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Internal adaptation of fixed prosthodontics at shoulder preparation level evaluated by horizontal sections.

AIM
The precision of fixed prosthodontic restorations is fundamental for clinical success: well-fitting crowns reduce the risk of recurrent caries and periodontal disease. The aim of this study is to evaluate the internal fit of fixed prosthodontics at the shoulder preparation level by examining horizontal sections.


METHODS
Twenty-four extracted teeth were resin-embedded and prepared on the platform of an iso-parallelometer with a 90 degrees shoulder with a rounded internal angle. Auro Galva Crown (AGC) copings were cemented in place. The preparations were observed by 3 different assessors at 8 points, first externally and then internally at 2 levels by grinding the specimen perpendicular to the long axis at 0.5 mm and at 0.2 mm from the margin of the preparation. A correction factor was calculated to derive real values from measured values. The results were analyzed using a linear regression with robust standard errors, accounting for within-subject correlation introduced by multiple measurements. Shrout-Fleiss Intraclass Correlation Coefficient (ICC) for Inter-Rater Reliability were calculated at each stage.


RESULTS
Internal measurements at 0.5 and 0.2 mm from the margin provided data similar to the external margin data. Average inter-assessor differences were in the range of 2 mm. ICC ranged from 0.93 for the 0.5 mm level to 0.97 for the external level.


CONCLUSIONS
External measurements effectively predict the internal precision at the shoulder level. Horizontal perimarginal sections allow the fit to be studied through the evaluation of a great number of points. Traditional vertical sections for the evaluation of internal fit enable only a few points to be observed. This internal observation method may be suitable for testing new materials.