Common Side Effects of Routine Orthodontic Mechanics

The application of simple orthodontic mechanics often result in side effects that move teeth in directions that are difficult to predict. However, with an understanding of some basic wire-bracket relationships combined with the mechanical principles that underlie how teeth move, these seemingly bizarre movements can be not only predicted, but also eliminated.

In this article, we will focus on two very common types of tooth movement that occur in the routine application of orthodontic forces. Although the interplay between these movements complicates understanding of the reasons behind these movements, the final result of the force systems can be understood and accounted for.

The first common movement occurs because orthodontic forces are rarely directed through the center of resistance of the teeth. Any force that is applied away from the center of resistance of a body produces a rotational tendency, or moment, on that body(1). Dr. Tom Mulligan, in his book "Common Sense Mechanics", refers to this phenomenon as the "cue ball concept." (2). If you wish to explore this concept in greater detail, you can refer to Mulligan's textbook, which is listed as a reference at the end of this article. For our purposes, remember that any time an intrusive force is placed on a tooth, the moment produced creates a tendency for the crown of that tooth to flare. Conversely, when an extrusive force is applied to a tooth, the moment produced creates a tendency for the crown of that tooth to upright or roll lingually.

For example, when opening the bite with a reverse curve arch wire on the lower arch, the extrusive force on the molars, which is applied away from the center of resistance, results in a moment that rotates the crown of the tooth in a lingual direction. Additionally, the intrusive force placed on the incisors (again, the force is applied away from the center of resistance) results in a moment that can result in labial crown movement.

Expression of these side effects can lead to problems in the correction of malocclusions. For example, if, in an effort to open a deep bite, a practitioner places compensating curve in an upper arch wire, the rotational tendency as a result of the forces placed on the upper molars would produce a tendency for those molar crowns to roll to the lingual. This could result in a narrowing of the upper arch width. Since narrow upper arches are often associated with a retrusive mandibular position (3), this change in arch width could make a Class II problem worse. Additionally, the same wire, due to the intrusive force placed on the anterior teeth, could result in flaring of the anterior teeth. Clinically, spaces may appear between the anterior teeth after these forces are applied. So, a compensating curve applied to an upper arch in effort to open the bite could cause a serious worsening of a Class II malocclusion.

The second common movement occurs as a result of the angle of entry of a wire into a bracket. Dr. Gerry Samson identifies the potential problems as a result of angle of entry when he says "The greatest angle of entry determines the greatest moment of couple which determines the equilibrium forces."(4)

This very important quote can be best understood by breaking it down into its component parts. First, let us explore the concept of angle of entry.

The angle of entry of the wire to the tooth can be affected by the malocclusion, archwire bends, and by the tip built into the pre-adjusted bracket. Most often, the greatest angle of wire entry occurs on the upper canine. Many malocclusions have mesially tipped upper canines, while many malocclusions have distally tipped upper canines. Additionally, because in the pre-adjusted appliance the canine bracket has the greatest tip built in, the net result is a wide variety of wire angles of entry which can profoundly change the movements that will be expressed as leveling and aligning occur.

The second portion of Dr. Samson's quote relates to the greatest moment of couple. Dr. Samson defines a couple as "equal and opposite forces (separated by a perpendicular distance) that are not in the same plane." (5) The picture below demonstrates a common example of a couple.

In the bracket/wire relationship in the drawing, the "equal and opposite forces" are a result of the wire contacting the bracket slot in the upper left portion of the slot and the lower right portion of the slot. The two forces produced as a result of this contact are equal, and are directed in opposite directions (depicted by the arrows in the drawing on the right).The result of this force system is a rotation of the tooth in a mesial or distal direction (tipping of the tooth, if we use the orthodontic term).

The final part of Dr. Samson's quote refers to the equilibrium forces. This can be best understood by again viewing the picture above. Because of the angle of entry of the wire into the slot, the wire will be positioned above the brackets to the left of the slot in the picture, and below the brackets to the right of the slot in the picture. Deflecting that wire into those slots will result in (let us assume for discussion that this is an upper right canine bracket) an intrusive force to the teeth to the left of the slot (again, if this is an upper right canine those teeth would be the pre-molars and molars) and an extrusive force to the teeth to the right of the slot (anterior teeth in our example).

Now here is where the prediction of tooth response gets tricky. The intrusive and extrusive forces applied to the teeth as a result of the angle of entry will, because they are forces applied away from the center of resistance, result in rotational tendencies. To explain this, let us look at another example. The picture below is a pretreatment photo of a deep bite patient.

Let's study this malocclusion. First, where will the greatest angle of wire entry be? Look at the canine and imagine a bracket placed on that tooth. Clearly, because of the mesially tipped canine combined with the large amount of tip in the pre-adjusted canine bracket, the angle of wire entry onto that tooth will be a lot larger than on any other tooth. Therefore, this angle of entry will determine equilibrium (where the teeth will go). Visualize a straight wire placed through the canine bracket. Where would that wire lie? Well, it would lie significantly gingivally to the anterior teeth and significantly occlusally to the posterior teeth. Next, ask yourself what forces will be imparted to those teeth as a result of the position of this wire? The answer is an intrusive force to the upper anterior teeth and an extrusive force to the upper posterior teeth. Finally, what kind of tooth movement will occur as a result of these forces? The answer is the moment as a result of the intrusive force on the anterior teeth will result in flaring of those crowns and the moment as a result of the extrusive force on the upper posterior teeth will result in molar and bicuspid lingual crown rotation. So the net result should be bite opening, flaring of the upper anterior teeth and lingual movement (rolling in) of the upper posterior crowns. Let's look at what happened. Here is the initial bracketing photo and the picture after the first three months of treatment.

After 1 month of treatment.

After 2 months of treatment.

After 3 months of treatment.

What happened in this initial leveling and aligning and is it predictable? The bite opened and the upper anterior teeth flared. This is exactly what should have happened. Additionally, the arch width narrowed in the posterior. Look at the bicuspid position pretreatment and after three months. Evaluate the amount of overjet in the bicuspid area. Clearly, these crowns rotated lingually.

This case demonstrates in cases with mesially inclined upper canine crowns, bite opening can occur quickly. Let's look at another example.


After 2 months (top) and 3 months (bottom).

What happened in this treatment? Clearly, the bite opening did not progress as easily as it did in the previous case. Why not? The answer can be discerned by examining the angle of wire entry onto the upper canine. In this case, the distally inclined canine resulted in the wire sitting below the brackets in the anterior teeth, which resulted in an extrusive force to those teeth. Bite deepening and lingual anterior crown rotation (due to the moment produced) occurred. Also, study the position of the upper right 2nd bicuspid. It is much more lingually inclined in the pretreatment photo than it is after one month of treatment. Why did this happen? An understanding of moments and forces provides the answer. The wire angle of entry on the canine bracket resulted in the wire sitting above the posterior brackets. When this wire was engaged, the result was in intrusive force to the posterior teeth. The resulting moment leads to the tendency for buccal crown rotation. The 2nd bicuspid rotated so far to the buccal that it almost went to reverse cross bite.

Understanding the "hows" and "whys" of the side effects of routine orthodontic mechanics allow the practitioner to take steps to emphasize the side effects that will help a particular malocclusion and minimize the side effects that may worsen the malocclusion. For example, in an open bite case with mesially inclined upper canines (see photo below), the leveling and aligning that will occur could worsen the open bite.

To mitigate these effects, the practitioner may want to position the anterior brackets more to the gingival than the original malocclusion would dictate. The anterior gingival bracket position compensates for the bite opening that is common with mesially inclined upper canines.

One common way to help control deleterious side effects is to change the inclination of the canine brackets to make the angle of entry less severe. Another solution is to simply level and align without engaging the canine. Then, after progressing to a relatively stiff arch wire, use a tandem arch wire or spring to engage the canines. This, combined with a stiff base arch wire, will align the canines while not allowing the moment of couple due to the wire angle of entry to influence the rest of the teeth. As the canine uprights, the side effects will not be expressed because the stiff base arch does not allow the deleterious movements.


When an orthodontic practitioner understands wire-bracket relationships and the tooth movements that are a result of these relationships, he or she is better able to predict, and account for, tooth movements that are most likely to occur. This leads to less surprises day to day orthodontic treatment. Lack of surprises makes delivering of orthodontic treatment more enjoyable and rewarding.

  1. Mulligan, T. Common Sense Mechanics in Everyday Orthodontics. CSM publishing, Phoenix, p21, 1998
  2. Ibid., p24-26
  3. Bishara, S. Textbook of Orthodontics. WB Saunders, Philadelphia, Chapter 20, p333, 2001.
  4. Samson, G. Locomotion 1.
  5. Ibid.

The printed article in the magazine can be viewed HERE!