The response of the rabbit mandibular incisor to experimental shortening and to the prevention of its eruption

Measurements have been made of the eruption rates of the mandibular incisors of domestic rabbits, to examine the effects of shortening one or both incisors for a period of three weeks. Concomitant measurements were made of the rate of wear and the length of the incisors, and of a number of other values defining the occlusion of the lower incisors with the uppers. In occlusion the incisors erupt at rates around 280 $\mu /\text{day}$. On being shortened their rates rise within a day to about 600 $\mu /\text{day}$ and are maintained at about 700 $\mu /\text{day}$, while the incisor is kept out of occlusion. During such changes the growing base does not alter its position. Thus under these conditions eruption rate is an index of the rate of growth of tooth-forming tissues which, in turn, is controlled by the forces exerted, in biting, on the incisor tip. When eruption is prevented by the insertion of a pin through tooth and bone, the basal tissue does not stop growing; it becomes folded and the base of the socket may be resorbed. This indicates that the control of basal growth by occlusal pressure does not extend below a minimum level of eruption, which may be estimated as about 100 $\mu /\text{day}$. The eruption rates of the right and left incisors of an animal tend to vary together when both are in, or out of, occlusion. Different factors hold the rates together under these two conditions, since there is no significant correlation between the rates of a shortened incisor and its unshortened fellow. There is no consistent relationship between the eruption rates of an incisor in occlusion and the potential rates it exhibits when erupting freely, although in either condition there is significant variance between the eruption rates of individuals During a period of experimental shortening, the eruption rates show peak values about 7 days after the incisor is taken out of occlusion. This phenomenon is not related to a change in level of a circulating substance promoting incisor growth; an incisor shortened during the period of peak eruption of its fellow behaves independently in that it shows peak values also about 7 days after being freed from occlusion. The incisor shortened second erupts faster than its fellow and this difference is maintained. During a period of experimental shortening, most operated incisors become proclinated and increase their radius of curvature, and such teeth usually show marked depression of their eruption rates in the following period, possibly through an increased proportion of the force of the bite being borne by the proclinated incisor.