Cross-bites are a common problem in young kids and if not corrected, they can cause real problems with facial growth and development, jaw joints, and oral function. Obviously, some cross-bites are more detrimental than others, but none of them are good. You may be wondering what a cross-bite is, why it is so detrimental and why there is so much urgency to correct it as soon as it is discovered. In this blog, we will address each of these questions and some of the remedies that may be recommended.
Cross-bite is a generic term used to describe the adverse relationship between teeth when the teeth in the upper jaw are not centered and biting on the teeth in the lower jaw with the upper front teeth just outside the lower teeth, and the upper back teeth biting with their inner bumps or cusps meshing in the grooves and pits in the center of the lower teeth. The upper teeth should naturally bite outside the lower teeth with the some overbite and some overhang outside the lower teeth all the way around the arch or semi-circle that the teeth follow.
This provides maximum aesthetics, chewing power, functional protection for the teeth and jaw joints, and overall oral health. A single tooth, multiple various individual teeth, or a continuous group of teeth may be in crossbite. There are two types of cross-bites; buccal and lingual. If the upper tooth bites inside the lower teeth, or behind the lower front teeth, this is a lingual cross-bite. Technically it should be called a lingual cross-bite, but since this is the most common type of cross-bite it is just called a cross-bite, as it is understood to be referring to a lingual cross-bite. If all the upper front teeth bite behind the lower front teeth, it is commonly called an under-bite instead of a cross-bite.
If an upper side or back tooth bites completely outside the lower teeth, that is referred to as a buccal cross-bite. Since the upper front teeth bite outside the lower front teeth naturally, if there is excessive space between the front teeth it is commonly referred to as over-bite rather than cross-bite. Only the upper back teeth biting outside the lower teeth is referred to as being in a buccal cross-bite. If all the upper back teeth bite completely outside the lower teeth, this is called a Brodie bite, named after Dr. Alan Brodie. This is also called a scissor-bite, because the teeth slide past each other to cut the food like blades on scissors, rather than chewing the food
by mashing it between the teeth.
Cross-bites can occur for many reasons. Oftentimes a single tooth can get misdirected in it’s eruption path and just grows into the arch towards the tongue rather than in line with the rest of the teeth. Usually this is because there is not enough room for the tooth to grow in where it belongs, so it grows in wherever it can work its way into the mouth. This can occur with the front teeth as well as the back teeth, and most often it involves only a single tooth rather than a group of continuous teeth. If a continuous group of teeth is in cross-bite, there is usually an underlying problem beyond just a lack of space or a path of eruption issue.
If there is a unilateral, or one sided, cross-bite of the back teeth it is usually not the teeth that are the problem, there is an underlying skeletal discrepancy that is the root cause. If the upper jaw is narrower than the lower jaw, then the upper teeth will follow an arch or semicircle that is narrower than the lower arch. This may cause either a unilateral or a bilateral cross-bite. If the upper arch is just narrow enough that the lower teeth will not fit inside the upper arch, but not so narrow that the upper arch will fit inside the lower arch, the patient will shift their jaw to one side causing a unilateral cross-bite. A unilateral cross-bite with a functional shift can be hard on the jaw joints and result in asymmetric jaw growth and future TMJ problems. This unilateral posterior cross-bite with a functional shift is much more detrimental than a bilateral cross-bite without a functional shift. The balanced bite, even if it is in cross-bite, is more stable and healthier for the joints and supports normal growth better than any bite with a functional shift.
A Brodie bite (bilateral posterior buccal cross-bite) or a unilateral scissor-bite or buccal crossbite with a functional shift may be due to an underlying skeletal discrepancy, but is most often due to the lower teeth following a severely collapsed arch form. The overall arch form may look more like an old fashioned key hole if the front teeth follow a normal arch form. In this situation, the best way to correct the bite is to get the lower side teeth upright in their supporting bone and establishing a normal arch form that coordinates with the natural upper arch form.
Cross-bites are detrimental in a number of different ways. All forms of cross-bite are detrimental, although some are worse than others. The single tooth cross-bites, especially in the front, can be very hard on the lower front teeth and everything that surrounds them. The small lower front teeth get beat up by the upper front tooth in cross-bite. This can cause the gums to recede, supporting bone to be reduced, and ultimately result in the tooth being lost. Correcting the cross-bite may allow the bone and gums to recover and be restored to a natural position, but this is not a certainty. For this reason, it is critical to correct these cross-bites as soon as they appear. If a front tooth looks like it will erupt into a cross-bite, it is advisable to see an orthodontist about modifying the tooth’s path of eruption and preventing it from growing into a cross-bite. With the front teeth, a preventive approach is always the best, although it may not be corrected successfully; depending on the method of correction and the individual circumstances.
Cross-bites in the back, whether lingual or buccal, single teeth, multiple teeth, unilateral or bilateral, they are all a concern. The single tooth cross-bites are a concern for the same detrimental reasons discussed above with the anterior or front teeth. Multiple tooth cross-bites are not usually as detrimental for the supporting tissues as a single tooth cross-bite, but they can cause some of the same damage if left untreated. Bilateral cross-bites are a concern because they are often a manifestation of an underlying growth discrepancy, and there are often functional deficits and interferences associated with the cross-bites. Because of the cross-bite, the teeth are unable to perform their protective functions and this puts the teeth at greater risk of chipping or fracturing chewing and during normal function.
With a unilateral cross-bite, there are additional functional interferences that cause the jaw to shift to one side in order to bite and chew. On top of the problems caused by a bilateral posterior cross-bite, the unilateral cross-bite has the significant additional problem of the jaw needing to shift to one side to achieve a bite. Constantly biting to one side encourages the teeth on one side to super-erupt, and the jaw to grow longer on one side, creating both a skeletal and a dental asymmetry. The problem with these asymmetries is that they are much easier to create than they are to fix. I the asymmetry persists throughout growth, it may require jaw surgery to correct.