Underbite and overbite

The incisor teeth will become out of alignment and the upper jaw will overshoot the lower jaw. Unless these developing hooks are removed and teeth are correctly re-aligned the condition will become progressively worse. The movement of the lower jaw against the upper jaw in a backwards and forwards motion will be lost. Furthermore, the molar hooks may well dig into the gums of the respective lower and upper jaws. This will cause pain and also prevent the acceptance of the bit.
The side-to-side grinding action of the molars causes them to develop spurs on the outside of the top molars and the inside of the bottom molars. In time this prevents the jaw moving sideways properly and also causes painful points which can ulcerate the tongue and insides of the cheeks.

Changes of the dental arcade with ageing

A horse's milk (or deciduous) teeth are replaced by permanent teeth by the time they are five years old. The teeth all grow until about six years old. The crowns are worn away, particularly the molars, through abrasion against the teeth on the opposite jaw. Thus the young horse has a much larger reserve molar crown than the older horse.
The incisors, which have relatively long crowns, do not continue to erupt to the same extent as the molars. Consequently, wear with age lowers the amount of crown, even to the gum in the very old horse. Unlike the molar they rarely fall out. The canines which are never in contact above and below do not wear out. With the backwards and forwards movement of the lower jaw, hooks may develop on the second upper premolar, which inhibits the forward movement of the lower jaw against the upper jaw. As a result of this inhibited movement the molar furthest back on the lower jaw will also develop a hook, which will then also prevent the lower jaw moving forwards.