Influence of repeated firings on marginal, axial, axio-occlusal, and occlusal fit of metal-ceramic restorations fabricated with different techniques.

STATEMENT OF PROBLEM: Ill-fitting restorations can jeopardize dental, periodontal, and systemic health. Repeated ceramic firing may influence the fitting accuracy of restorations, but scientific information regarding this issue is lacking.

PURPOSE: The purpose of this in vitro study was to assess and compare the influence of repeated firings on the marginal, axial, axio-occlusal, and occlusal fit of metal-ceramic 3-unit fixed partial dentures (FPDs) manufactured with different framework methods, including casting, fully sintered hard alloy milling (FHAM), presintered soft alloy milling (PSAM), and selective laser sintering (SLS).

MATERIAL AND METHODS: A total of 60 Co-Cr metal frameworks for 3-unit FPDs were fabricated with 3 different computer-aided design and computer-aided manufacturing (CAD-CAM) techniques and conventional casting for a typodont model. The discrepancy values for premolar and molar abutments were measured by using the silicone replica technique after fabrication of the framework, second firing, fourth firing, and seventh firing. The replicas were separated into mesiodistal and buccolingual cross-sections, and digital images of the cross-sections were obtained with a stereomicroscope at ×80 magnification. The measurement points were examined to record the discrepancy values (μm) with in-built measuring software. The recorded data were subjected to repeated-measures 4-way ANOVA and the Bonferroni post hoc test (α=.05).

RESULTS: For all measurement periods, the marginal, axial, axio-occlusal, and occlusal discrepancy values differed significantly among the manufacturing groups (P<.001). Moreover, the repeated firings had a statistically significant influence on the discrepancy values of PSAM, FHAM, and SLS groups (P<.05). The cast group was not significantly affected by repeated firings (P>.05). The PSAM group revealed the lowest discrepancy values at all measurement periods (P<.05), except after the second firing in the marginal and axial locations.

CONCLUSIONS: The PSAM-fabricated restorations demonstrated superior fitting accuracy. All manufacturing groups were influenced by repeated firings, except the cast group. The marginal discrepancy values of all manufacturing groups were within the range of clinical acceptability (<120 μm).