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Runx2/core binding factor alpha 1 (Cbfa1) and Osterix (Osx) are osteoblast-specific transcription factors essential for the development of a mature osteoblast phenotype and are thought to activate osteoblast marker genes in vivo to produce a bone-specific matrix. Dexamethasone (Dex) is known to be a potent stimulator of osteoblastic differentiation in vitro, however, the exact role is still unclear. To investigate the mechanisms of the stimulation of osteoblastic differentiation by Dex, we evaluated the effects of Dex on proliferation and mineralization as well as on mRNA expression of Cbfa1, Osx and osteoblast marker genes, osteocalcin (OC) and bone sialoprotein (BSP) mRNAs in differentiating foetal rat calvarial cells (FRCC), which were cultured for 35 days in the presence or absence of 10(-7) M Dex. Treatment of FRCC with Dex resulted in the stimulation of cell proliferation and increased the number of cells, which are able to produce bone-like nodules with a mineralized matrix when compared to untreated controls. Northern blot analysis revealed that, in the absence of Dex, Cbfa1 mRNA expressed at day 8, while Osx mRNA expressed at day 15. Subsequently expression of these mRNAs increased up to day 21, followed by constant expression during the culture period. The expression of OC and BSP mRNAs appeared to be synchronous with that of Osx mRNA and was detectable at day 15 with an increase thereafter. The presence of Dex resulted in an induction in Cbfa1 and Osx mRNA expression. The former appeared at day 5 and the latter appeared at day 11. Subsequently expression of Cbfa1 and Osx mRNAs increased up to day 15 with a decrease thereafter. Expression of OC and BSP mRNAs appeared to be coincident with that of Osx mRNA and was detectable at day 11 and reached a maximum at day 15 followed by constant expression. These observations indicate that induction of Cbfal and Osx mRNAs by Dex may be followed by activation of osteoblast marker genes such as OC and BSP mRNAs to produce a bone-specific matrix that subsequently becomes mineralized. Thus, it is likely that Dex may promote osteoblastic differentiation and mineralization of FRCC by inducing the expression of Cbfa1 and Osx genes in vitro.

Objectives: To estimate the risk of injury from radiofrequency (RF) heating of metallic dental devices in use during 3.0 T MRI. Methods: The whole-body specific absorption rate (WB-SAR) was calculated on the basis of saline temperature elevation under the maximum RF irradiation for 15 min to determine the operation parameters for the heating test. The temperature changes of three types of threeunit bridges, a full-arch fixed dental prosthesis and an orthodontic appliance in use during MRI with a 3.0 T MR system (Magnetom Ò Verio; Siemens AG, Erlangen, Germany) were then tested in accordance with the American Society for Testing and Materials F2182-09 standardized procedure under the maximum RF heating during 15 min RF irradiation. Results: The system console-predicted WB-SAR was approximately 1.4 W kg 21 and that measured with a saline phantom was 2.1 W kg 21. In the assessment of RF heating, the highest temperature increase was 11.80°C in the bridges, 11.59°C in the full-arch fixed dental prosthesis and 12.61°C in the orthodontic appliance. Conclusions: The relatively minor RF heating of dental casting material-based prostheses in Magnetom Verio systems in the normal operating mode should not pose a risk to patients. However, orthodontic appliances may exhibit RF heating above the industrial standard (CENELEC standard prEN45502-2-3); therefore, the wire should be removed from the bracket or a spacer should be used between the appliance and the oral mucosa during MRI.

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