ABSTRACT
The term ‘hypsodont’ refers to teeth in which the crown length is greater than that of the root, although the majority of it remains submerged and progressively erupts as the exposed crown is worn down. All the teeth of lagomorphs and hystricomorphs (guinea pigs and chinchillas) and the incisors of myomorphs (rats, mice, gerbils and hamsters) are aradicular hypsodont, characterized by the absence of a true root (aradicular), and are often misleadingly called ‘open rooted’. The germinal tissue at the tooth apex (relatively lucent areas on the lateral radiograph of the normal skull shown [49]) continuously produces new crown, which erupts as the occlusal surface is worn away during mastication of abrasive food material. The parts of the tooth are therefore properly referred to as ‘reserve crown’ (that which is embedded in alveolar bone) and ‘exposed crown’, rather than crown and root. The mandibular teeth grow and erupt faster than those of the maxilla. One study ascertained a rate of approximately 13 cm (5.2 inches) per year for the upper incisors and 20 cm (8 inches) per year for the lower. This process continues at a rate that ensures a constant exposed crown length, in equilibrium with the rate of attrition caused by the rabbit’s natural diet of grass and other fibrous herbage. If the rate of wear is reduced, either due to malocclusion or inappropriate food, the tooth will elongate abnormally. The rate of growth and attrition varies between individuals and is influenced by factors such as pregnancy, age and diet.
