Orthodontics is a dental specialty that prevents, diagnoses and treats dental irregularities. Orthodontics addresses problems such as overbite - upper front teeth stick out over the lower teeth; underbite - lower teeth or upper teeth are too far forward/back; crossbite - the teeth ends do not meet; open bite - space between the biting surfaces of the front and/or side teeth when the back teeth bite together; misplaced midline - center of the upper front teeth does not line up with the center of the lower front teeth; spacing - too much room between teeth or missing teeth; crowding not enough space for teeth to erupt. The premature loss of baby teeth, retention of baby teeth, thumb - or finger-sucking, accidents and certain types of dental disease can also result in a need for orthodontic treatment. These conditions are referred to as malocclusions, and can cause difficulty in chewing and speaking, wear away enamel on healthy teeth and putting excess stress on gum tissue and surrounding bone, and increase the risk of tooth decay and gum disease. Many different types of appliances, both fixed and removable, are used in orthodontics. These appliances function by placing gentle pressure on the teeth and jaws. Common appliances include: braces; special fixed appliances (control thumb sucking); fixed space maintainers - used in the premature loss of baby teeth to keep the space open until the permanent tooth erupts; removable space maintainers; jaw repositioning appliances - helps train the jaw to close in a more favorable position; lip and cheek bumpers - keep the lips or cheeks away from the teeth; palatal expander (widens the arch of the upper jaw); removable retainers; headgear.