Underbites occur when the top front teeth hit behind the bottom front teeth when the jaw is closed. This is an extremely rare problem which occurs usually because the person's lower jaw is longer than their upper jaw.
Underbites are most commonly treated between the ages of 7 and 10 so that the upper jaw can continue to grow properly. Underbites of all four front teeth suggest a jaw discrepancy that may require advanced treatment. Single tooth underbites are usually treated with simple appliances like retainers.