The biomechanical role of periodontal ligament in bonded and replanted vertically fractured teeth under cyclic biting forces

After teeth are replanted,there are two possible healing responses：periodontal ligament healing or ankylosis with subsequent replacement resorption.The purpose of this study was to compare the fatigue resistance of vertically fractured teeth after bonding the fragments under conditions simulating both healing modes.Thirty-two human premolars were vertically fractured and the fragments were bonded together with Super-Bond C＆B.They were then randomly distributed into four groups（BP,CP,CA,BA）.The BP and CP groups were used to investigate the periodontal ligament healing mode whilst the BA and CA groups simulated ankylosis.All teeth had root canal treatment performed.Metal crowns were constructed for the CP and CA groups.The BP and BA groups only had composite resin restorations in the access cavities.All specimens were subjected to a 260 N load at 4 Hz until failure of the bond or until 2 x 106 cycles had been reached if no fracture occurred.Cracks were detected by stereomicroscope imaging and also assessed via dye penetration tests.Finally,interfaces of the resin luting agent were examined by scanning electron microscope.The results confirmed that the fatigue resistance was higher in the groups with simulated periodontal ligament healing.Periodontal reattachment showed important biomechanical role in bonded and replanted vertically fractured teeth.

Iron casting skin management in no-bake mould – Effects of magnesium residual level and mould coating

The relative performance of coatings for furan resin sand moulds [P-toluol sulphonic acid(PTSA) as hardener] [FRS-PTSA moulds], was compared by analyzing the surface layer for degenerated graphite in Mg treated iron with 0.020 wt.% to 0.054 wt.% Mgres. It was found that the iron nodularising potential(Mg, Ce, La content) and whether the mould coatings contained S, or were capable of desulphurizing were important factors. These moulds have S in the PTSA binder, which aggravates graphite degeneration in the surface layer, depending strongly on the Mgres with lower Mgres increasing the layer thickness. The application of a mould coating strongly influenced graphite deterioration in the surface layer of castings. It either promoted graphite degeneration to less compact morphologies when using S-bearing coatings, or conversely, limited the surface layer thickness using desulphurization type coatings. Independently of the S-source at the metal – mould interface, the presence of sulphur had an adverse effect on graphite quality at the surface of Mg-treated irons, but its negative effect could also reach the graphite phase within the casting section. If the coatings employed desulphurization materials, such as Mg O, or a mixture(Ca O + Mg O + Talc) or Mgbearing Fe Si, they protected the graphite shape, improving graphite nodularity, at the metal – mould interface, and so decreased the average layer thickness in FRS-PTSA moulds. Fe Si Mg was highly efficient in minimizing the casting skin by improving graphite nodularity. It is presumed that the Mg O or(Mg O + Ca O + Talc) based coatings acted to remove any S released by the mould media. The Mg-Fe Si coatings also reacted with S from the mould but additionally supplemented the Mg nodularising potential prior to solidification. This dual activity is achievable with coatings containing active magnesium derived from fine Mg-Fe Si materials.