Advanced biomaterials used for a new telescopic retainer for removable dentures

Removable dentures supported by cast-metal telescopic crowns often exhibit an unpredictable increase or decrease in retentive force after being in clinical use for some time. The objective of the present in vitro study was to develop a new retainer for removable dentures and to evaluate its tribological properties. The new retainer is based on a tapered crown design and consists of a conical all-ceramic abutment crown and a coping made of electroplated gold. It was compared with conventional telescopic retainers made of cast metal. There were 30 specimens in groups of equal size by material used (abutment crown/coping): Group 1, gold/gold; Group 2, titanium/titanium; Group 3, ceramic/electroplated gold. Each specimen consisted of 2 conical-shaped abutment crowns (alpha =4 degrees; h = 6 mm; O(base) = 4,5 mm); their copings were rigidly connected at 25 mm intervals. Retentive forces were measured with a universal testing machine following axial loading to 5-400 N. Wear was simulated by 500-100, 000 joining and separating cycles in the presence of artificial saliva. Metallographic cross-sections were made to evaluate the specimens' fit and surfaces with an SEM. Retentive forces in Groups 1 and 2 increased with load, exhibiting nondirectional changes after induced wear. Sometimes the alloys' functional surfaces showed considerable tracks of wear. Neither load nor wear had any effect on Group 3 retentive forces (mean(force) = 5.03 N). The functional ceramic and gold surfaces showed no traces of wear and the best fit (median(gap) = 4.9 microm). Replacing cast metals by ceramics and electroplated gold results in retainers with clinically advantageous tribological effects, implying, in particular, high wear resistance.