Molar Axial Height Effect on Retention of CAD/CAM Ceramic Crowns

Objectives: To evaluate the significance of maxillary molar axial wall height with retention of adhesively-luted, all-ceramic CAD/CAM crowns.
Methods: Recently-extracted maxillary third molars were randomly divided into 5 groups (n=12) with all-ceramic crown preparations accomplished using a high speed handpiece inserted into a milling device. Preparations contained a 10-degree taper with each group consisting of axial wall heights of 0,1,2,3,and 4 four millimeters. Completed preparation surface area was determined using a digital measuring microscope (Hirox). Scanned preparations (CEREC) were fitted with e.max CAD crowns and luted with RelyX Unicem after HF acid etching and silanation. All manufacturer recommendations were followed. Specimens were stored at 37C/98% humidity and tested to failure after 24 hours at a 45-degree angle on a universal testing machine. Failure load was converted in MPa using the available bonding surface area with mean data analyzed using ANOVA/ Tukey’s post hoc test (p=0.05).
Results: Results are listed in the table. 2mm and 3mm axial height preparations demonstrated the greatest failure resistance followed by the 1mm and 4mm groups.
Conclusions: Under the conditions of this study, maxillary molars restored with adhesively-luted, CAD/CAM fabricated, IPS eMax CAD crowns with axial wall heights of 2 and 3mm demonstrated higher failure resistance compared to either 1 or 4mm axial wall height. This study provides some evidence that adhesion may compensate for less axial wall height in all-ceramic preparations with a 10 degree taper.