Abstract
The aim of the study was to see whether the length of the enamel secretion zone in unimpeded rat incisors, measured precisely, is in agreement with the observed decrease in enamel thickness. Unimpeded eruption of mandibular incisors of five experimental and two control rats was induced by cutting off the erupted part of the incisors three times per week for 5weeks. The length of the zone of enamel secretion in unimpeded and impeded control incisors was measured on longitudinal and serial transverse histological sections of fixed, demineralised and embedded hemimandibles. Impeded contralateral incisors were also included in the study. The length of the zone of enamel secretion in unimpeded incisors showed an increase to 8,398±558µm, that is 161% of the length in control incisors (5,213±95µm). The contralateral incisor showed a reduction in eruption rate, in length of the secretion zone, and the whole tooth was shifted somewhat apically. The measured length of the secretion zone is in agreement with the observed thickness of enamel (98µm) in unimpeded incisors. The reduced eruption rate and the apical shift of the contralateral incisor are probably due to an increased occlusal load.
Highlights
Dentin thickness is halved, but enamel thickness is only reduced to about 80% (Risnes, Møinichen, Septier, & Goldberg, 1996; Risnes et al, 1995)
We measured the length of the enamel secretion zone in unimpeded and control incisors in order to be able to relate it to the previously measured enamel thickness
If the enamel apposition rate remains constant during unimpeded eruption, an enamel thickness of 98 μm will be elaborated in 7.3 days (98/13.5)
Summary
When the rat lower incisor is cut repeatedly out of occlusion, its rate of eruption is about doubled (Schour & Medak, 1951; Taylor & Butcher, 1951; Herrmann, 1953; Robinson, Kirkham, & Nutman, 1988; Steigman, Michaeli, Yitzhaki, & Weinreb, 1989; Risnes, Septier, & Goldberg, 1995; Gomes et al, 2013) the ameloblasts and odontoblasts, which are carried along with the erupting incisor as they produce enamel and dentin (Risnes, 1979), will, theoretically, only have about half the normal time available for their production. Dentin thickness is halved, but enamel thickness is only reduced to about 80% (Risnes, Møinichen, Septier, & Goldberg, 1996; Risnes et al, 1995) This discrepancy could possibly be explained by a concomitant lengthening of the zone of enamel secretion. A lengthening of the enamel secretion zone has been observed; Robinson, Kirkham, and Nutman (1988) found a lengthening to 139%, Kirkham et al (1993) to 142%, while Skobe et al (1993) did not quantify the lengthening None of these authors detected a decrease in enamel thickness in the unimpeded incisors and did not try to relate enamel thickness to length of the enamel secretion zone. The aim of the present study was to see whether the length of the enamel secretion zone in unimpeded incisors, measured precisely, is in agreement with the observed decrease in enamel thickness
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