Narrow palate or palate that is not large enough. What does it mean?
I put a Keynote presentation online entitled “déficiences transversales du maxillaire” (transverse maxillary deficiencies). The following text is a similar version with more descriptive text to help the uninitiated reader to understand, because the Keynote presentation is made for dentists. This Keynote is a course that I presented at the continuing education of the Université de Montréal and Université de Laval. Have a nice reading.
Unilateral posterior crossbite
The upper jaw is wider than the lower jaw in a way that the upper teeth are over the lower teeth (see picture of Class I normal dentition). When the maxilla is insufficiently developed in width, we will notice that the upper teeth are inside the lower teeth instead of being over them.
The case below illustrates a right posterior crossbite. The arch midlines do not fit (blue arrows). The mandibular midline deviates to the right (yellow arrow) on the same side as the crossbite.
In fact, the whole lower jaw deviates to the right. The crossbite starts at the upper right primary canine and spreads to the last right molar. The upper teeth are thus inside the lower teeth whereas it is not the case on the left side.
If we manipulate the lower jaw, we can put in evidence the prematurity on the primary canines (red arrow) that makes the lower jaw deviate.
We then say that the jaw is in centric relation, meaning that the left and right joints are centered in their respective articular fossa.
It is from this point of contact that prematurity causes an interference that is said to be functional that forces the child to deviate the lower jaw to the right until the complete jaw closure.
This interference probably appeared when the primary (lacteal) canines erupted, that is around the age of 16 to 22 months, thus limiting the development of the palate in width.
It is important to understand that this deviation of the lower jaw to the right or to the left has an impact on facial symmetry.
In fact, this causes asymmetry on the same side as the crossbite. On the picture on the opposite side, notice the left side of the jaw that seems longer than the right side. The left angle of the mandible is not equal to the right angle of the mandible.
The center of the chin (red line) does not fit with the center of the face.
This type of deviation is not permanent in a growing child. Correcting the width of the palate with a rapid palatal expander is enough to prevent the mandible from deviating on one side or another and for the face to regain a better symmetry.
However, when growth is complete, asymmetry caused by the functional shift has good chances of becoming a permanent skeletal asymmetry.
Bilateral posterior crossbite
The lack of development of the maxilla in width can be such that the relationship between the posterior and lower teeth is inverted on both sides instead of only one. Such a palatal constriction causes a bilateral posterior crossbite.
The maxilla is then significantly smaller, narrower. The lack of space, when all adult teeth are in the mouth, will be much more important. Often, a bilateral crossbite will be associated with chronic mouth breathing.
To breathe through the mouth, the tongue must not be in contact with the palate, otherwise air would not pass through. The tongue thus does not participate in the development of the palate in width. To understand the role of the tongue and mouth breathing, I often use the following metaphor: small child, small tongue, big child, bigger tongue.
As the child grows up, the tongue grows proportionally. Yet, if the tongue is never in contact with the palate because the child breathes almost exclusively through the mouth (the nose being blocked permanently either because of allergies or any other mechanical cause), the tongue does not participate in the development of the maxilla, because it is most often located between the teeth in the lower jaw. The treatment, apart from palatal expansion with an expansion appliance, has to include the search for causes of nasal obstruction and mouth breathing.
Nevertheless, there will be no functional shift on one side or another unlike what we find when a unilateral crossbite is present.
Rapid palatal expander
A rapid palatal expander is made of a micrometric screw that is incorporated into either a metal framework (Hyrax) or an acrylic structure (HAAS). The appliance is retained on the teeth either by bands, or by a covering made of acrylic covering the teeth completely and bonded to the posterior teeth. The screw allows the increase of the gap of both the left and right components of the appliance and, at the same time, causes a disjunction of the palatal suture.
The rapid palatal expander is bonded to the teeth to allow a better lateral pressure.
Sometimes, it is necessary to expand the palate at the same time as we have to move teeth in the sagittal plane (front-back). The Pendulum is this kind of expansion appliance that allows the molars to move back by using the palatal vault and the premolars as anchor points to push on the molars.
In image A of the figure above, you will see that the expansion screw is open and that the pink plastic pieces are separated from one another. The expansion phase is thus completed. The stabilization wires are cut at that moment to allow the distallization springs to move the molars back (green arrows, image A). In image B, you will see that a space was created between the premolars and the molars. The distallization springs which had been preactivated before the bonding of the appliance were deployed in the direction indicated by the blue arrows. The movement of the springs is a rotation movement (as to unscrew a lid) which is done in a plane parallel to the palatal vault, which prevents the molars from tipping over. Thus the molars are straighter after moving back.
The second premolars are then disengaged from their occlusal rest and they start moving back (green arrows, image B). In image C, the spaces are now between the first and second premolars. The occlusal rest on the first premolars are cut in turn and they will then be able to move back in the space newly created. This type of appliance was used in the treatment of the case explained in detail on the page of treated case/Class II division 2.
How is it possible to expand the palate?
The maxilla is a paired bone, meaning that there are 2 of them in the human body. It is made of a left maxillary bone and a right maxillary bone. These 2 hemimaxillas are connected together by the intermaxillary suture (red arrows) and to other bones of the skull, such as the zygomatic bone, the palatine bone, the sphenoid bone, to name only these ones.
When an individual is growing, the suture that connects 2 consecutive bones contains cartilage. The presence of this cartilage allows the disjunction of the hemimaxillas under the force created by the daily activation of the expansion jackscrew depending on the duration and the amount prescribed by the orthodontist to obtain the required width.
At the end of the expansion, a space between both incisors will often be seen (arrow #3). The presence of this space or diastema proves that the 2 hemimaxillas were adequately separated (arrows #1) at the intermaxillary suture level during the activation of the appliance (arrow #2).
The gap between both incisors can be seen well in front view. This space will close up with the traction of gingival fibers attached to teeth. This movement starts as soon as the appliance is no longer activated.
It is normal to observe sensitivity in the incisors while the gap increases during the activation of the appliance.
Some patients will even feel a sensation of pressure at the base of the nose between the eyes. As you can see it on the X-ray, the maxillary disjunction has repercussions on all other sutures of the maxilla and these sutures remotely feel the pressure applied by the disjunctor. The sutures adjust and remodel themselves to absorb the pressure.
Function of the palatal expansion device
The appliance is used to increase the width of the palatal vault and allows the coordination of the width of the upper dentition with the width of the mandibular dentition. On the opposite side, the expansion was obtained by a banded appliance for which the screw is soldered into acrylic plates surrounded by a metal framework. The bands of the appliance are bonded by spring helices which allow a “derotation” movement as shown by the blue arrows. This derotation movement allows the first molars to move back by 1 to 2 mm, because the appliance is held anteriorly by occlusal rests bonded to the premolars. Palatal expansion allows the anterior part of the dentition to be rounder. It becomes less ogival.
This appliance drawing was designed by James Hilgers and published in the Journal of Clinical Orthodontics en 1991. JCO 1991, v25, #8, p491-497 JCO-ONLINE.COM.
It is thus an appliance that I have been using for 20 years just like the Hyrax. Each has its benefits and disadvantages, but their efficiency in terms of expansion is completely similar. They have their precise indications and it is up to the orthodontist to choose one model over the other.
Note that another design of expansion appliance using occlusal coverage, bonded to posterior teeth, without bands, exists. I prefer banded appliances over bonded appliances simply because it is much easier and much shorter to bond and remove a banded appliance than an appliance using occlusal coverage. With that being said, appliances using occlusal coverage are good appliances and I sometimes use them. Even then, precise indications guide my choice.
Duration of the palatal expansion
The expansion of the micrometric screw is done with one activation per day which represents 0.2 mm to 0.25 mm per day. The appliance thus needs to be activated during about thirty consecutive days (1 month) to obtain a 6-mm expansion. A check-up visit is done 14 to 15 days after starting the expansion to ensure:
that everything is going well,
that activation is done correctly,
that the appliance is bonded correctly,
to confirm the number of remaining days of activation.
About 30 days after starting the activation, another check-up visit is necessary. The orthodontist then verifies the transverse relationship (width) of the maxilla with the mandible. You then stop activating the appliance.
The expansion appliance will stay in place for 6 months. Thus, this makes up a total of 7 months between the moment the appliance is put in place and when it is removed.
Thus, the duration of the treatment with this appliance is generally 1 month of activation and 6 months of retention.
Turns or activations of the expansion device
The posology of the number of days of activation (1 activation/day) is prescribed depending on the case. This can vary from 14 to 40 days of activation. It is important to understand that one activation corresponds to one-quarter (¼) turn and not a complete turn or revolution of the cylinder.
To understand well how it works, first it is useful to know how a palatal disjunctor screw is made.
A palatal disjunctor is made of rigid connectors and framework (green arrows). A never-ending screw joins both parts of the framework. A perforated cylinder is located in the middle of the screw, which makes 4 holes in the cylinder.
How to activate the screw of the expander
A plastic handle with a rigid metal rod in one end is used to activate the screw.
1- Insert the round rod into the little hole on the perforated cylinder. Push the rod in until it is stopped by the step bend of the rod. Do not be afraid to touch the palate, because the rod is stopped by the step bend.
2- Push the plastic handle backward. The cylinder will make a revolution backward. A revolution represents ¼ turn or 90°.
3- As this backward revolution of the screw occurs (blue arrow), the metal rod makes a revolution on the opposite side (red arrow) in the plastic handle.
4- At the end of the revolution, a new hole has appeared at the front. It is important to push the handle until the hole appears.
5- Remove the rod by making a vertical movement downward (red arrow). It is important that the child has his mouth wide open and does not push his tongue upward against the handle.