Marginal discrepancies of Ni/Cr crowns for a soft tissue-level, trans-mucosal implant system

To examine the marginal fit of frameworks for implant-supported single crowns built with or without the use of prefabricated plastic copings.

Sixty premolar sized metal crown frameworks for cemented implant restorations were prepared by two separate dental laboratories. Twenty were prepared with the use of original plastic copings for burn-out (OPC), twenty without any prefabricated plastic coping (WPC), and finally twenty with modified plastic copings (MPC). Specimens were inspected at 100× magnification. Marginal discrepancies can emerge at the vertical level as marginal gaps (MG) or horizontal differences in the circumferential precision of the restorations (HEX). HEX and MG were recorded at 10 different points in each metal crown. The ANOVA test was performed to estimate if there were statistically significant differences between the two dental laboratories. ANOVA Test was also used between Groups OPC, WPC, and MPC to evaluate differences in the mean MG and HEX values. The Tukey's post hoc test was also performed to determine whether couples of data sets were different or not.

No statistically significant differences regarding HEX and MG were found among the two dental laboratories. Significant differences were found among OPC, WPC, and MPC groups both for MG and HEX (P < 0.05). Tukey post hoc tests revealed that there were statistically significant differences (P-value < 0.05) in all pairs of groups and for both dimensions measured. The OPC group showed horizontal and vertical over-extension. The WPC group margins appeared vertically under-extended but horizontally over-extended. The WPC groups showed unpredictability of results in the vertical dimension. The MPC group margins demonstrated horizontal over-extension and appeared vertically slightly under-extended. The frameworks belonging to this group appeared the most precise in the vertical dimension.

Casting Ni/Cr alloys without the use of prefabricated plastic components leads to unpredictable precision in the vertical marginal discrepancies. The use of plastic copings results in more predictable but horizontally and vertical over-extended margins when the snap-on mechanism is not removed properly after the casting due to the hardness of the Ni/Cr alloys. The removal of the snap-on mechanism after the wax-up and before casting (MPC group) results in more uniform and smaller vertical marginal discrepancies.