In gnarled enamel consisting of a spiral arrangement of prisms subjacent to the enamel cusp, spiral prisms, some of which may be transformed into double-bordered prisms, were found by scanning electron microscopy. It has been reported that such abnormal prisms temporarily appear during the formation of normal prisms in the inner enamel. The spiral prisms in the gnarled enamel, however, alternatively appeared in each normal prism. Such an appearance may be related to the spiral arrangement of prisms in the gnarled enamel.

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The lateral oblique transcranial radiograph is valuable in the diagnosis of temporomandibular joint dysfunction and in the study of the joint of one patient during long-term treatment. Until recently, only earrods had been used at Showa University Dental Hospital to fixate a patient's head. A new head-positioning subattachment for use in radiography of the temporomandibular joint was designed. This subattachment consists of an orbital aligning aid, a nasion rest, and a back support. On measurement of the temporomandibular joint space, the mean difference between the first and the second exposure was approximately 0.1 mm, and the maximum difference was approximately 0.3 mm. Reproducibility was high. The results suggest that the newly developed positioning subattachment device is useful for longitudinal studies of the geometric relationship between the fossa and the condyle.

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This study investigated the effect of implant timing on bone tissue reactions after placement of hydroxyapatite-coated implants into the tibiae of ovariectomized rats. Female Wistar rats, aged 12 weeks, were subjected to bilateral ovariectomy or sham surgery. Seven or 21 days after surgery, implants were placed in the proximal part of the tibia. Rats were killed 14 days after implantation, and bone tissue reactions were examined histologically. Only a slight difference was noted between ovariectomized and sham-operated rats in bone formation around implants and at areas of contact between hydoxyapatite and bone. In ovariectomized rats, newly formed bone trabeculae tended to decrease more rapidly than in sham-operated rats ; the decrease was especially rapid in rats that had received implants 21 days after ovariectomy. Therefore, bone resorption might progress more rapidly around implants placed the usual interval after ovariectomy than around implants placed earlier after ovariectomy. These results suggest that the application of dental implants to postmenopausal patients who have increased bone turnover must be carefully considered with respect to bone resorption around the implant.

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Periapical inflammation of deciduous teeth is one factor that influences the succeeding permanent tooth germ. The course of eruption of the succeeding permanent tooth and its hypoplastic status were observed. Eighteen deciduous molars from 11 children, whose permanent tooth germs seemed to be infected because periapical inflammation of deciduous teeth, were examined in this study. From the formative stage of the permanent tooth germ during infection, the clinical course was continuously observed with special attention to the degree of permanent tooth hypoplasia immediately after eruption and the tooth's growth status during the 3 years after eruption. The results were as follows : 1) Hypoplastic molar teeth were more frequently observed in 1st premolars than in 2nd premolars and in the maxilla more frequently than in the mandible. 2) Infection occurred in most cases when enamel calcification was insufficient. 3) A possible relationship was found between the degree of permanent tooth hypoplasia and the tooth's formative stage when the permanent tooth germ seemed to have become. 4) In many cases permanent teeth, erupted earlier on the infected side than on the noninfected side.

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