A crossbite develops when one or more upper teeth sit inside or behind the lower teeth instead of slightly outside them. It affects roughly 7 to 9% of children and adolescents, making it one of the more common bite problems orthodontists see. The causes range from inherited jaw structure to childhood habits, breathing patterns, and even facial injuries.
How a Crossbite Differs From a Normal Bite
In a normal bite, the upper teeth rest just slightly outside the lower teeth, like a lid fitting over a box. A crossbite reverses that relationship in one or more spots. It can show up in the front teeth, the back teeth, or both, and it can affect one side of the mouth or both sides.
A posterior crossbite means upper back teeth sit inside the lower back teeth. This is the most common type, occurring in about 7.6% of children. A unilateral posterior crossbite (affecting only one side) is more frequent than a bilateral one, which affects roughly 2.5% of children. An anterior crossbite means one or more upper front teeth sit behind the lower front teeth. When all the upper front teeth are positioned behind the lower ones, it’s typically called an underbite. There’s also a less common variation called a buccal crossbite, where upper back teeth bite completely outside the lower teeth rather than inside them.
Genetics and Jaw Size
The single biggest factor behind most crossbites is the size and shape of the jaws you inherit. The upper jaw (maxilla) and lower jaw (mandible) are each partially genetically predetermined in their growth. When one jaw is too large or too small relative to the other, the teeth don’t line up the way they should. A narrow upper jaw paired with a normal or wide lower jaw is a classic recipe for a posterior crossbite.
Twin studies have been central to understanding this. Researchers comparing identical and fraternal twins found significant differences in jaw dimensions between the two groups, confirming that bone structure has a strong hereditary component. Certain areas of the jaw appear to be under greater genetic control than others. The side of the lower jaw and the chin region, for example, seem more genetically driven, while other areas respond more to environmental pressure over time.
That said, crossbites are almost always multifactorial. Genetics sets the stage, but environmental factors often determine whether a crossbite actually develops and how severe it becomes. Think of it as a spectrum: some people’s jaw shapes are so strongly inherited that a crossbite is nearly inevitable, while others develop one primarily because of habits or health conditions during childhood.
Thumb Sucking and Pacifier Use
Prolonged sucking habits are one of the most well-documented environmental causes of crossbites in young children. When a child sucks a thumb, finger, or pacifier for extended periods, the pressure reshapes the developing palate. The upper jaw narrows, and the lower jaw may shift forward or to one side, creating a posterior crossbite.
The critical factor is duration. Research consistently links posterior crossbite development to pacifier use lasting more than one to two years, with the risk climbing further when the habit continues past age three or four. Studies have found strong correlations with daily pacifier use beyond 36 months. The good news is that if the habit stops early enough, some of the changes to the palate can self-correct as the child grows. Orthodontic pacifiers, designed to reduce pressure on the palate, show weaker correlations with crossbite development, though they don’t eliminate the risk entirely.
Mouth Breathing and Airway Problems
Chronic mouth breathing during childhood can physically reshape the upper jaw over time, and this is a cause many parents don’t suspect. When a child breathes through the mouth instead of the nose, whether from allergies, enlarged tonsils or adenoids, or a deviated septum, a chain of postural changes follows.
The tongue drops to the floor of the mouth instead of resting against the palate. Normally, the tongue’s gentle upward pressure helps the upper jaw grow wide and flat. Without that pressure, the palate narrows. At the same time, the cheek muscles tighten from the open-mouth posture and push inward on the upper back teeth. The combination of lost tongue support and increased cheek pressure squeezes the upper arch, making the upper teeth collapse inward past the lower teeth. This creates a posterior crossbite and, in some cases, a forward positioning of the lower jaw that contributes to an anterior crossbite as well.
Children who mouth-breathe often develop a longer face shape, lips that don’t close comfortably at rest, and an increased vertical height of the lower face. Addressing the underlying airway problem is an important part of correcting the crossbite, because if the breathing pattern doesn’t change, orthodontic correction is more likely to relapse.
Tooth Size, Crowding, and Late Baby Teeth
Sometimes the jaws are fine but the teeth themselves create the problem. If the upper teeth are smaller than average or the lower teeth are larger, the size mismatch can push individual teeth into crossbite. Crowding in the upper arch, where there simply isn’t enough room for all the teeth to line up correctly, can force one or two teeth inward past the lower arch.
The timing of baby tooth loss matters too. If a baby tooth is lost too early, the surrounding teeth can drift into the empty space and alter the path of the permanent tooth coming in behind it. The permanent tooth may erupt in a crossbite position simply because its neighbors have shifted. Conversely, a baby tooth that hangs on too long can deflect the incoming permanent tooth into the wrong position.
Facial Trauma and Jaw Injuries
A significant blow to the face during childhood can alter jaw growth and lead to a crossbite later. In children, 50 to 80% of lower jaw fractures involve the joint area (the condyle) or the angle of the jaw. These are the growth centers of the mandible, and damage to them during development can cause one side of the jaw to grow differently than the other.
When a condyle fracture heals with the jaw slightly shortened on one side, the bite shifts. The lower jaw may deviate toward the injured side, creating a unilateral crossbite. In severe cases, the joint can fuse (a condition called ankylosis), which restricts jaw growth on that side entirely and leads to pronounced facial asymmetry. Even fractures that heal well can subtly change how the jaws relate to each other, especially if they occur before the child has finished growing.
What Happens if a Crossbite Goes Untreated
A crossbite doesn’t just affect how teeth look. When the bite is off, the jaw often shifts to one side to compensate, a pattern called a functional shift. Over time, this asymmetric jaw movement can lead to uneven wear on the teeth, jaw pain, and dysfunction in the jaw joint. Research has linked untreated unilateral posterior crossbites to temporomandibular disorders (TMD), including displaced joint discs, facial pain, and clicking or locking of the jaw. In one documented case, correcting the crossbite allowed a displaced joint disc to return to its normal position and resolved most TMD symptoms.
In growing children, the functional shift is especially concerning because the jaw is still developing. Years of chewing and resting in a shifted position can lead to skeletal asymmetry, where one side of the face grows differently than the other. This is one reason orthodontists generally recommend correcting crossbites early.
When Crossbites Are Typically Corrected
Most orthodontists prefer to treat posterior crossbites during the mixed dentition stage, when children still have a combination of baby and permanent teeth. The majority of studies on crossbite correction involve children between ages 5 and 10. At this age, the upper jaw’s midline suture hasn’t fully fused, making it easier to widen the palate with an expansion device.
Treatment usually involves a palatal expander, a fixed appliance that gradually widens the upper jaw over weeks to months. After expansion, retention is important. Research suggests that wearing a retainer 24 hours a day for about six months after correction is generally enough to prevent the palate from narrowing back down. The retention period across studies ranges from 4 weeks to 16 months, but six months appears to be the threshold for stable results in most patients.
Anterior crossbites can sometimes be corrected with simpler appliances or braces, depending on whether the problem is just the teeth or involves the jaw structure itself. In adults whose palatal suture has fully fused, widening the upper jaw may require a surgical assist before expansion, since the bone is no longer flexible enough to respond to an appliance alone.