PubMed · 7043433
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1981. [Engineering problems in making cast crowns].. https://pubmed.ncbi.nlm.nih.gov/7043433/
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Clinical experiences for the past 9 years have demonstrated the practicability of the O-ring coping attachment, a modification of the telescopic crown-and-sleeve coping retainers, for removable partial dentures. A circumferential groove placed in the primary coping receives an elastomeric O-ring that fits into a corresponding groove made in the internal surface of the telescopic crown. The O-ring not only provides controllable retention but also acts as a shock absorber. Long-term retention of the prostheses can be easily maintained by periodic replacements of the O-rings. Sophisticated procedures and expensive machines are not required to make the prostheses. Excellent patient acceptance and the versatility in clinical applications make this system one of the winning designs for removable partial prosthodontics.
It has been shown that iatrogenic preparation damage increases caries progression and the need for restorative therapy of the adjacent tooth. The aim of this study was to quantify the damage of the adjacent tooth using three different preparation techniques: (1) diamond bur alone; (2) diamond bur combined with the EVA system and the Cavishape file; (3) diamond bur combined with an axial margin trimmer. All 136 preparations were performed by dental practitioners in their own practice. Examination of the area directly opposite to the axial box margin revealed significantly less damage (p < 0.01) using additionally the EVA system with the Cavishape file or the axial margin trimmer compared to using only diamond burs. The area opposite between the two box margins, which was in all three methods prepared only by the diamond bur, showed no significant differences between the methods under study. There were only a few teeth (overall 5%) with no damage of the adjacent surface. In order to reduce iatrogenic damage for the adjacent teeth the preparation technique has to be modified.
The fracture resistance, surface hardness and microstructure of different manually and automatically mixed temporary crown and bridge resins and two light cured materials were examined in this study. The materials on butylmethacrylate (BMA) based showed low transverse strength compared to the materials on urethanmethacrylate (UDMA) and Bis-GMA base. Their values corresponded to the transverse strength of a manually mixed, chemically cured resin on PMMA base. The automatically mixed material on Bis-GMA base had lower strength values and surface hardness but fewer macroporosities compared to the manually mixed resins. The light cured material on UDMA base showed a transverse strength and surface hardness, which were higher than the values of the materials on Bis-GMA base. The strength values of the light cured material on PMMA base decreased within 7 days and were already after 24 h in the range of the materials on BMA base.