The purpose of this study was to evaluate the stress distribution of an upper central incisor treated with dental post and core. Three kinds of dental post and core systems（cast metal post and core, fiber post with composite resin core, and stainless steel post with composite resin core）were simulated through a 3-dimentional finite element analysis method. 100N of load was applied to the teeth from three different directions（center of the lingual surface of the crown at 45°, the incisal edge of the crown at 45°, and the incisal edge of the crown at 0°to the long axis of the tooth）. The following results were obtained. In the fiber post with composite resin core, maximum principal stress was more decreased than the cast metal post and stainless steel post, at the around the apical dentin area. Although maximum principal stress of each cores were almost same at the dentin around the alveolar bone area, the value of the fiber post with composite resin core was more increased than the value at the apical dentin area. These results suggested that the fiber post with composite resin core would be the best choice for core buildup to avoid vertical root fracture. In each cores, ferrule effect is very important to avoid horizontal root fracture.

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Periodontal mechanoreceptors are dually innervated by primary neurons whose cell bodies exist in the trigeminal ganglion（TG）and the mesencephalic trigeminal nucleus（MTN）. We obtained simultaneous recordings of single- unit responses of primary periodontal mechanoreceptive neurons located in the TG and MTN of the same animal to controlled experimental forces applied to a single tooth. Simultaneous single-unit recordings of electrophysiologically identified primary periodontal mechanoreceptive neurons in a rat’s TG and MTN were performed while isosceles trapezoidal or rectangular mechanical stimuli were being applied in either the linguolabial or the labiolingual direction to the ipsilateral maxillary incisor. Fifty-four single units were recorded in 27 rats and classified as rapidly adapting（RA）or slowly adapting（SA）periodontal neurons on the basis of their responses to a decisive rectangular mechanical stimulus applied in the linguolabial direction. Significantly more RA neurons existed in the MTN than the TG. The response characteristics of the SA and RA periodontal neurons were quite similar between the TG and MTN. The trapezoidal and rectangular stimuli applied in the linguolabial direction elicited a vigorous sustained spike discharge in SA periodontal neurons in both the TG and MTN, whereas both stimuli applied in the opposite（labiolingual）direction did not elicit any SA responses. Rectangular stimuli applied in both directions elicited high-threshold ON-OFF spike responses in the RA periodontal neurons in both the TG and MTN. We also found an unreported burst-type periodontal neuron; its responsiveness was characterized by a brief repetitive spike discharge of high frequency in response to a slight tapping applied in both directions. Six of the 17 TG-RA neurons examined were this burst-type neuron. From the above results, it is presumed that during incisive process of rat chewing cycle, rapidly repeated chopping or chiselling movements bring about transient, forceful periodontal- masseteric reflex, which can be produced by abundant periodontal inputs from both the MTN and TG.

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