Teeth are not nails!

This title seems a bit odd for an orthodontic article, but this analogy illustrates well what does not happen by moving the teeth in the jaw bone during orthodontic movements! However, this is what many believe is happening, that is to say that a tooth behaves like a nail that can be sunk into, pulled out or moved into a piece of wood as if it were hit with a hammer!

Orthodontic movements and dental displacements

The main movements we perform to move teeth orthodontically can be categorized as follows: Intrusion or ingression: moves a tooth vertically to the bone either upward for the upper teeth or downward for the lower teeth. This may be necessary, for example, when an excessive overbite of the anterior teeth is present as in the case of Class II division 2. The orthodontic intrusion of a tooth is a vertical movement in the apical direction (toward the root) of the tooth. Extrusion or egression: moves a tooth vertically toward the opposing arch either downward for an upper tooth or upward for a lower tooth. The orthodontic extrusion of a tooth is a vertical movement in the coronal direction (toward the crown) of the tooth. This type of orthodontic movement is often used to close certain types of anterior open bite where the front teeth do not touch. Translation or horizontal movement in the anteroposterior direction: this movement is indicated, for instance, to close a space left by an extraction or to move backward front teeth that are too protrusive (excessive overjet).

Vertical movements

Intrusion or ingression of teeth

It is often necessary to move a tooth in the vertical dimension. This may be necessary in the presence of an excessive overbite of the anterior teeth or when a posterior tooth got “longer” because it is no longer in contact with a tooth in the opposite arch.

(A) When a tooth is “brought downward” or intruded, there is a misconception that it gradually sinks into the bone as it gets lower. (B) If this were the case, at the end of the intrusion, it would be fully inserted into the bone and almost invisible.

(A) What happens instead is that the level of bone gets lower at the same rate as the tooth, which maintains its original length and does not disappear into the bone. (B) The end result is that the tooth AND the bone are at a lower level after the intrusion.

Examples of dental intrusion to reduce an excessive overbite.

In the following two cases, the upper and lower incisors have been “intruded” into the direction of the arrows; the upper ones were thus “brought upward” and the lower ones were “brought downward”. Despite several millimeters of vertical movement, the length of the teeth did not decrease because the teeth have not been moved into the bone. Instead, the gums and bone surrounding the teeth followed them in their vertical movement.

(A) Intrusion of incisors to uncover them and make them visible and functional. The upper incisors appear even longer after the treatment because the contour of the gums has been changed. (B) Similar case: after being intruded, the teeth have the same length.

(A) The upper molars, that are no longer supported, got longer (extrusion) beyond the upper arch plane (blue line). (B) After corrections, the molars were “lifted” by a few mm (intrusion) to a normal level which will allow adequate restoration of the lower toothless space. The teeth have maintained their length and the level of bone and gum followed the dental movement. The posterior devices are held in place to stabilize the teeth which have no lower support.

➡ To see more examples of vertical adjustment of the level of teeth that allows the optimization of the smile esthetics, the gingival contour and incisal edges of the teeth.

Extrusion or egression of teeth

This movement aims at bringing teeth of opposing arches (top and bottom) that do not touch closer to one another in order to create overbite to the anterior teeth or to enable posterior teeth to touch.
The typical example is the closure of anterior or posterior open bites.

(A) Extrusion of a tooth may suggest that this tooth gets longer and comes out of the bone during this process. (B) If this happened, the tooth would become longer as it would come out of the bone.

(A) During dental extrusion or egression, the level of bone migrates at the same rate as the tooth and the tooth does not come out of the bone. (B) After extrusion, the tooth has the same length as at the beginning, but the bone level was changed.

(A) Anterior open bite to be closed by the extrusion of the anterior teeth (arrows). (B) At the end of the corrections, the incisors were displaced vertically by several millimeters. The teeth have kept their same visible length and the bone and gum have followed them.

EXCEPTIONS

Normal eruption

The first exception to this “rule” would be during the process of normal tooth eruption. When a tooth breaks through the gum, it gets longer and comes out of the bone and gum until it reaches its final position, which can take several years. The tooth that erupts thus becomes gradually more visible as it comes out. The following example illustrates well this eruption phenomenon.

During normal eruption, the teeth come out of the bone and gums and become progressively longer as shown by this young woman’s incisors. The numbers indicate the patient’s age.

(A) In this example, an 8-year-old little boy has a significant open bite (teeth that do not touch in front). (B) The use of a tongue crib blocks the tongue and allows the upper incisors to “go down”, dragging the gingiva and bone with them. Both central incisors are lower, but of the same length because the bone and the gingiva followed. These changes have occurred in only 4 months. This vertical movement will be later supplemented by the natural eruption of teeth which will then allow the teeth to come out slightly of the gingiva and look longer. This may take several years to complete.

Periodontal diseases

Another situation where the vertical movement of a tooth would not be accompanied by an equivalent movement of the gingiva and the supporting bone surrounding the tooth is in the presence of periodontal diseases around a tooth to move. This state of inflammation around the tooth can destroy the tissues and prevent the bone and gingiva from remaining attached during movement of the tooth. This is why it is important to have a healthy periodontium during an orthodontic treatment to prevent the loss of tissues that support the teeth (bone and gum).

(A) In the presence of periodontal diseases affecting the alveolar bone (arrows), the tooth will “come out” of the bone and will get longer once out of the gingiva (B). (C) The part remaining in the bone (root) will be shorter that initially while the outer part, which can now include part of the root, will be longer. This undesirable phenomenon will not occur in the presence of a healthy bone free of infection or inflammation.

(A) The periodontal disease having caused loss of tissues (bone and gum), the affected teeth appear longer (arrows). (B) A posterior tooth got longer and lost bone. Therefore, it seems longer. (C) X-rays show the severity of bone loss, which is why the teeth appear longer. If we attempt to extrude more such a tooth affected by an active periodontal disease, it will come out more from the bone and will become longer. It is essential to control periodontal diseases before an orthodontic treatment to avoid additional loss of tissues.

(A) The upper molars lengthened in the absence of opposing teeth and appear longer because bone was loss around these teeth. (B) The anterior teeth also lost supporting tissues and are longer. (C) The upper teeth of this case worn down the gingiva and the bone in front of the lower incisors, making them appear longer. Moving all these teeth orthodontically will require special care and the periodontal diseases will have to be controlled to prevent the teeth from “coming out” of the bone.

Multiple simultaneous movements

The different types of movements described in this page do not occur individually and it is common to see that different teeth require different movements to move them toward their ultimate goal, contributing to obtain a normal or ideal occlusion.

Intrusion and extrusion movements often occur together in the same mouth. (A) This malocclusion presents incisors that are too elongated; they will have to be “intruded” in the direction of the arrows. The canines (*) that have not finished erupting will become longer as they come out of the gum, which is due to the normal eruption process. (B) After the corrections, the canines “went down” to an ideal vertical level. The incisors were moved several mm, but kept the same crown length because the bone and gingiva have followed the movement of the teeth.

Horizontal movements (translation)

(A) During the horizontal movement of a tooth (translation), you would think that the tooth is gradually moving (A, B and C) while staying straight in the bone, but it is not the case. (B) Because the bone resists to movement, the crown of the tooth (the part that is out of the bone) tilts forward or backward in the direction of movement. (C) The tooth must then be uprighted so the crown and the root become straight again, but in a new, more advanced position. This phenomenon is one of the reasons why the closing of large spaces by translational movement may take a relatively long time.

(A, B and C) Unfortunately, a tooth does not move right that way by remaining straight and migrating through the bone during a horizontal movement.

(D) To move a tooth over a large distance, (E) it must first tilt in the direction of movement and (C) be uprighted afterward.

(A) Closing of a large space by moving the posterior teeth forward by a horizontal movement (translation). The molar was already tilted at the beginning, so the roots had to be moved more than the crown to upright the tooth. (B) After the correction, the space is completely closed. (C) Closing of an 11-mm space by a forward horizontal movement of the molars.

Reality: a combination of simultaneous movements

• In fact, orthodontic movements are a combination of all these movements that can occur simultaneously: intrusion, extrusion, translation/uprighting.
• However, if the situation justifies it, a movement can be specifically performed on a tooth without moving others. The orthodontist will then adjust its “mechanics” (corrective appliances/braces) accordingly.

BEFORE: Most of the time, several teeth are uneven and orthodontic corrections are aimed at performing a combination of intrusion and extrusion movements to level everything.

AFTER: Intrusion and extrusion movements maintain the length of the tooth, but alter the bone (and gingiva) level which follows the tooth in its movement.

Buccolingual movements

• A buccolingual movement is performed when it is required to move teeth either forward (buccal) or toward the tongue (lingual) to align the teeth in the dental arch.

(A) View from above of misaligned teeth (nails). (B) After the corrections, the teeth are aligned along an imaginary line (blue dotted line). Each tooth should normally keep the bone around it while it is moved in one direction or another.

• This movement, no matter how small it is, is required in almost all cases of orthodontic corrections.

➡ For more Information on dental movement in orthodontics.

Teeth move in the direction of the applied force.

Different types of orthodontic movements

Depending on where an orthodontic force will be applied (arrows) with respect to the centre of rotation of the tooth (red dot), a different movement can be generated.