The upper teeth should fit outside the lower teeth like a lid on a box. If the upper jaw is too narrow, the lower jaw usually swings to one side to allow the back teeth to mesh. A posterior cross-bite results. Posterior cross-bites can result in uneven wear of the teeth and can place extra strain on the jaw joint resulting in tempero-mandibular joint problems. Orthodontic treatment involves the use of a fixed or removable appliance to widen the upper jaw (maxillary expansion). Once the upper jaw has been widened, the lower jaw can close normally.