Sintering temperature accuracy and its effect on translucent yttria-stabilized zirconia: Flexural strength, crystal structure, tetragonality and light transmission.

OBJECTIVES: A crucial step in the preparation of yttria-stabilized zirconia materials (YSZ) is the final sintering step. Sintering parameters affect phase composition, grain growth and porosity of the material which, in turn, influence both mechanical and optical properties. Discrepancies of + /- 5% are common between actual and displayed firing temperatures depending on sintering furnace brand and condition. The aim of this study was therefore to investigate how such sintering temperature deviations in dental furnaces compared to the recommended firing protocols affected certain material properties of different yttria-stabilized zirconia materials. METHODS: Disc-shaped specimens were made from four different translucent and high translucent YSZ-powders and analysed regarding crystal structure; unit cell volume for the c, t, and t' -phases along with the tetragonality for the t and t' phases. Biaxial flexure strength and the spectral transmittance through the ceramic specimens were also measured. RESULTS: Deviations of 5% from the sintering temperature recommended for YSZ materials with different yttria content influenced material properties such as light transmittance, flexural strength, crystal phase fractions, tetragonality, and grain growth. Too low temperature resulted in decreased strength for some of the translucent zirconia materials, while others where less affected. Light transmittance varied depending on several factors such as grain size, crystal phase fractions and binder content in the start material prior to pre-sintering. SIGNIFICANCE: The use of high quality, wellcalibrated furnaces is crucial when sintering YSZ materials to avoid unwanted material changes. CONCLUSIONS: Deviations of 5% from the sintering temperature recommended for YSZ materials with different yttria content influence material properties such as light transmittance, flexural strength, crystal phase fractions, tetragonality, and grain growth. Too low temperature results in decreased strength for some translucent zirconia materials, while others are less affected. Light transmittance varies depending on several factors such as grain size, crystal phase fractions and binder content in the start material prior to pre-sintering. Consequently, the use of high quality, well-calibrated furnaces is crucial when sintering YSZ materials to avoid unwanted material changes.

Is cigarette smoking a promotor of the menopause?

All women (n = 11,809) aged 44-53 years from a representative district of Denmark were asked by questionnaire about their date of birth, date of latest menstrual bleeding, possible gynaecological operations, smoking habits, and use of medications. A total of 9,411 (80%) questionnaires were returned. After exclusions because of incomplete information (n = 275), possible surgical menopause (n = 1,270), and treatment with gonadal hormones (n = 2,221), 5,645 women remained suitable for to study. The reproducibility of the information given by these women was excellent as judged by personal interviews of 315 participants. The study population was divided into five 2-year age groups, and the proportion of postmenopausal women among non-smokers and heavy smokers was calculated. Differences in respect to the menopausal status were not observable in the oldest and the youngest two age groups comprising just a few per cent of post-menopausal women. However, heavy smokers in the other two age groups had passed the menopause earlier than the non-smokers (p less than 0.001). This finding suggests cigarette smoking as a promotor of the menopause.