A posterior crossbite is a condition where the upper and lower back teeth do not mesh together properly due to the narrow width of one arch compared to the other. The top left picture shows a buccal crossbite (“scissor bite”) where the upper teeth are too far outside of the lower teeth and the chewing surfaces of the back teeth do not contact at all.
The bottom left picture shows the most common presentation of a posterior crossbite. The upper jaw is narrow in width compared to the lower jaw and the upper back teeth sit closer towards the middle of the mouth relative to their lower counterparts.
A posterior crossbite can present as a unilateral or bilateral condition. Most often, a unilateral posterior cross bite is due to the narrowness of both sides of the upper arch. However, the crossbite is present on one side only of the bite due to a shift of the bottom jaw to the affected side as shown in the above left picture. The above right picture shows a bilateral posterior crossbite with a constricted upper arch and coincident dental midlines, thus indicating a lack of a shift in the bite.
The mid palatal suture is an anatomical structure where the two halves of the palate meet and, in a young patient, is a growth center. A constricted palate can be expanded while this suture is still immature and patent. The two halves of the palate can be spread apart and the suture remodels and new bone is formed in the area of tension. Expanding the palate creates a larger circumference for the dental arch and allows more space for teeth to erupt into the widened arch.
This patient presented with a unilateral posterior crossbite on the left side and anterior crossbite. A fixed expander with partial braces or a removable expander with springs can be used to correct both the anterior and posterior crossbite.
The above right picture shows correction of the anterior and posterior crossbite which created sufficient space for the eruption of the upper lateral incisors.
Treatment of the posterior crossbite involves insertion of removeable or fixed appliances that deliver expansion forces to the mid palatal suture to increase the width of the upper jaw.
There are many variations of expanders that your orthodontist may choose from. This depends on the type of forces required to expand the palate, the age of patient, and stage of eruption of the permanent teeth.
The top left image is a removable expander which is often used in children in the early mixed dentition. The top right image is a quad helix appliance which is cemented to the primary or permanent molars. This appliance is fixed in place and can be activated by your orthodontist to increase the force of expansion as needed.
The bottom left image is a Haas expander. This expander is also fixed and has acrylic shelves that contact the palate to deliver the forces closer to the suture.
The bottom right image is the hyrax expander which is most commonly used fixed appliance.
Expansion of the palate increases the circumference of the arch and creates additional space for the permanent teeth to erupt. You can see in the right picture above the additional space created post-expansion which allowed the permanent lateral incisors to erupt into the arch.
This patient presented with a bilateral buccal crossbite or scissor bite. This bite relationship could be due to narrow lower arch, wide upper arch or, more likely, a combination of both. The picture on the right shows correction of the discrepancy in the width of the two arches and eruption of the permanent teeth into proper bite.
Expansion of the palate in adults is challenging since the mid palatal suture is closed and the two halves of the palate can not be split and pushed apart with conventional expanders.
The above left image shows application of Temporary Anchorage Devices (TADs) in the palate. Four mini implants are placed in the palate and the expander is directly attached to the implants and applies the force of expansion directly on the bone and not through a connection to the molar teeth.
The above right image shows Surgical Assisted Rapid Palatal Expansion (SARPE), whereby an orthognathic surgeon releases the sutures in the maxilla to allow expansion of the palate. This procedure is done under general anesthesia.