Conical crowns with electroplated gold copings: retention force changes caused by wear and combined off-axial load

The aim of this study was to examine the wear behavior of conical crowns of gold alloy and zirconium dioxide ceramics facing electroplated gold copings.

The conical crowns were milled in Group A of a cast gold alloy (Degunorm(®), DeguDent(®)) and B zirconium dioxide (Cercon(®), DeguDent(®)). Fifteen specimens were milled per group with a conicity of 2°. The electroplated coping was established by direct electroforming with the Solaris system (DeguDent(®)) and glued into a framework. An apparatus accomplishing 5000 wear cycles performed the wear test. After each of the cycles, extra axial forces (80 N) were loaded 1 million times on an extension of the framework. The retentive forces and the correlating distance during insertion and separation were measured. The wear test was separated in a start phase, an initial wear phase and the long-term wear period. The retention force value and the force-distance integral of the first 0.3 mm of each cycle were calculated.

The changes of retention force (ΔF) and integral (ΔF(d)) differed significantly between both groups in the start phase (ΔF: A -1.85 N, B +1.39 N, P=0.0028; ΔF(d): A -0.155 N mm, B +0.2685 N mm, P=0.0378) and the initial wear phase. The long-term wear period showed no differences in force and integral development. As a statistical test for the comparison of the median values of the two groups, the Mann-Whitney test was used. The median force never fell below 4 N or exceeded 12 N.

Within the limitations of this study, the conical crowns tested showed clinically acceptable retentive properties. The values of about 4-6 N mentioned as sufficient in the recent literature were always attained. As the most retentive elements for dentures, both groups showed the main changes in retentive forces during the first 2000 cycles. The retention force development of the zirconia group appears to be less excursive over the whole wear test.