Marginal fit of zirconia FPDs and influence of veneering process

Objectives: The aim of this in vitro study was to evaluate marginal and internal fit of zirconia posterior four-unit FPDs. Furthermore, possible changes in restorations' fit during the veneering process were determined.

Methods: Four groups of all-ceramic FPDs (teeth 24-27) with ten specimens each were fabricated. Thirty frameworks were made of partially sintered zirconia [Cercon (Ce), Everest (Ev), Vita/inLab (Vi)], ten frames of fully sintered and HIP'ed zirconia [Digizon (Di)]. Marginal and internal fit of the frames were determined by means of a replica technique with four measuring locations being received for every abutment. Overall, five different measurements (horizontal, vertical and marginal discrepancy as well as marginal and internal gap) per measuring location were made. Afterwards, FPDs were veneered and once again investigated concerning their fit. Influence of different systems was evaluated by one-way ANOVA, influence of veneering process by pairwise t-test for specific systems.

Results: With the frameworks mean values ranged between 38 (Ev) and 116 µm (Ce) for horizontal, between 24 (Di) and 157 µm (Ev) for vertical and between 1 (Di) and 206 µm (Ev) for marginal discrepancies. Marginal gap varied between 102 (Vi) and 138 µm (Di), internal gap between 81 (Vi) und 112 µm (Ev). Differences between several systems were partly significant. Veneering process caused significant changes with all measuring sections at Ev and with two measuring sections at Ce and Di, respectively. With the Vita/inLab system no significant changes due to veneering were discovered.

Conclusions: Marginal and internal fit of specimens were significantly dependent on the CAD/CAM system used. The most likely explanation for changes of marginal fit during the veneering process are discrepancies in thermal expansion between veneering and core ceramics. In particular for multi-unit FPDs marginal accuracy has to be improved to ensure long term success of the restorations.