., 1981) landmark, long term clinical study showing overwhelming success using titanium implants to support fixed prostheses, that implants became a real option for orthodontic anchorage. Since that study, multiple investigators have shown the potential for osseointegrated and dental implant-associated appliances in orthodontics (Shapiro and Kokich, 1988; Odman et al., 1988; Ong et al., 1988; Goodacre et al., 1997).
The addition of an implant, mini-screw, or micro-implant to an orthodontic treatment plan has four additional considerations: increased cost, bone availability problems, access problems, and increased treatment time (Goodacre et al., 1997). The Brånemark study found that long term stability of implants is achieved with a proper surgical technique followed by no loading for 3-4 months in the mandible and 5-6 months in the maxilla (Adell et al., 1981). Many interdisciplinary teams also advocate 10-12 weeks of healing before loading implants specifically for orthodontic anchorage (Shapiro and Kokich, 1988). Because they do not require osseointegration, treatment time is reduced with mini-screws and micro-implants. Increased cost, bone availability problems, and access problems, however, remain considerations.

Author Bio:

Dr. Matthew Dunn is a board-certified orthodontist with offices in Phoenix, Litchfield Park and Wickenburg, Arizona. He completed both his dental and orthodontic training at the University of Michigan. Dr. Dunn has won multiple national research awards for his studies on the effects of pharmacological agents on tooth movement. He lives in Goodyear, Arizona with his wife Courtney Dunn (also an orthodontist) and their three children.

http://www.dunn-orthodontics.com